1: \input texinfo @c -*-texinfo-*-
2: @comment The source is gforth.ds, from which gforth.texi is generated
3:
4: @comment TODO: nac29jan99 - a list of things to add in the next edit:
5: @comment 1. x-ref all ambiguous or implementation-defined features?
6: @comment 2. Describe the use of Auser Avariable AConstant A, etc.
7: @comment 3. words in miscellaneous section need a home.
8: @comment 4. search for TODO for other minor and major works required.
9: @comment 5. [rats] change all @var to @i in Forth source so that info
10: @comment file looks decent.
11: @c Not an improvement IMO - anton
12: @c and anyway, this should be taken up
13: @c with Karl Berry (the texinfo guy) - anton
14: @c
15: @c Karl Berry writes:
16: @c If they don't like the all-caps for @var Info output, all I can say is
17: @c that it's always been that way, and the usage of all-caps for
18: @c metavariables has a long tradition. I think it's best to just let it be
19: @c what it is, for the sake of consistency among manuals.
20: @c
21: @comment .. would be useful to have a word that identified all deferred words
22: @comment should semantics stuff in intro be moved to another section
23:
24: @c POSTPONE, COMPILE, [COMPILE], LITERAL should have their own section
25:
26: @comment %**start of header (This is for running Texinfo on a region.)
27: @setfilename gforth.info
28: @include version.texi
29: @settitle Gforth Manual
30: @c @syncodeindex pg cp
31:
32: @macro progstyle {}
33: Programming style note:
34: @end macro
35:
36: @macro assignment {}
37: @table @i
38: @item Assignment:
39: @end macro
40: @macro endassignment {}
41: @end table
42: @end macro
43:
44: @comment macros for beautifying glossary entries
45: @macro GLOSS-START {}
46: @iftex
47: @ninerm
48: @end iftex
49: @end macro
50:
51: @macro GLOSS-END {}
52: @iftex
53: @rm
54: @end iftex
55: @end macro
56:
57: @comment %**end of header (This is for running Texinfo on a region.)
58: @copying
59: This manual is for Gforth (version @value{VERSION}, @value{UPDATED}),
60: a fast and portable implementation of the ANS Forth language. It
61: serves as reference manual, but it also contains an introduction to
62: Forth and a Forth tutorial.
63:
64: Copyright @copyright{} 1995, 1996, 1997, 1998, 2000, 2003, 2004,2005,2006,2007,2008,2009,2010,2011 Free Software Foundation, Inc.
65:
66: @quotation
67: Permission is granted to copy, distribute and/or modify this document
68: under the terms of the GNU Free Documentation License, Version 1.1 or
69: any later version published by the Free Software Foundation; with no
70: Invariant Sections, with the Front-Cover texts being ``A GNU Manual,''
71: and with the Back-Cover Texts as in (a) below. A copy of the
72: license is included in the section entitled ``GNU Free Documentation
73: License.''
74:
75: (a) The FSF's Back-Cover Text is: ``You have freedom to copy and modify
76: this GNU Manual, like GNU software. Copies published by the Free
77: Software Foundation raise funds for GNU development.''
78: @end quotation
79: @end copying
80:
81: @dircategory Software development
82: @direntry
83: * Gforth: (gforth). A fast interpreter for the Forth language.
84: @end direntry
85: @c The Texinfo manual also recommends doing this, but for Gforth it may
86: @c not make much sense
87: @c @dircategory Individual utilities
88: @c @direntry
89: @c * Gforth: (gforth)Invoking Gforth. gforth, gforth-fast, gforthmi
90: @c @end direntry
91:
92: @titlepage
93: @title Gforth
94: @subtitle for version @value{VERSION}, @value{UPDATED}
95: @author Neal Crook
96: @author Anton Ertl
97: @author David Kuehling
98: @author Bernd Paysan
99: @author Jens Wilke
100: @page
101: @vskip 0pt plus 1filll
102: @insertcopying
103: @end titlepage
104:
105: @contents
106:
107: @ifnottex
108: @node Top, Goals, (dir), (dir)
109: @top Gforth
110:
111: @insertcopying
112: @end ifnottex
113:
114: @menu
115: * Goals:: About the Gforth Project
116: * Gforth Environment:: Starting (and exiting) Gforth
117: * Tutorial:: Hands-on Forth Tutorial
118: * Introduction:: An introduction to ANS Forth
119: * Words:: Forth words available in Gforth
120: * Error messages:: How to interpret them
121: * Tools:: Programming tools
122: * ANS conformance:: Implementation-defined options etc.
123: * Standard vs Extensions:: Should I use extensions?
124: * Model:: The abstract machine of Gforth
125: * Integrating Gforth:: Forth as scripting language for applications
126: * Emacs and Gforth:: The Gforth Mode
127: * Image Files:: @code{.fi} files contain compiled code
128: * Engine:: The inner interpreter and the primitives
129: * Cross Compiler:: The Cross Compiler
130: * Bugs:: How to report them
131: * Origin:: Authors and ancestors of Gforth
132: * Forth-related information:: Books and places to look on the WWW
133: * Licenses::
134: * Word Index:: An item for each Forth word
135: * Concept Index:: A menu covering many topics
136:
137: @detailmenu
138: --- The Detailed Node Listing ---
139:
140: Gforth Environment
141:
142: * Invoking Gforth:: Getting in
143: * Leaving Gforth:: Getting out
144: * Command-line editing::
145: * Environment variables:: that affect how Gforth starts up
146: * Gforth Files:: What gets installed and where
147: * Gforth in pipes::
148: * Startup speed:: When 14ms is not fast enough ...
149:
150: Forth Tutorial
151:
152: * Starting Gforth Tutorial::
153: * Syntax Tutorial::
154: * Crash Course Tutorial::
155: * Stack Tutorial::
156: * Arithmetics Tutorial::
157: * Stack Manipulation Tutorial::
158: * Using files for Forth code Tutorial::
159: * Comments Tutorial::
160: * Colon Definitions Tutorial::
161: * Decompilation Tutorial::
162: * Stack-Effect Comments Tutorial::
163: * Types Tutorial::
164: * Factoring Tutorial::
165: * Designing the stack effect Tutorial::
166: * Local Variables Tutorial::
167: * Conditional execution Tutorial::
168: * Flags and Comparisons Tutorial::
169: * General Loops Tutorial::
170: * Counted loops Tutorial::
171: * Recursion Tutorial::
172: * Leaving definitions or loops Tutorial::
173: * Return Stack Tutorial::
174: * Memory Tutorial::
175: * Characters and Strings Tutorial::
176: * Alignment Tutorial::
177: * Floating Point Tutorial::
178: * Files Tutorial::
179: * Interpretation and Compilation Semantics and Immediacy Tutorial::
180: * Execution Tokens Tutorial::
181: * Exceptions Tutorial::
182: * Defining Words Tutorial::
183: * Arrays and Records Tutorial::
184: * POSTPONE Tutorial::
185: * Literal Tutorial::
186: * Advanced macros Tutorial::
187: * Compilation Tokens Tutorial::
188: * Wordlists and Search Order Tutorial::
189:
190: An Introduction to ANS Forth
191:
192: * Introducing the Text Interpreter::
193: * Stacks and Postfix notation::
194: * Your first definition::
195: * How does that work?::
196: * Forth is written in Forth::
197: * Review - elements of a Forth system::
198: * Where to go next::
199: * Exercises::
200:
201: Forth Words
202:
203: * Notation::
204: * Case insensitivity::
205: * Comments::
206: * Boolean Flags::
207: * Arithmetic::
208: * Stack Manipulation::
209: * Memory::
210: * Control Structures::
211: * Defining Words::
212: * Interpretation and Compilation Semantics::
213: * Tokens for Words::
214: * Compiling words::
215: * The Text Interpreter::
216: * The Input Stream::
217: * Word Lists::
218: * Environmental Queries::
219: * Files::
220: * Blocks::
221: * Other I/O::
222: * OS command line arguments::
223: * Locals::
224: * Structures::
225: * Object-oriented Forth::
226: * Programming Tools::
227: * C Interface::
228: * Assembler and Code Words::
229: * Threading Words::
230: * Passing Commands to the OS::
231: * Keeping track of Time::
232: * Miscellaneous Words::
233:
234: Arithmetic
235:
236: * Single precision::
237: * Double precision:: Double-cell integer arithmetic
238: * Bitwise operations::
239: * Numeric comparison::
240: * Mixed precision:: Operations with single and double-cell integers
241: * Floating Point::
242:
243: Stack Manipulation
244:
245: * Data stack::
246: * Floating point stack::
247: * Return stack::
248: * Locals stack::
249: * Stack pointer manipulation::
250:
251: Memory
252:
253: * Memory model::
254: * Dictionary allocation::
255: * Heap Allocation::
256: * Memory Access::
257: * Address arithmetic::
258: * Memory Blocks::
259:
260: Control Structures
261:
262: * Selection:: IF ... ELSE ... ENDIF
263: * Simple Loops:: BEGIN ...
264: * Counted Loops:: DO
265: * Arbitrary control structures::
266: * Calls and returns::
267: * Exception Handling::
268:
269: Defining Words
270:
271: * CREATE::
272: * Variables:: Variables and user variables
273: * Constants::
274: * Values:: Initialised variables
275: * Colon Definitions::
276: * Anonymous Definitions:: Definitions without names
277: * Quotations::
278: * Supplying names:: Passing definition names as strings
279: * User-defined Defining Words::
280: * Deferred Words:: Allow forward references
281: * Aliases::
282:
283: User-defined Defining Words
284:
285: * CREATE..DOES> applications::
286: * CREATE..DOES> details::
287: * Advanced does> usage example::
288: * Const-does>::
289:
290: Interpretation and Compilation Semantics
291:
292: * Combined words::
293:
294: Tokens for Words
295:
296: * Execution token:: represents execution/interpretation semantics
297: * Compilation token:: represents compilation semantics
298: * Name token:: represents named words
299:
300: Compiling words
301:
302: * Literals:: Compiling data values
303: * Macros:: Compiling words
304:
305: The Text Interpreter
306:
307: * Input Sources::
308: * Number Conversion::
309: * Interpret/Compile states::
310: * Interpreter Directives::
311:
312: Word Lists
313:
314: * Vocabularies::
315: * Why use word lists?::
316: * Word list example::
317:
318: Files
319:
320: * Forth source files::
321: * General files::
322: * Redirection::
323: * Search Paths::
324:
325: Search Paths
326:
327: * Source Search Paths::
328: * General Search Paths::
329:
330: Other I/O
331:
332: * Simple numeric output:: Predefined formats
333: * Formatted numeric output:: Formatted (pictured) output
334: * String Formats:: How Forth stores strings in memory
335: * Displaying characters and strings:: Other stuff
336: * String words:: Gforth's little string library
337: * Terminal output:: Cursor positioning etc.
338: * Single-key input::
339: * Line input and conversion::
340: * Pipes:: How to create your own pipes
341: * Xchars and Unicode:: Non-ASCII characters
342:
343: Locals
344:
345: * Gforth locals::
346: * ANS Forth locals::
347:
348: Gforth locals
349:
350: * Where are locals visible by name?::
351: * How long do locals live?::
352: * Locals programming style::
353: * Locals implementation::
354:
355: Structures
356:
357: * Why explicit structure support?::
358: * Structure Usage::
359: * Structure Naming Convention::
360: * Structure Implementation::
361: * Structure Glossary::
362: * Forth200x Structures::
363:
364: Object-oriented Forth
365:
366: * Why object-oriented programming?::
367: * Object-Oriented Terminology::
368: * Objects::
369: * OOF::
370: * Mini-OOF::
371: * Comparison with other object models::
372:
373: The @file{objects.fs} model
374:
375: * Properties of the Objects model::
376: * Basic Objects Usage::
377: * The Objects base class::
378: * Creating objects::
379: * Object-Oriented Programming Style::
380: * Class Binding::
381: * Method conveniences::
382: * Classes and Scoping::
383: * Dividing classes::
384: * Object Interfaces::
385: * Objects Implementation::
386: * Objects Glossary::
387:
388: The @file{oof.fs} model
389:
390: * Properties of the OOF model::
391: * Basic OOF Usage::
392: * The OOF base class::
393: * Class Declaration::
394: * Class Implementation::
395:
396: The @file{mini-oof.fs} model
397:
398: * Basic Mini-OOF Usage::
399: * Mini-OOF Example::
400: * Mini-OOF Implementation::
401:
402: Programming Tools
403:
404: * Examining:: Data and Code.
405: * Forgetting words:: Usually before reloading.
406: * Debugging:: Simple and quick.
407: * Assertions:: Making your programs self-checking.
408: * Singlestep Debugger:: Executing your program word by word.
409:
410: C Interface
411:
412: * Calling C Functions::
413: * Declaring C Functions::
414: * Calling C function pointers::
415: * Defining library interfaces::
416: * Declaring OS-level libraries::
417: * Callbacks::
418: * C interface internals::
419: * Low-Level C Interface Words::
420:
421: Assembler and Code Words
422:
423: * Assembler Definitions:: Definitions in assembly language
424: * Common Assembler:: Assembler Syntax
425: * Common Disassembler::
426: * 386 Assembler:: Deviations and special cases
427: * AMD64 Assembler::
428: * Alpha Assembler:: Deviations and special cases
429: * MIPS assembler:: Deviations and special cases
430: * PowerPC assembler:: Deviations and special cases
431: * ARM Assembler:: Deviations and special cases
432: * Other assemblers:: How to write them
433:
434: Tools
435:
436: * ANS Report:: Report the words used, sorted by wordset.
437: * Stack depth changes:: Where does this stack item come from?
438:
439: ANS conformance
440:
441: * The Core Words::
442: * The optional Block word set::
443: * The optional Double Number word set::
444: * The optional Exception word set::
445: * The optional Facility word set::
446: * The optional File-Access word set::
447: * The optional Floating-Point word set::
448: * The optional Locals word set::
449: * The optional Memory-Allocation word set::
450: * The optional Programming-Tools word set::
451: * The optional Search-Order word set::
452:
453: The Core Words
454:
455: * core-idef:: Implementation Defined Options
456: * core-ambcond:: Ambiguous Conditions
457: * core-other:: Other System Documentation
458:
459: The optional Block word set
460:
461: * block-idef:: Implementation Defined Options
462: * block-ambcond:: Ambiguous Conditions
463: * block-other:: Other System Documentation
464:
465: The optional Double Number word set
466:
467: * double-ambcond:: Ambiguous Conditions
468:
469: The optional Exception word set
470:
471: * exception-idef:: Implementation Defined Options
472:
473: The optional Facility word set
474:
475: * facility-idef:: Implementation Defined Options
476: * facility-ambcond:: Ambiguous Conditions
477:
478: The optional File-Access word set
479:
480: * file-idef:: Implementation Defined Options
481: * file-ambcond:: Ambiguous Conditions
482:
483: The optional Floating-Point word set
484:
485: * floating-idef:: Implementation Defined Options
486: * floating-ambcond:: Ambiguous Conditions
487:
488: The optional Locals word set
489:
490: * locals-idef:: Implementation Defined Options
491: * locals-ambcond:: Ambiguous Conditions
492:
493: The optional Memory-Allocation word set
494:
495: * memory-idef:: Implementation Defined Options
496:
497: The optional Programming-Tools word set
498:
499: * programming-idef:: Implementation Defined Options
500: * programming-ambcond:: Ambiguous Conditions
501:
502: The optional Search-Order word set
503:
504: * search-idef:: Implementation Defined Options
505: * search-ambcond:: Ambiguous Conditions
506:
507: Emacs and Gforth
508:
509: * Installing gforth.el:: Making Emacs aware of Forth.
510: * Emacs Tags:: Viewing the source of a word in Emacs.
511: * Hilighting:: Making Forth code look prettier.
512: * Auto-Indentation:: Customizing auto-indentation.
513: * Blocks Files:: Reading and writing blocks files.
514:
515: Image Files
516:
517: * Image Licensing Issues:: Distribution terms for images.
518: * Image File Background:: Why have image files?
519: * Non-Relocatable Image Files:: don't always work.
520: * Data-Relocatable Image Files:: are better.
521: * Fully Relocatable Image Files:: better yet.
522: * Stack and Dictionary Sizes:: Setting the default sizes for an image.
523: * Running Image Files:: @code{gforth -i @i{file}} or @i{file}.
524: * Modifying the Startup Sequence:: and turnkey applications.
525:
526: Fully Relocatable Image Files
527:
528: * gforthmi:: The normal way
529: * cross.fs:: The hard way
530:
531: Engine
532:
533: * Portability::
534: * Threading::
535: * Primitives::
536: * Performance::
537:
538: Threading
539:
540: * Scheduling::
541: * Direct or Indirect Threaded?::
542: * Dynamic Superinstructions::
543: * DOES>::
544:
545: Primitives
546:
547: * Automatic Generation::
548: * TOS Optimization::
549: * Produced code::
550:
551: Cross Compiler
552:
553: * Using the Cross Compiler::
554: * How the Cross Compiler Works::
555:
556: Licenses
557:
558: * GNU Free Documentation License:: License for copying this manual.
559: * Copying:: GPL (for copying this software).
560:
561: @end detailmenu
562: @end menu
563:
564: @c ----------------------------------------------------------
565: @iftex
566: @unnumbered Preface
567: @cindex Preface
568: This manual documents Gforth. Some introductory material is provided for
569: readers who are unfamiliar with Forth or who are migrating to Gforth
570: from other Forth compilers. However, this manual is primarily a
571: reference manual.
572: @end iftex
573:
574: @comment TODO much more blurb here.
575:
576: @c ******************************************************************
577: @node Goals, Gforth Environment, Top, Top
578: @comment node-name, next, previous, up
579: @chapter Goals of Gforth
580: @cindex goals of the Gforth project
581: The goal of the Gforth Project is to develop a standard model for
582: ANS Forth. This can be split into several subgoals:
583:
584: @itemize @bullet
585: @item
586: Gforth should conform to the ANS Forth Standard.
587: @item
588: It should be a model, i.e. it should define all the
589: implementation-dependent things.
590: @item
591: It should become standard, i.e. widely accepted and used. This goal
592: is the most difficult one.
593: @end itemize
594:
595: To achieve these goals Gforth should be
596: @itemize @bullet
597: @item
598: Similar to previous models (fig-Forth, F83)
599: @item
600: Powerful. It should provide for all the things that are considered
601: necessary today and even some that are not yet considered necessary.
602: @item
603: Efficient. It should not get the reputation of being exceptionally
604: slow.
605: @item
606: Free.
607: @item
608: Available on many machines/easy to port.
609: @end itemize
610:
611: Have we achieved these goals? Gforth conforms to the ANS Forth
612: standard. It may be considered a model, but we have not yet documented
613: which parts of the model are stable and which parts we are likely to
614: change. It certainly has not yet become a de facto standard, but it
615: appears to be quite popular. It has some similarities to and some
616: differences from previous models. It has some powerful features, but not
617: yet everything that we envisioned. We certainly have achieved our
618: execution speed goals (@pxref{Performance})@footnote{However, in 1998
619: the bar was raised when the major commercial Forth vendors switched to
620: native code compilers.}. It is free and available on many machines.
621:
622: @c ******************************************************************
623: @node Gforth Environment, Tutorial, Goals, Top
624: @chapter Gforth Environment
625: @cindex Gforth environment
626:
627: Note: ultimately, the Gforth man page will be auto-generated from the
628: material in this chapter.
629:
630: @menu
631: * Invoking Gforth:: Getting in
632: * Leaving Gforth:: Getting out
633: * Command-line editing::
634: * Environment variables:: that affect how Gforth starts up
635: * Gforth Files:: What gets installed and where
636: * Gforth in pipes::
637: * Startup speed:: When 14ms is not fast enough ...
638: @end menu
639:
640: For related information about the creation of images see @ref{Image Files}.
641:
642: @comment ----------------------------------------------
643: @node Invoking Gforth, Leaving Gforth, Gforth Environment, Gforth Environment
644: @section Invoking Gforth
645: @cindex invoking Gforth
646: @cindex running Gforth
647: @cindex command-line options
648: @cindex options on the command line
649: @cindex flags on the command line
650:
651: Gforth is made up of two parts; an executable ``engine'' (named
652: @command{gforth} or @command{gforth-fast}) and an image file. To start it, you
653: will usually just say @code{gforth} -- this automatically loads the
654: default image file @file{gforth.fi}. In many other cases the default
655: Gforth image will be invoked like this:
656: @example
657: gforth [file | -e forth-code] ...
658: @end example
659: @noindent
660: This interprets the contents of the files and the Forth code in the order they
661: are given.
662:
663: In addition to the @command{gforth} engine, there is also an engine
664: called @command{gforth-fast}, which is faster, but gives less
665: informative error messages (@pxref{Error messages}) and may catch some
666: errors (in particular, stack underflows and integer division errors)
667: later or not at all. You should use it for debugged,
668: performance-critical programs.
669:
670: Moreover, there is an engine called @command{gforth-itc}, which is
671: useful in some backwards-compatibility situations (@pxref{Direct or
672: Indirect Threaded?}).
673:
674: In general, the command line looks like this:
675:
676: @example
677: gforth[-fast] [engine options] [image options]
678: @end example
679:
680: The engine options must come before the rest of the command
681: line. They are:
682:
683: @table @code
684: @cindex -i, command-line option
685: @cindex --image-file, command-line option
686: @item --image-file @i{file}
687: @itemx -i @i{file}
688: Loads the Forth image @i{file} instead of the default
689: @file{gforth.fi} (@pxref{Image Files}).
690:
691: @cindex --appl-image, command-line option
692: @item --appl-image @i{file}
693: Loads the image @i{file} and leaves all further command-line arguments
694: to the image (instead of processing them as engine options). This is
695: useful for building executable application images on Unix, built with
696: @code{gforthmi --application ...}.
697:
698: @cindex --path, command-line option
699: @cindex -p, command-line option
700: @item --path @i{path}
701: @itemx -p @i{path}
702: Uses @i{path} for searching the image file and Forth source code files
703: instead of the default in the environment variable @code{GFORTHPATH} or
704: the path specified at installation time (e.g.,
705: @file{/usr/local/share/gforth/0.2.0:.}). A path is given as a list of
706: directories, separated by @samp{:} (on Unix) or @samp{;} (on other OSs).
707:
708: @cindex --dictionary-size, command-line option
709: @cindex -m, command-line option
710: @cindex @i{size} parameters for command-line options
711: @cindex size of the dictionary and the stacks
712: @item --dictionary-size @i{size}
713: @itemx -m @i{size}
714: Allocate @i{size} space for the Forth dictionary space instead of
715: using the default specified in the image (typically 256K). The
716: @i{size} specification for this and subsequent options consists of
717: an integer and a unit (e.g.,
718: @code{4M}). The unit can be one of @code{b} (bytes), @code{e} (element
719: size, in this case Cells), @code{k} (kilobytes), @code{M} (Megabytes),
720: @code{G} (Gigabytes), and @code{T} (Terabytes). If no unit is specified,
721: @code{e} is used.
722:
723: @cindex --data-stack-size, command-line option
724: @cindex -d, command-line option
725: @item --data-stack-size @i{size}
726: @itemx -d @i{size}
727: Allocate @i{size} space for the data stack instead of using the
728: default specified in the image (typically 16K).
729:
730: @cindex --return-stack-size, command-line option
731: @cindex -r, command-line option
732: @item --return-stack-size @i{size}
733: @itemx -r @i{size}
734: Allocate @i{size} space for the return stack instead of using the
735: default specified in the image (typically 15K).
736:
737: @cindex --fp-stack-size, command-line option
738: @cindex -f, command-line option
739: @item --fp-stack-size @i{size}
740: @itemx -f @i{size}
741: Allocate @i{size} space for the floating point stack instead of
742: using the default specified in the image (typically 15.5K). In this case
743: the unit specifier @code{e} refers to floating point numbers.
744:
745: @cindex --locals-stack-size, command-line option
746: @cindex -l, command-line option
747: @item --locals-stack-size @i{size}
748: @itemx -l @i{size}
749: Allocate @i{size} space for the locals stack instead of using the
750: default specified in the image (typically 14.5K).
751:
752: @cindex --vm-commit, command-line option
753: @cindex overcommit memory for dictionary and stacks
754: @cindex memory overcommit for dictionary and stacks
755: @item --vm-commit
756: Normally, Gforth tries to start up even if there is not enough virtual
757: memory for the dictionary and the stacks (using @code{MAP_NORESERVE}
758: on OSs that support it); so you can ask for a really big dictionary
759: and/or stacks, and as long as you don't use more virtual memory than
760: is available, everything will be fine (but if you use more, processes
761: get killed). With this option you just use the default allocation
762: policy of the OS; for OSs that don't overcommit (e.g., Solaris), this
763: means that you cannot and should not ask for as big dictionary and
764: stacks, but once Gforth successfully starts up, out-of-memory won't
765: kill it.
766:
767: @cindex -h, command-line option
768: @cindex --help, command-line option
769: @item --help
770: @itemx -h
771: Print a message about the command-line options
772:
773: @cindex -v, command-line option
774: @cindex --version, command-line option
775: @item --version
776: @itemx -v
777: Print version and exit
778:
779: @cindex --debug, command-line option
780: @item --debug
781: Print some information useful for debugging on startup.
782:
783: @cindex --offset-image, command-line option
784: @item --offset-image
785: Start the dictionary at a slightly different position than would be used
786: otherwise (useful for creating data-relocatable images,
787: @pxref{Data-Relocatable Image Files}).
788:
789: @cindex --no-offset-im, command-line option
790: @item --no-offset-im
791: Start the dictionary at the normal position.
792:
793: @cindex --clear-dictionary, command-line option
794: @item --clear-dictionary
795: Initialize all bytes in the dictionary to 0 before loading the image
796: (@pxref{Data-Relocatable Image Files}).
797:
798: @cindex --die-on-signal, command-line-option
799: @item --die-on-signal
800: Normally Gforth handles most signals (e.g., the user interrupt SIGINT,
801: or the segmentation violation SIGSEGV) by translating it into a Forth
802: @code{THROW}. With this option, Gforth exits if it receives such a
803: signal. This option is useful when the engine and/or the image might be
804: severely broken (such that it causes another signal before recovering
805: from the first); this option avoids endless loops in such cases.
806:
807: @cindex --no-dynamic, command-line option
808: @cindex --dynamic, command-line option
809: @item --no-dynamic
810: @item --dynamic
811: Disable or enable dynamic superinstructions with replication
812: (@pxref{Dynamic Superinstructions}).
813:
814: @cindex --no-super, command-line option
815: @item --no-super
816: Disable dynamic superinstructions, use just dynamic replication; this is
817: useful if you want to patch threaded code (@pxref{Dynamic
818: Superinstructions}).
819:
820: @cindex --ss-number, command-line option
821: @item --ss-number=@var{N}
822: Use only the first @var{N} static superinstructions compiled into the
823: engine (default: use them all; note that only @code{gforth-fast} has
824: any). This option is useful for measuring the performance impact of
825: static superinstructions.
826:
827: @cindex --ss-min-..., command-line options
828: @item --ss-min-codesize
829: @item --ss-min-ls
830: @item --ss-min-lsu
831: @item --ss-min-nexts
832: Use specified metric for determining the cost of a primitive or static
833: superinstruction for static superinstruction selection. @code{Codesize}
834: is the native code size of the primive or static superinstruction,
835: @code{ls} is the number of loads and stores, @code{lsu} is the number of
836: loads, stores, and updates, and @code{nexts} is the number of dispatches
837: (not taking dynamic superinstructions into account), i.e. every
838: primitive or static superinstruction has cost 1. Default:
839: @code{codesize} if you use dynamic code generation, otherwise
840: @code{nexts}.
841:
842: @cindex --ss-greedy, command-line option
843: @item --ss-greedy
844: This option is useful for measuring the performance impact of static
845: superinstructions. By default, an optimal shortest-path algorithm is
846: used for selecting static superinstructions. With @option{--ss-greedy}
847: this algorithm is modified to assume that anything after the static
848: superinstruction currently under consideration is not combined into
849: static superinstructions. With @option{--ss-min-nexts} this produces
850: the same result as a greedy algorithm that always selects the longest
851: superinstruction available at the moment. E.g., if there are
852: superinstructions AB and BCD, then for the sequence A B C D the optimal
853: algorithm will select A BCD and the greedy algorithm will select AB C D.
854:
855: @cindex --print-metrics, command-line option
856: @item --print-metrics
857: Prints some metrics used during static superinstruction selection:
858: @code{code size} is the actual size of the dynamically generated code.
859: @code{Metric codesize} is the sum of the codesize metrics as seen by
860: static superinstruction selection; there is a difference from @code{code
861: size}, because not all primitives and static superinstructions are
862: compiled into dynamically generated code, and because of markers. The
863: other metrics correspond to the @option{ss-min-...} options. This
864: option is useful for evaluating the effects of the @option{--ss-...}
865: options.
866:
867: @end table
868:
869: @cindex loading files at startup
870: @cindex executing code on startup
871: @cindex batch processing with Gforth
872: As explained above, the image-specific command-line arguments for the
873: default image @file{gforth.fi} consist of a sequence of filenames and
874: @code{-e @var{forth-code}} options that are interpreted in the sequence
875: in which they are given. The @code{-e @var{forth-code}} or
876: @code{--evaluate @var{forth-code}} option evaluates the Forth code. This
877: option takes only one argument; if you want to evaluate more Forth
878: words, you have to quote them or use @code{-e} several times. To exit
879: after processing the command line (instead of entering interactive mode)
880: append @code{-e bye} to the command line. You can also process the
881: command-line arguments with a Forth program (@pxref{OS command line
882: arguments}).
883:
884: @cindex versions, invoking other versions of Gforth
885: If you have several versions of Gforth installed, @code{gforth} will
886: invoke the version that was installed last. @code{gforth-@i{version}}
887: invokes a specific version. If your environment contains the variable
888: @code{GFORTHPATH}, you may want to override it by using the
889: @code{--path} option.
890:
891: Not yet implemented:
892: On startup the system first executes the system initialization file
893: (unless the option @code{--no-init-file} is given; note that the system
894: resulting from using this option may not be ANS Forth conformant). Then
895: the user initialization file @file{.gforth.fs} is executed, unless the
896: option @code{--no-rc} is given; this file is searched for in @file{.},
897: then in @file{~}, then in the normal path (see above).
898:
899:
900:
901: @comment ----------------------------------------------
902: @node Leaving Gforth, Command-line editing, Invoking Gforth, Gforth Environment
903: @section Leaving Gforth
904: @cindex Gforth - leaving
905: @cindex leaving Gforth
906:
907: You can leave Gforth by typing @code{bye} or @kbd{Ctrl-d} (at the start
908: of a line) or (if you invoked Gforth with the @code{--die-on-signal}
909: option) @kbd{Ctrl-c}. When you leave Gforth, all of your definitions and
910: data are discarded. For ways of saving the state of the system before
911: leaving Gforth see @ref{Image Files}.
912:
913: doc-bye
914:
915:
916: @comment ----------------------------------------------
917: @node Command-line editing, Environment variables, Leaving Gforth, Gforth Environment
918: @section Command-line editing
919: @cindex command-line editing
920:
921: Gforth maintains a history file that records every line that you type to
922: the text interpreter. This file is preserved between sessions, and is
923: used to provide a command-line recall facility; if you type @kbd{Ctrl-P}
924: repeatedly you can recall successively older commands from this (or
925: previous) session(s). The full list of command-line editing facilities is:
926:
927: @itemize @bullet
928: @item
929: @kbd{Ctrl-p} (``previous'') (or up-arrow) to recall successively older
930: commands from the history buffer.
931: @item
932: @kbd{Ctrl-n} (``next'') (or down-arrow) to recall successively newer commands
933: from the history buffer.
934: @item
935: @kbd{Ctrl-f} (or right-arrow) to move the cursor right, non-destructively.
936: @item
937: @kbd{Ctrl-b} (or left-arrow) to move the cursor left, non-destructively.
938: @item
939: @kbd{Ctrl-h} (backspace) to delete the character to the left of the cursor,
940: closing up the line.
941: @item
942: @kbd{Ctrl-k} to delete (``kill'') from the cursor to the end of the line.
943: @item
944: @kbd{Ctrl-a} to move the cursor to the start of the line.
945: @item
946: @kbd{Ctrl-e} to move the cursor to the end of the line.
947: @item
948: @key{RET} (@kbd{Ctrl-m}) or @key{LFD} (@kbd{Ctrl-j}) to submit the current
949: line.
950: @item
951: @key{TAB} to step through all possible full-word completions of the word
952: currently being typed.
953: @item
954: @kbd{Ctrl-d} on an empty line line to terminate Gforth (gracefully,
955: using @code{bye}).
956: @item
957: @kbd{Ctrl-x} (or @code{Ctrl-d} on a non-empty line) to delete the
958: character under the cursor.
959: @end itemize
960:
961: When editing, displayable characters are inserted to the left of the
962: cursor position; the line is always in ``insert'' (as opposed to
963: ``overstrike'') mode.
964:
965: @cindex history file
966: @cindex @file{.gforth-history}
967: On Unix systems, the history file is @file{~/.gforth-history} by
968: default@footnote{i.e. it is stored in the user's home directory.}. You
969: can find out the name and location of your history file using:
970:
971: @example
972: history-file type \ Unix-class systems
973:
974: history-file type \ Other systems
975: history-dir type
976: @end example
977:
978: If you enter long definitions by hand, you can use a text editor to
979: paste them out of the history file into a Forth source file for reuse at
980: a later time.
981:
982: Gforth never trims the size of the history file, so you should do this
983: periodically, if necessary.
984:
985: @comment this is all defined in history.fs
986: @comment NAC TODO the ctrl-D behaviour can either do a bye or a beep.. how is that option
987: @comment chosen?
988:
989:
990: @comment ----------------------------------------------
991: @node Environment variables, Gforth Files, Command-line editing, Gforth Environment
992: @section Environment variables
993: @cindex environment variables
994:
995: Gforth uses these environment variables:
996:
997: @itemize @bullet
998: @item
999: @cindex @code{GFORTHHIST} -- environment variable
1000: @code{GFORTHHIST} -- (Unix systems only) specifies the directory in which to
1001: open/create the history file, @file{.gforth-history}. Default:
1002: @code{$HOME}.
1003:
1004: @item
1005: @cindex @code{GFORTHPATH} -- environment variable
1006: @code{GFORTHPATH} -- specifies the path used when searching for the gforth image file and
1007: for Forth source-code files.
1008:
1009: @item
1010: @cindex @code{LANG} -- environment variable
1011: @code{LANG} -- see @code{LC_CTYPE}
1012:
1013: @item
1014: @cindex @code{LC_ALL} -- environment variable
1015: @code{LC_ALL} -- see @code{LC_CTYPE}
1016:
1017: @item
1018: @cindex @code{LC_CTYPE} -- environment variable
1019: @code{LC_CTYPE} -- If this variable contains ``UTF-8'' on Gforth
1020: startup, Gforth uses the UTF-8 encoding for strings internally and
1021: expects its input and produces its output in UTF-8 encoding, otherwise
1022: the encoding is 8bit (see @pxref{Xchars and Unicode}). If this
1023: environment variable is unset, Gforth looks in @code{LC_ALL}, and if
1024: that is unset, in @code{LANG}.
1025:
1026: @item
1027: @cindex @code{GFORTHSYSTEMPREFIX} -- environment variable
1028:
1029: @code{GFORTHSYSTEMPREFIX} -- specifies what to prepend to the argument
1030: of @code{system} before passing it to C's @code{system()}. Default:
1031: @code{"./$COMSPEC /c "} on Windows, @code{""} on other OSs. The prefix
1032: and the command are directly concatenated, so if a space between them is
1033: necessary, append it to the prefix.
1034:
1035: @item
1036: @cindex @code{GFORTH} -- environment variable
1037: @code{GFORTH} -- used by @file{gforthmi}, @xref{gforthmi}.
1038:
1039: @item
1040: @cindex @code{GFORTHD} -- environment variable
1041: @code{GFORTHD} -- used by @file{gforthmi}, @xref{gforthmi}.
1042:
1043: @item
1044: @cindex @code{TMP}, @code{TEMP} - environment variable
1045: @code{TMP}, @code{TEMP} - (non-Unix systems only) used as a potential
1046: location for the history file.
1047: @end itemize
1048:
1049: @comment also POSIXELY_CORRECT LINES COLUMNS HOME but no interest in
1050: @comment mentioning these.
1051:
1052: All the Gforth environment variables default to sensible values if they
1053: are not set.
1054:
1055:
1056: @comment ----------------------------------------------
1057: @node Gforth Files, Gforth in pipes, Environment variables, Gforth Environment
1058: @section Gforth files
1059: @cindex Gforth files
1060:
1061: When you install Gforth on a Unix system, it installs files in these
1062: locations by default:
1063:
1064: @itemize @bullet
1065: @item
1066: @file{/usr/local/bin/gforth}
1067: @item
1068: @file{/usr/local/bin/gforthmi}
1069: @item
1070: @file{/usr/local/man/man1/gforth.1} - man page.
1071: @item
1072: @file{/usr/local/info} - the Info version of this manual.
1073: @item
1074: @file{/usr/local/lib/gforth/<version>/...} - Gforth @file{.fi} files.
1075: @item
1076: @file{/usr/local/share/gforth/<version>/TAGS} - Emacs TAGS file.
1077: @item
1078: @file{/usr/local/share/gforth/<version>/...} - Gforth source files.
1079: @item
1080: @file{.../emacs/site-lisp/gforth.el} - Emacs gforth mode.
1081: @end itemize
1082:
1083: You can select different places for installation by using
1084: @code{configure} options (listed with @code{configure --help}).
1085:
1086: @comment ----------------------------------------------
1087: @node Gforth in pipes, Startup speed, Gforth Files, Gforth Environment
1088: @section Gforth in pipes
1089: @cindex pipes, Gforth as part of
1090:
1091: Gforth can be used in pipes created elsewhere (described here). It can
1092: also create pipes on its own (@pxref{Pipes}).
1093:
1094: @cindex input from pipes
1095: If you pipe into Gforth, your program should read with @code{read-file}
1096: or @code{read-line} from @code{stdin} (@pxref{General files}).
1097: @code{Key} does not recognize the end of input. Words like
1098: @code{accept} echo the input and are therefore usually not useful for
1099: reading from a pipe. You have to invoke the Forth program with an OS
1100: command-line option, as you have no chance to use the Forth command line
1101: (the text interpreter would try to interpret the pipe input).
1102:
1103: @cindex output in pipes
1104: You can output to a pipe with @code{type}, @code{emit}, @code{cr} etc.
1105:
1106: @cindex silent exiting from Gforth
1107: When you write to a pipe that has been closed at the other end, Gforth
1108: receives a SIGPIPE signal (``pipe broken''). Gforth translates this
1109: into the exception @code{broken-pipe-error}. If your application does
1110: not catch that exception, the system catches it and exits, usually
1111: silently (unless you were working on the Forth command line; then it
1112: prints an error message and exits). This is usually the desired
1113: behaviour.
1114:
1115: If you do not like this behaviour, you have to catch the exception
1116: yourself, and react to it.
1117:
1118: Here's an example of an invocation of Gforth that is usable in a pipe:
1119:
1120: @example
1121: gforth -e ": foo begin pad dup 10 stdin read-file throw dup while \
1122: type repeat ; foo bye"
1123: @end example
1124:
1125: This example just copies the input verbatim to the output. A very
1126: simple pipe containing this example looks like this:
1127:
1128: @example
1129: cat startup.fs |
1130: gforth -e ": foo begin pad dup 80 stdin read-file throw dup while \
1131: type repeat ; foo bye"|
1132: head
1133: @end example
1134:
1135: @cindex stderr and pipes
1136: Pipes involving Gforth's @code{stderr} output do not work.
1137:
1138: @comment ----------------------------------------------
1139: @node Startup speed, , Gforth in pipes, Gforth Environment
1140: @section Startup speed
1141: @cindex Startup speed
1142: @cindex speed, startup
1143:
1144: If Gforth is used for CGI scripts or in shell scripts, its startup
1145: speed may become a problem. On a 3GHz Core 2 Duo E8400 under 64-bit
1146: Linux 2.6.27.8 with libc-2.7, @code{gforth-fast -e bye} takes 13.1ms
1147: user and 1.2ms system time (@code{gforth -e bye} is faster on startup
1148: with about 3.4ms user time and 1.2ms system time, because it subsumes
1149: some of the options discussed below).
1150:
1151: If startup speed is a problem, you may consider the following ways to
1152: improve it; or you may consider ways to reduce the number of startups
1153: (for example, by using Fast-CGI). Note that the first steps below
1154: improve the startup time at the cost of run-time (including
1155: compile-time), so whether they are profitable depends on the balance
1156: of these times in your application.
1157:
1158: An easy step that influences Gforth startup speed is the use of a
1159: number of options that increase run-time, but decrease image-loading
1160: time.
1161:
1162: The first of these that you should try is @code{--ss-number=0
1163: --ss-states=1} because this option buys relatively little run-time
1164: speedup and costs quite a bit of time at startup. @code{gforth-fast
1165: --ss-number=0 --ss-states=1 -e bye} takes about 2.8ms user and 1.5ms
1166: system time.
1167:
1168: The next option is @code{--no-dynamic} which has a substantial impact
1169: on run-time (about a factor of 2 on several platforms), but still
1170: makes startup speed a little faster: @code{gforth-fast --ss-number=0
1171: --ss-states=1 --no-dynamic -e bye} consumes about 2.6ms user and 1.2ms
1172: system time.
1173:
1174: The next step to improve startup speed is to use a data-relocatable
1175: image (@pxref{Data-Relocatable Image Files}). This avoids the
1176: relocation cost for the code in the image (but not for the data).
1177: Note that the image is then specific to the particular binary you are
1178: using (i.e., whether it is @code{gforth}, @code{gforth-fast}, and even
1179: the particular build). You create the data-relocatable image that
1180: works with @code{./gforth-fast} with @code{GFORTHD="./gforth-fast
1181: --no-dynamic" gforthmi gforthdr.fi} (the @code{--no-dynamic} is
1182: required here or the image will not work). And you run it with
1183: @code{gforth-fast -i gforthdr.fi ... -e bye} (the flags discussed
1184: above don't matter here, because they only come into play on
1185: relocatable code). @code{gforth-fast -i gforthdr.fi -e bye} takes
1186: about 1.1ms user and 1.2ms system time.
1187:
1188: One step further is to avoid all relocation cost and part of the
1189: copy-on-write cost through using a non-relocatable image
1190: (@pxref{Non-Relocatable Image Files}). However, this has the
1191: disadvantage that it does not work on operating systems with address
1192: space randomization (the default in, e.g., Linux nowadays), or if the
1193: dictionary moves for any other reason (e.g., because of a change of
1194: the OS kernel or an updated library), so we cannot really recommend
1195: it. You create a non-relocatable image with @code{gforth-fast
1196: --no-dynamic -e "savesystem gforthnr.fi bye"} (the @code{--no-dynamic}
1197: is required here, too). And you run it with @code{gforth-fast -i
1198: gforthnr.fi ... -e bye} (again the flags discussed above don't
1199: matter). @code{gforth-fast -i gforthdr.fi -e bye} takes
1200: about 0.9ms user and 0.9ms system time.
1201:
1202: If the script you want to execute contains a significant amount of
1203: code, it may be profitable to compile it into the image to avoid the
1204: cost of compiling it at startup time.
1205:
1206: @c ******************************************************************
1207: @node Tutorial, Introduction, Gforth Environment, Top
1208: @chapter Forth Tutorial
1209: @cindex Tutorial
1210: @cindex Forth Tutorial
1211:
1212: @c Topics from nac's Introduction that could be mentioned:
1213: @c press <ret> after each line
1214: @c Prompt
1215: @c numbers vs. words in dictionary on text interpretation
1216: @c what happens on redefinition
1217: @c parsing words (in particular, defining words)
1218:
1219: The difference of this chapter from the Introduction
1220: (@pxref{Introduction}) is that this tutorial is more fast-paced, should
1221: be used while sitting in front of a computer, and covers much more
1222: material, but does not explain how the Forth system works.
1223:
1224: This tutorial can be used with any ANS-compliant Forth; any
1225: Gforth-specific features are marked as such and you can skip them if
1226: you work with another Forth. This tutorial does not explain all
1227: features of Forth, just enough to get you started and give you some
1228: ideas about the facilities available in Forth. Read the rest of the
1229: manual when you are through this.
1230:
1231: The intended way to use this tutorial is that you work through it while
1232: sitting in front of the console, take a look at the examples and predict
1233: what they will do, then try them out; if the outcome is not as expected,
1234: find out why (e.g., by trying out variations of the example), so you
1235: understand what's going on. There are also some assignments that you
1236: should solve.
1237:
1238: This tutorial assumes that you have programmed before and know what,
1239: e.g., a loop is.
1240:
1241: @c !! explain compat library
1242:
1243: @menu
1244: * Starting Gforth Tutorial::
1245: * Syntax Tutorial::
1246: * Crash Course Tutorial::
1247: * Stack Tutorial::
1248: * Arithmetics Tutorial::
1249: * Stack Manipulation Tutorial::
1250: * Using files for Forth code Tutorial::
1251: * Comments Tutorial::
1252: * Colon Definitions Tutorial::
1253: * Decompilation Tutorial::
1254: * Stack-Effect Comments Tutorial::
1255: * Types Tutorial::
1256: * Factoring Tutorial::
1257: * Designing the stack effect Tutorial::
1258: * Local Variables Tutorial::
1259: * Conditional execution Tutorial::
1260: * Flags and Comparisons Tutorial::
1261: * General Loops Tutorial::
1262: * Counted loops Tutorial::
1263: * Recursion Tutorial::
1264: * Leaving definitions or loops Tutorial::
1265: * Return Stack Tutorial::
1266: * Memory Tutorial::
1267: * Characters and Strings Tutorial::
1268: * Alignment Tutorial::
1269: * Floating Point Tutorial::
1270: * Files Tutorial::
1271: * Interpretation and Compilation Semantics and Immediacy Tutorial::
1272: * Execution Tokens Tutorial::
1273: * Exceptions Tutorial::
1274: * Defining Words Tutorial::
1275: * Arrays and Records Tutorial::
1276: * POSTPONE Tutorial::
1277: * Literal Tutorial::
1278: * Advanced macros Tutorial::
1279: * Compilation Tokens Tutorial::
1280: * Wordlists and Search Order Tutorial::
1281: @end menu
1282:
1283: @node Starting Gforth Tutorial, Syntax Tutorial, Tutorial, Tutorial
1284: @section Starting Gforth
1285: @cindex starting Gforth tutorial
1286: You can start Gforth by typing its name:
1287:
1288: @example
1289: gforth
1290: @end example
1291:
1292: That puts you into interactive mode; you can leave Gforth by typing
1293: @code{bye}. While in Gforth, you can edit the command line and access
1294: the command line history with cursor keys, similar to bash.
1295:
1296:
1297: @node Syntax Tutorial, Crash Course Tutorial, Starting Gforth Tutorial, Tutorial
1298: @section Syntax
1299: @cindex syntax tutorial
1300:
1301: A @dfn{word} is a sequence of arbitrary characters (except white
1302: space). Words are separated by white space. E.g., each of the
1303: following lines contains exactly one word:
1304:
1305: @example
1306: word
1307: !@@#$%^&*()
1308: 1234567890
1309: 5!a
1310: @end example
1311:
1312: A frequent beginner's error is to leave out necessary white space,
1313: resulting in an error like @samp{Undefined word}; so if you see such an
1314: error, check if you have put spaces wherever necessary.
1315:
1316: @example
1317: ." hello, world" \ correct
1318: ."hello, world" \ gives an "Undefined word" error
1319: @end example
1320:
1321: Gforth and most other Forth systems ignore differences in case (they are
1322: case-insensitive), i.e., @samp{word} is the same as @samp{Word}. If
1323: your system is case-sensitive, you may have to type all the examples
1324: given here in upper case.
1325:
1326:
1327: @node Crash Course Tutorial, Stack Tutorial, Syntax Tutorial, Tutorial
1328: @section Crash Course
1329:
1330: Forth does not prevent you from shooting yourself in the foot. Let's
1331: try a few ways to crash Gforth:
1332:
1333: @example
1334: 0 0 !
1335: here execute
1336: ' catch >body 20 erase abort
1337: ' (quit) >body 20 erase
1338: @end example
1339:
1340: The last two examples are guaranteed to destroy important parts of
1341: Gforth (and most other systems), so you better leave Gforth afterwards
1342: (if it has not finished by itself). On some systems you may have to
1343: kill gforth from outside (e.g., in Unix with @code{kill}).
1344:
1345: You will find out later what these lines do and then you will get an
1346: idea why they produce crashes.
1347:
1348: Now that you know how to produce crashes (and that there's not much to
1349: them), let's learn how to produce meaningful programs.
1350:
1351:
1352: @node Stack Tutorial, Arithmetics Tutorial, Crash Course Tutorial, Tutorial
1353: @section Stack
1354: @cindex stack tutorial
1355:
1356: The most obvious feature of Forth is the stack. When you type in a
1357: number, it is pushed on the stack. You can display the contents of the
1358: stack with @code{.s}.
1359:
1360: @example
1361: 1 2 .s
1362: 3 .s
1363: @end example
1364:
1365: @code{.s} displays the top-of-stack to the right, i.e., the numbers
1366: appear in @code{.s} output as they appeared in the input.
1367:
1368: You can print the top element of the stack with @code{.}.
1369:
1370: @example
1371: 1 2 3 . . .
1372: @end example
1373:
1374: In general, words consume their stack arguments (@code{.s} is an
1375: exception).
1376:
1377: @quotation Assignment
1378: What does the stack contain after @code{5 6 7 .}?
1379: @end quotation
1380:
1381:
1382: @node Arithmetics Tutorial, Stack Manipulation Tutorial, Stack Tutorial, Tutorial
1383: @section Arithmetics
1384: @cindex arithmetics tutorial
1385:
1386: The words @code{+}, @code{-}, @code{*}, @code{/}, and @code{mod} always
1387: operate on the top two stack items:
1388:
1389: @example
1390: 2 2 .s
1391: + .s
1392: .
1393: 2 1 - .
1394: 7 3 mod .
1395: @end example
1396:
1397: The operands of @code{-}, @code{/}, and @code{mod} are in the same order
1398: as in the corresponding infix expression (this is generally the case in
1399: Forth).
1400:
1401: Parentheses are superfluous (and not available), because the order of
1402: the words unambiguously determines the order of evaluation and the
1403: operands:
1404:
1405: @example
1406: 3 4 + 5 * .
1407: 3 4 5 * + .
1408: @end example
1409:
1410: @quotation Assignment
1411: What are the infix expressions corresponding to the Forth code above?
1412: Write @code{6-7*8+9} in Forth notation@footnote{This notation is also
1413: known as Postfix or RPN (Reverse Polish Notation).}.
1414: @end quotation
1415:
1416: To change the sign, use @code{negate}:
1417:
1418: @example
1419: 2 negate .
1420: @end example
1421:
1422: @quotation Assignment
1423: Convert -(-3)*4-5 to Forth.
1424: @end quotation
1425:
1426: @code{/mod} performs both @code{/} and @code{mod}.
1427:
1428: @example
1429: 7 3 /mod . .
1430: @end example
1431:
1432: Reference: @ref{Arithmetic}.
1433:
1434:
1435: @node Stack Manipulation Tutorial, Using files for Forth code Tutorial, Arithmetics Tutorial, Tutorial
1436: @section Stack Manipulation
1437: @cindex stack manipulation tutorial
1438:
1439: Stack manipulation words rearrange the data on the stack.
1440:
1441: @example
1442: 1 .s drop .s
1443: 1 .s dup .s drop drop .s
1444: 1 2 .s over .s drop drop drop
1445: 1 2 .s swap .s drop drop
1446: 1 2 3 .s rot .s drop drop drop
1447: @end example
1448:
1449: These are the most important stack manipulation words. There are also
1450: variants that manipulate twice as many stack items:
1451:
1452: @example
1453: 1 2 3 4 .s 2swap .s 2drop 2drop
1454: @end example
1455:
1456: Two more stack manipulation words are:
1457:
1458: @example
1459: 1 2 .s nip .s drop
1460: 1 2 .s tuck .s 2drop drop
1461: @end example
1462:
1463: @quotation Assignment
1464: Replace @code{nip} and @code{tuck} with combinations of other stack
1465: manipulation words.
1466:
1467: @example
1468: Given: How do you get:
1469: 1 2 3 3 2 1
1470: 1 2 3 1 2 3 2
1471: 1 2 3 1 2 3 3
1472: 1 2 3 1 3 3
1473: 1 2 3 2 1 3
1474: 1 2 3 4 4 3 2 1
1475: 1 2 3 1 2 3 1 2 3
1476: 1 2 3 4 1 2 3 4 1 2
1477: 1 2 3
1478: 1 2 3 1 2 3 4
1479: 1 2 3 1 3
1480: @end example
1481: @end quotation
1482:
1483: @example
1484: 5 dup * .
1485: @end example
1486:
1487: @quotation Assignment
1488: Write 17^3 and 17^4 in Forth, without writing @code{17} more than once.
1489: Write a piece of Forth code that expects two numbers on the stack
1490: (@var{a} and @var{b}, with @var{b} on top) and computes
1491: @code{(a-b)(a+1)}.
1492: @end quotation
1493:
1494: Reference: @ref{Stack Manipulation}.
1495:
1496:
1497: @node Using files for Forth code Tutorial, Comments Tutorial, Stack Manipulation Tutorial, Tutorial
1498: @section Using files for Forth code
1499: @cindex loading Forth code, tutorial
1500: @cindex files containing Forth code, tutorial
1501:
1502: While working at the Forth command line is convenient for one-line
1503: examples and short one-off code, you probably want to store your source
1504: code in files for convenient editing and persistence. You can use your
1505: favourite editor (Gforth includes Emacs support, @pxref{Emacs and
1506: Gforth}) to create @var{file.fs} and use
1507:
1508: @example
1509: s" @var{file.fs}" included
1510: @end example
1511:
1512: to load it into your Forth system. The file name extension I use for
1513: Forth files is @samp{.fs}.
1514:
1515: You can easily start Gforth with some files loaded like this:
1516:
1517: @example
1518: gforth @var{file1.fs} @var{file2.fs}
1519: @end example
1520:
1521: If an error occurs during loading these files, Gforth terminates,
1522: whereas an error during @code{INCLUDED} within Gforth usually gives you
1523: a Gforth command line. Starting the Forth system every time gives you a
1524: clean start every time, without interference from the results of earlier
1525: tries.
1526:
1527: I often put all the tests in a file, then load the code and run the
1528: tests with
1529:
1530: @example
1531: gforth @var{code.fs} @var{tests.fs} -e bye
1532: @end example
1533:
1534: (often by performing this command with @kbd{C-x C-e} in Emacs). The
1535: @code{-e bye} ensures that Gforth terminates afterwards so that I can
1536: restart this command without ado.
1537:
1538: The advantage of this approach is that the tests can be repeated easily
1539: every time the program ist changed, making it easy to catch bugs
1540: introduced by the change.
1541:
1542: Reference: @ref{Forth source files}.
1543:
1544:
1545: @node Comments Tutorial, Colon Definitions Tutorial, Using files for Forth code Tutorial, Tutorial
1546: @section Comments
1547: @cindex comments tutorial
1548:
1549: @example
1550: \ That's a comment; it ends at the end of the line
1551: ( Another comment; it ends here: ) .s
1552: @end example
1553:
1554: @code{\} and @code{(} are ordinary Forth words and therefore have to be
1555: separated with white space from the following text.
1556:
1557: @example
1558: \This gives an "Undefined word" error
1559: @end example
1560:
1561: The first @code{)} ends a comment started with @code{(}, so you cannot
1562: nest @code{(}-comments; and you cannot comment out text containing a
1563: @code{)} with @code{( ... )}@footnote{therefore it's a good idea to
1564: avoid @code{)} in word names.}.
1565:
1566: I use @code{\}-comments for descriptive text and for commenting out code
1567: of one or more line; I use @code{(}-comments for describing the stack
1568: effect, the stack contents, or for commenting out sub-line pieces of
1569: code.
1570:
1571: The Emacs mode @file{gforth.el} (@pxref{Emacs and Gforth}) supports
1572: these uses by commenting out a region with @kbd{C-x \}, uncommenting a
1573: region with @kbd{C-u C-x \}, and filling a @code{\}-commented region
1574: with @kbd{M-q}.
1575:
1576: Reference: @ref{Comments}.
1577:
1578:
1579: @node Colon Definitions Tutorial, Decompilation Tutorial, Comments Tutorial, Tutorial
1580: @section Colon Definitions
1581: @cindex colon definitions, tutorial
1582: @cindex definitions, tutorial
1583: @cindex procedures, tutorial
1584: @cindex functions, tutorial
1585:
1586: are similar to procedures and functions in other programming languages.
1587:
1588: @example
1589: : squared ( n -- n^2 )
1590: dup * ;
1591: 5 squared .
1592: 7 squared .
1593: @end example
1594:
1595: @code{:} starts the colon definition; its name is @code{squared}. The
1596: following comment describes its stack effect. The words @code{dup *}
1597: are not executed, but compiled into the definition. @code{;} ends the
1598: colon definition.
1599:
1600: The newly-defined word can be used like any other word, including using
1601: it in other definitions:
1602:
1603: @example
1604: : cubed ( n -- n^3 )
1605: dup squared * ;
1606: -5 cubed .
1607: : fourth-power ( n -- n^4 )
1608: squared squared ;
1609: 3 fourth-power .
1610: @end example
1611:
1612: @quotation Assignment
1613: Write colon definitions for @code{nip}, @code{tuck}, @code{negate}, and
1614: @code{/mod} in terms of other Forth words, and check if they work (hint:
1615: test your tests on the originals first). Don't let the
1616: @samp{redefined}-Messages spook you, they are just warnings.
1617: @end quotation
1618:
1619: Reference: @ref{Colon Definitions}.
1620:
1621:
1622: @node Decompilation Tutorial, Stack-Effect Comments Tutorial, Colon Definitions Tutorial, Tutorial
1623: @section Decompilation
1624: @cindex decompilation tutorial
1625: @cindex see tutorial
1626:
1627: You can decompile colon definitions with @code{see}:
1628:
1629: @example
1630: see squared
1631: see cubed
1632: @end example
1633:
1634: In Gforth @code{see} shows you a reconstruction of the source code from
1635: the executable code. Informations that were present in the source, but
1636: not in the executable code, are lost (e.g., comments).
1637:
1638: You can also decompile the predefined words:
1639:
1640: @example
1641: see .
1642: see +
1643: @end example
1644:
1645:
1646: @node Stack-Effect Comments Tutorial, Types Tutorial, Decompilation Tutorial, Tutorial
1647: @section Stack-Effect Comments
1648: @cindex stack-effect comments, tutorial
1649: @cindex --, tutorial
1650: By convention the comment after the name of a definition describes the
1651: stack effect: The part in front of the @samp{--} describes the state of
1652: the stack before the execution of the definition, i.e., the parameters
1653: that are passed into the colon definition; the part behind the @samp{--}
1654: is the state of the stack after the execution of the definition, i.e.,
1655: the results of the definition. The stack comment only shows the top
1656: stack items that the definition accesses and/or changes.
1657:
1658: You should put a correct stack effect on every definition, even if it is
1659: just @code{( -- )}. You should also add some descriptive comment to
1660: more complicated words (I usually do this in the lines following
1661: @code{:}). If you don't do this, your code becomes unreadable (because
1662: you have to work through every definition before you can understand
1663: any).
1664:
1665: @quotation Assignment
1666: The stack effect of @code{swap} can be written like this: @code{x1 x2 --
1667: x2 x1}. Describe the stack effect of @code{-}, @code{drop}, @code{dup},
1668: @code{over}, @code{rot}, @code{nip}, and @code{tuck}. Hint: When you
1669: are done, you can compare your stack effects to those in this manual
1670: (@pxref{Word Index}).
1671: @end quotation
1672:
1673: Sometimes programmers put comments at various places in colon
1674: definitions that describe the contents of the stack at that place (stack
1675: comments); i.e., they are like the first part of a stack-effect
1676: comment. E.g.,
1677:
1678: @example
1679: : cubed ( n -- n^3 )
1680: dup squared ( n n^2 ) * ;
1681: @end example
1682:
1683: In this case the stack comment is pretty superfluous, because the word
1684: is simple enough. If you think it would be a good idea to add such a
1685: comment to increase readability, you should also consider factoring the
1686: word into several simpler words (@pxref{Factoring Tutorial,,
1687: Factoring}), which typically eliminates the need for the stack comment;
1688: however, if you decide not to refactor it, then having such a comment is
1689: better than not having it.
1690:
1691: The names of the stack items in stack-effect and stack comments in the
1692: standard, in this manual, and in many programs specify the type through
1693: a type prefix, similar to Fortran and Hungarian notation. The most
1694: frequent prefixes are:
1695:
1696: @table @code
1697: @item n
1698: signed integer
1699: @item u
1700: unsigned integer
1701: @item c
1702: character
1703: @item f
1704: Boolean flags, i.e. @code{false} or @code{true}.
1705: @item a-addr,a-
1706: Cell-aligned address
1707: @item c-addr,c-
1708: Char-aligned address (note that a Char may have two bytes in Windows NT)
1709: @item xt
1710: Execution token, same size as Cell
1711: @item w,x
1712: Cell, can contain an integer or an address. It usually takes 32, 64 or
1713: 16 bits (depending on your platform and Forth system). A cell is more
1714: commonly known as machine word, but the term @emph{word} already means
1715: something different in Forth.
1716: @item d
1717: signed double-cell integer
1718: @item ud
1719: unsigned double-cell integer
1720: @item r
1721: Float (on the FP stack)
1722: @end table
1723:
1724: You can find a more complete list in @ref{Notation}.
1725:
1726: @quotation Assignment
1727: Write stack-effect comments for all definitions you have written up to
1728: now.
1729: @end quotation
1730:
1731:
1732: @node Types Tutorial, Factoring Tutorial, Stack-Effect Comments Tutorial, Tutorial
1733: @section Types
1734: @cindex types tutorial
1735:
1736: In Forth the names of the operations are not overloaded; so similar
1737: operations on different types need different names; e.g., @code{+} adds
1738: integers, and you have to use @code{f+} to add floating-point numbers.
1739: The following prefixes are often used for related operations on
1740: different types:
1741:
1742: @table @code
1743: @item (none)
1744: signed integer
1745: @item u
1746: unsigned integer
1747: @item c
1748: character
1749: @item d
1750: signed double-cell integer
1751: @item ud, du
1752: unsigned double-cell integer
1753: @item 2
1754: two cells (not-necessarily double-cell numbers)
1755: @item m, um
1756: mixed single-cell and double-cell operations
1757: @item f
1758: floating-point (note that in stack comments @samp{f} represents flags,
1759: and @samp{r} represents FP numbers; also, you need to include the
1760: exponent part in literal FP numbers, @pxref{Floating Point Tutorial}).
1761: @end table
1762:
1763: If there are no differences between the signed and the unsigned variant
1764: (e.g., for @code{+}), there is only the prefix-less variant.
1765:
1766: Forth does not perform type checking, neither at compile time, nor at
1767: run time. If you use the wrong operation, the data are interpreted
1768: incorrectly:
1769:
1770: @example
1771: -1 u.
1772: @end example
1773:
1774: If you have only experience with type-checked languages until now, and
1775: have heard how important type-checking is, don't panic! In my
1776: experience (and that of other Forthers), type errors in Forth code are
1777: usually easy to find (once you get used to it), the increased vigilance
1778: of the programmer tends to catch some harder errors in addition to most
1779: type errors, and you never have to work around the type system, so in
1780: most situations the lack of type-checking seems to be a win (projects to
1781: add type checking to Forth have not caught on).
1782:
1783:
1784: @node Factoring Tutorial, Designing the stack effect Tutorial, Types Tutorial, Tutorial
1785: @section Factoring
1786: @cindex factoring tutorial
1787:
1788: If you try to write longer definitions, you will soon find it hard to
1789: keep track of the stack contents. Therefore, good Forth programmers
1790: tend to write only short definitions (e.g., three lines). The art of
1791: finding meaningful short definitions is known as factoring (as in
1792: factoring polynomials).
1793:
1794: Well-factored programs offer additional advantages: smaller, more
1795: general words, are easier to test and debug and can be reused more and
1796: better than larger, specialized words.
1797:
1798: So, if you run into difficulties with stack management, when writing
1799: code, try to define meaningful factors for the word, and define the word
1800: in terms of those. Even if a factor contains only two words, it is
1801: often helpful.
1802:
1803: Good factoring is not easy, and it takes some practice to get the knack
1804: for it; but even experienced Forth programmers often don't find the
1805: right solution right away, but only when rewriting the program. So, if
1806: you don't come up with a good solution immediately, keep trying, don't
1807: despair.
1808:
1809: @c example !!
1810:
1811:
1812: @node Designing the stack effect Tutorial, Local Variables Tutorial, Factoring Tutorial, Tutorial
1813: @section Designing the stack effect
1814: @cindex Stack effect design, tutorial
1815: @cindex design of stack effects, tutorial
1816:
1817: In other languages you can use an arbitrary order of parameters for a
1818: function; and since there is only one result, you don't have to deal with
1819: the order of results, either.
1820:
1821: In Forth (and other stack-based languages, e.g., PostScript) the
1822: parameter and result order of a definition is important and should be
1823: designed well. The general guideline is to design the stack effect such
1824: that the word is simple to use in most cases, even if that complicates
1825: the implementation of the word. Some concrete rules are:
1826:
1827: @itemize @bullet
1828:
1829: @item
1830: Words consume all of their parameters (e.g., @code{.}).
1831:
1832: @item
1833: If there is a convention on the order of parameters (e.g., from
1834: mathematics or another programming language), stick with it (e.g.,
1835: @code{-}).
1836:
1837: @item
1838: If one parameter usually requires only a short computation (e.g., it is
1839: a constant), pass it on the top of the stack. Conversely, parameters
1840: that usually require a long sequence of code to compute should be passed
1841: as the bottom (i.e., first) parameter. This makes the code easier to
1842: read, because the reader does not need to keep track of the bottom item
1843: through a long sequence of code (or, alternatively, through stack
1844: manipulations). E.g., @code{!} (store, @pxref{Memory}) expects the
1845: address on top of the stack because it is usually simpler to compute
1846: than the stored value (often the address is just a variable).
1847:
1848: @item
1849: Similarly, results that are usually consumed quickly should be returned
1850: on the top of stack, whereas a result that is often used in long
1851: computations should be passed as bottom result. E.g., the file words
1852: like @code{open-file} return the error code on the top of stack, because
1853: it is usually consumed quickly by @code{throw}; moreover, the error code
1854: has to be checked before doing anything with the other results.
1855:
1856: @end itemize
1857:
1858: These rules are just general guidelines, don't lose sight of the overall
1859: goal to make the words easy to use. E.g., if the convention rule
1860: conflicts with the computation-length rule, you might decide in favour
1861: of the convention if the word will be used rarely, and in favour of the
1862: computation-length rule if the word will be used frequently (because
1863: with frequent use the cost of breaking the computation-length rule would
1864: be quite high, and frequent use makes it easier to remember an
1865: unconventional order).
1866:
1867: @c example !! structure package
1868:
1869:
1870: @node Local Variables Tutorial, Conditional execution Tutorial, Designing the stack effect Tutorial, Tutorial
1871: @section Local Variables
1872: @cindex local variables, tutorial
1873:
1874: You can define local variables (@emph{locals}) in a colon definition:
1875:
1876: @example
1877: : swap @{ a b -- b a @}
1878: b a ;
1879: 1 2 swap .s 2drop
1880: @end example
1881:
1882: (If your Forth system does not support this syntax, include
1883: @file{compat/anslocal.fs} first).
1884:
1885: In this example @code{@{ a b -- b a @}} is the locals definition; it
1886: takes two cells from the stack, puts the top of stack in @code{b} and
1887: the next stack element in @code{a}. @code{--} starts a comment ending
1888: with @code{@}}. After the locals definition, using the name of the
1889: local will push its value on the stack. You can leave the comment
1890: part (@code{-- b a}) away:
1891:
1892: @example
1893: : swap ( x1 x2 -- x2 x1 )
1894: @{ a b @} b a ;
1895: @end example
1896:
1897: In Gforth you can have several locals definitions, anywhere in a colon
1898: definition; in contrast, in a standard program you can have only one
1899: locals definition per colon definition, and that locals definition must
1900: be outside any control structure.
1901:
1902: With locals you can write slightly longer definitions without running
1903: into stack trouble. However, I recommend trying to write colon
1904: definitions without locals for exercise purposes to help you gain the
1905: essential factoring skills.
1906:
1907: @quotation Assignment
1908: Rewrite your definitions until now with locals
1909: @end quotation
1910:
1911: Reference: @ref{Locals}.
1912:
1913:
1914: @node Conditional execution Tutorial, Flags and Comparisons Tutorial, Local Variables Tutorial, Tutorial
1915: @section Conditional execution
1916: @cindex conditionals, tutorial
1917: @cindex if, tutorial
1918:
1919: In Forth you can use control structures only inside colon definitions.
1920: An @code{if}-structure looks like this:
1921:
1922: @example
1923: : abs ( n1 -- +n2 )
1924: dup 0 < if
1925: negate
1926: endif ;
1927: 5 abs .
1928: -5 abs .
1929: @end example
1930:
1931: @code{if} takes a flag from the stack. If the flag is non-zero (true),
1932: the following code is performed, otherwise execution continues after the
1933: @code{endif} (or @code{else}). @code{<} compares the top two stack
1934: elements and produces a flag:
1935:
1936: @example
1937: 1 2 < .
1938: 2 1 < .
1939: 1 1 < .
1940: @end example
1941:
1942: Actually the standard name for @code{endif} is @code{then}. This
1943: tutorial presents the examples using @code{endif}, because this is often
1944: less confusing for people familiar with other programming languages
1945: where @code{then} has a different meaning. If your system does not have
1946: @code{endif}, define it with
1947:
1948: @example
1949: : endif postpone then ; immediate
1950: @end example
1951:
1952: You can optionally use an @code{else}-part:
1953:
1954: @example
1955: : min ( n1 n2 -- n )
1956: 2dup < if
1957: drop
1958: else
1959: nip
1960: endif ;
1961: 2 3 min .
1962: 3 2 min .
1963: @end example
1964:
1965: @quotation Assignment
1966: Write @code{min} without @code{else}-part (hint: what's the definition
1967: of @code{nip}?).
1968: @end quotation
1969:
1970: Reference: @ref{Selection}.
1971:
1972:
1973: @node Flags and Comparisons Tutorial, General Loops Tutorial, Conditional execution Tutorial, Tutorial
1974: @section Flags and Comparisons
1975: @cindex flags tutorial
1976: @cindex comparison tutorial
1977:
1978: In a false-flag all bits are clear (0 when interpreted as integer). In
1979: a canonical true-flag all bits are set (-1 as a twos-complement signed
1980: integer); in many contexts (e.g., @code{if}) any non-zero value is
1981: treated as true flag.
1982:
1983: @example
1984: false .
1985: true .
1986: true hex u. decimal
1987: @end example
1988:
1989: Comparison words produce canonical flags:
1990:
1991: @example
1992: 1 1 = .
1993: 1 0= .
1994: 0 1 < .
1995: 0 0 < .
1996: -1 1 u< . \ type error, u< interprets -1 as large unsigned number
1997: -1 1 < .
1998: @end example
1999:
2000: Gforth supports all combinations of the prefixes @code{0 u d d0 du f f0}
2001: (or none) and the comparisons @code{= <> < > <= >=}. Only a part of
2002: these combinations are standard (for details see the standard,
2003: @ref{Numeric comparison}, @ref{Floating Point} or @ref{Word Index}).
2004:
2005: You can use @code{and or xor invert} as operations on canonical flags.
2006: Actually they are bitwise operations:
2007:
2008: @example
2009: 1 2 and .
2010: 1 2 or .
2011: 1 3 xor .
2012: 1 invert .
2013: @end example
2014:
2015: You can convert a zero/non-zero flag into a canonical flag with
2016: @code{0<>} (and complement it on the way with @code{0=}).
2017:
2018: @example
2019: 1 0= .
2020: 1 0<> .
2021: @end example
2022:
2023: You can use the all-bits-set feature of canonical flags and the bitwise
2024: operation of the Boolean operations to avoid @code{if}s:
2025:
2026: @example
2027: : foo ( n1 -- n2 )
2028: 0= if
2029: 14
2030: else
2031: 0
2032: endif ;
2033: 0 foo .
2034: 1 foo .
2035:
2036: : foo ( n1 -- n2 )
2037: 0= 14 and ;
2038: 0 foo .
2039: 1 foo .
2040: @end example
2041:
2042: @quotation Assignment
2043: Write @code{min} without @code{if}.
2044: @end quotation
2045:
2046: For reference, see @ref{Boolean Flags}, @ref{Numeric comparison}, and
2047: @ref{Bitwise operations}.
2048:
2049:
2050: @node General Loops Tutorial, Counted loops Tutorial, Flags and Comparisons Tutorial, Tutorial
2051: @section General Loops
2052: @cindex loops, indefinite, tutorial
2053:
2054: The endless loop is the most simple one:
2055:
2056: @example
2057: : endless ( -- )
2058: 0 begin
2059: dup . 1+
2060: again ;
2061: endless
2062: @end example
2063:
2064: Terminate this loop by pressing @kbd{Ctrl-C} (in Gforth). @code{begin}
2065: does nothing at run-time, @code{again} jumps back to @code{begin}.
2066:
2067: A loop with one exit at any place looks like this:
2068:
2069: @example
2070: : log2 ( +n1 -- n2 )
2071: \ logarithmus dualis of n1>0, rounded down to the next integer
2072: assert( dup 0> )
2073: 2/ 0 begin
2074: over 0> while
2075: 1+ swap 2/ swap
2076: repeat
2077: nip ;
2078: 7 log2 .
2079: 8 log2 .
2080: @end example
2081:
2082: At run-time @code{while} consumes a flag; if it is 0, execution
2083: continues behind the @code{repeat}; if the flag is non-zero, execution
2084: continues behind the @code{while}. @code{Repeat} jumps back to
2085: @code{begin}, just like @code{again}.
2086:
2087: In Forth there are a number of combinations/abbreviations, like
2088: @code{1+}. However, @code{2/} is not one of them; it shifts its
2089: argument right by one bit (arithmetic shift right), and viewed as
2090: division that always rounds towards negative infinity (floored
2091: division). In contrast, @code{/} rounds towards zero on some systems
2092: (not on default installations of gforth (>=0.7.0), however).
2093:
2094: @example
2095: -5 2 / . \ -2 or -3
2096: -5 2/ . \ -3
2097: @end example
2098:
2099: @code{assert(} is no standard word, but you can get it on systems other
2100: than Gforth by including @file{compat/assert.fs}. You can see what it
2101: does by trying
2102:
2103: @example
2104: 0 log2 .
2105: @end example
2106:
2107: Here's a loop with an exit at the end:
2108:
2109: @example
2110: : log2 ( +n1 -- n2 )
2111: \ logarithmus dualis of n1>0, rounded down to the next integer
2112: assert( dup 0 > )
2113: -1 begin
2114: 1+ swap 2/ swap
2115: over 0 <=
2116: until
2117: nip ;
2118: @end example
2119:
2120: @code{Until} consumes a flag; if it is zero, execution continues at
2121: the @code{begin}, otherwise after the @code{until}.
2122:
2123: @quotation Assignment
2124: Write a definition for computing the greatest common divisor.
2125: @end quotation
2126:
2127: Reference: @ref{Simple Loops}.
2128:
2129:
2130: @node Counted loops Tutorial, Recursion Tutorial, General Loops Tutorial, Tutorial
2131: @section Counted loops
2132: @cindex loops, counted, tutorial
2133:
2134: @example
2135: : ^ ( n1 u -- n )
2136: \ n = the uth power of n1
2137: 1 swap 0 u+do
2138: over *
2139: loop
2140: nip ;
2141: 3 2 ^ .
2142: 4 3 ^ .
2143: @end example
2144:
2145: @code{U+do} (from @file{compat/loops.fs}, if your Forth system doesn't
2146: have it) takes two numbers of the stack @code{( u3 u4 -- )}, and then
2147: performs the code between @code{u+do} and @code{loop} for @code{u3-u4}
2148: times (or not at all, if @code{u3-u4<0}).
2149:
2150: You can see the stack effect design rules at work in the stack effect of
2151: the loop start words: Since the start value of the loop is more
2152: frequently constant than the end value, the start value is passed on
2153: the top-of-stack.
2154:
2155: You can access the counter of a counted loop with @code{i}:
2156:
2157: @example
2158: : fac ( u -- u! )
2159: 1 swap 1+ 1 u+do
2160: i *
2161: loop ;
2162: 5 fac .
2163: 7 fac .
2164: @end example
2165:
2166: There is also @code{+do}, which expects signed numbers (important for
2167: deciding whether to enter the loop).
2168:
2169: @quotation Assignment
2170: Write a definition for computing the nth Fibonacci number.
2171: @end quotation
2172:
2173: You can also use increments other than 1:
2174:
2175: @example
2176: : up2 ( n1 n2 -- )
2177: +do
2178: i .
2179: 2 +loop ;
2180: 10 0 up2
2181:
2182: : down2 ( n1 n2 -- )
2183: -do
2184: i .
2185: 2 -loop ;
2186: 0 10 down2
2187: @end example
2188:
2189: Reference: @ref{Counted Loops}.
2190:
2191:
2192: @node Recursion Tutorial, Leaving definitions or loops Tutorial, Counted loops Tutorial, Tutorial
2193: @section Recursion
2194: @cindex recursion tutorial
2195:
2196: Usually the name of a definition is not visible in the definition; but
2197: earlier definitions are usually visible:
2198:
2199: @example
2200: 1 0 / . \ "Floating-point unidentified fault" in Gforth on some platforms
2201: : / ( n1 n2 -- n )
2202: dup 0= if
2203: -10 throw \ report division by zero
2204: endif
2205: / \ old version
2206: ;
2207: 1 0 /
2208: @end example
2209:
2210: For recursive definitions you can use @code{recursive} (non-standard) or
2211: @code{recurse}:
2212:
2213: @example
2214: : fac1 ( n -- n! ) recursive
2215: dup 0> if
2216: dup 1- fac1 *
2217: else
2218: drop 1
2219: endif ;
2220: 7 fac1 .
2221:
2222: : fac2 ( n -- n! )
2223: dup 0> if
2224: dup 1- recurse *
2225: else
2226: drop 1
2227: endif ;
2228: 8 fac2 .
2229: @end example
2230:
2231: @quotation Assignment
2232: Write a recursive definition for computing the nth Fibonacci number.
2233: @end quotation
2234:
2235: Reference (including indirect recursion): @xref{Calls and returns}.
2236:
2237:
2238: @node Leaving definitions or loops Tutorial, Return Stack Tutorial, Recursion Tutorial, Tutorial
2239: @section Leaving definitions or loops
2240: @cindex leaving definitions, tutorial
2241: @cindex leaving loops, tutorial
2242:
2243: @code{EXIT} exits the current definition right away. For every counted
2244: loop that is left in this way, an @code{UNLOOP} has to be performed
2245: before the @code{EXIT}:
2246:
2247: @c !! real examples
2248: @example
2249: : ...
2250: ... u+do
2251: ... if
2252: ... unloop exit
2253: endif
2254: ...
2255: loop
2256: ... ;
2257: @end example
2258:
2259: @code{LEAVE} leaves the innermost counted loop right away:
2260:
2261: @example
2262: : ...
2263: ... u+do
2264: ... if
2265: ... leave
2266: endif
2267: ...
2268: loop
2269: ... ;
2270: @end example
2271:
2272: @c !! example
2273:
2274: Reference: @ref{Calls and returns}, @ref{Counted Loops}.
2275:
2276:
2277: @node Return Stack Tutorial, Memory Tutorial, Leaving definitions or loops Tutorial, Tutorial
2278: @section Return Stack
2279: @cindex return stack tutorial
2280:
2281: In addition to the data stack Forth also has a second stack, the return
2282: stack; most Forth systems store the return addresses of procedure calls
2283: there (thus its name). Programmers can also use this stack:
2284:
2285: @example
2286: : foo ( n1 n2 -- )
2287: .s
2288: >r .s
2289: r@@ .
2290: >r .s
2291: r@@ .
2292: r> .
2293: r@@ .
2294: r> . ;
2295: 1 2 foo
2296: @end example
2297:
2298: @code{>r} takes an element from the data stack and pushes it onto the
2299: return stack; conversely, @code{r>} moves an elementm from the return to
2300: the data stack; @code{r@@} pushes a copy of the top of the return stack
2301: on the data stack.
2302:
2303: Forth programmers usually use the return stack for storing data
2304: temporarily, if using the data stack alone would be too complex, and
2305: factoring and locals are not an option:
2306:
2307: @example
2308: : 2swap ( x1 x2 x3 x4 -- x3 x4 x1 x2 )
2309: rot >r rot r> ;
2310: @end example
2311:
2312: The return address of the definition and the loop control parameters of
2313: counted loops usually reside on the return stack, so you have to take
2314: all items, that you have pushed on the return stack in a colon
2315: definition or counted loop, from the return stack before the definition
2316: or loop ends. You cannot access items that you pushed on the return
2317: stack outside some definition or loop within the definition of loop.
2318:
2319: If you miscount the return stack items, this usually ends in a crash:
2320:
2321: @example
2322: : crash ( n -- )
2323: >r ;
2324: 5 crash
2325: @end example
2326:
2327: You cannot mix using locals and using the return stack (according to the
2328: standard; Gforth has no problem). However, they solve the same
2329: problems, so this shouldn't be an issue.
2330:
2331: @quotation Assignment
2332: Can you rewrite any of the definitions you wrote until now in a better
2333: way using the return stack?
2334: @end quotation
2335:
2336: Reference: @ref{Return stack}.
2337:
2338:
2339: @node Memory Tutorial, Characters and Strings Tutorial, Return Stack Tutorial, Tutorial
2340: @section Memory
2341: @cindex memory access/allocation tutorial
2342:
2343: You can create a global variable @code{v} with
2344:
2345: @example
2346: variable v ( -- addr )
2347: @end example
2348:
2349: @code{v} pushes the address of a cell in memory on the stack. This cell
2350: was reserved by @code{variable}. You can use @code{!} (store) to store
2351: values into this cell and @code{@@} (fetch) to load the value from the
2352: stack into memory:
2353:
2354: @example
2355: v .
2356: 5 v ! .s
2357: v @@ .
2358: @end example
2359:
2360: You can see a raw dump of memory with @code{dump}:
2361:
2362: @example
2363: v 1 cells .s dump
2364: @end example
2365:
2366: @code{Cells ( n1 -- n2 )} gives you the number of bytes (or, more
2367: generally, address units (aus)) that @code{n1 cells} occupy. You can
2368: also reserve more memory:
2369:
2370: @example
2371: create v2 20 cells allot
2372: v2 20 cells dump
2373: @end example
2374:
2375: creates a variable-like word @code{v2} and reserves 20 uninitialized
2376: cells; the address pushed by @code{v2} points to the start of these 20
2377: cells (@pxref{CREATE}). You can use address arithmetic to access
2378: these cells:
2379:
2380: @example
2381: 3 v2 5 cells + !
2382: v2 20 cells dump
2383: @end example
2384:
2385: You can reserve and initialize memory with @code{,}:
2386:
2387: @example
2388: create v3
2389: 5 , 4 , 3 , 2 , 1 ,
2390: v3 @@ .
2391: v3 cell+ @@ .
2392: v3 2 cells + @@ .
2393: v3 5 cells dump
2394: @end example
2395:
2396: @quotation Assignment
2397: Write a definition @code{vsum ( addr u -- n )} that computes the sum of
2398: @code{u} cells, with the first of these cells at @code{addr}, the next
2399: one at @code{addr cell+} etc.
2400: @end quotation
2401:
2402: The difference between @code{variable} and @code{create} is that
2403: @code{variable} allots a cell, and that you cannot allot additional
2404: memory to a variable in standard Forth.
2405:
2406: You can also reserve memory without creating a new word:
2407:
2408: @example
2409: here 10 cells allot .
2410: here .
2411: @end example
2412:
2413: The first @code{here} pushes the start address of the memory area, the
2414: second @code{here} the address after the dictionary area. You should
2415: store the start address somewhere, or you will have a hard time
2416: finding the memory area again.
2417:
2418: @code{Allot} manages dictionary memory. The dictionary memory contains
2419: the system's data structures for words etc. on Gforth and most other
2420: Forth systems. It is managed like a stack: You can free the memory that
2421: you have just @code{allot}ed with
2422:
2423: @example
2424: -10 cells allot
2425: here .
2426: @end example
2427:
2428: Note that you cannot do this if you have created a new word in the
2429: meantime (because then your @code{allot}ed memory is no longer on the
2430: top of the dictionary ``stack'').
2431:
2432: Alternatively, you can use @code{allocate} and @code{free} which allow
2433: freeing memory in any order:
2434:
2435: @example
2436: 10 cells allocate throw .s
2437: 20 cells allocate throw .s
2438: swap
2439: free throw
2440: free throw
2441: @end example
2442:
2443: The @code{throw}s deal with errors (e.g., out of memory).
2444:
2445: And there is also a
2446: @uref{http://www.complang.tuwien.ac.at/forth/garbage-collection.zip,
2447: garbage collector}, which eliminates the need to @code{free} memory
2448: explicitly.
2449:
2450: Reference: @ref{Memory}.
2451:
2452:
2453: @node Characters and Strings Tutorial, Alignment Tutorial, Memory Tutorial, Tutorial
2454: @section Characters and Strings
2455: @cindex strings tutorial
2456: @cindex characters tutorial
2457:
2458: On the stack characters take up a cell, like numbers. In memory they
2459: have their own size (one 8-bit byte on most systems), and therefore
2460: require their own words for memory access:
2461:
2462: @example
2463: create v4
2464: 104 c, 97 c, 108 c, 108 c, 111 c,
2465: v4 4 chars + c@@ .
2466: v4 5 chars dump
2467: @end example
2468:
2469: The preferred representation of strings on the stack is @code{addr
2470: u-count}, where @code{addr} is the address of the first character and
2471: @code{u-count} is the number of characters in the string.
2472:
2473: @example
2474: v4 5 type
2475: @end example
2476:
2477: You get a string constant with
2478:
2479: @example
2480: s" hello, world" .s
2481: type
2482: @end example
2483:
2484: Make sure you have a space between @code{s"} and the string; @code{s"}
2485: is a normal Forth word and must be delimited with white space (try what
2486: happens when you remove the space).
2487:
2488: However, this interpretive use of @code{s"} is quite restricted: the
2489: string exists only until the next call of @code{s"} (some Forth systems
2490: keep more than one of these strings, but usually they still have a
2491: limited lifetime).
2492:
2493: @example
2494: s" hello," s" world" .s
2495: type
2496: type
2497: @end example
2498:
2499: You can also use @code{s"} in a definition, and the resulting
2500: strings then live forever (well, for as long as the definition):
2501:
2502: @example
2503: : foo s" hello," s" world" ;
2504: foo .s
2505: type
2506: type
2507: @end example
2508:
2509: @quotation Assignment
2510: @code{Emit ( c -- )} types @code{c} as character (not a number).
2511: Implement @code{type ( addr u -- )}.
2512: @end quotation
2513:
2514: Reference: @ref{Memory Blocks}.
2515:
2516:
2517: @node Alignment Tutorial, Floating Point Tutorial, Characters and Strings Tutorial, Tutorial
2518: @section Alignment
2519: @cindex alignment tutorial
2520: @cindex memory alignment tutorial
2521:
2522: On many processors cells have to be aligned in memory, if you want to
2523: access them with @code{@@} and @code{!} (and even if the processor does
2524: not require alignment, access to aligned cells is faster).
2525:
2526: @code{Create} aligns @code{here} (i.e., the place where the next
2527: allocation will occur, and that the @code{create}d word points to).
2528: Likewise, the memory produced by @code{allocate} starts at an aligned
2529: address. Adding a number of @code{cells} to an aligned address produces
2530: another aligned address.
2531:
2532: However, address arithmetic involving @code{char+} and @code{chars} can
2533: create an address that is not cell-aligned. @code{Aligned ( addr --
2534: a-addr )} produces the next aligned address:
2535:
2536: @example
2537: v3 char+ aligned .s @@ .
2538: v3 char+ .s @@ .
2539: @end example
2540:
2541: Similarly, @code{align} advances @code{here} to the next aligned
2542: address:
2543:
2544: @example
2545: create v5 97 c,
2546: here .
2547: align here .
2548: 1000 ,
2549: @end example
2550:
2551: Note that you should use aligned addresses even if your processor does
2552: not require them, if you want your program to be portable.
2553:
2554: Reference: @ref{Address arithmetic}.
2555:
2556: @node Floating Point Tutorial, Files Tutorial, Alignment Tutorial, Tutorial
2557: @section Floating Point
2558: @cindex floating point tutorial
2559: @cindex FP tutorial
2560:
2561: Floating-point (FP) numbers and arithmetic in Forth works mostly as one
2562: might expect, but there are a few things worth noting:
2563:
2564: The first point is not specific to Forth, but so important and yet not
2565: universally known that I mention it here: FP numbers are not reals.
2566: Many properties (e.g., arithmetic laws) that reals have and that one
2567: expects of all kinds of numbers do not hold for FP numbers. If you
2568: want to use FP computations, you should learn about their problems and
2569: how to avoid them; a good starting point is @cite{David Goldberg,
2570: @uref{http://docs.sun.com/source/806-3568/ncg_goldberg.html,What Every
2571: Computer Scientist Should Know About Floating-Point Arithmetic}, ACM
2572: Computing Surveys 23(1):5@minus{}48, March 1991}.
2573:
2574: In Forth source code literal FP numbers need an exponent, e.g.,
2575: @code{1e0}; this can also be written shorter as @code{1e}, longer as
2576: @code{+1.0e+0}, and many variations in between. The reason for this is
2577: that, for historical reasons, Forth interprets a decimal point alone
2578: (e.g., @code{1.}) as indicating a double-cell integer. Examples:
2579:
2580: @example
2581: 2e 2e f+ f.
2582: @end example
2583:
2584: Another requirement for literal FP numbers is that the current base is
2585: decimal; with a hex base @code{1e} is interpreted as an integer.
2586:
2587: Forth has a separate stack for FP numbers.@footnote{Theoretically, an
2588: ANS Forth system may implement the FP stack on the data stack, but
2589: virtually all systems implement a separate FP stack; and programming
2590: in a way that accommodates all models is so cumbersome that nobody
2591: does it.} One advantage of this model is that cells are not in the
2592: way when accessing FP values, and vice versa. Forth has a set of
2593: words for manipulating the FP stack: @code{fdup fswap fdrop fover
2594: frot} and (non-standard) @code{fnip ftuck fpick}.
2595:
2596: FP arithmetic words are prefixed with @code{F}. There is the usual
2597: set @code{f+ f- f* f/ f** fnegate} as well as a number of words for
2598: other functions, e.g., @code{fsqrt fsin fln fmin}. One word that you
2599: might expect is @code{f=}; but @code{f=} is non-standard, because FP
2600: computation results are usually inaccurate, so exact comparison is
2601: usually a mistake, and one should use approximate comparison.
2602: Unfortunately, @code{f~}, the standard word for that purpose, is not
2603: well designed, so Gforth provides @code{f~abs} and @code{f~rel} as
2604: well.
2605:
2606: And of course there are words for accessing FP numbers in memory
2607: (@code{f@@ f!}), and for address arithmetic (@code{floats float+
2608: faligned}). There are also variants of these words with an @code{sf}
2609: and @code{df} prefix for accessing IEEE format single-precision and
2610: double-precision numbers in memory; their main purpose is for
2611: accessing external FP data (e.g., that has been read from or will be
2612: written to a file).
2613:
2614: Here is an example of a dot-product word and its use:
2615:
2616: @example
2617: : v* ( f_addr1 nstride1 f_addr2 nstride2 ucount -- r )
2618: >r swap 2swap swap 0e r> 0 ?DO
2619: dup f@@ over + 2swap dup f@@ f* f+ over + 2swap
2620: LOOP
2621: 2drop 2drop ;
2622:
2623: create v 1.23e f, 4.56e f, 7.89e f,
2624:
2625: v 1 floats v 1 floats 3 v* f.
2626: @end example
2627:
2628: @quotation Assignment
2629: Write a program to solve a quadratic equation. Then read @cite{Henry
2630: G. Baker,
2631: @uref{http://home.pipeline.com/~hbaker1/sigplannotices/sigcol05.ps.gz,You
2632: Could Learn a Lot from a Quadratic}, ACM SIGPLAN Notices,
2633: 33(1):30@minus{}39, January 1998}, and see if you can improve your
2634: program. Finally, find a test case where the original and the
2635: improved version produce different results.
2636: @end quotation
2637:
2638: Reference: @ref{Floating Point}; @ref{Floating point stack};
2639: @ref{Number Conversion}; @ref{Memory Access}; @ref{Address
2640: arithmetic}.
2641:
2642: @node Files Tutorial, Interpretation and Compilation Semantics and Immediacy Tutorial, Floating Point Tutorial, Tutorial
2643: @section Files
2644: @cindex files tutorial
2645:
2646: This section gives a short introduction into how to use files inside
2647: Forth. It's broken up into five easy steps:
2648:
2649: @enumerate 1
2650: @item Opened an ASCII text file for input
2651: @item Opened a file for output
2652: @item Read input file until string matched (or some other condition matched)
2653: @item Wrote some lines from input ( modified or not) to output
2654: @item Closed the files.
2655: @end enumerate
2656:
2657: Reference: @ref{General files}.
2658:
2659: @subsection Open file for input
2660:
2661: @example
2662: s" foo.in" r/o open-file throw Value fd-in
2663: @end example
2664:
2665: @subsection Create file for output
2666:
2667: @example
2668: s" foo.out" w/o create-file throw Value fd-out
2669: @end example
2670:
2671: The available file modes are r/o for read-only access, r/w for
2672: read-write access, and w/o for write-only access. You could open both
2673: files with r/w, too, if you like. All file words return error codes; for
2674: most applications, it's best to pass there error codes with @code{throw}
2675: to the outer error handler.
2676:
2677: If you want words for opening and assigning, define them as follows:
2678:
2679: @example
2680: 0 Value fd-in
2681: 0 Value fd-out
2682: : open-input ( addr u -- ) r/o open-file throw to fd-in ;
2683: : open-output ( addr u -- ) w/o create-file throw to fd-out ;
2684: @end example
2685:
2686: Usage example:
2687:
2688: @example
2689: s" foo.in" open-input
2690: s" foo.out" open-output
2691: @end example
2692:
2693: @subsection Scan file for a particular line
2694:
2695: @example
2696: 256 Constant max-line
2697: Create line-buffer max-line 2 + allot
2698:
2699: : scan-file ( addr u -- )
2700: begin
2701: line-buffer max-line fd-in read-line throw
2702: while
2703: >r 2dup line-buffer r> compare 0=
2704: until
2705: else
2706: drop
2707: then
2708: 2drop ;
2709: @end example
2710:
2711: @code{read-line ( addr u1 fd -- u2 flag ior )} reads up to u1 bytes into
2712: the buffer at addr, and returns the number of bytes read, a flag that is
2713: false when the end of file is reached, and an error code.
2714:
2715: @code{compare ( addr1 u1 addr2 u2 -- n )} compares two strings and
2716: returns zero if both strings are equal. It returns a positive number if
2717: the first string is lexically greater, a negative if the second string
2718: is lexically greater.
2719:
2720: We haven't seen this loop here; it has two exits. Since the @code{while}
2721: exits with the number of bytes read on the stack, we have to clean up
2722: that separately; that's after the @code{else}.
2723:
2724: Usage example:
2725:
2726: @example
2727: s" The text I search is here" scan-file
2728: @end example
2729:
2730: @subsection Copy input to output
2731:
2732: @example
2733: : copy-file ( -- )
2734: begin
2735: line-buffer max-line fd-in read-line throw
2736: while
2737: line-buffer swap fd-out write-line throw
2738: repeat
2739: drop ;
2740: @end example
2741: @c !! does not handle long lines, no newline at end of file
2742:
2743: @subsection Close files
2744:
2745: @example
2746: fd-in close-file throw
2747: fd-out close-file throw
2748: @end example
2749:
2750: Likewise, you can put that into definitions, too:
2751:
2752: @example
2753: : close-input ( -- ) fd-in close-file throw ;
2754: : close-output ( -- ) fd-out close-file throw ;
2755: @end example
2756:
2757: @quotation Assignment
2758: How could you modify @code{copy-file} so that it copies until a second line is
2759: matched? Can you write a program that extracts a section of a text file,
2760: given the line that starts and the line that terminates that section?
2761: @end quotation
2762:
2763: @node Interpretation and Compilation Semantics and Immediacy Tutorial, Execution Tokens Tutorial, Files Tutorial, Tutorial
2764: @section Interpretation and Compilation Semantics and Immediacy
2765: @cindex semantics tutorial
2766: @cindex interpretation semantics tutorial
2767: @cindex compilation semantics tutorial
2768: @cindex immediate, tutorial
2769:
2770: When a word is compiled, it behaves differently from being interpreted.
2771: E.g., consider @code{+}:
2772:
2773: @example
2774: 1 2 + .
2775: : foo + ;
2776: @end example
2777:
2778: These two behaviours are known as compilation and interpretation
2779: semantics. For normal words (e.g., @code{+}), the compilation semantics
2780: is to append the interpretation semantics to the currently defined word
2781: (@code{foo} in the example above). I.e., when @code{foo} is executed
2782: later, the interpretation semantics of @code{+} (i.e., adding two
2783: numbers) will be performed.
2784:
2785: However, there are words with non-default compilation semantics, e.g.,
2786: the control-flow words like @code{if}. You can use @code{immediate} to
2787: change the compilation semantics of the last defined word to be equal to
2788: the interpretation semantics:
2789:
2790: @example
2791: : [FOO] ( -- )
2792: 5 . ; immediate
2793:
2794: [FOO]
2795: : bar ( -- )
2796: [FOO] ;
2797: bar
2798: see bar
2799: @end example
2800:
2801: Two conventions to mark words with non-default compilation semantics are
2802: names with brackets (more frequently used) and to write them all in
2803: upper case (less frequently used).
2804:
2805: In Gforth (and many other systems) you can also remove the
2806: interpretation semantics with @code{compile-only} (the compilation
2807: semantics is derived from the original interpretation semantics):
2808:
2809: @example
2810: : flip ( -- )
2811: 6 . ; compile-only \ but not immediate
2812: flip
2813:
2814: : flop ( -- )
2815: flip ;
2816: flop
2817: @end example
2818:
2819: In this example the interpretation semantics of @code{flop} is equal to
2820: the original interpretation semantics of @code{flip}.
2821:
2822: The text interpreter has two states: in interpret state, it performs the
2823: interpretation semantics of words it encounters; in compile state, it
2824: performs the compilation semantics of these words.
2825:
2826: Among other things, @code{:} switches into compile state, and @code{;}
2827: switches back to interpret state. They contain the factors @code{]}
2828: (switch to compile state) and @code{[} (switch to interpret state), that
2829: do nothing but switch the state.
2830:
2831: @example
2832: : xxx ( -- )
2833: [ 5 . ]
2834: ;
2835:
2836: xxx
2837: see xxx
2838: @end example
2839:
2840: These brackets are also the source of the naming convention mentioned
2841: above.
2842:
2843: Reference: @ref{Interpretation and Compilation Semantics}.
2844:
2845:
2846: @node Execution Tokens Tutorial, Exceptions Tutorial, Interpretation and Compilation Semantics and Immediacy Tutorial, Tutorial
2847: @section Execution Tokens
2848: @cindex execution tokens tutorial
2849: @cindex XT tutorial
2850:
2851: @code{' word} gives you the execution token (XT) of a word. The XT is a
2852: cell representing the interpretation semantics of a word. You can
2853: execute this semantics with @code{execute}:
2854:
2855: @example
2856: ' + .s
2857: 1 2 rot execute .
2858: @end example
2859:
2860: The XT is similar to a function pointer in C. However, parameter
2861: passing through the stack makes it a little more flexible:
2862:
2863: @example
2864: : map-array ( ... addr u xt -- ... )
2865: \ executes xt ( ... x -- ... ) for every element of the array starting
2866: \ at addr and containing u elements
2867: @{ xt @}
2868: cells over + swap ?do
2869: i @@ xt execute
2870: 1 cells +loop ;
2871:
2872: create a 3 , 4 , 2 , -1 , 4 ,
2873: a 5 ' . map-array .s
2874: 0 a 5 ' + map-array .
2875: s" max-n" environment? drop .s
2876: a 5 ' min map-array .
2877: @end example
2878:
2879: You can use map-array with the XTs of words that consume one element
2880: more than they produce. In theory you can also use it with other XTs,
2881: but the stack effect then depends on the size of the array, which is
2882: hard to understand.
2883:
2884: Since XTs are cell-sized, you can store them in memory and manipulate
2885: them on the stack like other cells. You can also compile the XT into a
2886: word with @code{compile,}:
2887:
2888: @example
2889: : foo1 ( n1 n2 -- n )
2890: [ ' + compile, ] ;
2891: see foo1
2892: @end example
2893:
2894: This is non-standard, because @code{compile,} has no compilation
2895: semantics in the standard, but it works in good Forth systems. For the
2896: broken ones, use
2897:
2898: @example
2899: : [compile,] compile, ; immediate
2900:
2901: : foo1 ( n1 n2 -- n )
2902: [ ' + ] [compile,] ;
2903: see foo
2904: @end example
2905:
2906: @code{'} is a word with default compilation semantics; it parses the
2907: next word when its interpretation semantics are executed, not during
2908: compilation:
2909:
2910: @example
2911: : foo ( -- xt )
2912: ' ;
2913: see foo
2914: : bar ( ... "word" -- ... )
2915: ' execute ;
2916: see bar
2917: 1 2 bar + .
2918: @end example
2919:
2920: You often want to parse a word during compilation and compile its XT so
2921: it will be pushed on the stack at run-time. @code{[']} does this:
2922:
2923: @example
2924: : xt-+ ( -- xt )
2925: ['] + ;
2926: see xt-+
2927: 1 2 xt-+ execute .
2928: @end example
2929:
2930: Many programmers tend to see @code{'} and the word it parses as one
2931: unit, and expect it to behave like @code{[']} when compiled, and are
2932: confused by the actual behaviour. If you are, just remember that the
2933: Forth system just takes @code{'} as one unit and has no idea that it is
2934: a parsing word (attempts to convenience programmers in this issue have
2935: usually resulted in even worse pitfalls, see
2936: @uref{http://www.complang.tuwien.ac.at/papers/ertl98.ps.gz,
2937: @code{State}-smartness---Why it is evil and How to Exorcise it}).
2938:
2939: Note that the state of the interpreter does not come into play when
2940: creating and executing XTs. I.e., even when you execute @code{'} in
2941: compile state, it still gives you the interpretation semantics. And
2942: whatever that state is, @code{execute} performs the semantics
2943: represented by the XT (i.e., for XTs produced with @code{'} the
2944: interpretation semantics).
2945:
2946: Reference: @ref{Tokens for Words}.
2947:
2948:
2949: @node Exceptions Tutorial, Defining Words Tutorial, Execution Tokens Tutorial, Tutorial
2950: @section Exceptions
2951: @cindex exceptions tutorial
2952:
2953: @code{throw ( n -- )} causes an exception unless n is zero.
2954:
2955: @example
2956: 100 throw .s
2957: 0 throw .s
2958: @end example
2959:
2960: @code{catch ( ... xt -- ... n )} behaves similar to @code{execute}, but
2961: it catches exceptions and pushes the number of the exception on the
2962: stack (or 0, if the xt executed without exception). If there was an
2963: exception, the stacks have the same depth as when entering @code{catch}:
2964:
2965: @example
2966: .s
2967: 3 0 ' / catch .s
2968: 3 2 ' / catch .s
2969: @end example
2970:
2971: @quotation Assignment
2972: Try the same with @code{execute} instead of @code{catch}.
2973: @end quotation
2974:
2975: @code{Throw} always jumps to the dynamically next enclosing
2976: @code{catch}, even if it has to leave several call levels to achieve
2977: this:
2978:
2979: @example
2980: : foo 100 throw ;
2981: : foo1 foo ." after foo" ;
2982: : bar ['] foo1 catch ;
2983: bar .
2984: @end example
2985:
2986: It is often important to restore a value upon leaving a definition, even
2987: if the definition is left through an exception. You can ensure this
2988: like this:
2989:
2990: @example
2991: : ...
2992: save-x
2993: ['] word-changing-x catch ( ... n )
2994: restore-x
2995: ( ... n ) throw ;
2996: @end example
2997:
2998: However, this is still not safe against, e.g., the user pressing
2999: @kbd{Ctrl-C} when execution is between the @code{catch} and
3000: @code{restore-x}.
3001:
3002: Gforth provides an alternative exception handling syntax that is safe
3003: against such cases: @code{try ... restore ... endtry}. If the code
3004: between @code{try} and @code{endtry} has an exception, the stack
3005: depths are restored, the exception number is pushed on the stack, and
3006: the execution continues right after @code{restore}.
3007:
3008: The safer equivalent to the restoration code above is
3009:
3010: @example
3011: : ...
3012: save-x
3013: try
3014: word-changing-x 0
3015: restore
3016: restore-x
3017: endtry
3018: throw ;
3019: @end example
3020:
3021: Reference: @ref{Exception Handling}.
3022:
3023:
3024: @node Defining Words Tutorial, Arrays and Records Tutorial, Exceptions Tutorial, Tutorial
3025: @section Defining Words
3026: @cindex defining words tutorial
3027: @cindex does> tutorial
3028: @cindex create...does> tutorial
3029:
3030: @c before semantics?
3031:
3032: @code{:}, @code{create}, and @code{variable} are definition words: They
3033: define other words. @code{Constant} is another definition word:
3034:
3035: @example
3036: 5 constant foo
3037: foo .
3038: @end example
3039:
3040: You can also use the prefixes @code{2} (double-cell) and @code{f}
3041: (floating point) with @code{variable} and @code{constant}.
3042:
3043: You can also define your own defining words. E.g.:
3044:
3045: @example
3046: : variable ( "name" -- )
3047: create 0 , ;
3048: @end example
3049:
3050: You can also define defining words that create words that do something
3051: other than just producing their address:
3052:
3053: @example
3054: : constant ( n "name" -- )
3055: create ,
3056: does> ( -- n )
3057: ( addr ) @@ ;
3058:
3059: 5 constant foo
3060: foo .
3061: @end example
3062:
3063: The definition of @code{constant} above ends at the @code{does>}; i.e.,
3064: @code{does>} replaces @code{;}, but it also does something else: It
3065: changes the last defined word such that it pushes the address of the
3066: body of the word and then performs the code after the @code{does>}
3067: whenever it is called.
3068:
3069: In the example above, @code{constant} uses @code{,} to store 5 into the
3070: body of @code{foo}. When @code{foo} executes, it pushes the address of
3071: the body onto the stack, then (in the code after the @code{does>})
3072: fetches the 5 from there.
3073:
3074: The stack comment near the @code{does>} reflects the stack effect of the
3075: defined word, not the stack effect of the code after the @code{does>}
3076: (the difference is that the code expects the address of the body that
3077: the stack comment does not show).
3078:
3079: You can use these definition words to do factoring in cases that involve
3080: (other) definition words. E.g., a field offset is always added to an
3081: address. Instead of defining
3082:
3083: @example
3084: 2 cells constant offset-field1
3085: @end example
3086:
3087: and using this like
3088:
3089: @example
3090: ( addr ) offset-field1 +
3091: @end example
3092:
3093: you can define a definition word
3094:
3095: @example
3096: : simple-field ( n "name" -- )
3097: create ,
3098: does> ( n1 -- n1+n )
3099: ( addr ) @@ + ;
3100: @end example
3101:
3102: Definition and use of field offsets now look like this:
3103:
3104: @example
3105: 2 cells simple-field field1
3106: create mystruct 4 cells allot
3107: mystruct .s field1 .s drop
3108: @end example
3109:
3110: If you want to do something with the word without performing the code
3111: after the @code{does>}, you can access the body of a @code{create}d word
3112: with @code{>body ( xt -- addr )}:
3113:
3114: @example
3115: : value ( n "name" -- )
3116: create ,
3117: does> ( -- n1 )
3118: @@ ;
3119: : to ( n "name" -- )
3120: ' >body ! ;
3121:
3122: 5 value foo
3123: foo .
3124: 7 to foo
3125: foo .
3126: @end example
3127:
3128: @quotation Assignment
3129: Define @code{defer ( "name" -- )}, which creates a word that stores an
3130: XT (at the start the XT of @code{abort}), and upon execution
3131: @code{execute}s the XT. Define @code{is ( xt "name" -- )} that stores
3132: @code{xt} into @code{name}, a word defined with @code{defer}. Indirect
3133: recursion is one application of @code{defer}.
3134: @end quotation
3135:
3136: Reference: @ref{User-defined Defining Words}.
3137:
3138:
3139: @node Arrays and Records Tutorial, POSTPONE Tutorial, Defining Words Tutorial, Tutorial
3140: @section Arrays and Records
3141: @cindex arrays tutorial
3142: @cindex records tutorial
3143: @cindex structs tutorial
3144:
3145: Forth has no standard words for defining data structures such as arrays
3146: and records (structs in C terminology), but you can build them yourself
3147: based on address arithmetic. You can also define words for defining
3148: arrays and records (@pxref{Defining Words Tutorial,, Defining Words}).
3149:
3150: One of the first projects a Forth newcomer sets out upon when learning
3151: about defining words is an array defining word (possibly for
3152: n-dimensional arrays). Go ahead and do it, I did it, too; you will
3153: learn something from it. However, don't be disappointed when you later
3154: learn that you have little use for these words (inappropriate use would
3155: be even worse). I have not found a set of useful array words yet;
3156: the needs are just too diverse, and named, global arrays (the result of
3157: naive use of defining words) are often not flexible enough (e.g.,
3158: consider how to pass them as parameters). Another such project is a set
3159: of words to help dealing with strings.
3160:
3161: On the other hand, there is a useful set of record words, and it has
3162: been defined in @file{compat/struct.fs}; these words are predefined in
3163: Gforth. They are explained in depth elsewhere in this manual (see
3164: @pxref{Structures}). The @code{simple-field} example above is
3165: simplified variant of fields in this package.
3166:
3167:
3168: @node POSTPONE Tutorial, Literal Tutorial, Arrays and Records Tutorial, Tutorial
3169: @section @code{POSTPONE}
3170: @cindex postpone tutorial
3171:
3172: You can compile the compilation semantics (instead of compiling the
3173: interpretation semantics) of a word with @code{POSTPONE}:
3174:
3175: @example
3176: : MY-+ ( Compilation: -- ; Run-time of compiled code: n1 n2 -- n )
3177: POSTPONE + ; immediate
3178: : foo ( n1 n2 -- n )
3179: MY-+ ;
3180: 1 2 foo .
3181: see foo
3182: @end example
3183:
3184: During the definition of @code{foo} the text interpreter performs the
3185: compilation semantics of @code{MY-+}, which performs the compilation
3186: semantics of @code{+}, i.e., it compiles @code{+} into @code{foo}.
3187:
3188: This example also displays separate stack comments for the compilation
3189: semantics and for the stack effect of the compiled code. For words with
3190: default compilation semantics these stack effects are usually not
3191: displayed; the stack effect of the compilation semantics is always
3192: @code{( -- )} for these words, the stack effect for the compiled code is
3193: the stack effect of the interpretation semantics.
3194:
3195: Note that the state of the interpreter does not come into play when
3196: performing the compilation semantics in this way. You can also perform
3197: it interpretively, e.g.:
3198:
3199: @example
3200: : foo2 ( n1 n2 -- n )
3201: [ MY-+ ] ;
3202: 1 2 foo .
3203: see foo
3204: @end example
3205:
3206: However, there are some broken Forth systems where this does not always
3207: work, and therefore this practice was been declared non-standard in
3208: 1999.
3209: @c !! repair.fs
3210:
3211: Here is another example for using @code{POSTPONE}:
3212:
3213: @example
3214: : MY-- ( Compilation: -- ; Run-time of compiled code: n1 n2 -- n )
3215: POSTPONE negate POSTPONE + ; immediate compile-only
3216: : bar ( n1 n2 -- n )
3217: MY-- ;
3218: 2 1 bar .
3219: see bar
3220: @end example
3221:
3222: You can define @code{ENDIF} in this way:
3223:
3224: @example
3225: : ENDIF ( Compilation: orig -- )
3226: POSTPONE then ; immediate
3227: @end example
3228:
3229: @quotation Assignment
3230: Write @code{MY-2DUP} that has compilation semantics equivalent to
3231: @code{2dup}, but compiles @code{over over}.
3232: @end quotation
3233:
3234: @c !! @xref{Macros} for reference
3235:
3236:
3237: @node Literal Tutorial, Advanced macros Tutorial, POSTPONE Tutorial, Tutorial
3238: @section @code{Literal}
3239: @cindex literal tutorial
3240:
3241: You cannot @code{POSTPONE} numbers:
3242:
3243: @example
3244: : [FOO] POSTPONE 500 ; immediate
3245: @end example
3246:
3247: Instead, you can use @code{LITERAL (compilation: n --; run-time: -- n )}:
3248:
3249: @example
3250: : [FOO] ( compilation: --; run-time: -- n )
3251: 500 POSTPONE literal ; immediate
3252:
3253: : flip [FOO] ;
3254: flip .
3255: see flip
3256: @end example
3257:
3258: @code{LITERAL} consumes a number at compile-time (when it's compilation
3259: semantics are executed) and pushes it at run-time (when the code it
3260: compiled is executed). A frequent use of @code{LITERAL} is to compile a
3261: number computed at compile time into the current word:
3262:
3263: @example
3264: : bar ( -- n )
3265: [ 2 2 + ] literal ;
3266: see bar
3267: @end example
3268:
3269: @quotation Assignment
3270: Write @code{]L} which allows writing the example above as @code{: bar (
3271: -- n ) [ 2 2 + ]L ;}
3272: @end quotation
3273:
3274: @c !! @xref{Macros} for reference
3275:
3276:
3277: @node Advanced macros Tutorial, Compilation Tokens Tutorial, Literal Tutorial, Tutorial
3278: @section Advanced macros
3279: @cindex macros, advanced tutorial
3280: @cindex run-time code generation, tutorial
3281:
3282: Reconsider @code{map-array} from @ref{Execution Tokens Tutorial,,
3283: Execution Tokens}. It frequently performs @code{execute}, a relatively
3284: expensive operation in some Forth implementations. You can use
3285: @code{compile,} and @code{POSTPONE} to eliminate these @code{execute}s
3286: and produce a word that contains the word to be performed directly:
3287:
3288: @c use ]] ... [[
3289: @example
3290: : compile-map-array ( compilation: xt -- ; run-time: ... addr u -- ... )
3291: \ at run-time, execute xt ( ... x -- ... ) for each element of the
3292: \ array beginning at addr and containing u elements
3293: @{ xt @}
3294: POSTPONE cells POSTPONE over POSTPONE + POSTPONE swap POSTPONE ?do
3295: POSTPONE i POSTPONE @@ xt compile,
3296: 1 cells POSTPONE literal POSTPONE +loop ;
3297:
3298: : sum-array ( addr u -- n )
3299: 0 rot rot [ ' + compile-map-array ] ;
3300: see sum-array
3301: a 5 sum-array .
3302: @end example
3303:
3304: You can use the full power of Forth for generating the code; here's an
3305: example where the code is generated in a loop:
3306:
3307: @example
3308: : compile-vmul-step ( compilation: n --; run-time: n1 addr1 -- n2 addr2 )
3309: \ n2=n1+(addr1)*n, addr2=addr1+cell
3310: POSTPONE tuck POSTPONE @@
3311: POSTPONE literal POSTPONE * POSTPONE +
3312: POSTPONE swap POSTPONE cell+ ;
3313:
3314: : compile-vmul ( compilation: addr1 u -- ; run-time: addr2 -- n )
3315: \ n=v1*v2 (inner product), where the v_i are represented as addr_i u
3316: 0 postpone literal postpone swap
3317: [ ' compile-vmul-step compile-map-array ]
3318: postpone drop ;
3319: see compile-vmul
3320:
3321: : a-vmul ( addr -- n )
3322: \ n=a*v, where v is a vector that's as long as a and starts at addr
3323: [ a 5 compile-vmul ] ;
3324: see a-vmul
3325: a a-vmul .
3326: @end example
3327:
3328: This example uses @code{compile-map-array} to show off, but you could
3329: also use @code{map-array} instead (try it now!).
3330:
3331: You can use this technique for efficient multiplication of large
3332: matrices. In matrix multiplication, you multiply every line of one
3333: matrix with every column of the other matrix. You can generate the code
3334: for one line once, and use it for every column. The only downside of
3335: this technique is that it is cumbersome to recover the memory consumed
3336: by the generated code when you are done (and in more complicated cases
3337: it is not possible portably).
3338:
3339: @c !! @xref{Macros} for reference
3340:
3341:
3342: @node Compilation Tokens Tutorial, Wordlists and Search Order Tutorial, Advanced macros Tutorial, Tutorial
3343: @section Compilation Tokens
3344: @cindex compilation tokens, tutorial
3345: @cindex CT, tutorial
3346:
3347: This section is Gforth-specific. You can skip it.
3348:
3349: @code{' word compile,} compiles the interpretation semantics. For words
3350: with default compilation semantics this is the same as performing the
3351: compilation semantics. To represent the compilation semantics of other
3352: words (e.g., words like @code{if} that have no interpretation
3353: semantics), Gforth has the concept of a compilation token (CT,
3354: consisting of two cells), and words @code{comp'} and @code{[comp']}.
3355: You can perform the compilation semantics represented by a CT with
3356: @code{execute}:
3357:
3358: @example
3359: : foo2 ( n1 n2 -- n )
3360: [ comp' + execute ] ;
3361: see foo
3362: @end example
3363:
3364: You can compile the compilation semantics represented by a CT with
3365: @code{postpone,}:
3366:
3367: @example
3368: : foo3 ( -- )
3369: [ comp' + postpone, ] ;
3370: see foo3
3371: @end example
3372:
3373: @code{[ comp' word postpone, ]} is equivalent to @code{POSTPONE word}.
3374: @code{comp'} is particularly useful for words that have no
3375: interpretation semantics:
3376:
3377: @example
3378: ' if
3379: comp' if .s 2drop
3380: @end example
3381:
3382: Reference: @ref{Tokens for Words}.
3383:
3384:
3385: @node Wordlists and Search Order Tutorial, , Compilation Tokens Tutorial, Tutorial
3386: @section Wordlists and Search Order
3387: @cindex wordlists tutorial
3388: @cindex search order, tutorial
3389:
3390: The dictionary is not just a memory area that allows you to allocate
3391: memory with @code{allot}, it also contains the Forth words, arranged in
3392: several wordlists. When searching for a word in a wordlist,
3393: conceptually you start searching at the youngest and proceed towards
3394: older words (in reality most systems nowadays use hash-tables); i.e., if
3395: you define a word with the same name as an older word, the new word
3396: shadows the older word.
3397:
3398: Which wordlists are searched in which order is determined by the search
3399: order. You can display the search order with @code{order}. It displays
3400: first the search order, starting with the wordlist searched first, then
3401: it displays the wordlist that will contain newly defined words.
3402:
3403: You can create a new, empty wordlist with @code{wordlist ( -- wid )}:
3404:
3405: @example
3406: wordlist constant mywords
3407: @end example
3408:
3409: @code{Set-current ( wid -- )} sets the wordlist that will contain newly
3410: defined words (the @emph{current} wordlist):
3411:
3412: @example
3413: mywords set-current
3414: order
3415: @end example
3416:
3417: Gforth does not display a name for the wordlist in @code{mywords}
3418: because this wordlist was created anonymously with @code{wordlist}.
3419:
3420: You can get the current wordlist with @code{get-current ( -- wid)}. If
3421: you want to put something into a specific wordlist without overall
3422: effect on the current wordlist, this typically looks like this:
3423:
3424: @example
3425: get-current mywords set-current ( wid )
3426: create someword
3427: ( wid ) set-current
3428: @end example
3429:
3430: You can write the search order with @code{set-order ( wid1 .. widn n --
3431: )} and read it with @code{get-order ( -- wid1 .. widn n )}. The first
3432: searched wordlist is topmost.
3433:
3434: @example
3435: get-order mywords swap 1+ set-order
3436: order
3437: @end example
3438:
3439: Yes, the order of wordlists in the output of @code{order} is reversed
3440: from stack comments and the output of @code{.s} and thus unintuitive.
3441:
3442: @quotation Assignment
3443: Define @code{>order ( wid -- )} with adds @code{wid} as first searched
3444: wordlist to the search order. Define @code{previous ( -- )}, which
3445: removes the first searched wordlist from the search order. Experiment
3446: with boundary conditions (you will see some crashes or situations that
3447: are hard or impossible to leave).
3448: @end quotation
3449:
3450: The search order is a powerful foundation for providing features similar
3451: to Modula-2 modules and C++ namespaces. However, trying to modularize
3452: programs in this way has disadvantages for debugging and reuse/factoring
3453: that overcome the advantages in my experience (I don't do huge projects,
3454: though). These disadvantages are not so clear in other
3455: languages/programming environments, because these languages are not so
3456: strong in debugging and reuse.
3457:
3458: @c !! example
3459:
3460: Reference: @ref{Word Lists}.
3461:
3462: @c ******************************************************************
3463: @node Introduction, Words, Tutorial, Top
3464: @comment node-name, next, previous, up
3465: @chapter An Introduction to ANS Forth
3466: @cindex Forth - an introduction
3467:
3468: The difference of this chapter from the Tutorial (@pxref{Tutorial}) is
3469: that it is slower-paced in its examples, but uses them to dive deep into
3470: explaining Forth internals (not covered by the Tutorial). Apart from
3471: that, this chapter covers far less material. It is suitable for reading
3472: without using a computer.
3473:
3474: The primary purpose of this manual is to document Gforth. However, since
3475: Forth is not a widely-known language and there is a lack of up-to-date
3476: teaching material, it seems worthwhile to provide some introductory
3477: material. For other sources of Forth-related
3478: information, see @ref{Forth-related information}.
3479:
3480: The examples in this section should work on any ANS Forth; the
3481: output shown was produced using Gforth. Each example attempts to
3482: reproduce the exact output that Gforth produces. If you try out the
3483: examples (and you should), what you should type is shown @kbd{like this}
3484: and Gforth's response is shown @code{like this}. The single exception is
3485: that, where the example shows @key{RET} it means that you should
3486: press the ``carriage return'' key. Unfortunately, some output formats for
3487: this manual cannot show the difference between @kbd{this} and
3488: @code{this} which will make trying out the examples harder (but not
3489: impossible).
3490:
3491: Forth is an unusual language. It provides an interactive development
3492: environment which includes both an interpreter and compiler. Forth
3493: programming style encourages you to break a problem down into many
3494: @cindex factoring
3495: small fragments (@dfn{factoring}), and then to develop and test each
3496: fragment interactively. Forth advocates assert that breaking the
3497: edit-compile-test cycle used by conventional programming languages can
3498: lead to great productivity improvements.
3499:
3500: @menu
3501: * Introducing the Text Interpreter::
3502: * Stacks and Postfix notation::
3503: * Your first definition::
3504: * How does that work?::
3505: * Forth is written in Forth::
3506: * Review - elements of a Forth system::
3507: * Where to go next::
3508: * Exercises::
3509: @end menu
3510:
3511: @comment ----------------------------------------------
3512: @node Introducing the Text Interpreter, Stacks and Postfix notation, Introduction, Introduction
3513: @section Introducing the Text Interpreter
3514: @cindex text interpreter
3515: @cindex outer interpreter
3516:
3517: @c IMO this is too detailed and the pace is too slow for
3518: @c an introduction. If you know German, take a look at
3519: @c http://www.complang.tuwien.ac.at/anton/lvas/skriptum-stack.html
3520: @c to see how I do it - anton
3521:
3522: @c nac-> Where I have accepted your comments 100% and modified the text
3523: @c accordingly, I have deleted your comments. Elsewhere I have added a
3524: @c response like this to attempt to rationalise what I have done. Of
3525: @c course, this is a very clumsy mechanism for something that would be
3526: @c done far more efficiently over a beer. Please delete any dialogue
3527: @c you consider closed.
3528:
3529: When you invoke the Forth image, you will see a startup banner printed
3530: and nothing else (if you have Gforth installed on your system, try
3531: invoking it now, by typing @kbd{gforth@key{RET}}). Forth is now running
3532: its command line interpreter, which is called the @dfn{Text Interpreter}
3533: (also known as the @dfn{Outer Interpreter}). (You will learn a lot
3534: about the text interpreter as you read through this chapter, for more
3535: detail @pxref{The Text Interpreter}).
3536:
3537: Although it's not obvious, Forth is actually waiting for your
3538: input. Type a number and press the @key{RET} key:
3539:
3540: @example
3541: @kbd{45@key{RET}} ok
3542: @end example
3543:
3544: Rather than give you a prompt to invite you to input something, the text
3545: interpreter prints a status message @i{after} it has processed a line
3546: of input. The status message in this case (``@code{ ok}'' followed by
3547: carriage-return) indicates that the text interpreter was able to process
3548: all of your input successfully. Now type something illegal:
3549:
3550: @example
3551: @kbd{qwer341@key{RET}}
3552: *the terminal*:2: Undefined word
3553: >>>qwer341<<<
3554: Backtrace:
3555: $2A95B42A20 throw
3556: $2A95B57FB8 no.extensions
3557: @end example
3558:
3559: The exact text, other than the ``Undefined word'' may differ slightly
3560: on your system, but the effect is the same; when the text interpreter
3561: detects an error, it discards any remaining text on a line, resets
3562: certain internal state and prints an error message. For a detailed
3563: description of error messages see @ref{Error messages}.
3564:
3565: The text interpreter waits for you to press carriage-return, and then
3566: processes your input line. Starting at the beginning of the line, it
3567: breaks the line into groups of characters separated by spaces. For each
3568: group of characters in turn, it makes two attempts to do something:
3569:
3570: @itemize @bullet
3571: @item
3572: @cindex name dictionary
3573: It tries to treat it as a command. It does this by searching a @dfn{name
3574: dictionary}. If the group of characters matches an entry in the name
3575: dictionary, the name dictionary provides the text interpreter with
3576: information that allows the text interpreter perform some actions. In
3577: Forth jargon, we say that the group
3578: @cindex word
3579: @cindex definition
3580: @cindex execution token
3581: @cindex xt
3582: of characters names a @dfn{word}, that the dictionary search returns an
3583: @dfn{execution token (xt)} corresponding to the @dfn{definition} of the
3584: word, and that the text interpreter executes the xt. Often, the terms
3585: @dfn{word} and @dfn{definition} are used interchangeably.
3586: @item
3587: If the text interpreter fails to find a match in the name dictionary, it
3588: tries to treat the group of characters as a number in the current number
3589: base (when you start up Forth, the current number base is base 10). If
3590: the group of characters legitimately represents a number, the text
3591: interpreter pushes the number onto a stack (we'll learn more about that
3592: in the next section).
3593: @end itemize
3594:
3595: If the text interpreter is unable to do either of these things with any
3596: group of characters, it discards the group of characters and the rest of
3597: the line, then prints an error message. If the text interpreter reaches
3598: the end of the line without error, it prints the status message ``@code{ ok}''
3599: followed by carriage-return.
3600:
3601: This is the simplest command we can give to the text interpreter:
3602:
3603: @example
3604: @key{RET} ok
3605: @end example
3606:
3607: The text interpreter did everything we asked it to do (nothing) without
3608: an error, so it said that everything is ``@code{ ok}''. Try a slightly longer
3609: command:
3610:
3611: @example
3612: @kbd{12 dup fred dup@key{RET}}
3613: *the terminal*:3: Undefined word
3614: 12 dup >>>fred<<< dup
3615: Backtrace:
3616: $2A95B42A20 throw
3617: $2A95B57FB8 no.extensions
3618: @end example
3619:
3620: When you press the carriage-return key, the text interpreter starts to
3621: work its way along the line:
3622:
3623: @itemize @bullet
3624: @item
3625: When it gets to the space after the @code{2}, it takes the group of
3626: characters @code{12} and looks them up in the name
3627: dictionary@footnote{We can't tell if it found them or not, but assume
3628: for now that it did not}. There is no match for this group of characters
3629: in the name dictionary, so it tries to treat them as a number. It is
3630: able to do this successfully, so it puts the number, 12, ``on the stack''
3631: (whatever that means).
3632: @item
3633: The text interpreter resumes scanning the line and gets the next group
3634: of characters, @code{dup}. It looks it up in the name dictionary and
3635: (you'll have to take my word for this) finds it, and executes the word
3636: @code{dup} (whatever that means).
3637: @item
3638: Once again, the text interpreter resumes scanning the line and gets the
3639: group of characters @code{fred}. It looks them up in the name
3640: dictionary, but can't find them. It tries to treat them as a number, but
3641: they don't represent any legal number.
3642: @end itemize
3643:
3644: At this point, the text interpreter gives up and prints an error
3645: message. The error message shows exactly how far the text interpreter
3646: got in processing the line. In particular, it shows that the text
3647: interpreter made no attempt to do anything with the final character
3648: group, @code{dup}, even though we have good reason to believe that the
3649: text interpreter would have no problem looking that word up and
3650: executing it a second time.
3651:
3652:
3653: @comment ----------------------------------------------
3654: @node Stacks and Postfix notation, Your first definition, Introducing the Text Interpreter, Introduction
3655: @section Stacks, postfix notation and parameter passing
3656: @cindex text interpreter
3657: @cindex outer interpreter
3658:
3659: In procedural programming languages (like C and Pascal), the
3660: building-block of programs is the @dfn{function} or @dfn{procedure}. These
3661: functions or procedures are called with @dfn{explicit parameters}. For
3662: example, in C we might write:
3663:
3664: @example
3665: total = total + new_volume(length,height,depth);
3666: @end example
3667:
3668: @noindent
3669: where new_volume is a function-call to another piece of code, and total,
3670: length, height and depth are all variables. length, height and depth are
3671: parameters to the function-call.
3672:
3673: In Forth, the equivalent of the function or procedure is the
3674: @dfn{definition} and parameters are implicitly passed between
3675: definitions using a shared stack that is visible to the
3676: programmer. Although Forth does support variables, the existence of the
3677: stack means that they are used far less often than in most other
3678: programming languages. When the text interpreter encounters a number, it
3679: will place (@dfn{push}) it on the stack. There are several stacks (the
3680: actual number is implementation-dependent ...) and the particular stack
3681: used for any operation is implied unambiguously by the operation being
3682: performed. The stack used for all integer operations is called the @dfn{data
3683: stack} and, since this is the stack used most commonly, references to
3684: ``the data stack'' are often abbreviated to ``the stack''.
3685:
3686: The stacks have a last-in, first-out (LIFO) organisation. If you type:
3687:
3688: @example
3689: @kbd{1 2 3@key{RET}} ok
3690: @end example
3691:
3692: Then this instructs the text interpreter to placed three numbers on the
3693: (data) stack. An analogy for the behaviour of the stack is to take a
3694: pack of playing cards and deal out the ace (1), 2 and 3 into a pile on
3695: the table. The 3 was the last card onto the pile (``last-in'') and if
3696: you take a card off the pile then, unless you're prepared to fiddle a
3697: bit, the card that you take off will be the 3 (``first-out''). The
3698: number that will be first-out of the stack is called the @dfn{top of
3699: stack}, which
3700: @cindex TOS definition
3701: is often abbreviated to @dfn{TOS}.
3702:
3703: To understand how parameters are passed in Forth, consider the
3704: behaviour of the definition @code{+} (pronounced ``plus''). You will not
3705: be surprised to learn that this definition performs addition. More
3706: precisely, it adds two number together and produces a result. Where does
3707: it get the two numbers from? It takes the top two numbers off the
3708: stack. Where does it place the result? On the stack. You can act-out the
3709: behaviour of @code{+} with your playing cards like this:
3710:
3711: @itemize @bullet
3712: @item
3713: Pick up two cards from the stack on the table
3714: @item
3715: Stare at them intently and ask yourself ``what @i{is} the sum of these two
3716: numbers''
3717: @item
3718: Decide that the answer is 5
3719: @item
3720: Shuffle the two cards back into the pack and find a 5
3721: @item
3722: Put a 5 on the remaining ace that's on the table.
3723: @end itemize
3724:
3725: If you don't have a pack of cards handy but you do have Forth running,
3726: you can use the definition @code{.s} to show the current state of the stack,
3727: without affecting the stack. Type:
3728:
3729: @example
3730: @kbd{clearstacks 1 2 3@key{RET}} ok
3731: @kbd{.s@key{RET}} <3> 1 2 3 ok
3732: @end example
3733:
3734: The text interpreter looks up the word @code{clearstacks} and executes
3735: it; it tidies up the stacks and removes any entries that may have been
3736: left on it by earlier examples. The text interpreter pushes each of the
3737: three numbers in turn onto the stack. Finally, the text interpreter
3738: looks up the word @code{.s} and executes it. The effect of executing
3739: @code{.s} is to print the ``<3>'' (the total number of items on the stack)
3740: followed by a list of all the items on the stack; the item on the far
3741: right-hand side is the TOS.
3742:
3743: You can now type:
3744:
3745: @example
3746: @kbd{+ .s@key{RET}} <2> 1 5 ok
3747: @end example
3748:
3749: @noindent
3750: which is correct; there are now 2 items on the stack and the result of
3751: the addition is 5.
3752:
3753: If you're playing with cards, try doing a second addition: pick up the
3754: two cards, work out that their sum is 6, shuffle them into the pack,
3755: look for a 6 and place that on the table. You now have just one item on
3756: the stack. What happens if you try to do a third addition? Pick up the
3757: first card, pick up the second card -- ah! There is no second card. This
3758: is called a @dfn{stack underflow} and consitutes an error. If you try to
3759: do the same thing with Forth it often reports an error (probably a Stack
3760: Underflow or an Invalid Memory Address error).
3761:
3762: The opposite situation to a stack underflow is a @dfn{stack overflow},
3763: which simply accepts that there is a finite amount of storage space
3764: reserved for the stack. To stretch the playing card analogy, if you had
3765: enough packs of cards and you piled the cards up on the table, you would
3766: eventually be unable to add another card; you'd hit the ceiling. Gforth
3767: allows you to set the maximum size of the stacks. In general, the only
3768: time that you will get a stack overflow is because a definition has a
3769: bug in it and is generating data on the stack uncontrollably.
3770:
3771: There's one final use for the playing card analogy. If you model your
3772: stack using a pack of playing cards, the maximum number of items on
3773: your stack will be 52 (I assume you didn't use the Joker). The maximum
3774: @i{value} of any item on the stack is 13 (the King). In fact, the only
3775: possible numbers are positive integer numbers 1 through 13; you can't
3776: have (for example) 0 or 27 or 3.52 or -2. If you change the way you
3777: think about some of the cards, you can accommodate different
3778: numbers. For example, you could think of the Jack as representing 0,
3779: the Queen as representing -1 and the King as representing -2. Your
3780: @i{range} remains unchanged (you can still only represent a total of 13
3781: numbers) but the numbers that you can represent are -2 through 10.
3782:
3783: In that analogy, the limit was the amount of information that a single
3784: stack entry could hold, and Forth has a similar limit. In Forth, the
3785: size of a stack entry is called a @dfn{cell}. The actual size of a cell is
3786: implementation dependent and affects the maximum value that a stack
3787: entry can hold. A Standard Forth provides a cell size of at least
3788: 16-bits, and most desktop systems use a cell size of 32-bits.
3789:
3790: Forth does not do any type checking for you, so you are free to
3791: manipulate and combine stack items in any way you wish. A convenient way
3792: of treating stack items is as 2's complement signed integers, and that
3793: is what Standard words like @code{+} do. Therefore you can type:
3794:
3795: @example
3796: @kbd{-5 12 + .s@key{RET}} <1> 7 ok
3797: @end example
3798:
3799: If you use numbers and definitions like @code{+} in order to turn Forth
3800: into a great big pocket calculator, you will realise that it's rather
3801: different from a normal calculator. Rather than typing 2 + 3 = you had
3802: to type 2 3 + (ignore the fact that you had to use @code{.s} to see the
3803: result). The terminology used to describe this difference is to say that
3804: your calculator uses @dfn{Infix Notation} (parameters and operators are
3805: mixed) whilst Forth uses @dfn{Postfix Notation} (parameters and
3806: operators are separate), also called @dfn{Reverse Polish Notation}.
3807:
3808: Whilst postfix notation might look confusing to begin with, it has
3809: several important advantages:
3810:
3811: @itemize @bullet
3812: @item
3813: it is unambiguous
3814: @item
3815: it is more concise
3816: @item
3817: it fits naturally with a stack-based system
3818: @end itemize
3819:
3820: To examine these claims in more detail, consider these sums:
3821:
3822: @example
3823: 6 + 5 * 4 =
3824: 4 * 5 + 6 =
3825: @end example
3826:
3827: If you're just learning maths or your maths is very rusty, you will
3828: probably come up with the answer 44 for the first and 26 for the
3829: second. If you are a bit of a whizz at maths you will remember the
3830: @i{convention} that multiplication takes precendence over addition, and
3831: you'd come up with the answer 26 both times. To explain the answer 26
3832: to someone who got the answer 44, you'd probably rewrite the first sum
3833: like this:
3834:
3835: @example
3836: 6 + (5 * 4) =
3837: @end example
3838:
3839: If what you really wanted was to perform the addition before the
3840: multiplication, you would have to use parentheses to force it.
3841:
3842: If you did the first two sums on a pocket calculator you would probably
3843: get the right answers, unless you were very cautious and entered them using
3844: these keystroke sequences:
3845:
3846: 6 + 5 = * 4 =
3847: 4 * 5 = + 6 =
3848:
3849: Postfix notation is unambiguous because the order that the operators
3850: are applied is always explicit; that also means that parentheses are
3851: never required. The operators are @i{active} (the act of quoting the
3852: operator makes the operation occur) which removes the need for ``=''.
3853:
3854: The sum 6 + 5 * 4 can be written (in postfix notation) in two
3855: equivalent ways:
3856:
3857: @example
3858: 6 5 4 * + or:
3859: 5 4 * 6 +
3860: @end example
3861:
3862: An important thing that you should notice about this notation is that
3863: the @i{order} of the numbers does not change; if you want to subtract
3864: 2 from 10 you type @code{10 2 -}.
3865:
3866: The reason that Forth uses postfix notation is very simple to explain: it
3867: makes the implementation extremely simple, and it follows naturally from
3868: using the stack as a mechanism for passing parameters. Another way of
3869: thinking about this is to realise that all Forth definitions are
3870: @i{active}; they execute as they are encountered by the text
3871: interpreter. The result of this is that the syntax of Forth is trivially
3872: simple.
3873:
3874:
3875:
3876: @comment ----------------------------------------------
3877: @node Your first definition, How does that work?, Stacks and Postfix notation, Introduction
3878: @section Your first Forth definition
3879: @cindex first definition
3880:
3881: Until now, the examples we've seen have been trivial; we've just been
3882: using Forth as a bigger-than-pocket calculator. Also, each calculation
3883: we've shown has been a ``one-off'' -- to repeat it we'd need to type it in
3884: again@footnote{That's not quite true. If you press the up-arrow key on
3885: your keyboard you should be able to scroll back to any earlier command,
3886: edit it and re-enter it.} In this section we'll see how to add new
3887: words to Forth's vocabulary.
3888:
3889: The easiest way to create a new word is to use a @dfn{colon
3890: definition}. We'll define a few and try them out before worrying too
3891: much about how they work. Try typing in these examples; be careful to
3892: copy the spaces accurately:
3893:
3894: @example
3895: : add-two 2 + . ;
3896: : greet ." Hello and welcome" ;
3897: : demo 5 add-two ;
3898: @end example
3899:
3900: @noindent
3901: Now try them out:
3902:
3903: @example
3904: @kbd{greet@key{RET}} Hello and welcome ok
3905: @kbd{greet greet@key{RET}} Hello and welcomeHello and welcome ok
3906: @kbd{4 add-two@key{RET}} 6 ok
3907: @kbd{demo@key{RET}} 7 ok
3908: @kbd{9 greet demo add-two@key{RET}} Hello and welcome7 11 ok
3909: @end example
3910:
3911: The first new thing that we've introduced here is the pair of words
3912: @code{:} and @code{;}. These are used to start and terminate a new
3913: definition, respectively. The first word after the @code{:} is the name
3914: for the new definition.
3915:
3916: As you can see from the examples, a definition is built up of words that
3917: have already been defined; Forth makes no distinction between
3918: definitions that existed when you started the system up, and those that
3919: you define yourself.
3920:
3921: The examples also introduce the words @code{.} (dot), @code{."}
3922: (dot-quote) and @code{dup} (dewp). Dot takes the value from the top of
3923: the stack and displays it. It's like @code{.s} except that it only
3924: displays the top item of the stack and it is destructive; after it has
3925: executed, the number is no longer on the stack. There is always one
3926: space printed after the number, and no spaces before it. Dot-quote
3927: defines a string (a sequence of characters) that will be printed when
3928: the word is executed. The string can contain any printable characters
3929: except @code{"}. A @code{"} has a special function; it is not a Forth
3930: word but it acts as a delimiter (the way that delimiters work is
3931: described in the next section). Finally, @code{dup} duplicates the value
3932: at the top of the stack. Try typing @code{5 dup .s} to see what it does.
3933:
3934: We already know that the text interpreter searches through the
3935: dictionary to locate names. If you've followed the examples earlier, you
3936: will already have a definition called @code{add-two}. Lets try modifying
3937: it by typing in a new definition:
3938:
3939: @example
3940: @kbd{: add-two dup . ." + 2 =" 2 + . ;@key{RET}} redefined add-two ok
3941: @end example
3942:
3943: Forth recognised that we were defining a word that already exists, and
3944: printed a message to warn us of that fact. Let's try out the new
3945: definition:
3946:
3947: @example
3948: @kbd{9 add-two@key{RET}} 9 + 2 =11 ok
3949: @end example
3950:
3951: @noindent
3952: All that we've actually done here, though, is to create a new
3953: definition, with a particular name. The fact that there was already a
3954: definition with the same name did not make any difference to the way
3955: that the new definition was created (except that Forth printed a warning
3956: message). The old definition of add-two still exists (try @code{demo}
3957: again to see that this is true). Any new definition will use the new
3958: definition of @code{add-two}, but old definitions continue to use the
3959: version that already existed at the time that they were @code{compiled}.
3960:
3961: Before you go on to the next section, try defining and redefining some
3962: words of your own.
3963:
3964: @comment ----------------------------------------------
3965: @node How does that work?, Forth is written in Forth, Your first definition, Introduction
3966: @section How does that work?
3967: @cindex parsing words
3968:
3969: @c That's pretty deep (IMO way too deep) for an introduction. - anton
3970:
3971: @c Is it a good idea to talk about the interpretation semantics of a
3972: @c number? We don't have an xt to go along with it. - anton
3973:
3974: @c Now that I have eliminated execution semantics, I wonder if it would not
3975: @c be better to keep them (or add run-time semantics), to make it easier to
3976: @c explain what compilation semantics usually does. - anton
3977:
3978: @c nac-> I removed the term ``default compilation sematics'' from the
3979: @c introductory chapter. Removing ``execution semantics'' was making
3980: @c everything simpler to explain, then I think the use of this term made
3981: @c everything more complex again. I replaced it with ``default
3982: @c semantics'' (which is used elsewhere in the manual) by which I mean
3983: @c ``a definition that has neither the immediate nor the compile-only
3984: @c flag set''.
3985:
3986: @c anton: I have eliminated default semantics (except in one place where it
3987: @c means "default interpretation and compilation semantics"), because it
3988: @c makes no sense in the presence of combined words. I reverted to
3989: @c "execution semantics" where necessary.
3990:
3991: @c nac-> I reworded big chunks of the ``how does that work''
3992: @c section (and, unusually for me, I think I even made it shorter!). See
3993: @c what you think -- I know I have not addressed your primary concern
3994: @c that it is too heavy-going for an introduction. From what I understood
3995: @c of your course notes it looks as though they might be a good framework.
3996: @c Things that I've tried to capture here are some things that came as a
3997: @c great revelation here when I first understood them. Also, I like the
3998: @c fact that a very simple code example shows up almost all of the issues
3999: @c that you need to understand to see how Forth works. That's unique and
4000: @c worthwhile to emphasise.
4001:
4002: @c anton: I think it's a good idea to present the details, especially those
4003: @c that you found to be a revelation, and probably the tutorial tries to be
4004: @c too superficial and does not get some of the things across that make
4005: @c Forth special. I do believe that most of the time these things should
4006: @c be discussed at the end of a section or in separate sections instead of
4007: @c in the middle of a section (e.g., the stuff you added in "User-defined
4008: @c defining words" leads in a completely different direction from the rest
4009: @c of the section).
4010:
4011: Now we're going to take another look at the definition of @code{add-two}
4012: from the previous section. From our knowledge of the way that the text
4013: interpreter works, we would have expected this result when we tried to
4014: define @code{add-two}:
4015:
4016: @example
4017: @kbd{: add-two 2 + . ;@key{RET}}
4018: *the terminal*:4: Undefined word
4019: : >>>add-two<<< 2 + . ;
4020: @end example
4021:
4022: The reason that this didn't happen is bound up in the way that @code{:}
4023: works. The word @code{:} does two special things. The first special
4024: thing that it does prevents the text interpreter from ever seeing the
4025: characters @code{add-two}. The text interpreter uses a variable called
4026: @cindex modifying >IN
4027: @code{>IN} (pronounced ``to-in'') to keep track of where it is in the
4028: input line. When it encounters the word @code{:} it behaves in exactly
4029: the same way as it does for any other word; it looks it up in the name
4030: dictionary, finds its xt and executes it. When @code{:} executes, it
4031: looks at the input buffer, finds the word @code{add-two} and advances the
4032: value of @code{>IN} to point past it. It then does some other stuff
4033: associated with creating the new definition (including creating an entry
4034: for @code{add-two} in the name dictionary). When the execution of @code{:}
4035: completes, control returns to the text interpreter, which is oblivious
4036: to the fact that it has been tricked into ignoring part of the input
4037: line.
4038:
4039: @cindex parsing words
4040: Words like @code{:} -- words that advance the value of @code{>IN} and so
4041: prevent the text interpreter from acting on the whole of the input line
4042: -- are called @dfn{parsing words}.
4043:
4044: @cindex @code{state} - effect on the text interpreter
4045: @cindex text interpreter - effect of state
4046: The second special thing that @code{:} does is change the value of a
4047: variable called @code{state}, which affects the way that the text
4048: interpreter behaves. When Gforth starts up, @code{state} has the value
4049: 0, and the text interpreter is said to be @dfn{interpreting}. During a
4050: colon definition (started with @code{:}), @code{state} is set to -1 and
4051: the text interpreter is said to be @dfn{compiling}.
4052:
4053: In this example, the text interpreter is compiling when it processes the
4054: string ``@code{2 + . ;}''. It still breaks the string down into
4055: character sequences in the same way. However, instead of pushing the
4056: number @code{2} onto the stack, it lays down (@dfn{compiles}) some magic
4057: into the definition of @code{add-two} that will make the number @code{2} get
4058: pushed onto the stack when @code{add-two} is @dfn{executed}. Similarly,
4059: the behaviours of @code{+} and @code{.} are also compiled into the
4060: definition.
4061:
4062: One category of words don't get compiled. These so-called @dfn{immediate
4063: words} get executed (performed @i{now}) regardless of whether the text
4064: interpreter is interpreting or compiling. The word @code{;} is an
4065: immediate word. Rather than being compiled into the definition, it
4066: executes. Its effect is to terminate the current definition, which
4067: includes changing the value of @code{state} back to 0.
4068:
4069: When you execute @code{add-two}, it has a @dfn{run-time effect} that is
4070: exactly the same as if you had typed @code{2 + . @key{RET}} outside of a
4071: definition.
4072:
4073: In Forth, every word or number can be described in terms of two
4074: properties:
4075:
4076: @itemize @bullet
4077: @item
4078: @cindex interpretation semantics
4079: Its @dfn{interpretation semantics} describe how it will behave when the
4080: text interpreter encounters it in @dfn{interpret} state. The
4081: interpretation semantics of a word are represented by an @dfn{execution
4082: token}.
4083: @item
4084: @cindex compilation semantics
4085: Its @dfn{compilation semantics} describe how it will behave when the
4086: text interpreter encounters it in @dfn{compile} state. The compilation
4087: semantics of a word are represented in an implementation-dependent way;
4088: Gforth uses a @dfn{compilation token}.
4089: @end itemize
4090:
4091: @noindent
4092: Numbers are always treated in a fixed way:
4093:
4094: @itemize @bullet
4095: @item
4096: When the number is @dfn{interpreted}, its behaviour is to push the
4097: number onto the stack.
4098: @item
4099: When the number is @dfn{compiled}, a piece of code is appended to the
4100: current definition that pushes the number when it runs. (In other words,
4101: the compilation semantics of a number are to postpone its interpretation
4102: semantics until the run-time of the definition that it is being compiled
4103: into.)
4104: @end itemize
4105:
4106: Words don't behave in such a regular way, but most have @i{default
4107: semantics} which means that they behave like this:
4108:
4109: @itemize @bullet
4110: @item
4111: The @dfn{interpretation semantics} of the word are to do something useful.
4112: @item
4113: The @dfn{compilation semantics} of the word are to append its
4114: @dfn{interpretation semantics} to the current definition (so that its
4115: run-time behaviour is to do something useful).
4116: @end itemize
4117:
4118: @cindex immediate words
4119: The actual behaviour of any particular word can be controlled by using
4120: the words @code{immediate} and @code{compile-only} when the word is
4121: defined. These words set flags in the name dictionary entry of the most
4122: recently defined word, and these flags are retrieved by the text
4123: interpreter when it finds the word in the name dictionary.
4124:
4125: A word that is marked as @dfn{immediate} has compilation semantics that
4126: are identical to its interpretation semantics. In other words, it
4127: behaves like this:
4128:
4129: @itemize @bullet
4130: @item
4131: The @dfn{interpretation semantics} of the word are to do something useful.
4132: @item
4133: The @dfn{compilation semantics} of the word are to do something useful
4134: (and actually the same thing); i.e., it is executed during compilation.
4135: @end itemize
4136:
4137: Marking a word as @dfn{compile-only} prohibits the text interpreter from
4138: performing the interpretation semantics of the word directly; an attempt
4139: to do so will generate an error. It is never necessary to use
4140: @code{compile-only} (and it is not even part of ANS Forth, though it is
4141: provided by many implementations) but it is good etiquette to apply it
4142: to a word that will not behave correctly (and might have unexpected
4143: side-effects) in interpret state. For example, it is only legal to use
4144: the conditional word @code{IF} within a definition. If you forget this
4145: and try to use it elsewhere, the fact that (in Gforth) it is marked as
4146: @code{compile-only} allows the text interpreter to generate a helpful
4147: error message rather than subjecting you to the consequences of your
4148: folly.
4149:
4150: This example shows the difference between an immediate and a
4151: non-immediate word:
4152:
4153: @example
4154: : show-state state @@ . ;
4155: : show-state-now show-state ; immediate
4156: : word1 show-state ;
4157: : word2 show-state-now ;
4158: @end example
4159:
4160: The word @code{immediate} after the definition of @code{show-state-now}
4161: makes that word an immediate word. These definitions introduce a new
4162: word: @code{@@} (pronounced ``fetch''). This word fetches the value of a
4163: variable, and leaves it on the stack. Therefore, the behaviour of
4164: @code{show-state} is to print a number that represents the current value
4165: of @code{state}.
4166:
4167: When you execute @code{word1}, it prints the number 0, indicating that
4168: the system is interpreting. When the text interpreter compiled the
4169: definition of @code{word1}, it encountered @code{show-state} whose
4170: compilation semantics are to append its interpretation semantics to the
4171: current definition. When you execute @code{word1}, it performs the
4172: interpretation semantics of @code{show-state}. At the time that @code{word1}
4173: (and therefore @code{show-state}) are executed, the system is
4174: interpreting.
4175:
4176: When you pressed @key{RET} after entering the definition of @code{word2},
4177: you should have seen the number -1 printed, followed by ``@code{
4178: ok}''. When the text interpreter compiled the definition of
4179: @code{word2}, it encountered @code{show-state-now}, an immediate word,
4180: whose compilation semantics are therefore to perform its interpretation
4181: semantics. It is executed straight away (even before the text
4182: interpreter has moved on to process another group of characters; the
4183: @code{;} in this example). The effect of executing it are to display the
4184: value of @code{state} @i{at the time that the definition of}
4185: @code{word2} @i{is being defined}. Printing -1 demonstrates that the
4186: system is compiling at this time. If you execute @code{word2} it does
4187: nothing at all.
4188:
4189: @cindex @code{."}, how it works
4190: Before leaving the subject of immediate words, consider the behaviour of
4191: @code{."} in the definition of @code{greet}, in the previous
4192: section. This word is both a parsing word and an immediate word. Notice
4193: that there is a space between @code{."} and the start of the text
4194: @code{Hello and welcome}, but that there is no space between the last
4195: letter of @code{welcome} and the @code{"} character. The reason for this
4196: is that @code{."} is a Forth word; it must have a space after it so that
4197: the text interpreter can identify it. The @code{"} is not a Forth word;
4198: it is a @dfn{delimiter}. The examples earlier show that, when the string
4199: is displayed, there is neither a space before the @code{H} nor after the
4200: @code{e}. Since @code{."} is an immediate word, it executes at the time
4201: that @code{greet} is defined. When it executes, its behaviour is to
4202: search forward in the input line looking for the delimiter. When it
4203: finds the delimiter, it updates @code{>IN} to point past the
4204: delimiter. It also compiles some magic code into the definition of
4205: @code{greet}; the xt of a run-time routine that prints a text string. It
4206: compiles the string @code{Hello and welcome} into memory so that it is
4207: available to be printed later. When the text interpreter gains control,
4208: the next word it finds in the input stream is @code{;} and so it
4209: terminates the definition of @code{greet}.
4210:
4211:
4212: @comment ----------------------------------------------
4213: @node Forth is written in Forth, Review - elements of a Forth system, How does that work?, Introduction
4214: @section Forth is written in Forth
4215: @cindex structure of Forth programs
4216:
4217: When you start up a Forth compiler, a large number of definitions
4218: already exist. In Forth, you develop a new application using bottom-up
4219: programming techniques to create new definitions that are defined in
4220: terms of existing definitions. As you create each definition you can
4221: test and debug it interactively.
4222:
4223: If you have tried out the examples in this section, you will probably
4224: have typed them in by hand; when you leave Gforth, your definitions will
4225: be lost. You can avoid this by using a text editor to enter Forth source
4226: code into a file, and then loading code from the file using
4227: @code{include} (@pxref{Forth source files}). A Forth source file is
4228: processed by the text interpreter, just as though you had typed it in by
4229: hand@footnote{Actually, there are some subtle differences -- see
4230: @ref{The Text Interpreter}.}.
4231:
4232: Gforth also supports the traditional Forth alternative to using text
4233: files for program entry (@pxref{Blocks}).
4234:
4235: In common with many, if not most, Forth compilers, most of Gforth is
4236: actually written in Forth. All of the @file{.fs} files in the
4237: installation directory@footnote{For example,
4238: @file{/usr/local/share/gforth...}} are Forth source files, which you can
4239: study to see examples of Forth programming.
4240:
4241: Gforth maintains a history file that records every line that you type to
4242: the text interpreter. This file is preserved between sessions, and is
4243: used to provide a command-line recall facility. If you enter long
4244: definitions by hand, you can use a text editor to paste them out of the
4245: history file into a Forth source file for reuse at a later time
4246: (for more information @pxref{Command-line editing}).
4247:
4248:
4249: @comment ----------------------------------------------
4250: @node Review - elements of a Forth system, Where to go next, Forth is written in Forth, Introduction
4251: @section Review - elements of a Forth system
4252: @cindex elements of a Forth system
4253:
4254: To summarise this chapter:
4255:
4256: @itemize @bullet
4257: @item
4258: Forth programs use @dfn{factoring} to break a problem down into small
4259: fragments called @dfn{words} or @dfn{definitions}.
4260: @item
4261: Forth program development is an interactive process.
4262: @item
4263: The main command loop that accepts input, and controls both
4264: interpretation and compilation, is called the @dfn{text interpreter}
4265: (also known as the @dfn{outer interpreter}).
4266: @item
4267: Forth has a very simple syntax, consisting of words and numbers
4268: separated by spaces or carriage-return characters. Any additional syntax
4269: is imposed by @dfn{parsing words}.
4270: @item
4271: Forth uses a stack to pass parameters between words. As a result, it
4272: uses postfix notation.
4273: @item
4274: To use a word that has previously been defined, the text interpreter
4275: searches for the word in the @dfn{name dictionary}.
4276: @item
4277: Words have @dfn{interpretation semantics} and @dfn{compilation semantics}.
4278: @item
4279: The text interpreter uses the value of @code{state} to select between
4280: the use of the @dfn{interpretation semantics} and the @dfn{compilation
4281: semantics} of a word that it encounters.
4282: @item
4283: The relationship between the @dfn{interpretation semantics} and
4284: @dfn{compilation semantics} for a word
4285: depend upon the way in which the word was defined (for example, whether
4286: it is an @dfn{immediate} word).
4287: @item
4288: Forth definitions can be implemented in Forth (called @dfn{high-level
4289: definitions}) or in some other way (usually a lower-level language and
4290: as a result often called @dfn{low-level definitions}, @dfn{code
4291: definitions} or @dfn{primitives}).
4292: @item
4293: Many Forth systems are implemented mainly in Forth.
4294: @end itemize
4295:
4296:
4297: @comment ----------------------------------------------
4298: @node Where to go next, Exercises, Review - elements of a Forth system, Introduction
4299: @section Where To Go Next
4300: @cindex where to go next
4301:
4302: Amazing as it may seem, if you have read (and understood) this far, you
4303: know almost all the fundamentals about the inner workings of a Forth
4304: system. You certainly know enough to be able to read and understand the
4305: rest of this manual and the ANS Forth document, to learn more about the
4306: facilities that Forth in general and Gforth in particular provide. Even
4307: scarier, you know almost enough to implement your own Forth system.
4308: However, that's not a good idea just yet... better to try writing some
4309: programs in Gforth.
4310:
4311: Forth has such a rich vocabulary that it can be hard to know where to
4312: start in learning it. This section suggests a few sets of words that are
4313: enough to write small but useful programs. Use the word index in this
4314: document to learn more about each word, then try it out and try to write
4315: small definitions using it. Start by experimenting with these words:
4316:
4317: @itemize @bullet
4318: @item
4319: Arithmetic: @code{+ - * / /MOD */ ABS INVERT}
4320: @item
4321: Comparison: @code{MIN MAX =}
4322: @item
4323: Logic: @code{AND OR XOR NOT}
4324: @item
4325: Stack manipulation: @code{DUP DROP SWAP OVER}
4326: @item
4327: Loops and decisions: @code{IF ELSE ENDIF ?DO I LOOP}
4328: @item
4329: Input/Output: @code{. ." EMIT CR KEY}
4330: @item
4331: Defining words: @code{: ; CREATE}
4332: @item
4333: Memory allocation words: @code{ALLOT ,}
4334: @item
4335: Tools: @code{SEE WORDS .S MARKER}
4336: @end itemize
4337:
4338: When you have mastered those, go on to:
4339:
4340: @itemize @bullet
4341: @item
4342: More defining words: @code{VARIABLE CONSTANT VALUE TO CREATE DOES>}
4343: @item
4344: Memory access: @code{@@ !}
4345: @end itemize
4346:
4347: When you have mastered these, there's nothing for it but to read through
4348: the whole of this manual and find out what you've missed.
4349:
4350: @comment ----------------------------------------------
4351: @node Exercises, , Where to go next, Introduction
4352: @section Exercises
4353: @cindex exercises
4354:
4355: TODO: provide a set of programming excercises linked into the stuff done
4356: already and into other sections of the manual. Provide solutions to all
4357: the exercises in a .fs file in the distribution.
4358:
4359: @c Get some inspiration from Starting Forth and Kelly&Spies.
4360:
4361: @c excercises:
4362: @c 1. take inches and convert to feet and inches.
4363: @c 2. take temperature and convert from fahrenheight to celcius;
4364: @c may need to care about symmetric vs floored??
4365: @c 3. take input line and do character substitution
4366: @c to encipher or decipher
4367: @c 4. as above but work on a file for in and out
4368: @c 5. take input line and convert to pig-latin
4369: @c
4370: @c thing of sets of things to exercise then come up with
4371: @c problems that need those things.
4372:
4373:
4374: @c ******************************************************************
4375: @node Words, Error messages, Introduction, Top
4376: @chapter Forth Words
4377: @cindex words
4378:
4379: @menu
4380: * Notation::
4381: * Case insensitivity::
4382: * Comments::
4383: * Boolean Flags::
4384: * Arithmetic::
4385: * Stack Manipulation::
4386: * Memory::
4387: * Control Structures::
4388: * Defining Words::
4389: * Interpretation and Compilation Semantics::
4390: * Tokens for Words::
4391: * Compiling words::
4392: * The Text Interpreter::
4393: * The Input Stream::
4394: * Word Lists::
4395: * Environmental Queries::
4396: * Files::
4397: * Blocks::
4398: * Other I/O::
4399: * OS command line arguments::
4400: * Locals::
4401: * Structures::
4402: * Object-oriented Forth::
4403: * Programming Tools::
4404: * C Interface::
4405: * Assembler and Code Words::
4406: * Threading Words::
4407: * Passing Commands to the OS::
4408: * Keeping track of Time::
4409: * Miscellaneous Words::
4410: @end menu
4411:
4412: @node Notation, Case insensitivity, Words, Words
4413: @section Notation
4414: @cindex notation of glossary entries
4415: @cindex format of glossary entries
4416: @cindex glossary notation format
4417: @cindex word glossary entry format
4418:
4419: The Forth words are described in this section in the glossary notation
4420: that has become a de-facto standard for Forth texts:
4421:
4422: @format
4423: @i{word} @i{Stack effect} @i{wordset} @i{pronunciation}
4424: @end format
4425: @i{Description}
4426:
4427: @table @var
4428: @item word
4429: The name of the word.
4430:
4431: @item Stack effect
4432: @cindex stack effect
4433: The stack effect is written in the notation @code{@i{before} --
4434: @i{after}}, where @i{before} and @i{after} describe the top of
4435: stack entries before and after the execution of the word. The rest of
4436: the stack is not touched by the word. The top of stack is rightmost,
4437: i.e., a stack sequence is written as it is typed in. Note that Gforth
4438: uses a separate floating point stack, but a unified stack
4439: notation. Also, return stack effects are not shown in @i{stack
4440: effect}, but in @i{Description}. The name of a stack item describes
4441: the type and/or the function of the item. See below for a discussion of
4442: the types.
4443:
4444: All words have two stack effects: A compile-time stack effect and a
4445: run-time stack effect. The compile-time stack-effect of most words is
4446: @i{ -- }. If the compile-time stack-effect of a word deviates from
4447: this standard behaviour, or the word does other unusual things at
4448: compile time, both stack effects are shown; otherwise only the run-time
4449: stack effect is shown.
4450:
4451: Also note that in code templates or examples there can be comments in
4452: parentheses that display the stack picture at this point; there is no
4453: @code{--} in these places, because there is no before-after situation.
4454:
4455: @cindex pronounciation of words
4456: @item pronunciation
4457: How the word is pronounced.
4458:
4459: @cindex wordset
4460: @cindex environment wordset
4461: @item wordset
4462: The ANS Forth standard is divided into several word sets. A standard
4463: system need not support all of them. Therefore, in theory, the fewer
4464: word sets your program uses the more portable it will be. However, we
4465: suspect that most ANS Forth systems on personal machines will feature
4466: all word sets. Words that are not defined in ANS Forth have
4467: @code{gforth} or @code{gforth-internal} as word set. @code{gforth}
4468: describes words that will work in future releases of Gforth;
4469: @code{gforth-internal} words are more volatile. Environmental query
4470: strings are also displayed like words; you can recognize them by the
4471: @code{environment} in the word set field.
4472:
4473: @item Description
4474: A description of the behaviour of the word.
4475: @end table
4476:
4477: @cindex types of stack items
4478: @cindex stack item types
4479: The type of a stack item is specified by the character(s) the name
4480: starts with:
4481:
4482: @table @code
4483: @item f
4484: @cindex @code{f}, stack item type
4485: Boolean flags, i.e. @code{false} or @code{true}.
4486: @item c
4487: @cindex @code{c}, stack item type
4488: Char
4489: @item w
4490: @cindex @code{w}, stack item type
4491: Cell, can contain an integer or an address
4492: @item n
4493: @cindex @code{n}, stack item type
4494: signed integer
4495: @item u
4496: @cindex @code{u}, stack item type
4497: unsigned integer
4498: @item d
4499: @cindex @code{d}, stack item type
4500: double sized signed integer
4501: @item ud
4502: @cindex @code{ud}, stack item type
4503: double sized unsigned integer
4504: @item r
4505: @cindex @code{r}, stack item type
4506: Float (on the FP stack)
4507: @item a-
4508: @cindex @code{a_}, stack item type
4509: Cell-aligned address
4510: @item c-
4511: @cindex @code{c_}, stack item type
4512: Char-aligned address (note that a Char may have two bytes in Windows NT)
4513: @item f-
4514: @cindex @code{f_}, stack item type
4515: Float-aligned address
4516: @item df-
4517: @cindex @code{df_}, stack item type
4518: Address aligned for IEEE double precision float
4519: @item sf-
4520: @cindex @code{sf_}, stack item type
4521: Address aligned for IEEE single precision float
4522: @item xt
4523: @cindex @code{xt}, stack item type
4524: Execution token, same size as Cell
4525: @item wid
4526: @cindex @code{wid}, stack item type
4527: Word list ID, same size as Cell
4528: @item ior, wior
4529: @cindex ior type description
4530: @cindex wior type description
4531: I/O result code, cell-sized. In Gforth, you can @code{throw} iors.
4532: @item f83name
4533: @cindex @code{f83name}, stack item type
4534: Pointer to a name structure
4535: @item "
4536: @cindex @code{"}, stack item type
4537: string in the input stream (not on the stack). The terminating character
4538: is a blank by default. If it is not a blank, it is shown in @code{<>}
4539: quotes.
4540: @end table
4541:
4542: @comment ----------------------------------------------
4543: @node Case insensitivity, Comments, Notation, Words
4544: @section Case insensitivity
4545: @cindex case sensitivity
4546: @cindex upper and lower case
4547:
4548: Gforth is case-insensitive; you can enter definitions and invoke
4549: Standard words using upper, lower or mixed case (however,
4550: @pxref{core-idef, Implementation-defined options, Implementation-defined
4551: options}).
4552:
4553: ANS Forth only @i{requires} implementations to recognise Standard words
4554: when they are typed entirely in upper case. Therefore, a Standard
4555: program must use upper case for all Standard words. You can use whatever
4556: case you like for words that you define, but in a Standard program you
4557: have to use the words in the same case that you defined them.
4558:
4559: Gforth supports case sensitivity through @code{table}s (case-sensitive
4560: wordlists, @pxref{Word Lists}).
4561:
4562: Two people have asked how to convert Gforth to be case-sensitive; while
4563: we think this is a bad idea, you can change all wordlists into tables
4564: like this:
4565:
4566: @example
4567: ' table-find forth-wordlist wordlist-map @ !
4568: @end example
4569:
4570: Note that you now have to type the predefined words in the same case
4571: that we defined them, which are varying. You may want to convert them
4572: to your favourite case before doing this operation (I won't explain how,
4573: because if you are even contemplating doing this, you'd better have
4574: enough knowledge of Forth systems to know this already).
4575:
4576: @node Comments, Boolean Flags, Case insensitivity, Words
4577: @section Comments
4578: @cindex comments
4579:
4580: Forth supports two styles of comment; the traditional @i{in-line} comment,
4581: @code{(} and its modern cousin, the @i{comment to end of line}; @code{\}.
4582:
4583:
4584: doc-(
4585: doc-\
4586: doc-\G
4587:
4588:
4589: @node Boolean Flags, Arithmetic, Comments, Words
4590: @section Boolean Flags
4591: @cindex Boolean flags
4592:
4593: A Boolean flag is cell-sized. A cell with all bits clear represents the
4594: flag @code{false} and a flag with all bits set represents the flag
4595: @code{true}. Words that check a flag (for example, @code{IF}) will treat
4596: a cell that has @i{any} bit set as @code{true}.
4597: @c on and off to Memory?
4598: @c true and false to "Bitwise operations" or "Numeric comparison"?
4599:
4600: doc-true
4601: doc-false
4602: doc-on
4603: doc-off
4604:
4605:
4606: @node Arithmetic, Stack Manipulation, Boolean Flags, Words
4607: @section Arithmetic
4608: @cindex arithmetic words
4609:
4610: @cindex division with potentially negative operands
4611: Forth arithmetic is not checked, i.e., you will not hear about integer
4612: overflow on addition or multiplication, you may hear about division by
4613: zero if you are lucky. The operator is written after the operands, but
4614: the operands are still in the original order. I.e., the infix @code{2-1}
4615: corresponds to @code{2 1 -}. Forth offers a variety of division
4616: operators. If you perform division with potentially negative operands,
4617: you do not want to use @code{/} or @code{/mod} with its undefined
4618: behaviour, but rather @code{fm/mod} or @code{sm/mod} (probably the
4619: former, @pxref{Mixed precision}).
4620: @comment TODO discuss the different division forms and the std approach
4621:
4622: @menu
4623: * Single precision::
4624: * Double precision:: Double-cell integer arithmetic
4625: * Bitwise operations::
4626: * Numeric comparison::
4627: * Mixed precision:: Operations with single and double-cell integers
4628: * Floating Point::
4629: @end menu
4630:
4631: @node Single precision, Double precision, Arithmetic, Arithmetic
4632: @subsection Single precision
4633: @cindex single precision arithmetic words
4634:
4635: @c !! cell undefined
4636:
4637: By default, numbers in Forth are single-precision integers that are one
4638: cell in size. They can be signed or unsigned, depending upon how you
4639: treat them. For the rules used by the text interpreter for recognising
4640: single-precision integers see @ref{Number Conversion}.
4641:
4642: These words are all defined for signed operands, but some of them also
4643: work for unsigned numbers: @code{+}, @code{1+}, @code{-}, @code{1-},
4644: @code{*}.
4645:
4646: doc-+
4647: doc-1+
4648: doc-under+
4649: doc--
4650: doc-1-
4651: doc-*
4652: doc-/
4653: doc-mod
4654: doc-/mod
4655: doc-negate
4656: doc-abs
4657: doc-min
4658: doc-max
4659: doc-floored
4660:
4661:
4662: @node Double precision, Bitwise operations, Single precision, Arithmetic
4663: @subsection Double precision
4664: @cindex double precision arithmetic words
4665:
4666: For the rules used by the text interpreter for
4667: recognising double-precision integers, see @ref{Number Conversion}.
4668:
4669: A double precision number is represented by a cell pair, with the most
4670: significant cell at the TOS. It is trivial to convert an unsigned single
4671: to a double: simply push a @code{0} onto the TOS. Since numbers are
4672: represented by Gforth using 2's complement arithmetic, converting a
4673: signed single to a (signed) double requires sign-extension across the
4674: most significant cell. This can be achieved using @code{s>d}. The moral
4675: of the story is that you cannot convert a number without knowing whether
4676: it represents an unsigned or a signed number.
4677:
4678: These words are all defined for signed operands, but some of them also
4679: work for unsigned numbers: @code{d+}, @code{d-}.
4680:
4681: doc-s>d
4682: doc-d>s
4683: doc-d+
4684: doc-d-
4685: doc-dnegate
4686: doc-dabs
4687: doc-dmin
4688: doc-dmax
4689:
4690:
4691: @node Bitwise operations, Numeric comparison, Double precision, Arithmetic
4692: @subsection Bitwise operations
4693: @cindex bitwise operation words
4694:
4695:
4696: doc-and
4697: doc-or
4698: doc-xor
4699: doc-invert
4700: doc-lshift
4701: doc-rshift
4702: doc-2*
4703: doc-d2*
4704: doc-2/
4705: doc-d2/
4706:
4707:
4708: @node Numeric comparison, Mixed precision, Bitwise operations, Arithmetic
4709: @subsection Numeric comparison
4710: @cindex numeric comparison words
4711:
4712: Note that the words that compare for equality (@code{= <> 0= 0<> d= d<>
4713: d0= d0<>}) work for for both signed and unsigned numbers.
4714:
4715: doc-<
4716: doc-<=
4717: doc-<>
4718: doc-=
4719: doc->
4720: doc->=
4721:
4722: doc-0<
4723: doc-0<=
4724: doc-0<>
4725: doc-0=
4726: doc-0>
4727: doc-0>=
4728:
4729: doc-u<
4730: doc-u<=
4731: @c u<> and u= exist but are the same as <> and =
4732: @c doc-u<>
4733: @c doc-u=
4734: doc-u>
4735: doc-u>=
4736:
4737: doc-within
4738:
4739: doc-d<
4740: doc-d<=
4741: doc-d<>
4742: doc-d=
4743: doc-d>
4744: doc-d>=
4745:
4746: doc-d0<
4747: doc-d0<=
4748: doc-d0<>
4749: doc-d0=
4750: doc-d0>
4751: doc-d0>=
4752:
4753: doc-du<
4754: doc-du<=
4755: @c du<> and du= exist but are the same as d<> and d=
4756: @c doc-du<>
4757: @c doc-du=
4758: doc-du>
4759: doc-du>=
4760:
4761:
4762: @node Mixed precision, Floating Point, Numeric comparison, Arithmetic
4763: @subsection Mixed precision
4764: @cindex mixed precision arithmetic words
4765:
4766:
4767: doc-m+
4768: doc-*/
4769: doc-*/mod
4770: doc-m*
4771: doc-um*
4772: doc-m*/
4773: doc-um/mod
4774: doc-fm/mod
4775: doc-sm/rem
4776:
4777:
4778: @node Floating Point, , Mixed precision, Arithmetic
4779: @subsection Floating Point
4780: @cindex floating point arithmetic words
4781:
4782: For the rules used by the text interpreter for
4783: recognising floating-point numbers see @ref{Number Conversion}.
4784:
4785: Gforth has a separate floating point stack, but the documentation uses
4786: the unified notation.@footnote{It's easy to generate the separate
4787: notation from that by just separating the floating-point numbers out:
4788: e.g. @code{( n r1 u r2 -- r3 )} becomes @code{( n u -- ) ( F: r1 r2 --
4789: r3 )}.}
4790:
4791: @cindex floating-point arithmetic, pitfalls
4792: Floating point numbers have a number of unpleasant surprises for the
4793: unwary (e.g., floating point addition is not associative) and even a
4794: few for the wary. You should not use them unless you know what you are
4795: doing or you don't care that the results you get are totally bogus. If
4796: you want to learn about the problems of floating point numbers (and
4797: how to avoid them), you might start with @cite{David Goldberg,
4798: @uref{http://docs.sun.com/source/806-3568/ncg_goldberg.html,What Every
4799: Computer Scientist Should Know About Floating-Point Arithmetic}, ACM
4800: Computing Surveys 23(1):5@minus{}48, March 1991}.
4801:
4802:
4803: doc-d>f
4804: doc-f>d
4805: doc-f+
4806: doc-f-
4807: doc-f*
4808: doc-f/
4809: doc-fnegate
4810: doc-fabs
4811: doc-fmax
4812: doc-fmin
4813: doc-floor
4814: doc-fround
4815: doc-f**
4816: doc-fsqrt
4817: doc-fexp
4818: doc-fexpm1
4819: doc-fln
4820: doc-flnp1
4821: doc-flog
4822: doc-falog
4823: doc-f2*
4824: doc-f2/
4825: doc-1/f
4826: doc-precision
4827: doc-set-precision
4828:
4829: @cindex angles in trigonometric operations
4830: @cindex trigonometric operations
4831: Angles in floating point operations are given in radians (a full circle
4832: has 2 pi radians).
4833:
4834: doc-fsin
4835: doc-fcos
4836: doc-fsincos
4837: doc-ftan
4838: doc-fasin
4839: doc-facos
4840: doc-fatan
4841: doc-fatan2
4842: doc-fsinh
4843: doc-fcosh
4844: doc-ftanh
4845: doc-fasinh
4846: doc-facosh
4847: doc-fatanh
4848: doc-pi
4849:
4850: @cindex equality of floats
4851: @cindex floating-point comparisons
4852: One particular problem with floating-point arithmetic is that comparison
4853: for equality often fails when you would expect it to succeed. For this
4854: reason approximate equality is often preferred (but you still have to
4855: know what you are doing). Also note that IEEE NaNs may compare
4856: differently from what you might expect. The comparison words are:
4857:
4858: doc-f~rel
4859: doc-f~abs
4860: doc-f~
4861: doc-f=
4862: doc-f<>
4863:
4864: doc-f<
4865: doc-f<=
4866: doc-f>
4867: doc-f>=
4868:
4869: doc-f0<
4870: doc-f0<=
4871: doc-f0<>
4872: doc-f0=
4873: doc-f0>
4874: doc-f0>=
4875:
4876:
4877: @node Stack Manipulation, Memory, Arithmetic, Words
4878: @section Stack Manipulation
4879: @cindex stack manipulation words
4880:
4881: @cindex floating-point stack in the standard
4882: Gforth maintains a number of separate stacks:
4883:
4884: @cindex data stack
4885: @cindex parameter stack
4886: @itemize @bullet
4887: @item
4888: A data stack (also known as the @dfn{parameter stack}) -- for
4889: characters, cells, addresses, and double cells.
4890:
4891: @cindex floating-point stack
4892: @item
4893: A floating point stack -- for holding floating point (FP) numbers.
4894:
4895: @cindex return stack
4896: @item
4897: A return stack -- for holding the return addresses of colon
4898: definitions and other (non-FP) data.
4899:
4900: @cindex locals stack
4901: @item
4902: A locals stack -- for holding local variables.
4903: @end itemize
4904:
4905: @menu
4906: * Data stack::
4907: * Floating point stack::
4908: * Return stack::
4909: * Locals stack::
4910: * Stack pointer manipulation::
4911: @end menu
4912:
4913: @node Data stack, Floating point stack, Stack Manipulation, Stack Manipulation
4914: @subsection Data stack
4915: @cindex data stack manipulation words
4916: @cindex stack manipulations words, data stack
4917:
4918:
4919: doc-drop
4920: doc-nip
4921: doc-dup
4922: doc-over
4923: doc-tuck
4924: doc-swap
4925: doc-pick
4926: doc-rot
4927: doc--rot
4928: doc-?dup
4929: doc-roll
4930: doc-2drop
4931: doc-2nip
4932: doc-2dup
4933: doc-2over
4934: doc-2tuck
4935: doc-2swap
4936: doc-2rot
4937:
4938:
4939: @node Floating point stack, Return stack, Data stack, Stack Manipulation
4940: @subsection Floating point stack
4941: @cindex floating-point stack manipulation words
4942: @cindex stack manipulation words, floating-point stack
4943:
4944: Whilst every sane Forth has a separate floating-point stack, it is not
4945: strictly required; an ANS Forth system could theoretically keep
4946: floating-point numbers on the data stack. As an additional difficulty,
4947: you don't know how many cells a floating-point number takes. It is
4948: reportedly possible to write words in a way that they work also for a
4949: unified stack model, but we do not recommend trying it. Instead, just
4950: say that your program has an environmental dependency on a separate
4951: floating-point stack.
4952:
4953: doc-floating-stack
4954:
4955: doc-fdrop
4956: doc-fnip
4957: doc-fdup
4958: doc-fover
4959: doc-ftuck
4960: doc-fswap
4961: doc-fpick
4962: doc-frot
4963:
4964:
4965: @node Return stack, Locals stack, Floating point stack, Stack Manipulation
4966: @subsection Return stack
4967: @cindex return stack manipulation words
4968: @cindex stack manipulation words, return stack
4969:
4970: @cindex return stack and locals
4971: @cindex locals and return stack
4972: A Forth system is allowed to keep local variables on the
4973: return stack. This is reasonable, as local variables usually eliminate
4974: the need to use the return stack explicitly. So, if you want to produce
4975: a standard compliant program and you are using local variables in a
4976: word, forget about return stack manipulations in that word (refer to the
4977: standard document for the exact rules).
4978:
4979: doc->r
4980: doc-r>
4981: doc-r@
4982: doc-rdrop
4983: doc-2>r
4984: doc-2r>
4985: doc-2r@
4986: doc-2rdrop
4987:
4988:
4989: @node Locals stack, Stack pointer manipulation, Return stack, Stack Manipulation
4990: @subsection Locals stack
4991:
4992: Gforth uses an extra locals stack. It is described, along with the
4993: reasons for its existence, in @ref{Locals implementation}.
4994:
4995: @node Stack pointer manipulation, , Locals stack, Stack Manipulation
4996: @subsection Stack pointer manipulation
4997: @cindex stack pointer manipulation words
4998:
4999: @c removed s0 r0 l0 -- they are obsolete aliases for sp0 rp0 lp0
5000: doc-sp0
5001: doc-sp@
5002: doc-sp!
5003: doc-fp0
5004: doc-fp@
5005: doc-fp!
5006: doc-rp0
5007: doc-rp@
5008: doc-rp!
5009: doc-lp0
5010: doc-lp@
5011: doc-lp!
5012:
5013:
5014: @node Memory, Control Structures, Stack Manipulation, Words
5015: @section Memory
5016: @cindex memory words
5017:
5018: @menu
5019: * Memory model::
5020: * Dictionary allocation::
5021: * Heap Allocation::
5022: * Memory Access::
5023: * Address arithmetic::
5024: * Memory Blocks::
5025: @end menu
5026:
5027: In addition to the standard Forth memory allocation words, there is also
5028: a @uref{http://www.complang.tuwien.ac.at/forth/garbage-collection.zip,
5029: garbage collector}.
5030:
5031: @node Memory model, Dictionary allocation, Memory, Memory
5032: @subsection ANS Forth and Gforth memory models
5033:
5034: @c The ANS Forth description is a mess (e.g., is the heap part of
5035: @c the dictionary?), so let's not stick to closely with it.
5036:
5037: ANS Forth considers a Forth system as consisting of several address
5038: spaces, of which only @dfn{data space} is managed and accessible with
5039: the memory words. Memory not necessarily in data space includes the
5040: stacks, the code (called code space) and the headers (called name
5041: space). In Gforth everything is in data space, but the code for the
5042: primitives is usually read-only.
5043:
5044: Data space is divided into a number of areas: The (data space portion of
5045: the) dictionary@footnote{Sometimes, the term @dfn{dictionary} is used to
5046: refer to the search data structure embodied in word lists and headers,
5047: because it is used for looking up names, just as you would in a
5048: conventional dictionary.}, the heap, and a number of system-allocated
5049: buffers.
5050:
5051: @cindex address arithmetic restrictions, ANS vs. Gforth
5052: @cindex contiguous regions, ANS vs. Gforth
5053: In ANS Forth data space is also divided into contiguous regions. You
5054: can only use address arithmetic within a contiguous region, not between
5055: them. Usually each allocation gives you one contiguous region, but the
5056: dictionary allocation words have additional rules (@pxref{Dictionary
5057: allocation}).
5058:
5059: Gforth provides one big address space, and address arithmetic can be
5060: performed between any addresses. However, in the dictionary headers or
5061: code are interleaved with data, so almost the only contiguous data space
5062: regions there are those described by ANS Forth as contiguous; but you
5063: can be sure that the dictionary is allocated towards increasing
5064: addresses even between contiguous regions. The memory order of
5065: allocations in the heap is platform-dependent (and possibly different
5066: from one run to the next).
5067:
5068:
5069: @node Dictionary allocation, Heap Allocation, Memory model, Memory
5070: @subsection Dictionary allocation
5071: @cindex reserving data space
5072: @cindex data space - reserving some
5073:
5074: Dictionary allocation is a stack-oriented allocation scheme, i.e., if
5075: you want to deallocate X, you also deallocate everything
5076: allocated after X.
5077:
5078: @cindex contiguous regions in dictionary allocation
5079: The allocations using the words below are contiguous and grow the region
5080: towards increasing addresses. Other words that allocate dictionary
5081: memory of any kind (i.e., defining words including @code{:noname}) end
5082: the contiguous region and start a new one.
5083:
5084: In ANS Forth only @code{create}d words are guaranteed to produce an
5085: address that is the start of the following contiguous region. In
5086: particular, the cell allocated by @code{variable} is not guaranteed to
5087: be contiguous with following @code{allot}ed memory.
5088:
5089: You can deallocate memory by using @code{allot} with a negative argument
5090: (with some restrictions, see @code{allot}). For larger deallocations use
5091: @code{marker}.
5092:
5093:
5094: doc-here
5095: doc-unused
5096: doc-allot
5097: doc-c,
5098: doc-f,
5099: doc-,
5100: doc-2,
5101:
5102: Memory accesses have to be aligned (@pxref{Address arithmetic}). So of
5103: course you should allocate memory in an aligned way, too. I.e., before
5104: allocating allocating a cell, @code{here} must be cell-aligned, etc.
5105: The words below align @code{here} if it is not already. Basically it is
5106: only already aligned for a type, if the last allocation was a multiple
5107: of the size of this type and if @code{here} was aligned for this type
5108: before.
5109:
5110: After freshly @code{create}ing a word, @code{here} is @code{align}ed in
5111: ANS Forth (@code{maxalign}ed in Gforth).
5112:
5113: doc-align
5114: doc-falign
5115: doc-sfalign
5116: doc-dfalign
5117: doc-maxalign
5118: doc-cfalign
5119:
5120:
5121: @node Heap Allocation, Memory Access, Dictionary allocation, Memory
5122: @subsection Heap allocation
5123: @cindex heap allocation
5124: @cindex dynamic allocation of memory
5125: @cindex memory-allocation word set
5126:
5127: @cindex contiguous regions and heap allocation
5128: Heap allocation supports deallocation of allocated memory in any
5129: order. Dictionary allocation is not affected by it (i.e., it does not
5130: end a contiguous region). In Gforth, these words are implemented using
5131: the standard C library calls malloc(), free() and resize().
5132:
5133: The memory region produced by one invocation of @code{allocate} or
5134: @code{resize} is internally contiguous. There is no contiguity between
5135: such a region and any other region (including others allocated from the
5136: heap).
5137:
5138: doc-allocate
5139: doc-free
5140: doc-resize
5141:
5142:
5143: @node Memory Access, Address arithmetic, Heap Allocation, Memory
5144: @subsection Memory Access
5145: @cindex memory access words
5146:
5147: doc-@
5148: doc-!
5149: doc-+!
5150: doc-c@
5151: doc-c!
5152: doc-2@
5153: doc-2!
5154: doc-f@
5155: doc-f!
5156: doc-sf@
5157: doc-sf!
5158: doc-df@
5159: doc-df!
5160: doc-sw@
5161: doc-uw@
5162: doc-w!
5163: doc-sl@
5164: doc-ul@
5165: doc-l!
5166:
5167: @node Address arithmetic, Memory Blocks, Memory Access, Memory
5168: @subsection Address arithmetic
5169: @cindex address arithmetic words
5170:
5171: Address arithmetic is the foundation on which you can build data
5172: structures like arrays, records (@pxref{Structures}) and objects
5173: (@pxref{Object-oriented Forth}).
5174:
5175: @cindex address unit
5176: @cindex au (address unit)
5177: ANS Forth does not specify the sizes of the data types. Instead, it
5178: offers a number of words for computing sizes and doing address
5179: arithmetic. Address arithmetic is performed in terms of address units
5180: (aus); on most systems the address unit is one byte. Note that a
5181: character may have more than one au, so @code{chars} is no noop (on
5182: platforms where it is a noop, it compiles to nothing).
5183:
5184: The basic address arithmetic words are @code{+} and @code{-}. E.g., if
5185: you have the address of a cell, perform @code{1 cells +}, and you will
5186: have the address of the next cell.
5187:
5188: @cindex contiguous regions and address arithmetic
5189: In ANS Forth you can perform address arithmetic only within a contiguous
5190: region, i.e., if you have an address into one region, you can only add
5191: and subtract such that the result is still within the region; you can
5192: only subtract or compare addresses from within the same contiguous
5193: region. Reasons: several contiguous regions can be arranged in memory
5194: in any way; on segmented systems addresses may have unusual
5195: representations, such that address arithmetic only works within a
5196: region. Gforth provides a few more guarantees (linear address space,
5197: dictionary grows upwards), but in general I have found it easy to stay
5198: within contiguous regions (exception: computing and comparing to the
5199: address just beyond the end of an array).
5200:
5201: @cindex alignment of addresses for types
5202: ANS Forth also defines words for aligning addresses for specific
5203: types. Many computers require that accesses to specific data types
5204: must only occur at specific addresses; e.g., that cells may only be
5205: accessed at addresses divisible by 4. Even if a machine allows unaligned
5206: accesses, it can usually perform aligned accesses faster.
5207:
5208: For the performance-conscious: alignment operations are usually only
5209: necessary during the definition of a data structure, not during the
5210: (more frequent) accesses to it.
5211:
5212: ANS Forth defines no words for character-aligning addresses. This is not
5213: an oversight, but reflects the fact that addresses that are not
5214: char-aligned have no use in the standard and therefore will not be
5215: created.
5216:
5217: @cindex @code{CREATE} and alignment
5218: ANS Forth guarantees that addresses returned by @code{CREATE}d words
5219: are cell-aligned; in addition, Gforth guarantees that these addresses
5220: are aligned for all purposes.
5221:
5222: Note that the ANS Forth word @code{char} has nothing to do with address
5223: arithmetic.
5224:
5225:
5226: doc-chars
5227: doc-char+
5228: doc-cells
5229: doc-cell+
5230: doc-cell
5231: doc-aligned
5232: doc-floats
5233: doc-float+
5234: doc-float
5235: doc-faligned
5236: doc-sfloats
5237: doc-sfloat+
5238: doc-sfaligned
5239: doc-dfloats
5240: doc-dfloat+
5241: doc-dfaligned
5242: doc-maxaligned
5243: doc-cfaligned
5244: doc-address-unit-bits
5245: doc-/w
5246: doc-/l
5247:
5248: @node Memory Blocks, , Address arithmetic, Memory
5249: @subsection Memory Blocks
5250: @cindex memory block words
5251: @cindex character strings - moving and copying
5252:
5253: Memory blocks often represent character strings; For ways of storing
5254: character strings in memory see @ref{String Formats}. For other
5255: string-processing words see @ref{Displaying characters and strings}.
5256:
5257: A few of these words work on address unit blocks. In that case, you
5258: usually have to insert @code{CHARS} before the word when working on
5259: character strings. Most words work on character blocks, and expect a
5260: char-aligned address.
5261:
5262: When copying characters between overlapping memory regions, use
5263: @code{chars move} or choose carefully between @code{cmove} and
5264: @code{cmove>}.
5265:
5266: doc-move
5267: doc-erase
5268: doc-cmove
5269: doc-cmove>
5270: doc-fill
5271: doc-blank
5272: doc-compare
5273: doc-str=
5274: doc-str<
5275: doc-string-prefix?
5276: doc-search
5277: doc--trailing
5278: doc-/string
5279: doc-bounds
5280: doc-pad
5281:
5282: @comment TODO examples
5283:
5284:
5285: @node Control Structures, Defining Words, Memory, Words
5286: @section Control Structures
5287: @cindex control structures
5288:
5289: Control structures in Forth cannot be used interpretively, only in a
5290: colon definition@footnote{To be precise, they have no interpretation
5291: semantics (@pxref{Interpretation and Compilation Semantics}).}. We do
5292: not like this limitation, but have not seen a satisfying way around it
5293: yet, although many schemes have been proposed.
5294:
5295: @menu
5296: * Selection:: IF ... ELSE ... ENDIF
5297: * Simple Loops:: BEGIN ...
5298: * Counted Loops:: DO
5299: * Arbitrary control structures::
5300: * Calls and returns::
5301: * Exception Handling::
5302: @end menu
5303:
5304: @node Selection, Simple Loops, Control Structures, Control Structures
5305: @subsection Selection
5306: @cindex selection control structures
5307: @cindex control structures for selection
5308:
5309: @cindex @code{IF} control structure
5310: @example
5311: @i{flag}
5312: IF
5313: @i{code}
5314: ENDIF
5315: @end example
5316: @noindent
5317:
5318: If @i{flag} is non-zero (as far as @code{IF} etc. are concerned, a cell
5319: with any bit set represents truth) @i{code} is executed.
5320:
5321: @example
5322: @i{flag}
5323: IF
5324: @i{code1}
5325: ELSE
5326: @i{code2}
5327: ENDIF
5328: @end example
5329:
5330: If @var{flag} is true, @i{code1} is executed, otherwise @i{code2} is
5331: executed.
5332:
5333: You can use @code{THEN} instead of @code{ENDIF}. Indeed, @code{THEN} is
5334: standard, and @code{ENDIF} is not, although it is quite popular. We
5335: recommend using @code{ENDIF}, because it is less confusing for people
5336: who also know other languages (and is not prone to reinforcing negative
5337: prejudices against Forth in these people). Adding @code{ENDIF} to a
5338: system that only supplies @code{THEN} is simple:
5339: @example
5340: : ENDIF POSTPONE then ; immediate
5341: @end example
5342:
5343: [According to @cite{Webster's New Encyclopedic Dictionary}, @dfn{then
5344: (adv.)} has the following meanings:
5345: @quotation
5346: ... 2b: following next after in order ... 3d: as a necessary consequence
5347: (if you were there, then you saw them).
5348: @end quotation
5349: Forth's @code{THEN} has the meaning 2b, whereas @code{THEN} in Pascal
5350: and many other programming languages has the meaning 3d.]
5351:
5352: Gforth also provides the words @code{?DUP-IF} and @code{?DUP-0=-IF}, so
5353: you can avoid using @code{?dup}. Using these alternatives is also more
5354: efficient than using @code{?dup}. Definitions in ANS Forth
5355: for @code{ENDIF}, @code{?DUP-IF} and @code{?DUP-0=-IF} are provided in
5356: @file{compat/control.fs}.
5357:
5358: @cindex @code{CASE} control structure
5359: @example
5360: @i{x}
5361: CASE
5362: @i{x1} OF @i{code1} ENDOF
5363: @i{x2} OF @i{code2} ENDOF
5364: @dots{}
5365: ( x ) @i{default-code} ( x )
5366: ENDCASE ( )
5367: @end example
5368:
5369: Executes the first @i{codei}, where the @i{xi} is equal to @i{x}. If no
5370: @i{xi} matches, the optional @i{default-code} is executed. The optional
5371: default case can be added by simply writing the code after the last
5372: @code{ENDOF}. It may use @i{x}, which is on top of the stack, but must
5373: not consume it. The value @i{x} is consumed by this construction
5374: (either by an @code{OF} that matches, or by the @code{ENDCASE}, if no OF
5375: matches). Example:
5376:
5377: @example
5378: : num-name ( n -- c-addr u )
5379: case
5380: 0 of s" zero " endof
5381: 1 of s" one " endof
5382: 2 of s" two " endof
5383: \ default case:
5384: s" other number"
5385: rot \ get n on top so ENDCASE can drop it
5386: endcase ;
5387: @end example
5388:
5389: You can also use (the non-standard) @code{?of} to use @code{case} as a
5390: general selection structure for more than two alternatives.
5391: @code{?Of} takes a flag. Example:
5392:
5393: @example
5394: : sgn ( n1 -- n2 )
5395: \ sign function
5396: case
5397: dup 0< ?of drop -1 endof
5398: dup 0> ?of drop 1 endof
5399: dup \ n1=0 -> n2=0; dup an item, to be consumed by ENDCASE
5400: endcase ;
5401: @end example
5402:
5403: @progstyle
5404: To keep the code understandable, you should ensure that you change the
5405: stack in the same way (wrt. number and types of stack items consumed
5406: and pushed) on all paths through a selection structure.
5407:
5408: @node Simple Loops, Counted Loops, Selection, Control Structures
5409: @subsection Simple Loops
5410: @cindex simple loops
5411: @cindex loops without count
5412:
5413: @cindex @code{WHILE} loop
5414: @example
5415: BEGIN
5416: @i{code1}
5417: @i{flag}
5418: WHILE
5419: @i{code2}
5420: REPEAT
5421: @end example
5422:
5423: @i{code1} is executed and @i{flag} is computed. If it is true,
5424: @i{code2} is executed and the loop is restarted; If @i{flag} is
5425: false, execution continues after the @code{REPEAT}.
5426:
5427: @cindex @code{UNTIL} loop
5428: @example
5429: BEGIN
5430: @i{code}
5431: @i{flag}
5432: UNTIL
5433: @end example
5434:
5435: @i{code} is executed. The loop is restarted if @code{flag} is false.
5436:
5437: @progstyle
5438: To keep the code understandable, a complete iteration of the loop should
5439: not change the number and types of the items on the stacks.
5440:
5441: @cindex endless loop
5442: @cindex loops, endless
5443: @example
5444: BEGIN
5445: @i{code}
5446: AGAIN
5447: @end example
5448:
5449: This is an endless loop.
5450:
5451: @node Counted Loops, Arbitrary control structures, Simple Loops, Control Structures
5452: @subsection Counted Loops
5453: @cindex counted loops
5454: @cindex loops, counted
5455: @cindex @code{DO} loops
5456:
5457: The basic counted loop is:
5458: @example
5459: @i{limit} @i{start}
5460: ?DO
5461: @i{body}
5462: LOOP
5463: @end example
5464:
5465: This performs one iteration for every integer, starting from @i{start}
5466: and up to, but excluding @i{limit}. The counter, or @i{index}, can be
5467: accessed with @code{i}. For example, the loop:
5468: @example
5469: 10 0 ?DO
5470: i .
5471: LOOP
5472: @end example
5473: @noindent
5474: prints @code{0 1 2 3 4 5 6 7 8 9}
5475:
5476: The index of the innermost loop can be accessed with @code{i}, the index
5477: of the next loop with @code{j}, and the index of the third loop with
5478: @code{k}.
5479:
5480:
5481: doc-i
5482: doc-j
5483: doc-k
5484:
5485:
5486: The loop control data are kept on the return stack, so there are some
5487: restrictions on mixing return stack accesses and counted loop words. In
5488: particuler, if you put values on the return stack outside the loop, you
5489: cannot read them inside the loop@footnote{well, not in a way that is
5490: portable.}. If you put values on the return stack within a loop, you
5491: have to remove them before the end of the loop and before accessing the
5492: index of the loop.
5493:
5494: There are several variations on the counted loop:
5495:
5496: @itemize @bullet
5497: @item
5498: @code{LEAVE} leaves the innermost counted loop immediately; execution
5499: continues after the associated @code{LOOP} or @code{NEXT}. For example:
5500:
5501: @example
5502: 10 0 ?DO i DUP . 3 = IF LEAVE THEN LOOP
5503: @end example
5504: prints @code{0 1 2 3}
5505:
5506:
5507: @item
5508: @code{UNLOOP} prepares for an abnormal loop exit, e.g., via
5509: @code{EXIT}. @code{UNLOOP} removes the loop control parameters from the
5510: return stack so @code{EXIT} can get to its return address. For example:
5511:
5512: @example
5513: : demo 10 0 ?DO i DUP . 3 = IF UNLOOP EXIT THEN LOOP ." Done" ;
5514: @end example
5515: prints @code{0 1 2 3}
5516:
5517:
5518: @item
5519: If @i{start} is greater than @i{limit}, a @code{?DO} loop is entered
5520: (and @code{LOOP} iterates until they become equal by wrap-around
5521: arithmetic). This behaviour is usually not what you want. Therefore,
5522: Gforth offers @code{+DO} and @code{U+DO} (as replacements for
5523: @code{?DO}), which do not enter the loop if @i{start} is greater than
5524: @i{limit}; @code{+DO} is for signed loop parameters, @code{U+DO} for
5525: unsigned loop parameters.
5526:
5527: @item
5528: @code{?DO} can be replaced by @code{DO}. @code{DO} always enters
5529: the loop, independent of the loop parameters. Do not use @code{DO}, even
5530: if you know that the loop is entered in any case. Such knowledge tends
5531: to become invalid during maintenance of a program, and then the
5532: @code{DO} will make trouble.
5533:
5534: @item
5535: @code{LOOP} can be replaced with @code{@i{n} +LOOP}; this updates the
5536: index by @i{n} instead of by 1. The loop is terminated when the border
5537: between @i{limit-1} and @i{limit} is crossed. E.g.:
5538:
5539: @example
5540: 4 0 +DO i . 2 +LOOP
5541: @end example
5542: @noindent
5543: prints @code{0 2}
5544:
5545: @example
5546: 4 1 +DO i . 2 +LOOP
5547: @end example
5548: @noindent
5549: prints @code{1 3}
5550:
5551: @item
5552: @cindex negative increment for counted loops
5553: @cindex counted loops with negative increment
5554: The behaviour of @code{@i{n} +LOOP} is peculiar when @i{n} is negative:
5555:
5556: @example
5557: -1 0 ?DO i . -1 +LOOP
5558: @end example
5559: @noindent
5560: prints @code{0 -1}
5561:
5562: @example
5563: 0 0 ?DO i . -1 +LOOP
5564: @end example
5565: prints nothing.
5566:
5567: Therefore we recommend avoiding @code{@i{n} +LOOP} with negative
5568: @i{n}. One alternative is @code{@i{u} -LOOP}, which reduces the
5569: index by @i{u} each iteration. The loop is terminated when the border
5570: between @i{limit+1} and @i{limit} is crossed. Gforth also provides
5571: @code{-DO} and @code{U-DO} for down-counting loops. E.g.:
5572:
5573: @example
5574: -2 0 -DO i . 1 -LOOP
5575: @end example
5576: @noindent
5577: prints @code{0 -1}
5578:
5579: @example
5580: -1 0 -DO i . 1 -LOOP
5581: @end example
5582: @noindent
5583: prints @code{0}
5584:
5585: @example
5586: 0 0 -DO i . 1 -LOOP
5587: @end example
5588: @noindent
5589: prints nothing.
5590:
5591: @end itemize
5592:
5593: Unfortunately, @code{+DO}, @code{U+DO}, @code{-DO}, @code{U-DO} and
5594: @code{-LOOP} are not defined in ANS Forth. However, an implementation
5595: for these words that uses only standard words is provided in
5596: @file{compat/loops.fs}.
5597:
5598:
5599: @cindex @code{FOR} loops
5600: Another counted loop is:
5601: @example
5602: @i{n}
5603: FOR
5604: @i{body}
5605: NEXT
5606: @end example
5607: This is the preferred loop of native code compiler writers who are too
5608: lazy to optimize @code{?DO} loops properly. This loop structure is not
5609: defined in ANS Forth. In Gforth, this loop iterates @i{n+1} times;
5610: @code{i} produces values starting with @i{n} and ending with 0. Other
5611: Forth systems may behave differently, even if they support @code{FOR}
5612: loops. To avoid problems, don't use @code{FOR} loops.
5613:
5614: @node Arbitrary control structures, Calls and returns, Counted Loops, Control Structures
5615: @subsection Arbitrary control structures
5616: @cindex control structures, user-defined
5617:
5618: @cindex control-flow stack
5619: ANS Forth permits and supports using control structures in a non-nested
5620: way. Information about incomplete control structures is stored on the
5621: control-flow stack. This stack may be implemented on the Forth data
5622: stack, and this is what we have done in Gforth.
5623:
5624: @cindex @code{orig}, control-flow stack item
5625: @cindex @code{dest}, control-flow stack item
5626: An @i{orig} entry represents an unresolved forward branch, a @i{dest}
5627: entry represents a backward branch target. A few words are the basis for
5628: building any control structure possible (except control structures that
5629: need storage, like calls, coroutines, and backtracking).
5630:
5631:
5632: doc-if
5633: doc-ahead
5634: doc-then
5635: doc-begin
5636: doc-until
5637: doc-again
5638: doc-cs-pick
5639: doc-cs-roll
5640:
5641:
5642: The Standard words @code{CS-PICK} and @code{CS-ROLL} allow you to
5643: manipulate the control-flow stack in a portable way. Without them, you
5644: would need to know how many stack items are occupied by a control-flow
5645: entry (many systems use one cell. In Gforth they currently take three,
5646: but this may change in the future).
5647:
5648: Some standard control structure words are built from these words:
5649:
5650:
5651: doc-else
5652: doc-while
5653: doc-repeat
5654:
5655:
5656: @noindent
5657: Gforth adds some more control-structure words:
5658:
5659:
5660: doc-endif
5661: doc-?dup-if
5662: doc-?dup-0=-if
5663:
5664:
5665: @noindent
5666: Counted loop words constitute a separate group of words:
5667:
5668:
5669: doc-?do
5670: doc-+do
5671: doc-u+do
5672: doc--do
5673: doc-u-do
5674: doc-do
5675: doc-for
5676: doc-loop
5677: doc-+loop
5678: doc--loop
5679: doc-next
5680: doc-leave
5681: doc-?leave
5682: doc-unloop
5683: doc-done
5684:
5685:
5686: The standard does not allow using @code{CS-PICK} and @code{CS-ROLL} on
5687: @i{do-sys}. Gforth allows it, but it's your job to ensure that for
5688: every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path
5689: through the definition (@code{LOOP} etc. compile an @code{UNLOOP} on the
5690: fall-through path). Also, you have to ensure that all @code{LEAVE}s are
5691: resolved (by using one of the loop-ending words or @code{DONE}).
5692:
5693: @noindent
5694: Another group of control structure words are:
5695:
5696:
5697: doc-case
5698: doc-endcase
5699: doc-of
5700: doc-?ofx
5701: doc-endof
5702:
5703:
5704: @i{case-sys} and @i{of-sys} cannot be processed using @code{CS-PICK} and
5705: @code{CS-ROLL}.
5706:
5707: @subsubsection Programming Style
5708: @cindex control structures programming style
5709: @cindex programming style, arbitrary control structures
5710:
5711: In order to ensure readability we recommend that you do not create
5712: arbitrary control structures directly, but define new control structure
5713: words for the control structure you want and use these words in your
5714: program. For example, instead of writing:
5715:
5716: @example
5717: BEGIN
5718: ...
5719: IF [ 1 CS-ROLL ]
5720: ...
5721: AGAIN THEN
5722: @end example
5723:
5724: @noindent
5725: we recommend defining control structure words, e.g.,
5726:
5727: @example
5728: : WHILE ( DEST -- ORIG DEST )
5729: POSTPONE IF
5730: 1 CS-ROLL ; immediate
5731:
5732: : REPEAT ( orig dest -- )
5733: POSTPONE AGAIN
5734: POSTPONE THEN ; immediate
5735: @end example
5736:
5737: @noindent
5738: and then using these to create the control structure:
5739:
5740: @example
5741: BEGIN
5742: ...
5743: WHILE
5744: ...
5745: REPEAT
5746: @end example
5747:
5748: That's much easier to read, isn't it? Of course, @code{REPEAT} and
5749: @code{WHILE} are predefined, so in this example it would not be
5750: necessary to define them.
5751:
5752: @node Calls and returns, Exception Handling, Arbitrary control structures, Control Structures
5753: @subsection Calls and returns
5754: @cindex calling a definition
5755: @cindex returning from a definition
5756:
5757: @cindex recursive definitions
5758: A definition can be called simply be writing the name of the definition
5759: to be called. Normally a definition is invisible during its own
5760: definition. If you want to write a directly recursive definition, you
5761: can use @code{recursive} to make the current definition visible, or
5762: @code{recurse} to call the current definition directly.
5763:
5764:
5765: doc-recursive
5766: doc-recurse
5767:
5768:
5769: @comment TODO add example of the two recursion methods
5770: @quotation
5771: @progstyle
5772: I prefer using @code{recursive} to @code{recurse}, because calling the
5773: definition by name is more descriptive (if the name is well-chosen) than
5774: the somewhat cryptic @code{recurse}. E.g., in a quicksort
5775: implementation, it is much better to read (and think) ``now sort the
5776: partitions'' than to read ``now do a recursive call''.
5777: @end quotation
5778:
5779: For mutual recursion, use @code{Defer}red words, like this:
5780:
5781: @example
5782: Defer foo
5783:
5784: : bar ( ... -- ... )
5785: ... foo ... ;
5786:
5787: :noname ( ... -- ... )
5788: ... bar ... ;
5789: IS foo
5790: @end example
5791:
5792: Deferred words are discussed in more detail in @ref{Deferred Words}.
5793:
5794: The current definition returns control to the calling definition when
5795: the end of the definition is reached or @code{EXIT} is encountered.
5796:
5797: doc-exit
5798: doc-;s
5799:
5800:
5801: @node Exception Handling, , Calls and returns, Control Structures
5802: @subsection Exception Handling
5803: @cindex exceptions
5804:
5805: @c quit is a very bad idea for error handling,
5806: @c because it does not translate into a THROW
5807: @c it also does not belong into this chapter
5808:
5809: If a word detects an error condition that it cannot handle, it can
5810: @code{throw} an exception. In the simplest case, this will terminate
5811: your program, and report an appropriate error.
5812:
5813: doc-throw
5814:
5815: @code{Throw} consumes a cell-sized error number on the stack. There are
5816: some predefined error numbers in ANS Forth (see @file{errors.fs}). In
5817: Gforth (and most other systems) you can use the iors produced by various
5818: words as error numbers (e.g., a typical use of @code{allocate} is
5819: @code{allocate throw}). Gforth also provides the word @code{exception}
5820: to define your own error numbers (with decent error reporting); an ANS
5821: Forth version of this word (but without the error messages) is available
5822: in @code{compat/except.fs}. And finally, you can use your own error
5823: numbers (anything outside the range -4095..0), but won't get nice error
5824: messages, only numbers. For example, try:
5825:
5826: @example
5827: -10 throw \ ANS defined
5828: -267 throw \ system defined
5829: s" my error" exception throw \ user defined
5830: 7 throw \ arbitrary number
5831: @end example
5832:
5833: doc---exception-exception
5834:
5835: A common idiom to @code{THROW} a specific error if a flag is true is
5836: this:
5837:
5838: @example
5839: @code{( flag ) 0<> @i{errno} and throw}
5840: @end example
5841:
5842: Your program can provide exception handlers to catch exceptions. An
5843: exception handler can be used to correct the problem, or to clean up
5844: some data structures and just throw the exception to the next exception
5845: handler. Note that @code{throw} jumps to the dynamically innermost
5846: exception handler. The system's exception handler is outermost, and just
5847: prints an error and restarts command-line interpretation (or, in batch
5848: mode (i.e., while processing the shell command line), leaves Gforth).
5849:
5850: The ANS Forth way to catch exceptions is @code{catch}:
5851:
5852: doc-catch
5853: doc-nothrow
5854:
5855: The most common use of exception handlers is to clean up the state when
5856: an error happens. E.g.,
5857:
5858: @example
5859: base @ >r hex \ actually the hex should be inside foo, or we h
5860: ['] foo catch ( nerror|0 )
5861: r> base !
5862: ( nerror|0 ) throw \ pass it on
5863: @end example
5864:
5865: A use of @code{catch} for handling the error @code{myerror} might look
5866: like this:
5867:
5868: @example
5869: ['] foo catch
5870: CASE
5871: myerror OF ... ( do something about it ) nothrow ENDOF
5872: dup throw \ default: pass other errors on, do nothing on non-errors
5873: ENDCASE
5874: @end example
5875:
5876: Having to wrap the code into a separate word is often cumbersome,
5877: therefore Gforth provides an alternative syntax:
5878:
5879: @example
5880: TRY
5881: @i{code1}
5882: IFERROR
5883: @i{code2}
5884: THEN
5885: @i{code3}
5886: ENDTRY
5887: @end example
5888:
5889: This performs @i{code1}. If @i{code1} completes normally, execution
5890: continues with @i{code3}. If there is an exception in @i{code1} or
5891: before @code{endtry}, the stacks are reset to the depth during
5892: @code{try}, the throw value is pushed on the data stack, and execution
5893: continues at @i{code2}, and finally falls through to @i{code3}.
5894:
5895: doc-try
5896: doc-endtry
5897: doc-iferror
5898:
5899: If you don't need @i{code2}, you can write @code{restore} instead of
5900: @code{iferror then}:
5901:
5902: @example
5903: TRY
5904: @i{code1}
5905: RESTORE
5906: @i{code3}
5907: ENDTRY
5908: @end example
5909:
5910: @cindex unwind-protect
5911: The cleanup example from above in this syntax:
5912:
5913: @example
5914: base @@ @{ oldbase @}
5915: TRY
5916: hex foo \ now the hex is placed correctly
5917: 0 \ value for throw
5918: RESTORE
5919: oldbase base !
5920: ENDTRY
5921: throw
5922: @end example
5923:
5924: An additional advantage of this variant is that an exception between
5925: @code{restore} and @code{endtry} (e.g., from the user pressing
5926: @kbd{Ctrl-C}) restarts the execution of the code after @code{restore},
5927: so the base will be restored under all circumstances.
5928:
5929: However, you have to ensure that this code does not cause an exception
5930: itself, otherwise the @code{iferror}/@code{restore} code will loop.
5931: Moreover, you should also make sure that the stack contents needed by
5932: the @code{iferror}/@code{restore} code exist everywhere between
5933: @code{try} and @code{endtry}; in our example this is achived by
5934: putting the data in a local before the @code{try} (you cannot use the
5935: return stack because the exception frame (@i{sys1}) is in the way
5936: there).
5937:
5938: This kind of usage corresponds to Lisp's @code{unwind-protect}.
5939:
5940: @cindex @code{recover} (old Gforth versions)
5941: If you do not want this exception-restarting behaviour, you achieve
5942: this as follows:
5943:
5944: @example
5945: TRY
5946: @i{code1}
5947: ENDTRY-IFERROR
5948: @i{code2}
5949: THEN
5950: @end example
5951:
5952: If there is an exception in @i{code1}, then @i{code2} is executed,
5953: otherwise execution continues behind the @code{then} (or in a possible
5954: @code{else} branch). This corresponds to the construct
5955:
5956: @example
5957: TRY
5958: @i{code1}
5959: RECOVER
5960: @i{code2}
5961: ENDTRY
5962: @end example
5963:
5964: in Gforth before version 0.7. So you can directly replace
5965: @code{recover}-using code; however, we recommend that you check if it
5966: would not be better to use one of the other @code{try} variants while
5967: you are at it.
5968:
5969: To ease the transition, Gforth provides two compatibility files:
5970: @file{endtry-iferror.fs} provides the @code{try ... endtry-iferror
5971: ... then} syntax (but not @code{iferror} or @code{restore}) for old
5972: systems; @file{recover-endtry.fs} provides the @code{try ... recover
5973: ... endtry} syntax on new systems, so you can use that file as a
5974: stopgap to run old programs. Both files work on any system (they just
5975: do nothing if the system already has the syntax it implements), so you
5976: can unconditionally @code{require} one of these files, even if you use
5977: a mix old and new systems.
5978:
5979: doc-restore
5980: doc-endtry-iferror
5981:
5982: Here's the error handling example:
5983:
5984: @example
5985: TRY
5986: foo
5987: ENDTRY-IFERROR
5988: CASE
5989: myerror OF ... ( do something about it ) nothrow ENDOF
5990: throw \ pass other errors on
5991: ENDCASE
5992: THEN
5993: @end example
5994:
5995: @progstyle
5996: As usual, you should ensure that the stack depth is statically known at
5997: the end: either after the @code{throw} for passing on errors, or after
5998: the @code{ENDTRY} (or, if you use @code{catch}, after the end of the
5999: selection construct for handling the error).
6000:
6001: There are two alternatives to @code{throw}: @code{Abort"} is conditional
6002: and you can provide an error message. @code{Abort} just produces an
6003: ``Aborted'' error.
6004:
6005: The problem with these words is that exception handlers cannot
6006: differentiate between different @code{abort"}s; they just look like
6007: @code{-2 throw} to them (the error message cannot be accessed by
6008: standard programs). Similar @code{abort} looks like @code{-1 throw} to
6009: exception handlers.
6010:
6011: doc-abort"
6012: doc-abort
6013:
6014:
6015:
6016: @c -------------------------------------------------------------
6017: @node Defining Words, Interpretation and Compilation Semantics, Control Structures, Words
6018: @section Defining Words
6019: @cindex defining words
6020:
6021: Defining words are used to extend Forth by creating new entries in the dictionary.
6022:
6023: @menu
6024: * CREATE::
6025: * Variables:: Variables and user variables
6026: * Constants::
6027: * Values:: Initialised variables
6028: * Colon Definitions::
6029: * Anonymous Definitions:: Definitions without names
6030: * Quotations::
6031: * Supplying names:: Passing definition names as strings
6032: * User-defined Defining Words::
6033: * Deferred Words:: Allow forward references
6034: * Aliases::
6035: @end menu
6036:
6037: @node CREATE, Variables, Defining Words, Defining Words
6038: @subsection @code{CREATE}
6039: @cindex simple defining words
6040: @cindex defining words, simple
6041:
6042: Defining words are used to create new entries in the dictionary. The
6043: simplest defining word is @code{CREATE}. @code{CREATE} is used like
6044: this:
6045:
6046: @example
6047: CREATE new-word1
6048: @end example
6049:
6050: @code{CREATE} is a parsing word, i.e., it takes an argument from the
6051: input stream (@code{new-word1} in our example). It generates a
6052: dictionary entry for @code{new-word1}. When @code{new-word1} is
6053: executed, all that it does is leave an address on the stack. The address
6054: represents the value of the data space pointer (@code{HERE}) at the time
6055: that @code{new-word1} was defined. Therefore, @code{CREATE} is a way of
6056: associating a name with the address of a region of memory.
6057:
6058: doc-create
6059:
6060: Note that in ANS Forth guarantees only for @code{create} that its body
6061: is in dictionary data space (i.e., where @code{here}, @code{allot}
6062: etc. work, @pxref{Dictionary allocation}). Also, in ANS Forth only
6063: @code{create}d words can be modified with @code{does>}
6064: (@pxref{User-defined Defining Words}). And in ANS Forth @code{>body}
6065: can only be applied to @code{create}d words.
6066:
6067: By extending this example to reserve some memory in data space, we end
6068: up with something like a @i{variable}. Here are two different ways to do
6069: it:
6070:
6071: @example
6072: CREATE new-word2 1 cells allot \ reserve 1 cell - initial value undefined
6073: CREATE new-word3 4 , \ reserve 1 cell and initialise it (to 4)
6074: @end example
6075:
6076: The variable can be examined and modified using @code{@@} (``fetch'') and
6077: @code{!} (``store'') like this:
6078:
6079: @example
6080: new-word2 @@ . \ get address, fetch from it and display
6081: 1234 new-word2 ! \ new value, get address, store to it
6082: @end example
6083:
6084: @cindex arrays
6085: A similar mechanism can be used to create arrays. For example, an
6086: 80-character text input buffer:
6087:
6088: @example
6089: CREATE text-buf 80 chars allot
6090:
6091: text-buf 0 chars + c@@ \ the 1st character (offset 0)
6092: text-buf 3 chars + c@@ \ the 4th character (offset 3)
6093: @end example
6094:
6095: You can build arbitrarily complex data structures by allocating
6096: appropriate areas of memory. For further discussions of this, and to
6097: learn about some Gforth tools that make it easier,
6098: @xref{Structures}.
6099:
6100:
6101: @node Variables, Constants, CREATE, Defining Words
6102: @subsection Variables
6103: @cindex variables
6104:
6105: The previous section showed how a sequence of commands could be used to
6106: generate a variable. As a final refinement, the whole code sequence can
6107: be wrapped up in a defining word (pre-empting the subject of the next
6108: section), making it easier to create new variables:
6109:
6110: @example
6111: : myvariableX ( "name" -- a-addr ) CREATE 1 cells allot ;
6112: : myvariable0 ( "name" -- a-addr ) CREATE 0 , ;
6113:
6114: myvariableX foo \ variable foo starts off with an unknown value
6115: myvariable0 joe \ whilst joe is initialised to 0
6116:
6117: 45 3 * foo ! \ set foo to 135
6118: 1234 joe ! \ set joe to 1234
6119: 3 joe +! \ increment joe by 3.. to 1237
6120: @end example
6121:
6122: Not surprisingly, there is no need to define @code{myvariable}, since
6123: Forth already has a definition @code{Variable}. ANS Forth does not
6124: guarantee that a @code{Variable} is initialised when it is created
6125: (i.e., it may behave like @code{myvariableX}). In contrast, Gforth's
6126: @code{Variable} initialises the variable to 0 (i.e., it behaves exactly
6127: like @code{myvariable0}). Forth also provides @code{2Variable} and
6128: @code{fvariable} for double and floating-point variables, respectively
6129: -- they are initialised to 0. and 0e in Gforth. If you use a @code{Variable} to
6130: store a boolean, you can use @code{on} and @code{off} to toggle its
6131: state.
6132:
6133: doc-variable
6134: doc-2variable
6135: doc-fvariable
6136:
6137: @cindex user variables
6138: @cindex user space
6139: The defining word @code{User} behaves in the same way as @code{Variable}.
6140: The difference is that it reserves space in @i{user (data) space} rather
6141: than normal data space. In a Forth system that has a multi-tasker, each
6142: task has its own set of user variables.
6143:
6144: doc-user
6145: @c doc-udp
6146: @c doc-uallot
6147:
6148: @comment TODO is that stuff about user variables strictly correct? Is it
6149: @comment just terminal tasks that have user variables?
6150: @comment should document tasker.fs (with some examples) elsewhere
6151: @comment in this manual, then expand on user space and user variables.
6152:
6153: @node Constants, Values, Variables, Defining Words
6154: @subsection Constants
6155: @cindex constants
6156:
6157: @code{Constant} allows you to declare a fixed value and refer to it by
6158: name. For example:
6159:
6160: @example
6161: 12 Constant INCHES-PER-FOOT
6162: 3E+08 fconstant SPEED-O-LIGHT
6163: @end example
6164:
6165: A @code{Variable} can be both read and written, so its run-time
6166: behaviour is to supply an address through which its current value can be
6167: manipulated. In contrast, the value of a @code{Constant} cannot be
6168: changed once it has been declared@footnote{Well, often it can be -- but
6169: not in a Standard, portable way. It's safer to use a @code{Value} (read
6170: on).} so it's not necessary to supply the address -- it is more
6171: efficient to return the value of the constant directly. That's exactly
6172: what happens; the run-time effect of a constant is to put its value on
6173: the top of the stack (You can find one
6174: way of implementing @code{Constant} in @ref{User-defined Defining Words}).
6175:
6176: Forth also provides @code{2Constant} and @code{fconstant} for defining
6177: double and floating-point constants, respectively.
6178:
6179: doc-constant
6180: doc-2constant
6181: doc-fconstant
6182:
6183: @c that's too deep, and it's not necessarily true for all ANS Forths. - anton
6184: @c nac-> How could that not be true in an ANS Forth? You can't define a
6185: @c constant, use it and then delete the definition of the constant..
6186:
6187: @c anton->An ANS Forth system can compile a constant to a literal; On
6188: @c decompilation you would see only the number, just as if it had been used
6189: @c in the first place. The word will stay, of course, but it will only be
6190: @c used by the text interpreter (no run-time duties, except when it is
6191: @c POSTPONEd or somesuch).
6192:
6193: @c nac:
6194: @c I agree that it's rather deep, but IMO it is an important difference
6195: @c relative to other programming languages.. often it's annoying: it
6196: @c certainly changes my programming style relative to C.
6197:
6198: @c anton: In what way?
6199:
6200: Constants in Forth behave differently from their equivalents in other
6201: programming languages. In other languages, a constant (such as an EQU in
6202: assembler or a #define in C) only exists at compile-time; in the
6203: executable program the constant has been translated into an absolute
6204: number and, unless you are using a symbolic debugger, it's impossible to
6205: know what abstract thing that number represents. In Forth a constant has
6206: an entry in the header space and remains there after the code that uses
6207: it has been defined. In fact, it must remain in the dictionary since it
6208: has run-time duties to perform. For example:
6209:
6210: @example
6211: 12 Constant INCHES-PER-FOOT
6212: : FEET-TO-INCHES ( n1 -- n2 ) INCHES-PER-FOOT * ;
6213: @end example
6214:
6215: @cindex in-lining of constants
6216: When @code{FEET-TO-INCHES} is executed, it will in turn execute the xt
6217: associated with the constant @code{INCHES-PER-FOOT}. If you use
6218: @code{see} to decompile the definition of @code{FEET-TO-INCHES}, you can
6219: see that it makes a call to @code{INCHES-PER-FOOT}. Some Forth compilers
6220: attempt to optimise constants by in-lining them where they are used. You
6221: can force Gforth to in-line a constant like this:
6222:
6223: @example
6224: : FEET-TO-INCHES ( n1 -- n2 ) [ INCHES-PER-FOOT ] LITERAL * ;
6225: @end example
6226:
6227: If you use @code{see} to decompile @i{this} version of
6228: @code{FEET-TO-INCHES}, you can see that @code{INCHES-PER-FOOT} is no
6229: longer present. To understand how this works, read
6230: @ref{Interpret/Compile states}, and @ref{Literals}.
6231:
6232: In-lining constants in this way might improve execution time
6233: fractionally, and can ensure that a constant is now only referenced at
6234: compile-time. However, the definition of the constant still remains in
6235: the dictionary. Some Forth compilers provide a mechanism for controlling
6236: a second dictionary for holding transient words such that this second
6237: dictionary can be deleted later in order to recover memory
6238: space. However, there is no standard way of doing this.
6239:
6240:
6241: @node Values, Colon Definitions, Constants, Defining Words
6242: @subsection Values
6243: @cindex values
6244:
6245: A @code{Value} behaves like a @code{Constant}, but it can be changed.
6246: @code{TO} is a parsing word that changes a @code{Values}. In Gforth
6247: (not in ANS Forth) you can access (and change) a @code{value} also with
6248: @code{>body}.
6249:
6250: Here are some
6251: examples:
6252:
6253: @example
6254: 12 Value APPLES \ Define APPLES with an initial value of 12
6255: 34 TO APPLES \ Change the value of APPLES. TO is a parsing word
6256: 1 ' APPLES >body +! \ Increment APPLES. Non-standard usage.
6257: APPLES \ puts 35 on the top of the stack.
6258: @end example
6259:
6260: doc-value
6261: doc-to
6262:
6263:
6264:
6265: @node Colon Definitions, Anonymous Definitions, Values, Defining Words
6266: @subsection Colon Definitions
6267: @cindex colon definitions
6268:
6269: @example
6270: : name ( ... -- ... )
6271: word1 word2 word3 ;
6272: @end example
6273:
6274: @noindent
6275: Creates a word called @code{name} that, upon execution, executes
6276: @code{word1 word2 word3}. @code{name} is a @dfn{(colon) definition}.
6277:
6278: The explanation above is somewhat superficial. For simple examples of
6279: colon definitions see @ref{Your first definition}. For an in-depth
6280: discussion of some of the issues involved, @xref{Interpretation and
6281: Compilation Semantics}.
6282:
6283: doc-:
6284: doc-;
6285:
6286:
6287: @node Anonymous Definitions, Quotations, Colon Definitions, Defining Words
6288: @subsection Anonymous Definitions
6289: @cindex colon definitions
6290: @cindex defining words without name
6291:
6292: Sometimes you want to define an @dfn{anonymous word}; a word without a
6293: name. You can do this with:
6294:
6295: doc-:noname
6296:
6297: This leaves the execution token for the word on the stack after the
6298: closing @code{;}. Here's an example in which a deferred word is
6299: initialised with an @code{xt} from an anonymous colon definition:
6300:
6301: @example
6302: Defer deferred
6303: :noname ( ... -- ... )
6304: ... ;
6305: IS deferred
6306: @end example
6307:
6308: @noindent
6309: Gforth provides an alternative way of doing this, using two separate
6310: words:
6311:
6312: doc-noname
6313: @cindex execution token of last defined word
6314: doc-latestxt
6315:
6316: @noindent
6317: The previous example can be rewritten using @code{noname} and
6318: @code{latestxt}:
6319:
6320: @example
6321: Defer deferred
6322: noname : ( ... -- ... )
6323: ... ;
6324: latestxt IS deferred
6325: @end example
6326:
6327: @noindent
6328: @code{noname} works with any defining word, not just @code{:}.
6329:
6330: @code{latestxt} also works when the last word was not defined as
6331: @code{noname}. It does not work for combined words, though. It also has
6332: the useful property that is is valid as soon as the header for a
6333: definition has been built. Thus:
6334:
6335: @example
6336: latestxt . : foo [ latestxt . ] ; ' foo .
6337: @end example
6338:
6339: @noindent
6340: prints 3 numbers; the last two are the same.
6341:
6342:
6343: @node Quotations, Supplying names, Anonymous Definitions, Defining Words
6344: @subsection Quotations
6345: @cindex quotations
6346: @cindex nested colon definitions
6347: @cindex colon definitions, nesting
6348:
6349: A quotation is an anonymous colon definition inside another colon
6350: definition. Quotations are useful when dealing with words that
6351: consume an execution token, like @code{catch} or
6352: @code{outfile-execute}. E.g. consider the following example of using
6353: @code{outfile-execute} (@pxref{Redirection}):
6354:
6355: @example
6356: : some-warning ( n -- )
6357: cr ." warning# " . ;
6358:
6359: : print-some-warning ( n -- )
6360: ['] some-warning stderr outfile-execute ;
6361: @end example
6362:
6363: Here we defined @code{some-warning} as a helper word whose xt we could
6364: pass to outfile-execute. Instead, we can use a quotation to define
6365: such a word anonymously inside @code{print-some-warning}:
6366:
6367: @example
6368: : print-some-warning ( n -- )
6369: [: cr ." warning# " . ;] stderr outfile-execute ;
6370: @end example
6371:
6372: The quotation is bouded by @code{[:} and @code{;]}. It produces an
6373: execution token at run-time.
6374:
6375: doc-[:
6376: doc-;]
6377:
6378:
6379: @node Supplying names, User-defined Defining Words, Quotations, Defining Words
6380: @subsection Supplying the name of a defined word
6381: @cindex names for defined words
6382: @cindex defining words, name given in a string
6383:
6384: By default, a defining word takes the name for the defined word from the
6385: input stream. Sometimes you want to supply the name from a string. You
6386: can do this with:
6387:
6388: doc-nextname
6389:
6390: For example:
6391:
6392: @example
6393: s" foo" nextname create
6394: @end example
6395:
6396: @noindent
6397: is equivalent to:
6398:
6399: @example
6400: create foo
6401: @end example
6402:
6403: @noindent
6404: @code{nextname} works with any defining word.
6405:
6406:
6407: @node User-defined Defining Words, Deferred Words, Supplying names, Defining Words
6408: @subsection User-defined Defining Words
6409: @cindex user-defined defining words
6410: @cindex defining words, user-defined
6411:
6412: You can create a new defining word by wrapping defining-time code around
6413: an existing defining word and putting the sequence in a colon
6414: definition.
6415:
6416: @c anton: This example is very complex and leads in a quite different
6417: @c direction from the CREATE-DOES> stuff that follows. It should probably
6418: @c be done elsewhere, or as a subsubsection of this subsection (or as a
6419: @c subsection of Defining Words)
6420:
6421: For example, suppose that you have a word @code{stats} that
6422: gathers statistics about colon definitions given the @i{xt} of the
6423: definition, and you want every colon definition in your application to
6424: make a call to @code{stats}. You can define and use a new version of
6425: @code{:} like this:
6426:
6427: @example
6428: : stats ( xt -- ) DUP ." (Gathering statistics for " . ." )"
6429: ... ; \ other code
6430:
6431: : my: : latestxt postpone literal ['] stats compile, ;
6432:
6433: my: foo + - ;
6434: @end example
6435:
6436: When @code{foo} is defined using @code{my:} these steps occur:
6437:
6438: @itemize @bullet
6439: @item
6440: @code{my:} is executed.
6441: @item
6442: The @code{:} within the definition (the one between @code{my:} and
6443: @code{latestxt}) is executed, and does just what it always does; it parses
6444: the input stream for a name, builds a dictionary header for the name
6445: @code{foo} and switches @code{state} from interpret to compile.
6446: @item
6447: The word @code{latestxt} is executed. It puts the @i{xt} for the word that is
6448: being defined -- @code{foo} -- onto the stack.
6449: @item
6450: The code that was produced by @code{postpone literal} is executed; this
6451: causes the value on the stack to be compiled as a literal in the code
6452: area of @code{foo}.
6453: @item
6454: The code @code{['] stats} compiles a literal into the definition of
6455: @code{my:}. When @code{compile,} is executed, that literal -- the
6456: execution token for @code{stats} -- is layed down in the code area of
6457: @code{foo} , following the literal@footnote{Strictly speaking, the
6458: mechanism that @code{compile,} uses to convert an @i{xt} into something
6459: in the code area is implementation-dependent. A threaded implementation
6460: might spit out the execution token directly whilst another
6461: implementation might spit out a native code sequence.}.
6462: @item
6463: At this point, the execution of @code{my:} is complete, and control
6464: returns to the text interpreter. The text interpreter is in compile
6465: state, so subsequent text @code{+ -} is compiled into the definition of
6466: @code{foo} and the @code{;} terminates the definition as always.
6467: @end itemize
6468:
6469: You can use @code{see} to decompile a word that was defined using
6470: @code{my:} and see how it is different from a normal @code{:}
6471: definition. For example:
6472:
6473: @example
6474: : bar + - ; \ like foo but using : rather than my:
6475: see bar
6476: : bar
6477: + - ;
6478: see foo
6479: : foo
6480: 107645672 stats + - ;
6481:
6482: \ use ' foo . to show that 107645672 is the xt for foo
6483: @end example
6484:
6485: You can use techniques like this to make new defining words in terms of
6486: @i{any} existing defining word.
6487:
6488:
6489: @cindex defining defining words
6490: @cindex @code{CREATE} ... @code{DOES>}
6491: If you want the words defined with your defining words to behave
6492: differently from words defined with standard defining words, you can
6493: write your defining word like this:
6494:
6495: @example
6496: : def-word ( "name" -- )
6497: CREATE @i{code1}
6498: DOES> ( ... -- ... )
6499: @i{code2} ;
6500:
6501: def-word name
6502: @end example
6503:
6504: @cindex child words
6505: This fragment defines a @dfn{defining word} @code{def-word} and then
6506: executes it. When @code{def-word} executes, it @code{CREATE}s a new
6507: word, @code{name}, and executes the code @i{code1}. The code @i{code2}
6508: is not executed at this time. The word @code{name} is sometimes called a
6509: @dfn{child} of @code{def-word}.
6510:
6511: When you execute @code{name}, the address of the body of @code{name} is
6512: put on the data stack and @i{code2} is executed (the address of the body
6513: of @code{name} is the address @code{HERE} returns immediately after the
6514: @code{CREATE}, i.e., the address a @code{create}d word returns by
6515: default).
6516:
6517: @c anton:
6518: @c www.dictionary.com says:
6519: @c at·a·vism: 1.The reappearance of a characteristic in an organism after
6520: @c several generations of absence, usually caused by the chance
6521: @c recombination of genes. 2.An individual or a part that exhibits
6522: @c atavism. Also called throwback. 3.The return of a trait or recurrence
6523: @c of previous behavior after a period of absence.
6524: @c
6525: @c Doesn't seem to fit.
6526:
6527: @c @cindex atavism in child words
6528: You can use @code{def-word} to define a set of child words that behave
6529: similarly; they all have a common run-time behaviour determined by
6530: @i{code2}. Typically, the @i{code1} sequence builds a data area in the
6531: body of the child word. The structure of the data is common to all
6532: children of @code{def-word}, but the data values are specific -- and
6533: private -- to each child word. When a child word is executed, the
6534: address of its private data area is passed as a parameter on TOS to be
6535: used and manipulated@footnote{It is legitimate both to read and write to
6536: this data area.} by @i{code2}.
6537:
6538: The two fragments of code that make up the defining words act (are
6539: executed) at two completely separate times:
6540:
6541: @itemize @bullet
6542: @item
6543: At @i{define time}, the defining word executes @i{code1} to generate a
6544: child word
6545: @item
6546: At @i{child execution time}, when a child word is invoked, @i{code2}
6547: is executed, using parameters (data) that are private and specific to
6548: the child word.
6549: @end itemize
6550:
6551: Another way of understanding the behaviour of @code{def-word} and
6552: @code{name} is to say that, if you make the following definitions:
6553: @example
6554: : def-word1 ( "name" -- )
6555: CREATE @i{code1} ;
6556:
6557: : action1 ( ... -- ... )
6558: @i{code2} ;
6559:
6560: def-word1 name1
6561: @end example
6562:
6563: @noindent
6564: Then using @code{name1 action1} is equivalent to using @code{name}.
6565:
6566: The classic example is that you can define @code{CONSTANT} in this way:
6567:
6568: @example
6569: : CONSTANT ( w "name" -- )
6570: CREATE ,
6571: DOES> ( -- w )
6572: @@ ;
6573: @end example
6574:
6575: @comment There is a beautiful description of how this works and what
6576: @comment it does in the Forthwrite 100th edition.. as well as an elegant
6577: @comment commentary on the Counting Fruits problem.
6578:
6579: When you create a constant with @code{5 CONSTANT five}, a set of
6580: define-time actions take place; first a new word @code{five} is created,
6581: then the value 5 is laid down in the body of @code{five} with
6582: @code{,}. When @code{five} is executed, the address of the body is put on
6583: the stack, and @code{@@} retrieves the value 5. The word @code{five} has
6584: no code of its own; it simply contains a data field and a pointer to the
6585: code that follows @code{DOES>} in its defining word. That makes words
6586: created in this way very compact.
6587:
6588: The final example in this section is intended to remind you that space
6589: reserved in @code{CREATE}d words is @i{data} space and therefore can be
6590: both read and written by a Standard program@footnote{Exercise: use this
6591: example as a starting point for your own implementation of @code{Value}
6592: and @code{TO} -- if you get stuck, investigate the behaviour of @code{'} and
6593: @code{[']}.}:
6594:
6595: @example
6596: : foo ( "name" -- )
6597: CREATE -1 ,
6598: DOES> ( -- )
6599: @@ . ;
6600:
6601: foo first-word
6602: foo second-word
6603:
6604: 123 ' first-word >BODY !
6605: @end example
6606:
6607: If @code{first-word} had been a @code{CREATE}d word, we could simply
6608: have executed it to get the address of its data field. However, since it
6609: was defined to have @code{DOES>} actions, its execution semantics are to
6610: perform those @code{DOES>} actions. To get the address of its data field
6611: it's necessary to use @code{'} to get its xt, then @code{>BODY} to
6612: translate the xt into the address of the data field. When you execute
6613: @code{first-word}, it will display @code{123}. When you execute
6614: @code{second-word} it will display @code{-1}.
6615:
6616: @cindex stack effect of @code{DOES>}-parts
6617: @cindex @code{DOES>}-parts, stack effect
6618: In the examples above the stack comment after the @code{DOES>} specifies
6619: the stack effect of the defined words, not the stack effect of the
6620: following code (the following code expects the address of the body on
6621: the top of stack, which is not reflected in the stack comment). This is
6622: the convention that I use and recommend (it clashes a bit with using
6623: locals declarations for stack effect specification, though).
6624:
6625: @menu
6626: * CREATE..DOES> applications::
6627: * CREATE..DOES> details::
6628: * Advanced does> usage example::
6629: * Const-does>::
6630: @end menu
6631:
6632: @node CREATE..DOES> applications, CREATE..DOES> details, User-defined Defining Words, User-defined Defining Words
6633: @subsubsection Applications of @code{CREATE..DOES>}
6634: @cindex @code{CREATE} ... @code{DOES>}, applications
6635:
6636: You may wonder how to use this feature. Here are some usage patterns:
6637:
6638: @cindex factoring similar colon definitions
6639: When you see a sequence of code occurring several times, and you can
6640: identify a meaning, you will factor it out as a colon definition. When
6641: you see similar colon definitions, you can factor them using
6642: @code{CREATE..DOES>}. E.g., an assembler usually defines several words
6643: that look very similar:
6644: @example
6645: : ori, ( reg-target reg-source n -- )
6646: 0 asm-reg-reg-imm ;
6647: : andi, ( reg-target reg-source n -- )
6648: 1 asm-reg-reg-imm ;
6649: @end example
6650:
6651: @noindent
6652: This could be factored with:
6653: @example
6654: : reg-reg-imm ( op-code -- )
6655: CREATE ,
6656: DOES> ( reg-target reg-source n -- )
6657: @@ asm-reg-reg-imm ;
6658:
6659: 0 reg-reg-imm ori,
6660: 1 reg-reg-imm andi,
6661: @end example
6662:
6663: @cindex currying
6664: Another view of @code{CREATE..DOES>} is to consider it as a crude way to
6665: supply a part of the parameters for a word (known as @dfn{currying} in
6666: the functional language community). E.g., @code{+} needs two
6667: parameters. Creating versions of @code{+} with one parameter fixed can
6668: be done like this:
6669:
6670: @example
6671: : curry+ ( n1 "name" -- )
6672: CREATE ,
6673: DOES> ( n2 -- n1+n2 )
6674: @@ + ;
6675:
6676: 3 curry+ 3+
6677: -2 curry+ 2-
6678: @end example
6679:
6680:
6681: @node CREATE..DOES> details, Advanced does> usage example, CREATE..DOES> applications, User-defined Defining Words
6682: @subsubsection The gory details of @code{CREATE..DOES>}
6683: @cindex @code{CREATE} ... @code{DOES>}, details
6684:
6685: doc-does>
6686:
6687: @cindex @code{DOES>} in a separate definition
6688: This means that you need not use @code{CREATE} and @code{DOES>} in the
6689: same definition; you can put the @code{DOES>}-part in a separate
6690: definition. This allows us to, e.g., select among different @code{DOES>}-parts:
6691: @example
6692: : does1
6693: DOES> ( ... -- ... )
6694: ... ;
6695:
6696: : does2
6697: DOES> ( ... -- ... )
6698: ... ;
6699:
6700: : def-word ( ... -- ... )
6701: create ...
6702: IF
6703: does1
6704: ELSE
6705: does2
6706: ENDIF ;
6707: @end example
6708:
6709: In this example, the selection of whether to use @code{does1} or
6710: @code{does2} is made at definition-time; at the time that the child word is
6711: @code{CREATE}d.
6712:
6713: @cindex @code{DOES>} in interpretation state
6714: In a standard program you can apply a @code{DOES>}-part only if the last
6715: word was defined with @code{CREATE}. In Gforth, the @code{DOES>}-part
6716: will override the behaviour of the last word defined in any case. In a
6717: standard program, you can use @code{DOES>} only in a colon
6718: definition. In Gforth, you can also use it in interpretation state, in a
6719: kind of one-shot mode; for example:
6720: @example
6721: CREATE name ( ... -- ... )
6722: @i{initialization}
6723: DOES>
6724: @i{code} ;
6725: @end example
6726:
6727: @noindent
6728: is equivalent to the standard:
6729: @example
6730: :noname
6731: DOES>
6732: @i{code} ;
6733: CREATE name EXECUTE ( ... -- ... )
6734: @i{initialization}
6735: @end example
6736:
6737: doc->body
6738:
6739: @node Advanced does> usage example, Const-does>, CREATE..DOES> details, User-defined Defining Words
6740: @subsubsection Advanced does> usage example
6741:
6742: The MIPS disassembler (@file{arch/mips/disasm.fs}) contains many words
6743: for disassembling instructions, that follow a very repetetive scheme:
6744:
6745: @example
6746: :noname @var{disasm-operands} s" @var{inst-name}" type ;
6747: @var{entry-num} cells @var{table} + !
6748: @end example
6749:
6750: Of course, this inspires the idea to factor out the commonalities to
6751: allow a definition like
6752:
6753: @example
6754: @var{disasm-operands} @var{entry-num} @var{table} define-inst @var{inst-name}
6755: @end example
6756:
6757: The parameters @var{disasm-operands} and @var{table} are usually
6758: correlated. Moreover, before I wrote the disassembler, there already
6759: existed code that defines instructions like this:
6760:
6761: @example
6762: @var{entry-num} @var{inst-format} @var{inst-name}
6763: @end example
6764:
6765: This code comes from the assembler and resides in
6766: @file{arch/mips/insts.fs}.
6767:
6768: So I had to define the @var{inst-format} words that performed the scheme
6769: above when executed. At first I chose to use run-time code-generation:
6770:
6771: @example
6772: : @var{inst-format} ( entry-num "name" -- ; compiled code: addr w -- )
6773: :noname Postpone @var{disasm-operands}
6774: name Postpone sliteral Postpone type Postpone ;
6775: swap cells @var{table} + ! ;
6776: @end example
6777:
6778: Note that this supplies the other two parameters of the scheme above.
6779:
6780: An alternative would have been to write this using
6781: @code{create}/@code{does>}:
6782:
6783: @example
6784: : @var{inst-format} ( entry-num "name" -- )
6785: here name string, ( entry-num c-addr ) \ parse and save "name"
6786: noname create , ( entry-num )
6787: latestxt swap cells @var{table} + !
6788: does> ( addr w -- )
6789: \ disassemble instruction w at addr
6790: @@ >r
6791: @var{disasm-operands}
6792: r> count type ;
6793: @end example
6794:
6795: Somehow the first solution is simpler, mainly because it's simpler to
6796: shift a string from definition-time to use-time with @code{sliteral}
6797: than with @code{string,} and friends.
6798:
6799: I wrote a lot of words following this scheme and soon thought about
6800: factoring out the commonalities among them. Note that this uses a
6801: two-level defining word, i.e., a word that defines ordinary defining
6802: words.
6803:
6804: This time a solution involving @code{postpone} and friends seemed more
6805: difficult (try it as an exercise), so I decided to use a
6806: @code{create}/@code{does>} word; since I was already at it, I also used
6807: @code{create}/@code{does>} for the lower level (try using
6808: @code{postpone} etc. as an exercise), resulting in the following
6809: definition:
6810:
6811: @example
6812: : define-format ( disasm-xt table-xt -- )
6813: \ define an instruction format that uses disasm-xt for
6814: \ disassembling and enters the defined instructions into table
6815: \ table-xt
6816: create 2,
6817: does> ( u "inst" -- )
6818: \ defines an anonymous word for disassembling instruction inst,
6819: \ and enters it as u-th entry into table-xt
6820: 2@@ swap here name string, ( u table-xt disasm-xt c-addr ) \ remember string
6821: noname create 2, \ define anonymous word
6822: execute latestxt swap ! \ enter xt of defined word into table-xt
6823: does> ( addr w -- )
6824: \ disassemble instruction w at addr
6825: 2@@ >r ( addr w disasm-xt R: c-addr )
6826: execute ( R: c-addr ) \ disassemble operands
6827: r> count type ; \ print name
6828: @end example
6829:
6830: Note that the tables here (in contrast to above) do the @code{cells +}
6831: by themselves (that's why you have to pass an xt). This word is used in
6832: the following way:
6833:
6834: @example
6835: ' @var{disasm-operands} ' @var{table} define-format @var{inst-format}
6836: @end example
6837:
6838: As shown above, the defined instruction format is then used like this:
6839:
6840: @example
6841: @var{entry-num} @var{inst-format} @var{inst-name}
6842: @end example
6843:
6844: In terms of currying, this kind of two-level defining word provides the
6845: parameters in three stages: first @var{disasm-operands} and @var{table},
6846: then @var{entry-num} and @var{inst-name}, finally @code{addr w}, i.e.,
6847: the instruction to be disassembled.
6848:
6849: Of course this did not quite fit all the instruction format names used
6850: in @file{insts.fs}, so I had to define a few wrappers that conditioned
6851: the parameters into the right form.
6852:
6853: If you have trouble following this section, don't worry. First, this is
6854: involved and takes time (and probably some playing around) to
6855: understand; second, this is the first two-level
6856: @code{create}/@code{does>} word I have written in seventeen years of
6857: Forth; and if I did not have @file{insts.fs} to start with, I may well
6858: have elected to use just a one-level defining word (with some repeating
6859: of parameters when using the defining word). So it is not necessary to
6860: understand this, but it may improve your understanding of Forth.
6861:
6862:
6863: @node Const-does>, , Advanced does> usage example, User-defined Defining Words
6864: @subsubsection @code{Const-does>}
6865:
6866: A frequent use of @code{create}...@code{does>} is for transferring some
6867: values from definition-time to run-time. Gforth supports this use with
6868:
6869: doc-const-does>
6870:
6871: A typical use of this word is:
6872:
6873: @example
6874: : curry+ ( n1 "name" -- )
6875: 1 0 CONST-DOES> ( n2 -- n1+n2 )
6876: + ;
6877:
6878: 3 curry+ 3+
6879: @end example
6880:
6881: Here the @code{1 0} means that 1 cell and 0 floats are transferred from
6882: definition to run-time.
6883:
6884: The advantages of using @code{const-does>} are:
6885:
6886: @itemize
6887:
6888: @item
6889: You don't have to deal with storing and retrieving the values, i.e.,
6890: your program becomes more writable and readable.
6891:
6892: @item
6893: When using @code{does>}, you have to introduce a @code{@@} that cannot
6894: be optimized away (because you could change the data using
6895: @code{>body}...@code{!}); @code{const-does>} avoids this problem.
6896:
6897: @end itemize
6898:
6899: An ANS Forth implementation of @code{const-does>} is available in
6900: @file{compat/const-does.fs}.
6901:
6902:
6903: @node Deferred Words, Aliases, User-defined Defining Words, Defining Words
6904: @subsection Deferred Words
6905: @cindex deferred words
6906:
6907: The defining word @code{Defer} allows you to define a word by name
6908: without defining its behaviour; the definition of its behaviour is
6909: deferred. Here are two situation where this can be useful:
6910:
6911: @itemize @bullet
6912: @item
6913: Where you want to allow the behaviour of a word to be altered later, and
6914: for all precompiled references to the word to change when its behaviour
6915: is changed.
6916: @item
6917: For mutual recursion; @xref{Calls and returns}.
6918: @end itemize
6919:
6920: In the following example, @code{foo} always invokes the version of
6921: @code{greet} that prints ``@code{Good morning}'' whilst @code{bar}
6922: always invokes the version that prints ``@code{Hello}''. There is no way
6923: of getting @code{foo} to use the later version without re-ordering the
6924: source code and recompiling it.
6925:
6926: @example
6927: : greet ." Good morning" ;
6928: : foo ... greet ... ;
6929: : greet ." Hello" ;
6930: : bar ... greet ... ;
6931: @end example
6932:
6933: This problem can be solved by defining @code{greet} as a @code{Defer}red
6934: word. The behaviour of a @code{Defer}red word can be defined and
6935: redefined at any time by using @code{IS} to associate the xt of a
6936: previously-defined word with it. The previous example becomes:
6937:
6938: @example
6939: Defer greet ( -- )
6940: : foo ... greet ... ;
6941: : bar ... greet ... ;
6942: : greet1 ( -- ) ." Good morning" ;
6943: : greet2 ( -- ) ." Hello" ;
6944: ' greet2 IS greet \ make greet behave like greet2
6945: @end example
6946:
6947: @progstyle
6948: You should write a stack comment for every deferred word, and put only
6949: XTs into deferred words that conform to this stack effect. Otherwise
6950: it's too difficult to use the deferred word.
6951:
6952: A deferred word can be used to improve the statistics-gathering example
6953: from @ref{User-defined Defining Words}; rather than edit the
6954: application's source code to change every @code{:} to a @code{my:}, do
6955: this:
6956:
6957: @example
6958: : real: : ; \ retain access to the original
6959: defer : \ redefine as a deferred word
6960: ' my: IS : \ use special version of :
6961: \
6962: \ load application here
6963: \
6964: ' real: IS : \ go back to the original
6965: @end example
6966:
6967:
6968: One thing to note is that @code{IS} has special compilation semantics,
6969: such that it parses the name at compile time (like @code{TO}):
6970:
6971: @example
6972: : set-greet ( xt -- )
6973: IS greet ;
6974:
6975: ' greet1 set-greet
6976: @end example
6977:
6978: In situations where @code{IS} does not fit, use @code{defer!} instead.
6979:
6980: A deferred word can only inherit execution semantics from the xt
6981: (because that is all that an xt can represent -- for more discussion of
6982: this @pxref{Tokens for Words}); by default it will have default
6983: interpretation and compilation semantics deriving from this execution
6984: semantics. However, you can change the interpretation and compilation
6985: semantics of the deferred word in the usual ways:
6986:
6987: @example
6988: : bar .... ; immediate
6989: Defer fred immediate
6990: Defer jim
6991:
6992: ' bar IS jim \ jim has default semantics
6993: ' bar IS fred \ fred is immediate
6994: @end example
6995:
6996: doc-defer
6997: doc-defer!
6998: doc-is
6999: doc-defer@
7000: doc-action-of
7001: @comment TODO document these: what's defers [is]
7002: doc-defers
7003:
7004: @c Use @code{words-deferred} to see a list of deferred words.
7005:
7006: Definitions of these words (except @code{defers}) in ANS Forth are
7007: provided in @file{compat/defer.fs}.
7008:
7009:
7010: @node Aliases, , Deferred Words, Defining Words
7011: @subsection Aliases
7012: @cindex aliases
7013:
7014: The defining word @code{Alias} allows you to define a word by name that
7015: has the same behaviour as some other word. Here are two situation where
7016: this can be useful:
7017:
7018: @itemize @bullet
7019: @item
7020: When you want access to a word's definition from a different word list
7021: (for an example of this, see the definition of the @code{Root} word list
7022: in the Gforth source).
7023: @item
7024: When you want to create a synonym; a definition that can be known by
7025: either of two names (for example, @code{THEN} and @code{ENDIF} are
7026: aliases).
7027: @end itemize
7028:
7029: Like deferred words, an alias has default compilation and interpretation
7030: semantics at the beginning (not the modifications of the other word),
7031: but you can change them in the usual ways (@code{immediate},
7032: @code{compile-only}). For example:
7033:
7034: @example
7035: : foo ... ; immediate
7036:
7037: ' foo Alias bar \ bar is not an immediate word
7038: ' foo Alias fooby immediate \ fooby is an immediate word
7039: @end example
7040:
7041: Words that are aliases have the same xt, different headers in the
7042: dictionary, and consequently different name tokens (@pxref{Tokens for
7043: Words}) and possibly different immediate flags. An alias can only have
7044: default or immediate compilation semantics; you can define aliases for
7045: combined words with @code{interpret/compile:} -- see @ref{Combined words}.
7046:
7047: doc-alias
7048:
7049:
7050: @node Interpretation and Compilation Semantics, Tokens for Words, Defining Words, Words
7051: @section Interpretation and Compilation Semantics
7052: @cindex semantics, interpretation and compilation
7053:
7054: @c !! state and ' are used without explanation
7055: @c example for immediate/compile-only? or is the tutorial enough
7056:
7057: @cindex interpretation semantics
7058: The @dfn{interpretation semantics} of a (named) word are what the text
7059: interpreter does when it encounters the word in interpret state. It also
7060: appears in some other contexts, e.g., the execution token returned by
7061: @code{' @i{word}} identifies the interpretation semantics of @i{word}
7062: (in other words, @code{' @i{word} execute} is equivalent to
7063: interpret-state text interpretation of @code{@i{word}}).
7064:
7065: @cindex compilation semantics
7066: The @dfn{compilation semantics} of a (named) word are what the text
7067: interpreter does when it encounters the word in compile state. It also
7068: appears in other contexts, e.g, @code{POSTPONE @i{word}}
7069: compiles@footnote{In standard terminology, ``appends to the current
7070: definition''.} the compilation semantics of @i{word}.
7071:
7072: @cindex execution semantics
7073: The standard also talks about @dfn{execution semantics}. They are used
7074: only for defining the interpretation and compilation semantics of many
7075: words. By default, the interpretation semantics of a word are to
7076: @code{execute} its execution semantics, and the compilation semantics of
7077: a word are to @code{compile,} its execution semantics.@footnote{In
7078: standard terminology: The default interpretation semantics are its
7079: execution semantics; the default compilation semantics are to append its
7080: execution semantics to the execution semantics of the current
7081: definition.}
7082:
7083: Unnamed words (@pxref{Anonymous Definitions}) cannot be encountered by
7084: the text interpreter, ticked, or @code{postpone}d, so they have no
7085: interpretation or compilation semantics. Their behaviour is represented
7086: by their XT (@pxref{Tokens for Words}), and we call it execution
7087: semantics, too.
7088:
7089: @comment TODO expand, make it co-operate with new sections on text interpreter.
7090:
7091: @cindex immediate words
7092: @cindex compile-only words
7093: You can change the semantics of the most-recently defined word:
7094:
7095:
7096: doc-immediate
7097: doc-compile-only
7098: doc-restrict
7099:
7100: By convention, words with non-default compilation semantics (e.g.,
7101: immediate words) often have names surrounded with brackets (e.g.,
7102: @code{[']}, @pxref{Execution token}).
7103:
7104: Note that ticking (@code{'}) a compile-only word gives an error
7105: (``Interpreting a compile-only word'').
7106:
7107: @menu
7108: * Combined words::
7109: @end menu
7110:
7111:
7112: @node Combined words, , Interpretation and Compilation Semantics, Interpretation and Compilation Semantics
7113: @subsection Combined Words
7114: @cindex combined words
7115:
7116: Gforth allows you to define @dfn{combined words} -- words that have an
7117: arbitrary combination of interpretation and compilation semantics.
7118:
7119: doc-interpret/compile:
7120:
7121: This feature was introduced for implementing @code{TO} and @code{S"}. I
7122: recommend that you do not define such words, as cute as they may be:
7123: they make it hard to get at both parts of the word in some contexts.
7124: E.g., assume you want to get an execution token for the compilation
7125: part. Instead, define two words, one that embodies the interpretation
7126: part, and one that embodies the compilation part. Once you have done
7127: that, you can define a combined word with @code{interpret/compile:} for
7128: the convenience of your users.
7129:
7130: You might try to use this feature to provide an optimizing
7131: implementation of the default compilation semantics of a word. For
7132: example, by defining:
7133: @example
7134: :noname
7135: foo bar ;
7136: :noname
7137: POSTPONE foo POSTPONE bar ;
7138: interpret/compile: opti-foobar
7139: @end example
7140:
7141: @noindent
7142: as an optimizing version of:
7143:
7144: @example
7145: : foobar
7146: foo bar ;
7147: @end example
7148:
7149: Unfortunately, this does not work correctly with @code{[compile]},
7150: because @code{[compile]} assumes that the compilation semantics of all
7151: @code{interpret/compile:} words are non-default. I.e., @code{[compile]
7152: opti-foobar} would compile compilation semantics, whereas
7153: @code{[compile] foobar} would compile interpretation semantics.
7154:
7155: @cindex state-smart words (are a bad idea)
7156: @anchor{state-smartness}
7157: Some people try to use @dfn{state-smart} words to emulate the feature provided
7158: by @code{interpret/compile:} (words are state-smart if they check
7159: @code{STATE} during execution). E.g., they would try to code
7160: @code{foobar} like this:
7161:
7162: @example
7163: : foobar
7164: STATE @@
7165: IF ( compilation state )
7166: POSTPONE foo POSTPONE bar
7167: ELSE
7168: foo bar
7169: ENDIF ; immediate
7170: @end example
7171:
7172: Although this works if @code{foobar} is only processed by the text
7173: interpreter, it does not work in other contexts (like @code{'} or
7174: @code{POSTPONE}). E.g., @code{' foobar} will produce an execution token
7175: for a state-smart word, not for the interpretation semantics of the
7176: original @code{foobar}; when you execute this execution token (directly
7177: with @code{EXECUTE} or indirectly through @code{COMPILE,}) in compile
7178: state, the result will not be what you expected (i.e., it will not
7179: perform @code{foo bar}). State-smart words are a bad idea. Simply don't
7180: write them@footnote{For a more detailed discussion of this topic, see
7181: M. Anton Ertl,
7182: @cite{@uref{http://www.complang.tuwien.ac.at/papers/ertl98.ps.gz,@code{State}-smartness---Why
7183: it is Evil and How to Exorcise it}}, EuroForth '98.}!
7184:
7185: @cindex defining words with arbitrary semantics combinations
7186: It is also possible to write defining words that define words with
7187: arbitrary combinations of interpretation and compilation semantics. In
7188: general, they look like this:
7189:
7190: @example
7191: : def-word
7192: create-interpret/compile
7193: @i{code1}
7194: interpretation>
7195: @i{code2}
7196: <interpretation
7197: compilation>
7198: @i{code3}
7199: <compilation ;
7200: @end example
7201:
7202: For a @i{word} defined with @code{def-word}, the interpretation
7203: semantics are to push the address of the body of @i{word} and perform
7204: @i{code2}, and the compilation semantics are to push the address of
7205: the body of @i{word} and perform @i{code3}. E.g., @code{constant}
7206: can also be defined like this (except that the defined constants don't
7207: behave correctly when @code{[compile]}d):
7208:
7209: @example
7210: : constant ( n "name" -- )
7211: create-interpret/compile
7212: ,
7213: interpretation> ( -- n )
7214: @@
7215: <interpretation
7216: compilation> ( compilation. -- ; run-time. -- n )
7217: @@ postpone literal
7218: <compilation ;
7219: @end example
7220:
7221:
7222: doc-create-interpret/compile
7223: doc-interpretation>
7224: doc-<interpretation
7225: doc-compilation>
7226: doc-<compilation
7227:
7228:
7229: Words defined with @code{interpret/compile:} and
7230: @code{create-interpret/compile} have an extended header structure that
7231: differs from other words; however, unless you try to access them with
7232: plain address arithmetic, you should not notice this. Words for
7233: accessing the header structure usually know how to deal with this; e.g.,
7234: @code{'} @i{word} @code{>body} also gives you the body of a word created
7235: with @code{create-interpret/compile}.
7236:
7237:
7238: @c -------------------------------------------------------------
7239: @node Tokens for Words, Compiling words, Interpretation and Compilation Semantics, Words
7240: @section Tokens for Words
7241: @cindex tokens for words
7242:
7243: This section describes the creation and use of tokens that represent
7244: words.
7245:
7246: @menu
7247: * Execution token:: represents execution/interpretation semantics
7248: * Compilation token:: represents compilation semantics
7249: * Name token:: represents named words
7250: @end menu
7251:
7252: @node Execution token, Compilation token, Tokens for Words, Tokens for Words
7253: @subsection Execution token
7254:
7255: @cindex xt
7256: @cindex execution token
7257: An @dfn{execution token} (@i{XT}) represents some behaviour of a word.
7258: You can use @code{execute} to invoke this behaviour.
7259:
7260: @cindex tick (')
7261: You can use @code{'} to get an execution token that represents the
7262: interpretation semantics of a named word:
7263:
7264: @example
7265: 5 ' . ( n xt )
7266: execute ( ) \ execute the xt (i.e., ".")
7267: @end example
7268:
7269: doc-'
7270:
7271: @code{'} parses at run-time; there is also a word @code{[']} that parses
7272: when it is compiled, and compiles the resulting XT:
7273:
7274: @example
7275: : foo ['] . execute ;
7276: 5 foo
7277: : bar ' execute ; \ by contrast,
7278: 5 bar . \ ' parses "." when bar executes
7279: @end example
7280:
7281: doc-[']
7282:
7283: If you want the execution token of @i{word}, write @code{['] @i{word}}
7284: in compiled code and @code{' @i{word}} in interpreted code. Gforth's
7285: @code{'} and @code{[']} behave somewhat unusually by complaining about
7286: compile-only words (because these words have no interpretation
7287: semantics). You might get what you want by using @code{COMP' @i{word}
7288: DROP} or @code{[COMP'] @i{word} DROP} (for details @pxref{Compilation
7289: token}).
7290:
7291: Another way to get an XT is @code{:noname} or @code{latestxt}
7292: (@pxref{Anonymous Definitions}). For anonymous words this gives an xt
7293: for the only behaviour the word has (the execution semantics). For
7294: named words, @code{latestxt} produces an XT for the same behaviour it
7295: would produce if the word was defined anonymously.
7296:
7297: @example
7298: :noname ." hello" ;
7299: execute
7300: @end example
7301:
7302: An XT occupies one cell and can be manipulated like any other cell.
7303:
7304: @cindex code field address
7305: @cindex CFA
7306: In ANS Forth the XT is just an abstract data type (i.e., defined by the
7307: operations that produce or consume it). For old hands: In Gforth, the
7308: XT is implemented as a code field address (CFA).
7309:
7310: doc-execute
7311: doc-perform
7312:
7313: @node Compilation token, Name token, Execution token, Tokens for Words
7314: @subsection Compilation token
7315:
7316: @cindex compilation token
7317: @cindex CT (compilation token)
7318: Gforth represents the compilation semantics of a named word by a
7319: @dfn{compilation token} consisting of two cells: @i{w xt}. The top cell
7320: @i{xt} is an execution token. The compilation semantics represented by
7321: the compilation token can be performed with @code{execute}, which
7322: consumes the whole compilation token, with an additional stack effect
7323: determined by the represented compilation semantics.
7324:
7325: At present, the @i{w} part of a compilation token is an execution token,
7326: and the @i{xt} part represents either @code{execute} or
7327: @code{compile,}@footnote{Depending upon the compilation semantics of the
7328: word. If the word has default compilation semantics, the @i{xt} will
7329: represent @code{compile,}. Otherwise (e.g., for immediate words), the
7330: @i{xt} will represent @code{execute}.}. However, don't rely on that
7331: knowledge, unless necessary; future versions of Gforth may introduce
7332: unusual compilation tokens (e.g., a compilation token that represents
7333: the compilation semantics of a literal).
7334:
7335: You can perform the compilation semantics represented by the compilation
7336: token with @code{execute}. You can compile the compilation semantics
7337: with @code{postpone,}. I.e., @code{COMP' @i{word} postpone,} is
7338: equivalent to @code{postpone @i{word}}.
7339:
7340: doc-[comp']
7341: doc-comp'
7342: doc-postpone,
7343:
7344: @node Name token, , Compilation token, Tokens for Words
7345: @subsection Name token
7346:
7347: @cindex name token
7348: Gforth represents named words by the @dfn{name token}, (@i{nt}). Name
7349: token is an abstract data type that occurs as argument or result of the
7350: words below.
7351:
7352: @c !! put this elswhere?
7353: @cindex name field address
7354: @cindex NFA
7355: The closest thing to the nt in older Forth systems is the name field
7356: address (NFA), but there are significant differences: in older Forth
7357: systems each word had a unique NFA, LFA, CFA and PFA (in this order, or
7358: LFA, NFA, CFA, PFA) and there were words for getting from one to the
7359: next. In contrast, in Gforth 0@dots{}n nts correspond to one xt; there
7360: is a link field in the structure identified by the name token, but
7361: searching usually uses a hash table external to these structures; the
7362: name in Gforth has a cell-wide count-and-flags field, and the nt is not
7363: implemented as the address of that count field.
7364:
7365: doc-find-name
7366: doc-latest
7367: doc->name
7368: doc-name>int
7369: doc-name?int
7370: doc-name>comp
7371: doc-name>string
7372: doc-id.
7373: doc-.name
7374: doc-.id
7375:
7376: @c ----------------------------------------------------------
7377: @node Compiling words, The Text Interpreter, Tokens for Words, Words
7378: @section Compiling words
7379: @cindex compiling words
7380: @cindex macros
7381:
7382: In contrast to most other languages, Forth has no strict boundary
7383: between compilation and run-time. E.g., you can run arbitrary code
7384: between defining words (or for computing data used by defining words
7385: like @code{constant}). Moreover, @code{Immediate} (@pxref{Interpretation
7386: and Compilation Semantics} and @code{[}...@code{]} (see below) allow
7387: running arbitrary code while compiling a colon definition (exception:
7388: you must not allot dictionary space).
7389:
7390: @menu
7391: * Literals:: Compiling data values
7392: * Macros:: Compiling words
7393: @end menu
7394:
7395: @node Literals, Macros, Compiling words, Compiling words
7396: @subsection Literals
7397: @cindex Literals
7398:
7399: The simplest and most frequent example is to compute a literal during
7400: compilation. E.g., the following definition prints an array of strings,
7401: one string per line:
7402:
7403: @example
7404: : .strings ( addr u -- ) \ gforth
7405: 2* cells bounds U+DO
7406: cr i 2@@ type
7407: 2 cells +LOOP ;
7408: @end example
7409:
7410: With a simple-minded compiler like Gforth's, this computes @code{2
7411: cells} on every loop iteration. You can compute this value once and for
7412: all at compile time and compile it into the definition like this:
7413:
7414: @example
7415: : .strings ( addr u -- ) \ gforth
7416: 2* cells bounds U+DO
7417: cr i 2@@ type
7418: [ 2 cells ] literal +LOOP ;
7419: @end example
7420:
7421: @code{[} switches the text interpreter to interpret state (you will get
7422: an @code{ok} prompt if you type this example interactively and insert a
7423: newline between @code{[} and @code{]}), so it performs the
7424: interpretation semantics of @code{2 cells}; this computes a number.
7425: @code{]} switches the text interpreter back into compile state. It then
7426: performs @code{Literal}'s compilation semantics, which are to compile
7427: this number into the current word. You can decompile the word with
7428: @code{see .strings} to see the effect on the compiled code.
7429:
7430: You can also optimize the @code{2* cells} into @code{[ 2 cells ] literal
7431: *} in this way.
7432:
7433: doc-[
7434: doc-]
7435: doc-literal
7436: doc-]L
7437:
7438: There are also words for compiling other data types than single cells as
7439: literals:
7440:
7441: doc-2literal
7442: doc-fliteral
7443: doc-sliteral
7444:
7445: @cindex colon-sys, passing data across @code{:}
7446: @cindex @code{:}, passing data across
7447: You might be tempted to pass data from outside a colon definition to the
7448: inside on the data stack. This does not work, because @code{:} puhes a
7449: colon-sys, making stuff below unaccessible. E.g., this does not work:
7450:
7451: @example
7452: 5 : foo literal ; \ error: "unstructured"
7453: @end example
7454:
7455: Instead, you have to pass the value in some other way, e.g., through a
7456: variable:
7457:
7458: @example
7459: variable temp
7460: 5 temp !
7461: : foo [ temp @@ ] literal ;
7462: @end example
7463:
7464:
7465: @node Macros, , Literals, Compiling words
7466: @subsection Macros
7467: @cindex Macros
7468: @cindex compiling compilation semantics
7469:
7470: @code{Literal} and friends compile data values into the current
7471: definition. You can also write words that compile other words into the
7472: current definition. E.g.,
7473:
7474: @example
7475: : compile-+ ( -- ) \ compiled code: ( n1 n2 -- n )
7476: POSTPONE + ;
7477:
7478: : foo ( n1 n2 -- n )
7479: [ compile-+ ] ;
7480: 1 2 foo .
7481: @end example
7482:
7483: This is equivalent to @code{: foo + ;} (@code{see foo} to check this).
7484: What happens in this example? @code{Postpone} compiles the compilation
7485: semantics of @code{+} into @code{compile-+}; later the text interpreter
7486: executes @code{compile-+} and thus the compilation semantics of +, which
7487: compile (the execution semantics of) @code{+} into
7488: @code{foo}.@footnote{A recent RFI answer requires that compiling words
7489: should only be executed in compile state, so this example is not
7490: guaranteed to work on all standard systems, but on any decent system it
7491: will work.}
7492:
7493: doc-postpone
7494:
7495: Compiling words like @code{compile-+} are usually immediate (or similar)
7496: so you do not have to switch to interpret state to execute them;
7497: modifying the last example accordingly produces:
7498:
7499: @example
7500: : [compile-+] ( compilation: --; interpretation: -- )
7501: \ compiled code: ( n1 n2 -- n )
7502: POSTPONE + ; immediate
7503:
7504: : foo ( n1 n2 -- n )
7505: [compile-+] ;
7506: 1 2 foo .
7507: @end example
7508:
7509: You will occassionally find the need to POSTPONE several words;
7510: putting POSTPONE before each such word is cumbersome, so Gforth
7511: provides a more convenient syntax: @code{]] ... [[}. This
7512: allows us to write @code{[compile-+]} as:
7513:
7514: @example
7515: : [compile-+] ( compilation: --; interpretation: -- )
7516: ]] + [[ ; immediate
7517: @end example
7518:
7519: doc-]]
7520: doc-[[
7521:
7522: The unusual direction of the brackets indicates their function:
7523: @code{]]} switches from compilation to postponing (i.e., compilation
7524: of compilation), just like @code{]} switches from immediate execution
7525: (interpretation) to compilation. Conversely, @code{[[} switches from
7526: postponing to compilation, ananlogous to @code{[} which switches from
7527: compilation to immediate execution.
7528:
7529: The real advantage of @code{]] }...@code{ [[} becomes apparent when
7530: there are many words to POSTPONE. E.g., the word
7531: @code{compile-map-array} (@pxref{Advanced macros Tutorial}) can be
7532: written much shorter as follows:
7533:
7534: @example
7535: : compile-map-array ( compilation: xt -- ; run-time: ... addr u -- ... )
7536: \ at run-time, execute xt ( ... x -- ... ) for each element of the
7537: \ array beginning at addr and containing u elements
7538: @{ xt @}
7539: ]] cells over + swap ?do
7540: i @@ [[ xt compile,
7541: 1 cells ]]L +loop [[ ;
7542: @end example
7543:
7544: This example also uses @code{]]L} as a shortcut for @code{]] literal}.
7545: There are also other shortcuts
7546:
7547: doc-]]L
7548: doc-]]2L
7549: doc-]]FL
7550: doc-]]SL
7551:
7552: Note that parsing words don't parse at postpone time; if you want to
7553: provide the parsed string right away, you have to switch back to
7554: compilation:
7555:
7556: @example
7557: ]] ... [[ s" some string" ]]2L ... [[
7558: ]] ... [[ ['] + ]]L ... [[
7559: @end example
7560:
7561: Definitions of @code{]]} and friends in ANS Forth are provided in
7562: @file{compat/macros.fs}.
7563:
7564: Immediate compiling words are similar to macros in other languages (in
7565: particular, Lisp). The important differences to macros in, e.g., C are:
7566:
7567: @itemize @bullet
7568:
7569: @item
7570: You use the same language for defining and processing macros, not a
7571: separate preprocessing language and processor.
7572:
7573: @item
7574: Consequently, the full power of Forth is available in macro definitions.
7575: E.g., you can perform arbitrarily complex computations, or generate
7576: different code conditionally or in a loop (e.g., @pxref{Advanced macros
7577: Tutorial}). This power is very useful when writing a parser generators
7578: or other code-generating software.
7579:
7580: @item
7581: Macros defined using @code{postpone} etc. deal with the language at a
7582: higher level than strings; name binding happens at macro definition
7583: time, so you can avoid the pitfalls of name collisions that can happen
7584: in C macros. Of course, Forth is a liberal language and also allows to
7585: shoot yourself in the foot with text-interpreted macros like
7586:
7587: @example
7588: : [compile-+] s" +" evaluate ; immediate
7589: @end example
7590:
7591: Apart from binding the name at macro use time, using @code{evaluate}
7592: also makes your definition @code{state}-smart (@pxref{state-smartness}).
7593: @end itemize
7594:
7595: You may want the macro to compile a number into a word. The word to do
7596: it is @code{literal}, but you have to @code{postpone} it, so its
7597: compilation semantics take effect when the macro is executed, not when
7598: it is compiled:
7599:
7600: @example
7601: : [compile-5] ( -- ) \ compiled code: ( -- n )
7602: 5 POSTPONE literal ; immediate
7603:
7604: : foo [compile-5] ;
7605: foo .
7606: @end example
7607:
7608: You may want to pass parameters to a macro, that the macro should
7609: compile into the current definition. If the parameter is a number, then
7610: you can use @code{postpone literal} (similar for other values).
7611:
7612: If you want to pass a word that is to be compiled, the usual way is to
7613: pass an execution token and @code{compile,} it:
7614:
7615: @example
7616: : twice1 ( xt -- ) \ compiled code: ... -- ...
7617: dup compile, compile, ;
7618:
7619: : 2+ ( n1 -- n2 )
7620: [ ' 1+ twice1 ] ;
7621: @end example
7622:
7623: doc-compile,
7624:
7625: An alternative available in Gforth, that allows you to pass compile-only
7626: words as parameters is to use the compilation token (@pxref{Compilation
7627: token}). The same example in this technique:
7628:
7629: @example
7630: : twice ( ... ct -- ... ) \ compiled code: ... -- ...
7631: 2dup 2>r execute 2r> execute ;
7632:
7633: : 2+ ( n1 -- n2 )
7634: [ comp' 1+ twice ] ;
7635: @end example
7636:
7637: In the example above @code{2>r} and @code{2r>} ensure that @code{twice}
7638: works even if the executed compilation semantics has an effect on the
7639: data stack.
7640:
7641: You can also define complete definitions with these words; this provides
7642: an alternative to using @code{does>} (@pxref{User-defined Defining
7643: Words}). E.g., instead of
7644:
7645: @example
7646: : curry+ ( n1 "name" -- )
7647: CREATE ,
7648: DOES> ( n2 -- n1+n2 )
7649: @@ + ;
7650: @end example
7651:
7652: you could define
7653:
7654: @example
7655: : curry+ ( n1 "name" -- )
7656: \ name execution: ( n2 -- n1+n2 )
7657: >r : r> POSTPONE literal POSTPONE + POSTPONE ; ;
7658:
7659: -3 curry+ 3-
7660: see 3-
7661: @end example
7662:
7663: The sequence @code{>r : r>} is necessary, because @code{:} puts a
7664: colon-sys on the data stack that makes everything below it unaccessible.
7665:
7666: This way of writing defining words is sometimes more, sometimes less
7667: convenient than using @code{does>} (@pxref{Advanced does> usage
7668: example}). One advantage of this method is that it can be optimized
7669: better, because the compiler knows that the value compiled with
7670: @code{literal} is fixed, whereas the data associated with a
7671: @code{create}d word can be changed.
7672:
7673: @c doc-[compile] !! not properly documented
7674:
7675: @c ----------------------------------------------------------
7676: @node The Text Interpreter, The Input Stream, Compiling words, Words
7677: @section The Text Interpreter
7678: @cindex interpreter - outer
7679: @cindex text interpreter
7680: @cindex outer interpreter
7681:
7682: @c Should we really describe all these ugly details? IMO the text
7683: @c interpreter should be much cleaner, but that may not be possible within
7684: @c ANS Forth. - anton
7685: @c nac-> I wanted to explain how it works to show how you can exploit
7686: @c it in your own programs. When I was writing a cross-compiler, figuring out
7687: @c some of these gory details was very helpful to me. None of the textbooks
7688: @c I've seen cover it, and the most modern Forth textbook -- Forth Inc's,
7689: @c seems to positively avoid going into too much detail for some of
7690: @c the internals.
7691:
7692: @c anton: ok. I wonder, though, if this is the right place; for some stuff
7693: @c it is; for the ugly details, I would prefer another place. I wonder
7694: @c whether we should have a chapter before "Words" that describes some
7695: @c basic concepts referred to in words, and a chapter after "Words" that
7696: @c describes implementation details.
7697:
7698: The text interpreter@footnote{This is an expanded version of the
7699: material in @ref{Introducing the Text Interpreter}.} is an endless loop
7700: that processes input from the current input device. It is also called
7701: the outer interpreter, in contrast to the inner interpreter
7702: (@pxref{Engine}) which executes the compiled Forth code on interpretive
7703: implementations.
7704:
7705: @cindex interpret state
7706: @cindex compile state
7707: The text interpreter operates in one of two states: @dfn{interpret
7708: state} and @dfn{compile state}. The current state is defined by the
7709: aptly-named variable @code{state}.
7710:
7711: This section starts by describing how the text interpreter behaves when
7712: it is in interpret state, processing input from the user input device --
7713: the keyboard. This is the mode that a Forth system is in after it starts
7714: up.
7715:
7716: @cindex input buffer
7717: @cindex terminal input buffer
7718: The text interpreter works from an area of memory called the @dfn{input
7719: buffer}@footnote{When the text interpreter is processing input from the
7720: keyboard, this area of memory is called the @dfn{terminal input buffer}
7721: (TIB) and is addressed by the (obsolescent) words @code{TIB} and
7722: @code{#TIB}.}, which stores your keyboard input when you press the
7723: @key{RET} key. Starting at the beginning of the input buffer, it skips
7724: leading spaces (called @dfn{delimiters}) then parses a string (a
7725: sequence of non-space characters) until it reaches either a space
7726: character or the end of the buffer. Having parsed a string, it makes two
7727: attempts to process it:
7728:
7729: @cindex dictionary
7730: @itemize @bullet
7731: @item
7732: It looks for the string in a @dfn{dictionary} of definitions. If the
7733: string is found, the string names a @dfn{definition} (also known as a
7734: @dfn{word}) and the dictionary search returns information that allows
7735: the text interpreter to perform the word's @dfn{interpretation
7736: semantics}. In most cases, this simply means that the word will be
7737: executed.
7738: @item
7739: If the string is not found in the dictionary, the text interpreter
7740: attempts to treat it as a number, using the rules described in
7741: @ref{Number Conversion}. If the string represents a legal number in the
7742: current radix, the number is pushed onto a parameter stack (the data
7743: stack for integers, the floating-point stack for floating-point
7744: numbers).
7745: @end itemize
7746:
7747: If both attempts fail, or if the word is found in the dictionary but has
7748: no interpretation semantics@footnote{This happens if the word was
7749: defined as @code{COMPILE-ONLY}.} the text interpreter discards the
7750: remainder of the input buffer, issues an error message and waits for
7751: more input. If one of the attempts succeeds, the text interpreter
7752: repeats the parsing process until the whole of the input buffer has been
7753: processed, at which point it prints the status message ``@code{ ok}''
7754: and waits for more input.
7755:
7756: @c anton: this should be in the input stream subsection (or below it)
7757:
7758: @cindex parse area
7759: The text interpreter keeps track of its position in the input buffer by
7760: updating a variable called @code{>IN} (pronounced ``to-in''). The value
7761: of @code{>IN} starts out as 0, indicating an offset of 0 from the start
7762: of the input buffer. The region from offset @code{>IN @@} to the end of
7763: the input buffer is called the @dfn{parse area}@footnote{In other words,
7764: the text interpreter processes the contents of the input buffer by
7765: parsing strings from the parse area until the parse area is empty.}.
7766: This example shows how @code{>IN} changes as the text interpreter parses
7767: the input buffer:
7768:
7769: @example
7770: : remaining >IN @@ SOURCE 2 PICK - -ROT + SWAP
7771: CR ." ->" TYPE ." <-" ; IMMEDIATE
7772:
7773: 1 2 3 remaining + remaining .
7774:
7775: : foo 1 2 3 remaining SWAP remaining ;
7776: @end example
7777:
7778: @noindent
7779: The result is:
7780:
7781: @example
7782: ->+ remaining .<-
7783: ->.<-5 ok
7784:
7785: ->SWAP remaining ;-<
7786: ->;<- ok
7787: @end example
7788:
7789: @cindex parsing words
7790: The value of @code{>IN} can also be modified by a word in the input
7791: buffer that is executed by the text interpreter. This means that a word
7792: can ``trick'' the text interpreter into either skipping a section of the
7793: input buffer@footnote{This is how parsing words work.} or into parsing a
7794: section twice. For example:
7795:
7796: @example
7797: : lat ." <<foo>>" ;
7798: : flat ." <<bar>>" >IN DUP @@ 3 - SWAP ! ;
7799: @end example
7800:
7801: @noindent
7802: When @code{flat} is executed, this output is produced@footnote{Exercise
7803: for the reader: what would happen if the @code{3} were replaced with
7804: @code{4}?}:
7805:
7806: @example
7807: <<bar>><<foo>>
7808: @end example
7809:
7810: This technique can be used to work around some of the interoperability
7811: problems of parsing words. Of course, it's better to avoid parsing
7812: words where possible.
7813:
7814: @noindent
7815: Two important notes about the behaviour of the text interpreter:
7816:
7817: @itemize @bullet
7818: @item
7819: It processes each input string to completion before parsing additional
7820: characters from the input buffer.
7821: @item
7822: It treats the input buffer as a read-only region (and so must your code).
7823: @end itemize
7824:
7825: @noindent
7826: When the text interpreter is in compile state, its behaviour changes in
7827: these ways:
7828:
7829: @itemize @bullet
7830: @item
7831: If a parsed string is found in the dictionary, the text interpreter will
7832: perform the word's @dfn{compilation semantics}. In most cases, this
7833: simply means that the execution semantics of the word will be appended
7834: to the current definition.
7835: @item
7836: When a number is encountered, it is compiled into the current definition
7837: (as a literal) rather than being pushed onto a parameter stack.
7838: @item
7839: If an error occurs, @code{state} is modified to put the text interpreter
7840: back into interpret state.
7841: @item
7842: Each time a line is entered from the keyboard, Gforth prints
7843: ``@code{ compiled}'' rather than `` @code{ok}''.
7844: @end itemize
7845:
7846: @cindex text interpreter - input sources
7847: When the text interpreter is using an input device other than the
7848: keyboard, its behaviour changes in these ways:
7849:
7850: @itemize @bullet
7851: @item
7852: When the parse area is empty, the text interpreter attempts to refill
7853: the input buffer from the input source. When the input source is
7854: exhausted, the input source is set back to the previous input source.
7855: @item
7856: It doesn't print out ``@code{ ok}'' or ``@code{ compiled}'' messages each
7857: time the parse area is emptied.
7858: @item
7859: If an error occurs, the input source is set back to the user input
7860: device.
7861: @end itemize
7862:
7863: You can read about this in more detail in @ref{Input Sources}.
7864:
7865: doc->in
7866: doc-source
7867:
7868: doc-tib
7869: doc-#tib
7870:
7871:
7872: @menu
7873: * Input Sources::
7874: * Number Conversion::
7875: * Interpret/Compile states::
7876: * Interpreter Directives::
7877: @end menu
7878:
7879: @node Input Sources, Number Conversion, The Text Interpreter, The Text Interpreter
7880: @subsection Input Sources
7881: @cindex input sources
7882: @cindex text interpreter - input sources
7883:
7884: By default, the text interpreter processes input from the user input
7885: device (the keyboard) when Forth starts up. The text interpreter can
7886: process input from any of these sources:
7887:
7888: @itemize @bullet
7889: @item
7890: The user input device -- the keyboard.
7891: @item
7892: A file, using the words described in @ref{Forth source files}.
7893: @item
7894: A block, using the words described in @ref{Blocks}.
7895: @item
7896: A text string, using @code{evaluate}.
7897: @end itemize
7898:
7899: A program can identify the current input device from the values of
7900: @code{source-id} and @code{blk}.
7901:
7902:
7903: doc-source-id
7904: doc-blk
7905:
7906: doc-save-input
7907: doc-restore-input
7908:
7909: doc-evaluate
7910: doc-query
7911:
7912:
7913:
7914: @node Number Conversion, Interpret/Compile states, Input Sources, The Text Interpreter
7915: @subsection Number Conversion
7916: @cindex number conversion
7917: @cindex double-cell numbers, input format
7918: @cindex input format for double-cell numbers
7919: @cindex single-cell numbers, input format
7920: @cindex input format for single-cell numbers
7921: @cindex floating-point numbers, input format
7922: @cindex input format for floating-point numbers
7923:
7924: This section describes the rules that the text interpreter uses when it
7925: tries to convert a string into a number.
7926:
7927: Let <digit> represent any character that is a legal digit in the current
7928: number base@footnote{For example, 0-9 when the number base is decimal or
7929: 0-9, A-F when the number base is hexadecimal.}.
7930:
7931: Let <decimal digit> represent any character in the range 0-9.
7932:
7933: Let @{@i{a b}@} represent the @i{optional} presence of any of the characters
7934: in the braces (@i{a} or @i{b} or neither).
7935:
7936: Let * represent any number of instances of the previous character
7937: (including none).
7938:
7939: Let any other character represent itself.
7940:
7941: @noindent
7942: Now, the conversion rules are:
7943:
7944: @itemize @bullet
7945: @item
7946: A string of the form <digit><digit>* is treated as a single-precision
7947: (cell-sized) positive integer. Examples are 0 123 6784532 32343212343456 42
7948: @item
7949: A string of the form -<digit><digit>* is treated as a single-precision
7950: (cell-sized) negative integer, and is represented using 2's-complement
7951: arithmetic. Examples are -45 -5681 -0
7952: @item
7953: A string of the form <digit><digit>*.<digit>* is treated as a double-precision
7954: (double-cell-sized) positive integer. Examples are 3465. 3.465 34.65
7955: (all three of these represent the same number).
7956: @item
7957: A string of the form -<digit><digit>*.<digit>* is treated as a
7958: double-precision (double-cell-sized) negative integer, and is
7959: represented using 2's-complement arithmetic. Examples are -3465. -3.465
7960: -34.65 (all three of these represent the same number).
7961: @item
7962: A string of the form @{+ -@}<decimal digit>@{.@}<decimal digit>*@{e
7963: E@}@{+ -@}<decimal digit><decimal digit>* is treated as a floating-point
7964: number. Examples are 1e 1e0 1.e 1.e0 +1e+0 (which all represent the same
7965: number) +12.E-4
7966: @end itemize
7967:
7968: By default, the number base used for integer number conversion is
7969: given by the contents of the variable @code{base}. Note that a lot of
7970: confusion can result from unexpected values of @code{base}. If you
7971: change @code{base} anywhere, make sure to save the old value and
7972: restore it afterwards; better yet, use @code{base-execute}, which does
7973: this for you. In general I recommend keeping @code{base} decimal, and
7974: using the prefixes described below for the popular non-decimal bases.
7975:
7976: doc-dpl
7977: doc-base-execute
7978: doc-base
7979: doc-hex
7980: doc-decimal
7981:
7982: @cindex '-prefix for character strings
7983: @cindex &-prefix for decimal numbers
7984: @cindex #-prefix for decimal numbers
7985: @cindex %-prefix for binary numbers
7986: @cindex $-prefix for hexadecimal numbers
7987: @cindex 0x-prefix for hexadecimal numbers
7988: Gforth allows you to override the value of @code{base} by using a
7989: prefix@footnote{Some Forth implementations provide a similar scheme by
7990: implementing @code{$} etc. as parsing words that process the subsequent
7991: number in the input stream and push it onto the stack. For example, see
7992: @cite{Number Conversion and Literals}, by Wil Baden; Forth Dimensions
7993: 20(3) pages 26--27. In such implementations, unlike in Gforth, a space
7994: is required between the prefix and the number.} before the first digit
7995: of an (integer) number. The following prefixes are supported:
7996:
7997: @itemize @bullet
7998: @item
7999: @code{&} -- decimal
8000: @item
8001: @code{#} -- decimal
8002: @item
8003: @code{%} -- binary
8004: @item
8005: @code{$} -- hexadecimal
8006: @item
8007: @code{0x} -- hexadecimal, if base<33.
8008: @item
8009: @code{'} -- numeric value (e.g., ASCII code) of next character; an
8010: optional @code{'} may be present after the character.
8011: @end itemize
8012:
8013: Here are some examples, with the equivalent decimal number shown after
8014: in braces:
8015:
8016: -$41 (-65), %1001101 (205), %1001.0001 (145 - a double-precision number),
8017: 'A (65),
8018: -'a' (-97),
8019: &905 (905), $abc (2478), $ABC (2478).
8020:
8021: @cindex number conversion - traps for the unwary
8022: @noindent
8023: Number conversion has a number of traps for the unwary:
8024:
8025: @itemize @bullet
8026: @item
8027: You cannot determine the current number base using the code sequence
8028: @code{base @@ .} -- the number base is always 10 in the current number
8029: base. Instead, use something like @code{base @@ dec.}
8030: @item
8031: If the number base is set to a value greater than 14 (for example,
8032: hexadecimal), the number 123E4 is ambiguous; the conversion rules allow
8033: it to be intepreted as either a single-precision integer or a
8034: floating-point number (Gforth treats it as an integer). The ambiguity
8035: can be resolved by explicitly stating the sign of the mantissa and/or
8036: exponent: 123E+4 or +123E4 -- if the number base is decimal, no
8037: ambiguity arises; either representation will be treated as a
8038: floating-point number.
8039: @item
8040: There is a word @code{bin} but it does @i{not} set the number base!
8041: It is used to specify file types.
8042: @item
8043: ANS Forth requires the @code{.} of a double-precision number to be the
8044: final character in the string. Gforth allows the @code{.} to be
8045: anywhere after the first digit.
8046: @item
8047: The number conversion process does not check for overflow.
8048: @item
8049: In an ANS Forth program @code{base} is required to be decimal when
8050: converting floating-point numbers. In Gforth, number conversion to
8051: floating-point numbers always uses base &10, irrespective of the value
8052: of @code{base}.
8053: @end itemize
8054:
8055: You can read numbers into your programs with the words described in
8056: @ref{Line input and conversion}.
8057:
8058: @node Interpret/Compile states, Interpreter Directives, Number Conversion, The Text Interpreter
8059: @subsection Interpret/Compile states
8060: @cindex Interpret/Compile states
8061:
8062: A standard program is not permitted to change @code{state}
8063: explicitly. However, it can change @code{state} implicitly, using the
8064: words @code{[} and @code{]}. When @code{[} is executed it switches
8065: @code{state} to interpret state, and therefore the text interpreter
8066: starts interpreting. When @code{]} is executed it switches @code{state}
8067: to compile state and therefore the text interpreter starts
8068: compiling. The most common usage for these words is for switching into
8069: interpret state and back from within a colon definition; this technique
8070: can be used to compile a literal (for an example, @pxref{Literals}) or
8071: for conditional compilation (for an example, @pxref{Interpreter
8072: Directives}).
8073:
8074:
8075: @c This is a bad example: It's non-standard, and it's not necessary.
8076: @c However, I can't think of a good example for switching into compile
8077: @c state when there is no current word (@code{state}-smart words are not a
8078: @c good reason). So maybe we should use an example for switching into
8079: @c interpret @code{state} in a colon def. - anton
8080: @c nac-> I agree. I started out by putting in the example, then realised
8081: @c that it was non-ANS, so wrote more words around it. I hope this
8082: @c re-written version is acceptable to you. I do want to keep the example
8083: @c as it is helpful for showing what is and what is not portable, particularly
8084: @c where it outlaws a style in common use.
8085:
8086: @c anton: it's more important to show what's portable. After we have done
8087: @c that, we can also show what's not. In any case, I have written a
8088: @c section Compiling Words which also deals with [ ].
8089:
8090: @c !! The following example does not work in Gforth 0.5.9 or later.
8091:
8092: @c @code{[} and @code{]} also give you the ability to switch into compile
8093: @c state and back, but we cannot think of any useful Standard application
8094: @c for this ability. Pre-ANS Forth textbooks have examples like this:
8095:
8096: @c @example
8097: @c : AA ." this is A" ;
8098: @c : BB ." this is B" ;
8099: @c : CC ." this is C" ;
8100:
8101: @c create table ] aa bb cc [
8102:
8103: @c : go ( n -- ) \ n is offset into table.. 0 for 1st entry
8104: @c cells table + @@ execute ;
8105: @c @end example
8106:
8107: @c This example builds a jump table; @code{0 go} will display ``@code{this
8108: @c is A}''. Using @code{[} and @code{]} in this example is equivalent to
8109: @c defining @code{table} like this:
8110:
8111: @c @example
8112: @c create table ' aa COMPILE, ' bb COMPILE, ' cc COMPILE,
8113: @c @end example
8114:
8115: @c The problem with this code is that the definition of @code{table} is not
8116: @c portable -- it @i{compile}s execution tokens into code space. Whilst it
8117: @c @i{may} work on systems where code space and data space co-incide, the
8118: @c Standard only allows data space to be assigned for a @code{CREATE}d
8119: @c word. In addition, the Standard only allows @code{@@} to access data
8120: @c space, whilst this example is using it to access code space. The only
8121: @c portable, Standard way to build this table is to build it in data space,
8122: @c like this:
8123:
8124: @c @example
8125: @c create table ' aa , ' bb , ' cc ,
8126: @c @end example
8127:
8128: @c doc-state
8129:
8130:
8131: @node Interpreter Directives, , Interpret/Compile states, The Text Interpreter
8132: @subsection Interpreter Directives
8133: @cindex interpreter directives
8134: @cindex conditional compilation
8135:
8136: These words are usually used in interpret state; typically to control
8137: which parts of a source file are processed by the text
8138: interpreter. There are only a few ANS Forth Standard words, but Gforth
8139: supplements these with a rich set of immediate control structure words
8140: to compensate for the fact that the non-immediate versions can only be
8141: used in compile state (@pxref{Control Structures}). Typical usages:
8142:
8143: @example
8144: FALSE Constant HAVE-ASSEMBLER
8145: .
8146: .
8147: HAVE-ASSEMBLER [IF]
8148: : ASSEMBLER-FEATURE
8149: ...
8150: ;
8151: [ENDIF]
8152: .
8153: .
8154: : SEE
8155: ... \ general-purpose SEE code
8156: [ HAVE-ASSEMBLER [IF] ]
8157: ... \ assembler-specific SEE code
8158: [ [ENDIF] ]
8159: ;
8160: @end example
8161:
8162:
8163: doc-[IF]
8164: doc-[ELSE]
8165: doc-[THEN]
8166: doc-[ENDIF]
8167:
8168: doc-[IFDEF]
8169: doc-[IFUNDEF]
8170:
8171: doc-[?DO]
8172: doc-[DO]
8173: doc-[FOR]
8174: doc-[LOOP]
8175: doc-[+LOOP]
8176: doc-[NEXT]
8177:
8178: doc-[BEGIN]
8179: doc-[UNTIL]
8180: doc-[AGAIN]
8181: doc-[WHILE]
8182: doc-[REPEAT]
8183:
8184:
8185: @c -------------------------------------------------------------
8186: @node The Input Stream, Word Lists, The Text Interpreter, Words
8187: @section The Input Stream
8188: @cindex input stream
8189:
8190: @c !! integrate this better with the "Text Interpreter" section
8191: The text interpreter reads from the input stream, which can come from
8192: several sources (@pxref{Input Sources}). Some words, in particular
8193: defining words, but also words like @code{'}, read parameters from the
8194: input stream instead of from the stack.
8195:
8196: Such words are called parsing words, because they parse the input
8197: stream. Parsing words are hard to use in other words, because it is
8198: hard to pass program-generated parameters through the input stream.
8199: They also usually have an unintuitive combination of interpretation and
8200: compilation semantics when implemented naively, leading to various
8201: approaches that try to produce a more intuitive behaviour
8202: (@pxref{Combined words}).
8203:
8204: It should be obvious by now that parsing words are a bad idea. If you
8205: want to implement a parsing word for convenience, also provide a factor
8206: of the word that does not parse, but takes the parameters on the stack.
8207: To implement the parsing word on top if it, you can use the following
8208: words:
8209:
8210: @c anton: these belong in the input stream section
8211: doc-parse
8212: doc-parse-name
8213: doc-parse-word
8214: doc-name
8215: doc-word
8216: doc-refill
8217:
8218: Conversely, if you have the bad luck (or lack of foresight) to have to
8219: deal with parsing words without having such factors, how do you pass a
8220: string that is not in the input stream to it?
8221:
8222: doc-execute-parsing
8223:
8224: A definition of this word in ANS Forth is provided in
8225: @file{compat/execute-parsing.fs}.
8226:
8227: If you want to run a parsing word on a file, the following word should
8228: help:
8229:
8230: doc-execute-parsing-file
8231:
8232: @c -------------------------------------------------------------
8233: @node Word Lists, Environmental Queries, The Input Stream, Words
8234: @section Word Lists
8235: @cindex word lists
8236: @cindex header space
8237:
8238: A wordlist is a list of named words; you can add new words and look up
8239: words by name (and you can remove words in a restricted way with
8240: markers). Every named (and @code{reveal}ed) word is in one wordlist.
8241:
8242: @cindex search order stack
8243: The text interpreter searches the wordlists present in the search order
8244: (a stack of wordlists), from the top to the bottom. Within each
8245: wordlist, the search starts conceptually at the newest word; i.e., if
8246: two words in a wordlist have the same name, the newer word is found.
8247:
8248: @cindex compilation word list
8249: New words are added to the @dfn{compilation wordlist} (aka current
8250: wordlist).
8251:
8252: @cindex wid
8253: A word list is identified by a cell-sized word list identifier (@i{wid})
8254: in much the same way as a file is identified by a file handle. The
8255: numerical value of the wid has no (portable) meaning, and might change
8256: from session to session.
8257:
8258: The ANS Forth ``Search order'' word set is intended to provide a set of
8259: low-level tools that allow various different schemes to be
8260: implemented. Gforth also provides @code{vocabulary}, a traditional Forth
8261: word. @file{compat/vocabulary.fs} provides an implementation in ANS
8262: Forth.
8263:
8264: @comment TODO: locals section refers to here, saying that every word list (aka
8265: @comment vocabulary) has its own methods for searching etc. Need to document that.
8266: @c anton: but better in a separate subsection on wordlist internals
8267:
8268: @comment TODO: document markers, reveal, tables, mappedwordlist
8269:
8270: @comment the gforthman- prefix is used to pick out the true definition of a
8271: @comment word from the source files, rather than some alias.
8272:
8273: doc-forth-wordlist
8274: doc-definitions
8275: doc-get-current
8276: doc-set-current
8277: doc-get-order
8278: doc-set-order
8279: doc-wordlist
8280: doc-table
8281: doc->order
8282: doc-previous
8283: doc-also
8284: doc-forth
8285: doc-only
8286: doc-order
8287:
8288: doc-find
8289: doc-search-wordlist
8290:
8291: doc-words
8292: doc-vlist
8293: @c doc-words-deferred
8294:
8295: @c doc-mappedwordlist @c map-structure undefined, implemantation-specific
8296: doc-root
8297: doc-vocabulary
8298: doc-seal
8299: doc-vocs
8300: doc-current
8301: doc-context
8302:
8303:
8304: @menu
8305: * Vocabularies::
8306: * Why use word lists?::
8307: * Word list example::
8308: @end menu
8309:
8310: @node Vocabularies, Why use word lists?, Word Lists, Word Lists
8311: @subsection Vocabularies
8312: @cindex Vocabularies, detailed explanation
8313:
8314: Here is an example of creating and using a new wordlist using ANS
8315: Forth words:
8316:
8317: @example
8318: wordlist constant my-new-words-wordlist
8319: : my-new-words get-order nip my-new-words-wordlist swap set-order ;
8320:
8321: \ add it to the search order
8322: also my-new-words
8323:
8324: \ alternatively, add it to the search order and make it
8325: \ the compilation word list
8326: also my-new-words definitions
8327: \ type "order" to see the problem
8328: @end example
8329:
8330: The problem with this example is that @code{order} has no way to
8331: associate the name @code{my-new-words} with the wid of the word list (in
8332: Gforth, @code{order} and @code{vocs} will display @code{???} for a wid
8333: that has no associated name). There is no Standard way of associating a
8334: name with a wid.
8335:
8336: In Gforth, this example can be re-coded using @code{vocabulary}, which
8337: associates a name with a wid:
8338:
8339: @example
8340: vocabulary my-new-words
8341:
8342: \ add it to the search order
8343: also my-new-words
8344:
8345: \ alternatively, add it to the search order and make it
8346: \ the compilation word list
8347: my-new-words definitions
8348: \ type "order" to see that the problem is solved
8349: @end example
8350:
8351:
8352: @node Why use word lists?, Word list example, Vocabularies, Word Lists
8353: @subsection Why use word lists?
8354: @cindex word lists - why use them?
8355:
8356: Here are some reasons why people use wordlists:
8357:
8358: @itemize @bullet
8359:
8360: @c anton: Gforth's hashing implementation makes the search speed
8361: @c independent from the number of words. But it is linear with the number
8362: @c of wordlists that have to be searched, so in effect using more wordlists
8363: @c actually slows down compilation.
8364:
8365: @c @item
8366: @c To improve compilation speed by reducing the number of header space
8367: @c entries that must be searched. This is achieved by creating a new
8368: @c word list that contains all of the definitions that are used in the
8369: @c definition of a Forth system but which would not usually be used by
8370: @c programs running on that system. That word list would be on the search
8371: @c list when the Forth system was compiled but would be removed from the
8372: @c search list for normal operation. This can be a useful technique for
8373: @c low-performance systems (for example, 8-bit processors in embedded
8374: @c systems) but is unlikely to be necessary in high-performance desktop
8375: @c systems.
8376:
8377: @item
8378: To prevent a set of words from being used outside the context in which
8379: they are valid. Two classic examples of this are an integrated editor
8380: (all of the edit commands are defined in a separate word list; the
8381: search order is set to the editor word list when the editor is invoked;
8382: the old search order is restored when the editor is terminated) and an
8383: integrated assembler (the op-codes for the machine are defined in a
8384: separate word list which is used when a @code{CODE} word is defined).
8385:
8386: @item
8387: To organize the words of an application or library into a user-visible
8388: set (in @code{forth-wordlist} or some other common wordlist) and a set
8389: of helper words used just for the implementation (hidden in a separate
8390: wordlist). This keeps @code{words}' output smaller, separates
8391: implementation and interface, and reduces the chance of name conflicts
8392: within the common wordlist.
8393:
8394: @item
8395: To prevent a name-space clash between multiple definitions with the same
8396: name. For example, when building a cross-compiler you might have a word
8397: @code{IF} that generates conditional code for your target system. By
8398: placing this definition in a different word list you can control whether
8399: the host system's @code{IF} or the target system's @code{IF} get used in
8400: any particular context by controlling the order of the word lists on the
8401: search order stack.
8402:
8403: @end itemize
8404:
8405: The downsides of using wordlists are:
8406:
8407: @itemize
8408:
8409: @item
8410: Debugging becomes more cumbersome.
8411:
8412: @item
8413: Name conflicts worked around with wordlists are still there, and you
8414: have to arrange the search order carefully to get the desired results;
8415: if you forget to do that, you get hard-to-find errors (as in any case
8416: where you read the code differently from the compiler; @code{see} can
8417: help seeing which of several possible words the name resolves to in such
8418: cases). @code{See} displays just the name of the words, not what
8419: wordlist they belong to, so it might be misleading. Using unique names
8420: is a better approach to avoid name conflicts.
8421:
8422: @item
8423: You have to explicitly undo any changes to the search order. In many
8424: cases it would be more convenient if this happened implicitly. Gforth
8425: currently does not provide such a feature, but it may do so in the
8426: future.
8427: @end itemize
8428:
8429:
8430: @node Word list example, , Why use word lists?, Word Lists
8431: @subsection Word list example
8432: @cindex word lists - example
8433:
8434: The following example is from the
8435: @uref{http://www.complang.tuwien.ac.at/forth/garbage-collection.zip,
8436: garbage collector} and uses wordlists to separate public words from
8437: helper words:
8438:
8439: @example
8440: get-current ( wid )
8441: vocabulary garbage-collector also garbage-collector definitions
8442: ... \ define helper words
8443: ( wid ) set-current \ restore original (i.e., public) compilation wordlist
8444: ... \ define the public (i.e., API) words
8445: \ they can refer to the helper words
8446: previous \ restore original search order (helper words become invisible)
8447: @end example
8448:
8449: @c -------------------------------------------------------------
8450: @node Environmental Queries, Files, Word Lists, Words
8451: @section Environmental Queries
8452: @cindex environmental queries
8453:
8454: ANS Forth introduced the idea of ``environmental queries'' as a way
8455: for a program running on a system to determine certain characteristics of the system.
8456: The Standard specifies a number of strings that might be recognised by a system.
8457:
8458: The Standard requires that the header space used for environmental queries
8459: be distinct from the header space used for definitions.
8460:
8461: Typically, environmental queries are supported by creating a set of
8462: definitions in a word list that is @i{only} used during environmental
8463: queries; that is what Gforth does. There is no Standard way of adding
8464: definitions to the set of recognised environmental queries, but any
8465: implementation that supports the loading of optional word sets must have
8466: some mechanism for doing this (after loading the word set, the
8467: associated environmental query string must return @code{true}). In
8468: Gforth, the word list used to honour environmental queries can be
8469: manipulated just like any other word list.
8470:
8471:
8472: doc-environment?
8473: doc-environment-wordlist
8474:
8475: doc-gforth
8476: doc-os-class
8477:
8478:
8479: Note that, whilst the documentation for (e.g.) @code{gforth} shows it
8480: returning two items on the stack, querying it using @code{environment?}
8481: will return an additional item; the @code{true} flag that shows that the
8482: string was recognised.
8483:
8484: @comment TODO Document the standard strings or note where they are documented herein
8485:
8486: Here are some examples of using environmental queries:
8487:
8488: @example
8489: s" address-unit-bits" environment? 0=
8490: [IF]
8491: cr .( environmental attribute address-units-bits unknown... ) cr
8492: [ELSE]
8493: drop \ ensure balanced stack effect
8494: [THEN]
8495:
8496: \ this might occur in the prelude of a standard program that uses THROW
8497: s" exception" environment? [IF]
8498: 0= [IF]
8499: : throw abort" exception thrown" ;
8500: [THEN]
8501: [ELSE] \ we don't know, so make sure
8502: : throw abort" exception thrown" ;
8503: [THEN]
8504:
8505: s" gforth" environment? [IF] .( Gforth version ) TYPE
8506: [ELSE] .( Not Gforth..) [THEN]
8507:
8508: \ a program using v*
8509: s" gforth" environment? [IF]
8510: s" 0.5.0" compare 0< [IF] \ v* is a primitive since 0.5.0
8511: : v* ( f_addr1 nstride1 f_addr2 nstride2 ucount -- r )
8512: >r swap 2swap swap 0e r> 0 ?DO
8513: dup f@@ over + 2swap dup f@@ f* f+ over + 2swap
8514: LOOP
8515: 2drop 2drop ;
8516: [THEN]
8517: [ELSE] \
8518: : v* ( f_addr1 nstride1 f_addr2 nstride2 ucount -- r )
8519: ...
8520: [THEN]
8521: @end example
8522:
8523: Here is an example of adding a definition to the environment word list:
8524:
8525: @example
8526: get-current environment-wordlist set-current
8527: true constant block
8528: true constant block-ext
8529: set-current
8530: @end example
8531:
8532: You can see what definitions are in the environment word list like this:
8533:
8534: @example
8535: environment-wordlist >order words previous
8536: @end example
8537:
8538:
8539: @c -------------------------------------------------------------
8540: @node Files, Blocks, Environmental Queries, Words
8541: @section Files
8542: @cindex files
8543: @cindex I/O - file-handling
8544:
8545: Gforth provides facilities for accessing files that are stored in the
8546: host operating system's file-system. Files that are processed by Gforth
8547: can be divided into two categories:
8548:
8549: @itemize @bullet
8550: @item
8551: Files that are processed by the Text Interpreter (@dfn{Forth source files}).
8552: @item
8553: Files that are processed by some other program (@dfn{general files}).
8554: @end itemize
8555:
8556: @menu
8557: * Forth source files::
8558: * General files::
8559: * Redirection::
8560: * Search Paths::
8561: @end menu
8562:
8563: @c -------------------------------------------------------------
8564: @node Forth source files, General files, Files, Files
8565: @subsection Forth source files
8566: @cindex including files
8567: @cindex Forth source files
8568:
8569: The simplest way to interpret the contents of a file is to use one of
8570: these two formats:
8571:
8572: @example
8573: include mysource.fs
8574: s" mysource.fs" included
8575: @end example
8576:
8577: You usually want to include a file only if it is not included already
8578: (by, say, another source file). In that case, you can use one of these
8579: three formats:
8580:
8581: @example
8582: require mysource.fs
8583: needs mysource.fs
8584: s" mysource.fs" required
8585: @end example
8586:
8587: @cindex stack effect of included files
8588: @cindex including files, stack effect
8589: It is good practice to write your source files such that interpreting them
8590: does not change the stack. Source files designed in this way can be used with
8591: @code{required} and friends without complications. For example:
8592:
8593: @example
8594: 1024 require foo.fs drop
8595: @end example
8596:
8597: Here you want to pass the argument 1024 (e.g., a buffer size) to
8598: @file{foo.fs}. Interpreting @file{foo.fs} has the stack effect ( n -- n
8599: ), which allows its use with @code{require}. Of course with such
8600: parameters to required files, you have to ensure that the first
8601: @code{require} fits for all uses (i.e., @code{require} it early in the
8602: master load file).
8603:
8604: doc-include-file
8605: doc-included
8606: doc-included?
8607: doc-include
8608: doc-required
8609: doc-require
8610: doc-needs
8611: @c doc-init-included-files @c internal
8612: doc-sourcefilename
8613: doc-sourceline#
8614:
8615: A definition in ANS Forth for @code{required} is provided in
8616: @file{compat/required.fs}.
8617:
8618: @c -------------------------------------------------------------
8619: @node General files, Redirection, Forth source files, Files
8620: @subsection General files
8621: @cindex general files
8622: @cindex file-handling
8623:
8624: Files are opened/created by name and type. The following file access
8625: methods (FAMs) are recognised:
8626:
8627: @cindex fam (file access method)
8628: doc-r/o
8629: doc-r/w
8630: doc-w/o
8631: doc-bin
8632:
8633:
8634: When a file is opened/created, it returns a file identifier,
8635: @i{wfileid} that is used for all other file commands. All file
8636: commands also return a status value, @i{wior}, that is 0 for a
8637: successful operation and an implementation-defined non-zero value in the
8638: case of an error.
8639:
8640:
8641: doc-open-file
8642: doc-create-file
8643:
8644: doc-close-file
8645: doc-delete-file
8646: doc-rename-file
8647: doc-read-file
8648: doc-read-line
8649: doc-key-file
8650: doc-key?-file
8651: doc-write-file
8652: doc-write-line
8653: doc-emit-file
8654: doc-flush-file
8655:
8656: doc-file-status
8657: doc-file-position
8658: doc-reposition-file
8659: doc-file-size
8660: doc-resize-file
8661:
8662: doc-slurp-file
8663: doc-slurp-fid
8664: doc-stdin
8665: doc-stdout
8666: doc-stderr
8667:
8668: @c ---------------------------------------------------------
8669: @node Redirection, Search Paths, General files, Files
8670: @subsection Redirection
8671: @cindex Redirection
8672: @cindex Input Redirection
8673: @cindex Output Redirection
8674:
8675: You can redirect the output of @code{type} and @code{emit} and all the
8676: words that use them (all output words that don't have an explicit
8677: target file) to an arbitrary file with the @code{outfile-execute},
8678: used like this:
8679:
8680: @example
8681: : some-warning ( n -- )
8682: cr ." warning# " . ;
8683:
8684: : print-some-warning ( n -- )
8685: ['] some-warning stderr outfile-execute ;
8686: @end example
8687:
8688: After @code{some-warning} is executed, the original output direction
8689: is restored; this construct is safe against exceptions. Similarly,
8690: there is @code{infile-execute} for redirecting the input of @code{key}
8691: and its users (any input word that does not take a file explicitly).
8692:
8693: doc-outfile-execute
8694: doc-infile-execute
8695:
8696: If you do not want to redirect the input or output to a file, you can
8697: also make use of the fact that @code{key}, @code{emit} and @code{type}
8698: are deferred words (@pxref{Deferred Words}). However, in that case
8699: you have to worry about the restoration and the protection against
8700: exceptions yourself; also, note that for redirecting the output in
8701: this way, you have to redirect both @code{emit} and @code{type}.
8702:
8703: @c ---------------------------------------------------------
8704: @node Search Paths, , Redirection, Files
8705: @subsection Search Paths
8706: @cindex path for @code{included}
8707: @cindex file search path
8708: @cindex @code{include} search path
8709: @cindex search path for files
8710:
8711: If you specify an absolute filename (i.e., a filename starting with
8712: @file{/} or @file{~}, or with @file{:} in the second position (as in
8713: @samp{C:...})) for @code{included} and friends, that file is included
8714: just as you would expect.
8715:
8716: If the filename starts with @file{./}, this refers to the directory that
8717: the present file was @code{included} from. This allows files to include
8718: other files relative to their own position (irrespective of the current
8719: working directory or the absolute position). This feature is essential
8720: for libraries consisting of several files, where a file may include
8721: other files from the library. It corresponds to @code{#include "..."}
8722: in C. If the current input source is not a file, @file{.} refers to the
8723: directory of the innermost file being included, or, if there is no file
8724: being included, to the current working directory.
8725:
8726: For relative filenames (not starting with @file{./}), Gforth uses a
8727: search path similar to Forth's search order (@pxref{Word Lists}). It
8728: tries to find the given filename in the directories present in the path,
8729: and includes the first one it finds. There are separate search paths for
8730: Forth source files and general files. If the search path contains the
8731: directory @file{.}, this refers to the directory of the current file, or
8732: the working directory, as if the file had been specified with @file{./}.
8733:
8734: Use @file{~+} to refer to the current working directory (as in the
8735: @code{bash}).
8736:
8737: @c anton: fold the following subsubsections into this subsection?
8738:
8739: @menu
8740: * Source Search Paths::
8741: * General Search Paths::
8742: @end menu
8743:
8744: @c ---------------------------------------------------------
8745: @node Source Search Paths, General Search Paths, Search Paths, Search Paths
8746: @subsubsection Source Search Paths
8747: @cindex search path control, source files
8748:
8749: The search path is initialized when you start Gforth (@pxref{Invoking
8750: Gforth}). You can display it and change it using @code{fpath} in
8751: combination with the general path handling words.
8752:
8753: doc-fpath
8754: @c the functionality of the following words is easily available through
8755: @c fpath and the general path words. The may go away.
8756: @c doc-.fpath
8757: @c doc-fpath+
8758: @c doc-fpath=
8759: @c doc-open-fpath-file
8760:
8761: @noindent
8762: Here is an example of using @code{fpath} and @code{require}:
8763:
8764: @example
8765: fpath path= /usr/lib/forth/|./
8766: require timer.fs
8767: @end example
8768:
8769:
8770: @c ---------------------------------------------------------
8771: @node General Search Paths, , Source Search Paths, Search Paths
8772: @subsubsection General Search Paths
8773: @cindex search path control, source files
8774:
8775: Your application may need to search files in several directories, like
8776: @code{included} does. To facilitate this, Gforth allows you to define
8777: and use your own search paths, by providing generic equivalents of the
8778: Forth search path words:
8779:
8780: doc-open-path-file
8781: doc-path-allot
8782: doc-clear-path
8783: doc-also-path
8784: doc-.path
8785: doc-path+
8786: doc-path=
8787:
8788: @c anton: better define a word for it, say "path-allot ( ucount -- path-addr )
8789:
8790: Here's an example of creating an empty search path:
8791: @c
8792: @example
8793: create mypath 500 path-allot \ maximum length 500 chars (is checked)
8794: @end example
8795:
8796: @c -------------------------------------------------------------
8797: @node Blocks, Other I/O, Files, Words
8798: @section Blocks
8799: @cindex I/O - blocks
8800: @cindex blocks
8801:
8802: When you run Gforth on a modern desk-top computer, it runs under the
8803: control of an operating system which provides certain services. One of
8804: these services is @var{file services}, which allows Forth source code
8805: and data to be stored in files and read into Gforth (@pxref{Files}).
8806:
8807: Traditionally, Forth has been an important programming language on
8808: systems where it has interfaced directly to the underlying hardware with
8809: no intervening operating system. Forth provides a mechanism, called
8810: @dfn{blocks}, for accessing mass storage on such systems.
8811:
8812: A block is a 1024-byte data area, which can be used to hold data or
8813: Forth source code. No structure is imposed on the contents of the
8814: block. A block is identified by its number; blocks are numbered
8815: contiguously from 1 to an implementation-defined maximum.
8816:
8817: A typical system that used blocks but no operating system might use a
8818: single floppy-disk drive for mass storage, with the disks formatted to
8819: provide 256-byte sectors. Blocks would be implemented by assigning the
8820: first four sectors of the disk to block 1, the second four sectors to
8821: block 2 and so on, up to the limit of the capacity of the disk. The disk
8822: would not contain any file system information, just the set of blocks.
8823:
8824: @cindex blocks file
8825: On systems that do provide file services, blocks are typically
8826: implemented by storing a sequence of blocks within a single @dfn{blocks
8827: file}. The size of the blocks file will be an exact multiple of 1024
8828: bytes, corresponding to the number of blocks it contains. This is the
8829: mechanism that Gforth uses.
8830:
8831: @cindex @file{blocks.fb}
8832: Only one blocks file can be open at a time. If you use block words without
8833: having specified a blocks file, Gforth defaults to the blocks file
8834: @file{blocks.fb}. Gforth uses the Forth search path when attempting to
8835: locate a blocks file (@pxref{Source Search Paths}).
8836:
8837: @cindex block buffers
8838: When you read and write blocks under program control, Gforth uses a
8839: number of @dfn{block buffers} as intermediate storage. These buffers are
8840: not used when you use @code{load} to interpret the contents of a block.
8841:
8842: The behaviour of the block buffers is analagous to that of a cache.
8843: Each block buffer has three states:
8844:
8845: @itemize @bullet
8846: @item
8847: Unassigned
8848: @item
8849: Assigned-clean
8850: @item
8851: Assigned-dirty
8852: @end itemize
8853:
8854: Initially, all block buffers are @i{unassigned}. In order to access a
8855: block, the block (specified by its block number) must be assigned to a
8856: block buffer.
8857:
8858: The assignment of a block to a block buffer is performed by @code{block}
8859: or @code{buffer}. Use @code{block} when you wish to modify the existing
8860: contents of a block. Use @code{buffer} when you don't care about the
8861: existing contents of the block@footnote{The ANS Forth definition of
8862: @code{buffer} is intended not to cause disk I/O; if the data associated
8863: with the particular block is already stored in a block buffer due to an
8864: earlier @code{block} command, @code{buffer} will return that block
8865: buffer and the existing contents of the block will be
8866: available. Otherwise, @code{buffer} will simply assign a new, empty
8867: block buffer for the block.}.
8868:
8869: Once a block has been assigned to a block buffer using @code{block} or
8870: @code{buffer}, that block buffer becomes the @i{current block
8871: buffer}. Data may only be manipulated (read or written) within the
8872: current block buffer.
8873:
8874: When the contents of the current block buffer has been modified it is
8875: necessary, @emph{before calling @code{block} or @code{buffer} again}, to
8876: either abandon the changes (by doing nothing) or mark the block as
8877: changed (assigned-dirty), using @code{update}. Using @code{update} does
8878: not change the blocks file; it simply changes a block buffer's state to
8879: @i{assigned-dirty}. The block will be written implicitly when it's
8880: buffer is needed for another block, or explicitly by @code{flush} or
8881: @code{save-buffers}.
8882:
8883: word @code{Flush} writes all @i{assigned-dirty} blocks back to the
8884: blocks file on disk. Leaving Gforth with @code{bye} also performs a
8885: @code{flush}.
8886:
8887: In Gforth, @code{block} and @code{buffer} use a @i{direct-mapped}
8888: algorithm to assign a block buffer to a block. That means that any
8889: particular block can only be assigned to one specific block buffer,
8890: called (for the particular operation) the @i{victim buffer}. If the
8891: victim buffer is @i{unassigned} or @i{assigned-clean} it is allocated to
8892: the new block immediately. If it is @i{assigned-dirty} its current
8893: contents are written back to the blocks file on disk before it is
8894: allocated to the new block.
8895:
8896: Although no structure is imposed on the contents of a block, it is
8897: traditional to display the contents as 16 lines each of 64 characters. A
8898: block provides a single, continuous stream of input (for example, it
8899: acts as a single parse area) -- there are no end-of-line characters
8900: within a block, and no end-of-file character at the end of a
8901: block. There are two consequences of this:
8902:
8903: @itemize @bullet
8904: @item
8905: The last character of one line wraps straight into the first character
8906: of the following line
8907: @item
8908: The word @code{\} -- comment to end of line -- requires special
8909: treatment; in the context of a block it causes all characters until the
8910: end of the current 64-character ``line'' to be ignored.
8911: @end itemize
8912:
8913: In Gforth, when you use @code{block} with a non-existent block number,
8914: the current blocks file will be extended to the appropriate size and the
8915: block buffer will be initialised with spaces.
8916:
8917: Gforth includes a simple block editor (type @code{use blocked.fb 0 list}
8918: for details) but doesn't encourage the use of blocks; the mechanism is
8919: only provided for backward compatibility -- ANS Forth requires blocks to
8920: be available when files are.
8921:
8922: Common techniques that are used when working with blocks include:
8923:
8924: @itemize @bullet
8925: @item
8926: A screen editor that allows you to edit blocks without leaving the Forth
8927: environment.
8928: @item
8929: Shadow screens; where every code block has an associated block
8930: containing comments (for example: code in odd block numbers, comments in
8931: even block numbers). Typically, the block editor provides a convenient
8932: mechanism to toggle between code and comments.
8933: @item
8934: Load blocks; a single block (typically block 1) contains a number of
8935: @code{thru} commands which @code{load} the whole of the application.
8936: @end itemize
8937:
8938: See Frank Sergeant's Pygmy Forth to see just how well blocks can be
8939: integrated into a Forth programming environment.
8940:
8941: @comment TODO what about errors on open-blocks?
8942:
8943: doc-open-blocks
8944: doc-use
8945: doc-block-offset
8946: doc-get-block-fid
8947: doc-block-position
8948:
8949: doc-list
8950: doc-scr
8951:
8952: doc-block
8953: doc-buffer
8954:
8955: doc-empty-buffers
8956: doc-empty-buffer
8957: doc-update
8958: doc-updated?
8959: doc-save-buffers
8960: doc-save-buffer
8961: doc-flush
8962:
8963: doc-load
8964: doc-thru
8965: doc-+load
8966: doc-+thru
8967: doc---gforthman--->
8968: doc-block-included
8969:
8970:
8971: @c -------------------------------------------------------------
8972: @node Other I/O, OS command line arguments, Blocks, Words
8973: @section Other I/O
8974: @cindex I/O - keyboard and display
8975:
8976: @menu
8977: * Simple numeric output:: Predefined formats
8978: * Formatted numeric output:: Formatted (pictured) output
8979: * String Formats:: How Forth stores strings in memory
8980: * Displaying characters and strings:: Other stuff
8981: * String words:: Gforth's little string library
8982: * Terminal output:: Cursor positioning etc.
8983: * Single-key input::
8984: * Line input and conversion::
8985: * Pipes:: How to create your own pipes
8986: * Xchars and Unicode:: Non-ASCII characters
8987: @end menu
8988:
8989: @node Simple numeric output, Formatted numeric output, Other I/O, Other I/O
8990: @subsection Simple numeric output
8991: @cindex numeric output - simple/free-format
8992:
8993: The simplest output functions are those that display numbers from the
8994: data or floating-point stacks. Floating-point output is always displayed
8995: using base 10. Numbers displayed from the data stack use the value stored
8996: in @code{base}.
8997:
8998:
8999: doc-.
9000: doc-dec.
9001: doc-hex.
9002: doc-u.
9003: doc-.r
9004: doc-u.r
9005: doc-d.
9006: doc-ud.
9007: doc-d.r
9008: doc-ud.r
9009: doc-f.
9010: doc-fe.
9011: doc-fs.
9012: doc-f.rdp
9013:
9014: Examples of printing the number 1234.5678E23 in the different floating-point output
9015: formats are shown below:
9016:
9017: @example
9018: f. 123456779999999000000000000.
9019: fe. 123.456779999999E24
9020: fs. 1.23456779999999E26
9021: @end example
9022:
9023:
9024: @node Formatted numeric output, String Formats, Simple numeric output, Other I/O
9025: @subsection Formatted numeric output
9026: @cindex formatted numeric output
9027: @cindex pictured numeric output
9028: @cindex numeric output - formatted
9029:
9030: Forth traditionally uses a technique called @dfn{pictured numeric
9031: output} for formatted printing of integers. In this technique, digits
9032: are extracted from the number (using the current output radix defined by
9033: @code{base}), converted to ASCII codes and appended to a string that is
9034: built in a scratch-pad area of memory (@pxref{core-idef,
9035: Implementation-defined options, Implementation-defined
9036: options}). Arbitrary characters can be appended to the string during the
9037: extraction process. The completed string is specified by an address
9038: and length and can be manipulated (@code{TYPE}ed, copied, modified)
9039: under program control.
9040:
9041: All of the integer output words described in the previous section
9042: (@pxref{Simple numeric output}) are implemented in Gforth using pictured
9043: numeric output.
9044:
9045: Three important things to remember about pictured numeric output:
9046:
9047: @itemize @bullet
9048: @item
9049: It always operates on double-precision numbers; to display a
9050: single-precision number, convert it first (for ways of doing this
9051: @pxref{Double precision}).
9052: @item
9053: It always treats the double-precision number as though it were
9054: unsigned. The examples below show ways of printing signed numbers.
9055: @item
9056: The string is built up from right to left; least significant digit first.
9057: @end itemize
9058:
9059:
9060: doc-<#
9061: doc-<<#
9062: doc-#
9063: doc-#s
9064: doc-hold
9065: doc-sign
9066: doc-#>
9067: doc-#>>
9068:
9069: doc-represent
9070: doc-f>str-rdp
9071: doc-f>buf-rdp
9072:
9073:
9074: @noindent
9075: Here are some examples of using pictured numeric output:
9076:
9077: @example
9078: : my-u. ( u -- )
9079: \ Simplest use of pns.. behaves like Standard u.
9080: 0 \ convert to unsigned double
9081: <<# \ start conversion
9082: #s \ convert all digits
9083: #> \ complete conversion
9084: TYPE SPACE \ display, with trailing space
9085: #>> ; \ release hold area
9086:
9087: : cents-only ( u -- )
9088: 0 \ convert to unsigned double
9089: <<# \ start conversion
9090: # # \ convert two least-significant digits
9091: #> \ complete conversion, discard other digits
9092: TYPE SPACE \ display, with trailing space
9093: #>> ; \ release hold area
9094:
9095: : dollars-and-cents ( u -- )
9096: 0 \ convert to unsigned double
9097: <<# \ start conversion
9098: # # \ convert two least-significant digits
9099: [char] . hold \ insert decimal point
9100: #s \ convert remaining digits
9101: [char] $ hold \ append currency symbol
9102: #> \ complete conversion
9103: TYPE SPACE \ display, with trailing space
9104: #>> ; \ release hold area
9105:
9106: : my-. ( n -- )
9107: \ handling negatives.. behaves like Standard .
9108: s>d \ convert to signed double
9109: swap over dabs \ leave sign byte followed by unsigned double
9110: <<# \ start conversion
9111: #s \ convert all digits
9112: rot sign \ get at sign byte, append "-" if needed
9113: #> \ complete conversion
9114: TYPE SPACE \ display, with trailing space
9115: #>> ; \ release hold area
9116:
9117: : account. ( n -- )
9118: \ accountants don't like minus signs, they use parentheses
9119: \ for negative numbers
9120: s>d \ convert to signed double
9121: swap over dabs \ leave sign byte followed by unsigned double
9122: <<# \ start conversion
9123: 2 pick \ get copy of sign byte
9124: 0< IF [char] ) hold THEN \ right-most character of output
9125: #s \ convert all digits
9126: rot \ get at sign byte
9127: 0< IF [char] ( hold THEN
9128: #> \ complete conversion
9129: TYPE SPACE \ display, with trailing space
9130: #>> ; \ release hold area
9131:
9132: @end example
9133:
9134: Here are some examples of using these words:
9135:
9136: @example
9137: 1 my-u. 1
9138: hex -1 my-u. decimal FFFFFFFF
9139: 1 cents-only 01
9140: 1234 cents-only 34
9141: 2 dollars-and-cents $0.02
9142: 1234 dollars-and-cents $12.34
9143: 123 my-. 123
9144: -123 my. -123
9145: 123 account. 123
9146: -456 account. (456)
9147: @end example
9148:
9149:
9150: @node String Formats, Displaying characters and strings, Formatted numeric output, Other I/O
9151: @subsection String Formats
9152: @cindex strings - see character strings
9153: @cindex character strings - formats
9154: @cindex I/O - see character strings
9155: @cindex counted strings
9156:
9157: @c anton: this does not really belong here; maybe the memory section,
9158: @c or the principles chapter
9159:
9160: Forth commonly uses two different methods for representing character
9161: strings:
9162:
9163: @itemize @bullet
9164: @item
9165: @cindex address of counted string
9166: @cindex counted string
9167: As a @dfn{counted string}, represented by a @i{c-addr}. The char
9168: addressed by @i{c-addr} contains a character-count, @i{n}, of the
9169: string and the string occupies the subsequent @i{n} char addresses in
9170: memory.
9171: @item
9172: As cell pair on the stack; @i{c-addr u}, where @i{u} is the length
9173: of the string in characters, and @i{c-addr} is the address of the
9174: first byte of the string.
9175: @end itemize
9176:
9177: ANS Forth encourages the use of the second format when representing
9178: strings.
9179:
9180:
9181: doc-count
9182:
9183:
9184: For words that move, copy and search for strings see @ref{Memory
9185: Blocks}. For words that display characters and strings see
9186: @ref{Displaying characters and strings}.
9187:
9188: @node Displaying characters and strings, String words, String Formats, Other I/O
9189: @subsection Displaying characters and strings
9190: @cindex characters - compiling and displaying
9191: @cindex character strings - compiling and displaying
9192:
9193: This section starts with a glossary of Forth words and ends with a set
9194: of examples.
9195:
9196: doc-bl
9197: doc-space
9198: doc-spaces
9199: doc-emit
9200: doc-toupper
9201: doc-."
9202: doc-.(
9203: doc-.\"
9204: doc-type
9205: doc-typewhite
9206: doc-cr
9207: @cindex cursor control
9208: doc-s"
9209: doc-s\"
9210: doc-c"
9211: doc-char
9212: doc-[char]
9213:
9214:
9215: @noindent
9216: As an example, consider the following text, stored in a file @file{test.fs}:
9217:
9218: @example
9219: .( text-1)
9220: : my-word
9221: ." text-2" cr
9222: .( text-3)
9223: ;
9224:
9225: ." text-4"
9226:
9227: : my-char
9228: [char] ALPHABET emit
9229: char emit
9230: ;
9231: @end example
9232:
9233: When you load this code into Gforth, the following output is generated:
9234:
9235: @example
9236: @kbd{include test.fs @key{RET}} text-1text-3text-4 ok
9237: @end example
9238:
9239: @itemize @bullet
9240: @item
9241: Messages @code{text-1} and @code{text-3} are displayed because @code{.(}
9242: is an immediate word; it behaves in the same way whether it is used inside
9243: or outside a colon definition.
9244: @item
9245: Message @code{text-4} is displayed because of Gforth's added interpretation
9246: semantics for @code{."}.
9247: @item
9248: Message @code{text-2} is @i{not} displayed, because the text interpreter
9249: performs the compilation semantics for @code{."} within the definition of
9250: @code{my-word}.
9251: @end itemize
9252:
9253: Here are some examples of executing @code{my-word} and @code{my-char}:
9254:
9255: @example
9256: @kbd{my-word @key{RET}} text-2
9257: ok
9258: @kbd{my-char fred @key{RET}} Af ok
9259: @kbd{my-char jim @key{RET}} Aj ok
9260: @end example
9261:
9262: @itemize @bullet
9263: @item
9264: Message @code{text-2} is displayed because of the run-time behaviour of
9265: @code{."}.
9266: @item
9267: @code{[char]} compiles the ``A'' from ``ALPHABET'' and puts its display code
9268: on the stack at run-time. @code{emit} always displays the character
9269: when @code{my-char} is executed.
9270: @item
9271: @code{char} parses a string at run-time and the second @code{emit} displays
9272: the first character of the string.
9273: @item
9274: If you type @code{see my-char} you can see that @code{[char]} discarded
9275: the text ``LPHABET'' and only compiled the display code for ``A'' into the
9276: definition of @code{my-char}.
9277: @end itemize
9278:
9279: @node String words, Terminal output, Displaying characters and strings, Other I/O
9280: @subsection String words
9281: @cindex string words
9282:
9283: The following string library stores strings in ordinary variables,
9284: which then contain a pointer to a counted string stored allocated from
9285: the heap. Instead of a count byte, there's a whole count cell,
9286: sufficient for all normal use. The string library originates from
9287: bigFORTH.
9288:
9289: doc-delete
9290: doc-insert
9291: doc-$!
9292: doc-$@
9293: doc-$@len
9294: doc-$!len
9295: doc-$del
9296: doc-$ins
9297: doc-$+!
9298: doc-$off
9299: doc-$init
9300: doc-$split
9301: doc-$iter
9302: doc-$over
9303: doc-$[]
9304: doc-$[]!
9305: doc-$[]+!
9306: doc-$[]@
9307:
9308: @node Terminal output, Single-key input, String words, Other I/O
9309: @subsection Terminal output
9310: @cindex output to terminal
9311: @cindex terminal output
9312:
9313: If you are outputting to a terminal, you may want to control the
9314: positioning of the cursor:
9315: @cindex cursor positioning
9316:
9317: doc-at-xy
9318:
9319: In order to know where to position the cursor, it is often helpful to
9320: know the size of the screen:
9321: @cindex terminal size
9322:
9323: doc-form
9324:
9325: And sometimes you want to use:
9326: @cindex clear screen
9327:
9328: doc-page
9329:
9330: Note that on non-terminals you should use @code{12 emit}, not
9331: @code{page}, to get a form feed.
9332:
9333:
9334: @node Single-key input, Line input and conversion, Terminal output, Other I/O
9335: @subsection Single-key input
9336: @cindex single-key input
9337: @cindex input, single-key
9338:
9339: If you want to get a single printable character, you can use
9340: @code{key}; to check whether a character is available for @code{key},
9341: you can use @code{key?}.
9342:
9343: doc-key
9344: doc-key?
9345:
9346: If you want to process a mix of printable and non-printable
9347: characters, you can do that with @code{ekey} and friends. @code{Ekey}
9348: produces a keyboard event that you have to convert into a character
9349: with @code{ekey>char} or into a key identifier with @code{ekey>fkey}.
9350:
9351: Typical code for using EKEY looks like this:
9352:
9353: @example
9354: ekey ekey>char if ( c )
9355: ... \ do something with the character
9356: else ekey>fkey if ( key-id )
9357: case
9358: k-up of ... endof
9359: k-f1 of ... endof
9360: k-left k-shift-mask or k-ctrl-mask or of ... endof
9361: ...
9362: endcase
9363: else ( keyboard-event )
9364: drop \ just ignore an unknown keyboard event type
9365: then then
9366: @end example
9367:
9368: doc-ekey
9369: doc-ekey>char
9370: doc-ekey>fkey
9371: doc-ekey?
9372:
9373: The key identifiers for cursor keys are:
9374:
9375: doc-k-left
9376: doc-k-right
9377: doc-k-up
9378: doc-k-down
9379: doc-k-home
9380: doc-k-end
9381: doc-k-prior
9382: doc-k-next
9383: doc-k-insert
9384: doc-k-delete
9385:
9386: The key identifiers for function keys (aka keypad keys) are:
9387:
9388: doc-k-f1
9389: doc-k-f2
9390: doc-k-f3
9391: doc-k-f4
9392: doc-k-f5
9393: doc-k-f6
9394: doc-k-f7
9395: doc-k-f8
9396: doc-k-f9
9397: doc-k-f10
9398: doc-k-f11
9399: doc-k-f12
9400:
9401: Note that @code{k-f11} and @code{k-f12} are not as widely available.
9402:
9403: You can combine these key identifiers with masks for various shift keys:
9404:
9405: doc-k-shift-mask
9406: doc-k-ctrl-mask
9407: doc-k-alt-mask
9408:
9409: Note that, even if a Forth system has @code{ekey>fkey} and the key
9410: identifier words, the keys are not necessarily available or it may not
9411: necessarily be able to report all the keys and all the possible
9412: combinations with shift masks. Therefore, write your programs in such
9413: a way that they are still useful even if the keys and key combinations
9414: cannot be pressed or are not recognized.
9415:
9416: Examples: Older keyboards often do not have an F11 and F12 key. If
9417: you run Gforth in an xterm, the xterm catches a number of combinations
9418: (e.g., @key{Shift-Up}), and never passes it to Gforth. Finally,
9419: Gforth currently does not recognize and report combinations with
9420: multiple shift keys (so the @key{shift-ctrl-left} case in the example
9421: above would never be entered).
9422:
9423: Gforth recognizes various keys available on ANSI terminals (in MS-DOS
9424: you need the ANSI.SYS driver to get that behaviour); it works by
9425: recognizing the escape sequences that ANSI terminals send when such a
9426: key is pressed. If you have a terminal that sends other escape
9427: sequences, you will not get useful results on Gforth. Other Forth
9428: systems may work in a different way.
9429:
9430: Gforth also provides a few words for outputting names of function
9431: keys:
9432:
9433: doc-fkey.
9434: doc-simple-fkey-string
9435:
9436:
9437: @node Line input and conversion, Pipes, Single-key input, Other I/O
9438: @subsection Line input and conversion
9439: @cindex line input from terminal
9440: @cindex input, linewise from terminal
9441: @cindex convertin strings to numbers
9442: @cindex I/O - see input
9443:
9444: For ways of storing character strings in memory see @ref{String Formats}.
9445:
9446: @comment TODO examples for >number >float accept key key? pad parse word refill
9447: @comment then index them
9448:
9449: Words for inputting one line from the keyboard:
9450:
9451: doc-accept
9452: doc-edit-line
9453:
9454: Conversion words:
9455:
9456: doc-s>number?
9457: doc-s>unumber?
9458: doc->number
9459: doc->float
9460: doc->float1
9461:
9462: @comment obsolescent words..
9463: Obsolescent input and conversion words:
9464:
9465: doc-convert
9466: doc-expect
9467: doc-span
9468:
9469:
9470: @node Pipes, Xchars and Unicode, Line input and conversion, Other I/O
9471: @subsection Pipes
9472: @cindex pipes, creating your own
9473:
9474: In addition to using Gforth in pipes created by other processes
9475: (@pxref{Gforth in pipes}), you can create your own pipe with
9476: @code{open-pipe}, and read from or write to it.
9477:
9478: doc-open-pipe
9479: doc-close-pipe
9480:
9481: If you write to a pipe, Gforth can throw a @code{broken-pipe-error}; if
9482: you don't catch this exception, Gforth will catch it and exit, usually
9483: silently (@pxref{Gforth in pipes}). Since you probably do not want
9484: this, you should wrap a @code{catch} or @code{try} block around the code
9485: from @code{open-pipe} to @code{close-pipe}, so you can deal with the
9486: problem yourself, and then return to regular processing.
9487:
9488: doc-broken-pipe-error
9489:
9490: @node Xchars and Unicode, , Pipes, Other I/O
9491: @subsection Xchars and Unicode
9492:
9493: ASCII is only appropriate for the English language. Most western
9494: languages however fit somewhat into the Forth frame, since a byte is
9495: sufficient to encode the few special characters in each (though not
9496: always the same encoding can be used; latin-1 is most widely used,
9497: though). For other languages, different char-sets have to be used,
9498: several of them variable-width. Most prominent representant is
9499: UTF-8. Let's call these extended characters xchars. The primitive
9500: fixed-size characters stored as bytes are called pchars in this
9501: section.
9502:
9503: The xchar words add a few data types:
9504:
9505: @itemize
9506:
9507: @item
9508: @var{xc} is an extended char (xchar) on the stack. It occupies one cell,
9509: and is a subset of unsigned cell. Note: UTF-8 can not store more that
9510: 31 bits; on 16 bit systems, only the UCS16 subset of the UTF-8
9511: character set can be used.
9512:
9513: @item
9514: @var{xc-addr} is the address of an xchar in memory. Alignment
9515: requirements are the same as @var{c-addr}. The memory representation of an
9516: xchar differs from the stack representation, and depends on the
9517: encoding used. An xchar may use a variable number of pchars in memory.
9518:
9519: @item
9520: @var{xc-addr} @var{u} is a buffer of xchars in memory, starting at
9521: @var{xc-addr}, @var{u} pchars long.
9522:
9523: @end itemize
9524:
9525: doc-xc-size
9526: doc-x-size
9527: doc-xc@+
9528: doc-xc!+?
9529: doc-xchar+
9530: doc-xchar-
9531: doc-+x/string
9532: doc-x\string-
9533: doc--trailing-garbage
9534: doc-x-width
9535: doc-xkey
9536: doc-xemit
9537:
9538: There's a new environment query
9539:
9540: doc-xchar-encoding
9541:
9542: @node OS command line arguments, Locals, Other I/O, Words
9543: @section OS command line arguments
9544: @cindex OS command line arguments
9545: @cindex command line arguments, OS
9546: @cindex arguments, OS command line
9547:
9548: The usual way to pass arguments to Gforth programs on the command line
9549: is via the @option{-e} option, e.g.
9550:
9551: @example
9552: gforth -e "123 456" foo.fs -e bye
9553: @end example
9554:
9555: However, you may want to interpret the command-line arguments directly.
9556: In that case, you can access the (image-specific) command-line arguments
9557: through @code{next-arg}:
9558:
9559: doc-next-arg
9560:
9561: Here's an example program @file{echo.fs} for @code{next-arg}:
9562:
9563: @example
9564: : echo ( -- )
9565: begin
9566: next-arg 2dup 0 0 d<> while
9567: type space
9568: repeat
9569: 2drop ;
9570:
9571: echo cr bye
9572: @end example
9573:
9574: This can be invoked with
9575:
9576: @example
9577: gforth echo.fs hello world
9578: @end example
9579:
9580: and it will print
9581:
9582: @example
9583: hello world
9584: @end example
9585:
9586: The next lower level of dealing with the OS command line are the
9587: following words:
9588:
9589: doc-arg
9590: doc-shift-args
9591:
9592: Finally, at the lowest level Gforth provides the following words:
9593:
9594: doc-argc
9595: doc-argv
9596:
9597: @c -------------------------------------------------------------
9598: @node Locals, Structures, OS command line arguments, Words
9599: @section Locals
9600: @cindex locals
9601:
9602: Local variables can make Forth programming more enjoyable and Forth
9603: programs easier to read. Unfortunately, the locals of ANS Forth are
9604: laden with restrictions. Therefore, we provide not only the ANS Forth
9605: locals wordset, but also our own, more powerful locals wordset (we
9606: implemented the ANS Forth locals wordset through our locals wordset).
9607:
9608: The ideas in this section have also been published in M. Anton Ertl,
9609: @cite{@uref{http://www.complang.tuwien.ac.at/papers/ertl94l.ps.gz,
9610: Automatic Scoping of Local Variables}}, EuroForth '94.
9611:
9612: @menu
9613: * Gforth locals::
9614: * ANS Forth locals::
9615: @end menu
9616:
9617: @node Gforth locals, ANS Forth locals, Locals, Locals
9618: @subsection Gforth locals
9619: @cindex Gforth locals
9620: @cindex locals, Gforth style
9621:
9622: Locals can be defined with
9623:
9624: @example
9625: @{ local1 local2 ... -- comment @}
9626: @end example
9627: or
9628: @example
9629: @{ local1 local2 ... @}
9630: @end example
9631:
9632: E.g.,
9633: @example
9634: : max @{ n1 n2 -- n3 @}
9635: n1 n2 > if
9636: n1
9637: else
9638: n2
9639: endif ;
9640: @end example
9641:
9642: The similarity of locals definitions with stack comments is intended. A
9643: locals definition often replaces the stack comment of a word. The order
9644: of the locals corresponds to the order in a stack comment and everything
9645: after the @code{--} is really a comment.
9646:
9647: This similarity has one disadvantage: It is too easy to confuse locals
9648: declarations with stack comments, causing bugs and making them hard to
9649: find. However, this problem can be avoided by appropriate coding
9650: conventions: Do not use both notations in the same program. If you do,
9651: they should be distinguished using additional means, e.g. by position.
9652:
9653: @cindex types of locals
9654: @cindex locals types
9655: The name of the local may be preceded by a type specifier, e.g.,
9656: @code{F:} for a floating point value:
9657:
9658: @example
9659: : CX* @{ F: Ar F: Ai F: Br F: Bi -- Cr Ci @}
9660: \ complex multiplication
9661: Ar Br f* Ai Bi f* f-
9662: Ar Bi f* Ai Br f* f+ ;
9663: @end example
9664:
9665: @cindex flavours of locals
9666: @cindex locals flavours
9667: @cindex value-flavoured locals
9668: @cindex variable-flavoured locals
9669: Gforth currently supports cells (@code{W:}, @code{W^}), doubles
9670: (@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters
9671: (@code{C:}, @code{C^}) in two flavours: a value-flavoured local (defined
9672: with @code{W:}, @code{D:} etc.) produces its value and can be changed
9673: with @code{TO}. A variable-flavoured local (defined with @code{W^} etc.)
9674: produces its address (which becomes invalid when the variable's scope is
9675: left). E.g., the standard word @code{emit} can be defined in terms of
9676: @code{type} like this:
9677:
9678: @example
9679: : emit @{ C^ char* -- @}
9680: char* 1 type ;
9681: @end example
9682:
9683: @cindex default type of locals
9684: @cindex locals, default type
9685: A local without type specifier is a @code{W:} local. Both flavours of
9686: locals are initialized with values from the data or FP stack.
9687:
9688: Currently there is no way to define locals with user-defined data
9689: structures, but we are working on it.
9690:
9691: Gforth allows defining locals everywhere in a colon definition. This
9692: poses the following questions:
9693:
9694: @menu
9695: * Where are locals visible by name?::
9696: * How long do locals live?::
9697: * Locals programming style::
9698: * Locals implementation::
9699: @end menu
9700:
9701: @node Where are locals visible by name?, How long do locals live?, Gforth locals, Gforth locals
9702: @subsubsection Where are locals visible by name?
9703: @cindex locals visibility
9704: @cindex visibility of locals
9705: @cindex scope of locals
9706:
9707: Basically, the answer is that locals are visible where you would expect
9708: it in block-structured languages, and sometimes a little longer. If you
9709: want to restrict the scope of a local, enclose its definition in
9710: @code{SCOPE}...@code{ENDSCOPE}.
9711:
9712:
9713: doc-scope
9714: doc-endscope
9715:
9716:
9717: These words behave like control structure words, so you can use them
9718: with @code{CS-PICK} and @code{CS-ROLL} to restrict the scope in
9719: arbitrary ways.
9720:
9721: If you want a more exact answer to the visibility question, here's the
9722: basic principle: A local is visible in all places that can only be
9723: reached through the definition of the local@footnote{In compiler
9724: construction terminology, all places dominated by the definition of the
9725: local.}. In other words, it is not visible in places that can be reached
9726: without going through the definition of the local. E.g., locals defined
9727: in @code{IF}...@code{ENDIF} are visible until the @code{ENDIF}, locals
9728: defined in @code{BEGIN}...@code{UNTIL} are visible after the
9729: @code{UNTIL} (until, e.g., a subsequent @code{ENDSCOPE}).
9730:
9731: The reasoning behind this solution is: We want to have the locals
9732: visible as long as it is meaningful. The user can always make the
9733: visibility shorter by using explicit scoping. In a place that can
9734: only be reached through the definition of a local, the meaning of a
9735: local name is clear. In other places it is not: How is the local
9736: initialized at the control flow path that does not contain the
9737: definition? Which local is meant, if the same name is defined twice in
9738: two independent control flow paths?
9739:
9740: This should be enough detail for nearly all users, so you can skip the
9741: rest of this section. If you really must know all the gory details and
9742: options, read on.
9743:
9744: In order to implement this rule, the compiler has to know which places
9745: are unreachable. It knows this automatically after @code{AHEAD},
9746: @code{AGAIN}, @code{EXIT} and @code{LEAVE}; in other cases (e.g., after
9747: most @code{THROW}s), you can use the word @code{UNREACHABLE} to tell the
9748: compiler that the control flow never reaches that place. If
9749: @code{UNREACHABLE} is not used where it could, the only consequence is
9750: that the visibility of some locals is more limited than the rule above
9751: says. If @code{UNREACHABLE} is used where it should not (i.e., if you
9752: lie to the compiler), buggy code will be produced.
9753:
9754:
9755: doc-unreachable
9756:
9757:
9758: Another problem with this rule is that at @code{BEGIN}, the compiler
9759: does not know which locals will be visible on the incoming
9760: back-edge. All problems discussed in the following are due to this
9761: ignorance of the compiler (we discuss the problems using @code{BEGIN}
9762: loops as examples; the discussion also applies to @code{?DO} and other
9763: loops). Perhaps the most insidious example is:
9764: @example
9765: AHEAD
9766: BEGIN
9767: x
9768: [ 1 CS-ROLL ] THEN
9769: @{ x @}
9770: ...
9771: UNTIL
9772: @end example
9773:
9774: This should be legal according to the visibility rule. The use of
9775: @code{x} can only be reached through the definition; but that appears
9776: textually below the use.
9777:
9778: From this example it is clear that the visibility rules cannot be fully
9779: implemented without major headaches. Our implementation treats common
9780: cases as advertised and the exceptions are treated in a safe way: The
9781: compiler makes a reasonable guess about the locals visible after a
9782: @code{BEGIN}; if it is too pessimistic, the
9783: user will get a spurious error about the local not being defined; if the
9784: compiler is too optimistic, it will notice this later and issue a
9785: warning. In the case above the compiler would complain about @code{x}
9786: being undefined at its use. You can see from the obscure examples in
9787: this section that it takes quite unusual control structures to get the
9788: compiler into trouble, and even then it will often do fine.
9789:
9790: If the @code{BEGIN} is reachable from above, the most optimistic guess
9791: is that all locals visible before the @code{BEGIN} will also be
9792: visible after the @code{BEGIN}. This guess is valid for all loops that
9793: are entered only through the @code{BEGIN}, in particular, for normal
9794: @code{BEGIN}...@code{WHILE}...@code{REPEAT} and
9795: @code{BEGIN}...@code{UNTIL} loops and it is implemented in our
9796: compiler. When the branch to the @code{BEGIN} is finally generated by
9797: @code{AGAIN} or @code{UNTIL}, the compiler checks the guess and
9798: warns the user if it was too optimistic:
9799: @example
9800: IF
9801: @{ x @}
9802: BEGIN
9803: \ x ?
9804: [ 1 cs-roll ] THEN
9805: ...
9806: UNTIL
9807: @end example
9808:
9809: Here, @code{x} lives only until the @code{BEGIN}, but the compiler
9810: optimistically assumes that it lives until the @code{THEN}. It notices
9811: this difference when it compiles the @code{UNTIL} and issues a
9812: warning. The user can avoid the warning, and make sure that @code{x}
9813: is not used in the wrong area by using explicit scoping:
9814: @example
9815: IF
9816: SCOPE
9817: @{ x @}
9818: ENDSCOPE
9819: BEGIN
9820: [ 1 cs-roll ] THEN
9821: ...
9822: UNTIL
9823: @end example
9824:
9825: Since the guess is optimistic, there will be no spurious error messages
9826: about undefined locals.
9827:
9828: If the @code{BEGIN} is not reachable from above (e.g., after
9829: @code{AHEAD} or @code{EXIT}), the compiler cannot even make an
9830: optimistic guess, as the locals visible after the @code{BEGIN} may be
9831: defined later. Therefore, the compiler assumes that no locals are
9832: visible after the @code{BEGIN}. However, the user can use
9833: @code{ASSUME-LIVE} to make the compiler assume that the same locals are
9834: visible at the BEGIN as at the point where the top control-flow stack
9835: item was created.
9836:
9837:
9838: doc-assume-live
9839:
9840:
9841: @noindent
9842: E.g.,
9843: @example
9844: @{ x @}
9845: AHEAD
9846: ASSUME-LIVE
9847: BEGIN
9848: x
9849: [ 1 CS-ROLL ] THEN
9850: ...
9851: UNTIL
9852: @end example
9853:
9854: Other cases where the locals are defined before the @code{BEGIN} can be
9855: handled by inserting an appropriate @code{CS-ROLL} before the
9856: @code{ASSUME-LIVE} (and changing the control-flow stack manipulation
9857: behind the @code{ASSUME-LIVE}).
9858:
9859: Cases where locals are defined after the @code{BEGIN} (but should be
9860: visible immediately after the @code{BEGIN}) can only be handled by
9861: rearranging the loop. E.g., the ``most insidious'' example above can be
9862: arranged into:
9863: @example
9864: BEGIN
9865: @{ x @}
9866: ... 0=
9867: WHILE
9868: x
9869: REPEAT
9870: @end example
9871:
9872: @node How long do locals live?, Locals programming style, Where are locals visible by name?, Gforth locals
9873: @subsubsection How long do locals live?
9874: @cindex locals lifetime
9875: @cindex lifetime of locals
9876:
9877: The right answer for the lifetime question would be: A local lives at
9878: least as long as it can be accessed. For a value-flavoured local this
9879: means: until the end of its visibility. However, a variable-flavoured
9880: local could be accessed through its address far beyond its visibility
9881: scope. Ultimately, this would mean that such locals would have to be
9882: garbage collected. Since this entails un-Forth-like implementation
9883: complexities, I adopted the same cowardly solution as some other
9884: languages (e.g., C): The local lives only as long as it is visible;
9885: afterwards its address is invalid (and programs that access it
9886: afterwards are erroneous).
9887:
9888: @node Locals programming style, Locals implementation, How long do locals live?, Gforth locals
9889: @subsubsection Locals programming style
9890: @cindex locals programming style
9891: @cindex programming style, locals
9892:
9893: The freedom to define locals anywhere has the potential to change
9894: programming styles dramatically. In particular, the need to use the
9895: return stack for intermediate storage vanishes. Moreover, all stack
9896: manipulations (except @code{PICK}s and @code{ROLL}s with run-time
9897: determined arguments) can be eliminated: If the stack items are in the
9898: wrong order, just write a locals definition for all of them; then
9899: write the items in the order you want.
9900:
9901: This seems a little far-fetched and eliminating stack manipulations is
9902: unlikely to become a conscious programming objective. Still, the number
9903: of stack manipulations will be reduced dramatically if local variables
9904: are used liberally (e.g., compare @code{max} (@pxref{Gforth locals}) with
9905: a traditional implementation of @code{max}).
9906:
9907: This shows one potential benefit of locals: making Forth programs more
9908: readable. Of course, this benefit will only be realized if the
9909: programmers continue to honour the principle of factoring instead of
9910: using the added latitude to make the words longer.
9911:
9912: @cindex single-assignment style for locals
9913: Using @code{TO} can and should be avoided. Without @code{TO},
9914: every value-flavoured local has only a single assignment and many
9915: advantages of functional languages apply to Forth. I.e., programs are
9916: easier to analyse, to optimize and to read: It is clear from the
9917: definition what the local stands for, it does not turn into something
9918: different later.
9919:
9920: E.g., a definition using @code{TO} might look like this:
9921: @example
9922: : strcmp @{ addr1 u1 addr2 u2 -- n @}
9923: u1 u2 min 0
9924: ?do
9925: addr1 c@@ addr2 c@@ -
9926: ?dup-if
9927: unloop exit
9928: then
9929: addr1 char+ TO addr1
9930: addr2 char+ TO addr2
9931: loop
9932: u1 u2 - ;
9933: @end example
9934: Here, @code{TO} is used to update @code{addr1} and @code{addr2} at
9935: every loop iteration. @code{strcmp} is a typical example of the
9936: readability problems of using @code{TO}. When you start reading
9937: @code{strcmp}, you think that @code{addr1} refers to the start of the
9938: string. Only near the end of the loop you realize that it is something
9939: else.
9940:
9941: This can be avoided by defining two locals at the start of the loop that
9942: are initialized with the right value for the current iteration.
9943: @example
9944: : strcmp @{ addr1 u1 addr2 u2 -- n @}
9945: addr1 addr2
9946: u1 u2 min 0
9947: ?do @{ s1 s2 @}
9948: s1 c@@ s2 c@@ -
9949: ?dup-if
9950: unloop exit
9951: then
9952: s1 char+ s2 char+
9953: loop
9954: 2drop
9955: u1 u2 - ;
9956: @end example
9957: Here it is clear from the start that @code{s1} has a different value
9958: in every loop iteration.
9959:
9960: @node Locals implementation, , Locals programming style, Gforth locals
9961: @subsubsection Locals implementation
9962: @cindex locals implementation
9963: @cindex implementation of locals
9964:
9965: @cindex locals stack
9966: Gforth uses an extra locals stack. The most compelling reason for
9967: this is that the return stack is not float-aligned; using an extra stack
9968: also eliminates the problems and restrictions of using the return stack
9969: as locals stack. Like the other stacks, the locals stack grows toward
9970: lower addresses. A few primitives allow an efficient implementation:
9971:
9972:
9973: doc-@local#
9974: doc-f@local#
9975: doc-laddr#
9976: doc-lp+!#
9977: doc-lp!
9978: doc->l
9979: doc-f>l
9980:
9981:
9982: In addition to these primitives, some specializations of these
9983: primitives for commonly occurring inline arguments are provided for
9984: efficiency reasons, e.g., @code{@@local0} as specialization of
9985: @code{@@local#} for the inline argument 0. The following compiling words
9986: compile the right specialized version, or the general version, as
9987: appropriate:
9988:
9989:
9990: @c doc-compile-@local
9991: @c doc-compile-f@local
9992: doc-compile-lp+!
9993:
9994:
9995: Combinations of conditional branches and @code{lp+!#} like
9996: @code{?branch-lp+!#} (the locals pointer is only changed if the branch
9997: is taken) are provided for efficiency and correctness in loops.
9998:
9999: A special area in the dictionary space is reserved for keeping the
10000: local variable names. @code{@{} switches the dictionary pointer to this
10001: area and @code{@}} switches it back and generates the locals
10002: initializing code. @code{W:} etc.@ are normal defining words. This
10003: special area is cleared at the start of every colon definition.
10004:
10005: @cindex word list for defining locals
10006: A special feature of Gforth's dictionary is used to implement the
10007: definition of locals without type specifiers: every word list (aka
10008: vocabulary) has its own methods for searching
10009: etc. (@pxref{Word Lists}). For the present purpose we defined a word list
10010: with a special search method: When it is searched for a word, it
10011: actually creates that word using @code{W:}. @code{@{} changes the search
10012: order to first search the word list containing @code{@}}, @code{W:} etc.,
10013: and then the word list for defining locals without type specifiers.
10014:
10015: The lifetime rules support a stack discipline within a colon
10016: definition: The lifetime of a local is either nested with other locals
10017: lifetimes or it does not overlap them.
10018:
10019: At @code{BEGIN}, @code{IF}, and @code{AHEAD} no code for locals stack
10020: pointer manipulation is generated. Between control structure words
10021: locals definitions can push locals onto the locals stack. @code{AGAIN}
10022: is the simplest of the other three control flow words. It has to
10023: restore the locals stack depth of the corresponding @code{BEGIN}
10024: before branching. The code looks like this:
10025: @format
10026: @code{lp+!#} current-locals-size @minus{} dest-locals-size
10027: @code{branch} <begin>
10028: @end format
10029:
10030: @code{UNTIL} is a little more complicated: If it branches back, it
10031: must adjust the stack just like @code{AGAIN}. But if it falls through,
10032: the locals stack must not be changed. The compiler generates the
10033: following code:
10034: @format
10035: @code{?branch-lp+!#} <begin> current-locals-size @minus{} dest-locals-size
10036: @end format
10037: The locals stack pointer is only adjusted if the branch is taken.
10038:
10039: @code{THEN} can produce somewhat inefficient code:
10040: @format
10041: @code{lp+!#} current-locals-size @minus{} orig-locals-size
10042: <orig target>:
10043: @code{lp+!#} orig-locals-size @minus{} new-locals-size
10044: @end format
10045: The second @code{lp+!#} adjusts the locals stack pointer from the
10046: level at the @i{orig} point to the level after the @code{THEN}. The
10047: first @code{lp+!#} adjusts the locals stack pointer from the current
10048: level to the level at the orig point, so the complete effect is an
10049: adjustment from the current level to the right level after the
10050: @code{THEN}.
10051:
10052: @cindex locals information on the control-flow stack
10053: @cindex control-flow stack items, locals information
10054: In a conventional Forth implementation a dest control-flow stack entry
10055: is just the target address and an orig entry is just the address to be
10056: patched. Our locals implementation adds a word list to every orig or dest
10057: item. It is the list of locals visible (or assumed visible) at the point
10058: described by the entry. Our implementation also adds a tag to identify
10059: the kind of entry, in particular to differentiate between live and dead
10060: (reachable and unreachable) orig entries.
10061:
10062: A few unusual operations have to be performed on locals word lists:
10063:
10064:
10065: doc-common-list
10066: doc-sub-list?
10067: doc-list-size
10068:
10069:
10070: Several features of our locals word list implementation make these
10071: operations easy to implement: The locals word lists are organised as
10072: linked lists; the tails of these lists are shared, if the lists
10073: contain some of the same locals; and the address of a name is greater
10074: than the address of the names behind it in the list.
10075:
10076: Another important implementation detail is the variable
10077: @code{dead-code}. It is used by @code{BEGIN} and @code{THEN} to
10078: determine if they can be reached directly or only through the branch
10079: that they resolve. @code{dead-code} is set by @code{UNREACHABLE},
10080: @code{AHEAD}, @code{EXIT} etc., and cleared at the start of a colon
10081: definition, by @code{BEGIN} and usually by @code{THEN}.
10082:
10083: Counted loops are similar to other loops in most respects, but
10084: @code{LEAVE} requires special attention: It performs basically the same
10085: service as @code{AHEAD}, but it does not create a control-flow stack
10086: entry. Therefore the information has to be stored elsewhere;
10087: traditionally, the information was stored in the target fields of the
10088: branches created by the @code{LEAVE}s, by organizing these fields into a
10089: linked list. Unfortunately, this clever trick does not provide enough
10090: space for storing our extended control flow information. Therefore, we
10091: introduce another stack, the leave stack. It contains the control-flow
10092: stack entries for all unresolved @code{LEAVE}s.
10093:
10094: Local names are kept until the end of the colon definition, even if
10095: they are no longer visible in any control-flow path. In a few cases
10096: this may lead to increased space needs for the locals name area, but
10097: usually less than reclaiming this space would cost in code size.
10098:
10099:
10100: @node ANS Forth locals, , Gforth locals, Locals
10101: @subsection ANS Forth locals
10102: @cindex locals, ANS Forth style
10103:
10104: The ANS Forth locals wordset does not define a syntax for locals, but
10105: words that make it possible to define various syntaxes. One of the
10106: possible syntaxes is a subset of the syntax we used in the Gforth locals
10107: wordset, i.e.:
10108:
10109: @example
10110: @{ local1 local2 ... -- comment @}
10111: @end example
10112: @noindent
10113: or
10114: @example
10115: @{ local1 local2 ... @}
10116: @end example
10117:
10118: The order of the locals corresponds to the order in a stack comment. The
10119: restrictions are:
10120:
10121: @itemize @bullet
10122: @item
10123: Locals can only be cell-sized values (no type specifiers are allowed).
10124: @item
10125: Locals can be defined only outside control structures.
10126: @item
10127: Locals can interfere with explicit usage of the return stack. For the
10128: exact (and long) rules, see the standard. If you don't use return stack
10129: accessing words in a definition using locals, you will be all right. The
10130: purpose of this rule is to make locals implementation on the return
10131: stack easier.
10132: @item
10133: The whole definition must be in one line.
10134: @end itemize
10135:
10136: Locals defined in ANS Forth behave like @code{VALUE}s
10137: (@pxref{Values}). I.e., they are initialized from the stack. Using their
10138: name produces their value. Their value can be changed using @code{TO}.
10139:
10140: Since the syntax above is supported by Gforth directly, you need not do
10141: anything to use it. If you want to port a program using this syntax to
10142: another ANS Forth system, use @file{compat/anslocal.fs} to implement the
10143: syntax on the other system.
10144:
10145: Note that a syntax shown in the standard, section A.13 looks
10146: similar, but is quite different in having the order of locals
10147: reversed. Beware!
10148:
10149: The ANS Forth locals wordset itself consists of one word:
10150:
10151: doc-(local)
10152:
10153: The ANS Forth locals extension wordset defines a syntax using
10154: @code{locals|}, but it is so awful that we strongly recommend not to use
10155: it. We have implemented this syntax to make porting to Gforth easy, but
10156: do not document it here. The problem with this syntax is that the locals
10157: are defined in an order reversed with respect to the standard stack
10158: comment notation, making programs harder to read, and easier to misread
10159: and miswrite. The only merit of this syntax is that it is easy to
10160: implement using the ANS Forth locals wordset.
10161:
10162:
10163: @c ----------------------------------------------------------
10164: @node Structures, Object-oriented Forth, Locals, Words
10165: @section Structures
10166: @cindex structures
10167: @cindex records
10168:
10169: This section presents the structure package that comes with Gforth. A
10170: version of the package implemented in ANS Forth is available in
10171: @file{compat/struct.fs}. This package was inspired by a posting on
10172: comp.lang.forth in 1989 (unfortunately I don't remember, by whom;
10173: possibly John Hayes). A version of this section has been published in
10174: M. Anton Ertl,
10175: @uref{http://www.complang.tuwien.ac.at/forth/objects/structs.html, Yet
10176: Another Forth Structures Package}, Forth Dimensions 19(3), pages
10177: 13--16. Marcel Hendrix provided helpful comments.
10178:
10179: @menu
10180: * Why explicit structure support?::
10181: * Structure Usage::
10182: * Structure Naming Convention::
10183: * Structure Implementation::
10184: * Structure Glossary::
10185: * Forth200x Structures::
10186: @end menu
10187:
10188: @node Why explicit structure support?, Structure Usage, Structures, Structures
10189: @subsection Why explicit structure support?
10190:
10191: @cindex address arithmetic for structures
10192: @cindex structures using address arithmetic
10193: If we want to use a structure containing several fields, we could simply
10194: reserve memory for it, and access the fields using address arithmetic
10195: (@pxref{Address arithmetic}). As an example, consider a structure with
10196: the following fields
10197:
10198: @table @code
10199: @item a
10200: is a float
10201: @item b
10202: is a cell
10203: @item c
10204: is a float
10205: @end table
10206:
10207: Given the (float-aligned) base address of the structure we get the
10208: address of the field
10209:
10210: @table @code
10211: @item a
10212: without doing anything further.
10213: @item b
10214: with @code{float+}
10215: @item c
10216: with @code{float+ cell+ faligned}
10217: @end table
10218:
10219: It is easy to see that this can become quite tiring.
10220:
10221: Moreover, it is not very readable, because seeing a
10222: @code{cell+} tells us neither which kind of structure is
10223: accessed nor what field is accessed; we have to somehow infer the kind
10224: of structure, and then look up in the documentation, which field of
10225: that structure corresponds to that offset.
10226:
10227: Finally, this kind of address arithmetic also causes maintenance
10228: troubles: If you add or delete a field somewhere in the middle of the
10229: structure, you have to find and change all computations for the fields
10230: afterwards.
10231:
10232: So, instead of using @code{cell+} and friends directly, how
10233: about storing the offsets in constants:
10234:
10235: @example
10236: 0 constant a-offset
10237: 0 float+ constant b-offset
10238: 0 float+ cell+ faligned c-offset
10239: @end example
10240:
10241: Now we can get the address of field @code{x} with @code{x-offset
10242: +}. This is much better in all respects. Of course, you still
10243: have to change all later offset definitions if you add a field. You can
10244: fix this by declaring the offsets in the following way:
10245:
10246: @example
10247: 0 constant a-offset
10248: a-offset float+ constant b-offset
10249: b-offset cell+ faligned constant c-offset
10250: @end example
10251:
10252: Since we always use the offsets with @code{+}, we could use a defining
10253: word @code{cfield} that includes the @code{+} in the action of the
10254: defined word:
10255:
10256: @example
10257: : cfield ( n "name" -- )
10258: create ,
10259: does> ( name execution: addr1 -- addr2 )
10260: @@ + ;
10261:
10262: 0 cfield a
10263: 0 a float+ cfield b
10264: 0 b cell+ faligned cfield c
10265: @end example
10266:
10267: Instead of @code{x-offset +}, we now simply write @code{x}.
10268:
10269: The structure field words now can be used quite nicely. However,
10270: their definition is still a bit cumbersome: We have to repeat the
10271: name, the information about size and alignment is distributed before
10272: and after the field definitions etc. The structure package presented
10273: here addresses these problems.
10274:
10275: @node Structure Usage, Structure Naming Convention, Why explicit structure support?, Structures
10276: @subsection Structure Usage
10277: @cindex structure usage
10278:
10279: @cindex @code{field} usage
10280: @cindex @code{struct} usage
10281: @cindex @code{end-struct} usage
10282: You can define a structure for a (data-less) linked list with:
10283: @example
10284: struct
10285: cell% field list-next
10286: end-struct list%
10287: @end example
10288:
10289: With the address of the list node on the stack, you can compute the
10290: address of the field that contains the address of the next node with
10291: @code{list-next}. E.g., you can determine the length of a list
10292: with:
10293:
10294: @example
10295: : list-length ( list -- n )
10296: \ "list" is a pointer to the first element of a linked list
10297: \ "n" is the length of the list
10298: 0 BEGIN ( list1 n1 )
10299: over
10300: WHILE ( list1 n1 )
10301: 1+ swap list-next @@ swap
10302: REPEAT
10303: nip ;
10304: @end example
10305:
10306: You can reserve memory for a list node in the dictionary with
10307: @code{list% %allot}, which leaves the address of the list node on the
10308: stack. For the equivalent allocation on the heap you can use @code{list%
10309: %alloc} (or, for an @code{allocate}-like stack effect (i.e., with ior),
10310: use @code{list% %allocate}). You can get the the size of a list
10311: node with @code{list% %size} and its alignment with @code{list%
10312: %alignment}.
10313:
10314: Note that in ANS Forth the body of a @code{create}d word is
10315: @code{aligned} but not necessarily @code{faligned};
10316: therefore, if you do a:
10317:
10318: @example
10319: create @emph{name} foo% %allot drop
10320: @end example
10321:
10322: @noindent
10323: then the memory alloted for @code{foo%} is guaranteed to start at the
10324: body of @code{@emph{name}} only if @code{foo%} contains only character,
10325: cell and double fields. Therefore, if your structure contains floats,
10326: better use
10327:
10328: @example
10329: foo% %allot constant @emph{name}
10330: @end example
10331:
10332: @cindex structures containing structures
10333: You can include a structure @code{foo%} as a field of
10334: another structure, like this:
10335: @example
10336: struct
10337: ...
10338: foo% field ...
10339: ...
10340: end-struct ...
10341: @end example
10342:
10343: @cindex structure extension
10344: @cindex extended records
10345: Instead of starting with an empty structure, you can extend an
10346: existing structure. E.g., a plain linked list without data, as defined
10347: above, is hardly useful; You can extend it to a linked list of integers,
10348: like this:@footnote{This feature is also known as @emph{extended
10349: records}. It is the main innovation in the Oberon language; in other
10350: words, adding this feature to Modula-2 led Wirth to create a new
10351: language, write a new compiler etc. Adding this feature to Forth just
10352: required a few lines of code.}
10353:
10354: @example
10355: list%
10356: cell% field intlist-int
10357: end-struct intlist%
10358: @end example
10359:
10360: @code{intlist%} is a structure with two fields:
10361: @code{list-next} and @code{intlist-int}.
10362:
10363: @cindex structures containing arrays
10364: You can specify an array type containing @emph{n} elements of
10365: type @code{foo%} like this:
10366:
10367: @example
10368: foo% @emph{n} *
10369: @end example
10370:
10371: You can use this array type in any place where you can use a normal
10372: type, e.g., when defining a @code{field}, or with
10373: @code{%allot}.
10374:
10375: @cindex first field optimization
10376: The first field is at the base address of a structure and the word for
10377: this field (e.g., @code{list-next}) actually does not change the address
10378: on the stack. You may be tempted to leave it away in the interest of
10379: run-time and space efficiency. This is not necessary, because the
10380: structure package optimizes this case: If you compile a first-field
10381: words, no code is generated. So, in the interest of readability and
10382: maintainability you should include the word for the field when accessing
10383: the field.
10384:
10385:
10386: @node Structure Naming Convention, Structure Implementation, Structure Usage, Structures
10387: @subsection Structure Naming Convention
10388: @cindex structure naming convention
10389:
10390: The field names that come to (my) mind are often quite generic, and,
10391: if used, would cause frequent name clashes. E.g., many structures
10392: probably contain a @code{counter} field. The structure names
10393: that come to (my) mind are often also the logical choice for the names
10394: of words that create such a structure.
10395:
10396: Therefore, I have adopted the following naming conventions:
10397:
10398: @itemize @bullet
10399: @cindex field naming convention
10400: @item
10401: The names of fields are of the form
10402: @code{@emph{struct}-@emph{field}}, where
10403: @code{@emph{struct}} is the basic name of the structure, and
10404: @code{@emph{field}} is the basic name of the field. You can
10405: think of field words as converting the (address of the)
10406: structure into the (address of the) field.
10407:
10408: @cindex structure naming convention
10409: @item
10410: The names of structures are of the form
10411: @code{@emph{struct}%}, where
10412: @code{@emph{struct}} is the basic name of the structure.
10413: @end itemize
10414:
10415: This naming convention does not work that well for fields of extended
10416: structures; e.g., the integer list structure has a field
10417: @code{intlist-int}, but has @code{list-next}, not
10418: @code{intlist-next}.
10419:
10420: @node Structure Implementation, Structure Glossary, Structure Naming Convention, Structures
10421: @subsection Structure Implementation
10422: @cindex structure implementation
10423: @cindex implementation of structures
10424:
10425: The central idea in the implementation is to pass the data about the
10426: structure being built on the stack, not in some global
10427: variable. Everything else falls into place naturally once this design
10428: decision is made.
10429:
10430: The type description on the stack is of the form @emph{align
10431: size}. Keeping the size on the top-of-stack makes dealing with arrays
10432: very simple.
10433:
10434: @code{field} is a defining word that uses @code{Create}
10435: and @code{DOES>}. The body of the field contains the offset
10436: of the field, and the normal @code{DOES>} action is simply:
10437:
10438: @example
10439: @@ +
10440: @end example
10441:
10442: @noindent
10443: i.e., add the offset to the address, giving the stack effect
10444: @i{addr1 -- addr2} for a field.
10445:
10446: @cindex first field optimization, implementation
10447: This simple structure is slightly complicated by the optimization
10448: for fields with offset 0, which requires a different
10449: @code{DOES>}-part (because we cannot rely on there being
10450: something on the stack if such a field is invoked during
10451: compilation). Therefore, we put the different @code{DOES>}-parts
10452: in separate words, and decide which one to invoke based on the
10453: offset. For a zero offset, the field is basically a noop; it is
10454: immediate, and therefore no code is generated when it is compiled.
10455:
10456: @node Structure Glossary, Forth200x Structures, Structure Implementation, Structures
10457: @subsection Structure Glossary
10458: @cindex structure glossary
10459:
10460:
10461: doc-%align
10462: doc-%alignment
10463: doc-%alloc
10464: doc-%allocate
10465: doc-%allot
10466: doc-cell%
10467: doc-char%
10468: doc-dfloat%
10469: doc-double%
10470: doc-end-struct
10471: doc-field
10472: doc-float%
10473: doc-naligned
10474: doc-sfloat%
10475: doc-%size
10476: doc-struct
10477:
10478:
10479: @node Forth200x Structures, , Structure Glossary, Structures
10480: @subsection Forth200x Structures
10481: @cindex Structures in Forth200x
10482:
10483: The Forth 200x standard defines a slightly less convenient form of
10484: structures. In general (when using @code{field+}, you have to perform
10485: the alignment yourself, but there are a number of convenience words
10486: (e.g., @code{field:} that perform the alignment for you.
10487:
10488: A typical usage example is:
10489:
10490: @example
10491: 0
10492: field: s-a
10493: faligned 2 floats +field s-b
10494: constant s-struct
10495: @end example
10496:
10497: An alternative way of writing this structure is:
10498:
10499: @example
10500: begin-structure s-struct
10501: field: s-a
10502: faligned 2 floats +field s-b
10503: end-structure
10504: @end example
10505:
10506: doc-begin-structure
10507: doc-end-structure
10508: doc-+field
10509: doc-cfield:
10510: doc-field:
10511: doc-2field:
10512: doc-ffield:
10513: doc-sffield:
10514: doc-dffield:
10515:
10516: @c -------------------------------------------------------------
10517: @node Object-oriented Forth, Programming Tools, Structures, Words
10518: @section Object-oriented Forth
10519:
10520: Gforth comes with three packages for object-oriented programming:
10521: @file{objects.fs}, @file{oof.fs}, and @file{mini-oof.fs}; none of them
10522: is preloaded, so you have to @code{include} them before use. The most
10523: important differences between these packages (and others) are discussed
10524: in @ref{Comparison with other object models}. All packages are written
10525: in ANS Forth and can be used with any other ANS Forth.
10526:
10527: @menu
10528: * Why object-oriented programming?::
10529: * Object-Oriented Terminology::
10530: * Objects::
10531: * OOF::
10532: * Mini-OOF::
10533: * Comparison with other object models::
10534: @end menu
10535:
10536: @c ----------------------------------------------------------------
10537: @node Why object-oriented programming?, Object-Oriented Terminology, Object-oriented Forth, Object-oriented Forth
10538: @subsection Why object-oriented programming?
10539: @cindex object-oriented programming motivation
10540: @cindex motivation for object-oriented programming
10541:
10542: Often we have to deal with several data structures (@emph{objects}),
10543: that have to be treated similarly in some respects, but differently in
10544: others. Graphical objects are the textbook example: circles, triangles,
10545: dinosaurs, icons, and others, and we may want to add more during program
10546: development. We want to apply some operations to any graphical object,
10547: e.g., @code{draw} for displaying it on the screen. However, @code{draw}
10548: has to do something different for every kind of object.
10549: @comment TODO add some other operations eg perimeter, area
10550: @comment and tie in to concrete examples later..
10551:
10552: We could implement @code{draw} as a big @code{CASE}
10553: control structure that executes the appropriate code depending on the
10554: kind of object to be drawn. This would be not be very elegant, and,
10555: moreover, we would have to change @code{draw} every time we add
10556: a new kind of graphical object (say, a spaceship).
10557:
10558: What we would rather do is: When defining spaceships, we would tell
10559: the system: ``Here's how you @code{draw} a spaceship; you figure
10560: out the rest''.
10561:
10562: This is the problem that all systems solve that (rightfully) call
10563: themselves object-oriented; the object-oriented packages presented here
10564: solve this problem (and not much else).
10565: @comment TODO ?list properties of oo systems.. oo vs o-based?
10566:
10567: @c ------------------------------------------------------------------------
10568: @node Object-Oriented Terminology, Objects, Why object-oriented programming?, Object-oriented Forth
10569: @subsection Object-Oriented Terminology
10570: @cindex object-oriented terminology
10571: @cindex terminology for object-oriented programming
10572:
10573: This section is mainly for reference, so you don't have to understand
10574: all of it right away. The terminology is mainly Smalltalk-inspired. In
10575: short:
10576:
10577: @table @emph
10578: @cindex class
10579: @item class
10580: a data structure definition with some extras.
10581:
10582: @cindex object
10583: @item object
10584: an instance of the data structure described by the class definition.
10585:
10586: @cindex instance variables
10587: @item instance variables
10588: fields of the data structure.
10589:
10590: @cindex selector
10591: @cindex method selector
10592: @cindex virtual function
10593: @item selector
10594: (or @emph{method selector}) a word (e.g.,
10595: @code{draw}) that performs an operation on a variety of data
10596: structures (classes). A selector describes @emph{what} operation to
10597: perform. In C++ terminology: a (pure) virtual function.
10598:
10599: @cindex method
10600: @item method
10601: the concrete definition that performs the operation
10602: described by the selector for a specific class. A method specifies
10603: @emph{how} the operation is performed for a specific class.
10604:
10605: @cindex selector invocation
10606: @cindex message send
10607: @cindex invoking a selector
10608: @item selector invocation
10609: a call of a selector. One argument of the call (the TOS (top-of-stack))
10610: is used for determining which method is used. In Smalltalk terminology:
10611: a message (consisting of the selector and the other arguments) is sent
10612: to the object.
10613:
10614: @cindex receiving object
10615: @item receiving object
10616: the object used for determining the method executed by a selector
10617: invocation. In the @file{objects.fs} model, it is the object that is on
10618: the TOS when the selector is invoked. (@emph{Receiving} comes from
10619: the Smalltalk @emph{message} terminology.)
10620:
10621: @cindex child class
10622: @cindex parent class
10623: @cindex inheritance
10624: @item child class
10625: a class that has (@emph{inherits}) all properties (instance variables,
10626: selectors, methods) from a @emph{parent class}. In Smalltalk
10627: terminology: The subclass inherits from the superclass. In C++
10628: terminology: The derived class inherits from the base class.
10629:
10630: @end table
10631:
10632: @c If you wonder about the message sending terminology, it comes from
10633: @c a time when each object had it's own task and objects communicated via
10634: @c message passing; eventually the Smalltalk developers realized that
10635: @c they can do most things through simple (indirect) calls. They kept the
10636: @c terminology.
10637:
10638: @c --------------------------------------------------------------
10639: @node Objects, OOF, Object-Oriented Terminology, Object-oriented Forth
10640: @subsection The @file{objects.fs} model
10641: @cindex objects
10642: @cindex object-oriented programming
10643:
10644: @cindex @file{objects.fs}
10645: @cindex @file{oof.fs}
10646:
10647: This section describes the @file{objects.fs} package. This material also
10648: has been published in M. Anton Ertl,
10649: @cite{@uref{http://www.complang.tuwien.ac.at/forth/objects/objects.html,
10650: Yet Another Forth Objects Package}}, Forth Dimensions 19(2), pages
10651: 37--43.
10652: @c McKewan's and Zsoter's packages
10653:
10654: This section assumes that you have read @ref{Structures}.
10655:
10656: The techniques on which this model is based have been used to implement
10657: the parser generator, Gray, and have also been used in Gforth for
10658: implementing the various flavours of word lists (hashed or not,
10659: case-sensitive or not, special-purpose word lists for locals etc.).
10660:
10661:
10662: @menu
10663: * Properties of the Objects model::
10664: * Basic Objects Usage::
10665: * The Objects base class::
10666: * Creating objects::
10667: * Object-Oriented Programming Style::
10668: * Class Binding::
10669: * Method conveniences::
10670: * Classes and Scoping::
10671: * Dividing classes::
10672: * Object Interfaces::
10673: * Objects Implementation::
10674: * Objects Glossary::
10675: @end menu
10676:
10677: Marcel Hendrix provided helpful comments on this section.
10678:
10679: @node Properties of the Objects model, Basic Objects Usage, Objects, Objects
10680: @subsubsection Properties of the @file{objects.fs} model
10681: @cindex @file{objects.fs} properties
10682:
10683: @itemize @bullet
10684: @item
10685: It is straightforward to pass objects on the stack. Passing
10686: selectors on the stack is a little less convenient, but possible.
10687:
10688: @item
10689: Objects are just data structures in memory, and are referenced by their
10690: address. You can create words for objects with normal defining words
10691: like @code{constant}. Likewise, there is no difference between instance
10692: variables that contain objects and those that contain other data.
10693:
10694: @item
10695: Late binding is efficient and easy to use.
10696:
10697: @item
10698: It avoids parsing, and thus avoids problems with state-smartness
10699: and reduced extensibility; for convenience there are a few parsing
10700: words, but they have non-parsing counterparts. There are also a few
10701: defining words that parse. This is hard to avoid, because all standard
10702: defining words parse (except @code{:noname}); however, such
10703: words are not as bad as many other parsing words, because they are not
10704: state-smart.
10705:
10706: @item
10707: It does not try to incorporate everything. It does a few things and does
10708: them well (IMO). In particular, this model was not designed to support
10709: information hiding (although it has features that may help); you can use
10710: a separate package for achieving this.
10711:
10712: @item
10713: It is layered; you don't have to learn and use all features to use this
10714: model. Only a few features are necessary (@pxref{Basic Objects Usage},
10715: @pxref{The Objects base class}, @pxref{Creating objects}.), the others
10716: are optional and independent of each other.
10717:
10718: @item
10719: An implementation in ANS Forth is available.
10720:
10721: @end itemize
10722:
10723:
10724: @node Basic Objects Usage, The Objects base class, Properties of the Objects model, Objects
10725: @subsubsection Basic @file{objects.fs} Usage
10726: @cindex basic objects usage
10727: @cindex objects, basic usage
10728:
10729: You can define a class for graphical objects like this:
10730:
10731: @cindex @code{class} usage
10732: @cindex @code{end-class} usage
10733: @cindex @code{selector} usage
10734: @example
10735: object class \ "object" is the parent class
10736: selector draw ( x y graphical -- )
10737: end-class graphical
10738: @end example
10739:
10740: This code defines a class @code{graphical} with an
10741: operation @code{draw}. We can perform the operation
10742: @code{draw} on any @code{graphical} object, e.g.:
10743:
10744: @example
10745: 100 100 t-rex draw
10746: @end example
10747:
10748: @noindent
10749: where @code{t-rex} is a word (say, a constant) that produces a
10750: graphical object.
10751:
10752: @comment TODO add a 2nd operation eg perimeter.. and use for
10753: @comment a concrete example
10754:
10755: @cindex abstract class
10756: How do we create a graphical object? With the present definitions,
10757: we cannot create a useful graphical object. The class
10758: @code{graphical} describes graphical objects in general, but not
10759: any concrete graphical object type (C++ users would call it an
10760: @emph{abstract class}); e.g., there is no method for the selector
10761: @code{draw} in the class @code{graphical}.
10762:
10763: For concrete graphical objects, we define child classes of the
10764: class @code{graphical}, e.g.:
10765:
10766: @cindex @code{overrides} usage
10767: @cindex @code{field} usage in class definition
10768: @example
10769: graphical class \ "graphical" is the parent class
10770: cell% field circle-radius
10771:
10772: :noname ( x y circle -- )
10773: circle-radius @@ draw-circle ;
10774: overrides draw
10775:
10776: :noname ( n-radius circle -- )
10777: circle-radius ! ;
10778: overrides construct
10779:
10780: end-class circle
10781: @end example
10782:
10783: Here we define a class @code{circle} as a child of @code{graphical},
10784: with field @code{circle-radius} (which behaves just like a field
10785: (@pxref{Structures}); it defines (using @code{overrides}) new methods
10786: for the selectors @code{draw} and @code{construct} (@code{construct} is
10787: defined in @code{object}, the parent class of @code{graphical}).
10788:
10789: Now we can create a circle on the heap (i.e.,
10790: @code{allocate}d memory) with:
10791:
10792: @cindex @code{heap-new} usage
10793: @example
10794: 50 circle heap-new constant my-circle
10795: @end example
10796:
10797: @noindent
10798: @code{heap-new} invokes @code{construct}, thus
10799: initializing the field @code{circle-radius} with 50. We can draw
10800: this new circle at (100,100) with:
10801:
10802: @example
10803: 100 100 my-circle draw
10804: @end example
10805:
10806: @cindex selector invocation, restrictions
10807: @cindex class definition, restrictions
10808: Note: You can only invoke a selector if the object on the TOS
10809: (the receiving object) belongs to the class where the selector was
10810: defined or one of its descendents; e.g., you can invoke
10811: @code{draw} only for objects belonging to @code{graphical}
10812: or its descendents (e.g., @code{circle}). Immediately before
10813: @code{end-class}, the search order has to be the same as
10814: immediately after @code{class}.
10815:
10816: @node The Objects base class, Creating objects, Basic Objects Usage, Objects
10817: @subsubsection The @file{object.fs} base class
10818: @cindex @code{object} class
10819:
10820: When you define a class, you have to specify a parent class. So how do
10821: you start defining classes? There is one class available from the start:
10822: @code{object}. It is ancestor for all classes and so is the
10823: only class that has no parent. It has two selectors: @code{construct}
10824: and @code{print}.
10825:
10826: @node Creating objects, Object-Oriented Programming Style, The Objects base class, Objects
10827: @subsubsection Creating objects
10828: @cindex creating objects
10829: @cindex object creation
10830: @cindex object allocation options
10831:
10832: @cindex @code{heap-new} discussion
10833: @cindex @code{dict-new} discussion
10834: @cindex @code{construct} discussion
10835: You can create and initialize an object of a class on the heap with
10836: @code{heap-new} ( ... class -- object ) and in the dictionary
10837: (allocation with @code{allot}) with @code{dict-new} (
10838: ... class -- object ). Both words invoke @code{construct}, which
10839: consumes the stack items indicated by "..." above.
10840:
10841: @cindex @code{init-object} discussion
10842: @cindex @code{class-inst-size} discussion
10843: If you want to allocate memory for an object yourself, you can get its
10844: alignment and size with @code{class-inst-size 2@@} ( class --
10845: align size ). Once you have memory for an object, you can initialize
10846: it with @code{init-object} ( ... class object -- );
10847: @code{construct} does only a part of the necessary work.
10848:
10849: @node Object-Oriented Programming Style, Class Binding, Creating objects, Objects
10850: @subsubsection Object-Oriented Programming Style
10851: @cindex object-oriented programming style
10852: @cindex programming style, object-oriented
10853:
10854: This section is not exhaustive.
10855:
10856: @cindex stack effects of selectors
10857: @cindex selectors and stack effects
10858: In general, it is a good idea to ensure that all methods for the
10859: same selector have the same stack effect: when you invoke a selector,
10860: you often have no idea which method will be invoked, so, unless all
10861: methods have the same stack effect, you will not know the stack effect
10862: of the selector invocation.
10863:
10864: One exception to this rule is methods for the selector
10865: @code{construct}. We know which method is invoked, because we
10866: specify the class to be constructed at the same place. Actually, I
10867: defined @code{construct} as a selector only to give the users a
10868: convenient way to specify initialization. The way it is used, a
10869: mechanism different from selector invocation would be more natural
10870: (but probably would take more code and more space to explain).
10871:
10872: @node Class Binding, Method conveniences, Object-Oriented Programming Style, Objects
10873: @subsubsection Class Binding
10874: @cindex class binding
10875: @cindex early binding
10876:
10877: @cindex late binding
10878: Normal selector invocations determine the method at run-time depending
10879: on the class of the receiving object. This run-time selection is called
10880: @i{late binding}.
10881:
10882: Sometimes it's preferable to invoke a different method. For example,
10883: you might want to use the simple method for @code{print}ing
10884: @code{object}s instead of the possibly long-winded @code{print} method
10885: of the receiver class. You can achieve this by replacing the invocation
10886: of @code{print} with:
10887:
10888: @cindex @code{[bind]} usage
10889: @example
10890: [bind] object print
10891: @end example
10892:
10893: @noindent
10894: in compiled code or:
10895:
10896: @cindex @code{bind} usage
10897: @example
10898: bind object print
10899: @end example
10900:
10901: @cindex class binding, alternative to
10902: @noindent
10903: in interpreted code. Alternatively, you can define the method with a
10904: name (e.g., @code{print-object}), and then invoke it through the
10905: name. Class binding is just a (often more convenient) way to achieve
10906: the same effect; it avoids name clutter and allows you to invoke
10907: methods directly without naming them first.
10908:
10909: @cindex superclass binding
10910: @cindex parent class binding
10911: A frequent use of class binding is this: When we define a method
10912: for a selector, we often want the method to do what the selector does
10913: in the parent class, and a little more. There is a special word for
10914: this purpose: @code{[parent]}; @code{[parent]
10915: @emph{selector}} is equivalent to @code{[bind] @emph{parent
10916: selector}}, where @code{@emph{parent}} is the parent
10917: class of the current class. E.g., a method definition might look like:
10918:
10919: @cindex @code{[parent]} usage
10920: @example
10921: :noname
10922: dup [parent] foo \ do parent's foo on the receiving object
10923: ... \ do some more
10924: ; overrides foo
10925: @end example
10926:
10927: @cindex class binding as optimization
10928: In @cite{Object-oriented programming in ANS Forth} (Forth Dimensions,
10929: March 1997), Andrew McKewan presents class binding as an optimization
10930: technique. I recommend not using it for this purpose unless you are in
10931: an emergency. Late binding is pretty fast with this model anyway, so the
10932: benefit of using class binding is small; the cost of using class binding
10933: where it is not appropriate is reduced maintainability.
10934:
10935: While we are at programming style questions: You should bind
10936: selectors only to ancestor classes of the receiving object. E.g., say,
10937: you know that the receiving object is of class @code{foo} or its
10938: descendents; then you should bind only to @code{foo} and its
10939: ancestors.
10940:
10941: @node Method conveniences, Classes and Scoping, Class Binding, Objects
10942: @subsubsection Method conveniences
10943: @cindex method conveniences
10944:
10945: In a method you usually access the receiving object pretty often. If
10946: you define the method as a plain colon definition (e.g., with
10947: @code{:noname}), you may have to do a lot of stack
10948: gymnastics. To avoid this, you can define the method with @code{m:
10949: ... ;m}. E.g., you could define the method for
10950: @code{draw}ing a @code{circle} with
10951:
10952: @cindex @code{this} usage
10953: @cindex @code{m:} usage
10954: @cindex @code{;m} usage
10955: @example
10956: m: ( x y circle -- )
10957: ( x y ) this circle-radius @@ draw-circle ;m
10958: @end example
10959:
10960: @cindex @code{exit} in @code{m: ... ;m}
10961: @cindex @code{exitm} discussion
10962: @cindex @code{catch} in @code{m: ... ;m}
10963: When this method is executed, the receiver object is removed from the
10964: stack; you can access it with @code{this} (admittedly, in this
10965: example the use of @code{m: ... ;m} offers no advantage). Note
10966: that I specify the stack effect for the whole method (i.e. including
10967: the receiver object), not just for the code between @code{m:}
10968: and @code{;m}. You cannot use @code{exit} in
10969: @code{m:...;m}; instead, use
10970: @code{exitm}.@footnote{Moreover, for any word that calls
10971: @code{catch} and was defined before loading
10972: @code{objects.fs}, you have to redefine it like I redefined
10973: @code{catch}: @code{: catch this >r catch r> to-this ;}}
10974:
10975: @cindex @code{inst-var} usage
10976: You will frequently use sequences of the form @code{this
10977: @emph{field}} (in the example above: @code{this
10978: circle-radius}). If you use the field only in this way, you can
10979: define it with @code{inst-var} and eliminate the
10980: @code{this} before the field name. E.g., the @code{circle}
10981: class above could also be defined with:
10982:
10983: @example
10984: graphical class
10985: cell% inst-var radius
10986:
10987: m: ( x y circle -- )
10988: radius @@ draw-circle ;m
10989: overrides draw
10990:
10991: m: ( n-radius circle -- )
10992: radius ! ;m
10993: overrides construct
10994:
10995: end-class circle
10996: @end example
10997:
10998: @code{radius} can only be used in @code{circle} and its
10999: descendent classes and inside @code{m:...;m}.
11000:
11001: @cindex @code{inst-value} usage
11002: You can also define fields with @code{inst-value}, which is
11003: to @code{inst-var} what @code{value} is to
11004: @code{variable}. You can change the value of such a field with
11005: @code{[to-inst]}. E.g., we could also define the class
11006: @code{circle} like this:
11007:
11008: @example
11009: graphical class
11010: inst-value radius
11011:
11012: m: ( x y circle -- )
11013: radius draw-circle ;m
11014: overrides draw
11015:
11016: m: ( n-radius circle -- )
11017: [to-inst] radius ;m
11018: overrides construct
11019:
11020: end-class circle
11021: @end example
11022:
11023: @c !! :m is easy to confuse with m:. Another name would be better.
11024:
11025: @c Finally, you can define named methods with @code{:m}. One use of this
11026: @c feature is the definition of words that occur only in one class and are
11027: @c not intended to be overridden, but which still need method context
11028: @c (e.g., for accessing @code{inst-var}s). Another use is for methods that
11029: @c would be bound frequently, if defined anonymously.
11030:
11031:
11032: @node Classes and Scoping, Dividing classes, Method conveniences, Objects
11033: @subsubsection Classes and Scoping
11034: @cindex classes and scoping
11035: @cindex scoping and classes
11036:
11037: Inheritance is frequent, unlike structure extension. This exacerbates
11038: the problem with the field name convention (@pxref{Structure Naming
11039: Convention}): One always has to remember in which class the field was
11040: originally defined; changing a part of the class structure would require
11041: changes for renaming in otherwise unaffected code.
11042:
11043: @cindex @code{inst-var} visibility
11044: @cindex @code{inst-value} visibility
11045: To solve this problem, I added a scoping mechanism (which was not in my
11046: original charter): A field defined with @code{inst-var} (or
11047: @code{inst-value}) is visible only in the class where it is defined and in
11048: the descendent classes of this class. Using such fields only makes
11049: sense in @code{m:}-defined methods in these classes anyway.
11050:
11051: This scoping mechanism allows us to use the unadorned field name,
11052: because name clashes with unrelated words become much less likely.
11053:
11054: @cindex @code{protected} discussion
11055: @cindex @code{private} discussion
11056: Once we have this mechanism, we can also use it for controlling the
11057: visibility of other words: All words defined after
11058: @code{protected} are visible only in the current class and its
11059: descendents. @code{public} restores the compilation
11060: (i.e. @code{current}) word list that was in effect before. If you
11061: have several @code{protected}s without an intervening
11062: @code{public} or @code{set-current}, @code{public}
11063: will restore the compilation word list in effect before the first of
11064: these @code{protected}s.
11065:
11066: @node Dividing classes, Object Interfaces, Classes and Scoping, Objects
11067: @subsubsection Dividing classes
11068: @cindex Dividing classes
11069: @cindex @code{methods}...@code{end-methods}
11070:
11071: You may want to do the definition of methods separate from the
11072: definition of the class, its selectors, fields, and instance variables,
11073: i.e., separate the implementation from the definition. You can do this
11074: in the following way:
11075:
11076: @example
11077: graphical class
11078: inst-value radius
11079: end-class circle
11080:
11081: ... \ do some other stuff
11082:
11083: circle methods \ now we are ready
11084:
11085: m: ( x y circle -- )
11086: radius draw-circle ;m
11087: overrides draw
11088:
11089: m: ( n-radius circle -- )
11090: [to-inst] radius ;m
11091: overrides construct
11092:
11093: end-methods
11094: @end example
11095:
11096: You can use several @code{methods}...@code{end-methods} sections. The
11097: only things you can do to the class in these sections are: defining
11098: methods, and overriding the class's selectors. You must not define new
11099: selectors or fields.
11100:
11101: Note that you often have to override a selector before using it. In
11102: particular, you usually have to override @code{construct} with a new
11103: method before you can invoke @code{heap-new} and friends. E.g., you
11104: must not create a circle before the @code{overrides construct} sequence
11105: in the example above.
11106:
11107: @node Object Interfaces, Objects Implementation, Dividing classes, Objects
11108: @subsubsection Object Interfaces
11109: @cindex object interfaces
11110: @cindex interfaces for objects
11111:
11112: In this model you can only call selectors defined in the class of the
11113: receiving objects or in one of its ancestors. If you call a selector
11114: with a receiving object that is not in one of these classes, the
11115: result is undefined; if you are lucky, the program crashes
11116: immediately.
11117:
11118: @cindex selectors common to hardly-related classes
11119: Now consider the case when you want to have a selector (or several)
11120: available in two classes: You would have to add the selector to a
11121: common ancestor class, in the worst case to @code{object}. You
11122: may not want to do this, e.g., because someone else is responsible for
11123: this ancestor class.
11124:
11125: The solution for this problem is interfaces. An interface is a
11126: collection of selectors. If a class implements an interface, the
11127: selectors become available to the class and its descendents. A class
11128: can implement an unlimited number of interfaces. For the problem
11129: discussed above, we would define an interface for the selector(s), and
11130: both classes would implement the interface.
11131:
11132: As an example, consider an interface @code{storage} for
11133: writing objects to disk and getting them back, and a class
11134: @code{foo} that implements it. The code would look like this:
11135:
11136: @cindex @code{interface} usage
11137: @cindex @code{end-interface} usage
11138: @cindex @code{implementation} usage
11139: @example
11140: interface
11141: selector write ( file object -- )
11142: selector read1 ( file object -- )
11143: end-interface storage
11144:
11145: bar class
11146: storage implementation
11147:
11148: ... overrides write
11149: ... overrides read1
11150: ...
11151: end-class foo
11152: @end example
11153:
11154: @noindent
11155: (I would add a word @code{read} @i{( file -- object )} that uses
11156: @code{read1} internally, but that's beyond the point illustrated
11157: here.)
11158:
11159: Note that you cannot use @code{protected} in an interface; and
11160: of course you cannot define fields.
11161:
11162: In the Neon model, all selectors are available for all classes;
11163: therefore it does not need interfaces. The price you pay in this model
11164: is slower late binding, and therefore, added complexity to avoid late
11165: binding.
11166:
11167: @node Objects Implementation, Objects Glossary, Object Interfaces, Objects
11168: @subsubsection @file{objects.fs} Implementation
11169: @cindex @file{objects.fs} implementation
11170:
11171: @cindex @code{object-map} discussion
11172: An object is a piece of memory, like one of the data structures
11173: described with @code{struct...end-struct}. It has a field
11174: @code{object-map} that points to the method map for the object's
11175: class.
11176:
11177: @cindex method map
11178: @cindex virtual function table
11179: The @emph{method map}@footnote{This is Self terminology; in C++
11180: terminology: virtual function table.} is an array that contains the
11181: execution tokens (@i{xt}s) of the methods for the object's class. Each
11182: selector contains an offset into a method map.
11183:
11184: @cindex @code{selector} implementation, class
11185: @code{selector} is a defining word that uses
11186: @code{CREATE} and @code{DOES>}. The body of the
11187: selector contains the offset; the @code{DOES>} action for a
11188: class selector is, basically:
11189:
11190: @example
11191: ( object addr ) @@ over object-map @@ + @@ execute
11192: @end example
11193:
11194: Since @code{object-map} is the first field of the object, it
11195: does not generate any code. As you can see, calling a selector has a
11196: small, constant cost.
11197:
11198: @cindex @code{current-interface} discussion
11199: @cindex class implementation and representation
11200: A class is basically a @code{struct} combined with a method
11201: map. During the class definition the alignment and size of the class
11202: are passed on the stack, just as with @code{struct}s, so
11203: @code{field} can also be used for defining class
11204: fields. However, passing more items on the stack would be
11205: inconvenient, so @code{class} builds a data structure in memory,
11206: which is accessed through the variable
11207: @code{current-interface}. After its definition is complete, the
11208: class is represented on the stack by a pointer (e.g., as parameter for
11209: a child class definition).
11210:
11211: A new class starts off with the alignment and size of its parent,
11212: and a copy of the parent's method map. Defining new fields extends the
11213: size and alignment; likewise, defining new selectors extends the
11214: method map. @code{overrides} just stores a new @i{xt} in the method
11215: map at the offset given by the selector.
11216:
11217: @cindex class binding, implementation
11218: Class binding just gets the @i{xt} at the offset given by the selector
11219: from the class's method map and @code{compile,}s (in the case of
11220: @code{[bind]}) it.
11221:
11222: @cindex @code{this} implementation
11223: @cindex @code{catch} and @code{this}
11224: @cindex @code{this} and @code{catch}
11225: I implemented @code{this} as a @code{value}. At the
11226: start of an @code{m:...;m} method the old @code{this} is
11227: stored to the return stack and restored at the end; and the object on
11228: the TOS is stored @code{TO this}. This technique has one
11229: disadvantage: If the user does not leave the method via
11230: @code{;m}, but via @code{throw} or @code{exit},
11231: @code{this} is not restored (and @code{exit} may
11232: crash). To deal with the @code{throw} problem, I have redefined
11233: @code{catch} to save and restore @code{this}; the same
11234: should be done with any word that can catch an exception. As for
11235: @code{exit}, I simply forbid it (as a replacement, there is
11236: @code{exitm}).
11237:
11238: @cindex @code{inst-var} implementation
11239: @code{inst-var} is just the same as @code{field}, with
11240: a different @code{DOES>} action:
11241: @example
11242: @@ this +
11243: @end example
11244: Similar for @code{inst-value}.
11245:
11246: @cindex class scoping implementation
11247: Each class also has a word list that contains the words defined with
11248: @code{inst-var} and @code{inst-value}, and its protected
11249: words. It also has a pointer to its parent. @code{class} pushes
11250: the word lists of the class and all its ancestors onto the search order stack,
11251: and @code{end-class} drops them.
11252:
11253: @cindex interface implementation
11254: An interface is like a class without fields, parent and protected
11255: words; i.e., it just has a method map. If a class implements an
11256: interface, its method map contains a pointer to the method map of the
11257: interface. The positive offsets in the map are reserved for class
11258: methods, therefore interface map pointers have negative
11259: offsets. Interfaces have offsets that are unique throughout the
11260: system, unlike class selectors, whose offsets are only unique for the
11261: classes where the selector is available (invokable).
11262:
11263: This structure means that interface selectors have to perform one
11264: indirection more than class selectors to find their method. Their body
11265: contains the interface map pointer offset in the class method map, and
11266: the method offset in the interface method map. The
11267: @code{does>} action for an interface selector is, basically:
11268:
11269: @example
11270: ( object selector-body )
11271: 2dup selector-interface @@ ( object selector-body object interface-offset )
11272: swap object-map @@ + @@ ( object selector-body map )
11273: swap selector-offset @@ + @@ execute
11274: @end example
11275:
11276: where @code{object-map} and @code{selector-offset} are
11277: first fields and generate no code.
11278:
11279: As a concrete example, consider the following code:
11280:
11281: @example
11282: interface
11283: selector if1sel1
11284: selector if1sel2
11285: end-interface if1
11286:
11287: object class
11288: if1 implementation
11289: selector cl1sel1
11290: cell% inst-var cl1iv1
11291:
11292: ' m1 overrides construct
11293: ' m2 overrides if1sel1
11294: ' m3 overrides if1sel2
11295: ' m4 overrides cl1sel2
11296: end-class cl1
11297:
11298: create obj1 object dict-new drop
11299: create obj2 cl1 dict-new drop
11300: @end example
11301:
11302: The data structure created by this code (including the data structure
11303: for @code{object}) is shown in the
11304: @uref{objects-implementation.eps,figure}, assuming a cell size of 4.
11305: @comment TODO add this diagram..
11306:
11307: @node Objects Glossary, , Objects Implementation, Objects
11308: @subsubsection @file{objects.fs} Glossary
11309: @cindex @file{objects.fs} Glossary
11310:
11311:
11312: doc---objects-bind
11313: doc---objects-<bind>
11314: doc---objects-bind'
11315: doc---objects-[bind]
11316: doc---objects-class
11317: doc---objects-class->map
11318: doc---objects-class-inst-size
11319: doc---objects-class-override!
11320: doc---objects-class-previous
11321: doc---objects-class>order
11322: doc---objects-construct
11323: doc---objects-current'
11324: doc---objects-[current]
11325: doc---objects-current-interface
11326: doc---objects-dict-new
11327: doc---objects-end-class
11328: doc---objects-end-class-noname
11329: doc---objects-end-interface
11330: doc---objects-end-interface-noname
11331: doc---objects-end-methods
11332: doc---objects-exitm
11333: doc---objects-heap-new
11334: doc---objects-implementation
11335: doc---objects-init-object
11336: doc---objects-inst-value
11337: doc---objects-inst-var
11338: doc---objects-interface
11339: doc---objects-m:
11340: doc---objects-:m
11341: doc---objects-;m
11342: doc---objects-method
11343: doc---objects-methods
11344: doc---objects-object
11345: doc---objects-overrides
11346: doc---objects-[parent]
11347: doc---objects-print
11348: doc---objects-protected
11349: doc---objects-public
11350: doc---objects-selector
11351: doc---objects-this
11352: doc---objects-<to-inst>
11353: doc---objects-[to-inst]
11354: doc---objects-to-this
11355: doc---objects-xt-new
11356:
11357:
11358: @c -------------------------------------------------------------
11359: @node OOF, Mini-OOF, Objects, Object-oriented Forth
11360: @subsection The @file{oof.fs} model
11361: @cindex oof
11362: @cindex object-oriented programming
11363:
11364: @cindex @file{objects.fs}
11365: @cindex @file{oof.fs}
11366:
11367: This section describes the @file{oof.fs} package.
11368:
11369: The package described in this section has been used in bigFORTH since 1991, and
11370: used for two large applications: a chromatographic system used to
11371: create new medicaments, and a graphic user interface library (MINOS).
11372:
11373: You can find a description (in German) of @file{oof.fs} in @cite{Object
11374: oriented bigFORTH} by Bernd Paysan, published in @cite{Vierte Dimension}
11375: 10(2), 1994.
11376:
11377: @menu
11378: * Properties of the OOF model::
11379: * Basic OOF Usage::
11380: * The OOF base class::
11381: * Class Declaration::
11382: * Class Implementation::
11383: @end menu
11384:
11385: @node Properties of the OOF model, Basic OOF Usage, OOF, OOF
11386: @subsubsection Properties of the @file{oof.fs} model
11387: @cindex @file{oof.fs} properties
11388:
11389: @itemize @bullet
11390: @item
11391: This model combines object oriented programming with information
11392: hiding. It helps you writing large application, where scoping is
11393: necessary, because it provides class-oriented scoping.
11394:
11395: @item
11396: Named objects, object pointers, and object arrays can be created,
11397: selector invocation uses the ``object selector'' syntax. Selector invocation
11398: to objects and/or selectors on the stack is a bit less convenient, but
11399: possible.
11400:
11401: @item
11402: Selector invocation and instance variable usage of the active object is
11403: straightforward, since both make use of the active object.
11404:
11405: @item
11406: Late binding is efficient and easy to use.
11407:
11408: @item
11409: State-smart objects parse selectors. However, extensibility is provided
11410: using a (parsing) selector @code{postpone} and a selector @code{'}.
11411:
11412: @item
11413: An implementation in ANS Forth is available.
11414:
11415: @end itemize
11416:
11417:
11418: @node Basic OOF Usage, The OOF base class, Properties of the OOF model, OOF
11419: @subsubsection Basic @file{oof.fs} Usage
11420: @cindex @file{oof.fs} usage
11421:
11422: This section uses the same example as for @code{objects} (@pxref{Basic Objects Usage}).
11423:
11424: You can define a class for graphical objects like this:
11425:
11426: @cindex @code{class} usage
11427: @cindex @code{class;} usage
11428: @cindex @code{method} usage
11429: @example
11430: object class graphical \ "object" is the parent class
11431: method draw ( x y -- )
11432: class;
11433: @end example
11434:
11435: This code defines a class @code{graphical} with an
11436: operation @code{draw}. We can perform the operation
11437: @code{draw} on any @code{graphical} object, e.g.:
11438:
11439: @example
11440: 100 100 t-rex draw
11441: @end example
11442:
11443: @noindent
11444: where @code{t-rex} is an object or object pointer, created with e.g.
11445: @code{graphical : t-rex}.
11446:
11447: @cindex abstract class
11448: How do we create a graphical object? With the present definitions,
11449: we cannot create a useful graphical object. The class
11450: @code{graphical} describes graphical objects in general, but not
11451: any concrete graphical object type (C++ users would call it an
11452: @emph{abstract class}); e.g., there is no method for the selector
11453: @code{draw} in the class @code{graphical}.
11454:
11455: For concrete graphical objects, we define child classes of the
11456: class @code{graphical}, e.g.:
11457:
11458: @example
11459: graphical class circle \ "graphical" is the parent class
11460: cell var circle-radius
11461: how:
11462: : draw ( x y -- )
11463: circle-radius @@ draw-circle ;
11464:
11465: : init ( n-radius -- )
11466: circle-radius ! ;
11467: class;
11468: @end example
11469:
11470: Here we define a class @code{circle} as a child of @code{graphical},
11471: with a field @code{circle-radius}; it defines new methods for the
11472: selectors @code{draw} and @code{init} (@code{init} is defined in
11473: @code{object}, the parent class of @code{graphical}).
11474:
11475: Now we can create a circle in the dictionary with:
11476:
11477: @example
11478: 50 circle : my-circle
11479: @end example
11480:
11481: @noindent
11482: @code{:} invokes @code{init}, thus initializing the field
11483: @code{circle-radius} with 50. We can draw this new circle at (100,100)
11484: with:
11485:
11486: @example
11487: 100 100 my-circle draw
11488: @end example
11489:
11490: @cindex selector invocation, restrictions
11491: @cindex class definition, restrictions
11492: Note: You can only invoke a selector if the receiving object belongs to
11493: the class where the selector was defined or one of its descendents;
11494: e.g., you can invoke @code{draw} only for objects belonging to
11495: @code{graphical} or its descendents (e.g., @code{circle}). The scoping
11496: mechanism will check if you try to invoke a selector that is not
11497: defined in this class hierarchy, so you'll get an error at compilation
11498: time.
11499:
11500:
11501: @node The OOF base class, Class Declaration, Basic OOF Usage, OOF
11502: @subsubsection The @file{oof.fs} base class
11503: @cindex @file{oof.fs} base class
11504:
11505: When you define a class, you have to specify a parent class. So how do
11506: you start defining classes? There is one class available from the start:
11507: @code{object}. You have to use it as ancestor for all classes. It is the
11508: only class that has no parent. Classes are also objects, except that
11509: they don't have instance variables; class manipulation such as
11510: inheritance or changing definitions of a class is handled through
11511: selectors of the class @code{object}.
11512:
11513: @code{object} provides a number of selectors:
11514:
11515: @itemize @bullet
11516: @item
11517: @code{class} for subclassing, @code{definitions} to add definitions
11518: later on, and @code{class?} to get type informations (is the class a
11519: subclass of the class passed on the stack?).
11520:
11521: doc---object-class
11522: doc---object-definitions
11523: doc---object-class?
11524:
11525:
11526: @item
11527: @code{init} and @code{dispose} as constructor and destructor of the
11528: object. @code{init} is invocated after the object's memory is allocated,
11529: while @code{dispose} also handles deallocation. Thus if you redefine
11530: @code{dispose}, you have to call the parent's dispose with @code{super
11531: dispose}, too.
11532:
11533: doc---object-init
11534: doc---object-dispose
11535:
11536:
11537: @item
11538: @code{new}, @code{new[]}, @code{:}, @code{ptr}, @code{asptr}, and
11539: @code{[]} to create named and unnamed objects and object arrays or
11540: object pointers.
11541:
11542: doc---object-new
11543: doc---object-new[]
11544: doc---object-:
11545: doc---object-ptr
11546: doc---object-asptr
11547: doc---object-[]
11548:
11549:
11550: @item
11551: @code{::} and @code{super} for explicit scoping. You should use explicit
11552: scoping only for super classes or classes with the same set of instance
11553: variables. Explicitly-scoped selectors use early binding.
11554:
11555: doc---object-::
11556: doc---object-super
11557:
11558:
11559: @item
11560: @code{self} to get the address of the object
11561:
11562: doc---object-self
11563:
11564:
11565: @item
11566: @code{bind}, @code{bound}, @code{link}, and @code{is} to assign object
11567: pointers and instance defers.
11568:
11569: doc---object-bind
11570: doc---object-bound
11571: doc---object-link
11572: doc---object-is
11573:
11574:
11575: @item
11576: @code{'} to obtain selector tokens, @code{send} to invocate selectors
11577: form the stack, and @code{postpone} to generate selector invocation code.
11578:
11579: doc---object-'
11580: doc---object-postpone
11581:
11582:
11583: @item
11584: @code{with} and @code{endwith} to select the active object from the
11585: stack, and enable its scope. Using @code{with} and @code{endwith}
11586: also allows you to create code using selector @code{postpone} without being
11587: trapped by the state-smart objects.
11588:
11589: doc---object-with
11590: doc---object-endwith
11591:
11592:
11593: @end itemize
11594:
11595: @node Class Declaration, Class Implementation, The OOF base class, OOF
11596: @subsubsection Class Declaration
11597: @cindex class declaration
11598:
11599: @itemize @bullet
11600: @item
11601: Instance variables
11602:
11603: doc---oof-var
11604:
11605:
11606: @item
11607: Object pointers
11608:
11609: doc---oof-ptr
11610: doc---oof-asptr
11611:
11612:
11613: @item
11614: Instance defers
11615:
11616: doc---oof-defer
11617:
11618:
11619: @item
11620: Method selectors
11621:
11622: doc---oof-early
11623: doc---oof-method
11624:
11625:
11626: @item
11627: Class-wide variables
11628:
11629: doc---oof-static
11630:
11631:
11632: @item
11633: End declaration
11634:
11635: doc---oof-how:
11636: doc---oof-class;
11637:
11638:
11639: @end itemize
11640:
11641: @c -------------------------------------------------------------
11642: @node Class Implementation, , Class Declaration, OOF
11643: @subsubsection Class Implementation
11644: @cindex class implementation
11645:
11646: @c -------------------------------------------------------------
11647: @node Mini-OOF, Comparison with other object models, OOF, Object-oriented Forth
11648: @subsection The @file{mini-oof.fs} model
11649: @cindex mini-oof
11650:
11651: Gforth's third object oriented Forth package is a 12-liner. It uses a
11652: mixture of the @file{objects.fs} and the @file{oof.fs} syntax,
11653: and reduces to the bare minimum of features. This is based on a posting
11654: of Bernd Paysan in comp.lang.forth.
11655:
11656: @menu
11657: * Basic Mini-OOF Usage::
11658: * Mini-OOF Example::
11659: * Mini-OOF Implementation::
11660: @end menu
11661:
11662: @c -------------------------------------------------------------
11663: @node Basic Mini-OOF Usage, Mini-OOF Example, Mini-OOF, Mini-OOF
11664: @subsubsection Basic @file{mini-oof.fs} Usage
11665: @cindex mini-oof usage
11666:
11667: There is a base class (@code{class}, which allocates one cell for the
11668: object pointer) plus seven other words: to define a method, a variable,
11669: a class; to end a class, to resolve binding, to allocate an object and
11670: to compile a class method.
11671: @comment TODO better description of the last one
11672:
11673:
11674: doc-object
11675: doc-method
11676: doc-var
11677: doc-class
11678: doc-end-class
11679: doc-defines
11680: doc-new
11681: doc-::
11682:
11683:
11684:
11685: @c -------------------------------------------------------------
11686: @node Mini-OOF Example, Mini-OOF Implementation, Basic Mini-OOF Usage, Mini-OOF
11687: @subsubsection Mini-OOF Example
11688: @cindex mini-oof example
11689:
11690: A short example shows how to use this package. This example, in slightly
11691: extended form, is supplied as @file{moof-exm.fs}
11692: @comment TODO could flesh this out with some comments from the Forthwrite article
11693:
11694: @example
11695: object class
11696: method init
11697: method draw
11698: end-class graphical
11699: @end example
11700:
11701: This code defines a class @code{graphical} with an
11702: operation @code{draw}. We can perform the operation
11703: @code{draw} on any @code{graphical} object, e.g.:
11704:
11705: @example
11706: 100 100 t-rex draw
11707: @end example
11708:
11709: where @code{t-rex} is an object or object pointer, created with e.g.
11710: @code{graphical new Constant t-rex}.
11711:
11712: For concrete graphical objects, we define child classes of the
11713: class @code{graphical}, e.g.:
11714:
11715: @example
11716: graphical class
11717: cell var circle-radius
11718: end-class circle \ "graphical" is the parent class
11719:
11720: :noname ( x y -- )
11721: circle-radius @@ draw-circle ; circle defines draw
11722: :noname ( r -- )
11723: circle-radius ! ; circle defines init
11724: @end example
11725:
11726: There is no implicit init method, so we have to define one. The creation
11727: code of the object now has to call init explicitely.
11728:
11729: @example
11730: circle new Constant my-circle
11731: 50 my-circle init
11732: @end example
11733:
11734: It is also possible to add a function to create named objects with
11735: automatic call of @code{init}, given that all objects have @code{init}
11736: on the same place:
11737:
11738: @example
11739: : new: ( .. o "name" -- )
11740: new dup Constant init ;
11741: 80 circle new: large-circle
11742: @end example
11743:
11744: We can draw this new circle at (100,100) with:
11745:
11746: @example
11747: 100 100 my-circle draw
11748: @end example
11749:
11750: @node Mini-OOF Implementation, , Mini-OOF Example, Mini-OOF
11751: @subsubsection @file{mini-oof.fs} Implementation
11752:
11753: Object-oriented systems with late binding typically use a
11754: ``vtable''-approach: the first variable in each object is a pointer to a
11755: table, which contains the methods as function pointers. The vtable
11756: may also contain other information.
11757:
11758: So first, let's declare selectors:
11759:
11760: @example
11761: : method ( m v "name" -- m' v ) Create over , swap cell+ swap
11762: DOES> ( ... o -- ... ) @@ over @@ + @@ execute ;
11763: @end example
11764:
11765: During selector declaration, the number of selectors and instance
11766: variables is on the stack (in address units). @code{method} creates one
11767: selector and increments the selector number. To execute a selector, it
11768: takes the object, fetches the vtable pointer, adds the offset, and
11769: executes the method @i{xt} stored there. Each selector takes the object
11770: it is invoked with as top of stack parameter; it passes the parameters
11771: (including the object) unchanged to the appropriate method which should
11772: consume that object.
11773:
11774: Now, we also have to declare instance variables
11775:
11776: @example
11777: : var ( m v size "name" -- m v' ) Create over , +
11778: DOES> ( o -- addr ) @@ + ;
11779: @end example
11780:
11781: As before, a word is created with the current offset. Instance
11782: variables can have different sizes (cells, floats, doubles, chars), so
11783: all we do is take the size and add it to the offset. If your machine
11784: has alignment restrictions, put the proper @code{aligned} or
11785: @code{faligned} before the variable, to adjust the variable
11786: offset. That's why it is on the top of stack.
11787:
11788: We need a starting point (the base object) and some syntactic sugar:
11789:
11790: @example
11791: Create object 1 cells , 2 cells ,
11792: : class ( class -- class selectors vars ) dup 2@@ ;
11793: @end example
11794:
11795: For inheritance, the vtable of the parent object has to be
11796: copied when a new, derived class is declared. This gives all the
11797: methods of the parent class, which can be overridden, though.
11798:
11799: @example
11800: : end-class ( class selectors vars "name" -- )
11801: Create here >r , dup , 2 cells ?DO ['] noop , 1 cells +LOOP
11802: cell+ dup cell+ r> rot @@ 2 cells /string move ;
11803: @end example
11804:
11805: The first line creates the vtable, initialized with
11806: @code{noop}s. The second line is the inheritance mechanism, it
11807: copies the xts from the parent vtable.
11808:
11809: We still have no way to define new methods, let's do that now:
11810:
11811: @example
11812: : defines ( xt class "name" -- ) ' >body @@ + ! ;
11813: @end example
11814:
11815: To allocate a new object, we need a word, too:
11816:
11817: @example
11818: : new ( class -- o ) here over @@ allot swap over ! ;
11819: @end example
11820:
11821: Sometimes derived classes want to access the method of the
11822: parent object. There are two ways to achieve this with Mini-OOF:
11823: first, you could use named words, and second, you could look up the
11824: vtable of the parent object.
11825:
11826: @example
11827: : :: ( class "name" -- ) ' >body @@ + @@ compile, ;
11828: @end example
11829:
11830:
11831: Nothing can be more confusing than a good example, so here is
11832: one. First let's declare a text object (called
11833: @code{button}), that stores text and position:
11834:
11835: @example
11836: object class
11837: cell var text
11838: cell var len
11839: cell var x
11840: cell var y
11841: method init
11842: method draw
11843: end-class button
11844: @end example
11845:
11846: @noindent
11847: Now, implement the two methods, @code{draw} and @code{init}:
11848:
11849: @example
11850: :noname ( o -- )
11851: >r r@@ x @@ r@@ y @@ at-xy r@@ text @@ r> len @@ type ;
11852: button defines draw
11853: :noname ( addr u o -- )
11854: >r 0 r@@ x ! 0 r@@ y ! r@@ len ! r> text ! ;
11855: button defines init
11856: @end example
11857:
11858: @noindent
11859: To demonstrate inheritance, we define a class @code{bold-button}, with no
11860: new data and no new selectors:
11861:
11862: @example
11863: button class
11864: end-class bold-button
11865:
11866: : bold 27 emit ." [1m" ;
11867: : normal 27 emit ." [0m" ;
11868: @end example
11869:
11870: @noindent
11871: The class @code{bold-button} has a different draw method to
11872: @code{button}, but the new method is defined in terms of the draw method
11873: for @code{button}:
11874:
11875: @example
11876: :noname bold [ button :: draw ] normal ; bold-button defines draw
11877: @end example
11878:
11879: @noindent
11880: Finally, create two objects and apply selectors:
11881:
11882: @example
11883: button new Constant foo
11884: s" thin foo" foo init
11885: page
11886: foo draw
11887: bold-button new Constant bar
11888: s" fat bar" bar init
11889: 1 bar y !
11890: bar draw
11891: @end example
11892:
11893:
11894: @node Comparison with other object models, , Mini-OOF, Object-oriented Forth
11895: @subsection Comparison with other object models
11896: @cindex comparison of object models
11897: @cindex object models, comparison
11898:
11899: Many object-oriented Forth extensions have been proposed (@cite{A survey
11900: of object-oriented Forths} (SIGPLAN Notices, April 1996) by Bradford
11901: J. Rodriguez and W. F. S. Poehlman lists 17). This section discusses the
11902: relation of the object models described here to two well-known and two
11903: closely-related (by the use of method maps) models. Andras Zsoter
11904: helped us with this section.
11905:
11906: @cindex Neon model
11907: The most popular model currently seems to be the Neon model (see
11908: @cite{Object-oriented programming in ANS Forth} (Forth Dimensions, March
11909: 1997) by Andrew McKewan) but this model has a number of limitations
11910: @footnote{A longer version of this critique can be
11911: found in @cite{On Standardizing Object-Oriented Forth Extensions} (Forth
11912: Dimensions, May 1997) by Anton Ertl.}:
11913:
11914: @itemize @bullet
11915: @item
11916: It uses a @code{@emph{selector object}} syntax, which makes it unnatural
11917: to pass objects on the stack.
11918:
11919: @item
11920: It requires that the selector parses the input stream (at
11921: compile time); this leads to reduced extensibility and to bugs that are
11922: hard to find.
11923:
11924: @item
11925: It allows using every selector on every object; this eliminates the
11926: need for interfaces, but makes it harder to create efficient
11927: implementations.
11928: @end itemize
11929:
11930: @cindex Pountain's object-oriented model
11931: Another well-known publication is @cite{Object-Oriented Forth} (Academic
11932: Press, London, 1987) by Dick Pountain. However, it is not really about
11933: object-oriented programming, because it hardly deals with late
11934: binding. Instead, it focuses on features like information hiding and
11935: overloading that are characteristic of modular languages like Ada (83).
11936:
11937: @cindex Zsoter's object-oriented model
11938: In @uref{http://www.forth.org/oopf.html, Does late binding have to be
11939: slow?} (Forth Dimensions 18(1) 1996, pages 31-35) Andras Zsoter
11940: describes a model that makes heavy use of an active object (like
11941: @code{this} in @file{objects.fs}): The active object is not only used
11942: for accessing all fields, but also specifies the receiving object of
11943: every selector invocation; you have to change the active object
11944: explicitly with @code{@{ ... @}}, whereas in @file{objects.fs} it
11945: changes more or less implicitly at @code{m: ... ;m}. Such a change at
11946: the method entry point is unnecessary with Zsoter's model, because the
11947: receiving object is the active object already. On the other hand, the
11948: explicit change is absolutely necessary in that model, because otherwise
11949: no one could ever change the active object. An ANS Forth implementation
11950: of this model is available through
11951: @uref{http://www.forth.org/oopf.html}.
11952:
11953: @cindex @file{oof.fs}, differences to other models
11954: The @file{oof.fs} model combines information hiding and overloading
11955: resolution (by keeping names in various word lists) with object-oriented
11956: programming. It sets the active object implicitly on method entry, but
11957: also allows explicit changing (with @code{>o...o>} or with
11958: @code{with...endwith}). It uses parsing and state-smart objects and
11959: classes for resolving overloading and for early binding: the object or
11960: class parses the selector and determines the method from this. If the
11961: selector is not parsed by an object or class, it performs a call to the
11962: selector for the active object (late binding), like Zsoter's model.
11963: Fields are always accessed through the active object. The big
11964: disadvantage of this model is the parsing and the state-smartness, which
11965: reduces extensibility and increases the opportunities for subtle bugs;
11966: essentially, you are only safe if you never tick or @code{postpone} an
11967: object or class (Bernd disagrees, but I (Anton) am not convinced).
11968:
11969: @cindex @file{mini-oof.fs}, differences to other models
11970: The @file{mini-oof.fs} model is quite similar to a very stripped-down
11971: version of the @file{objects.fs} model, but syntactically it is a
11972: mixture of the @file{objects.fs} and @file{oof.fs} models.
11973:
11974:
11975: @c -------------------------------------------------------------
11976: @node Programming Tools, C Interface, Object-oriented Forth, Words
11977: @section Programming Tools
11978: @cindex programming tools
11979:
11980: @c !! move this and assembler down below OO stuff.
11981:
11982: @menu
11983: * Examining:: Data and Code.
11984: * Forgetting words:: Usually before reloading.
11985: * Debugging:: Simple and quick.
11986: * Assertions:: Making your programs self-checking.
11987: * Singlestep Debugger:: Executing your program word by word.
11988: @end menu
11989:
11990: @node Examining, Forgetting words, Programming Tools, Programming Tools
11991: @subsection Examining data and code
11992: @cindex examining data and code
11993: @cindex data examination
11994: @cindex code examination
11995:
11996: The following words inspect the stack non-destructively:
11997:
11998: doc-.s
11999: doc-f.s
12000: doc-maxdepth-.s
12001:
12002: There is a word @code{.r} but it does @i{not} display the return stack!
12003: It is used for formatted numeric output (@pxref{Simple numeric output}).
12004:
12005: doc-depth
12006: doc-fdepth
12007: doc-clearstack
12008: doc-clearstacks
12009:
12010: The following words inspect memory.
12011:
12012: doc-?
12013: doc-dump
12014:
12015: And finally, @code{see} allows to inspect code:
12016:
12017: doc-see
12018: doc-xt-see
12019: doc-simple-see
12020: doc-simple-see-range
12021: doc-see-code
12022: doc-see-code-range
12023:
12024: @node Forgetting words, Debugging, Examining, Programming Tools
12025: @subsection Forgetting words
12026: @cindex words, forgetting
12027: @cindex forgeting words
12028:
12029: @c anton: other, maybe better places for this subsection: Defining Words;
12030: @c Dictionary allocation. At least a reference should be there.
12031:
12032: Forth allows you to forget words (and everything that was alloted in the
12033: dictonary after them) in a LIFO manner.
12034:
12035: doc-marker
12036:
12037: The most common use of this feature is during progam development: when
12038: you change a source file, forget all the words it defined and load it
12039: again (since you also forget everything defined after the source file
12040: was loaded, you have to reload that, too). Note that effects like
12041: storing to variables and destroyed system words are not undone when you
12042: forget words. With a system like Gforth, that is fast enough at
12043: starting up and compiling, I find it more convenient to exit and restart
12044: Gforth, as this gives me a clean slate.
12045:
12046: Here's an example of using @code{marker} at the start of a source file
12047: that you are debugging; it ensures that you only ever have one copy of
12048: the file's definitions compiled at any time:
12049:
12050: @example
12051: [IFDEF] my-code
12052: my-code
12053: [ENDIF]
12054:
12055: marker my-code
12056: init-included-files
12057:
12058: \ .. definitions start here
12059: \ .
12060: \ .
12061: \ end
12062: @end example
12063:
12064:
12065: @node Debugging, Assertions, Forgetting words, Programming Tools
12066: @subsection Debugging
12067: @cindex debugging
12068:
12069: Languages with a slow edit/compile/link/test development loop tend to
12070: require sophisticated tracing/stepping debuggers to facilate debugging.
12071:
12072: A much better (faster) way in fast-compiling languages is to add
12073: printing code at well-selected places, let the program run, look at
12074: the output, see where things went wrong, add more printing code, etc.,
12075: until the bug is found.
12076:
12077: The simple debugging aids provided in @file{debugs.fs}
12078: are meant to support this style of debugging.
12079:
12080: The word @code{~~} prints debugging information (by default the source
12081: location and the stack contents). It is easy to insert. If you use Emacs
12082: it is also easy to remove (@kbd{C-x ~} in the Emacs Forth mode to
12083: query-replace them with nothing). The deferred words
12084: @code{printdebugdata} and @code{.debugline} control the output of
12085: @code{~~}. The default source location output format works well with
12086: Emacs' compilation mode, so you can step through the program at the
12087: source level using @kbd{C-x `} (the advantage over a stepping debugger
12088: is that you can step in any direction and you know where the crash has
12089: happened or where the strange data has occurred).
12090:
12091: doc-~~
12092: doc-printdebugdata
12093: doc-.debugline
12094: doc-debug-fid
12095:
12096: @cindex filenames in @code{~~} output
12097: @code{~~} (and assertions) will usually print the wrong file name if a
12098: marker is executed in the same file after their occurance. They will
12099: print @samp{*somewhere*} as file name if a marker is executed in the
12100: same file before their occurance.
12101:
12102:
12103: @node Assertions, Singlestep Debugger, Debugging, Programming Tools
12104: @subsection Assertions
12105: @cindex assertions
12106:
12107: It is a good idea to make your programs self-checking, especially if you
12108: make an assumption that may become invalid during maintenance (for
12109: example, that a certain field of a data structure is never zero). Gforth
12110: supports @dfn{assertions} for this purpose. They are used like this:
12111:
12112: @example
12113: assert( @i{flag} )
12114: @end example
12115:
12116: The code between @code{assert(} and @code{)} should compute a flag, that
12117: should be true if everything is alright and false otherwise. It should
12118: not change anything else on the stack. The overall stack effect of the
12119: assertion is @code{( -- )}. E.g.
12120:
12121: @example
12122: assert( 1 1 + 2 = ) \ what we learn in school
12123: assert( dup 0<> ) \ assert that the top of stack is not zero
12124: assert( false ) \ this code should not be reached
12125: @end example
12126:
12127: The need for assertions is different at different times. During
12128: debugging, we want more checking, in production we sometimes care more
12129: for speed. Therefore, assertions can be turned off, i.e., the assertion
12130: becomes a comment. Depending on the importance of an assertion and the
12131: time it takes to check it, you may want to turn off some assertions and
12132: keep others turned on. Gforth provides several levels of assertions for
12133: this purpose:
12134:
12135:
12136: doc-assert0(
12137: doc-assert1(
12138: doc-assert2(
12139: doc-assert3(
12140: doc-assert(
12141: doc-)
12142:
12143:
12144: The variable @code{assert-level} specifies the highest assertions that
12145: are turned on. I.e., at the default @code{assert-level} of one,
12146: @code{assert0(} and @code{assert1(} assertions perform checking, while
12147: @code{assert2(} and @code{assert3(} assertions are treated as comments.
12148:
12149: The value of @code{assert-level} is evaluated at compile-time, not at
12150: run-time. Therefore you cannot turn assertions on or off at run-time;
12151: you have to set the @code{assert-level} appropriately before compiling a
12152: piece of code. You can compile different pieces of code at different
12153: @code{assert-level}s (e.g., a trusted library at level 1 and
12154: newly-written code at level 3).
12155:
12156:
12157: doc-assert-level
12158:
12159:
12160: If an assertion fails, a message compatible with Emacs' compilation mode
12161: is produced and the execution is aborted (currently with @code{ABORT"}.
12162: If there is interest, we will introduce a special throw code. But if you
12163: intend to @code{catch} a specific condition, using @code{throw} is
12164: probably more appropriate than an assertion).
12165:
12166: @cindex filenames in assertion output
12167: Assertions (and @code{~~}) will usually print the wrong file name if a
12168: marker is executed in the same file after their occurance. They will
12169: print @samp{*somewhere*} as file name if a marker is executed in the
12170: same file before their occurance.
12171:
12172: Definitions in ANS Forth for these assertion words are provided
12173: in @file{compat/assert.fs}.
12174:
12175:
12176: @node Singlestep Debugger, , Assertions, Programming Tools
12177: @subsection Singlestep Debugger
12178: @cindex singlestep Debugger
12179: @cindex debugging Singlestep
12180:
12181: The singlestep debugger works only with the engine @code{gforth-itc}.
12182:
12183: When you create a new word there's often the need to check whether it
12184: behaves correctly or not. You can do this by typing @code{dbg
12185: badword}. A debug session might look like this:
12186:
12187: @example
12188: : badword 0 DO i . LOOP ; ok
12189: 2 dbg badword
12190: : badword
12191: Scanning code...
12192:
12193: Nesting debugger ready!
12194:
12195: 400D4738 8049BC4 0 -> [ 2 ] 00002 00000
12196: 400D4740 8049F68 DO -> [ 0 ]
12197: 400D4744 804A0C8 i -> [ 1 ] 00000
12198: 400D4748 400C5E60 . -> 0 [ 0 ]
12199: 400D474C 8049D0C LOOP -> [ 0 ]
12200: 400D4744 804A0C8 i -> [ 1 ] 00001
12201: 400D4748 400C5E60 . -> 1 [ 0 ]
12202: 400D474C 8049D0C LOOP -> [ 0 ]
12203: 400D4758 804B384 ; -> ok
12204: @end example
12205:
12206: Each line displayed is one step. You always have to hit return to
12207: execute the next word that is displayed. If you don't want to execute
12208: the next word in a whole, you have to type @kbd{n} for @code{nest}. Here is
12209: an overview what keys are available:
12210:
12211: @table @i
12212:
12213: @item @key{RET}
12214: Next; Execute the next word.
12215:
12216: @item n
12217: Nest; Single step through next word.
12218:
12219: @item u
12220: Unnest; Stop debugging and execute rest of word. If we got to this word
12221: with nest, continue debugging with the calling word.
12222:
12223: @item d
12224: Done; Stop debugging and execute rest.
12225:
12226: @item s
12227: Stop; Abort immediately.
12228:
12229: @end table
12230:
12231: Debugging large application with this mechanism is very difficult, because
12232: you have to nest very deeply into the program before the interesting part
12233: begins. This takes a lot of time.
12234:
12235: To do it more directly put a @code{BREAK:} command into your source code.
12236: When program execution reaches @code{BREAK:} the single step debugger is
12237: invoked and you have all the features described above.
12238:
12239: If you have more than one part to debug it is useful to know where the
12240: program has stopped at the moment. You can do this by the
12241: @code{BREAK" string"} command. This behaves like @code{BREAK:} except that
12242: string is typed out when the ``breakpoint'' is reached.
12243:
12244:
12245: doc-dbg
12246: doc-break:
12247: doc-break"
12248:
12249: @c ------------------------------------------------------------
12250: @node C Interface, Assembler and Code Words, Programming Tools, Words
12251: @section C Interface
12252: @cindex C interface
12253: @cindex foreign language interface
12254: @cindex interface to C functions
12255:
12256: Note that the C interface is not yet complete; callbacks are missing,
12257: as well as a way of declaring structs, unions, and their fields.
12258:
12259: @menu
12260: * Calling C Functions::
12261: * Declaring C Functions::
12262: * Calling C function pointers::
12263: * Defining library interfaces::
12264: * Declaring OS-level libraries::
12265: * Callbacks::
12266: * C interface internals::
12267: * Low-Level C Interface Words::
12268: @end menu
12269:
12270: @node Calling C Functions, Declaring C Functions, C Interface, C Interface
12271: @subsection Calling C functions
12272: @cindex C functions, calls to
12273: @cindex calling C functions
12274:
12275: Once a C function is declared (see @pxref{Declaring C Functions}), you
12276: can call it as follows: You push the arguments on the stack(s), and
12277: then call the word for the C function. The arguments have to be
12278: pushed in the same order as the arguments appear in the C
12279: documentation (i.e., the first argument is deepest on the stack).
12280: Integer and pointer arguments have to be pushed on the data stack,
12281: floating-point arguments on the FP stack; these arguments are consumed
12282: by the called C function.
12283:
12284: On returning from the C function, the return value, if any, resides on
12285: the appropriate stack: an integer return value is pushed on the data
12286: stack, an FP return value on the FP stack, and a void return value
12287: results in not pushing anything. Note that most C functions have a
12288: return value, even if that is often not used in C; in Forth, you have
12289: to @code{drop} this return value explicitly if you do not use it.
12290:
12291: The C interface automatically converts between the C type and the
12292: Forth type as necessary, on a best-effort basis (in some cases, there
12293: may be some loss).
12294:
12295: As an example, consider the POSIX function @code{lseek()}:
12296:
12297: @example
12298: off_t lseek(int fd, off_t offset, int whence);
12299: @end example
12300:
12301: This function takes three integer arguments, and returns an integer
12302: argument, so a Forth call for setting the current file offset to the
12303: start of the file could look like this:
12304:
12305: @example
12306: fd @@ 0 SEEK_SET lseek -1 = if
12307: ... \ error handling
12308: then
12309: @end example
12310:
12311: You might be worried that an @code{off_t} does not fit into a cell, so
12312: you could not pass larger offsets to lseek, and might get only a part
12313: of the return values. In that case, in your declaration of the
12314: function (@pxref{Declaring C Functions}) you should declare it to use
12315: double-cells for the off_t argument and return value, and maybe give
12316: the resulting Forth word a different name, like @code{dlseek}; the
12317: result could be called like this:
12318:
12319: @example
12320: fd @@ 0. SEEK_SET dlseek -1. d= if
12321: ... \ error handling
12322: then
12323: @end example
12324:
12325: Passing and returning structs or unions is currently not supported by
12326: our interface@footnote{If you know the calling convention of your C
12327: compiler, you usually can call such functions in some way, but that
12328: way is usually not portable between platforms, and sometimes not even
12329: between C compilers.}.
12330:
12331: Calling functions with a variable number of arguments (@emph{variadic}
12332: functions, e.g., @code{printf()}) is only supported by having you
12333: declare one function-calling word for each argument pattern, and
12334: calling the appropriate word for the desired pattern.
12335:
12336:
12337:
12338: @node Declaring C Functions, Calling C function pointers, Calling C Functions, C Interface
12339: @subsection Declaring C Functions
12340: @cindex C functions, declarations
12341: @cindex declaring C functions
12342:
12343: Before you can call @code{lseek} or @code{dlseek}, you have to declare
12344: it. The declaration consists of two parts:
12345:
12346: @table @b
12347:
12348: @item The C part
12349: is the C declaration of the function, or more typically and portably,
12350: a C-style @code{#include} of a file that contains the declaration of
12351: the C function.
12352:
12353: @item The Forth part
12354: declares the Forth types of the parameters and the Forth word name
12355: corresponding to the C function.
12356:
12357: @end table
12358:
12359: For the words @code{lseek} and @code{dlseek} mentioned earlier, the
12360: declarations are:
12361:
12362: @example
12363: \c #define _FILE_OFFSET_BITS 64
12364: \c #include <sys/types.h>
12365: \c #include <unistd.h>
12366: c-function lseek lseek n n n -- n
12367: c-function dlseek lseek n d n -- d
12368: @end example
12369:
12370: The C part of the declarations is prefixed by @code{\c}, and the rest
12371: of the line is ordinary C code. You can use as many lines of C
12372: declarations as you like, and they are visible for all further
12373: function declarations.
12374:
12375: The Forth part declares each interface word with @code{c-function},
12376: followed by the Forth name of the word, the C name of the called
12377: function, and the stack effect of the word. The stack effect contains
12378: an arbitrary number of types of parameters, then @code{--}, and then
12379: exactly one type for the return value. The possible types are:
12380:
12381: @table @code
12382:
12383: @item n
12384: single-cell integer
12385:
12386: @item a
12387: address (single-cell)
12388:
12389: @item d
12390: double-cell integer
12391:
12392: @item r
12393: floating-point value
12394:
12395: @item func
12396: C function pointer
12397:
12398: @item void
12399: no value (used as return type for void functions)
12400:
12401: @end table
12402:
12403: @cindex variadic C functions
12404:
12405: To deal with variadic C functions, you can declare one Forth word for
12406: every pattern you want to use, e.g.:
12407:
12408: @example
12409: \c #include <stdio.h>
12410: c-function printf-nr printf a n r -- n
12411: c-function printf-rn printf a r n -- n
12412: @end example
12413:
12414: Note that with C functions declared as variadic (or if you don't
12415: provide a prototype), the C interface has no C type to convert to, so
12416: no automatic conversion happens, which may lead to portability
12417: problems in some cases. In such cases you can perform the conversion
12418: explicitly on the C level, e.g., as follows:
12419:
12420: @example
12421: \c #define printfll(s,ll) printf(s,(long long)ll)
12422: c-function printfll printfll a n -- n
12423: @end example
12424:
12425: Here, instead of calling @code{printf()} directly, we define a macro
12426: that casts (converts) the Forth single-cell integer into a
12427: C @code{long long} before calling @code{printf()}.
12428:
12429: doc-\c
12430: doc-c-function
12431: doc-c-value
12432: doc-c-variable
12433:
12434: In order to work, this C interface invokes GCC at run-time and uses
12435: dynamic linking. If these features are not available, there are
12436: other, less convenient and less portable C interfaces in @file{lib.fs}
12437: and @file{oldlib.fs}. These interfaces are mostly undocumented and
12438: mostly incompatible with each other and with the documented C
12439: interface; you can find some examples for the @file{lib.fs} interface
12440: in @file{lib.fs}.
12441:
12442:
12443: @node Calling C function pointers, Defining library interfaces, Declaring C Functions, C Interface
12444: @subsection Calling C function pointers from Forth
12445: @cindex C function pointers, calling from Forth
12446:
12447: If you come across a C function pointer (e.g., in some C-constructed
12448: structure) and want to call it from your Forth program, you can also
12449: use the features explained until now to achieve that, as follows:
12450:
12451: Let us assume that there is a C function pointer type @code{func1}
12452: defined in some header file @file{func1.h}, and you know that these
12453: functions take one integer argument and return an integer result; and
12454: you want to call functions through such pointers. Just define
12455:
12456: @example
12457: \c #include <func1.h>
12458: \c #define call_func1(par1,fptr) ((func1)fptr)(par1)
12459: c-function call-func1 call_func1 n func -- n
12460: @end example
12461:
12462: and then you can call a function pointed to by, say @code{func1a} as
12463: follows:
12464:
12465: @example
12466: -5 func1a call-func1 .
12467: @end example
12468:
12469: In the C part, @code{call_func} is defined as a macro to avoid having
12470: to declare the exact parameter and return types, so the C compiler
12471: knows them from the declaration of @code{func1}.
12472:
12473: The Forth word @code{call-func1} is similar to @code{execute}, except
12474: that it takes a C @code{func1} pointer instead of a Forth execution
12475: token, and it is specific to @code{func1} pointers. For each type of
12476: function pointer you want to call from Forth, you have to define
12477: a separate calling word.
12478:
12479:
12480: @node Defining library interfaces, Declaring OS-level libraries, Calling C function pointers, C Interface
12481: @subsection Defining library interfaces
12482: @cindex giving a name to a library interface
12483: @cindex library interface names
12484:
12485: You can give a name to a bunch of C function declarations (a library
12486: interface), as follows:
12487:
12488: @example
12489: c-library lseek-lib
12490: \c #define _FILE_OFFSET_BITS 64
12491: ...
12492: end-c-library
12493: @end example
12494:
12495: The effect of giving such a name to the interface is that the names of
12496: the generated files will contain that name, and when you use the
12497: interface a second time, it will use the existing files instead of
12498: generating and compiling them again, saving you time. Note that even
12499: if you change the declarations, the old (stale) files will be used,
12500: probably leading to errors. So, during development of the
12501: declarations we recommend not using @code{c-library}. Normally these
12502: files are cached in @file{$HOME/.gforth/libcc-named}, so by deleting
12503: that directory you can get rid of stale files.
12504:
12505: Note that you should use @code{c-library} before everything else
12506: having anything to do with that library, as it resets some setup
12507: stuff. The idea is that the typical use is to put each
12508: @code{c-library}...@code{end-library} unit in its own file, and to be
12509: able to include these files in any order.
12510:
12511: Note that the library name is not allocated in the dictionary and
12512: therefore does not shadow dictionary names. It is used in the file
12513: system, so you have to use naming conventions appropriate for file
12514: systems. Also, you must not call a function you declare after
12515: @code{c-library} before you perform @code{end-c-library}.
12516:
12517: A major benefit of these named library interfaces is that, once they
12518: are generated, the tools used to generated them (in particular, the C
12519: compiler and libtool) are no longer needed, so the interface can be
12520: used even on machines that do not have the tools installed.
12521:
12522: doc-c-library-name
12523: doc-c-library
12524: doc-end-c-library
12525:
12526:
12527: @node Declaring OS-level libraries, Callbacks, Defining library interfaces, C Interface
12528: @subsection Declaring OS-level libraries
12529: @cindex Shared libraries in C interface
12530: @cindex Dynamically linked libraries in C interface
12531: @cindex Libraries in C interface
12532:
12533: For calling some C functions, you need to link with a specific
12534: OS-level library that contains that function. E.g., the @code{sin}
12535: function requires linking a special library by using the command line
12536: switch @code{-lm}. In our C iterface you do the equivalent thing by
12537: calling @code{add-lib} as follows:
12538:
12539: @example
12540: clear-libs
12541: s" m" add-lib
12542: \c #include <math.h>
12543: c-function sin sin r -- r
12544: @end example
12545:
12546: First, you clear any libraries that may have been declared earlier
12547: (you don't need them for @code{sin}); then you add the @code{m}
12548: library (actually @code{libm.so} or somesuch) to the currently
12549: declared libraries; you can add as many as you need. Finally you
12550: declare the function as shown above. Typically you will use the same
12551: set of library declarations for many function declarations; you need
12552: to write only one set for that, right at the beginning.
12553:
12554: Note that you must not call @code{clear-libs} inside
12555: @code{c-library...end-c-library}; however, @code{c-library} performs
12556: the function of @code{clear-libs}, so @code{clear-libs} is not
12557: necessary, and you usually want to put @code{add-lib} calls inside
12558: @code{c-library...end-c-library}.
12559:
12560: doc-clear-libs
12561: doc-add-lib
12562:
12563:
12564: @node Callbacks, C interface internals, Declaring OS-level libraries, C Interface
12565: @subsection Callbacks
12566: @cindex Callback functions written in Forth
12567: @cindex C function pointers to Forth words
12568:
12569: Callbacks are not yet supported by the documented C interface. You
12570: can use the undocumented @file{lib.fs} interface for callbacks.
12571:
12572: In some cases you have to pass a function pointer to a C function,
12573: i.e., the library wants to call back to your application (and the
12574: pointed-to function is called a callback function). You can pass the
12575: address of an existing C function (that you get with @code{lib-sym},
12576: @pxref{Low-Level C Interface Words}), but if there is no appropriate C
12577: function, you probably want to define the function as a Forth word.
12578:
12579: @c I don't understand the existing callback interface from the example - anton
12580:
12581:
12582: @c > > Und dann gibt's noch die fptr-Deklaration, die einem
12583: @c > > C-Funktionspointer entspricht (Deklaration gleich wie bei
12584: @c > > Library-Funktionen, nur ohne den C-Namen, Aufruf mit der
12585: @c > > C-Funktionsadresse auf dem TOS).
12586: @c >
12587: @c > Ja, da bin ich dann ausgestiegen, weil ich aus dem Beispiel nicht
12588: @c > gesehen habe, wozu das gut ist.
12589: @c
12590: @c Irgendwie muss ich den Callback ja testen. Und es soll ja auch
12591: @c vorkommen, dass man von irgendwelchen kranken Interfaces einen
12592: @c Funktionspointer übergeben bekommt, den man dann bei Gelegenheit
12593: @c aufrufen muss. Also kann man den deklarieren, und das damit deklarierte
12594: @c Wort verhält sich dann wie ein EXECUTE für alle C-Funktionen mit
12595: @c demselben Prototyp.
12596:
12597:
12598: @node C interface internals, Low-Level C Interface Words, Callbacks, C Interface
12599: @subsection How the C interface works
12600:
12601: The documented C interface works by generating a C code out of the
12602: declarations.
12603:
12604: In particular, for every Forth word declared with @code{c-function},
12605: it generates a wrapper function in C that takes the Forth data from
12606: the Forth stacks, and calls the target C function with these data as
12607: arguments. The C compiler then performs an implicit conversion
12608: between the Forth type from the stack, and the C type for the
12609: parameter, which is given by the C function prototype. After the C
12610: function returns, the return value is likewise implicitly converted to
12611: a Forth type and written back on the stack.
12612:
12613: The @code{\c} lines are literally included in the C code (but without
12614: the @code{\c}), and provide the necessary declarations so that the C
12615: compiler knows the C types and has enough information to perform the
12616: conversion.
12617:
12618: These wrapper functions are eventually compiled and dynamically linked
12619: into Gforth, and then they can be called.
12620:
12621: The libraries added with @code{add-lib} are used in the compile
12622: command line to specify dependent libraries with @code{-l@var{lib}},
12623: causing these libraries to be dynamically linked when the wrapper
12624: function is linked.
12625:
12626:
12627: @node Low-Level C Interface Words, , C interface internals, C Interface
12628: @subsection Low-Level C Interface Words
12629:
12630: doc-open-lib
12631: doc-lib-sym
12632: doc-lib-error
12633: doc-call-c
12634:
12635: @c -------------------------------------------------------------
12636: @node Assembler and Code Words, Threading Words, C Interface, Words
12637: @section Assembler and Code Words
12638: @cindex assembler
12639: @cindex code words
12640:
12641: @menu
12642: * Assembler Definitions:: Definitions in assembly language
12643: * Common Assembler:: Assembler Syntax
12644: * Common Disassembler::
12645: * 386 Assembler:: Deviations and special cases
12646: * AMD64 Assembler::
12647: * Alpha Assembler:: Deviations and special cases
12648: * MIPS assembler:: Deviations and special cases
12649: * PowerPC assembler:: Deviations and special cases
12650: * ARM Assembler:: Deviations and special cases
12651: * Other assemblers:: How to write them
12652: @end menu
12653:
12654: @node Assembler Definitions, Common Assembler, Assembler and Code Words, Assembler and Code Words
12655: @subsection Definitions in assembly language
12656:
12657: Gforth provides ways to implement words in assembly language (using
12658: @code{abi-code}...@code{end-code}), and also ways to define defining
12659: words with arbitrary run-time behaviour (like @code{does>}), where
12660: (unlike @code{does>}) the behaviour is not defined in Forth, but in
12661: assembly language (with @code{;code}).
12662:
12663: However, the machine-independent nature of Gforth poses a few
12664: problems: First of all, Gforth runs on several architectures, so it
12665: can provide no standard assembler. It does provide assemblers for
12666: several of the architectures it runs on, though. Moreover, you can
12667: use a system-independent assembler in Gforth, or compile machine code
12668: directly with @code{,} and @code{c,}.
12669:
12670: Another problem is that the virtual machine registers of Gforth (the
12671: stack pointers and the virtual machine instruction pointer) depend on
12672: the installation and engine. Also, which registers are free to use
12673: also depend on the installation and engine. So any code written to
12674: run in the context of the Gforth virtual machine is essentially
12675: limited to the installation and engine it was developed for (it may
12676: run elsewhere, but you cannot rely on that).
12677:
12678: Fortunately, you can define @code{abi-code} words in Gforth that are
12679: portable to any Gforth running on a platform with the same calling
12680: convention (ABI); typically this means portability to the same
12681: architecture/OS combination, sometimes crossing OS boundaries).
12682:
12683: doc-assembler
12684: doc-init-asm
12685: doc-abi-code
12686: doc-end-code
12687: doc-code
12688: doc-;code
12689: doc-flush-icache
12690:
12691:
12692: If @code{flush-icache} does not work correctly, @code{abi-code} words
12693: etc. will not work (reliably), either.
12694:
12695: The typical usage of these words can be shown most easily by analogy
12696: to the equivalent high-level defining words:
12697:
12698: @example
12699: : foo abi-code foo
12700: <high-level Forth words> <assembler>
12701: ; end-code
12702:
12703: : bar : bar
12704: <high-level Forth words> <high-level Forth words>
12705: CREATE CREATE
12706: <high-level Forth words> <high-level Forth words>
12707: DOES> ;code
12708: <high-level Forth words> <assembler>
12709: ; end-code
12710: @end example
12711:
12712: For using @code{abi-code}, take a look at the ABI documentation of
12713: your platform to see how the parameters are passed (so you know where
12714: you get the stack pointers) and how the return value is passed (so you
12715: know where the data stack pointer is returned). The ABI documentation
12716: also tells you which registers are saved by the caller (caller-saved),
12717: so you are free to destroy them in your code, and which registers have
12718: to be preserved by the called word (callee-saved), so you have to save
12719: them before using them, and restore them afterwards. For some
12720: architectures and OSs we give short summaries of the parts of the
12721: calling convention in the appropriate sections. More
12722: reverse-engineering oriented people can also find out about the
12723: passing and returning of the stack pointers through @code{see
12724: abi-call}.
12725:
12726: Most ABIs pass the parameters through registers, but some (in
12727: particular the most common 386 (aka IA-32) calling conventions) pass
12728: them on the architectural stack. The common ABIs all pass the return
12729: value in a register.
12730:
12731: Other things you need to know for using @code{abi-code} is that both
12732: the data and the FP stack grow downwards (towards lower addresses) in
12733: Gforth, with @code{1 cells} size per cell, and @code{1 floats} size
12734: per FP value.
12735:
12736: Here's an example of using @code{abi-code} on the 386 architecture:
12737:
12738: @example
12739: abi-code my+ ( n1 n2 -- n )
12740: 4 sp d) ax mov \ sp into return reg
12741: ax ) cx mov \ tos
12742: 4 # ax add \ update sp (pop)
12743: cx ax ) add \ sec = sec+tos
12744: ret \ return from my+
12745: end-code
12746: @end example
12747:
12748: An AMD64 variant of this example can be found in @ref{AMD64 Assembler}.
12749:
12750: Here's a 386 example that deals with FP values:
12751:
12752: @example
12753: abi-code my-f+ ( r1 r2 -- r )
12754: 8 sp d) cx mov \ load address of fp
12755: cx ) dx mov \ load fp
12756: .fl dx ) fld \ r2
12757: 8 # dx add \ update fp
12758: .fl dx ) fadd \ r1+r2
12759: .fl dx ) fstp \ store r
12760: dx cx ) mov \ store new fp
12761: 4 sp d) ax mov \ sp into return reg
12762: ret \ return from my-f+
12763: end-code
12764: @end example
12765:
12766:
12767: @node Common Assembler, Common Disassembler, Assembler Definitions, Assembler and Code Words
12768: @subsection Common Assembler
12769:
12770: The assemblers in Gforth generally use a postfix syntax, i.e., the
12771: instruction name follows the operands.
12772:
12773: The operands are passed in the usual order (the same that is used in the
12774: manual of the architecture). Since they all are Forth words, they have
12775: to be separated by spaces; you can also use Forth words to compute the
12776: operands.
12777:
12778: The instruction names usually end with a @code{,}. This makes it easier
12779: to visually separate instructions if you put several of them on one
12780: line; it also avoids shadowing other Forth words (e.g., @code{and}).
12781:
12782: Registers are usually specified by number; e.g., (decimal) @code{11}
12783: specifies registers R11 and F11 on the Alpha architecture (which one,
12784: depends on the instruction). The usual names are also available, e.g.,
12785: @code{s2} for R11 on Alpha.
12786:
12787: Control flow is specified similar to normal Forth code (@pxref{Arbitrary
12788: control structures}), with @code{if,}, @code{ahead,}, @code{then,},
12789: @code{begin,}, @code{until,}, @code{again,}, @code{cs-roll},
12790: @code{cs-pick}, @code{else,}, @code{while,}, and @code{repeat,}. The
12791: conditions are specified in a way specific to each assembler.
12792:
12793: The rest of this section is of interest mainly for those who want to
12794: define @code{code} words (instead of the more portable @code{abi-code}
12795: words).
12796:
12797: Note that the register assignments of the Gforth engine can change
12798: between Gforth versions, or even between different compilations of the
12799: same Gforth version (e.g., if you use a different GCC version). If
12800: you are using @code{CODE} instead of @code{ABI-CODE}, and you want to
12801: refer to Gforth's registers (e.g., the stack pointer or TOS), I
12802: recommend defining your own words for refering to these registers, and
12803: using them later on; then you can adapt to a changed register
12804: assignment.
12805:
12806: The most common use of these registers is to end a @code{code}
12807: definition with a dispatch to the next word (the @code{next} routine).
12808: A portable way to do this is to jump to @code{' noop >code-address}
12809: (of course, this is less efficient than integrating the @code{next}
12810: code and scheduling it well). When using @code{ABI-CODE}, you can
12811: just assemble a normal subroutine return (but make sure you return the
12812: data stack pointer).
12813:
12814: Another difference between Gforth versions is that the top of stack is
12815: kept in memory in @code{gforth} and, on most platforms, in a register
12816: in @code{gforth-fast}. For @code{ABI-CODE} definitions, any stack
12817: caching registers are guaranteed to be flushed to the stack, allowing
12818: you to reliably access the top of stack in memory.
12819:
12820: @node Common Disassembler, 386 Assembler, Common Assembler, Assembler and Code Words
12821: @subsection Common Disassembler
12822: @cindex disassembler, general
12823: @cindex gdb disassembler
12824:
12825: You can disassemble a @code{code} word with @code{see}
12826: (@pxref{Debugging}). You can disassemble a section of memory with
12827:
12828: doc-discode
12829:
12830: There are two kinds of disassembler for Gforth: The Forth disassembler
12831: (available on some CPUs) and the gdb disassembler (available on
12832: platforms with @command{gdb} and @command{mktemp}). If both are
12833: available, the Forth disassembler is used by default. If you prefer
12834: the gdb disassembler, say
12835:
12836: @example
12837: ' disasm-gdb is discode
12838: @end example
12839:
12840: If neither is available, @code{discode} performs @code{dump}.
12841:
12842: The Forth disassembler generally produces output that can be fed into the
12843: assembler (i.e., same syntax, etc.). It also includes additional
12844: information in comments. In particular, the address of the instruction
12845: is given in a comment before the instruction.
12846:
12847: The gdb disassembler produces output in the same format as the gdb
12848: @code{disassemble} command (@pxref{Machine Code,,Source and machine
12849: code,gdb,Debugging with GDB}), in the default flavour (AT&T syntax for
12850: the 386 and AMD64 architectures).
12851:
12852: @code{See} may display more or less than the actual code of the word,
12853: because the recognition of the end of the code is unreliable. You can
12854: use @code{discode} if it did not display enough. It may display more, if
12855: the code word is not immediately followed by a named word. If you have
12856: something else there, you can follow the word with @code{align latest ,}
12857: to ensure that the end is recognized.
12858:
12859: @node 386 Assembler, AMD64 Assembler, Common Disassembler, Assembler and Code Words
12860: @subsection 386 Assembler
12861:
12862: The 386 assembler included in Gforth was written by Bernd Paysan, it's
12863: available under GPL, and originally part of bigFORTH.
12864:
12865: The 386 disassembler included in Gforth was written by Andrew McKewan
12866: and is in the public domain.
12867:
12868: The disassembler displays code in an Intel-like prefix syntax.
12869:
12870: The assembler uses a postfix syntax with AT&T-style parameter order
12871: (i.e., destination last).
12872:
12873: The assembler includes all instruction of the Athlon, i.e. 486 core
12874: instructions, Pentium and PPro extensions, floating point, MMX, 3Dnow!,
12875: but not ISSE. It's an integrated 16- and 32-bit assembler. Default is 32
12876: bit, you can switch to 16 bit with .86 and back to 32 bit with .386.
12877:
12878: There are several prefixes to switch between different operation sizes,
12879: @code{.b} for byte accesses, @code{.w} for word accesses, @code{.d} for
12880: double-word accesses. Addressing modes can be switched with @code{.wa}
12881: for 16 bit addresses, and @code{.da} for 32 bit addresses. You don't
12882: need a prefix for byte register names (@code{AL} et al).
12883:
12884: For floating point operations, the prefixes are @code{.fs} (IEEE
12885: single), @code{.fl} (IEEE double), @code{.fx} (extended), @code{.fw}
12886: (word), @code{.fd} (double-word), and @code{.fq} (quad-word). The
12887: default is @code{.fx}, so you need to specify @code{.fl} explicitly
12888: when dealing with Gforth FP values.
12889:
12890: The MMX opcodes don't have size prefixes, they are spelled out like in
12891: the Intel assembler. Instead of move from and to memory, there are
12892: PLDQ/PLDD and PSTQ/PSTD.
12893:
12894: The registers lack the 'e' prefix; even in 32 bit mode, eax is called
12895: ax. Immediate values are indicated by postfixing them with @code{#},
12896: e.g., @code{3 #}. Here are some examples of addressing modes in various
12897: syntaxes:
12898:
12899: @example
12900: Gforth Intel (NASM) AT&T (gas) Name
12901: .w ax ax %ax register (16 bit)
12902: ax eax %eax register (32 bit)
12903: 3 # offset 3 $3 immediate
12904: 1000 #) byte ptr 1000 1000 displacement
12905: bx ) [ebx] (%ebx) base
12906: 100 di d) 100[edi] 100(%edi) base+displacement
12907: 20 ax *4 i#) 20[eax*4] 20(,%eax,4) (index*scale)+displacement
12908: di ax *4 i) [edi][eax*4] (%edi,%eax,4) base+(index*scale)
12909: 4 bx cx di) 4[ebx][ecx] 4(%ebx,%ecx) base+index+displacement
12910: 12 sp ax *2 di) 12[esp][eax*2] 12(%esp,%eax,2) base+(index*scale)+displacement
12911: @end example
12912:
12913: You can use @code{L)} and @code{LI)} instead of @code{D)} and
12914: @code{DI)} to enforce 32-bit displacement fields (useful for
12915: later patching).
12916:
12917: Some example of instructions are:
12918:
12919: @example
12920: ax bx mov \ move ebx,eax
12921: 3 # ax mov \ mov eax,3
12922: 100 di d) ax mov \ mov eax,100[edi]
12923: 4 bx cx di) ax mov \ mov eax,4[ebx][ecx]
12924: .w ax bx mov \ mov bx,ax
12925: @end example
12926:
12927: The following forms are supported for binary instructions:
12928:
12929: @example
12930: <reg> <reg> <inst>
12931: <n> # <reg> <inst>
12932: <mem> <reg> <inst>
12933: <reg> <mem> <inst>
12934: <n> # <mem> <inst>
12935: @end example
12936:
12937: The shift/rotate syntax is:
12938:
12939: @example
12940: <reg/mem> 1 # shl \ shortens to shift without immediate
12941: <reg/mem> 4 # shl
12942: <reg/mem> cl shl
12943: @end example
12944:
12945: Precede string instructions (@code{movs} etc.) with @code{.b} to get
12946: the byte version.
12947:
12948: The control structure words @code{IF} @code{UNTIL} etc. must be preceded
12949: by one of these conditions: @code{vs vc u< u>= 0= 0<> u<= u> 0< 0>= ps
12950: pc < >= <= >}. (Note that most of these words shadow some Forth words
12951: when @code{assembler} is in front of @code{forth} in the search path,
12952: e.g., in @code{code} words). Currently the control structure words use
12953: one stack item, so you have to use @code{roll} instead of @code{cs-roll}
12954: to shuffle them (you can also use @code{swap} etc.).
12955:
12956: Based on the Intel ABI (used in Linux), @code{abi-code} words can find
12957: the data stack pointer at @code{4 sp d)}, and the address of the FP
12958: stack pointer at @code{8 sp d)}; the data stack pointer is returned in
12959: @code{ax}; @code{Ax}, @code{cx}, and @code{dx} are caller-saved, so
12960: you do not need to preserve their values inside the word. You can
12961: return from the word with @code{ret}, the parameters are cleaned up by
12962: the caller.
12963:
12964: For examples of 386 @code{abi-code} words, see @ref{Assembler Definitions}.
12965:
12966:
12967: @node AMD64 Assembler, Alpha Assembler, 386 Assembler, Assembler and Code Words
12968: @subsection AMD64 (x86_64) Assembler
12969:
12970: The AMD64 assembler is a slightly modified version of the 386
12971: assembler, and as such shares most of the syntax. Two new prefixes,
12972: @code{.q} and @code{.qa}, are provided to select 64-bit operand and
12973: address sizes respectively. 64-bit sizes are the default, so normally
12974: you only have to use the other prefixes. Also there are additional
12975: register operands @code{R8}-@code{R15}.
12976:
12977: The registers lack the 'e' or 'r' prefix; even in 64 bit mode,
12978: @code{rax} is called @code{ax}. Additional register operands are
12979: available to refer to the lowest-significant byte of all registers:
12980: @code{R8L}-@code{R15L}, @code{SPL}, @code{BPL}, @code{SIL},
12981: @code{DIL}.
12982:
12983: The Linux-AMD64 calling convention is to pass the first 6 integer
12984: parameters in rdi, rsi, rdx, rcx, r8 and r9 and to return the result
12985: in rax and rdx; to pass the first 8 FP parameters in xmm0--xmm7 and to
12986: return FP results in xmm0--xmm1. So @code{abi-code} words get the
12987: data stack pointer in @code{di} and the address of the FP stack
12988: pointer in @code{si}, and return the data stack pointer in @code{ax}.
12989: The other caller-saved registers are: r10, r11, xmm8-xmm15. This
12990: calling convention reportedly is also used in other non-Microsoft OSs.
12991: @c source: http://en.wikipedia.org/wiki/X86_calling_conventions#AMD64_ABI_convention
12992:
12993: @c source: http://msdn.microsoft.com/en-us/library/9b372w95(v=VS.90).aspx
12994: Windows x64 passes the first four integer parameters in rcx, rdx, r8
12995: and r9 and return the integer result in rax. The other caller-saved
12996: registers are r10 and r11.
12997:
12998: Here is an example of an AMD64 @code{abi-code} word:
12999:
13000: @example
13001: abi-code my+ ( n1 n2 -- n3 )
13002: \ SP passed in di, returned in ax, address of FP passed in si
13003: 8 di d) ax lea \ compute new sp in result reg
13004: di ) dx mov \ get old tos
13005: dx ax ) add \ add to new tos
13006: ret
13007: end-code
13008: @end example
13009:
13010: Here's a AMD64 example that deals with FP values:
13011:
13012: @example
13013: abi-code my-f+ ( r1 r2 -- r )
13014: \ SP passed in di, returned in ax, address of FP passed in si
13015: si ) dx mov \ load fp
13016: 8 dx d) xmm0 movsd \ r2
13017: dx ) xmm0 addsd \ r1+r2
13018: xmm0 8 dx d) movsd \ store r
13019: 8 # si ) add \ update fp
13020: di ax mov \ sp into return reg
13021: ret
13022: end-code
13023: @end example
13024:
13025: @node Alpha Assembler, MIPS assembler, AMD64 Assembler, Assembler and Code Words
13026: @subsection Alpha Assembler
13027:
13028: The Alpha assembler and disassembler were originally written by Bernd
13029: Thallner.
13030:
13031: The register names @code{a0}--@code{a5} are not available to avoid
13032: shadowing hex numbers.
13033:
13034: Immediate forms of arithmetic instructions are distinguished by a
13035: @code{#} just before the @code{,}, e.g., @code{and#,} (note: @code{lda,}
13036: does not count as arithmetic instruction).
13037:
13038: You have to specify all operands to an instruction, even those that
13039: other assemblers consider optional, e.g., the destination register for
13040: @code{br,}, or the destination register and hint for @code{jmp,}.
13041:
13042: You can specify conditions for @code{if,} by removing the first @code{b}
13043: and the trailing @code{,} from a branch with a corresponding name; e.g.,
13044:
13045: @example
13046: 11 fgt if, \ if F11>0e
13047: ...
13048: endif,
13049: @end example
13050:
13051: @code{fbgt,} gives @code{fgt}.
13052:
13053: @node MIPS assembler, PowerPC assembler, Alpha Assembler, Assembler and Code Words
13054: @subsection MIPS assembler
13055:
13056: The MIPS assembler was originally written by Christian Pirker.
13057:
13058: Currently the assembler and disassembler covers most of the MIPS32
13059: architecture and doesn't support FP instructions.
13060:
13061: The register names @code{$a0}--@code{$a3} are not available to avoid
13062: shadowing hex numbers. Use register numbers @code{$4}--@code{$7}
13063: instead.
13064:
13065: Nothing distinguishes registers from immediate values. Use explicit
13066: opcode names with the @code{i} suffix for instructions with immediate
13067: argument. E.g. @code{addiu,} in place of @code{addu,}.
13068:
13069: Where the architecture manual specifies several formats for the
13070: instruction (e.g., for @code{jalr,}),use the one with more arguments
13071: (i.e. two for @code{jalr,}). When in doubt, see
13072: @code{arch/mips/testasm.fs} for an example of correct use.
13073:
13074: Branches and jumps in the MIPS architecture have a delay slot. You
13075: have to fill it manually (the simplest way is to use @code{nop,}), the
13076: assembler does not do it for you (unlike @command{as}). Even
13077: @code{if,}, @code{ahead,}, @code{until,}, @code{again,},
13078: @code{while,}, @code{else,} and @code{repeat,} need a delay slot.
13079: Since @code{begin,} and @code{then,} just specify branch targets, they
13080: are not affected. For branches the argument specifying the target is
13081: a relative address. Add the address of the delay slot to get the
13082: absolute address.
13083:
13084: Note that you must not put branches nor jumps (nor control-flow
13085: instructions) into the delay slot. Also it is a bad idea to put
13086: pseudo-ops such as @code{li,} into a delay slot, as these may expand
13087: to several instructions. The MIPS I architecture also had load delay
13088: slots, and newer MIPSes still have restrictions on using @code{mfhi,}
13089: and @code{mflo,}. Be careful to satisfy these restrictions, the
13090: assembler does not do it for you.
13091:
13092: Some example of instructions are:
13093:
13094: @example
13095: $ra 12 $sp sw, \ sw ra,12(sp)
13096: $4 8 $s0 lw, \ lw a0,8(s0)
13097: $v0 $0 lui, \ lui v0,0x0
13098: $s0 $s4 $12 addiu, \ addiu s0,s4,0x12
13099: $s0 $s4 $4 addu, \ addu s0,s4,$a0
13100: $ra $t9 jalr, \ jalr t9
13101: @end example
13102:
13103: You can specify the conditions for @code{if,} etc. by taking a
13104: conditional branch and leaving away the @code{b} at the start and the
13105: @code{,} at the end. E.g.,
13106:
13107: @example
13108: 4 5 eq if,
13109: ... \ do something if $4 equals $5
13110: then,
13111: @end example
13112:
13113: The calling conventions for 32-bit MIPS machines is to pass the first
13114: 4 arguments in registers @code{$4}..@code{$7}, and to use
13115: @code{$v0}-@code{$v1} for return values. In addition to these
13116: registers, it is ok to clobber registers @code{$t0}-@code{$t8} without
13117: saving and restoring them.
13118:
13119: If you use @code{jalr,} to call into dynamic library routines, you
13120: must first load the called function's address into @code{$t9}, which
13121: is used by position-indirect code to do relative memory accesses.
13122:
13123: Here is an example of a MIPS32 @code{abi-code} word:
13124:
13125: @example
13126: abi-code my+ ( n1 n2 -- n3 )
13127: \ SP passed in $4, returned in $v0
13128: $t0 4 $4 lw, \ load n1, n2 from stack
13129: $t1 0 $4 lw,
13130: $t0 $t0 $t1 addu, \ add n1+n2, result in $t0
13131: $t0 4 $4 sw, \ store result (overwriting n1)
13132: $ra jr, \ return to caller
13133: $v0 $4 4 addiu, \ (delay slot) return uptated SP in $v0
13134: end-code
13135: @end example
13136:
13137: @node PowerPC assembler, ARM Assembler, MIPS assembler, Assembler and Code Words
13138: @subsection PowerPC assembler
13139:
13140: The PowerPC assembler and disassembler were contributed by Michal
13141: Revucky.
13142:
13143: This assembler does not follow the convention of ending mnemonic names
13144: with a ``,'', so some mnemonic names shadow regular Forth words (in
13145: particular: @code{and or xor fabs}); so if you want to use the Forth
13146: words, you have to make them visible first, e.g., with @code{also
13147: forth}.
13148:
13149: Registers are referred to by their number, e.g., @code{9} means the
13150: integer register 9 or the FP register 9 (depending on the
13151: instruction).
13152:
13153: Because there is no way to distinguish registers from immediate values,
13154: you have to explicitly use the immediate forms of instructions, i.e.,
13155: @code{addi,}, not just @code{add,}.
13156:
13157: The assembler and disassembler usually support the most general form
13158: of an instruction, but usually not the shorter forms (especially for
13159: branches).
13160:
13161:
13162: @node ARM Assembler, Other assemblers, PowerPC assembler, Assembler and Code Words
13163: @subsection ARM Assembler
13164:
13165: The ARM assembler includes all instruction of ARM architecture version
13166: 4, and the BLX instruction from architecture 5. It does not (yet)
13167: have support for Thumb instructions. It also lacks support for any
13168: co-processors.
13169:
13170: The assembler uses a postfix syntax with the same operand order as
13171: used in the ARM Architecture Reference Manual. Mnemonics are suffixed
13172: by a comma.
13173:
13174: Registers are specified by their names @code{r0} through @code{r15},
13175: with the aliases @code{pc}, @code{lr}, @code{sp}, @code{ip} and
13176: @code{fp} provided for convenience. Note that @code{ip} refers to
13177: the``intra procedure call scratch register'' (@code{r12}) and does not
13178: refer to an instruction pointer. @code{sp} refers to the ARM ABI
13179: stack pointer (@code{r13}) and not the Forth stack pointer.
13180:
13181: Condition codes can be specified anywhere in the instruction, but will
13182: be most readable if specified just in front of the mnemonic. The 'S'
13183: flag is not a separate word, but encoded into instruction mnemonics,
13184: ie. just use @code{adds,} instead of @code{add,} if you want the
13185: status register to be updated.
13186:
13187: The following table lists the syntax of operands for general
13188: instructions:
13189:
13190: @example
13191: Gforth normal assembler description
13192: 123 # #123 immediate
13193: r12 r12 register
13194: r12 4 #LSL r12, LSL #4 shift left by immediate
13195: r12 r1 #LSL r12, LSL r1 shift left by register
13196: r12 4 #LSR r12, LSR #4 shift right by immediate
13197: r12 r1 #LSR r12, LSR r1 shift right by register
13198: r12 4 #ASR r12, ASR #4 arithmetic shift right
13199: r12 r1 #ASR r12, ASR r1 ... by register
13200: r12 4 #ROR r12, ROR #4 rotate right by immediate
13201: r12 r1 #ROR r12, ROR r1 ... by register
13202: r12 RRX r12, RRX rotate right with extend by 1
13203: @end example
13204:
13205: Memory operand syntax is listed in this table:
13206:
13207: @example
13208: Gforth normal assembler description
13209: r4 ] [r4] register
13210: r4 4 #] [r4, #+4] register with immediate offset
13211: r4 -4 #] [r4, #-4] with negative offset
13212: r4 r1 +] [r4, +r1] register with register offset
13213: r4 r1 -] [r4, -r1] with negated register offset
13214: r4 r1 2 #LSL -] [r4, -r1, LSL #2] with negated and shifted offset
13215: r4 4 #]! [r4, #+4]! immediate preincrement
13216: r4 r1 +]! [r4, +r1]! register preincrement
13217: r4 r1 -]! [r4, +r1]! register predecrement
13218: r4 r1 2 #LSL +]! [r4, +r1, LSL #2]! shifted preincrement
13219: r4 -4 ]# [r4], #-4 immediate postdecrement
13220: r4 r1 ]+ [r4], r1 register postincrement
13221: r4 r1 ]- [r4], -r1 register postdecrement
13222: r4 r1 2 #LSL ]- [r4], -r1, LSL #2 shifted postdecrement
13223: ' xyz >body [#] xyz PC-relative addressing
13224: @end example
13225:
13226: Register lists for load/store multiple instructions are started and
13227: terminated by using the words @code{@{} and @code{@}} respectively.
13228: Between braces, register names can be listed one by one or register
13229: ranges can be formed by using the postfix operator @code{r-r}. The
13230: @code{^} flag is not encoded in the register list operand, but instead
13231: directly encoded into the instruction mnemonic, ie. use @code{^ldm,}
13232: and @code{^stm,}.
13233:
13234: Addressing modes for load/store multiple are not encoded as
13235: instruction suffixes, but instead specified like an addressing mode,
13236: Use one of @code{DA}, @code{IA}, @code{DB}, @code{IB}, @code{DA!},
13237: @code{IA!}, @code{DB!} or @code{IB!}.
13238:
13239: The following table gives some examples:
13240:
13241: @example
13242: Gforth normal assembler
13243: r4 ia @{ r0 r7 r8 @} stm, stmia r4, @{r0,r7,r8@}
13244: r4 db! @{ r0 r7 r8 @} ldm, ldmdb r4!, @{r0,r7,r8@}
13245: sp ia! @{ r0 r15 r-r @} ^ldm, ldmfd sp!, @{r0-r15@}^
13246: @end example
13247:
13248: Control structure words typical for Forth assemblers are available:
13249: @code{if,} @code{ahead,} @code{then,} @code{else,} @code{begin,}
13250: @code{until,} @code{again,} @code{while,} @code{repeat,}
13251: @code{repeat-until,}. Conditions are specified in front of these words:
13252:
13253: @example
13254: r1 r2 cmp, \ compare r1 and r2
13255: eq if, \ equal?
13256: ... \ code executed if r1 == r2
13257: then,
13258: @end example
13259:
13260: Example of a definition using the ARM assembler:
13261:
13262: @example
13263: abi-code my+ ( n1 n2 -- n3 )
13264: \ arm abi: r0=SP, r1=&FP, r2,r3,r12 saved by caller
13265: r0 IA! @{ r2 r3 @} ldm, \ pop r2 = n2, r3 = n1
13266: r3 r2 r3 add, \ r3 = n1+n1
13267: r3 r0 -4 #]! str, \ push r3
13268: pc lr mov, \ return to caller, new SP in r0
13269: end-code
13270: @end example
13271:
13272: @node Other assemblers, , ARM Assembler, Assembler and Code Words
13273: @subsection Other assemblers
13274:
13275: If you want to contribute another assembler/disassembler, please contact
13276: us (@email{anton@@mips.complang.tuwien.ac.at}) to check if we have such
13277: an assembler already. If you are writing them from scratch, please use
13278: a similar syntax style as the one we use (i.e., postfix, commas at the
13279: end of the instruction names, @pxref{Common Assembler}); make the output
13280: of the disassembler be valid input for the assembler, and keep the style
13281: similar to the style we used.
13282:
13283: Hints on implementation: The most important part is to have a good test
13284: suite that contains all instructions. Once you have that, the rest is
13285: easy. For actual coding you can take a look at
13286: @file{arch/mips/disasm.fs} to get some ideas on how to use data for both
13287: the assembler and disassembler, avoiding redundancy and some potential
13288: bugs. You can also look at that file (and @pxref{Advanced does> usage
13289: example}) to get ideas how to factor a disassembler.
13290:
13291: Start with the disassembler, because it's easier to reuse data from the
13292: disassembler for the assembler than the other way round.
13293:
13294: For the assembler, take a look at @file{arch/alpha/asm.fs}, which shows
13295: how simple it can be.
13296:
13297:
13298:
13299:
13300: @c -------------------------------------------------------------
13301: @node Threading Words, Passing Commands to the OS, Assembler and Code Words, Words
13302: @section Threading Words
13303: @cindex threading words
13304:
13305: @cindex code address
13306: These words provide access to code addresses and other threading stuff
13307: in Gforth (and, possibly, other interpretive Forths). It more or less
13308: abstracts away the differences between direct and indirect threading
13309: (and, for direct threading, the machine dependences). However, at
13310: present this wordset is still incomplete. It is also pretty low-level;
13311: some day it will hopefully be made unnecessary by an internals wordset
13312: that abstracts implementation details away completely.
13313:
13314: The terminology used here stems from indirect threaded Forth systems; in
13315: such a system, the XT of a word is represented by the CFA (code field
13316: address) of a word; the CFA points to a cell that contains the code
13317: address. The code address is the address of some machine code that
13318: performs the run-time action of invoking the word (e.g., the
13319: @code{dovar:} routine pushes the address of the body of the word (a
13320: variable) on the stack
13321: ).
13322:
13323: @cindex code address
13324: @cindex code field address
13325: In an indirect threaded Forth, you can get the code address of @i{name}
13326: with @code{' @i{name} @@}; in Gforth you can get it with @code{' @i{name}
13327: >code-address}, independent of the threading method.
13328:
13329: doc-threading-method
13330: doc->code-address
13331: doc-code-address!
13332:
13333: @cindex @code{does>}-handler
13334: @cindex @code{does>}-code
13335: For a word defined with @code{DOES>}, the code address usually points to
13336: a jump instruction (the @dfn{does-handler}) that jumps to the dodoes
13337: routine (in Gforth on some platforms, it can also point to the dodoes
13338: routine itself). What you are typically interested in, though, is
13339: whether a word is a @code{DOES>}-defined word, and what Forth code it
13340: executes; @code{>does-code} tells you that.
13341:
13342: doc->does-code
13343:
13344: To create a @code{DOES>}-defined word with the following basic words,
13345: you have to set up a @code{DOES>}-handler with @code{does-handler!};
13346: @code{/does-handler} aus behind you have to place your executable Forth
13347: code. Finally you have to create a word and modify its behaviour with
13348: @code{does-handler!}.
13349:
13350: doc-does-code!
13351: doc-does-handler!
13352: doc-/does-handler
13353:
13354: The code addresses produced by various defining words are produced by
13355: the following words:
13356:
13357: doc-docol:
13358: doc-docon:
13359: doc-dovar:
13360: doc-douser:
13361: doc-dodefer:
13362: doc-dofield:
13363:
13364: @cindex definer
13365: The following two words generalize @code{>code-address},
13366: @code{>does-code}, @code{code-address!}, and @code{does-code!}:
13367:
13368: doc->definer
13369: doc-definer!
13370:
13371: @c -------------------------------------------------------------
13372: @node Passing Commands to the OS, Keeping track of Time, Threading Words, Words
13373: @section Passing Commands to the Operating System
13374: @cindex operating system - passing commands
13375: @cindex shell commands
13376:
13377: Gforth allows you to pass an arbitrary string to the host operating
13378: system shell (if such a thing exists) for execution.
13379:
13380: doc-sh
13381: doc-system
13382: doc-$?
13383: doc-getenv
13384:
13385: @c -------------------------------------------------------------
13386: @node Keeping track of Time, Miscellaneous Words, Passing Commands to the OS, Words
13387: @section Keeping track of Time
13388: @cindex time-related words
13389:
13390: doc-ms
13391: doc-time&date
13392: doc-utime
13393: doc-cputime
13394:
13395:
13396: @c -------------------------------------------------------------
13397: @node Miscellaneous Words, , Keeping track of Time, Words
13398: @section Miscellaneous Words
13399: @cindex miscellaneous words
13400:
13401: @comment TODO find homes for these
13402:
13403: These section lists the ANS Forth words that are not documented
13404: elsewhere in this manual. Ultimately, they all need proper homes.
13405:
13406: doc-quit
13407:
13408: The following ANS Forth words are not currently supported by Gforth
13409: (@pxref{ANS conformance}):
13410:
13411: @code{EDITOR}
13412: @code{EMIT?}
13413: @code{FORGET}
13414:
13415: @c ******************************************************************
13416: @node Error messages, Tools, Words, Top
13417: @chapter Error messages
13418: @cindex error messages
13419: @cindex backtrace
13420:
13421: A typical Gforth error message looks like this:
13422:
13423: @example
13424: in file included from \evaluated string/:-1
13425: in file included from ./yyy.fs:1
13426: ./xxx.fs:4: Invalid memory address
13427: >>>bar<<<
13428: Backtrace:
13429: $400E664C @@
13430: $400E6664 foo
13431: @end example
13432:
13433: The message identifying the error is @code{Invalid memory address}. The
13434: error happened when text-interpreting line 4 of the file
13435: @file{./xxx.fs}. This line is given (it contains @code{bar}), and the
13436: word on the line where the error happened, is pointed out (with
13437: @code{>>>} and @code{<<<}).
13438:
13439: The file containing the error was included in line 1 of @file{./yyy.fs},
13440: and @file{yyy.fs} was included from a non-file (in this case, by giving
13441: @file{yyy.fs} as command-line parameter to Gforth).
13442:
13443: At the end of the error message you find a return stack dump that can be
13444: interpreted as a backtrace (possibly empty). On top you find the top of
13445: the return stack when the @code{throw} happened, and at the bottom you
13446: find the return stack entry just above the return stack of the topmost
13447: text interpreter.
13448:
13449: To the right of most return stack entries you see a guess for the word
13450: that pushed that return stack entry as its return address. This gives a
13451: backtrace. In our case we see that @code{bar} called @code{foo}, and
13452: @code{foo} called @code{@@} (and @code{@@} had an @emph{Invalid memory
13453: address} exception).
13454:
13455: Note that the backtrace is not perfect: We don't know which return stack
13456: entries are return addresses (so we may get false positives); and in
13457: some cases (e.g., for @code{abort"}) we cannot determine from the return
13458: address the word that pushed the return address, so for some return
13459: addresses you see no names in the return stack dump.
13460:
13461: @cindex @code{catch} and backtraces
13462: The return stack dump represents the return stack at the time when a
13463: specific @code{throw} was executed. In programs that make use of
13464: @code{catch}, it is not necessarily clear which @code{throw} should be
13465: used for the return stack dump (e.g., consider one @code{throw} that
13466: indicates an error, which is caught, and during recovery another error
13467: happens; which @code{throw} should be used for the stack dump?).
13468: Gforth presents the return stack dump for the first @code{throw} after
13469: the last executed (not returned-to) @code{catch} or @code{nothrow};
13470: this works well in the usual case. To get the right backtrace, you
13471: usually want to insert @code{nothrow} or @code{['] false catch drop}
13472: after a @code{catch} if the error is not rethrown.
13473:
13474: @cindex @code{gforth-fast} and backtraces
13475: @cindex @code{gforth-fast}, difference from @code{gforth}
13476: @cindex backtraces with @code{gforth-fast}
13477: @cindex return stack dump with @code{gforth-fast}
13478: @code{Gforth} is able to do a return stack dump for throws generated
13479: from primitives (e.g., invalid memory address, stack empty etc.);
13480: @code{gforth-fast} is only able to do a return stack dump from a
13481: directly called @code{throw} (including @code{abort} etc.). Given an
13482: exception caused by a primitive in @code{gforth-fast}, you will
13483: typically see no return stack dump at all; however, if the exception is
13484: caught by @code{catch} (e.g., for restoring some state), and then
13485: @code{throw}n again, the return stack dump will be for the first such
13486: @code{throw}.
13487:
13488: @c ******************************************************************
13489: @node Tools, ANS conformance, Error messages, Top
13490: @chapter Tools
13491:
13492: @menu
13493: * ANS Report:: Report the words used, sorted by wordset.
13494: * Stack depth changes:: Where does this stack item come from?
13495: @end menu
13496:
13497: See also @ref{Emacs and Gforth}.
13498:
13499: @node ANS Report, Stack depth changes, Tools, Tools
13500: @section @file{ans-report.fs}: Report the words used, sorted by wordset
13501: @cindex @file{ans-report.fs}
13502: @cindex report the words used in your program
13503: @cindex words used in your program
13504:
13505: If you want to label a Forth program as ANS Forth Program, you must
13506: document which wordsets the program uses; for extension wordsets, it is
13507: helpful to list the words the program requires from these wordsets
13508: (because Forth systems are allowed to provide only some words of them).
13509:
13510: The @file{ans-report.fs} tool makes it easy for you to determine which
13511: words from which wordset and which non-ANS words your application
13512: uses. You simply have to include @file{ans-report.fs} before loading the
13513: program you want to check. After loading your program, you can get the
13514: report with @code{print-ans-report}. A typical use is to run this as
13515: batch job like this:
13516: @example
13517: gforth ans-report.fs myprog.fs -e "print-ans-report bye"
13518: @end example
13519:
13520: The output looks like this (for @file{compat/control.fs}):
13521: @example
13522: The program uses the following words
13523: from CORE :
13524: : POSTPONE THEN ; immediate ?dup IF 0=
13525: from BLOCK-EXT :
13526: \
13527: from FILE :
13528: (
13529: @end example
13530:
13531: @subsection Caveats
13532:
13533: Note that @file{ans-report.fs} just checks which words are used, not whether
13534: they are used in an ANS Forth conforming way!
13535:
13536: Some words are defined in several wordsets in the
13537: standard. @file{ans-report.fs} reports them for only one of the
13538: wordsets, and not necessarily the one you expect. It depends on usage
13539: which wordset is the right one to specify. E.g., if you only use the
13540: compilation semantics of @code{S"}, it is a Core word; if you also use
13541: its interpretation semantics, it is a File word.
13542:
13543:
13544: @node Stack depth changes, , ANS Report, Tools
13545: @section Stack depth changes during interpretation
13546: @cindex @file{depth-changes.fs}
13547: @cindex depth changes during interpretation
13548: @cindex stack depth changes during interpretation
13549: @cindex items on the stack after interpretation
13550:
13551: Sometimes you notice that, after loading a file, there are items left
13552: on the stack. The tool @file{depth-changes.fs} helps you find out
13553: quickly where in the file these stack items are coming from.
13554:
13555: The simplest way of using @file{depth-changes.fs} is to include it
13556: before the file(s) you want to check, e.g.:
13557:
13558: @example
13559: gforth depth-changes.fs my-file.fs
13560: @end example
13561:
13562: This will compare the stack depths of the data and FP stack at every
13563: empty line (in interpretation state) against these depths at the last
13564: empty line (in interpretation state). If the depths are not equal,
13565: the position in the file and the stack contents are printed with
13566: @code{~~} (@pxref{Debugging}). This indicates that a stack depth
13567: change has occured in the paragraph of non-empty lines before the
13568: indicated line. It is a good idea to leave an empty line at the end
13569: of the file, so the last paragraph is checked, too.
13570:
13571: Checking only at empty lines usually works well, but sometimes you
13572: have big blocks of non-empty lines (e.g., when building a big table),
13573: and you want to know where in this block the stack depth changed. You
13574: can check all interpreted lines with
13575:
13576: @example
13577: gforth depth-changes.fs -e "' all-lines is depth-changes-filter" my-file.fs
13578: @end example
13579:
13580: This checks the stack depth at every end-of-line. So the depth change
13581: occured in the line reported by the @code{~~} (not in the line
13582: before).
13583:
13584: Note that, while this offers better accuracy in indicating where the
13585: stack depth changes, it will often report many intentional stack depth
13586: changes (e.g., when an interpreted computation stretches across
13587: several lines). You can suppress the checking of some lines by
13588: putting backslashes at the end of these lines (not followed by white
13589: space), and using
13590:
13591: @example
13592: gforth depth-changes.fs -e "' most-lines is depth-changes-filter" my-file.fs
13593: @end example
13594:
13595: @c ******************************************************************
13596: @node ANS conformance, Standard vs Extensions, Tools, Top
13597: @chapter ANS conformance
13598: @cindex ANS conformance of Gforth
13599:
13600: To the best of our knowledge, Gforth is an
13601:
13602: ANS Forth System
13603: @itemize @bullet
13604: @item providing the Core Extensions word set
13605: @item providing the Block word set
13606: @item providing the Block Extensions word set
13607: @item providing the Double-Number word set
13608: @item providing the Double-Number Extensions word set
13609: @item providing the Exception word set
13610: @item providing the Exception Extensions word set
13611: @item providing the Facility word set
13612: @item providing @code{EKEY}, @code{EKEY>CHAR}, @code{EKEY?}, @code{MS} and @code{TIME&DATE} from the Facility Extensions word set
13613: @item providing the File Access word set
13614: @item providing the File Access Extensions word set
13615: @item providing the Floating-Point word set
13616: @item providing the Floating-Point Extensions word set
13617: @item providing the Locals word set
13618: @item providing the Locals Extensions word set
13619: @item providing the Memory-Allocation word set
13620: @item providing the Memory-Allocation Extensions word set (that one's easy)
13621: @item providing the Programming-Tools word set
13622: @item providing @code{;CODE}, @code{AHEAD}, @code{ASSEMBLER}, @code{BYE}, @code{CODE}, @code{CS-PICK}, @code{CS-ROLL}, @code{STATE}, @code{[ELSE]}, @code{[IF]}, @code{[THEN]} from the Programming-Tools Extensions word set
13623: @item providing the Search-Order word set
13624: @item providing the Search-Order Extensions word set
13625: @item providing the String word set
13626: @item providing the String Extensions word set (another easy one)
13627: @end itemize
13628:
13629: Gforth has the following environmental restrictions:
13630:
13631: @cindex environmental restrictions
13632: @itemize @bullet
13633: @item
13634: While processing the OS command line, if an exception is not caught,
13635: Gforth exits with a non-zero exit code instyead of performing QUIT.
13636:
13637: @item
13638: When an @code{throw} is performed after a @code{query}, Gforth does not
13639: allways restore the input source specification in effect at the
13640: corresponding catch.
13641:
13642: @end itemize
13643:
13644:
13645: @cindex system documentation
13646: In addition, ANS Forth systems are required to document certain
13647: implementation choices. This chapter tries to meet these
13648: requirements. In many cases it gives a way to ask the system for the
13649: information instead of providing the information directly, in
13650: particular, if the information depends on the processor, the operating
13651: system or the installation options chosen, or if they are likely to
13652: change during the maintenance of Gforth.
13653:
13654: @comment The framework for the rest has been taken from pfe.
13655:
13656: @menu
13657: * The Core Words::
13658: * The optional Block word set::
13659: * The optional Double Number word set::
13660: * The optional Exception word set::
13661: * The optional Facility word set::
13662: * The optional File-Access word set::
13663: * The optional Floating-Point word set::
13664: * The optional Locals word set::
13665: * The optional Memory-Allocation word set::
13666: * The optional Programming-Tools word set::
13667: * The optional Search-Order word set::
13668: @end menu
13669:
13670:
13671: @c =====================================================================
13672: @node The Core Words, The optional Block word set, ANS conformance, ANS conformance
13673: @comment node-name, next, previous, up
13674: @section The Core Words
13675: @c =====================================================================
13676: @cindex core words, system documentation
13677: @cindex system documentation, core words
13678:
13679: @menu
13680: * core-idef:: Implementation Defined Options
13681: * core-ambcond:: Ambiguous Conditions
13682: * core-other:: Other System Documentation
13683: @end menu
13684:
13685: @c ---------------------------------------------------------------------
13686: @node core-idef, core-ambcond, The Core Words, The Core Words
13687: @subsection Implementation Defined Options
13688: @c ---------------------------------------------------------------------
13689: @cindex core words, implementation-defined options
13690: @cindex implementation-defined options, core words
13691:
13692:
13693: @table @i
13694: @item (Cell) aligned addresses:
13695: @cindex cell-aligned addresses
13696: @cindex aligned addresses
13697: processor-dependent. Gforth's alignment words perform natural alignment
13698: (e.g., an address aligned for a datum of size 8 is divisible by
13699: 8). Unaligned accesses usually result in a @code{-23 THROW}.
13700:
13701: @item @code{EMIT} and non-graphic characters:
13702: @cindex @code{EMIT} and non-graphic characters
13703: @cindex non-graphic characters and @code{EMIT}
13704: The character is output using the C library function (actually, macro)
13705: @code{putc}.
13706:
13707: @item character editing of @code{ACCEPT} and @code{EXPECT}:
13708: @cindex character editing of @code{ACCEPT} and @code{EXPECT}
13709: @cindex editing in @code{ACCEPT} and @code{EXPECT}
13710: @cindex @code{ACCEPT}, editing
13711: @cindex @code{EXPECT}, editing
13712: This is modeled on the GNU readline library (@pxref{Readline
13713: Interaction, , Command Line Editing, readline, The GNU Readline
13714: Library}) with Emacs-like key bindings. @kbd{Tab} deviates a little by
13715: producing a full word completion every time you type it (instead of
13716: producing the common prefix of all completions). @xref{Command-line editing}.
13717:
13718: @item character set:
13719: @cindex character set
13720: The character set of your computer and display device. Gforth is
13721: 8-bit-clean (but some other component in your system may make trouble).
13722:
13723: @item Character-aligned address requirements:
13724: @cindex character-aligned address requirements
13725: installation-dependent. Currently a character is represented by a C
13726: @code{unsigned char}; in the future we might switch to @code{wchar_t}
13727: (Comments on that requested).
13728:
13729: @item character-set extensions and matching of names:
13730: @cindex character-set extensions and matching of names
13731: @cindex case-sensitivity for name lookup
13732: @cindex name lookup, case-sensitivity
13733: @cindex locale and case-sensitivity
13734: Any character except the ASCII NUL character can be used in a
13735: name. Matching is case-insensitive (except in @code{TABLE}s). The
13736: matching is performed using the C library function @code{strncasecmp}, whose
13737: function is probably influenced by the locale. E.g., the @code{C} locale
13738: does not know about accents and umlauts, so they are matched
13739: case-sensitively in that locale. For portability reasons it is best to
13740: write programs such that they work in the @code{C} locale. Then one can
13741: use libraries written by a Polish programmer (who might use words
13742: containing ISO Latin-2 encoded characters) and by a French programmer
13743: (ISO Latin-1) in the same program (of course, @code{WORDS} will produce
13744: funny results for some of the words (which ones, depends on the font you
13745: are using)). Also, the locale you prefer may not be available in other
13746: operating systems. Hopefully, Unicode will solve these problems one day.
13747:
13748: @item conditions under which control characters match a space delimiter:
13749: @cindex space delimiters
13750: @cindex control characters as delimiters
13751: If @code{word} is called with the space character as a delimiter, all
13752: white-space characters (as identified by the C macro @code{isspace()})
13753: are delimiters. @code{Parse}, on the other hand, treats space like other
13754: delimiters. @code{Parse-name}, which is used by the outer
13755: interpreter (aka text interpreter) by default, treats all white-space
13756: characters as delimiters.
13757:
13758: @item format of the control-flow stack:
13759: @cindex control-flow stack, format
13760: The data stack is used as control-flow stack. The size of a control-flow
13761: stack item in cells is given by the constant @code{cs-item-size}. At the
13762: time of this writing, an item consists of a (pointer to a) locals list
13763: (third), an address in the code (second), and a tag for identifying the
13764: item (TOS). The following tags are used: @code{defstart},
13765: @code{live-orig}, @code{dead-orig}, @code{dest}, @code{do-dest},
13766: @code{scopestart}.
13767:
13768: @item conversion of digits > 35
13769: @cindex digits > 35
13770: The characters @code{[\]^_'} are the digits with the decimal value
13771: 36@minus{}41. There is no way to input many of the larger digits.
13772:
13773: @item display after input terminates in @code{ACCEPT} and @code{EXPECT}:
13774: @cindex @code{EXPECT}, display after end of input
13775: @cindex @code{ACCEPT}, display after end of input
13776: The cursor is moved to the end of the entered string. If the input is
13777: terminated using the @kbd{Return} key, a space is typed.
13778:
13779: @item exception abort sequence of @code{ABORT"}:
13780: @cindex exception abort sequence of @code{ABORT"}
13781: @cindex @code{ABORT"}, exception abort sequence
13782: The error string is stored into the variable @code{"error} and a
13783: @code{-2 throw} is performed.
13784:
13785: @item input line terminator:
13786: @cindex input line terminator
13787: @cindex line terminator on input
13788: @cindex newline character on input
13789: For interactive input, @kbd{C-m} (CR) and @kbd{C-j} (LF) terminate
13790: lines. One of these characters is typically produced when you type the
13791: @kbd{Enter} or @kbd{Return} key.
13792:
13793: @item maximum size of a counted string:
13794: @cindex maximum size of a counted string
13795: @cindex counted string, maximum size
13796: @code{s" /counted-string" environment? drop .}. Currently 255 characters
13797: on all platforms, but this may change.
13798:
13799: @item maximum size of a parsed string:
13800: @cindex maximum size of a parsed string
13801: @cindex parsed string, maximum size
13802: Given by the constant @code{/line}. Currently 255 characters.
13803:
13804: @item maximum size of a definition name, in characters:
13805: @cindex maximum size of a definition name, in characters
13806: @cindex name, maximum length
13807: MAXU/8
13808:
13809: @item maximum string length for @code{ENVIRONMENT?}, in characters:
13810: @cindex maximum string length for @code{ENVIRONMENT?}, in characters
13811: @cindex @code{ENVIRONMENT?} string length, maximum
13812: MAXU/8
13813:
13814: @item method of selecting the user input device:
13815: @cindex user input device, method of selecting
13816: The user input device is the standard input. There is currently no way to
13817: change it from within Gforth. However, the input can typically be
13818: redirected in the command line that starts Gforth.
13819:
13820: @item method of selecting the user output device:
13821: @cindex user output device, method of selecting
13822: @code{EMIT} and @code{TYPE} output to the file-id stored in the value
13823: @code{outfile-id} (@code{stdout} by default). Gforth uses unbuffered
13824: output when the user output device is a terminal, otherwise the output
13825: is buffered.
13826:
13827: @item methods of dictionary compilation:
13828: What are we expected to document here?
13829:
13830: @item number of bits in one address unit:
13831: @cindex number of bits in one address unit
13832: @cindex address unit, size in bits
13833: @code{s" address-units-bits" environment? drop .}. 8 in all current
13834: platforms.
13835:
13836: @item number representation and arithmetic:
13837: @cindex number representation and arithmetic
13838: Processor-dependent. Binary two's complement on all current platforms.
13839:
13840: @item ranges for integer types:
13841: @cindex ranges for integer types
13842: @cindex integer types, ranges
13843: Installation-dependent. Make environmental queries for @code{MAX-N},
13844: @code{MAX-U}, @code{MAX-D} and @code{MAX-UD}. The lower bounds for
13845: unsigned (and positive) types is 0. The lower bound for signed types on
13846: two's complement and one's complement machines machines can be computed
13847: by adding 1 to the upper bound.
13848:
13849: @item read-only data space regions:
13850: @cindex read-only data space regions
13851: @cindex data-space, read-only regions
13852: The whole Forth data space is writable.
13853:
13854: @item size of buffer at @code{WORD}:
13855: @cindex size of buffer at @code{WORD}
13856: @cindex @code{WORD} buffer size
13857: @code{PAD HERE - .}. 104 characters on 32-bit machines. The buffer is
13858: shared with the pictured numeric output string. If overwriting
13859: @code{PAD} is acceptable, it is as large as the remaining dictionary
13860: space, although only as much can be sensibly used as fits in a counted
13861: string.
13862:
13863: @item size of one cell in address units:
13864: @cindex cell size
13865: @code{1 cells .}.
13866:
13867: @item size of one character in address units:
13868: @cindex char size
13869: @code{1 chars .}. 1 on all current platforms.
13870:
13871: @item size of the keyboard terminal buffer:
13872: @cindex size of the keyboard terminal buffer
13873: @cindex terminal buffer, size
13874: Varies. You can determine the size at a specific time using @code{lp@@
13875: tib - .}. It is shared with the locals stack and TIBs of files that
13876: include the current file. You can change the amount of space for TIBs
13877: and locals stack at Gforth startup with the command line option
13878: @code{-l}.
13879:
13880: @item size of the pictured numeric output buffer:
13881: @cindex size of the pictured numeric output buffer
13882: @cindex pictured numeric output buffer, size
13883: @code{PAD HERE - .}. 104 characters on 32-bit machines. The buffer is
13884: shared with @code{WORD}.
13885:
13886: @item size of the scratch area returned by @code{PAD}:
13887: @cindex size of the scratch area returned by @code{PAD}
13888: @cindex @code{PAD} size
13889: The remainder of dictionary space. @code{unused pad here - - .}.
13890:
13891: @item system case-sensitivity characteristics:
13892: @cindex case-sensitivity characteristics
13893: Dictionary searches are case-insensitive (except in
13894: @code{TABLE}s). However, as explained above under @i{character-set
13895: extensions}, the matching for non-ASCII characters is determined by the
13896: locale you are using. In the default @code{C} locale all non-ASCII
13897: characters are matched case-sensitively.
13898:
13899: @item system prompt:
13900: @cindex system prompt
13901: @cindex prompt
13902: @code{ ok} in interpret state, @code{ compiled} in compile state.
13903:
13904: @item division rounding:
13905: @cindex division rounding
13906: The ordinary division words @code{/ mod /mod */ */mod} perform floored
13907: division (with the default installation of Gforth). You can check
13908: this with @code{s" floored" environment? drop .}. If you write
13909: programs that need a specific division rounding, best use
13910: @code{fm/mod} or @code{sm/rem} for portability.
13911:
13912: @item values of @code{STATE} when true:
13913: @cindex @code{STATE} values
13914: -1.
13915:
13916: @item values returned after arithmetic overflow:
13917: On two's complement machines, arithmetic is performed modulo
13918: 2**bits-per-cell for single arithmetic and 4**bits-per-cell for double
13919: arithmetic (with appropriate mapping for signed types). Division by
13920: zero typically results in a @code{-55 throw} (Floating-point
13921: unidentified fault) or @code{-10 throw} (divide by zero). Integer
13922: division overflow can result in these throws, or in @code{-11 throw};
13923: in @code{gforth-fast} division overflow and divide by zero may also
13924: result in returning bogus results without producing an exception.
13925:
13926: @item whether the current definition can be found after @t{DOES>}:
13927: @cindex @t{DOES>}, visibility of current definition
13928: No.
13929:
13930: @end table
13931:
13932: @c ---------------------------------------------------------------------
13933: @node core-ambcond, core-other, core-idef, The Core Words
13934: @subsection Ambiguous conditions
13935: @c ---------------------------------------------------------------------
13936: @cindex core words, ambiguous conditions
13937: @cindex ambiguous conditions, core words
13938:
13939: @table @i
13940:
13941: @item a name is neither a word nor a number:
13942: @cindex name not found
13943: @cindex undefined word
13944: @code{-13 throw} (Undefined word).
13945:
13946: @item a definition name exceeds the maximum length allowed:
13947: @cindex word name too long
13948: @code{-19 throw} (Word name too long)
13949:
13950: @item addressing a region not inside the various data spaces of the forth system:
13951: @cindex Invalid memory address
13952: The stacks, code space and header space are accessible. Machine code space is
13953: typically readable. Accessing other addresses gives results dependent on
13954: the operating system. On decent systems: @code{-9 throw} (Invalid memory
13955: address).
13956:
13957: @item argument type incompatible with parameter:
13958: @cindex argument type mismatch
13959: This is usually not caught. Some words perform checks, e.g., the control
13960: flow words, and issue a @code{ABORT"} or @code{-12 THROW} (Argument type
13961: mismatch).
13962:
13963: @item attempting to obtain the execution token of a word with undefined execution semantics:
13964: @cindex Interpreting a compile-only word, for @code{'} etc.
13965: @cindex execution token of words with undefined execution semantics
13966: @code{-14 throw} (Interpreting a compile-only word). In some cases, you
13967: get an execution token for @code{compile-only-error} (which performs a
13968: @code{-14 throw} when executed).
13969:
13970: @item dividing by zero:
13971: @cindex dividing by zero
13972: @cindex floating point unidentified fault, integer division
13973: On some platforms, this produces a @code{-10 throw} (Division by
13974: zero); on other systems, this typically results in a @code{-55 throw}
13975: (Floating-point unidentified fault).
13976:
13977: @item insufficient data stack or return stack space:
13978: @cindex insufficient data stack or return stack space
13979: @cindex stack overflow
13980: @cindex address alignment exception, stack overflow
13981: @cindex Invalid memory address, stack overflow
13982: Depending on the operating system, the installation, and the invocation
13983: of Gforth, this is either checked by the memory management hardware, or
13984: it is not checked. If it is checked, you typically get a @code{-3 throw}
13985: (Stack overflow), @code{-5 throw} (Return stack overflow), or @code{-9
13986: throw} (Invalid memory address) (depending on the platform and how you
13987: achieved the overflow) as soon as the overflow happens. If it is not
13988: checked, overflows typically result in mysterious illegal memory
13989: accesses, producing @code{-9 throw} (Invalid memory address) or
13990: @code{-23 throw} (Address alignment exception); they might also destroy
13991: the internal data structure of @code{ALLOCATE} and friends, resulting in
13992: various errors in these words.
13993:
13994: @item insufficient space for loop control parameters:
13995: @cindex insufficient space for loop control parameters
13996: Like other return stack overflows.
13997:
13998: @item insufficient space in the dictionary:
13999: @cindex insufficient space in the dictionary
14000: @cindex dictionary overflow
14001: If you try to allot (either directly with @code{allot}, or indirectly
14002: with @code{,}, @code{create} etc.) more memory than available in the
14003: dictionary, you get a @code{-8 throw} (Dictionary overflow). If you try
14004: to access memory beyond the end of the dictionary, the results are
14005: similar to stack overflows.
14006:
14007: @item interpreting a word with undefined interpretation semantics:
14008: @cindex interpreting a word with undefined interpretation semantics
14009: @cindex Interpreting a compile-only word
14010: For some words, we have defined interpretation semantics. For the
14011: others: @code{-14 throw} (Interpreting a compile-only word).
14012:
14013: @item modifying the contents of the input buffer or a string literal:
14014: @cindex modifying the contents of the input buffer or a string literal
14015: These are located in writable memory and can be modified.
14016:
14017: @item overflow of the pictured numeric output string:
14018: @cindex overflow of the pictured numeric output string
14019: @cindex pictured numeric output string, overflow
14020: @code{-17 throw} (Pictured numeric ouput string overflow).
14021:
14022: @item parsed string overflow:
14023: @cindex parsed string overflow
14024: @code{PARSE} cannot overflow. @code{WORD} does not check for overflow.
14025:
14026: @item producing a result out of range:
14027: @cindex result out of range
14028: On two's complement machines, arithmetic is performed modulo
14029: 2**bits-per-cell for single arithmetic and 4**bits-per-cell for double
14030: arithmetic (with appropriate mapping for signed types). Division by
14031: zero typically results in a @code{-10 throw} (divide by zero) or
14032: @code{-55 throw} (floating point unidentified fault). Overflow on
14033: division may result in these errors or in @code{-11 throw} (result out
14034: of range). @code{Gforth-fast} may silently produce bogus results on
14035: division overflow or division by zero. @code{Convert} and
14036: @code{>number} currently overflow silently.
14037:
14038: @item reading from an empty data or return stack:
14039: @cindex stack empty
14040: @cindex stack underflow
14041: @cindex return stack underflow
14042: The data stack is checked by the outer (aka text) interpreter after
14043: every word executed. If it has underflowed, a @code{-4 throw} (Stack
14044: underflow) is performed. Apart from that, stacks may be checked or not,
14045: depending on operating system, installation, and invocation. If they are
14046: caught by a check, they typically result in @code{-4 throw} (Stack
14047: underflow), @code{-6 throw} (Return stack underflow) or @code{-9 throw}
14048: (Invalid memory address), depending on the platform and which stack
14049: underflows and by how much. Note that even if the system uses checking
14050: (through the MMU), your program may have to underflow by a significant
14051: number of stack items to trigger the reaction (the reason for this is
14052: that the MMU, and therefore the checking, works with a page-size
14053: granularity). If there is no checking, the symptoms resulting from an
14054: underflow are similar to those from an overflow. Unbalanced return
14055: stack errors can result in a variety of symptoms, including @code{-9 throw}
14056: (Invalid memory address) and Illegal Instruction (typically @code{-260
14057: throw}).
14058:
14059: @item unexpected end of the input buffer, resulting in an attempt to use a zero-length string as a name:
14060: @cindex unexpected end of the input buffer
14061: @cindex zero-length string as a name
14062: @cindex Attempt to use zero-length string as a name
14063: @code{Create} and its descendants perform a @code{-16 throw} (Attempt to
14064: use zero-length string as a name). Words like @code{'} probably will not
14065: find what they search. Note that it is possible to create zero-length
14066: names with @code{nextname} (should it not?).
14067:
14068: @item @code{>IN} greater than input buffer:
14069: @cindex @code{>IN} greater than input buffer
14070: The next invocation of a parsing word returns a string with length 0.
14071:
14072: @item @code{RECURSE} appears after @code{DOES>}:
14073: @cindex @code{RECURSE} appears after @code{DOES>}
14074: Compiles a recursive call to the defining word, not to the defined word.
14075:
14076: @item argument input source different than current input source for @code{RESTORE-INPUT}:
14077: @cindex argument input source different than current input source for @code{RESTORE-INPUT}
14078: @cindex argument type mismatch, @code{RESTORE-INPUT}
14079: @cindex @code{RESTORE-INPUT}, Argument type mismatch
14080: @code{-12 THROW}. Note that, once an input file is closed (e.g., because
14081: the end of the file was reached), its source-id may be
14082: reused. Therefore, restoring an input source specification referencing a
14083: closed file may lead to unpredictable results instead of a @code{-12
14084: THROW}.
14085:
14086: In the future, Gforth may be able to restore input source specifications
14087: from other than the current input source.
14088:
14089: @item data space containing definitions gets de-allocated:
14090: @cindex data space containing definitions gets de-allocated
14091: Deallocation with @code{allot} is not checked. This typically results in
14092: memory access faults or execution of illegal instructions.
14093:
14094: @item data space read/write with incorrect alignment:
14095: @cindex data space read/write with incorrect alignment
14096: @cindex alignment faults
14097: @cindex address alignment exception
14098: Processor-dependent. Typically results in a @code{-23 throw} (Address
14099: alignment exception). Under Linux-Intel on a 486 or later processor with
14100: alignment turned on, incorrect alignment results in a @code{-9 throw}
14101: (Invalid memory address). There are reportedly some processors with
14102: alignment restrictions that do not report violations.
14103:
14104: @item data space pointer not properly aligned, @code{,}, @code{C,}:
14105: @cindex data space pointer not properly aligned, @code{,}, @code{C,}
14106: Like other alignment errors.
14107:
14108: @item less than u+2 stack items (@code{PICK} and @code{ROLL}):
14109: Like other stack underflows.
14110:
14111: @item loop control parameters not available:
14112: @cindex loop control parameters not available
14113: Not checked. The counted loop words simply assume that the top of return
14114: stack items are loop control parameters and behave accordingly.
14115:
14116: @item most recent definition does not have a name (@code{IMMEDIATE}):
14117: @cindex most recent definition does not have a name (@code{IMMEDIATE})
14118: @cindex last word was headerless
14119: @code{abort" last word was headerless"}.
14120:
14121: @item name not defined by @code{VALUE} used by @code{TO}:
14122: @cindex name not defined by @code{VALUE} used by @code{TO}
14123: @cindex @code{TO} on non-@code{VALUE}s
14124: @cindex Invalid name argument, @code{TO}
14125: @code{-32 throw} (Invalid name argument) (unless name is a local or was
14126: defined by @code{CONSTANT}; in the latter case it just changes the constant).
14127:
14128: @item name not found (@code{'}, @code{POSTPONE}, @code{[']}, @code{[COMPILE]}):
14129: @cindex name not found (@code{'}, @code{POSTPONE}, @code{[']}, @code{[COMPILE]})
14130: @cindex undefined word, @code{'}, @code{POSTPONE}, @code{[']}, @code{[COMPILE]}
14131: @code{-13 throw} (Undefined word)
14132:
14133: @item parameters are not of the same type (@code{DO}, @code{?DO}, @code{WITHIN}):
14134: @cindex parameters are not of the same type (@code{DO}, @code{?DO}, @code{WITHIN})
14135: Gforth behaves as if they were of the same type. I.e., you can predict
14136: the behaviour by interpreting all parameters as, e.g., signed.
14137:
14138: @item @code{POSTPONE} or @code{[COMPILE]} applied to @code{TO}:
14139: @cindex @code{POSTPONE} or @code{[COMPILE]} applied to @code{TO}
14140: Assume @code{: X POSTPONE TO ; IMMEDIATE}. @code{X} performs the
14141: compilation semantics of @code{TO}.
14142:
14143: @item String longer than a counted string returned by @code{WORD}:
14144: @cindex string longer than a counted string returned by @code{WORD}
14145: @cindex @code{WORD}, string overflow
14146: Not checked. The string will be ok, but the count will, of course,
14147: contain only the least significant bits of the length.
14148:
14149: @item u greater than or equal to the number of bits in a cell (@code{LSHIFT}, @code{RSHIFT}):
14150: @cindex @code{LSHIFT}, large shift counts
14151: @cindex @code{RSHIFT}, large shift counts
14152: Processor-dependent. Typical behaviours are returning 0 and using only
14153: the low bits of the shift count.
14154:
14155: @item word not defined via @code{CREATE}:
14156: @cindex @code{>BODY} of non-@code{CREATE}d words
14157: @code{>BODY} produces the PFA of the word no matter how it was defined.
14158:
14159: @cindex @code{DOES>} of non-@code{CREATE}d words
14160: @code{DOES>} changes the execution semantics of the last defined word no
14161: matter how it was defined. E.g., @code{CONSTANT DOES>} is equivalent to
14162: @code{CREATE , DOES>}.
14163:
14164: @item words improperly used outside @code{<#} and @code{#>}:
14165: Not checked. As usual, you can expect memory faults.
14166:
14167: @end table
14168:
14169:
14170: @c ---------------------------------------------------------------------
14171: @node core-other, , core-ambcond, The Core Words
14172: @subsection Other system documentation
14173: @c ---------------------------------------------------------------------
14174: @cindex other system documentation, core words
14175: @cindex core words, other system documentation
14176:
14177: @table @i
14178: @item nonstandard words using @code{PAD}:
14179: @cindex @code{PAD} use by nonstandard words
14180: None.
14181:
14182: @item operator's terminal facilities available:
14183: @cindex operator's terminal facilities available
14184: After processing the OS's command line, Gforth goes into interactive mode,
14185: and you can give commands to Gforth interactively. The actual facilities
14186: available depend on how you invoke Gforth.
14187:
14188: @item program data space available:
14189: @cindex program data space available
14190: @cindex data space available
14191: @code{UNUSED .} gives the remaining dictionary space. The total
14192: dictionary space can be specified with the @code{-m} switch
14193: (@pxref{Invoking Gforth}) when Gforth starts up.
14194:
14195: @item return stack space available:
14196: @cindex return stack space available
14197: You can compute the total return stack space in cells with
14198: @code{s" RETURN-STACK-CELLS" environment? drop .}. You can specify it at
14199: startup time with the @code{-r} switch (@pxref{Invoking Gforth}).
14200:
14201: @item stack space available:
14202: @cindex stack space available
14203: You can compute the total data stack space in cells with
14204: @code{s" STACK-CELLS" environment? drop .}. You can specify it at
14205: startup time with the @code{-d} switch (@pxref{Invoking Gforth}).
14206:
14207: @item system dictionary space required, in address units:
14208: @cindex system dictionary space required, in address units
14209: Type @code{here forthstart - .} after startup. At the time of this
14210: writing, this gives 80080 (bytes) on a 32-bit system.
14211: @end table
14212:
14213:
14214: @c =====================================================================
14215: @node The optional Block word set, The optional Double Number word set, The Core Words, ANS conformance
14216: @section The optional Block word set
14217: @c =====================================================================
14218: @cindex system documentation, block words
14219: @cindex block words, system documentation
14220:
14221: @menu
14222: * block-idef:: Implementation Defined Options
14223: * block-ambcond:: Ambiguous Conditions
14224: * block-other:: Other System Documentation
14225: @end menu
14226:
14227:
14228: @c ---------------------------------------------------------------------
14229: @node block-idef, block-ambcond, The optional Block word set, The optional Block word set
14230: @subsection Implementation Defined Options
14231: @c ---------------------------------------------------------------------
14232: @cindex implementation-defined options, block words
14233: @cindex block words, implementation-defined options
14234:
14235: @table @i
14236: @item the format for display by @code{LIST}:
14237: @cindex @code{LIST} display format
14238: First the screen number is displayed, then 16 lines of 64 characters,
14239: each line preceded by the line number.
14240:
14241: @item the length of a line affected by @code{\}:
14242: @cindex length of a line affected by @code{\}
14243: @cindex @code{\}, line length in blocks
14244: 64 characters.
14245: @end table
14246:
14247:
14248: @c ---------------------------------------------------------------------
14249: @node block-ambcond, block-other, block-idef, The optional Block word set
14250: @subsection Ambiguous conditions
14251: @c ---------------------------------------------------------------------
14252: @cindex block words, ambiguous conditions
14253: @cindex ambiguous conditions, block words
14254:
14255: @table @i
14256: @item correct block read was not possible:
14257: @cindex block read not possible
14258: Typically results in a @code{throw} of some OS-derived value (between
14259: -512 and -2048). If the blocks file was just not long enough, blanks are
14260: supplied for the missing portion.
14261:
14262: @item I/O exception in block transfer:
14263: @cindex I/O exception in block transfer
14264: @cindex block transfer, I/O exception
14265: Typically results in a @code{throw} of some OS-derived value (between
14266: -512 and -2048).
14267:
14268: @item invalid block number:
14269: @cindex invalid block number
14270: @cindex block number invalid
14271: @code{-35 throw} (Invalid block number)
14272:
14273: @item a program directly alters the contents of @code{BLK}:
14274: @cindex @code{BLK}, altering @code{BLK}
14275: The input stream is switched to that other block, at the same
14276: position. If the storing to @code{BLK} happens when interpreting
14277: non-block input, the system will get quite confused when the block ends.
14278:
14279: @item no current block buffer for @code{UPDATE}:
14280: @cindex @code{UPDATE}, no current block buffer
14281: @code{UPDATE} has no effect.
14282:
14283: @end table
14284:
14285: @c ---------------------------------------------------------------------
14286: @node block-other, , block-ambcond, The optional Block word set
14287: @subsection Other system documentation
14288: @c ---------------------------------------------------------------------
14289: @cindex other system documentation, block words
14290: @cindex block words, other system documentation
14291:
14292: @table @i
14293: @item any restrictions a multiprogramming system places on the use of buffer addresses:
14294: No restrictions (yet).
14295:
14296: @item the number of blocks available for source and data:
14297: depends on your disk space.
14298:
14299: @end table
14300:
14301:
14302: @c =====================================================================
14303: @node The optional Double Number word set, The optional Exception word set, The optional Block word set, ANS conformance
14304: @section The optional Double Number word set
14305: @c =====================================================================
14306: @cindex system documentation, double words
14307: @cindex double words, system documentation
14308:
14309: @menu
14310: * double-ambcond:: Ambiguous Conditions
14311: @end menu
14312:
14313:
14314: @c ---------------------------------------------------------------------
14315: @node double-ambcond, , The optional Double Number word set, The optional Double Number word set
14316: @subsection Ambiguous conditions
14317: @c ---------------------------------------------------------------------
14318: @cindex double words, ambiguous conditions
14319: @cindex ambiguous conditions, double words
14320:
14321: @table @i
14322: @item @i{d} outside of range of @i{n} in @code{D>S}:
14323: @cindex @code{D>S}, @i{d} out of range of @i{n}
14324: The least significant cell of @i{d} is produced.
14325:
14326: @end table
14327:
14328:
14329: @c =====================================================================
14330: @node The optional Exception word set, The optional Facility word set, The optional Double Number word set, ANS conformance
14331: @section The optional Exception word set
14332: @c =====================================================================
14333: @cindex system documentation, exception words
14334: @cindex exception words, system documentation
14335:
14336: @menu
14337: * exception-idef:: Implementation Defined Options
14338: @end menu
14339:
14340:
14341: @c ---------------------------------------------------------------------
14342: @node exception-idef, , The optional Exception word set, The optional Exception word set
14343: @subsection Implementation Defined Options
14344: @c ---------------------------------------------------------------------
14345: @cindex implementation-defined options, exception words
14346: @cindex exception words, implementation-defined options
14347:
14348: @table @i
14349: @item @code{THROW}-codes used in the system:
14350: @cindex @code{THROW}-codes used in the system
14351: The codes -256@minus{}-511 are used for reporting signals. The mapping
14352: from OS signal numbers to throw codes is -256@minus{}@i{signal}. The
14353: codes -512@minus{}-2047 are used for OS errors (for file and memory
14354: allocation operations). The mapping from OS error numbers to throw codes
14355: is -512@minus{}@code{errno}. One side effect of this mapping is that
14356: undefined OS errors produce a message with a strange number; e.g.,
14357: @code{-1000 THROW} results in @code{Unknown error 488} on my system.
14358: @end table
14359:
14360: @c =====================================================================
14361: @node The optional Facility word set, The optional File-Access word set, The optional Exception word set, ANS conformance
14362: @section The optional Facility word set
14363: @c =====================================================================
14364: @cindex system documentation, facility words
14365: @cindex facility words, system documentation
14366:
14367: @menu
14368: * facility-idef:: Implementation Defined Options
14369: * facility-ambcond:: Ambiguous Conditions
14370: @end menu
14371:
14372:
14373: @c ---------------------------------------------------------------------
14374: @node facility-idef, facility-ambcond, The optional Facility word set, The optional Facility word set
14375: @subsection Implementation Defined Options
14376: @c ---------------------------------------------------------------------
14377: @cindex implementation-defined options, facility words
14378: @cindex facility words, implementation-defined options
14379:
14380: @table @i
14381: @item encoding of keyboard events (@code{EKEY}):
14382: @cindex keyboard events, encoding in @code{EKEY}
14383: @cindex @code{EKEY}, encoding of keyboard events
14384: Keys corresponding to ASCII characters are encoded as ASCII characters.
14385: Other keys are encoded with the constants @code{k-left}, @code{k-right},
14386: @code{k-up}, @code{k-down}, @code{k-home}, @code{k-end}, @code{k1},
14387: @code{k2}, @code{k3}, @code{k4}, @code{k5}, @code{k6}, @code{k7},
14388: @code{k8}, @code{k9}, @code{k10}, @code{k11}, @code{k12}.
14389:
14390:
14391: @item duration of a system clock tick:
14392: @cindex duration of a system clock tick
14393: @cindex clock tick duration
14394: System dependent. With respect to @code{MS}, the time is specified in
14395: microseconds. How well the OS and the hardware implement this, is
14396: another question.
14397:
14398: @item repeatability to be expected from the execution of @code{MS}:
14399: @cindex repeatability to be expected from the execution of @code{MS}
14400: @cindex @code{MS}, repeatability to be expected
14401: System dependent. On Unix, a lot depends on load. If the system is
14402: lightly loaded, and the delay is short enough that Gforth does not get
14403: swapped out, the performance should be acceptable. Under MS-DOS and
14404: other single-tasking systems, it should be good.
14405:
14406: @end table
14407:
14408:
14409: @c ---------------------------------------------------------------------
14410: @node facility-ambcond, , facility-idef, The optional Facility word set
14411: @subsection Ambiguous conditions
14412: @c ---------------------------------------------------------------------
14413: @cindex facility words, ambiguous conditions
14414: @cindex ambiguous conditions, facility words
14415:
14416: @table @i
14417: @item @code{AT-XY} can't be performed on user output device:
14418: @cindex @code{AT-XY} can't be performed on user output device
14419: Largely terminal dependent. No range checks are done on the arguments.
14420: No errors are reported. You may see some garbage appearing, you may see
14421: simply nothing happen.
14422:
14423: @end table
14424:
14425:
14426: @c =====================================================================
14427: @node The optional File-Access word set, The optional Floating-Point word set, The optional Facility word set, ANS conformance
14428: @section The optional File-Access word set
14429: @c =====================================================================
14430: @cindex system documentation, file words
14431: @cindex file words, system documentation
14432:
14433: @menu
14434: * file-idef:: Implementation Defined Options
14435: * file-ambcond:: Ambiguous Conditions
14436: @end menu
14437:
14438: @c ---------------------------------------------------------------------
14439: @node file-idef, file-ambcond, The optional File-Access word set, The optional File-Access word set
14440: @subsection Implementation Defined Options
14441: @c ---------------------------------------------------------------------
14442: @cindex implementation-defined options, file words
14443: @cindex file words, implementation-defined options
14444:
14445: @table @i
14446: @item file access methods used:
14447: @cindex file access methods used
14448: @code{R/O}, @code{R/W} and @code{BIN} work as you would
14449: expect. @code{W/O} translates into the C file opening mode @code{w} (or
14450: @code{wb}): The file is cleared, if it exists, and created, if it does
14451: not (with both @code{open-file} and @code{create-file}). Under Unix
14452: @code{create-file} creates a file with 666 permissions modified by your
14453: umask.
14454:
14455: @item file exceptions:
14456: @cindex file exceptions
14457: The file words do not raise exceptions (except, perhaps, memory access
14458: faults when you pass illegal addresses or file-ids).
14459:
14460: @item file line terminator:
14461: @cindex file line terminator
14462: System-dependent. Gforth uses C's newline character as line
14463: terminator. What the actual character code(s) of this are is
14464: system-dependent.
14465:
14466: @item file name format:
14467: @cindex file name format
14468: System dependent. Gforth just uses the file name format of your OS.
14469:
14470: @item information returned by @code{FILE-STATUS}:
14471: @cindex @code{FILE-STATUS}, returned information
14472: @code{FILE-STATUS} returns the most powerful file access mode allowed
14473: for the file: Either @code{R/O}, @code{W/O} or @code{R/W}. If the file
14474: cannot be accessed, @code{R/O BIN} is returned. @code{BIN} is applicable
14475: along with the returned mode.
14476:
14477: @item input file state after an exception when including source:
14478: @cindex exception when including source
14479: All files that are left via the exception are closed.
14480:
14481: @item @i{ior} values and meaning:
14482: @cindex @i{ior} values and meaning
14483: @cindex @i{wior} values and meaning
14484: The @i{ior}s returned by the file and memory allocation words are
14485: intended as throw codes. They typically are in the range
14486: -512@minus{}-2047 of OS errors. The mapping from OS error numbers to
14487: @i{ior}s is -512@minus{}@i{errno}.
14488:
14489: @item maximum depth of file input nesting:
14490: @cindex maximum depth of file input nesting
14491: @cindex file input nesting, maximum depth
14492: limited by the amount of return stack, locals/TIB stack, and the number
14493: of open files available. This should not give you troubles.
14494:
14495: @item maximum size of input line:
14496: @cindex maximum size of input line
14497: @cindex input line size, maximum
14498: @code{/line}. Currently 255.
14499:
14500: @item methods of mapping block ranges to files:
14501: @cindex mapping block ranges to files
14502: @cindex files containing blocks
14503: @cindex blocks in files
14504: By default, blocks are accessed in the file @file{blocks.fb} in the
14505: current working directory. The file can be switched with @code{USE}.
14506:
14507: @item number of string buffers provided by @code{S"}:
14508: @cindex @code{S"}, number of string buffers
14509: 1
14510:
14511: @item size of string buffer used by @code{S"}:
14512: @cindex @code{S"}, size of string buffer
14513: @code{/line}. currently 255.
14514:
14515: @end table
14516:
14517: @c ---------------------------------------------------------------------
14518: @node file-ambcond, , file-idef, The optional File-Access word set
14519: @subsection Ambiguous conditions
14520: @c ---------------------------------------------------------------------
14521: @cindex file words, ambiguous conditions
14522: @cindex ambiguous conditions, file words
14523:
14524: @table @i
14525: @item attempting to position a file outside its boundaries:
14526: @cindex @code{REPOSITION-FILE}, outside the file's boundaries
14527: @code{REPOSITION-FILE} is performed as usual: Afterwards,
14528: @code{FILE-POSITION} returns the value given to @code{REPOSITION-FILE}.
14529:
14530: @item attempting to read from file positions not yet written:
14531: @cindex reading from file positions not yet written
14532: End-of-file, i.e., zero characters are read and no error is reported.
14533:
14534: @item @i{file-id} is invalid (@code{INCLUDE-FILE}):
14535: @cindex @code{INCLUDE-FILE}, @i{file-id} is invalid
14536: An appropriate exception may be thrown, but a memory fault or other
14537: problem is more probable.
14538:
14539: @item I/O exception reading or closing @i{file-id} (@code{INCLUDE-FILE}, @code{INCLUDED}):
14540: @cindex @code{INCLUDE-FILE}, I/O exception reading or closing @i{file-id}
14541: @cindex @code{INCLUDED}, I/O exception reading or closing @i{file-id}
14542: The @i{ior} produced by the operation, that discovered the problem, is
14543: thrown.
14544:
14545: @item named file cannot be opened (@code{INCLUDED}):
14546: @cindex @code{INCLUDED}, named file cannot be opened
14547: The @i{ior} produced by @code{open-file} is thrown.
14548:
14549: @item requesting an unmapped block number:
14550: @cindex unmapped block numbers
14551: There are no unmapped legal block numbers. On some operating systems,
14552: writing a block with a large number may overflow the file system and
14553: have an error message as consequence.
14554:
14555: @item using @code{source-id} when @code{blk} is non-zero:
14556: @cindex @code{SOURCE-ID}, behaviour when @code{BLK} is non-zero
14557: @code{source-id} performs its function. Typically it will give the id of
14558: the source which loaded the block. (Better ideas?)
14559:
14560: @end table
14561:
14562:
14563: @c =====================================================================
14564: @node The optional Floating-Point word set, The optional Locals word set, The optional File-Access word set, ANS conformance
14565: @section The optional Floating-Point word set
14566: @c =====================================================================
14567: @cindex system documentation, floating-point words
14568: @cindex floating-point words, system documentation
14569:
14570: @menu
14571: * floating-idef:: Implementation Defined Options
14572: * floating-ambcond:: Ambiguous Conditions
14573: @end menu
14574:
14575:
14576: @c ---------------------------------------------------------------------
14577: @node floating-idef, floating-ambcond, The optional Floating-Point word set, The optional Floating-Point word set
14578: @subsection Implementation Defined Options
14579: @c ---------------------------------------------------------------------
14580: @cindex implementation-defined options, floating-point words
14581: @cindex floating-point words, implementation-defined options
14582:
14583: @table @i
14584: @item format and range of floating point numbers:
14585: @cindex format and range of floating point numbers
14586: @cindex floating point numbers, format and range
14587: System-dependent; the @code{double} type of C.
14588:
14589: @item results of @code{REPRESENT} when @i{float} is out of range:
14590: @cindex @code{REPRESENT}, results when @i{float} is out of range
14591: System dependent; @code{REPRESENT} is implemented using the C library
14592: function @code{ecvt()} and inherits its behaviour in this respect.
14593:
14594: @item rounding or truncation of floating-point numbers:
14595: @cindex rounding of floating-point numbers
14596: @cindex truncation of floating-point numbers
14597: @cindex floating-point numbers, rounding or truncation
14598: System dependent; the rounding behaviour is inherited from the hosting C
14599: compiler. IEEE-FP-based (i.e., most) systems by default round to
14600: nearest, and break ties by rounding to even (i.e., such that the last
14601: bit of the mantissa is 0).
14602:
14603: @item size of floating-point stack:
14604: @cindex floating-point stack size
14605: @code{s" FLOATING-STACK" environment? drop .} gives the total size of
14606: the floating-point stack (in floats). You can specify this on startup
14607: with the command-line option @code{-f} (@pxref{Invoking Gforth}).
14608:
14609: @item width of floating-point stack:
14610: @cindex floating-point stack width
14611: @code{1 floats}.
14612:
14613: @end table
14614:
14615:
14616: @c ---------------------------------------------------------------------
14617: @node floating-ambcond, , floating-idef, The optional Floating-Point word set
14618: @subsection Ambiguous conditions
14619: @c ---------------------------------------------------------------------
14620: @cindex floating-point words, ambiguous conditions
14621: @cindex ambiguous conditions, floating-point words
14622:
14623: @table @i
14624: @item @code{df@@} or @code{df!} used with an address that is not double-float aligned:
14625: @cindex @code{df@@} or @code{df!} used with an address that is not double-float aligned
14626: System-dependent. Typically results in a @code{-23 THROW} like other
14627: alignment violations.
14628:
14629: @item @code{f@@} or @code{f!} used with an address that is not float aligned:
14630: @cindex @code{f@@} used with an address that is not float aligned
14631: @cindex @code{f!} used with an address that is not float aligned
14632: System-dependent. Typically results in a @code{-23 THROW} like other
14633: alignment violations.
14634:
14635: @item floating-point result out of range:
14636: @cindex floating-point result out of range
14637: System-dependent. Can result in a @code{-43 throw} (floating point
14638: overflow), @code{-54 throw} (floating point underflow), @code{-41 throw}
14639: (floating point inexact result), @code{-55 THROW} (Floating-point
14640: unidentified fault), or can produce a special value representing, e.g.,
14641: Infinity.
14642:
14643: @item @code{sf@@} or @code{sf!} used with an address that is not single-float aligned:
14644: @cindex @code{sf@@} or @code{sf!} used with an address that is not single-float aligned
14645: System-dependent. Typically results in an alignment fault like other
14646: alignment violations.
14647:
14648: @item @code{base} is not decimal (@code{REPRESENT}, @code{F.}, @code{FE.}, @code{FS.}):
14649: @cindex @code{base} is not decimal (@code{REPRESENT}, @code{F.}, @code{FE.}, @code{FS.})
14650: The floating-point number is converted into decimal nonetheless.
14651:
14652: @item Both arguments are equal to zero (@code{FATAN2}):
14653: @cindex @code{FATAN2}, both arguments are equal to zero
14654: System-dependent. @code{FATAN2} is implemented using the C library
14655: function @code{atan2()}.
14656:
14657: @item Using @code{FTAN} on an argument @i{r1} where cos(@i{r1}) is zero:
14658: @cindex @code{FTAN} on an argument @i{r1} where cos(@i{r1}) is zero
14659: System-dependent. Anyway, typically the cos of @i{r1} will not be zero
14660: because of small errors and the tan will be a very large (or very small)
14661: but finite number.
14662:
14663: @item @i{d} cannot be presented precisely as a float in @code{D>F}:
14664: @cindex @code{D>F}, @i{d} cannot be presented precisely as a float
14665: The result is rounded to the nearest float.
14666:
14667: @item dividing by zero:
14668: @cindex dividing by zero, floating-point
14669: @cindex floating-point dividing by zero
14670: @cindex floating-point unidentified fault, FP divide-by-zero
14671: Platform-dependent; can produce an Infinity, NaN, @code{-42 throw}
14672: (floating point divide by zero) or @code{-55 throw} (Floating-point
14673: unidentified fault).
14674:
14675: @item exponent too big for conversion (@code{DF!}, @code{DF@@}, @code{SF!}, @code{SF@@}):
14676: @cindex exponent too big for conversion (@code{DF!}, @code{DF@@}, @code{SF!}, @code{SF@@})
14677: System dependent. On IEEE-FP based systems the number is converted into
14678: an infinity.
14679:
14680: @item @i{float}<1 (@code{FACOSH}):
14681: @cindex @code{FACOSH}, @i{float}<1
14682: @cindex floating-point unidentified fault, @code{FACOSH}
14683: Platform-dependent; on IEEE-FP systems typically produces a NaN.
14684:
14685: @item @i{float}=<-1 (@code{FLNP1}):
14686: @cindex @code{FLNP1}, @i{float}=<-1
14687: @cindex floating-point unidentified fault, @code{FLNP1}
14688: Platform-dependent; on IEEE-FP systems typically produces a NaN (or a
14689: negative infinity for @i{float}=-1).
14690:
14691: @item @i{float}=<0 (@code{FLN}, @code{FLOG}):
14692: @cindex @code{FLN}, @i{float}=<0
14693: @cindex @code{FLOG}, @i{float}=<0
14694: @cindex floating-point unidentified fault, @code{FLN} or @code{FLOG}
14695: Platform-dependent; on IEEE-FP systems typically produces a NaN (or a
14696: negative infinity for @i{float}=0).
14697:
14698: @item @i{float}<0 (@code{FASINH}, @code{FSQRT}):
14699: @cindex @code{FASINH}, @i{float}<0
14700: @cindex @code{FSQRT}, @i{float}<0
14701: @cindex floating-point unidentified fault, @code{FASINH} or @code{FSQRT}
14702: Platform-dependent; for @code{fsqrt} this typically gives a NaN, for
14703: @code{fasinh} some platforms produce a NaN, others a number (bug in the
14704: C library?).
14705:
14706: @item |@i{float}|>1 (@code{FACOS}, @code{FASIN}, @code{FATANH}):
14707: @cindex @code{FACOS}, |@i{float}|>1
14708: @cindex @code{FASIN}, |@i{float}|>1
14709: @cindex @code{FATANH}, |@i{float}|>1
14710: @cindex floating-point unidentified fault, @code{FACOS}, @code{FASIN} or @code{FATANH}
14711: Platform-dependent; IEEE-FP systems typically produce a NaN.
14712:
14713: @item integer part of float cannot be represented by @i{d} in @code{F>D}:
14714: @cindex @code{F>D}, integer part of float cannot be represented by @i{d}
14715: @cindex floating-point unidentified fault, @code{F>D}
14716: Platform-dependent; typically, some double number is produced and no
14717: error is reported.
14718:
14719: @item string larger than pictured numeric output area (@code{f.}, @code{fe.}, @code{fs.}):
14720: @cindex string larger than pictured numeric output area (@code{f.}, @code{fe.}, @code{fs.})
14721: @code{Precision} characters of the numeric output area are used. If
14722: @code{precision} is too high, these words will smash the data or code
14723: close to @code{here}.
14724: @end table
14725:
14726: @c =====================================================================
14727: @node The optional Locals word set, The optional Memory-Allocation word set, The optional Floating-Point word set, ANS conformance
14728: @section The optional Locals word set
14729: @c =====================================================================
14730: @cindex system documentation, locals words
14731: @cindex locals words, system documentation
14732:
14733: @menu
14734: * locals-idef:: Implementation Defined Options
14735: * locals-ambcond:: Ambiguous Conditions
14736: @end menu
14737:
14738:
14739: @c ---------------------------------------------------------------------
14740: @node locals-idef, locals-ambcond, The optional Locals word set, The optional Locals word set
14741: @subsection Implementation Defined Options
14742: @c ---------------------------------------------------------------------
14743: @cindex implementation-defined options, locals words
14744: @cindex locals words, implementation-defined options
14745:
14746: @table @i
14747: @item maximum number of locals in a definition:
14748: @cindex maximum number of locals in a definition
14749: @cindex locals, maximum number in a definition
14750: @code{s" #locals" environment? drop .}. Currently 15. This is a lower
14751: bound, e.g., on a 32-bit machine there can be 41 locals of up to 8
14752: characters. The number of locals in a definition is bounded by the size
14753: of locals-buffer, which contains the names of the locals.
14754:
14755: @end table
14756:
14757:
14758: @c ---------------------------------------------------------------------
14759: @node locals-ambcond, , locals-idef, The optional Locals word set
14760: @subsection Ambiguous conditions
14761: @c ---------------------------------------------------------------------
14762: @cindex locals words, ambiguous conditions
14763: @cindex ambiguous conditions, locals words
14764:
14765: @table @i
14766: @item executing a named local in interpretation state:
14767: @cindex local in interpretation state
14768: @cindex Interpreting a compile-only word, for a local
14769: Locals have no interpretation semantics. If you try to perform the
14770: interpretation semantics, you will get a @code{-14 throw} somewhere
14771: (Interpreting a compile-only word). If you perform the compilation
14772: semantics, the locals access will be compiled (irrespective of state).
14773:
14774: @item @i{name} not defined by @code{VALUE} or @code{(LOCAL)} (@code{TO}):
14775: @cindex name not defined by @code{VALUE} or @code{(LOCAL)} used by @code{TO}
14776: @cindex @code{TO} on non-@code{VALUE}s and non-locals
14777: @cindex Invalid name argument, @code{TO}
14778: @code{-32 throw} (Invalid name argument)
14779:
14780: @end table
14781:
14782:
14783: @c =====================================================================
14784: @node The optional Memory-Allocation word set, The optional Programming-Tools word set, The optional Locals word set, ANS conformance
14785: @section The optional Memory-Allocation word set
14786: @c =====================================================================
14787: @cindex system documentation, memory-allocation words
14788: @cindex memory-allocation words, system documentation
14789:
14790: @menu
14791: * memory-idef:: Implementation Defined Options
14792: @end menu
14793:
14794:
14795: @c ---------------------------------------------------------------------
14796: @node memory-idef, , The optional Memory-Allocation word set, The optional Memory-Allocation word set
14797: @subsection Implementation Defined Options
14798: @c ---------------------------------------------------------------------
14799: @cindex implementation-defined options, memory-allocation words
14800: @cindex memory-allocation words, implementation-defined options
14801:
14802: @table @i
14803: @item values and meaning of @i{ior}:
14804: @cindex @i{ior} values and meaning
14805: The @i{ior}s returned by the file and memory allocation words are
14806: intended as throw codes. They typically are in the range
14807: -512@minus{}-2047 of OS errors. The mapping from OS error numbers to
14808: @i{ior}s is -512@minus{}@i{errno}.
14809:
14810: @end table
14811:
14812: @c =====================================================================
14813: @node The optional Programming-Tools word set, The optional Search-Order word set, The optional Memory-Allocation word set, ANS conformance
14814: @section The optional Programming-Tools word set
14815: @c =====================================================================
14816: @cindex system documentation, programming-tools words
14817: @cindex programming-tools words, system documentation
14818:
14819: @menu
14820: * programming-idef:: Implementation Defined Options
14821: * programming-ambcond:: Ambiguous Conditions
14822: @end menu
14823:
14824:
14825: @c ---------------------------------------------------------------------
14826: @node programming-idef, programming-ambcond, The optional Programming-Tools word set, The optional Programming-Tools word set
14827: @subsection Implementation Defined Options
14828: @c ---------------------------------------------------------------------
14829: @cindex implementation-defined options, programming-tools words
14830: @cindex programming-tools words, implementation-defined options
14831:
14832: @table @i
14833: @item ending sequence for input following @code{;CODE} and @code{CODE}:
14834: @cindex @code{;CODE} ending sequence
14835: @cindex @code{CODE} ending sequence
14836: @code{END-CODE}
14837:
14838: @item manner of processing input following @code{;CODE} and @code{CODE}:
14839: @cindex @code{;CODE}, processing input
14840: @cindex @code{CODE}, processing input
14841: The @code{ASSEMBLER} vocabulary is pushed on the search order stack, and
14842: the input is processed by the text interpreter, (starting) in interpret
14843: state.
14844:
14845: @item search order capability for @code{EDITOR} and @code{ASSEMBLER}:
14846: @cindex @code{ASSEMBLER}, search order capability
14847: The ANS Forth search order word set.
14848:
14849: @item source and format of display by @code{SEE}:
14850: @cindex @code{SEE}, source and format of output
14851: The source for @code{see} is the executable code used by the inner
14852: interpreter. The current @code{see} tries to output Forth source code
14853: (and on some platforms, assembly code for primitives) as well as
14854: possible.
14855:
14856: @end table
14857:
14858: @c ---------------------------------------------------------------------
14859: @node programming-ambcond, , programming-idef, The optional Programming-Tools word set
14860: @subsection Ambiguous conditions
14861: @c ---------------------------------------------------------------------
14862: @cindex programming-tools words, ambiguous conditions
14863: @cindex ambiguous conditions, programming-tools words
14864:
14865: @table @i
14866:
14867: @item deleting the compilation word list (@code{FORGET}):
14868: @cindex @code{FORGET}, deleting the compilation word list
14869: Not implemented (yet).
14870:
14871: @item fewer than @i{u}+1 items on the control-flow stack (@code{CS-PICK}, @code{CS-ROLL}):
14872: @cindex @code{CS-PICK}, fewer than @i{u}+1 items on the control flow-stack
14873: @cindex @code{CS-ROLL}, fewer than @i{u}+1 items on the control flow-stack
14874: @cindex control-flow stack underflow
14875: This typically results in an @code{abort"} with a descriptive error
14876: message (may change into a @code{-22 throw} (Control structure mismatch)
14877: in the future). You may also get a memory access error. If you are
14878: unlucky, this ambiguous condition is not caught.
14879:
14880: @item @i{name} can't be found (@code{FORGET}):
14881: @cindex @code{FORGET}, @i{name} can't be found
14882: Not implemented (yet).
14883:
14884: @item @i{name} not defined via @code{CREATE}:
14885: @cindex @code{;CODE}, @i{name} not defined via @code{CREATE}
14886: @code{;CODE} behaves like @code{DOES>} in this respect, i.e., it changes
14887: the execution semantics of the last defined word no matter how it was
14888: defined.
14889:
14890: @item @code{POSTPONE} applied to @code{[IF]}:
14891: @cindex @code{POSTPONE} applied to @code{[IF]}
14892: @cindex @code{[IF]} and @code{POSTPONE}
14893: After defining @code{: X POSTPONE [IF] ; IMMEDIATE}. @code{X} is
14894: equivalent to @code{[IF]}.
14895:
14896: @item reaching the end of the input source before matching @code{[ELSE]} or @code{[THEN]}:
14897: @cindex @code{[IF]}, end of the input source before matching @code{[ELSE]} or @code{[THEN]}
14898: Continue in the same state of conditional compilation in the next outer
14899: input source. Currently there is no warning to the user about this.
14900:
14901: @item removing a needed definition (@code{FORGET}):
14902: @cindex @code{FORGET}, removing a needed definition
14903: Not implemented (yet).
14904:
14905: @end table
14906:
14907:
14908: @c =====================================================================
14909: @node The optional Search-Order word set, , The optional Programming-Tools word set, ANS conformance
14910: @section The optional Search-Order word set
14911: @c =====================================================================
14912: @cindex system documentation, search-order words
14913: @cindex search-order words, system documentation
14914:
14915: @menu
14916: * search-idef:: Implementation Defined Options
14917: * search-ambcond:: Ambiguous Conditions
14918: @end menu
14919:
14920:
14921: @c ---------------------------------------------------------------------
14922: @node search-idef, search-ambcond, The optional Search-Order word set, The optional Search-Order word set
14923: @subsection Implementation Defined Options
14924: @c ---------------------------------------------------------------------
14925: @cindex implementation-defined options, search-order words
14926: @cindex search-order words, implementation-defined options
14927:
14928: @table @i
14929: @item maximum number of word lists in search order:
14930: @cindex maximum number of word lists in search order
14931: @cindex search order, maximum depth
14932: @code{s" wordlists" environment? drop .}. Currently 16.
14933:
14934: @item minimum search order:
14935: @cindex minimum search order
14936: @cindex search order, minimum
14937: @code{root root}.
14938:
14939: @end table
14940:
14941: @c ---------------------------------------------------------------------
14942: @node search-ambcond, , search-idef, The optional Search-Order word set
14943: @subsection Ambiguous conditions
14944: @c ---------------------------------------------------------------------
14945: @cindex search-order words, ambiguous conditions
14946: @cindex ambiguous conditions, search-order words
14947:
14948: @table @i
14949: @item changing the compilation word list (during compilation):
14950: @cindex changing the compilation word list (during compilation)
14951: @cindex compilation word list, change before definition ends
14952: The word is entered into the word list that was the compilation word list
14953: at the start of the definition. Any changes to the name field (e.g.,
14954: @code{immediate}) or the code field (e.g., when executing @code{DOES>})
14955: are applied to the latest defined word (as reported by @code{latest} or
14956: @code{latestxt}), if possible, irrespective of the compilation word list.
14957:
14958: @item search order empty (@code{previous}):
14959: @cindex @code{previous}, search order empty
14960: @cindex vocstack empty, @code{previous}
14961: @code{abort" Vocstack empty"}.
14962:
14963: @item too many word lists in search order (@code{also}):
14964: @cindex @code{also}, too many word lists in search order
14965: @cindex vocstack full, @code{also}
14966: @code{abort" Vocstack full"}.
14967:
14968: @end table
14969:
14970: @c ***************************************************************
14971: @node Standard vs Extensions, Model, ANS conformance, Top
14972: @chapter Should I use Gforth extensions?
14973: @cindex Gforth extensions
14974:
14975: As you read through the rest of this manual, you will see documentation
14976: for @i{Standard} words, and documentation for some appealing Gforth
14977: @i{extensions}. You might ask yourself the question: @i{``Should I
14978: restrict myself to the standard, or should I use the extensions?''}
14979:
14980: The answer depends on the goals you have for the program you are working
14981: on:
14982:
14983: @itemize @bullet
14984:
14985: @item Is it just for yourself or do you want to share it with others?
14986:
14987: @item
14988: If you want to share it, do the others all use Gforth?
14989:
14990: @item
14991: If it is just for yourself, do you want to restrict yourself to Gforth?
14992:
14993: @end itemize
14994:
14995: If restricting the program to Gforth is ok, then there is no reason not
14996: to use extensions. It is still a good idea to keep to the standard
14997: where it is easy, in case you want to reuse these parts in another
14998: program that you want to be portable.
14999:
15000: If you want to be able to port the program to other Forth systems, there
15001: are the following points to consider:
15002:
15003: @itemize @bullet
15004:
15005: @item
15006: Most Forth systems that are being maintained support the ANS Forth
15007: standard. So if your program complies with the standard, it will be
15008: portable among many systems.
15009:
15010: @item
15011: A number of the Gforth extensions can be implemented in ANS Forth using
15012: public-domain files provided in the @file{compat/} directory. These are
15013: mentioned in the text in passing. There is no reason not to use these
15014: extensions, your program will still be ANS Forth compliant; just include
15015: the appropriate compat files with your program.
15016:
15017: @item
15018: The tool @file{ans-report.fs} (@pxref{ANS Report}) makes it easy to
15019: analyse your program and determine what non-Standard words it relies
15020: upon. However, it does not check whether you use standard words in a
15021: non-standard way.
15022:
15023: @item
15024: Some techniques are not standardized by ANS Forth, and are hard or
15025: impossible to implement in a standard way, but can be implemented in
15026: most Forth systems easily, and usually in similar ways (e.g., accessing
15027: word headers). Forth has a rich historical precedent for programmers
15028: taking advantage of implementation-dependent features of their tools
15029: (for example, relying on a knowledge of the dictionary
15030: structure). Sometimes these techniques are necessary to extract every
15031: last bit of performance from the hardware, sometimes they are just a
15032: programming shorthand.
15033:
15034: @item
15035: Does using a Gforth extension save more work than the porting this part
15036: to other Forth systems (if any) will cost?
15037:
15038: @item
15039: Is the additional functionality worth the reduction in portability and
15040: the additional porting problems?
15041:
15042: @end itemize
15043:
15044: In order to perform these considerations, you need to know what's
15045: standard and what's not. This manual generally states if something is
15046: non-standard, but the authoritative source is the
15047: @uref{http://www.taygeta.com/forth/dpans.html,standard document}.
15048: Appendix A of the Standard (@var{Rationale}) provides a valuable insight
15049: into the thought processes of the technical committee.
15050:
15051: Note also that portability between Forth systems is not the only
15052: portability issue; there is also the issue of portability between
15053: different platforms (processor/OS combinations).
15054:
15055: @c ***************************************************************
15056: @node Model, Integrating Gforth, Standard vs Extensions, Top
15057: @chapter Model
15058:
15059: This chapter has yet to be written. It will contain information, on
15060: which internal structures you can rely.
15061:
15062: @c ***************************************************************
15063: @node Integrating Gforth, Emacs and Gforth, Model, Top
15064: @chapter Integrating Gforth into C programs
15065:
15066: This is not yet implemented.
15067:
15068: Several people like to use Forth as scripting language for applications
15069: that are otherwise written in C, C++, or some other language.
15070:
15071: The Forth system ATLAST provides facilities for embedding it into
15072: applications; unfortunately it has several disadvantages: most
15073: importantly, it is not based on ANS Forth, and it is apparently dead
15074: (i.e., not developed further and not supported). The facilities
15075: provided by Gforth in this area are inspired by ATLAST's facilities, so
15076: making the switch should not be hard.
15077:
15078: We also tried to design the interface such that it can easily be
15079: implemented by other Forth systems, so that we may one day arrive at a
15080: standardized interface. Such a standard interface would allow you to
15081: replace the Forth system without having to rewrite C code.
15082:
15083: You embed the Gforth interpreter by linking with the library
15084: @code{libgforth.a} (give the compiler the option @code{-lgforth}). All
15085: global symbols in this library that belong to the interface, have the
15086: prefix @code{forth_}. (Global symbols that are used internally have the
15087: prefix @code{gforth_}).
15088:
15089: You can include the declarations of Forth types and the functions and
15090: variables of the interface with @code{#include <forth.h>}.
15091:
15092: Types.
15093:
15094: Variables.
15095:
15096: Data and FP Stack pointer. Area sizes.
15097:
15098: functions.
15099:
15100: forth_init(imagefile)
15101: forth_evaluate(string) exceptions?
15102: forth_goto(address) (or forth_execute(xt)?)
15103: forth_continue() (a corountining mechanism)
15104:
15105: Adding primitives.
15106:
15107: No checking.
15108:
15109: Signals?
15110:
15111: Accessing the Stacks
15112:
15113: @c ******************************************************************
15114: @node Emacs and Gforth, Image Files, Integrating Gforth, Top
15115: @chapter Emacs and Gforth
15116: @cindex Emacs and Gforth
15117:
15118: @cindex @file{gforth.el}
15119: @cindex @file{forth.el}
15120: @cindex Rydqvist, Goran
15121: @cindex Kuehling, David
15122: @cindex comment editing commands
15123: @cindex @code{\}, editing with Emacs
15124: @cindex debug tracer editing commands
15125: @cindex @code{~~}, removal with Emacs
15126: @cindex Forth mode in Emacs
15127:
15128: Gforth comes with @file{gforth.el}, an improved version of
15129: @file{forth.el} by Goran Rydqvist (included in the TILE package). The
15130: improvements are:
15131:
15132: @itemize @bullet
15133: @item
15134: A better handling of indentation.
15135: @item
15136: A custom hilighting engine for Forth-code.
15137: @item
15138: Comment paragraph filling (@kbd{M-q})
15139: @item
15140: Commenting (@kbd{C-x \}) and uncommenting (@kbd{C-u C-x \}) of regions
15141: @item
15142: Removal of debugging tracers (@kbd{C-x ~}, @pxref{Debugging}).
15143: @item
15144: Support of the @code{info-lookup} feature for looking up the
15145: documentation of a word.
15146: @item
15147: Support for reading and writing blocks files.
15148: @end itemize
15149:
15150: To get a basic description of these features, enter Forth mode and
15151: type @kbd{C-h m}.
15152:
15153: @cindex source location of error or debugging output in Emacs
15154: @cindex error output, finding the source location in Emacs
15155: @cindex debugging output, finding the source location in Emacs
15156: In addition, Gforth supports Emacs quite well: The source code locations
15157: given in error messages, debugging output (from @code{~~}) and failed
15158: assertion messages are in the right format for Emacs' compilation mode
15159: (@pxref{Compilation, , Running Compilations under Emacs, emacs, Emacs
15160: Manual}) so the source location corresponding to an error or other
15161: message is only a few keystrokes away (@kbd{C-x `} for the next error,
15162: @kbd{C-c C-c} for the error under the cursor).
15163:
15164: @cindex viewing the documentation of a word in Emacs
15165: @cindex context-sensitive help
15166: Moreover, for words documented in this manual, you can look up the
15167: glossary entry quickly by using @kbd{C-h TAB}
15168: (@code{info-lookup-symbol}, @pxref{Documentation, ,Documentation
15169: Commands, emacs, Emacs Manual}). This feature requires Emacs 20.3 or
15170: later and does not work for words containing @code{:}.
15171:
15172: @menu
15173: * Installing gforth.el:: Making Emacs aware of Forth.
15174: * Emacs Tags:: Viewing the source of a word in Emacs.
15175: * Hilighting:: Making Forth code look prettier.
15176: * Auto-Indentation:: Customizing auto-indentation.
15177: * Blocks Files:: Reading and writing blocks files.
15178: @end menu
15179:
15180: @c ----------------------------------
15181: @node Installing gforth.el, Emacs Tags, Emacs and Gforth, Emacs and Gforth
15182: @section Installing gforth.el
15183: @cindex @file{.emacs}
15184: @cindex @file{gforth.el}, installation
15185: To make the features from @file{gforth.el} available in Emacs, add
15186: the following lines to your @file{.emacs} file:
15187:
15188: @example
15189: (autoload 'forth-mode "gforth.el")
15190: (setq auto-mode-alist (cons '("\\.fs\\'" . forth-mode)
15191: auto-mode-alist))
15192: (autoload 'forth-block-mode "gforth.el")
15193: (setq auto-mode-alist (cons '("\\.fb\\'" . forth-block-mode)
15194: auto-mode-alist))
15195: (add-hook 'forth-mode-hook (function (lambda ()
15196: ;; customize variables here:
15197: (setq forth-indent-level 4)
15198: (setq forth-minor-indent-level 2)
15199: (setq forth-hilight-level 3)
15200: ;;; ...
15201: )))
15202: @end example
15203:
15204: @c ----------------------------------
15205: @node Emacs Tags, Hilighting, Installing gforth.el, Emacs and Gforth
15206: @section Emacs Tags
15207: @cindex @file{TAGS} file
15208: @cindex @file{etags.fs}
15209: @cindex viewing the source of a word in Emacs
15210: @cindex @code{require}, placement in files
15211: @cindex @code{include}, placement in files
15212: If you @code{require} @file{etags.fs}, a new @file{TAGS} file will be
15213: produced (@pxref{Tags, , Tags Tables, emacs, Emacs Manual}) that
15214: contains the definitions of all words defined afterwards. You can then
15215: find the source for a word using @kbd{M-.}. Note that Emacs can use
15216: several tags files at the same time (e.g., one for the Gforth sources
15217: and one for your program, @pxref{Select Tags Table,,Selecting a Tags
15218: Table,emacs, Emacs Manual}). The TAGS file for the preloaded words is
15219: @file{$(datadir)/gforth/$(VERSION)/TAGS} (e.g.,
15220: @file{/usr/local/share/gforth/0.2.0/TAGS}). To get the best behaviour
15221: with @file{etags.fs}, you should avoid putting definitions both before
15222: and after @code{require} etc., otherwise you will see the same file
15223: visited several times by commands like @code{tags-search}.
15224:
15225: @c ----------------------------------
15226: @node Hilighting, Auto-Indentation, Emacs Tags, Emacs and Gforth
15227: @section Hilighting
15228: @cindex hilighting Forth code in Emacs
15229: @cindex highlighting Forth code in Emacs
15230: @file{gforth.el} comes with a custom source hilighting engine. When
15231: you open a file in @code{forth-mode}, it will be completely parsed,
15232: assigning faces to keywords, comments, strings etc. While you edit
15233: the file, modified regions get parsed and updated on-the-fly.
15234:
15235: Use the variable `forth-hilight-level' to change the level of
15236: decoration from 0 (no hilighting at all) to 3 (the default). Even if
15237: you set the hilighting level to 0, the parser will still work in the
15238: background, collecting information about whether regions of text are
15239: ``compiled'' or ``interpreted''. Those information are required for
15240: auto-indentation to work properly. Set `forth-disable-parser' to
15241: non-nil if your computer is too slow to handle parsing. This will
15242: have an impact on the smartness of the auto-indentation engine,
15243: though.
15244:
15245: Sometimes Forth sources define new features that should be hilighted,
15246: new control structures, defining-words etc. You can use the variable
15247: `forth-custom-words' to make @code{forth-mode} hilight additional
15248: words and constructs. See the docstring of `forth-words' for details
15249: (in Emacs, type @kbd{C-h v forth-words}).
15250:
15251: `forth-custom-words' is meant to be customized in your
15252: @file{.emacs} file. To customize hilighing in a file-specific manner,
15253: set `forth-local-words' in a local-variables section at the end of
15254: your source file (@pxref{Local Variables in Files,, Variables, emacs, Emacs Manual}).
15255:
15256: Example:
15257: @example
15258: 0 [IF]
15259: Local Variables:
15260: forth-local-words:
15261: ((("t:") definition-starter (font-lock-keyword-face . 1)
15262: "[ \t\n]" t name (font-lock-function-name-face . 3))
15263: ((";t") definition-ender (font-lock-keyword-face . 1)))
15264: End:
15265: [THEN]
15266: @end example
15267:
15268: @c ----------------------------------
15269: @node Auto-Indentation, Blocks Files, Hilighting, Emacs and Gforth
15270: @section Auto-Indentation
15271: @cindex auto-indentation of Forth code in Emacs
15272: @cindex indentation of Forth code in Emacs
15273: @code{forth-mode} automatically tries to indent lines in a smart way,
15274: whenever you type @key{TAB} or break a line with @kbd{C-m}.
15275:
15276: Simple customization can be achieved by setting
15277: `forth-indent-level' and `forth-minor-indent-level' in your
15278: @file{.emacs} file. For historical reasons @file{gforth.el} indents
15279: per default by multiples of 4 columns. To use the more traditional
15280: 3-column indentation, add the following lines to your @file{.emacs}:
15281:
15282: @example
15283: (add-hook 'forth-mode-hook (function (lambda ()
15284: ;; customize variables here:
15285: (setq forth-indent-level 3)
15286: (setq forth-minor-indent-level 1)
15287: )))
15288: @end example
15289:
15290: If you want indentation to recognize non-default words, customize it
15291: by setting `forth-custom-indent-words' in your @file{.emacs}. See the
15292: docstring of `forth-indent-words' for details (in Emacs, type @kbd{C-h
15293: v forth-indent-words}).
15294:
15295: To customize indentation in a file-specific manner, set
15296: `forth-local-indent-words' in a local-variables section at the end of
15297: your source file (@pxref{Local Variables in Files, Variables,,emacs,
15298: Emacs Manual}).
15299:
15300: Example:
15301: @example
15302: 0 [IF]
15303: Local Variables:
15304: forth-local-indent-words:
15305: ((("t:") (0 . 2) (0 . 2))
15306: ((";t") (-2 . 0) (0 . -2)))
15307: End:
15308: [THEN]
15309: @end example
15310:
15311: @c ----------------------------------
15312: @node Blocks Files, , Auto-Indentation, Emacs and Gforth
15313: @section Blocks Files
15314: @cindex blocks files, use with Emacs
15315: @code{forth-mode} Autodetects blocks files by checking whether the
15316: length of the first line exceeds 1023 characters. It then tries to
15317: convert the file into normal text format. When you save the file, it
15318: will be written to disk as normal stream-source file.
15319:
15320: If you want to write blocks files, use @code{forth-blocks-mode}. It
15321: inherits all the features from @code{forth-mode}, plus some additions:
15322:
15323: @itemize @bullet
15324: @item
15325: Files are written to disk in blocks file format.
15326: @item
15327: Screen numbers are displayed in the mode line (enumerated beginning
15328: with the value of `forth-block-base')
15329: @item
15330: Warnings are displayed when lines exceed 64 characters.
15331: @item
15332: The beginning of the currently edited block is marked with an
15333: overlay-arrow.
15334: @end itemize
15335:
15336: There are some restrictions you should be aware of. When you open a
15337: blocks file that contains tabulator or newline characters, these
15338: characters will be translated into spaces when the file is written
15339: back to disk. If tabs or newlines are encountered during blocks file
15340: reading, an error is output to the echo area. So have a look at the
15341: `*Messages*' buffer, when Emacs' bell rings during reading.
15342:
15343: Please consult the docstring of @code{forth-blocks-mode} for more
15344: information by typing @kbd{C-h v forth-blocks-mode}).
15345:
15346: @c ******************************************************************
15347: @node Image Files, Engine, Emacs and Gforth, Top
15348: @chapter Image Files
15349: @cindex image file
15350: @cindex @file{.fi} files
15351: @cindex precompiled Forth code
15352: @cindex dictionary in persistent form
15353: @cindex persistent form of dictionary
15354:
15355: An image file is a file containing an image of the Forth dictionary,
15356: i.e., compiled Forth code and data residing in the dictionary. By
15357: convention, we use the extension @code{.fi} for image files.
15358:
15359: @menu
15360: * Image Licensing Issues:: Distribution terms for images.
15361: * Image File Background:: Why have image files?
15362: * Non-Relocatable Image Files:: don't always work.
15363: * Data-Relocatable Image Files:: are better.
15364: * Fully Relocatable Image Files:: better yet.
15365: * Stack and Dictionary Sizes:: Setting the default sizes for an image.
15366: * Running Image Files:: @code{gforth -i @i{file}} or @i{file}.
15367: * Modifying the Startup Sequence:: and turnkey applications.
15368: @end menu
15369:
15370: @node Image Licensing Issues, Image File Background, Image Files, Image Files
15371: @section Image Licensing Issues
15372: @cindex license for images
15373: @cindex image license
15374:
15375: An image created with @code{gforthmi} (@pxref{gforthmi}) or
15376: @code{savesystem} (@pxref{Non-Relocatable Image Files}) includes the
15377: original image; i.e., according to copyright law it is a derived work of
15378: the original image.
15379:
15380: Since Gforth is distributed under the GNU GPL, the newly created image
15381: falls under the GNU GPL, too. In particular, this means that if you
15382: distribute the image, you have to make all of the sources for the image
15383: available, including those you wrote. For details see @ref{Copying, ,
15384: GNU General Public License (Section 3)}.
15385:
15386: If you create an image with @code{cross} (@pxref{cross.fs}), the image
15387: contains only code compiled from the sources you gave it; if none of
15388: these sources is under the GPL, the terms discussed above do not apply
15389: to the image. However, if your image needs an engine (a gforth binary)
15390: that is under the GPL, you should make sure that you distribute both in
15391: a way that is at most a @emph{mere aggregation}, if you don't want the
15392: terms of the GPL to apply to the image.
15393:
15394: @node Image File Background, Non-Relocatable Image Files, Image Licensing Issues, Image Files
15395: @section Image File Background
15396: @cindex image file background
15397:
15398: Gforth consists not only of primitives (in the engine), but also of
15399: definitions written in Forth. Since the Forth compiler itself belongs to
15400: those definitions, it is not possible to start the system with the
15401: engine and the Forth source alone. Therefore we provide the Forth
15402: code as an image file in nearly executable form. When Gforth starts up,
15403: a C routine loads the image file into memory, optionally relocates the
15404: addresses, then sets up the memory (stacks etc.) according to
15405: information in the image file, and (finally) starts executing Forth
15406: code.
15407:
15408: The default image file is @file{gforth.fi} (in the @code{GFORTHPATH}).
15409: You can use a different image by using the @code{-i},
15410: @code{--image-file} or @code{--appl-image} options (@pxref{Invoking
15411: Gforth}), e.g.:
15412:
15413: @example
15414: gforth-fast -i myimage.fi
15415: @end example
15416:
15417: There are different variants of image files, and they represent
15418: different compromises between the goals of making it easy to generate
15419: image files and making them portable.
15420:
15421: @cindex relocation at run-time
15422: Win32Forth 3.4 and Mitch Bradley's @code{cforth} use relocation at
15423: run-time. This avoids many of the complications discussed below (image
15424: files are data relocatable without further ado), but costs performance
15425: (one addition per memory access) and makes it difficult to pass
15426: addresses between Forth and library calls or other programs.
15427:
15428: @cindex relocation at load-time
15429: By contrast, the Gforth loader performs relocation at image load time. The
15430: loader also has to replace tokens that represent primitive calls with the
15431: appropriate code-field addresses (or code addresses in the case of
15432: direct threading).
15433:
15434: There are three kinds of image files, with different degrees of
15435: relocatability: non-relocatable, data-relocatable, and fully relocatable
15436: image files.
15437:
15438: @cindex image file loader
15439: @cindex relocating loader
15440: @cindex loader for image files
15441: These image file variants have several restrictions in common; they are
15442: caused by the design of the image file loader:
15443:
15444: @itemize @bullet
15445: @item
15446: There is only one segment; in particular, this means, that an image file
15447: cannot represent @code{ALLOCATE}d memory chunks (and pointers to
15448: them). The contents of the stacks are not represented, either.
15449:
15450: @item
15451: The only kinds of relocation supported are: adding the same offset to
15452: all cells that represent data addresses; and replacing special tokens
15453: with code addresses or with pieces of machine code.
15454:
15455: If any complex computations involving addresses are performed, the
15456: results cannot be represented in the image file. Several applications that
15457: use such computations come to mind:
15458:
15459: @itemize @minus
15460: @item
15461: Hashing addresses (or data structures which contain addresses) for table
15462: lookup. If you use Gforth's @code{table}s or @code{wordlist}s for this
15463: purpose, you will have no problem, because the hash tables are
15464: recomputed automatically when the system is started. If you use your own
15465: hash tables, you will have to do something similar.
15466:
15467: @item
15468: There's a cute implementation of doubly-linked lists that uses
15469: @code{XOR}ed addresses. You could represent such lists as singly-linked
15470: in the image file, and restore the doubly-linked representation on
15471: startup.@footnote{In my opinion, though, you should think thrice before
15472: using a doubly-linked list (whatever implementation).}
15473:
15474: @item
15475: The code addresses of run-time routines like @code{docol:} cannot be
15476: represented in the image file (because their tokens would be replaced by
15477: machine code in direct threaded implementations). As a workaround,
15478: compute these addresses at run-time with @code{>code-address} from the
15479: executions tokens of appropriate words (see the definitions of
15480: @code{docol:} and friends in @file{kernel/getdoers.fs}).
15481:
15482: @item
15483: On many architectures addresses are represented in machine code in some
15484: shifted or mangled form. You cannot put @code{CODE} words that contain
15485: absolute addresses in this form in a relocatable image file. Workarounds
15486: are representing the address in some relative form (e.g., relative to
15487: the CFA, which is present in some register), or loading the address from
15488: a place where it is stored in a non-mangled form.
15489: @end itemize
15490: @end itemize
15491:
15492: @node Non-Relocatable Image Files, Data-Relocatable Image Files, Image File Background, Image Files
15493: @section Non-Relocatable Image Files
15494: @cindex non-relocatable image files
15495: @cindex image file, non-relocatable
15496:
15497: These files are simple memory dumps of the dictionary. They are
15498: specific to the executable (i.e., @file{gforth} file) they were
15499: created with. What's worse, they are specific to the place on which
15500: the dictionary resided when the image was created. Now, there is no
15501: guarantee that the dictionary will reside at the same place the next
15502: time you start Gforth, so there's no guarantee that a non-relocatable
15503: image will work the next time (Gforth will complain instead of
15504: crashing, though). Indeed, on OSs with (enabled) address-space
15505: randomization non-relocatable images are unlikely to work.
15506:
15507: You can create a non-relocatable image file with @code{savesystem}, e.g.:
15508:
15509: @example
15510: gforth app.fs -e "savesystem app.fi bye"
15511: @end example
15512:
15513: doc-savesystem
15514:
15515:
15516: @node Data-Relocatable Image Files, Fully Relocatable Image Files, Non-Relocatable Image Files, Image Files
15517: @section Data-Relocatable Image Files
15518: @cindex data-relocatable image files
15519: @cindex image file, data-relocatable
15520:
15521: These files contain relocatable data addresses, but fixed code
15522: addresses (instead of tokens). They are specific to the executable
15523: (i.e., @file{gforth} file) they were created with. Also, they disable
15524: dynamic native code generation (typically a factor of 2 in speed).
15525: You get a data-relocatable image, if you pass the engine you want to
15526: use through the @code{GFORTHD} environment variable to @file{gforthmi}
15527: (@pxref{gforthmi}), e.g.
15528:
15529: @example
15530: GFORTHD="/usr/bin/gforth-fast --no-dynamic" gforthmi myimage.fi source.fs
15531: @end example
15532:
15533: Note that the @code{--no-dynamic} is required here for the image to
15534: work (otherwise it will contain references to dynamically generated
15535: code that is not saved in the image).
15536:
15537:
15538: @node Fully Relocatable Image Files, Stack and Dictionary Sizes, Data-Relocatable Image Files, Image Files
15539: @section Fully Relocatable Image Files
15540: @cindex fully relocatable image files
15541: @cindex image file, fully relocatable
15542:
15543: @cindex @file{kern*.fi}, relocatability
15544: @cindex @file{gforth.fi}, relocatability
15545: These image files have relocatable data addresses, and tokens for code
15546: addresses. They can be used with different binaries (e.g., with and
15547: without debugging) on the same machine, and even across machines with
15548: the same data formats (byte order, cell size, floating point format),
15549: and they work with dynamic native code generation. However, they are
15550: usually specific to the version of Gforth they were created with. The
15551: files @file{gforth.fi} and @file{kernl*.fi} are fully relocatable.
15552:
15553: There are two ways to create a fully relocatable image file:
15554:
15555: @menu
15556: * gforthmi:: The normal way
15557: * cross.fs:: The hard way
15558: @end menu
15559:
15560: @node gforthmi, cross.fs, Fully Relocatable Image Files, Fully Relocatable Image Files
15561: @subsection @file{gforthmi}
15562: @cindex @file{comp-i.fs}
15563: @cindex @file{gforthmi}
15564:
15565: You will usually use @file{gforthmi}. If you want to create an
15566: image @i{file} that contains everything you would load by invoking
15567: Gforth with @code{gforth @i{options}}, you simply say:
15568: @example
15569: gforthmi @i{file} @i{options}
15570: @end example
15571:
15572: E.g., if you want to create an image @file{asm.fi} that has the file
15573: @file{asm.fs} loaded in addition to the usual stuff, you could do it
15574: like this:
15575:
15576: @example
15577: gforthmi asm.fi asm.fs
15578: @end example
15579:
15580: @file{gforthmi} is implemented as a sh script and works like this: It
15581: produces two non-relocatable images for different addresses and then
15582: compares them. Its output reflects this: first you see the output (if
15583: any) of the two Gforth invocations that produce the non-relocatable image
15584: files, then you see the output of the comparing program: It displays the
15585: offset used for data addresses and the offset used for code addresses;
15586: moreover, for each cell that cannot be represented correctly in the
15587: image files, it displays a line like this:
15588:
15589: @example
15590: 78DC BFFFFA50 BFFFFA40
15591: @end example
15592:
15593: This means that at offset $78dc from @code{forthstart}, one input image
15594: contains $bffffa50, and the other contains $bffffa40. Since these cells
15595: cannot be represented correctly in the output image, you should examine
15596: these places in the dictionary and verify that these cells are dead
15597: (i.e., not read before they are written).
15598:
15599: @cindex --application, @code{gforthmi} option
15600: If you insert the option @code{--application} in front of the image file
15601: name, you will get an image that uses the @code{--appl-image} option
15602: instead of the @code{--image-file} option (@pxref{Invoking
15603: Gforth}). When you execute such an image on Unix (by typing the image
15604: name as command), the Gforth engine will pass all options to the image
15605: instead of trying to interpret them as engine options.
15606:
15607: If you type @file{gforthmi} with no arguments, it prints some usage
15608: instructions.
15609:
15610: @cindex @code{savesystem} during @file{gforthmi}
15611: @cindex @code{bye} during @file{gforthmi}
15612: @cindex doubly indirect threaded code
15613: @cindex environment variables
15614: @cindex @code{GFORTHD} -- environment variable
15615: @cindex @code{GFORTH} -- environment variable
15616: @cindex @code{gforth-ditc}
15617: There are a few wrinkles: After processing the passed @i{options}, the
15618: words @code{savesystem} and @code{bye} must be visible. A special
15619: doubly indirect threaded version of the @file{gforth} executable is
15620: used for creating the non-relocatable images; you can pass the exact
15621: filename of this executable through the environment variable
15622: @code{GFORTHD} (default: @file{gforth-ditc}); if you pass a version
15623: that is not doubly indirect threaded, you will not get a fully
15624: relocatable image, but a data-relocatable image
15625: (@pxref{Data-Relocatable Image Files}), because there is no code
15626: address offset). The normal @file{gforth} executable is used for
15627: creating the relocatable image; you can pass the exact filename of
15628: this executable through the environment variable @code{GFORTH}.
15629:
15630: @node cross.fs, , gforthmi, Fully Relocatable Image Files
15631: @subsection @file{cross.fs}
15632: @cindex @file{cross.fs}
15633: @cindex cross-compiler
15634: @cindex metacompiler
15635: @cindex target compiler
15636:
15637: You can also use @code{cross}, a batch compiler that accepts a Forth-like
15638: programming language (@pxref{Cross Compiler}).
15639:
15640: @code{cross} allows you to create image files for machines with
15641: different data sizes and data formats than the one used for generating
15642: the image file. You can also use it to create an application image that
15643: does not contain a Forth compiler. These features are bought with
15644: restrictions and inconveniences in programming. E.g., addresses have to
15645: be stored in memory with special words (@code{A!}, @code{A,}, etc.) in
15646: order to make the code relocatable.
15647:
15648:
15649: @node Stack and Dictionary Sizes, Running Image Files, Fully Relocatable Image Files, Image Files
15650: @section Stack and Dictionary Sizes
15651: @cindex image file, stack and dictionary sizes
15652: @cindex dictionary size default
15653: @cindex stack size default
15654:
15655: If you invoke Gforth with a command line flag for the size
15656: (@pxref{Invoking Gforth}), the size you specify is stored in the
15657: dictionary. If you save the dictionary with @code{savesystem} or create
15658: an image with @file{gforthmi}, this size will become the default
15659: for the resulting image file. E.g., the following will create a
15660: fully relocatable version of @file{gforth.fi} with a 1MB dictionary:
15661:
15662: @example
15663: gforthmi gforth.fi -m 1M
15664: @end example
15665:
15666: In other words, if you want to set the default size for the dictionary
15667: and the stacks of an image, just invoke @file{gforthmi} with the
15668: appropriate options when creating the image.
15669:
15670: @cindex stack size, cache-friendly
15671: Note: For cache-friendly behaviour (i.e., good performance), you should
15672: make the sizes of the stacks modulo, say, 2K, somewhat different. E.g.,
15673: the default stack sizes are: data: 16k (mod 2k=0); fp: 15.5k (mod
15674: 2k=1.5k); return: 15k(mod 2k=1k); locals: 14.5k (mod 2k=0.5k).
15675:
15676: @node Running Image Files, Modifying the Startup Sequence, Stack and Dictionary Sizes, Image Files
15677: @section Running Image Files
15678: @cindex running image files
15679: @cindex invoking image files
15680: @cindex image file invocation
15681:
15682: @cindex -i, invoke image file
15683: @cindex --image file, invoke image file
15684: You can invoke Gforth with an image file @i{image} instead of the
15685: default @file{gforth.fi} with the @code{-i} flag (@pxref{Invoking Gforth}):
15686: @example
15687: gforth -i @i{image}
15688: @end example
15689:
15690: @cindex executable image file
15691: @cindex image file, executable
15692: If your operating system supports starting scripts with a line of the
15693: form @code{#! ...}, you just have to type the image file name to start
15694: Gforth with this image file (note that the file extension @code{.fi} is
15695: just a convention). I.e., to run Gforth with the image file @i{image},
15696: you can just type @i{image} instead of @code{gforth -i @i{image}}.
15697: This works because every @code{.fi} file starts with a line of this
15698: format:
15699:
15700: @example
15701: #! /usr/local/bin/gforth-0.4.0 -i
15702: @end example
15703:
15704: The file and pathname for the Gforth engine specified on this line is
15705: the specific Gforth executable that it was built against; i.e. the value
15706: of the environment variable @code{GFORTH} at the time that
15707: @file{gforthmi} was executed.
15708:
15709: You can make use of the same shell capability to make a Forth source
15710: file into an executable. For example, if you place this text in a file:
15711:
15712: @example
15713: #! /usr/local/bin/gforth
15714:
15715: ." Hello, world" CR
15716: bye
15717: @end example
15718:
15719: @noindent
15720: and then make the file executable (chmod +x in Unix), you can run it
15721: directly from the command line. The sequence @code{#!} is used in two
15722: ways; firstly, it is recognised as a ``magic sequence'' by the operating
15723: system@footnote{The Unix kernel actually recognises two types of files:
15724: executable files and files of data, where the data is processed by an
15725: interpreter that is specified on the ``interpreter line'' -- the first
15726: line of the file, starting with the sequence #!. There may be a small
15727: limit (e.g., 32) on the number of characters that may be specified on
15728: the interpreter line.} secondly it is treated as a comment character by
15729: Gforth. Because of the second usage, a space is required between
15730: @code{#!} and the path to the executable (moreover, some Unixes
15731: require the sequence @code{#! /}).
15732:
15733: Most Unix systems (including Linux) support exactly one option after
15734: the binary name. If that is not enough, you can use the following
15735: trick:
15736:
15737: @example
15738: #! /bin/sh
15739: : ## ; 0 [if]
15740: exec gforth -m 10M -d 1M $0 "$@@"
15741: [then]
15742: ." Hello, world" cr
15743: bye \ caution: this prevents (further) processing of "$@@"
15744: @end example
15745:
15746: First this script is interpreted as shell script, which treats the
15747: first two lines as (mostly) comments, then performs the third line,
15748: which invokes gforth with this script (@code{$0}) as parameter and its
15749: parameters as additional parameters (@code{"$@@"}). Then this script
15750: is interpreted as Forth script, which first defines a colon definition
15751: @code{##}, then ignores everything up to @code{[then]} and finally
15752: processes the following Forth code. You can also use
15753:
15754: @example
15755: #0 [if]
15756: @end example
15757:
15758: in the second line, but this works only in Gforth-0.7.0 and later.
15759:
15760: The @file{gforthmi} approach is the fastest one, the shell-based one
15761: is slowest (needs to start an additional shell). An additional
15762: advantage of the shell approach is that it is unnecessary to know
15763: where the Gforth binary resides, as long as it is in the @code{$PATH}.
15764:
15765: doc-#!
15766:
15767:
15768: @node Modifying the Startup Sequence, , Running Image Files, Image Files
15769: @section Modifying the Startup Sequence
15770: @cindex startup sequence for image file
15771: @cindex image file initialization sequence
15772: @cindex initialization sequence of image file
15773:
15774: You can add your own initialization to the startup sequence of an image
15775: through the deferred word @code{'cold}. @code{'cold} is invoked just
15776: before the image-specific command line processing (i.e., loading files
15777: and evaluating (@code{-e}) strings) starts.
15778:
15779: A sequence for adding your initialization usually looks like this:
15780:
15781: @example
15782: :noname
15783: Defers 'cold \ do other initialization stuff (e.g., rehashing wordlists)
15784: ... \ your stuff
15785: ; IS 'cold
15786: @end example
15787:
15788: After @code{'cold}, Gforth processes the image options
15789: (@pxref{Invoking Gforth}), and then it performs @code{bootmessage},
15790: another deferred word. This normally prints Gforth's startup message
15791: and does nothing else.
15792:
15793: @cindex turnkey image files
15794: @cindex image file, turnkey applications
15795: So, if you want to make a turnkey image (i.e., an image for an
15796: application instead of an extended Forth system), you can do this in
15797: two ways:
15798:
15799: @itemize @bullet
15800:
15801: @item
15802: If you want to do your interpretation of the OS command-line
15803: arguments, hook into @code{'cold}. In that case you probably also
15804: want to build the image with @code{gforthmi --application}
15805: (@pxref{gforthmi}) to keep the engine from processing OS command line
15806: options. You can then do your own command-line processing with
15807: @code{next-arg}
15808:
15809: @item
15810: If you want to have the normal Gforth processing of OS command-line
15811: arguments, hook into @code{bootmessage}.
15812:
15813: @end itemize
15814:
15815: In either case, you probably do not want the word that you execute in
15816: these hooks to exit normally, but use @code{bye} or @code{throw}.
15817: Otherwise the Gforth startup process would continue and eventually
15818: present the Forth command line to the user.
15819:
15820: doc-'cold
15821: doc-bootmessage
15822:
15823: @c ******************************************************************
15824: @node Engine, Cross Compiler, Image Files, Top
15825: @chapter Engine
15826: @cindex engine
15827: @cindex virtual machine
15828:
15829: Reading this chapter is not necessary for programming with Gforth. It
15830: may be helpful for finding your way in the Gforth sources.
15831:
15832: The ideas in this section have also been published in the following
15833: papers: Bernd Paysan, @cite{ANS fig/GNU/??? Forth} (in German),
15834: Forth-Tagung '93; M. Anton Ertl,
15835: @cite{@uref{http://www.complang.tuwien.ac.at/papers/ertl93.ps.Z, A
15836: Portable Forth Engine}}, EuroForth '93; M. Anton Ertl,
15837: @cite{@uref{http://www.complang.tuwien.ac.at/papers/ertl02.ps.gz,
15838: Threaded code variations and optimizations (extended version)}},
15839: Forth-Tagung '02.
15840:
15841: @menu
15842: * Portability::
15843: * Threading::
15844: * Primitives::
15845: * Performance::
15846: @end menu
15847:
15848: @node Portability, Threading, Engine, Engine
15849: @section Portability
15850: @cindex engine portability
15851:
15852: An important goal of the Gforth Project is availability across a wide
15853: range of personal machines. fig-Forth, and, to a lesser extent, F83,
15854: achieved this goal by manually coding the engine in assembly language
15855: for several then-popular processors. This approach is very
15856: labor-intensive and the results are short-lived due to progress in
15857: computer architecture.
15858:
15859: @cindex C, using C for the engine
15860: Others have avoided this problem by coding in C, e.g., Mitch Bradley
15861: (cforth), Mikael Patel (TILE) and Dirk Zoller (pfe). This approach is
15862: particularly popular for UNIX-based Forths due to the large variety of
15863: architectures of UNIX machines. Unfortunately an implementation in C
15864: does not mix well with the goals of efficiency and with using
15865: traditional techniques: Indirect or direct threading cannot be expressed
15866: in C, and switch threading, the fastest technique available in C, is
15867: significantly slower. Another problem with C is that it is very
15868: cumbersome to express double integer arithmetic.
15869:
15870: @cindex GNU C for the engine
15871: @cindex long long
15872: Fortunately, there is a portable language that does not have these
15873: limitations: GNU C, the version of C processed by the GNU C compiler
15874: (@pxref{C Extensions, , Extensions to the C Language Family, gcc.info,
15875: GNU C Manual}). Its labels as values feature (@pxref{Labels as Values, ,
15876: Labels as Values, gcc.info, GNU C Manual}) makes direct and indirect
15877: threading possible, its @code{long long} type (@pxref{Long Long, ,
15878: Double-Word Integers, gcc.info, GNU C Manual}) corresponds to Forth's
15879: double numbers on many systems. GNU C is freely available on all
15880: important (and many unimportant) UNIX machines, VMS, 80386s running
15881: MS-DOS, the Amiga, and the Atari ST, so a Forth written in GNU C can run
15882: on all these machines.
15883:
15884: Writing in a portable language has the reputation of producing code that
15885: is slower than assembly. For our Forth engine we repeatedly looked at
15886: the code produced by the compiler and eliminated most compiler-induced
15887: inefficiencies by appropriate changes in the source code.
15888:
15889: @cindex explicit register declarations
15890: @cindex --enable-force-reg, configuration flag
15891: @cindex -DFORCE_REG
15892: However, register allocation cannot be portably influenced by the
15893: programmer, leading to some inefficiencies on register-starved
15894: machines. We use explicit register declarations (@pxref{Explicit Reg
15895: Vars, , Variables in Specified Registers, gcc.info, GNU C Manual}) to
15896: improve the speed on some machines. They are turned on by using the
15897: configuration flag @code{--enable-force-reg} (@code{gcc} switch
15898: @code{-DFORCE_REG}). Unfortunately, this feature not only depends on the
15899: machine, but also on the compiler version: On some machines some
15900: compiler versions produce incorrect code when certain explicit register
15901: declarations are used. So by default @code{-DFORCE_REG} is not used.
15902:
15903: @node Threading, Primitives, Portability, Engine
15904: @section Threading
15905: @cindex inner interpreter implementation
15906: @cindex threaded code implementation
15907:
15908: @cindex labels as values
15909: GNU C's labels as values extension (available since @code{gcc-2.0},
15910: @pxref{Labels as Values, , Labels as Values, gcc.info, GNU C Manual})
15911: makes it possible to take the address of @i{label} by writing
15912: @code{&&@i{label}}. This address can then be used in a statement like
15913: @code{goto *@i{address}}. I.e., @code{goto *&&x} is the same as
15914: @code{goto x}.
15915:
15916: @cindex @code{NEXT}, indirect threaded
15917: @cindex indirect threaded inner interpreter
15918: @cindex inner interpreter, indirect threaded
15919: With this feature an indirect threaded @code{NEXT} looks like:
15920: @example
15921: cfa = *ip++;
15922: ca = *cfa;
15923: goto *ca;
15924: @end example
15925: @cindex instruction pointer
15926: For those unfamiliar with the names: @code{ip} is the Forth instruction
15927: pointer; the @code{cfa} (code-field address) corresponds to ANS Forths
15928: execution token and points to the code field of the next word to be
15929: executed; The @code{ca} (code address) fetched from there points to some
15930: executable code, e.g., a primitive or the colon definition handler
15931: @code{docol}.
15932:
15933: @cindex @code{NEXT}, direct threaded
15934: @cindex direct threaded inner interpreter
15935: @cindex inner interpreter, direct threaded
15936: Direct threading is even simpler:
15937: @example
15938: ca = *ip++;
15939: goto *ca;
15940: @end example
15941:
15942: Of course we have packaged the whole thing neatly in macros called
15943: @code{NEXT} and @code{NEXT1} (the part of @code{NEXT} after fetching the cfa).
15944:
15945: @menu
15946: * Scheduling::
15947: * Direct or Indirect Threaded?::
15948: * Dynamic Superinstructions::
15949: * DOES>::
15950: @end menu
15951:
15952: @node Scheduling, Direct or Indirect Threaded?, Threading, Threading
15953: @subsection Scheduling
15954: @cindex inner interpreter optimization
15955:
15956: There is a little complication: Pipelined and superscalar processors,
15957: i.e., RISC and some modern CISC machines can process independent
15958: instructions while waiting for the results of an instruction. The
15959: compiler usually reorders (schedules) the instructions in a way that
15960: achieves good usage of these delay slots. However, on our first tries
15961: the compiler did not do well on scheduling primitives. E.g., for
15962: @code{+} implemented as
15963: @example
15964: n=sp[0]+sp[1];
15965: sp++;
15966: sp[0]=n;
15967: NEXT;
15968: @end example
15969: the @code{NEXT} comes strictly after the other code, i.e., there is
15970: nearly no scheduling. After a little thought the problem becomes clear:
15971: The compiler cannot know that @code{sp} and @code{ip} point to different
15972: addresses (and the version of @code{gcc} we used would not know it even
15973: if it was possible), so it could not move the load of the cfa above the
15974: store to the TOS. Indeed the pointers could be the same, if code on or
15975: very near the top of stack were executed. In the interest of speed we
15976: chose to forbid this probably unused ``feature'' and helped the compiler
15977: in scheduling: @code{NEXT} is divided into several parts:
15978: @code{NEXT_P0}, @code{NEXT_P1} and @code{NEXT_P2}). @code{+} now looks
15979: like:
15980: @example
15981: NEXT_P0;
15982: n=sp[0]+sp[1];
15983: sp++;
15984: NEXT_P1;
15985: sp[0]=n;
15986: NEXT_P2;
15987: @end example
15988:
15989: There are various schemes that distribute the different operations of
15990: NEXT between these parts in several ways; in general, different schemes
15991: perform best on different processors. We use a scheme for most
15992: architectures that performs well for most processors of this
15993: architecture; in the future we may switch to benchmarking and chosing
15994: the scheme on installation time.
15995:
15996:
15997: @node Direct or Indirect Threaded?, Dynamic Superinstructions, Scheduling, Threading
15998: @subsection Direct or Indirect Threaded?
15999: @cindex threading, direct or indirect?
16000:
16001: Threaded forth code consists of references to primitives (simple machine
16002: code routines like @code{+}) and to non-primitives (e.g., colon
16003: definitions, variables, constants); for a specific class of
16004: non-primitives (e.g., variables) there is one code routine (e.g.,
16005: @code{dovar}), but each variable needs a separate reference to its data.
16006:
16007: Traditionally Forth has been implemented as indirect threaded code,
16008: because this allows to use only one cell to reference a non-primitive
16009: (basically you point to the data, and find the code address there).
16010:
16011: @cindex primitive-centric threaded code
16012: However, threaded code in Gforth (since 0.6.0) uses two cells for
16013: non-primitives, one for the code address, and one for the data address;
16014: the data pointer is an immediate argument for the virtual machine
16015: instruction represented by the code address. We call this
16016: @emph{primitive-centric} threaded code, because all code addresses point
16017: to simple primitives. E.g., for a variable, the code address is for
16018: @code{lit} (also used for integer literals like @code{99}).
16019:
16020: Primitive-centric threaded code allows us to use (faster) direct
16021: threading as dispatch method, completely portably (direct threaded code
16022: in Gforth before 0.6.0 required architecture-specific code). It also
16023: eliminates the performance problems related to I-cache consistency that
16024: 386 implementations have with direct threaded code, and allows
16025: additional optimizations.
16026:
16027: @cindex hybrid direct/indirect threaded code
16028: There is a catch, however: the @var{xt} parameter of @code{execute} can
16029: occupy only one cell, so how do we pass non-primitives with their code
16030: @emph{and} data addresses to them? Our answer is to use indirect
16031: threaded dispatch for @code{execute} and other words that use a
16032: single-cell xt. So, normal threaded code in colon definitions uses
16033: direct threading, and @code{execute} and similar words, which dispatch
16034: to xts on the data stack, use indirect threaded code. We call this
16035: @emph{hybrid direct/indirect} threaded code.
16036:
16037: @cindex engines, gforth vs. gforth-fast vs. gforth-itc
16038: @cindex gforth engine
16039: @cindex gforth-fast engine
16040: The engines @command{gforth} and @command{gforth-fast} use hybrid
16041: direct/indirect threaded code. This means that with these engines you
16042: cannot use @code{,} to compile an xt. Instead, you have to use
16043: @code{compile,}.
16044:
16045: @cindex gforth-itc engine
16046: If you want to compile xts with @code{,}, use @command{gforth-itc}.
16047: This engine uses plain old indirect threaded code. It still compiles in
16048: a primitive-centric style, so you cannot use @code{compile,} instead of
16049: @code{,} (e.g., for producing tables of xts with @code{] word1 word2
16050: ... [}). If you want to do that, you have to use @command{gforth-itc}
16051: and execute @code{' , is compile,}. Your program can check if it is
16052: running on a hybrid direct/indirect threaded engine or a pure indirect
16053: threaded engine with @code{threading-method} (@pxref{Threading Words}).
16054:
16055:
16056: @node Dynamic Superinstructions, DOES>, Direct or Indirect Threaded?, Threading
16057: @subsection Dynamic Superinstructions
16058: @cindex Dynamic superinstructions with replication
16059: @cindex Superinstructions
16060: @cindex Replication
16061:
16062: The engines @command{gforth} and @command{gforth-fast} use another
16063: optimization: Dynamic superinstructions with replication. As an
16064: example, consider the following colon definition:
16065:
16066: @example
16067: : squared ( n1 -- n2 )
16068: dup * ;
16069: @end example
16070:
16071: Gforth compiles this into the threaded code sequence
16072:
16073: @example
16074: dup
16075: *
16076: ;s
16077: @end example
16078:
16079: In normal direct threaded code there is a code address occupying one
16080: cell for each of these primitives. Each code address points to a
16081: machine code routine, and the interpreter jumps to this machine code in
16082: order to execute the primitive. The routines for these three
16083: primitives are (in @command{gforth-fast} on the 386):
16084:
16085: @example
16086: Code dup
16087: ( $804B950 ) add esi , # -4 \ $83 $C6 $FC
16088: ( $804B953 ) add ebx , # 4 \ $83 $C3 $4
16089: ( $804B956 ) mov dword ptr 4 [esi] , ecx \ $89 $4E $4
16090: ( $804B959 ) jmp dword ptr FC [ebx] \ $FF $63 $FC
16091: end-code
16092: Code *
16093: ( $804ACC4 ) mov eax , dword ptr 4 [esi] \ $8B $46 $4
16094: ( $804ACC7 ) add esi , # 4 \ $83 $C6 $4
16095: ( $804ACCA ) add ebx , # 4 \ $83 $C3 $4
16096: ( $804ACCD ) imul ecx , eax \ $F $AF $C8
16097: ( $804ACD0 ) jmp dword ptr FC [ebx] \ $FF $63 $FC
16098: end-code
16099: Code ;s
16100: ( $804A693 ) mov eax , dword ptr [edi] \ $8B $7
16101: ( $804A695 ) add edi , # 4 \ $83 $C7 $4
16102: ( $804A698 ) lea ebx , dword ptr 4 [eax] \ $8D $58 $4
16103: ( $804A69B ) jmp dword ptr FC [ebx] \ $FF $63 $FC
16104: end-code
16105: @end example
16106:
16107: With dynamic superinstructions and replication the compiler does not
16108: just lay down the threaded code, but also copies the machine code
16109: fragments, usually without the jump at the end.
16110:
16111: @example
16112: ( $4057D27D ) add esi , # -4 \ $83 $C6 $FC
16113: ( $4057D280 ) add ebx , # 4 \ $83 $C3 $4
16114: ( $4057D283 ) mov dword ptr 4 [esi] , ecx \ $89 $4E $4
16115: ( $4057D286 ) mov eax , dword ptr 4 [esi] \ $8B $46 $4
16116: ( $4057D289 ) add esi , # 4 \ $83 $C6 $4
16117: ( $4057D28C ) add ebx , # 4 \ $83 $C3 $4
16118: ( $4057D28F ) imul ecx , eax \ $F $AF $C8
16119: ( $4057D292 ) mov eax , dword ptr [edi] \ $8B $7
16120: ( $4057D294 ) add edi , # 4 \ $83 $C7 $4
16121: ( $4057D297 ) lea ebx , dword ptr 4 [eax] \ $8D $58 $4
16122: ( $4057D29A ) jmp dword ptr FC [ebx] \ $FF $63 $FC
16123: @end example
16124:
16125: Only when a threaded-code control-flow change happens (e.g., in
16126: @code{;s}), the jump is appended. This optimization eliminates many of
16127: these jumps and makes the rest much more predictable. The speedup
16128: depends on the processor and the application; on the Athlon and Pentium
16129: III this optimization typically produces a speedup by a factor of 2.
16130:
16131: The code addresses in the direct-threaded code are set to point to the
16132: appropriate points in the copied machine code, in this example like
16133: this:
16134:
16135: @example
16136: primitive code address
16137: dup $4057D27D
16138: * $4057D286
16139: ;s $4057D292
16140: @end example
16141:
16142: Thus there can be threaded-code jumps to any place in this piece of
16143: code. This also simplifies decompilation quite a bit.
16144:
16145: @cindex --no-dynamic command-line option
16146: @cindex --no-super command-line option
16147: You can disable this optimization with @option{--no-dynamic}. You can
16148: use the copying without eliminating the jumps (i.e., dynamic
16149: replication, but without superinstructions) with @option{--no-super};
16150: this gives the branch prediction benefit alone; the effect on
16151: performance depends on the CPU; on the Athlon and Pentium III the
16152: speedup is a little less than for dynamic superinstructions with
16153: replication.
16154:
16155: @cindex patching threaded code
16156: One use of these options is if you want to patch the threaded code.
16157: With superinstructions, many of the dispatch jumps are eliminated, so
16158: patching often has no effect. These options preserve all the dispatch
16159: jumps.
16160:
16161: @cindex --dynamic command-line option
16162: On some machines dynamic superinstructions are disabled by default,
16163: because it is unsafe on these machines. However, if you feel
16164: adventurous, you can enable it with @option{--dynamic}.
16165:
16166: @node DOES>, , Dynamic Superinstructions, Threading
16167: @subsection DOES>
16168: @cindex @code{DOES>} implementation
16169:
16170: @cindex @code{dodoes} routine
16171: @cindex @code{DOES>}-code
16172: One of the most complex parts of a Forth engine is @code{dodoes}, i.e.,
16173: the chunk of code executed by every word defined by a
16174: @code{CREATE}...@code{DOES>} pair; actually with primitive-centric code,
16175: this is only needed if the xt of the word is @code{execute}d. The main
16176: problem here is: How to find the Forth code to be executed, i.e. the
16177: code after the @code{DOES>} (the @code{DOES>}-code)? There are two
16178: solutions:
16179:
16180: In fig-Forth the code field points directly to the @code{dodoes} and the
16181: @code{DOES>}-code address is stored in the cell after the code address
16182: (i.e. at @code{@i{CFA} cell+}). It may seem that this solution is
16183: illegal in the Forth-79 and all later standards, because in fig-Forth
16184: this address lies in the body (which is illegal in these
16185: standards). However, by making the code field larger for all words this
16186: solution becomes legal again. We use this approach. Leaving a cell
16187: unused in most words is a bit wasteful, but on the machines we are
16188: targeting this is hardly a problem.
16189:
16190:
16191: @node Primitives, Performance, Threading, Engine
16192: @section Primitives
16193: @cindex primitives, implementation
16194: @cindex virtual machine instructions, implementation
16195:
16196: @menu
16197: * Automatic Generation::
16198: * TOS Optimization::
16199: * Produced code::
16200: @end menu
16201:
16202: @node Automatic Generation, TOS Optimization, Primitives, Primitives
16203: @subsection Automatic Generation
16204: @cindex primitives, automatic generation
16205:
16206: @cindex @file{prims2x.fs}
16207:
16208: Since the primitives are implemented in a portable language, there is no
16209: longer any need to minimize the number of primitives. On the contrary,
16210: having many primitives has an advantage: speed. In order to reduce the
16211: number of errors in primitives and to make programming them easier, we
16212: provide a tool, the primitive generator (@file{prims2x.fs} aka Vmgen,
16213: @pxref{Top, Vmgen, Introduction, vmgen, Vmgen}), that automatically
16214: generates most (and sometimes all) of the C code for a primitive from
16215: the stack effect notation. The source for a primitive has the following
16216: form:
16217:
16218: @cindex primitive source format
16219: @format
16220: @i{Forth-name} ( @i{stack-effect} ) @i{category} [@i{pronounc.}]
16221: [@code{""}@i{glossary entry}@code{""}]
16222: @i{C code}
16223: [@code{:}
16224: @i{Forth code}]
16225: @end format
16226:
16227: The items in brackets are optional. The category and glossary fields
16228: are there for generating the documentation, the Forth code is there
16229: for manual implementations on machines without GNU C. E.g., the source
16230: for the primitive @code{+} is:
16231: @example
16232: + ( n1 n2 -- n ) core plus
16233: n = n1+n2;
16234: @end example
16235:
16236: This looks like a specification, but in fact @code{n = n1+n2} is C
16237: code. Our primitive generation tool extracts a lot of information from
16238: the stack effect notations@footnote{We use a one-stack notation, even
16239: though we have separate data and floating-point stacks; The separate
16240: notation can be generated easily from the unified notation.}: The number
16241: of items popped from and pushed on the stack, their type, and by what
16242: name they are referred to in the C code. It then generates a C code
16243: prelude and postlude for each primitive. The final C code for @code{+}
16244: looks like this:
16245:
16246: @example
16247: I_plus: /* + ( n1 n2 -- n ) */ /* label, stack effect */
16248: /* */ /* documentation */
16249: NAME("+") /* debugging output (with -DDEBUG) */
16250: @{
16251: DEF_CA /* definition of variable ca (indirect threading) */
16252: Cell n1; /* definitions of variables */
16253: Cell n2;
16254: Cell n;
16255: NEXT_P0; /* NEXT part 0 */
16256: n1 = (Cell) sp[1]; /* input */
16257: n2 = (Cell) TOS;
16258: sp += 1; /* stack adjustment */
16259: @{
16260: n = n1+n2; /* C code taken from the source */
16261: @}
16262: NEXT_P1; /* NEXT part 1 */
16263: TOS = (Cell)n; /* output */
16264: NEXT_P2; /* NEXT part 2 */
16265: @}
16266: @end example
16267:
16268: This looks long and inefficient, but the GNU C compiler optimizes quite
16269: well and produces optimal code for @code{+} on, e.g., the R3000 and the
16270: HP RISC machines: Defining the @code{n}s does not produce any code, and
16271: using them as intermediate storage also adds no cost.
16272:
16273: There are also other optimizations that are not illustrated by this
16274: example: assignments between simple variables are usually for free (copy
16275: propagation). If one of the stack items is not used by the primitive
16276: (e.g. in @code{drop}), the compiler eliminates the load from the stack
16277: (dead code elimination). On the other hand, there are some things that
16278: the compiler does not do, therefore they are performed by
16279: @file{prims2x.fs}: The compiler does not optimize code away that stores
16280: a stack item to the place where it just came from (e.g., @code{over}).
16281:
16282: While programming a primitive is usually easy, there are a few cases
16283: where the programmer has to take the actions of the generator into
16284: account, most notably @code{?dup}, but also words that do not (always)
16285: fall through to @code{NEXT}.
16286:
16287: For more information
16288:
16289: @node TOS Optimization, Produced code, Automatic Generation, Primitives
16290: @subsection TOS Optimization
16291: @cindex TOS optimization for primitives
16292: @cindex primitives, keeping the TOS in a register
16293:
16294: An important optimization for stack machine emulators, e.g., Forth
16295: engines, is keeping one or more of the top stack items in
16296: registers. If a word has the stack effect @i{in1}...@i{inx} @code{--}
16297: @i{out1}...@i{outy}, keeping the top @i{n} items in registers
16298: @itemize @bullet
16299: @item
16300: is better than keeping @i{n-1} items, if @i{x>=n} and @i{y>=n},
16301: due to fewer loads from and stores to the stack.
16302: @item is slower than keeping @i{n-1} items, if @i{x<>y} and @i{x<n} and
16303: @i{y<n}, due to additional moves between registers.
16304: @end itemize
16305:
16306: @cindex -DUSE_TOS
16307: @cindex -DUSE_NO_TOS
16308: In particular, keeping one item in a register is never a disadvantage,
16309: if there are enough registers. Keeping two items in registers is a
16310: disadvantage for frequent words like @code{?branch}, constants,
16311: variables, literals and @code{i}. Therefore our generator only produces
16312: code that keeps zero or one items in registers. The generated C code
16313: covers both cases; the selection between these alternatives is made at
16314: C-compile time using the switch @code{-DUSE_TOS}. @code{TOS} in the C
16315: code for @code{+} is just a simple variable name in the one-item case,
16316: otherwise it is a macro that expands into @code{sp[0]}. Note that the
16317: GNU C compiler tries to keep simple variables like @code{TOS} in
16318: registers, and it usually succeeds, if there are enough registers.
16319:
16320: @cindex -DUSE_FTOS
16321: @cindex -DUSE_NO_FTOS
16322: The primitive generator performs the TOS optimization for the
16323: floating-point stack, too (@code{-DUSE_FTOS}). For floating-point
16324: operations the benefit of this optimization is even larger:
16325: floating-point operations take quite long on most processors, but can be
16326: performed in parallel with other operations as long as their results are
16327: not used. If the FP-TOS is kept in a register, this works. If
16328: it is kept on the stack, i.e., in memory, the store into memory has to
16329: wait for the result of the floating-point operation, lengthening the
16330: execution time of the primitive considerably.
16331:
16332: The TOS optimization makes the automatic generation of primitives a
16333: bit more complicated. Just replacing all occurrences of @code{sp[0]} by
16334: @code{TOS} is not sufficient. There are some special cases to
16335: consider:
16336: @itemize @bullet
16337: @item In the case of @code{dup ( w -- w w )} the generator must not
16338: eliminate the store to the original location of the item on the stack,
16339: if the TOS optimization is turned on.
16340: @item Primitives with stack effects of the form @code{--}
16341: @i{out1}...@i{outy} must store the TOS to the stack at the start.
16342: Likewise, primitives with the stack effect @i{in1}...@i{inx} @code{--}
16343: must load the TOS from the stack at the end. But for the null stack
16344: effect @code{--} no stores or loads should be generated.
16345: @end itemize
16346:
16347: @node Produced code, , TOS Optimization, Primitives
16348: @subsection Produced code
16349: @cindex primitives, assembly code listing
16350:
16351: @cindex @file{engine.s}
16352: To see what assembly code is produced for the primitives on your machine
16353: with your compiler and your flag settings, type @code{make engine.s} and
16354: look at the resulting file @file{engine.s}. Alternatively, you can also
16355: disassemble the code of primitives with @code{see} on some architectures.
16356:
16357: @node Performance, , Primitives, Engine
16358: @section Performance
16359: @cindex performance of some Forth interpreters
16360: @cindex engine performance
16361: @cindex benchmarking Forth systems
16362: @cindex Gforth performance
16363:
16364: On RISCs the Gforth engine is very close to optimal; i.e., it is usually
16365: impossible to write a significantly faster threaded-code engine.
16366:
16367: On register-starved machines like the 386 architecture processors
16368: improvements are possible, because @code{gcc} does not utilize the
16369: registers as well as a human, even with explicit register declarations;
16370: e.g., Bernd Beuster wrote a Forth system fragment in assembly language
16371: and hand-tuned it for the 486; this system is 1.19 times faster on the
16372: Sieve benchmark on a 486DX2/66 than Gforth compiled with
16373: @code{gcc-2.6.3} with @code{-DFORCE_REG}. The situation has improved
16374: with gcc-2.95 and gforth-0.4.9; now the most important virtual machine
16375: registers fit in real registers (and we can even afford to use the TOS
16376: optimization), resulting in a speedup of 1.14 on the sieve over the
16377: earlier results. And dynamic superinstructions provide another speedup
16378: (but only around a factor 1.2 on the 486).
16379:
16380: @cindex Win32Forth performance
16381: @cindex NT Forth performance
16382: @cindex eforth performance
16383: @cindex ThisForth performance
16384: @cindex PFE performance
16385: @cindex TILE performance
16386: The potential advantage of assembly language implementations is not
16387: necessarily realized in complete Forth systems: We compared Gforth-0.5.9
16388: (direct threaded, compiled with @code{gcc-2.95.1} and
16389: @code{-DFORCE_REG}) with Win32Forth 1.2093 (newer versions are
16390: reportedly much faster), LMI's NT Forth (Beta, May 1994) and Eforth
16391: (with and without peephole (aka pinhole) optimization of the threaded
16392: code); all these systems were written in assembly language. We also
16393: compared Gforth with three systems written in C: PFE-0.9.14 (compiled
16394: with @code{gcc-2.6.3} with the default configuration for Linux:
16395: @code{-O2 -fomit-frame-pointer -DUSE_REGS -DUNROLL_NEXT}), ThisForth
16396: Beta (compiled with @code{gcc-2.6.3 -O3 -fomit-frame-pointer}; ThisForth
16397: employs peephole optimization of the threaded code) and TILE (compiled
16398: with @code{make opt}). We benchmarked Gforth, PFE, ThisForth and TILE on
16399: a 486DX2/66 under Linux. Kenneth O'Heskin kindly provided the results
16400: for Win32Forth and NT Forth on a 486DX2/66 with similar memory
16401: performance under Windows NT. Marcel Hendrix ported Eforth to Linux,
16402: then extended it to run the benchmarks, added the peephole optimizer,
16403: ran the benchmarks and reported the results.
16404:
16405: We used four small benchmarks: the ubiquitous Sieve; bubble-sorting and
16406: matrix multiplication come from the Stanford integer benchmarks and have
16407: been translated into Forth by Martin Fraeman; we used the versions
16408: included in the TILE Forth package, but with bigger data set sizes; and
16409: a recursive Fibonacci number computation for benchmarking calling
16410: performance. The following table shows the time taken for the benchmarks
16411: scaled by the time taken by Gforth (in other words, it shows the speedup
16412: factor that Gforth achieved over the other systems).
16413:
16414: @example
16415: relative Win32- NT eforth This-
16416: time Gforth Forth Forth eforth +opt PFE Forth TILE
16417: sieve 1.00 2.16 1.78 2.16 1.32 2.46 4.96 13.37
16418: bubble 1.00 1.93 2.07 2.18 1.29 2.21 5.70
16419: matmul 1.00 1.92 1.76 1.90 0.96 2.06 5.32
16420: fib 1.00 2.32 2.03 1.86 1.31 2.64 4.55 6.54
16421: @end example
16422:
16423: You may be quite surprised by the good performance of Gforth when
16424: compared with systems written in assembly language. One important reason
16425: for the disappointing performance of these other systems is probably
16426: that they are not written optimally for the 486 (e.g., they use the
16427: @code{lods} instruction). In addition, Win32Forth uses a comfortable,
16428: but costly method for relocating the Forth image: like @code{cforth}, it
16429: computes the actual addresses at run time, resulting in two address
16430: computations per @code{NEXT} (@pxref{Image File Background}).
16431:
16432: The speedup of Gforth over PFE, ThisForth and TILE can be easily
16433: explained with the self-imposed restriction of the latter systems to
16434: standard C, which makes efficient threading impossible (however, the
16435: measured implementation of PFE uses a GNU C extension: @pxref{Global Reg
16436: Vars, , Defining Global Register Variables, gcc.info, GNU C Manual}).
16437: Moreover, current C compilers have a hard time optimizing other aspects
16438: of the ThisForth and the TILE source.
16439:
16440: The performance of Gforth on 386 architecture processors varies widely
16441: with the version of @code{gcc} used. E.g., @code{gcc-2.5.8} failed to
16442: allocate any of the virtual machine registers into real machine
16443: registers by itself and would not work correctly with explicit register
16444: declarations, giving a significantly slower engine (on a 486DX2/66
16445: running the Sieve) than the one measured above.
16446:
16447: Note that there have been several releases of Win32Forth since the
16448: release presented here, so the results presented above may have little
16449: predictive value for the performance of Win32Forth today (results for
16450: the current release on an i486DX2/66 are welcome).
16451:
16452: @cindex @file{Benchres}
16453: In
16454: @cite{@uref{http://www.complang.tuwien.ac.at/papers/ertl&maierhofer95.ps.gz,
16455: Translating Forth to Efficient C}} by M. Anton Ertl and Martin
16456: Maierhofer (presented at EuroForth '95), an indirect threaded version of
16457: Gforth is compared with Win32Forth, NT Forth, PFE, ThisForth, and
16458: several native code systems; that version of Gforth is slower on a 486
16459: than the version used here. You can find a newer version of these
16460: measurements at
16461: @uref{http://www.complang.tuwien.ac.at/forth/performance.html}. You can
16462: find numbers for Gforth on various machines in @file{Benchres}.
16463:
16464: @c ******************************************************************
16465: @c @node Binding to System Library, Cross Compiler, Engine, Top
16466: @c @chapter Binding to System Library
16467:
16468: @c ****************************************************************
16469: @node Cross Compiler, Bugs, Engine, Top
16470: @chapter Cross Compiler
16471: @cindex @file{cross.fs}
16472: @cindex cross-compiler
16473: @cindex metacompiler
16474: @cindex target compiler
16475:
16476: The cross compiler is used to bootstrap a Forth kernel. Since Gforth is
16477: mostly written in Forth, including crucial parts like the outer
16478: interpreter and compiler, it needs compiled Forth code to get
16479: started. The cross compiler allows to create new images for other
16480: architectures, even running under another Forth system.
16481:
16482: @menu
16483: * Using the Cross Compiler::
16484: * How the Cross Compiler Works::
16485: @end menu
16486:
16487: @node Using the Cross Compiler, How the Cross Compiler Works, Cross Compiler, Cross Compiler
16488: @section Using the Cross Compiler
16489:
16490: The cross compiler uses a language that resembles Forth, but isn't. The
16491: main difference is that you can execute Forth code after definition,
16492: while you usually can't execute the code compiled by cross, because the
16493: code you are compiling is typically for a different computer than the
16494: one you are compiling on.
16495:
16496: @c anton: This chapter is somewhat different from waht I would expect: I
16497: @c would expect an explanation of the cross language and how to create an
16498: @c application image with it. The section explains some aspects of
16499: @c creating a Gforth kernel.
16500:
16501: The Makefile is already set up to allow you to create kernels for new
16502: architectures with a simple make command. The generic kernels using the
16503: GCC compiled virtual machine are created in the normal build process
16504: with @code{make}. To create a embedded Gforth executable for e.g. the
16505: 8086 processor (running on a DOS machine), type
16506:
16507: @example
16508: make kernl-8086.fi
16509: @end example
16510:
16511: This will use the machine description from the @file{arch/8086}
16512: directory to create a new kernel. A machine file may look like that:
16513:
16514: @example
16515: \ Parameter for target systems 06oct92py
16516:
16517: 4 Constant cell \ cell size in bytes
16518: 2 Constant cell<< \ cell shift to bytes
16519: 5 Constant cell>bit \ cell shift to bits
16520: 8 Constant bits/char \ bits per character
16521: 8 Constant bits/byte \ bits per byte [default: 8]
16522: 8 Constant float \ bytes per float
16523: 8 Constant /maxalign \ maximum alignment in bytes
16524: false Constant bigendian \ byte order
16525: ( true=big, false=little )
16526:
16527: include machpc.fs \ feature list
16528: @end example
16529:
16530: This part is obligatory for the cross compiler itself, the feature list
16531: is used by the kernel to conditionally compile some features in and out,
16532: depending on whether the target supports these features.
16533:
16534: There are some optional features, if you define your own primitives,
16535: have an assembler, or need special, nonstandard preparation to make the
16536: boot process work. @code{asm-include} includes an assembler,
16537: @code{prims-include} includes primitives, and @code{>boot} prepares for
16538: booting.
16539:
16540: @example
16541: : asm-include ." Include assembler" cr
16542: s" arch/8086/asm.fs" included ;
16543:
16544: : prims-include ." Include primitives" cr
16545: s" arch/8086/prim.fs" included ;
16546:
16547: : >boot ." Prepare booting" cr
16548: s" ' boot >body into-forth 1+ !" evaluate ;
16549: @end example
16550:
16551: These words are used as sort of macro during the cross compilation in
16552: the file @file{kernel/main.fs}. Instead of using these macros, it would
16553: be possible --- but more complicated --- to write a new kernel project
16554: file, too.
16555:
16556: @file{kernel/main.fs} expects the machine description file name on the
16557: stack; the cross compiler itself (@file{cross.fs}) assumes that either
16558: @code{mach-file} leaves a counted string on the stack, or
16559: @code{machine-file} leaves an address, count pair of the filename on the
16560: stack.
16561:
16562: The feature list is typically controlled using @code{SetValue}, generic
16563: files that are used by several projects can use @code{DefaultValue}
16564: instead. Both functions work like @code{Value}, when the value isn't
16565: defined, but @code{SetValue} works like @code{to} if the value is
16566: defined, and @code{DefaultValue} doesn't set anything, if the value is
16567: defined.
16568:
16569: @example
16570: \ generic mach file for pc gforth 03sep97jaw
16571:
16572: true DefaultValue NIL \ relocating
16573:
16574: >ENVIRON
16575:
16576: true DefaultValue file \ controls the presence of the
16577: \ file access wordset
16578: true DefaultValue OS \ flag to indicate a operating system
16579:
16580: true DefaultValue prims \ true: primitives are c-code
16581:
16582: true DefaultValue floating \ floating point wordset is present
16583:
16584: true DefaultValue glocals \ gforth locals are present
16585: \ will be loaded
16586: true DefaultValue dcomps \ double number comparisons
16587:
16588: true DefaultValue hash \ hashing primitives are loaded/present
16589:
16590: true DefaultValue xconds \ used together with glocals,
16591: \ special conditionals supporting gforths'
16592: \ local variables
16593: true DefaultValue header \ save a header information
16594:
16595: true DefaultValue backtrace \ enables backtrace code
16596:
16597: false DefaultValue ec
16598: false DefaultValue crlf
16599:
16600: cell 2 = [IF] &32 [ELSE] &256 [THEN] KB DefaultValue kernel-size
16601:
16602: &16 KB DefaultValue stack-size
16603: &15 KB &512 + DefaultValue fstack-size
16604: &15 KB DefaultValue rstack-size
16605: &14 KB &512 + DefaultValue lstack-size
16606: @end example
16607:
16608: @node How the Cross Compiler Works, , Using the Cross Compiler, Cross Compiler
16609: @section How the Cross Compiler Works
16610:
16611: @node Bugs, Origin, Cross Compiler, Top
16612: @appendix Bugs
16613: @cindex bug reporting
16614:
16615: Known bugs are described in the file @file{BUGS} in the Gforth distribution.
16616:
16617: If you find a bug, please submit a bug report through
16618: @uref{https://savannah.gnu.org/bugs/?func=addbug&group=gforth}.
16619:
16620: @itemize @bullet
16621: @item
16622: A program (or a sequence of keyboard commands) that reproduces the bug.
16623: @item
16624: A description of what you think constitutes the buggy behaviour.
16625: @item
16626: The Gforth version used (it is announced at the start of an
16627: interactive Gforth session).
16628: @item
16629: The machine and operating system (on Unix
16630: systems @code{uname -a} will report this information).
16631: @item
16632: The installation options (you can find the configure options at the
16633: start of @file{config.status}) and configuration (@code{configure}
16634: output or @file{config.cache}).
16635: @item
16636: A complete list of changes (if any) you (or your installer) have made to the
16637: Gforth sources.
16638: @end itemize
16639:
16640: For a thorough guide on reporting bugs read @ref{Bug Reporting, , How
16641: to Report Bugs, gcc.info, GNU C Manual}.
16642:
16643:
16644: @node Origin, Forth-related information, Bugs, Top
16645: @appendix Authors and Ancestors of Gforth
16646:
16647: @section Authors and Contributors
16648: @cindex authors of Gforth
16649: @cindex contributors to Gforth
16650:
16651: The Gforth project was started in mid-1992 by Bernd Paysan and Anton
16652: Ertl. The third major author was Jens Wilke. Neal Crook contributed a
16653: lot to the manual. Assemblers and disassemblers were contributed by
16654: Andrew McKewan, Christian Pirker, Bernd Thallner, and Michal Revucky.
16655: Lennart Benschop (who was one of Gforth's first users, in mid-1993)
16656: and Stuart Ramsden inspired us with their continuous feedback. Lennart
16657: Benshop contributed @file{glosgen.fs}, while Stuart Ramsden has been
16658: working on automatic support for calling C libraries. Helpful comments
16659: also came from Paul Kleinrubatscher, Christian Pirker, Dirk Zoller,
16660: Marcel Hendrix, John Wavrik, Barrie Stott, Marc de Groot, Jorge
16661: Acerada, Bruce Hoyt, Robert Epprecht, Dennis Ruffer and David
16662: N. Williams. Since the release of Gforth-0.2.1 there were also helpful
16663: comments from many others; thank you all, sorry for not listing you
16664: here (but digging through my mailbox to extract your names is on my
16665: to-do list).
16666:
16667: Gforth also owes a lot to the authors of the tools we used (GCC, CVS,
16668: and autoconf, among others), and to the creators of the Internet: Gforth
16669: was developed across the Internet, and its authors did not meet
16670: physically for the first 4 years of development.
16671:
16672: @section Pedigree
16673: @cindex pedigree of Gforth
16674:
16675: Gforth descends from bigFORTH (1993) and fig-Forth. Of course, a
16676: significant part of the design of Gforth was prescribed by ANS Forth.
16677:
16678: Bernd Paysan wrote bigFORTH, a descendent from TurboForth, an unreleased
16679: 32 bit native code version of VolksForth for the Atari ST, written
16680: mostly by Dietrich Weineck.
16681:
16682: VolksForth was written by Klaus Schleisiek, Bernd Pennemann, Georg
16683: Rehfeld and Dietrich Weineck for the C64 (called UltraForth there) in
16684: the mid-80s and ported to the Atari ST in 1986. It descends from fig-Forth.
16685:
16686: @c Henry Laxen and Mike Perry wrote F83 as a model implementation of the
16687: @c Forth-83 standard. !! Pedigree? When?
16688:
16689: A team led by Bill Ragsdale implemented fig-Forth on many processors in
16690: 1979. Robert Selzer and Bill Ragsdale developed the original
16691: implementation of fig-Forth for the 6502 based on microForth.
16692:
16693: The principal architect of microForth was Dean Sanderson. microForth was
16694: FORTH, Inc.'s first off-the-shelf product. It was developed in 1976 for
16695: the 1802, and subsequently implemented on the 8080, the 6800 and the
16696: Z80.
16697:
16698: All earlier Forth systems were custom-made, usually by Charles Moore,
16699: who discovered (as he puts it) Forth during the late 60s. The first full
16700: Forth existed in 1971.
16701:
16702: A part of the information in this section comes from
16703: @cite{@uref{http://www.forth.com/resources/evolution/index.html,The
16704: Evolution of Forth}} by Elizabeth D. Rather, Donald R. Colburn and
16705: Charles H. Moore, presented at the HOPL-II conference and preprinted
16706: in SIGPLAN Notices 28(3), 1993. You can find more historical and
16707: genealogical information about Forth there. For a more general (and
16708: graphical) Forth family tree look see
16709: @cite{@uref{http://www.complang.tuwien.ac.at/forth/family-tree/},
16710: Forth Family Tree and Timeline}.
16711:
16712: @c ------------------------------------------------------------------
16713: @node Forth-related information, Licenses, Origin, Top
16714: @appendix Other Forth-related information
16715: @cindex Forth-related information
16716:
16717: @c anton: I threw most of this stuff out, because it can be found through
16718: @c the FAQ and the FAQ is more likely to be up-to-date.
16719:
16720: @cindex comp.lang.forth
16721: @cindex frequently asked questions
16722: There is an active news group (comp.lang.forth) discussing Forth
16723: (including Gforth) and Forth-related issues. Its
16724: @uref{http://www.complang.tuwien.ac.at/forth/faq/faq-general-2.html,FAQs}
16725: (frequently asked questions and their answers) contains a lot of
16726: information on Forth. You should read it before posting to
16727: comp.lang.forth.
16728:
16729: The ANS Forth standard is most usable in its
16730: @uref{http://www.taygeta.com/forth/dpans.html, HTML form}.
16731:
16732: @c ---------------------------------------------------
16733: @node Licenses, Word Index, Forth-related information, Top
16734: @appendix Licenses
16735:
16736: @menu
16737: * GNU Free Documentation License:: License for copying this manual.
16738: * Copying:: GPL (for copying this software).
16739: @end menu
16740:
16741: @node GNU Free Documentation License, Copying, Licenses, Licenses
16742: @appendixsec GNU Free Documentation License
16743: @include fdl.texi
16744:
16745: @node Copying, , GNU Free Documentation License, Licenses
16746: @appendixsec GNU GENERAL PUBLIC LICENSE
16747: @include gpl.texi
16748:
16749:
16750:
16751: @c ------------------------------------------------------------------
16752: @node Word Index, Concept Index, Licenses, Top
16753: @unnumbered Word Index
16754:
16755: This index is a list of Forth words that have ``glossary'' entries
16756: within this manual. Each word is listed with its stack effect and
16757: wordset.
16758:
16759: @printindex fn
16760:
16761: @c anton: the name index seems superfluous given the word and concept indices.
16762:
16763: @c @node Name Index, Concept Index, Word Index, Top
16764: @c @unnumbered Name Index
16765:
16766: @c This index is a list of Forth words that have ``glossary'' entries
16767: @c within this manual.
16768:
16769: @c @printindex ky
16770:
16771: @c -------------------------------------------------------
16772: @node Concept Index, , Word Index, Top
16773: @unnumbered Concept and Word Index
16774:
16775: Not all entries listed in this index are present verbatim in the
16776: text. This index also duplicates, in abbreviated form, all of the words
16777: listed in the Word Index (only the names are listed for the words here).
16778:
16779: @printindex cp
16780:
16781: @bye
16782:
16783:
16784:
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