Annotation of gforth/doc/gforth.ds, revision 1.120
1.1 anton 1: \input texinfo @c -*-texinfo-*-
2: @comment The source is gforth.ds, from which gforth.texi is generated
1.28 crook 3:
1.21 crook 4: @comment TODO: nac29jan99 - a list of things to add in the next edit:
1.28 crook 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.
1.36 anton 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
1.113 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
1.29 crook 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:
1.66 anton 24: @c POSTPONE, COMPILE, [COMPILE], LITERAL should have their own section
1.28 crook 25:
1.1 anton 26: @comment %**start of header (This is for running Texinfo on a region.)
27: @setfilename gforth.info
1.113 anton 28: @include version.texi
1.1 anton 29: @settitle Gforth Manual
1.113 anton 30: @c @syncodeindex pg cp
1.49 anton 31:
1.12 anton 32: @macro progstyle {}
33: Programming style note:
1.3 anton 34: @end macro
1.48 anton 35:
36: @macro assignment {}
37: @table @i
38: @item Assignment:
39: @end macro
40: @macro endassignment {}
41: @end table
42: @end macro
43:
1.29 crook 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:
1.113 anton 57: @comment %**end of header (This is for running Texinfo on a region.)
58: @copying
59: This manual is for Gforth
60: (version @value{VERSION}, @value{UPDATED}),
61: a fast and portable implementation of the ANS Forth language
1.29 crook 62:
1.113 anton 63: Copyright @copyright{} 1995, 1996, 1997, 1998, 2000, 2003 Free Software Foundation, Inc.
1.29 crook 64:
1.113 anton 65: @quotation
66: Permission is granted to copy, distribute and/or modify this document
67: under the terms of the GNU Free Documentation License, Version 1.1 or
68: any later version published by the Free Software Foundation; with no
69: Invariant Sections, with the Front-Cover texts being ``A GNU Manual,''
70: and with the Back-Cover Texts as in (a) below. A copy of the
71: license is included in the section entitled ``GNU Free Documentation
72: License.''
73:
74: (a) The FSF's Back-Cover Text is: ``You have freedom to copy and modify
75: this GNU Manual, like GNU software. Copies published by the Free
76: Software Foundation raise funds for GNU development.''
77: @end quotation
78: @end copying
1.10 anton 79:
1.113 anton 80: @dircategory Software development
81: @direntry
82: * Gforth: (gforth). A fast interpreter for the Forth language.
83: @end direntry
84: @c The Texinfo manual also recommends doing this, but for Gforth it may
85: @c not make much sense
86: @c @dircategory Individual utilities
87: @c @direntry
88: @c * Gforth: (gforth)Invoking Gforth. gforth, gforth-fast, gforthmi
89: @c @end direntry
1.1 anton 90:
91: @titlepage
1.113 anton 92: @title Gforth
93: @subtitle for version @value{VERSION}, @value{UPDATED}
94: @author Neal Crook
95: @author Anton Ertl
1.114 anton 96: @author David Kuehling
1.113 anton 97: @author Bernd Paysan
98: @author Jens Wilke
1.1 anton 99: @page
100: @vskip 0pt plus 1filll
1.113 anton 101: @insertcopying
102: @end titlepage
1.1 anton 103:
1.113 anton 104: @contents
1.1 anton 105:
1.113 anton 106: @ifnottex
107: @node Top, Goals, (dir), (dir)
108: @top Gforth
1.1 anton 109:
1.113 anton 110: @insertcopying
1.49 anton 111: @end ifnottex
1.1 anton 112:
113: @menu
1.26 crook 114: * Goals:: About the Gforth Project
1.29 crook 115: * Gforth Environment:: Starting (and exiting) Gforth
1.48 anton 116: * Tutorial:: Hands-on Forth Tutorial
1.21 crook 117: * Introduction:: An introduction to ANS Forth
1.1 anton 118: * Words:: Forth words available in Gforth
1.24 anton 119: * Error messages:: How to interpret them
1.1 anton 120: * Tools:: Programming tools
121: * ANS conformance:: Implementation-defined options etc.
1.65 anton 122: * Standard vs Extensions:: Should I use extensions?
1.1 anton 123: * Model:: The abstract machine of Gforth
124: * Integrating Gforth:: Forth as scripting language for applications
125: * Emacs and Gforth:: The Gforth Mode
126: * Image Files:: @code{.fi} files contain compiled code
127: * Engine:: The inner interpreter and the primitives
1.13 pazsan 128: * Cross Compiler:: The Cross Compiler
1.1 anton 129: * Bugs:: How to report them
130: * Origin:: Authors and ancestors of Gforth
1.21 crook 131: * Forth-related information:: Books and places to look on the WWW
1.113 anton 132: * Licenses::
1.1 anton 133: * Word Index:: An item for each Forth word
134: * Concept Index:: A menu covering many topics
1.12 anton 135:
1.91 anton 136: @detailmenu
137: --- The Detailed Node Listing ---
1.12 anton 138:
1.29 crook 139: Gforth Environment
140:
1.32 anton 141: * Invoking Gforth:: Getting in
142: * Leaving Gforth:: Getting out
143: * Command-line editing::
1.48 anton 144: * Environment variables:: that affect how Gforth starts up
1.32 anton 145: * Gforth Files:: What gets installed and where
1.112 anton 146: * Gforth in pipes::
1.48 anton 147: * Startup speed:: When 35ms is not fast enough ...
148:
149: Forth Tutorial
150:
151: * Starting Gforth Tutorial::
152: * Syntax Tutorial::
153: * Crash Course Tutorial::
154: * Stack Tutorial::
155: * Arithmetics Tutorial::
156: * Stack Manipulation Tutorial::
157: * Using files for Forth code Tutorial::
158: * Comments Tutorial::
159: * Colon Definitions Tutorial::
160: * Decompilation Tutorial::
161: * Stack-Effect Comments Tutorial::
162: * Types Tutorial::
163: * Factoring Tutorial::
164: * Designing the stack effect Tutorial::
165: * Local Variables Tutorial::
166: * Conditional execution Tutorial::
167: * Flags and Comparisons Tutorial::
168: * General Loops Tutorial::
169: * Counted loops Tutorial::
170: * Recursion Tutorial::
171: * Leaving definitions or loops Tutorial::
172: * Return Stack Tutorial::
173: * Memory Tutorial::
174: * Characters and Strings Tutorial::
175: * Alignment Tutorial::
1.87 anton 176: * Files Tutorial::
1.48 anton 177: * Interpretation and Compilation Semantics and Immediacy Tutorial::
178: * Execution Tokens Tutorial::
179: * Exceptions Tutorial::
180: * Defining Words Tutorial::
181: * Arrays and Records Tutorial::
182: * POSTPONE Tutorial::
183: * Literal Tutorial::
184: * Advanced macros Tutorial::
185: * Compilation Tokens Tutorial::
186: * Wordlists and Search Order Tutorial::
1.29 crook 187:
1.24 anton 188: An Introduction to ANS Forth
189:
1.67 anton 190: * Introducing the Text Interpreter::
191: * Stacks and Postfix notation::
192: * Your first definition::
193: * How does that work?::
194: * Forth is written in Forth::
195: * Review - elements of a Forth system::
196: * Where to go next::
197: * Exercises::
1.24 anton 198:
1.12 anton 199: Forth Words
200:
201: * Notation::
1.65 anton 202: * Case insensitivity::
203: * Comments::
204: * Boolean Flags::
1.12 anton 205: * Arithmetic::
206: * Stack Manipulation::
207: * Memory::
208: * Control Structures::
209: * Defining Words::
1.65 anton 210: * Interpretation and Compilation Semantics::
1.47 crook 211: * Tokens for Words::
1.81 anton 212: * Compiling words::
1.65 anton 213: * The Text Interpreter::
1.111 anton 214: * The Input Stream::
1.65 anton 215: * Word Lists::
216: * Environmental Queries::
1.12 anton 217: * Files::
218: * Blocks::
219: * Other I/O::
1.78 anton 220: * Locals::
221: * Structures::
222: * Object-oriented Forth::
1.12 anton 223: * Programming Tools::
224: * Assembler and Code Words::
225: * Threading Words::
1.65 anton 226: * Passing Commands to the OS::
227: * Keeping track of Time::
228: * Miscellaneous Words::
1.12 anton 229:
230: Arithmetic
231:
232: * Single precision::
1.67 anton 233: * Double precision:: Double-cell integer arithmetic
1.12 anton 234: * Bitwise operations::
1.67 anton 235: * Numeric comparison::
1.32 anton 236: * Mixed precision:: Operations with single and double-cell integers
1.12 anton 237: * Floating Point::
238:
239: Stack Manipulation
240:
241: * Data stack::
242: * Floating point stack::
243: * Return stack::
244: * Locals stack::
245: * Stack pointer manipulation::
246:
247: Memory
248:
1.32 anton 249: * Memory model::
250: * Dictionary allocation::
251: * Heap Allocation::
252: * Memory Access::
253: * Address arithmetic::
254: * Memory Blocks::
1.12 anton 255:
256: Control Structures
257:
1.41 anton 258: * Selection:: IF ... ELSE ... ENDIF
259: * Simple Loops:: BEGIN ...
1.32 anton 260: * Counted Loops:: DO
1.67 anton 261: * Arbitrary control structures::
262: * Calls and returns::
1.12 anton 263: * Exception Handling::
264:
265: Defining Words
266:
1.67 anton 267: * CREATE::
1.44 crook 268: * Variables:: Variables and user variables
1.67 anton 269: * Constants::
1.44 crook 270: * Values:: Initialised variables
1.67 anton 271: * Colon Definitions::
1.44 crook 272: * Anonymous Definitions:: Definitions without names
1.71 anton 273: * Supplying names:: Passing definition names as strings
1.67 anton 274: * User-defined Defining Words::
1.44 crook 275: * Deferred words:: Allow forward references
1.67 anton 276: * Aliases::
1.47 crook 277:
1.63 anton 278: User-defined Defining Words
279:
280: * CREATE..DOES> applications::
281: * CREATE..DOES> details::
282: * Advanced does> usage example::
1.91 anton 283: * @code{Const-does>}::
1.63 anton 284:
1.47 crook 285: Interpretation and Compilation Semantics
286:
1.67 anton 287: * Combined words::
1.12 anton 288:
1.71 anton 289: Tokens for Words
290:
291: * Execution token:: represents execution/interpretation semantics
292: * Compilation token:: represents compilation semantics
293: * Name token:: represents named words
294:
1.82 anton 295: Compiling words
296:
297: * Literals:: Compiling data values
298: * Macros:: Compiling words
299:
1.21 crook 300: The Text Interpreter
301:
1.67 anton 302: * Input Sources::
303: * Number Conversion::
304: * Interpret/Compile states::
305: * Interpreter Directives::
1.21 crook 306:
1.26 crook 307: Word Lists
308:
1.75 anton 309: * Vocabularies::
1.67 anton 310: * Why use word lists?::
1.75 anton 311: * Word list example::
1.26 crook 312:
313: Files
314:
1.48 anton 315: * Forth source files::
316: * General files::
317: * Search Paths::
318:
319: Search Paths
320:
1.75 anton 321: * Source Search Paths::
1.26 crook 322: * General Search Paths::
323:
324: Other I/O
325:
1.32 anton 326: * Simple numeric output:: Predefined formats
327: * Formatted numeric output:: Formatted (pictured) output
328: * String Formats:: How Forth stores strings in memory
1.67 anton 329: * Displaying characters and strings:: Other stuff
1.32 anton 330: * Input:: Input
1.112 anton 331: * Pipes:: How to create your own pipes
1.26 crook 332:
333: Locals
334:
335: * Gforth locals::
336: * ANS Forth locals::
337:
338: Gforth locals
339:
340: * Where are locals visible by name?::
341: * How long do locals live?::
1.78 anton 342: * Locals programming style::
343: * Locals implementation::
1.26 crook 344:
1.12 anton 345: Structures
346:
347: * Why explicit structure support?::
348: * Structure Usage::
349: * Structure Naming Convention::
350: * Structure Implementation::
351: * Structure Glossary::
352:
353: Object-oriented Forth
354:
1.48 anton 355: * Why object-oriented programming?::
356: * Object-Oriented Terminology::
357: * Objects::
358: * OOF::
359: * Mini-OOF::
1.23 crook 360: * Comparison with other object models::
1.12 anton 361:
1.24 anton 362: The @file{objects.fs} model
1.12 anton 363:
364: * Properties of the Objects model::
365: * Basic Objects Usage::
1.41 anton 366: * The Objects base class::
1.12 anton 367: * Creating objects::
368: * Object-Oriented Programming Style::
369: * Class Binding::
370: * Method conveniences::
371: * Classes and Scoping::
1.41 anton 372: * Dividing classes::
1.12 anton 373: * Object Interfaces::
374: * Objects Implementation::
375: * Objects Glossary::
376:
1.24 anton 377: The @file{oof.fs} model
1.12 anton 378:
1.67 anton 379: * Properties of the OOF model::
380: * Basic OOF Usage::
381: * The OOF base class::
382: * Class Declaration::
383: * Class Implementation::
1.12 anton 384:
1.24 anton 385: The @file{mini-oof.fs} model
1.23 crook 386:
1.48 anton 387: * Basic Mini-OOF Usage::
388: * Mini-OOF Example::
389: * Mini-OOF Implementation::
1.23 crook 390:
1.78 anton 391: Programming Tools
392:
393: * Examining::
394: * Forgetting words::
395: * Debugging:: Simple and quick.
396: * Assertions:: Making your programs self-checking.
397: * Singlestep Debugger:: Executing your program word by word.
398:
399: Assembler and Code Words
400:
401: * Code and ;code::
402: * Common Assembler:: Assembler Syntax
403: * Common Disassembler::
404: * 386 Assembler:: Deviations and special cases
405: * Alpha Assembler:: Deviations and special cases
406: * MIPS assembler:: Deviations and special cases
407: * Other assemblers:: How to write them
408:
1.12 anton 409: Tools
410:
411: * ANS Report:: Report the words used, sorted by wordset.
412:
413: ANS conformance
414:
415: * The Core Words::
416: * The optional Block word set::
417: * The optional Double Number word set::
418: * The optional Exception word set::
419: * The optional Facility word set::
420: * The optional File-Access word set::
421: * The optional Floating-Point word set::
422: * The optional Locals word set::
423: * The optional Memory-Allocation word set::
424: * The optional Programming-Tools word set::
425: * The optional Search-Order word set::
426:
427: The Core Words
428:
429: * core-idef:: Implementation Defined Options
430: * core-ambcond:: Ambiguous Conditions
431: * core-other:: Other System Documentation
432:
433: The optional Block word set
434:
435: * block-idef:: Implementation Defined Options
436: * block-ambcond:: Ambiguous Conditions
437: * block-other:: Other System Documentation
438:
439: The optional Double Number word set
440:
441: * double-ambcond:: Ambiguous Conditions
442:
443: The optional Exception word set
444:
445: * exception-idef:: Implementation Defined Options
446:
447: The optional Facility word set
448:
449: * facility-idef:: Implementation Defined Options
450: * facility-ambcond:: Ambiguous Conditions
451:
452: The optional File-Access word set
453:
454: * file-idef:: Implementation Defined Options
455: * file-ambcond:: Ambiguous Conditions
456:
457: The optional Floating-Point word set
458:
459: * floating-idef:: Implementation Defined Options
460: * floating-ambcond:: Ambiguous Conditions
461:
462: The optional Locals word set
463:
464: * locals-idef:: Implementation Defined Options
465: * locals-ambcond:: Ambiguous Conditions
466:
467: The optional Memory-Allocation word set
468:
469: * memory-idef:: Implementation Defined Options
470:
471: The optional Programming-Tools word set
472:
473: * programming-idef:: Implementation Defined Options
474: * programming-ambcond:: Ambiguous Conditions
475:
476: The optional Search-Order word set
477:
478: * search-idef:: Implementation Defined Options
479: * search-ambcond:: Ambiguous Conditions
480:
1.109 anton 481: Emacs and Gforth
482:
483: * Installing gforth.el:: Making Emacs aware of Forth.
484: * Emacs Tags:: Viewing the source of a word in Emacs.
485: * Hilighting:: Making Forth code look prettier.
486: * Auto-Indentation:: Customizing auto-indentation.
487: * Blocks Files:: Reading and writing blocks files.
488:
1.12 anton 489: Image Files
490:
1.24 anton 491: * Image Licensing Issues:: Distribution terms for images.
492: * Image File Background:: Why have image files?
1.67 anton 493: * Non-Relocatable Image Files:: don't always work.
1.24 anton 494: * Data-Relocatable Image Files:: are better.
1.67 anton 495: * Fully Relocatable Image Files:: better yet.
1.24 anton 496: * Stack and Dictionary Sizes:: Setting the default sizes for an image.
1.32 anton 497: * Running Image Files:: @code{gforth -i @i{file}} or @i{file}.
1.24 anton 498: * Modifying the Startup Sequence:: and turnkey applications.
1.12 anton 499:
500: Fully Relocatable Image Files
501:
1.27 crook 502: * gforthmi:: The normal way
1.12 anton 503: * cross.fs:: The hard way
504:
505: Engine
506:
507: * Portability::
508: * Threading::
509: * Primitives::
510: * Performance::
511:
512: Threading
513:
514: * Scheduling::
515: * Direct or Indirect Threaded?::
1.109 anton 516: * Dynamic Superinstructions::
1.12 anton 517: * DOES>::
518:
519: Primitives
520:
521: * Automatic Generation::
522: * TOS Optimization::
523: * Produced code::
1.13 pazsan 524:
525: Cross Compiler
526:
1.67 anton 527: * Using the Cross Compiler::
528: * How the Cross Compiler Works::
1.13 pazsan 529:
1.113 anton 530: Licenses
531:
532: * GNU Free Documentation License:: License for copying this manual.
533: * Copying:: GPL (for copying this software).
534:
1.24 anton 535: @end detailmenu
1.1 anton 536: @end menu
537:
1.113 anton 538: @c ----------------------------------------------------------
1.1 anton 539: @iftex
540: @unnumbered Preface
541: @cindex Preface
1.21 crook 542: This manual documents Gforth. Some introductory material is provided for
543: readers who are unfamiliar with Forth or who are migrating to Gforth
544: from other Forth compilers. However, this manual is primarily a
545: reference manual.
1.1 anton 546: @end iftex
547:
1.28 crook 548: @comment TODO much more blurb here.
1.26 crook 549:
550: @c ******************************************************************
1.113 anton 551: @node Goals, Gforth Environment, Top, Top
1.26 crook 552: @comment node-name, next, previous, up
553: @chapter Goals of Gforth
554: @cindex goals of the Gforth project
555: The goal of the Gforth Project is to develop a standard model for
556: ANS Forth. This can be split into several subgoals:
557:
558: @itemize @bullet
559: @item
560: Gforth should conform to the ANS Forth Standard.
561: @item
562: It should be a model, i.e. it should define all the
563: implementation-dependent things.
564: @item
565: It should become standard, i.e. widely accepted and used. This goal
566: is the most difficult one.
567: @end itemize
568:
569: To achieve these goals Gforth should be
570: @itemize @bullet
571: @item
572: Similar to previous models (fig-Forth, F83)
573: @item
574: Powerful. It should provide for all the things that are considered
575: necessary today and even some that are not yet considered necessary.
576: @item
577: Efficient. It should not get the reputation of being exceptionally
578: slow.
579: @item
580: Free.
581: @item
582: Available on many machines/easy to port.
583: @end itemize
584:
585: Have we achieved these goals? Gforth conforms to the ANS Forth
586: standard. It may be considered a model, but we have not yet documented
587: which parts of the model are stable and which parts we are likely to
588: change. It certainly has not yet become a de facto standard, but it
589: appears to be quite popular. It has some similarities to and some
590: differences from previous models. It has some powerful features, but not
591: yet everything that we envisioned. We certainly have achieved our
1.65 anton 592: execution speed goals (@pxref{Performance})@footnote{However, in 1998
593: the bar was raised when the major commercial Forth vendors switched to
594: native code compilers.}. It is free and available on many machines.
1.29 crook 595:
1.26 crook 596: @c ******************************************************************
1.48 anton 597: @node Gforth Environment, Tutorial, Goals, Top
1.29 crook 598: @chapter Gforth Environment
599: @cindex Gforth environment
1.21 crook 600:
1.45 crook 601: Note: ultimately, the Gforth man page will be auto-generated from the
1.29 crook 602: material in this chapter.
1.21 crook 603:
604: @menu
1.29 crook 605: * Invoking Gforth:: Getting in
606: * Leaving Gforth:: Getting out
607: * Command-line editing::
1.48 anton 608: * Environment variables:: that affect how Gforth starts up
1.29 crook 609: * Gforth Files:: What gets installed and where
1.112 anton 610: * Gforth in pipes::
1.48 anton 611: * Startup speed:: When 35ms is not fast enough ...
1.21 crook 612: @end menu
613:
1.49 anton 614: For related information about the creation of images see @ref{Image Files}.
1.29 crook 615:
1.21 crook 616: @comment ----------------------------------------------
1.48 anton 617: @node Invoking Gforth, Leaving Gforth, Gforth Environment, Gforth Environment
1.29 crook 618: @section Invoking Gforth
619: @cindex invoking Gforth
620: @cindex running Gforth
621: @cindex command-line options
622: @cindex options on the command line
623: @cindex flags on the command line
1.21 crook 624:
1.30 anton 625: Gforth is made up of two parts; an executable ``engine'' (named
1.109 anton 626: @command{gforth} or @command{gforth-fast}) and an image file. To start it, you
1.30 anton 627: will usually just say @code{gforth} -- this automatically loads the
628: default image file @file{gforth.fi}. In many other cases the default
629: Gforth image will be invoked like this:
1.21 crook 630: @example
1.30 anton 631: gforth [file | -e forth-code] ...
1.21 crook 632: @end example
1.29 crook 633: @noindent
634: This interprets the contents of the files and the Forth code in the order they
635: are given.
1.21 crook 636:
1.109 anton 637: In addition to the @command{gforth} engine, there is also an engine
638: called @command{gforth-fast}, which is faster, but gives less
639: informative error messages (@pxref{Error messages}) and may catch some
640: stack underflows later or not at all. You should use it for debugged,
641: performance-critical programs.
642:
643: Moreover, there is an engine called @command{gforth-itc}, which is
644: useful in some backwards-compatibility situations (@pxref{Direct or
645: Indirect Threaded?}).
1.30 anton 646:
1.29 crook 647: In general, the command line looks like this:
1.21 crook 648:
649: @example
1.30 anton 650: gforth[-fast] [engine options] [image options]
1.21 crook 651: @end example
652:
1.30 anton 653: The engine options must come before the rest of the command
1.29 crook 654: line. They are:
1.26 crook 655:
1.29 crook 656: @table @code
657: @cindex -i, command-line option
658: @cindex --image-file, command-line option
659: @item --image-file @i{file}
660: @itemx -i @i{file}
661: Loads the Forth image @i{file} instead of the default
662: @file{gforth.fi} (@pxref{Image Files}).
1.21 crook 663:
1.39 anton 664: @cindex --appl-image, command-line option
665: @item --appl-image @i{file}
666: Loads the image @i{file} and leaves all further command-line arguments
1.65 anton 667: to the image (instead of processing them as engine options). This is
668: useful for building executable application images on Unix, built with
1.39 anton 669: @code{gforthmi --application ...}.
670:
1.29 crook 671: @cindex --path, command-line option
672: @cindex -p, command-line option
673: @item --path @i{path}
674: @itemx -p @i{path}
675: Uses @i{path} for searching the image file and Forth source code files
676: instead of the default in the environment variable @code{GFORTHPATH} or
677: the path specified at installation time (e.g.,
678: @file{/usr/local/share/gforth/0.2.0:.}). A path is given as a list of
679: directories, separated by @samp{:} (on Unix) or @samp{;} (on other OSs).
1.21 crook 680:
1.29 crook 681: @cindex --dictionary-size, command-line option
682: @cindex -m, command-line option
683: @cindex @i{size} parameters for command-line options
684: @cindex size of the dictionary and the stacks
685: @item --dictionary-size @i{size}
686: @itemx -m @i{size}
687: Allocate @i{size} space for the Forth dictionary space instead of
688: using the default specified in the image (typically 256K). The
689: @i{size} specification for this and subsequent options consists of
690: an integer and a unit (e.g.,
691: @code{4M}). The unit can be one of @code{b} (bytes), @code{e} (element
692: size, in this case Cells), @code{k} (kilobytes), @code{M} (Megabytes),
693: @code{G} (Gigabytes), and @code{T} (Terabytes). If no unit is specified,
694: @code{e} is used.
1.21 crook 695:
1.29 crook 696: @cindex --data-stack-size, command-line option
697: @cindex -d, command-line option
698: @item --data-stack-size @i{size}
699: @itemx -d @i{size}
700: Allocate @i{size} space for the data stack instead of using the
701: default specified in the image (typically 16K).
1.21 crook 702:
1.29 crook 703: @cindex --return-stack-size, command-line option
704: @cindex -r, command-line option
705: @item --return-stack-size @i{size}
706: @itemx -r @i{size}
707: Allocate @i{size} space for the return stack instead of using the
708: default specified in the image (typically 15K).
1.21 crook 709:
1.29 crook 710: @cindex --fp-stack-size, command-line option
711: @cindex -f, command-line option
712: @item --fp-stack-size @i{size}
713: @itemx -f @i{size}
714: Allocate @i{size} space for the floating point stack instead of
715: using the default specified in the image (typically 15.5K). In this case
716: the unit specifier @code{e} refers to floating point numbers.
1.21 crook 717:
1.48 anton 718: @cindex --locals-stack-size, command-line option
719: @cindex -l, command-line option
720: @item --locals-stack-size @i{size}
721: @itemx -l @i{size}
722: Allocate @i{size} space for the locals stack instead of using the
723: default specified in the image (typically 14.5K).
724:
725: @cindex -h, command-line option
726: @cindex --help, command-line option
727: @item --help
728: @itemx -h
729: Print a message about the command-line options
730:
731: @cindex -v, command-line option
732: @cindex --version, command-line option
733: @item --version
734: @itemx -v
735: Print version and exit
736:
737: @cindex --debug, command-line option
738: @item --debug
739: Print some information useful for debugging on startup.
740:
741: @cindex --offset-image, command-line option
742: @item --offset-image
743: Start the dictionary at a slightly different position than would be used
744: otherwise (useful for creating data-relocatable images,
745: @pxref{Data-Relocatable Image Files}).
746:
747: @cindex --no-offset-im, command-line option
748: @item --no-offset-im
749: Start the dictionary at the normal position.
750:
751: @cindex --clear-dictionary, command-line option
752: @item --clear-dictionary
753: Initialize all bytes in the dictionary to 0 before loading the image
754: (@pxref{Data-Relocatable Image Files}).
755:
756: @cindex --die-on-signal, command-line-option
757: @item --die-on-signal
758: Normally Gforth handles most signals (e.g., the user interrupt SIGINT,
759: or the segmentation violation SIGSEGV) by translating it into a Forth
760: @code{THROW}. With this option, Gforth exits if it receives such a
761: signal. This option is useful when the engine and/or the image might be
762: severely broken (such that it causes another signal before recovering
763: from the first); this option avoids endless loops in such cases.
1.109 anton 764:
1.119 anton 765: @cindex --no-dynamic, command-line option
766: @cindex --dynamic, command-line option
1.109 anton 767: @item --no-dynamic
768: @item --dynamic
769: Disable or enable dynamic superinstructions with replication
770: (@pxref{Dynamic Superinstructions}).
771:
1.119 anton 772: @cindex --no-super, command-line option
1.109 anton 773: @item --no-super
1.110 anton 774: Disable dynamic superinstructions, use just dynamic replication; this is
775: useful if you want to patch threaded code (@pxref{Dynamic
776: Superinstructions}).
1.119 anton 777:
778: @cindex --ss-number, command-line option
779: @item --ss-number=@var{N}
780: Use only the first @var{N} static superinstructions compiled into the
781: engine (default: use them all; note that only @code{gforth-fast} has
782: any). This option is useful for measuring the performance impact of
783: static superinstructions.
784:
785: @cindex --ss-min-..., command-line options
786: @item --ss-min-codesize
787: @item --ss-min-ls
788: @item --ss-min-lsu
789: @item --ss-min-nexts
790: Use specified metric for determining the cost of a primitive or static
791: superinstruction for static superinstruction selection. @code{Codesize}
792: is the native code size of the primive or static superinstruction,
793: @code{ls} is the number of loads and stores, @code{lsu} is the number of
794: loads, stores, and updates, and @code{nexts} is the number of dispatches
795: (not taking dynamic superinstructions into account), i.e. every
796: primitive or static superinstruction has cost 1. Default:
797: @code{codesize} if you use dynamic code generation, otherwise
798: @code{nexts}.
799:
800: @cindex --ss-greedy, command-line option
801: @item --ss-greedy
802: This option is useful for measuring the performance impact of static
803: superinstructions. By default, an optimal shortest-path algorithm is
804: used for selecting static superinstructions. With @option{--ss-greedy}
805: this algorithm is modified to assume that anything after the static
806: superinstruction currently under consideration is not combined into
807: static superinstructions. With @option{--ss-min-nexts} this produces
808: the same result as a greedy algorithm that always selects the longest
809: superinstruction available at the moment. E.g., if there are
810: superinstructions AB and BCD, then for the sequence A B C D the optimal
811: algorithm will select A BCD and the greedy algorithm will select AB C D.
812:
813: @cindex --print-metrics, command-line option
814: @item --print-metrics
815: Prints some metrics used during static superinstruction selection:
816: @code{code size} is the actual size of the dynamically generated code.
817: @code{Metric codesize} is the sum of the codesize metrics as seen by
818: static superinstruction selection; there is a difference from @code{code
819: size}, because not all primitives and static superinstructions are
820: compiled into dynamically generated code, and because of markers. The
821: other metrics correspond to the @option{ss-min-...} options. This
822: option is useful for evaluating the effects of the @option{--ss-...}
823: options.
1.109 anton 824:
1.48 anton 825: @end table
826:
827: @cindex loading files at startup
828: @cindex executing code on startup
829: @cindex batch processing with Gforth
830: As explained above, the image-specific command-line arguments for the
831: default image @file{gforth.fi} consist of a sequence of filenames and
832: @code{-e @var{forth-code}} options that are interpreted in the sequence
833: in which they are given. The @code{-e @var{forth-code}} or
834: @code{--evaluate @var{forth-code}} option evaluates the Forth
835: code. This option takes only one argument; if you want to evaluate more
836: Forth words, you have to quote them or use @code{-e} several times. To exit
837: after processing the command line (instead of entering interactive mode)
838: append @code{-e bye} to the command line.
839:
840: @cindex versions, invoking other versions of Gforth
841: If you have several versions of Gforth installed, @code{gforth} will
842: invoke the version that was installed last. @code{gforth-@i{version}}
843: invokes a specific version. If your environment contains the variable
844: @code{GFORTHPATH}, you may want to override it by using the
845: @code{--path} option.
846:
847: Not yet implemented:
848: On startup the system first executes the system initialization file
849: (unless the option @code{--no-init-file} is given; note that the system
850: resulting from using this option may not be ANS Forth conformant). Then
851: the user initialization file @file{.gforth.fs} is executed, unless the
1.62 crook 852: option @code{--no-rc} is given; this file is searched for in @file{.},
1.48 anton 853: then in @file{~}, then in the normal path (see above).
854:
855:
856:
857: @comment ----------------------------------------------
858: @node Leaving Gforth, Command-line editing, Invoking Gforth, Gforth Environment
859: @section Leaving Gforth
860: @cindex Gforth - leaving
861: @cindex leaving Gforth
862:
863: You can leave Gforth by typing @code{bye} or @kbd{Ctrl-d} (at the start
864: of a line) or (if you invoked Gforth with the @code{--die-on-signal}
865: option) @kbd{Ctrl-c}. When you leave Gforth, all of your definitions and
1.49 anton 866: data are discarded. For ways of saving the state of the system before
867: leaving Gforth see @ref{Image Files}.
1.48 anton 868:
869: doc-bye
870:
871:
872: @comment ----------------------------------------------
1.65 anton 873: @node Command-line editing, Environment variables, Leaving Gforth, Gforth Environment
1.48 anton 874: @section Command-line editing
875: @cindex command-line editing
876:
877: Gforth maintains a history file that records every line that you type to
878: the text interpreter. This file is preserved between sessions, and is
879: used to provide a command-line recall facility; if you type @kbd{Ctrl-P}
880: repeatedly you can recall successively older commands from this (or
881: previous) session(s). The full list of command-line editing facilities is:
882:
883: @itemize @bullet
884: @item
885: @kbd{Ctrl-p} (``previous'') (or up-arrow) to recall successively older
886: commands from the history buffer.
887: @item
888: @kbd{Ctrl-n} (``next'') (or down-arrow) to recall successively newer commands
889: from the history buffer.
890: @item
891: @kbd{Ctrl-f} (or right-arrow) to move the cursor right, non-destructively.
892: @item
893: @kbd{Ctrl-b} (or left-arrow) to move the cursor left, non-destructively.
894: @item
895: @kbd{Ctrl-h} (backspace) to delete the character to the left of the cursor,
896: closing up the line.
897: @item
898: @kbd{Ctrl-k} to delete (``kill'') from the cursor to the end of the line.
899: @item
900: @kbd{Ctrl-a} to move the cursor to the start of the line.
901: @item
902: @kbd{Ctrl-e} to move the cursor to the end of the line.
903: @item
904: @key{RET} (@kbd{Ctrl-m}) or @key{LFD} (@kbd{Ctrl-j}) to submit the current
905: line.
906: @item
907: @key{TAB} to step through all possible full-word completions of the word
908: currently being typed.
909: @item
1.65 anton 910: @kbd{Ctrl-d} on an empty line line to terminate Gforth (gracefully,
911: using @code{bye}).
912: @item
913: @kbd{Ctrl-x} (or @code{Ctrl-d} on a non-empty line) to delete the
914: character under the cursor.
1.48 anton 915: @end itemize
916:
917: When editing, displayable characters are inserted to the left of the
918: cursor position; the line is always in ``insert'' (as opposed to
919: ``overstrike'') mode.
920:
921: @cindex history file
922: @cindex @file{.gforth-history}
923: On Unix systems, the history file is @file{~/.gforth-history} by
924: default@footnote{i.e. it is stored in the user's home directory.}. You
925: can find out the name and location of your history file using:
926:
927: @example
928: history-file type \ Unix-class systems
929:
930: history-file type \ Other systems
931: history-dir type
932: @end example
933:
934: If you enter long definitions by hand, you can use a text editor to
935: paste them out of the history file into a Forth source file for reuse at
936: a later time.
937:
938: Gforth never trims the size of the history file, so you should do this
939: periodically, if necessary.
940:
941: @comment this is all defined in history.fs
942: @comment NAC TODO the ctrl-D behaviour can either do a bye or a beep.. how is that option
943: @comment chosen?
944:
945:
946: @comment ----------------------------------------------
1.65 anton 947: @node Environment variables, Gforth Files, Command-line editing, Gforth Environment
1.48 anton 948: @section Environment variables
949: @cindex environment variables
950:
951: Gforth uses these environment variables:
952:
953: @itemize @bullet
954: @item
955: @cindex @code{GFORTHHIST} -- environment variable
956: @code{GFORTHHIST} -- (Unix systems only) specifies the directory in which to
957: open/create the history file, @file{.gforth-history}. Default:
958: @code{$HOME}.
959:
960: @item
961: @cindex @code{GFORTHPATH} -- environment variable
962: @code{GFORTHPATH} -- specifies the path used when searching for the gforth image file and
963: for Forth source-code files.
964:
965: @item
966: @cindex @code{GFORTH} -- environment variable
1.49 anton 967: @code{GFORTH} -- used by @file{gforthmi}, @xref{gforthmi}.
1.48 anton 968:
969: @item
970: @cindex @code{GFORTHD} -- environment variable
1.62 crook 971: @code{GFORTHD} -- used by @file{gforthmi}, @xref{gforthmi}.
1.48 anton 972:
973: @item
974: @cindex @code{TMP}, @code{TEMP} - environment variable
975: @code{TMP}, @code{TEMP} - (non-Unix systems only) used as a potential
976: location for the history file.
977: @end itemize
978:
979: @comment also POSIXELY_CORRECT LINES COLUMNS HOME but no interest in
980: @comment mentioning these.
981:
982: All the Gforth environment variables default to sensible values if they
983: are not set.
984:
985:
986: @comment ----------------------------------------------
1.112 anton 987: @node Gforth Files, Gforth in pipes, Environment variables, Gforth Environment
1.48 anton 988: @section Gforth files
989: @cindex Gforth files
990:
991: When you install Gforth on a Unix system, it installs files in these
992: locations by default:
993:
994: @itemize @bullet
995: @item
996: @file{/usr/local/bin/gforth}
997: @item
998: @file{/usr/local/bin/gforthmi}
999: @item
1000: @file{/usr/local/man/man1/gforth.1} - man page.
1001: @item
1002: @file{/usr/local/info} - the Info version of this manual.
1003: @item
1004: @file{/usr/local/lib/gforth/<version>/...} - Gforth @file{.fi} files.
1005: @item
1006: @file{/usr/local/share/gforth/<version>/TAGS} - Emacs TAGS file.
1007: @item
1008: @file{/usr/local/share/gforth/<version>/...} - Gforth source files.
1009: @item
1010: @file{.../emacs/site-lisp/gforth.el} - Emacs gforth mode.
1011: @end itemize
1012:
1013: You can select different places for installation by using
1014: @code{configure} options (listed with @code{configure --help}).
1015:
1016: @comment ----------------------------------------------
1.112 anton 1017: @node Gforth in pipes, Startup speed, Gforth Files, Gforth Environment
1018: @section Gforth in pipes
1019: @cindex pipes, Gforth as part of
1020:
1021: Gforth can be used in pipes created elsewhere (described here). It can
1022: also create pipes on its own (@pxref{Pipes}).
1023:
1024: @cindex input from pipes
1025: If you pipe into Gforth, your program should read with @code{read-file}
1026: or @code{read-line} from @code{stdin} (@pxref{General files}).
1027: @code{Key} does not recognize the end of input. Words like
1028: @code{accept} echo the input and are therefore usually not useful for
1029: reading from a pipe. You have to invoke the Forth program with an OS
1030: command-line option, as you have no chance to use the Forth command line
1031: (the text interpreter would try to interpret the pipe input).
1032:
1033: @cindex output in pipes
1034: You can output to a pipe with @code{type}, @code{emit}, @code{cr} etc.
1035:
1036: @cindex silent exiting from Gforth
1037: When you write to a pipe that has been closed at the other end, Gforth
1038: receives a SIGPIPE signal (``pipe broken''). Gforth translates this
1039: into the exception @code{broken-pipe-error}. If your application does
1040: not catch that exception, the system catches it and exits, usually
1041: silently (unless you were working on the Forth command line; then it
1042: prints an error message and exits). This is usually the desired
1043: behaviour.
1044:
1045: If you do not like this behaviour, you have to catch the exception
1046: yourself, and react to it.
1047:
1048: Here's an example of an invocation of Gforth that is usable in a pipe:
1049:
1050: @example
1051: gforth -e ": foo begin pad dup 10 stdin read-file throw dup while \
1052: type repeat ; foo bye"
1053: @end example
1054:
1055: This example just copies the input verbatim to the output. A very
1056: simple pipe containing this example looks like this:
1057:
1058: @example
1059: cat startup.fs |
1060: gforth -e ": foo begin pad dup 80 stdin read-file throw dup while \
1061: type repeat ; foo bye"|
1062: head
1063: @end example
1064:
1065: @cindex stderr and pipes
1066: Pipes involving Gforth's @code{stderr} output do not work.
1067:
1068: @comment ----------------------------------------------
1069: @node Startup speed, , Gforth in pipes, Gforth Environment
1.48 anton 1070: @section Startup speed
1071: @cindex Startup speed
1072: @cindex speed, startup
1073:
1074: If Gforth is used for CGI scripts or in shell scripts, its startup
1075: speed may become a problem. On a 300MHz 21064a under Linux-2.2.13 with
1076: glibc-2.0.7, @code{gforth -e bye} takes about 24.6ms user and 11.3ms
1077: system time.
1078:
1079: If startup speed is a problem, you may consider the following ways to
1080: improve it; or you may consider ways to reduce the number of startups
1.62 crook 1081: (for example, by using Fast-CGI).
1.48 anton 1082:
1.112 anton 1083: An easy step that influences Gforth startup speed is the use of the
1084: @option{--no-dynamic} option; this decreases image loading speed, but
1085: increases compile-time and run-time.
1086:
1087: Another step to improve startup speed is to statically link Gforth, by
1.48 anton 1088: building it with @code{XLDFLAGS=-static}. This requires more memory for
1089: the code and will therefore slow down the first invocation, but
1090: subsequent invocations avoid the dynamic linking overhead. Another
1091: disadvantage is that Gforth won't profit from library upgrades. As a
1092: result, @code{gforth-static -e bye} takes about 17.1ms user and
1093: 8.2ms system time.
1094:
1095: The next step to improve startup speed is to use a non-relocatable image
1.65 anton 1096: (@pxref{Non-Relocatable Image Files}). You can create this image with
1.48 anton 1097: @code{gforth -e "savesystem gforthnr.fi bye"} and later use it with
1098: @code{gforth -i gforthnr.fi ...}. This avoids the relocation overhead
1099: and a part of the copy-on-write overhead. The disadvantage is that the
1.62 crook 1100: non-relocatable image does not work if the OS gives Gforth a different
1.48 anton 1101: address for the dictionary, for whatever reason; so you better provide a
1102: fallback on a relocatable image. @code{gforth-static -i gforthnr.fi -e
1103: bye} takes about 15.3ms user and 7.5ms system time.
1104:
1105: The final step is to disable dictionary hashing in Gforth. Gforth
1106: builds the hash table on startup, which takes much of the startup
1107: overhead. You can do this by commenting out the @code{include hash.fs}
1108: in @file{startup.fs} and everything that requires @file{hash.fs} (at the
1109: moment @file{table.fs} and @file{ekey.fs}) and then doing @code{make}.
1110: The disadvantages are that functionality like @code{table} and
1111: @code{ekey} is missing and that text interpretation (e.g., compiling)
1112: now takes much longer. So, you should only use this method if there is
1113: no significant text interpretation to perform (the script should be
1.62 crook 1114: compiled into the image, amongst other things). @code{gforth-static -i
1.48 anton 1115: gforthnrnh.fi -e bye} takes about 2.1ms user and 6.1ms system time.
1116:
1117: @c ******************************************************************
1118: @node Tutorial, Introduction, Gforth Environment, Top
1119: @chapter Forth Tutorial
1120: @cindex Tutorial
1121: @cindex Forth Tutorial
1122:
1.67 anton 1123: @c Topics from nac's Introduction that could be mentioned:
1124: @c press <ret> after each line
1125: @c Prompt
1126: @c numbers vs. words in dictionary on text interpretation
1127: @c what happens on redefinition
1128: @c parsing words (in particular, defining words)
1129:
1.83 anton 1130: The difference of this chapter from the Introduction
1131: (@pxref{Introduction}) is that this tutorial is more fast-paced, should
1132: be used while sitting in front of a computer, and covers much more
1133: material, but does not explain how the Forth system works.
1134:
1.62 crook 1135: This tutorial can be used with any ANS-compliant Forth; any
1136: Gforth-specific features are marked as such and you can skip them if you
1137: work with another Forth. This tutorial does not explain all features of
1138: Forth, just enough to get you started and give you some ideas about the
1139: facilities available in Forth. Read the rest of the manual and the
1140: standard when you are through this.
1.48 anton 1141:
1142: The intended way to use this tutorial is that you work through it while
1143: sitting in front of the console, take a look at the examples and predict
1144: what they will do, then try them out; if the outcome is not as expected,
1145: find out why (e.g., by trying out variations of the example), so you
1146: understand what's going on. There are also some assignments that you
1147: should solve.
1148:
1149: This tutorial assumes that you have programmed before and know what,
1150: e.g., a loop is.
1151:
1152: @c !! explain compat library
1153:
1154: @menu
1155: * Starting Gforth Tutorial::
1156: * Syntax Tutorial::
1157: * Crash Course Tutorial::
1158: * Stack Tutorial::
1159: * Arithmetics Tutorial::
1160: * Stack Manipulation Tutorial::
1161: * Using files for Forth code Tutorial::
1162: * Comments Tutorial::
1163: * Colon Definitions Tutorial::
1164: * Decompilation Tutorial::
1165: * Stack-Effect Comments Tutorial::
1166: * Types Tutorial::
1167: * Factoring Tutorial::
1168: * Designing the stack effect Tutorial::
1169: * Local Variables Tutorial::
1170: * Conditional execution Tutorial::
1171: * Flags and Comparisons Tutorial::
1172: * General Loops Tutorial::
1173: * Counted loops Tutorial::
1174: * Recursion Tutorial::
1175: * Leaving definitions or loops Tutorial::
1176: * Return Stack Tutorial::
1177: * Memory Tutorial::
1178: * Characters and Strings Tutorial::
1179: * Alignment Tutorial::
1.87 anton 1180: * Files Tutorial::
1.48 anton 1181: * Interpretation and Compilation Semantics and Immediacy Tutorial::
1182: * Execution Tokens Tutorial::
1183: * Exceptions Tutorial::
1184: * Defining Words Tutorial::
1185: * Arrays and Records Tutorial::
1186: * POSTPONE Tutorial::
1187: * Literal Tutorial::
1188: * Advanced macros Tutorial::
1189: * Compilation Tokens Tutorial::
1190: * Wordlists and Search Order Tutorial::
1191: @end menu
1192:
1193: @node Starting Gforth Tutorial, Syntax Tutorial, Tutorial, Tutorial
1194: @section Starting Gforth
1.66 anton 1195: @cindex starting Gforth tutorial
1.48 anton 1196: You can start Gforth by typing its name:
1197:
1198: @example
1199: gforth
1200: @end example
1201:
1202: That puts you into interactive mode; you can leave Gforth by typing
1203: @code{bye}. While in Gforth, you can edit the command line and access
1204: the command line history with cursor keys, similar to bash.
1205:
1206:
1207: @node Syntax Tutorial, Crash Course Tutorial, Starting Gforth Tutorial, Tutorial
1208: @section Syntax
1.66 anton 1209: @cindex syntax tutorial
1.48 anton 1210:
1211: A @dfn{word} is a sequence of arbitrary characters (expcept white
1212: space). Words are separated by white space. E.g., each of the
1213: following lines contains exactly one word:
1214:
1215: @example
1216: word
1217: !@@#$%^&*()
1218: 1234567890
1219: 5!a
1220: @end example
1221:
1222: A frequent beginner's error is to leave away necessary white space,
1223: resulting in an error like @samp{Undefined word}; so if you see such an
1224: error, check if you have put spaces wherever necessary.
1225:
1226: @example
1227: ." hello, world" \ correct
1228: ."hello, world" \ gives an "Undefined word" error
1229: @end example
1230:
1.65 anton 1231: Gforth and most other Forth systems ignore differences in case (they are
1.48 anton 1232: case-insensitive), i.e., @samp{word} is the same as @samp{Word}. If
1233: your system is case-sensitive, you may have to type all the examples
1234: given here in upper case.
1235:
1236:
1237: @node Crash Course Tutorial, Stack Tutorial, Syntax Tutorial, Tutorial
1238: @section Crash Course
1239:
1240: Type
1241:
1242: @example
1243: 0 0 !
1244: here execute
1245: ' catch >body 20 erase abort
1246: ' (quit) >body 20 erase
1247: @end example
1248:
1249: The last two examples are guaranteed to destroy parts of Gforth (and
1250: most other systems), so you better leave Gforth afterwards (if it has
1251: not finished by itself). On some systems you may have to kill gforth
1252: from outside (e.g., in Unix with @code{kill}).
1253:
1254: Now that you know how to produce crashes (and that there's not much to
1255: them), let's learn how to produce meaningful programs.
1256:
1257:
1258: @node Stack Tutorial, Arithmetics Tutorial, Crash Course Tutorial, Tutorial
1259: @section Stack
1.66 anton 1260: @cindex stack tutorial
1.48 anton 1261:
1262: The most obvious feature of Forth is the stack. When you type in a
1263: number, it is pushed on the stack. You can display the content of the
1264: stack with @code{.s}.
1265:
1266: @example
1267: 1 2 .s
1268: 3 .s
1269: @end example
1270:
1271: @code{.s} displays the top-of-stack to the right, i.e., the numbers
1272: appear in @code{.s} output as they appeared in the input.
1273:
1274: You can print the top of stack element with @code{.}.
1275:
1276: @example
1277: 1 2 3 . . .
1278: @end example
1279:
1280: In general, words consume their stack arguments (@code{.s} is an
1281: exception).
1282:
1283: @assignment
1284: What does the stack contain after @code{5 6 7 .}?
1285: @endassignment
1286:
1287:
1288: @node Arithmetics Tutorial, Stack Manipulation Tutorial, Stack Tutorial, Tutorial
1289: @section Arithmetics
1.66 anton 1290: @cindex arithmetics tutorial
1.48 anton 1291:
1292: The words @code{+}, @code{-}, @code{*}, @code{/}, and @code{mod} always
1293: operate on the top two stack items:
1294:
1295: @example
1.67 anton 1296: 2 2 .s
1297: + .s
1298: .
1.48 anton 1299: 2 1 - .
1300: 7 3 mod .
1301: @end example
1302:
1303: The operands of @code{-}, @code{/}, and @code{mod} are in the same order
1304: as in the corresponding infix expression (this is generally the case in
1305: Forth).
1306:
1307: Parentheses are superfluous (and not available), because the order of
1308: the words unambiguously determines the order of evaluation and the
1309: operands:
1310:
1311: @example
1312: 3 4 + 5 * .
1313: 3 4 5 * + .
1314: @end example
1315:
1316: @assignment
1317: What are the infix expressions corresponding to the Forth code above?
1318: Write @code{6-7*8+9} in Forth notation@footnote{This notation is also
1319: known as Postfix or RPN (Reverse Polish Notation).}.
1320: @endassignment
1321:
1322: To change the sign, use @code{negate}:
1323:
1324: @example
1325: 2 negate .
1326: @end example
1327:
1328: @assignment
1329: Convert -(-3)*4-5 to Forth.
1330: @endassignment
1331:
1332: @code{/mod} performs both @code{/} and @code{mod}.
1333:
1334: @example
1335: 7 3 /mod . .
1336: @end example
1337:
1.66 anton 1338: Reference: @ref{Arithmetic}.
1339:
1340:
1.48 anton 1341: @node Stack Manipulation Tutorial, Using files for Forth code Tutorial, Arithmetics Tutorial, Tutorial
1342: @section Stack Manipulation
1.66 anton 1343: @cindex stack manipulation tutorial
1.48 anton 1344:
1345: Stack manipulation words rearrange the data on the stack.
1346:
1347: @example
1348: 1 .s drop .s
1349: 1 .s dup .s drop drop .s
1350: 1 2 .s over .s drop drop drop
1351: 1 2 .s swap .s drop drop
1352: 1 2 3 .s rot .s drop drop drop
1353: @end example
1354:
1355: These are the most important stack manipulation words. There are also
1356: variants that manipulate twice as many stack items:
1357:
1358: @example
1359: 1 2 3 4 .s 2swap .s 2drop 2drop
1360: @end example
1361:
1362: Two more stack manipulation words are:
1363:
1364: @example
1365: 1 2 .s nip .s drop
1366: 1 2 .s tuck .s 2drop drop
1367: @end example
1368:
1369: @assignment
1370: Replace @code{nip} and @code{tuck} with combinations of other stack
1371: manipulation words.
1372:
1373: @example
1374: Given: How do you get:
1375: 1 2 3 3 2 1
1376: 1 2 3 1 2 3 2
1377: 1 2 3 1 2 3 3
1378: 1 2 3 1 3 3
1379: 1 2 3 2 1 3
1380: 1 2 3 4 4 3 2 1
1381: 1 2 3 1 2 3 1 2 3
1382: 1 2 3 4 1 2 3 4 1 2
1383: 1 2 3
1384: 1 2 3 1 2 3 4
1385: 1 2 3 1 3
1386: @end example
1387: @endassignment
1388:
1389: @example
1390: 5 dup * .
1391: @end example
1392:
1393: @assignment
1394: Write 17^3 and 17^4 in Forth, without writing @code{17} more than once.
1395: Write a piece of Forth code that expects two numbers on the stack
1396: (@var{a} and @var{b}, with @var{b} on top) and computes
1397: @code{(a-b)(a+1)}.
1398: @endassignment
1399:
1.66 anton 1400: Reference: @ref{Stack Manipulation}.
1401:
1402:
1.48 anton 1403: @node Using files for Forth code Tutorial, Comments Tutorial, Stack Manipulation Tutorial, Tutorial
1404: @section Using files for Forth code
1.66 anton 1405: @cindex loading Forth code, tutorial
1406: @cindex files containing Forth code, tutorial
1.48 anton 1407:
1408: While working at the Forth command line is convenient for one-line
1409: examples and short one-off code, you probably want to store your source
1410: code in files for convenient editing and persistence. You can use your
1411: favourite editor (Gforth includes Emacs support, @pxref{Emacs and
1.102 anton 1412: Gforth}) to create @var{file.fs} and use
1.48 anton 1413:
1414: @example
1.102 anton 1415: s" @var{file.fs}" included
1.48 anton 1416: @end example
1417:
1418: to load it into your Forth system. The file name extension I use for
1419: Forth files is @samp{.fs}.
1420:
1421: You can easily start Gforth with some files loaded like this:
1422:
1423: @example
1.102 anton 1424: gforth @var{file1.fs} @var{file2.fs}
1.48 anton 1425: @end example
1426:
1427: If an error occurs during loading these files, Gforth terminates,
1428: whereas an error during @code{INCLUDED} within Gforth usually gives you
1429: a Gforth command line. Starting the Forth system every time gives you a
1430: clean start every time, without interference from the results of earlier
1431: tries.
1432:
1433: I often put all the tests in a file, then load the code and run the
1434: tests with
1435:
1436: @example
1.102 anton 1437: gforth @var{code.fs} @var{tests.fs} -e bye
1.48 anton 1438: @end example
1439:
1440: (often by performing this command with @kbd{C-x C-e} in Emacs). The
1441: @code{-e bye} ensures that Gforth terminates afterwards so that I can
1442: restart this command without ado.
1443:
1444: The advantage of this approach is that the tests can be repeated easily
1445: every time the program ist changed, making it easy to catch bugs
1446: introduced by the change.
1447:
1.66 anton 1448: Reference: @ref{Forth source files}.
1449:
1.48 anton 1450:
1451: @node Comments Tutorial, Colon Definitions Tutorial, Using files for Forth code Tutorial, Tutorial
1452: @section Comments
1.66 anton 1453: @cindex comments tutorial
1.48 anton 1454:
1455: @example
1456: \ That's a comment; it ends at the end of the line
1457: ( Another comment; it ends here: ) .s
1458: @end example
1459:
1460: @code{\} and @code{(} are ordinary Forth words and therefore have to be
1461: separated with white space from the following text.
1462:
1463: @example
1464: \This gives an "Undefined word" error
1465: @end example
1466:
1467: The first @code{)} ends a comment started with @code{(}, so you cannot
1468: nest @code{(}-comments; and you cannot comment out text containing a
1469: @code{)} with @code{( ... )}@footnote{therefore it's a good idea to
1470: avoid @code{)} in word names.}.
1471:
1472: I use @code{\}-comments for descriptive text and for commenting out code
1473: of one or more line; I use @code{(}-comments for describing the stack
1474: effect, the stack contents, or for commenting out sub-line pieces of
1475: code.
1476:
1477: The Emacs mode @file{gforth.el} (@pxref{Emacs and Gforth}) supports
1478: these uses by commenting out a region with @kbd{C-x \}, uncommenting a
1479: region with @kbd{C-u C-x \}, and filling a @code{\}-commented region
1480: with @kbd{M-q}.
1481:
1.66 anton 1482: Reference: @ref{Comments}.
1483:
1.48 anton 1484:
1485: @node Colon Definitions Tutorial, Decompilation Tutorial, Comments Tutorial, Tutorial
1486: @section Colon Definitions
1.66 anton 1487: @cindex colon definitions, tutorial
1488: @cindex definitions, tutorial
1489: @cindex procedures, tutorial
1490: @cindex functions, tutorial
1.48 anton 1491:
1492: are similar to procedures and functions in other programming languages.
1493:
1494: @example
1495: : squared ( n -- n^2 )
1496: dup * ;
1497: 5 squared .
1498: 7 squared .
1499: @end example
1500:
1501: @code{:} starts the colon definition; its name is @code{squared}. The
1502: following comment describes its stack effect. The words @code{dup *}
1503: are not executed, but compiled into the definition. @code{;} ends the
1504: colon definition.
1505:
1506: The newly-defined word can be used like any other word, including using
1507: it in other definitions:
1508:
1509: @example
1510: : cubed ( n -- n^3 )
1511: dup squared * ;
1512: -5 cubed .
1513: : fourth-power ( n -- n^4 )
1514: squared squared ;
1515: 3 fourth-power .
1516: @end example
1517:
1518: @assignment
1519: Write colon definitions for @code{nip}, @code{tuck}, @code{negate}, and
1520: @code{/mod} in terms of other Forth words, and check if they work (hint:
1521: test your tests on the originals first). Don't let the
1522: @samp{redefined}-Messages spook you, they are just warnings.
1523: @endassignment
1524:
1.66 anton 1525: Reference: @ref{Colon Definitions}.
1526:
1.48 anton 1527:
1528: @node Decompilation Tutorial, Stack-Effect Comments Tutorial, Colon Definitions Tutorial, Tutorial
1529: @section Decompilation
1.66 anton 1530: @cindex decompilation tutorial
1531: @cindex see tutorial
1.48 anton 1532:
1533: You can decompile colon definitions with @code{see}:
1534:
1535: @example
1536: see squared
1537: see cubed
1538: @end example
1539:
1540: In Gforth @code{see} shows you a reconstruction of the source code from
1541: the executable code. Informations that were present in the source, but
1542: not in the executable code, are lost (e.g., comments).
1543:
1.65 anton 1544: You can also decompile the predefined words:
1545:
1546: @example
1547: see .
1548: see +
1549: @end example
1550:
1551:
1.48 anton 1552: @node Stack-Effect Comments Tutorial, Types Tutorial, Decompilation Tutorial, Tutorial
1553: @section Stack-Effect Comments
1.66 anton 1554: @cindex stack-effect comments, tutorial
1555: @cindex --, tutorial
1.48 anton 1556: By convention the comment after the name of a definition describes the
1557: stack effect: The part in from of the @samp{--} describes the state of
1558: the stack before the execution of the definition, i.e., the parameters
1559: that are passed into the colon definition; the part behind the @samp{--}
1560: is the state of the stack after the execution of the definition, i.e.,
1561: the results of the definition. The stack comment only shows the top
1562: stack items that the definition accesses and/or changes.
1563:
1564: You should put a correct stack effect on every definition, even if it is
1565: just @code{( -- )}. You should also add some descriptive comment to
1566: more complicated words (I usually do this in the lines following
1567: @code{:}). If you don't do this, your code becomes unreadable (because
1.117 anton 1568: you have to work through every definition before you can understand
1.48 anton 1569: any).
1570:
1571: @assignment
1572: The stack effect of @code{swap} can be written like this: @code{x1 x2 --
1573: x2 x1}. Describe the stack effect of @code{-}, @code{drop}, @code{dup},
1574: @code{over}, @code{rot}, @code{nip}, and @code{tuck}. Hint: When you
1.65 anton 1575: are done, you can compare your stack effects to those in this manual
1.48 anton 1576: (@pxref{Word Index}).
1577: @endassignment
1578:
1579: Sometimes programmers put comments at various places in colon
1580: definitions that describe the contents of the stack at that place (stack
1581: comments); i.e., they are like the first part of a stack-effect
1582: comment. E.g.,
1583:
1584: @example
1585: : cubed ( n -- n^3 )
1586: dup squared ( n n^2 ) * ;
1587: @end example
1588:
1589: In this case the stack comment is pretty superfluous, because the word
1590: is simple enough. If you think it would be a good idea to add such a
1591: comment to increase readability, you should also consider factoring the
1592: word into several simpler words (@pxref{Factoring Tutorial,,
1.60 anton 1593: Factoring}), which typically eliminates the need for the stack comment;
1.48 anton 1594: however, if you decide not to refactor it, then having such a comment is
1595: better than not having it.
1596:
1597: The names of the stack items in stack-effect and stack comments in the
1598: standard, in this manual, and in many programs specify the type through
1599: a type prefix, similar to Fortran and Hungarian notation. The most
1600: frequent prefixes are:
1601:
1602: @table @code
1603: @item n
1604: signed integer
1605: @item u
1606: unsigned integer
1607: @item c
1608: character
1609: @item f
1610: Boolean flags, i.e. @code{false} or @code{true}.
1611: @item a-addr,a-
1612: Cell-aligned address
1613: @item c-addr,c-
1614: Char-aligned address (note that a Char may have two bytes in Windows NT)
1615: @item xt
1616: Execution token, same size as Cell
1617: @item w,x
1618: Cell, can contain an integer or an address. It usually takes 32, 64 or
1619: 16 bits (depending on your platform and Forth system). A cell is more
1620: commonly known as machine word, but the term @emph{word} already means
1621: something different in Forth.
1622: @item d
1623: signed double-cell integer
1624: @item ud
1625: unsigned double-cell integer
1626: @item r
1627: Float (on the FP stack)
1628: @end table
1629:
1630: You can find a more complete list in @ref{Notation}.
1631:
1632: @assignment
1633: Write stack-effect comments for all definitions you have written up to
1634: now.
1635: @endassignment
1636:
1637:
1638: @node Types Tutorial, Factoring Tutorial, Stack-Effect Comments Tutorial, Tutorial
1639: @section Types
1.66 anton 1640: @cindex types tutorial
1.48 anton 1641:
1642: In Forth the names of the operations are not overloaded; so similar
1643: operations on different types need different names; e.g., @code{+} adds
1644: integers, and you have to use @code{f+} to add floating-point numbers.
1645: The following prefixes are often used for related operations on
1646: different types:
1647:
1648: @table @code
1649: @item (none)
1650: signed integer
1651: @item u
1652: unsigned integer
1653: @item c
1654: character
1655: @item d
1656: signed double-cell integer
1657: @item ud, du
1658: unsigned double-cell integer
1659: @item 2
1660: two cells (not-necessarily double-cell numbers)
1661: @item m, um
1662: mixed single-cell and double-cell operations
1663: @item f
1664: floating-point (note that in stack comments @samp{f} represents flags,
1.66 anton 1665: and @samp{r} represents FP numbers).
1.48 anton 1666: @end table
1667:
1668: If there are no differences between the signed and the unsigned variant
1669: (e.g., for @code{+}), there is only the prefix-less variant.
1670:
1671: Forth does not perform type checking, neither at compile time, nor at
1672: run time. If you use the wrong oeration, the data are interpreted
1673: incorrectly:
1674:
1675: @example
1676: -1 u.
1677: @end example
1678:
1679: If you have only experience with type-checked languages until now, and
1680: have heard how important type-checking is, don't panic! In my
1681: experience (and that of other Forthers), type errors in Forth code are
1682: usually easy to find (once you get used to it), the increased vigilance
1683: of the programmer tends to catch some harder errors in addition to most
1684: type errors, and you never have to work around the type system, so in
1685: most situations the lack of type-checking seems to be a win (projects to
1686: add type checking to Forth have not caught on).
1687:
1688:
1689: @node Factoring Tutorial, Designing the stack effect Tutorial, Types Tutorial, Tutorial
1690: @section Factoring
1.66 anton 1691: @cindex factoring tutorial
1.48 anton 1692:
1693: If you try to write longer definitions, you will soon find it hard to
1694: keep track of the stack contents. Therefore, good Forth programmers
1695: tend to write only short definitions (e.g., three lines). The art of
1696: finding meaningful short definitions is known as factoring (as in
1697: factoring polynomials).
1698:
1699: Well-factored programs offer additional advantages: smaller, more
1700: general words, are easier to test and debug and can be reused more and
1701: better than larger, specialized words.
1702:
1703: So, if you run into difficulties with stack management, when writing
1704: code, try to define meaningful factors for the word, and define the word
1705: in terms of those. Even if a factor contains only two words, it is
1706: often helpful.
1707:
1.65 anton 1708: Good factoring is not easy, and it takes some practice to get the knack
1709: for it; but even experienced Forth programmers often don't find the
1710: right solution right away, but only when rewriting the program. So, if
1711: you don't come up with a good solution immediately, keep trying, don't
1712: despair.
1.48 anton 1713:
1714: @c example !!
1715:
1716:
1717: @node Designing the stack effect Tutorial, Local Variables Tutorial, Factoring Tutorial, Tutorial
1718: @section Designing the stack effect
1.66 anton 1719: @cindex Stack effect design, tutorial
1720: @cindex design of stack effects, tutorial
1.48 anton 1721:
1722: In other languages you can use an arbitrary order of parameters for a
1.65 anton 1723: function; and since there is only one result, you don't have to deal with
1.48 anton 1724: the order of results, either.
1725:
1.117 anton 1726: In Forth (and other stack-based languages, e.g., PostScript) the
1.48 anton 1727: parameter and result order of a definition is important and should be
1728: designed well. The general guideline is to design the stack effect such
1729: that the word is simple to use in most cases, even if that complicates
1730: the implementation of the word. Some concrete rules are:
1731:
1732: @itemize @bullet
1733:
1734: @item
1735: Words consume all of their parameters (e.g., @code{.}).
1736:
1737: @item
1738: If there is a convention on the order of parameters (e.g., from
1739: mathematics or another programming language), stick with it (e.g.,
1740: @code{-}).
1741:
1742: @item
1743: If one parameter usually requires only a short computation (e.g., it is
1744: a constant), pass it on the top of the stack. Conversely, parameters
1745: that usually require a long sequence of code to compute should be passed
1746: as the bottom (i.e., first) parameter. This makes the code easier to
1747: read, because reader does not need to keep track of the bottom item
1748: through a long sequence of code (or, alternatively, through stack
1.49 anton 1749: manipulations). E.g., @code{!} (store, @pxref{Memory}) expects the
1.48 anton 1750: address on top of the stack because it is usually simpler to compute
1751: than the stored value (often the address is just a variable).
1752:
1753: @item
1754: Similarly, results that are usually consumed quickly should be returned
1755: on the top of stack, whereas a result that is often used in long
1756: computations should be passed as bottom result. E.g., the file words
1757: like @code{open-file} return the error code on the top of stack, because
1758: it is usually consumed quickly by @code{throw}; moreover, the error code
1759: has to be checked before doing anything with the other results.
1760:
1761: @end itemize
1762:
1763: These rules are just general guidelines, don't lose sight of the overall
1764: goal to make the words easy to use. E.g., if the convention rule
1765: conflicts with the computation-length rule, you might decide in favour
1766: of the convention if the word will be used rarely, and in favour of the
1767: computation-length rule if the word will be used frequently (because
1768: with frequent use the cost of breaking the computation-length rule would
1769: be quite high, and frequent use makes it easier to remember an
1770: unconventional order).
1771:
1772: @c example !! structure package
1773:
1.65 anton 1774:
1.48 anton 1775: @node Local Variables Tutorial, Conditional execution Tutorial, Designing the stack effect Tutorial, Tutorial
1776: @section Local Variables
1.66 anton 1777: @cindex local variables, tutorial
1.48 anton 1778:
1779: You can define local variables (@emph{locals}) in a colon definition:
1780:
1781: @example
1782: : swap @{ a b -- b a @}
1783: b a ;
1784: 1 2 swap .s 2drop
1785: @end example
1786:
1787: (If your Forth system does not support this syntax, include
1788: @file{compat/anslocals.fs} first).
1789:
1790: In this example @code{@{ a b -- b a @}} is the locals definition; it
1791: takes two cells from the stack, puts the top of stack in @code{b} and
1792: the next stack element in @code{a}. @code{--} starts a comment ending
1793: with @code{@}}. After the locals definition, using the name of the
1794: local will push its value on the stack. You can leave the comment
1795: part (@code{-- b a}) away:
1796:
1797: @example
1798: : swap ( x1 x2 -- x2 x1 )
1799: @{ a b @} b a ;
1800: @end example
1801:
1802: In Gforth you can have several locals definitions, anywhere in a colon
1803: definition; in contrast, in a standard program you can have only one
1804: locals definition per colon definition, and that locals definition must
1805: be outside any controll structure.
1806:
1807: With locals you can write slightly longer definitions without running
1808: into stack trouble. However, I recommend trying to write colon
1809: definitions without locals for exercise purposes to help you gain the
1810: essential factoring skills.
1811:
1812: @assignment
1813: Rewrite your definitions until now with locals
1814: @endassignment
1815:
1.66 anton 1816: Reference: @ref{Locals}.
1817:
1.48 anton 1818:
1819: @node Conditional execution Tutorial, Flags and Comparisons Tutorial, Local Variables Tutorial, Tutorial
1820: @section Conditional execution
1.66 anton 1821: @cindex conditionals, tutorial
1822: @cindex if, tutorial
1.48 anton 1823:
1824: In Forth you can use control structures only inside colon definitions.
1825: An @code{if}-structure looks like this:
1826:
1827: @example
1828: : abs ( n1 -- +n2 )
1829: dup 0 < if
1830: negate
1831: endif ;
1832: 5 abs .
1833: -5 abs .
1834: @end example
1835:
1836: @code{if} takes a flag from the stack. If the flag is non-zero (true),
1837: the following code is performed, otherwise execution continues after the
1.51 pazsan 1838: @code{endif} (or @code{else}). @code{<} compares the top two stack
1.48 anton 1839: elements and prioduces a flag:
1840:
1841: @example
1842: 1 2 < .
1843: 2 1 < .
1844: 1 1 < .
1845: @end example
1846:
1847: Actually the standard name for @code{endif} is @code{then}. This
1848: tutorial presents the examples using @code{endif}, because this is often
1849: less confusing for people familiar with other programming languages
1850: where @code{then} has a different meaning. If your system does not have
1851: @code{endif}, define it with
1852:
1853: @example
1854: : endif postpone then ; immediate
1855: @end example
1856:
1857: You can optionally use an @code{else}-part:
1858:
1859: @example
1860: : min ( n1 n2 -- n )
1861: 2dup < if
1862: drop
1863: else
1864: nip
1865: endif ;
1866: 2 3 min .
1867: 3 2 min .
1868: @end example
1869:
1870: @assignment
1871: Write @code{min} without @code{else}-part (hint: what's the definition
1872: of @code{nip}?).
1873: @endassignment
1874:
1.66 anton 1875: Reference: @ref{Selection}.
1876:
1.48 anton 1877:
1878: @node Flags and Comparisons Tutorial, General Loops Tutorial, Conditional execution Tutorial, Tutorial
1879: @section Flags and Comparisons
1.66 anton 1880: @cindex flags tutorial
1881: @cindex comparison tutorial
1.48 anton 1882:
1883: In a false-flag all bits are clear (0 when interpreted as integer). In
1884: a canonical true-flag all bits are set (-1 as a twos-complement signed
1885: integer); in many contexts (e.g., @code{if}) any non-zero value is
1886: treated as true flag.
1887:
1888: @example
1889: false .
1890: true .
1891: true hex u. decimal
1892: @end example
1893:
1894: Comparison words produce canonical flags:
1895:
1896: @example
1897: 1 1 = .
1898: 1 0= .
1899: 0 1 < .
1900: 0 0 < .
1901: -1 1 u< . \ type error, u< interprets -1 as large unsigned number
1902: -1 1 < .
1903: @end example
1904:
1.66 anton 1905: Gforth supports all combinations of the prefixes @code{0 u d d0 du f f0}
1906: (or none) and the comparisons @code{= <> < > <= >=}. Only a part of
1907: these combinations are standard (for details see the standard,
1908: @ref{Numeric comparison}, @ref{Floating Point} or @ref{Word Index}).
1.48 anton 1909:
1910: You can use @code{and or xor invert} can be used as operations on
1911: canonical flags. Actually they are bitwise operations:
1912:
1913: @example
1914: 1 2 and .
1915: 1 2 or .
1916: 1 3 xor .
1917: 1 invert .
1918: @end example
1919:
1920: You can convert a zero/non-zero flag into a canonical flag with
1921: @code{0<>} (and complement it on the way with @code{0=}).
1922:
1923: @example
1924: 1 0= .
1925: 1 0<> .
1926: @end example
1927:
1.65 anton 1928: You can use the all-bits-set feature of canonical flags and the bitwise
1.48 anton 1929: operation of the Boolean operations to avoid @code{if}s:
1930:
1931: @example
1932: : foo ( n1 -- n2 )
1933: 0= if
1934: 14
1935: else
1936: 0
1937: endif ;
1938: 0 foo .
1939: 1 foo .
1940:
1941: : foo ( n1 -- n2 )
1942: 0= 14 and ;
1943: 0 foo .
1944: 1 foo .
1945: @end example
1946:
1947: @assignment
1948: Write @code{min} without @code{if}.
1949: @endassignment
1950:
1.66 anton 1951: For reference, see @ref{Boolean Flags}, @ref{Numeric comparison}, and
1952: @ref{Bitwise operations}.
1953:
1.48 anton 1954:
1955: @node General Loops Tutorial, Counted loops Tutorial, Flags and Comparisons Tutorial, Tutorial
1956: @section General Loops
1.66 anton 1957: @cindex loops, indefinite, tutorial
1.48 anton 1958:
1959: The endless loop is the most simple one:
1960:
1961: @example
1962: : endless ( -- )
1963: 0 begin
1964: dup . 1+
1965: again ;
1966: endless
1967: @end example
1968:
1969: Terminate this loop by pressing @kbd{Ctrl-C} (in Gforth). @code{begin}
1970: does nothing at run-time, @code{again} jumps back to @code{begin}.
1971:
1972: A loop with one exit at any place looks like this:
1973:
1974: @example
1975: : log2 ( +n1 -- n2 )
1976: \ logarithmus dualis of n1>0, rounded down to the next integer
1977: assert( dup 0> )
1978: 2/ 0 begin
1979: over 0> while
1980: 1+ swap 2/ swap
1981: repeat
1982: nip ;
1983: 7 log2 .
1984: 8 log2 .
1985: @end example
1986:
1987: At run-time @code{while} consumes a flag; if it is 0, execution
1.51 pazsan 1988: continues behind the @code{repeat}; if the flag is non-zero, execution
1.48 anton 1989: continues behind the @code{while}. @code{Repeat} jumps back to
1990: @code{begin}, just like @code{again}.
1991:
1992: In Forth there are many combinations/abbreviations, like @code{1+}.
1.90 anton 1993: However, @code{2/} is not one of them; it shifts its argument right by
1.48 anton 1994: one bit (arithmetic shift right):
1995:
1996: @example
1997: -5 2 / .
1998: -5 2/ .
1999: @end example
2000:
2001: @code{assert(} is no standard word, but you can get it on systems other
2002: then Gforth by including @file{compat/assert.fs}. You can see what it
2003: does by trying
2004:
2005: @example
2006: 0 log2 .
2007: @end example
2008:
2009: Here's a loop with an exit at the end:
2010:
2011: @example
2012: : log2 ( +n1 -- n2 )
2013: \ logarithmus dualis of n1>0, rounded down to the next integer
2014: assert( dup 0 > )
2015: -1 begin
2016: 1+ swap 2/ swap
2017: over 0 <=
2018: until
2019: nip ;
2020: @end example
2021:
2022: @code{Until} consumes a flag; if it is non-zero, execution continues at
2023: the @code{begin}, otherwise after the @code{until}.
2024:
2025: @assignment
2026: Write a definition for computing the greatest common divisor.
2027: @endassignment
2028:
1.66 anton 2029: Reference: @ref{Simple Loops}.
2030:
1.48 anton 2031:
2032: @node Counted loops Tutorial, Recursion Tutorial, General Loops Tutorial, Tutorial
2033: @section Counted loops
1.66 anton 2034: @cindex loops, counted, tutorial
1.48 anton 2035:
2036: @example
2037: : ^ ( n1 u -- n )
2038: \ n = the uth power of u1
2039: 1 swap 0 u+do
2040: over *
2041: loop
2042: nip ;
2043: 3 2 ^ .
2044: 4 3 ^ .
2045: @end example
2046:
2047: @code{U+do} (from @file{compat/loops.fs}, if your Forth system doesn't
2048: have it) takes two numbers of the stack @code{( u3 u4 -- )}, and then
2049: performs the code between @code{u+do} and @code{loop} for @code{u3-u4}
2050: times (or not at all, if @code{u3-u4<0}).
2051:
2052: You can see the stack effect design rules at work in the stack effect of
2053: the loop start words: Since the start value of the loop is more
2054: frequently constant than the end value, the start value is passed on
2055: the top-of-stack.
2056:
2057: You can access the counter of a counted loop with @code{i}:
2058:
2059: @example
2060: : fac ( u -- u! )
2061: 1 swap 1+ 1 u+do
2062: i *
2063: loop ;
2064: 5 fac .
2065: 7 fac .
2066: @end example
2067:
2068: There is also @code{+do}, which expects signed numbers (important for
2069: deciding whether to enter the loop).
2070:
2071: @assignment
2072: Write a definition for computing the nth Fibonacci number.
2073: @endassignment
2074:
1.65 anton 2075: You can also use increments other than 1:
2076:
2077: @example
2078: : up2 ( n1 n2 -- )
2079: +do
2080: i .
2081: 2 +loop ;
2082: 10 0 up2
2083:
2084: : down2 ( n1 n2 -- )
2085: -do
2086: i .
2087: 2 -loop ;
2088: 0 10 down2
2089: @end example
1.48 anton 2090:
1.66 anton 2091: Reference: @ref{Counted Loops}.
2092:
1.48 anton 2093:
2094: @node Recursion Tutorial, Leaving definitions or loops Tutorial, Counted loops Tutorial, Tutorial
2095: @section Recursion
1.66 anton 2096: @cindex recursion tutorial
1.48 anton 2097:
2098: Usually the name of a definition is not visible in the definition; but
2099: earlier definitions are usually visible:
2100:
2101: @example
2102: 1 0 / . \ "Floating-point unidentified fault" in Gforth on most platforms
2103: : / ( n1 n2 -- n )
2104: dup 0= if
2105: -10 throw \ report division by zero
2106: endif
2107: / \ old version
2108: ;
2109: 1 0 /
2110: @end example
2111:
2112: For recursive definitions you can use @code{recursive} (non-standard) or
2113: @code{recurse}:
2114:
2115: @example
2116: : fac1 ( n -- n! ) recursive
2117: dup 0> if
2118: dup 1- fac1 *
2119: else
2120: drop 1
2121: endif ;
2122: 7 fac1 .
2123:
2124: : fac2 ( n -- n! )
2125: dup 0> if
2126: dup 1- recurse *
2127: else
2128: drop 1
2129: endif ;
2130: 8 fac2 .
2131: @end example
2132:
2133: @assignment
2134: Write a recursive definition for computing the nth Fibonacci number.
2135: @endassignment
2136:
1.66 anton 2137: Reference (including indirect recursion): @xref{Calls and returns}.
2138:
1.48 anton 2139:
2140: @node Leaving definitions or loops Tutorial, Return Stack Tutorial, Recursion Tutorial, Tutorial
2141: @section Leaving definitions or loops
1.66 anton 2142: @cindex leaving definitions, tutorial
2143: @cindex leaving loops, tutorial
1.48 anton 2144:
2145: @code{EXIT} exits the current definition right away. For every counted
2146: loop that is left in this way, an @code{UNLOOP} has to be performed
2147: before the @code{EXIT}:
2148:
2149: @c !! real examples
2150: @example
2151: : ...
2152: ... u+do
2153: ... if
2154: ... unloop exit
2155: endif
2156: ...
2157: loop
2158: ... ;
2159: @end example
2160:
2161: @code{LEAVE} leaves the innermost counted loop right away:
2162:
2163: @example
2164: : ...
2165: ... u+do
2166: ... if
2167: ... leave
2168: endif
2169: ...
2170: loop
2171: ... ;
2172: @end example
2173:
1.65 anton 2174: @c !! example
1.48 anton 2175:
1.66 anton 2176: Reference: @ref{Calls and returns}, @ref{Counted Loops}.
2177:
2178:
1.48 anton 2179: @node Return Stack Tutorial, Memory Tutorial, Leaving definitions or loops Tutorial, Tutorial
2180: @section Return Stack
1.66 anton 2181: @cindex return stack tutorial
1.48 anton 2182:
2183: In addition to the data stack Forth also has a second stack, the return
2184: stack; most Forth systems store the return addresses of procedure calls
2185: there (thus its name). Programmers can also use this stack:
2186:
2187: @example
2188: : foo ( n1 n2 -- )
2189: .s
2190: >r .s
1.50 anton 2191: r@@ .
1.48 anton 2192: >r .s
1.50 anton 2193: r@@ .
1.48 anton 2194: r> .
1.50 anton 2195: r@@ .
1.48 anton 2196: r> . ;
2197: 1 2 foo
2198: @end example
2199:
2200: @code{>r} takes an element from the data stack and pushes it onto the
2201: return stack; conversely, @code{r>} moves an elementm from the return to
2202: the data stack; @code{r@@} pushes a copy of the top of the return stack
2203: on the return stack.
2204:
2205: Forth programmers usually use the return stack for storing data
2206: temporarily, if using the data stack alone would be too complex, and
2207: factoring and locals are not an option:
2208:
2209: @example
2210: : 2swap ( x1 x2 x3 x4 -- x3 x4 x1 x2 )
2211: rot >r rot r> ;
2212: @end example
2213:
2214: The return address of the definition and the loop control parameters of
2215: counted loops usually reside on the return stack, so you have to take
2216: all items, that you have pushed on the return stack in a colon
2217: definition or counted loop, from the return stack before the definition
2218: or loop ends. You cannot access items that you pushed on the return
2219: stack outside some definition or loop within the definition of loop.
2220:
2221: If you miscount the return stack items, this usually ends in a crash:
2222:
2223: @example
2224: : crash ( n -- )
2225: >r ;
2226: 5 crash
2227: @end example
2228:
2229: You cannot mix using locals and using the return stack (according to the
2230: standard; Gforth has no problem). However, they solve the same
2231: problems, so this shouldn't be an issue.
2232:
2233: @assignment
2234: Can you rewrite any of the definitions you wrote until now in a better
2235: way using the return stack?
2236: @endassignment
2237:
1.66 anton 2238: Reference: @ref{Return stack}.
2239:
1.48 anton 2240:
2241: @node Memory Tutorial, Characters and Strings Tutorial, Return Stack Tutorial, Tutorial
2242: @section Memory
1.66 anton 2243: @cindex memory access/allocation tutorial
1.48 anton 2244:
2245: You can create a global variable @code{v} with
2246:
2247: @example
2248: variable v ( -- addr )
2249: @end example
2250:
2251: @code{v} pushes the address of a cell in memory on the stack. This cell
2252: was reserved by @code{variable}. You can use @code{!} (store) to store
2253: values into this cell and @code{@@} (fetch) to load the value from the
2254: stack into memory:
2255:
2256: @example
2257: v .
2258: 5 v ! .s
1.50 anton 2259: v @@ .
1.48 anton 2260: @end example
2261:
1.65 anton 2262: You can see a raw dump of memory with @code{dump}:
2263:
2264: @example
2265: v 1 cells .s dump
2266: @end example
2267:
2268: @code{Cells ( n1 -- n2 )} gives you the number of bytes (or, more
2269: generally, address units (aus)) that @code{n1 cells} occupy. You can
2270: also reserve more memory:
1.48 anton 2271:
2272: @example
2273: create v2 20 cells allot
1.65 anton 2274: v2 20 cells dump
1.48 anton 2275: @end example
2276:
1.65 anton 2277: creates a word @code{v2} and reserves 20 uninitialized cells; the
2278: address pushed by @code{v2} points to the start of these 20 cells. You
2279: can use address arithmetic to access these cells:
1.48 anton 2280:
2281: @example
2282: 3 v2 5 cells + !
1.65 anton 2283: v2 20 cells dump
1.48 anton 2284: @end example
2285:
2286: You can reserve and initialize memory with @code{,}:
2287:
2288: @example
2289: create v3
2290: 5 , 4 , 3 , 2 , 1 ,
1.50 anton 2291: v3 @@ .
2292: v3 cell+ @@ .
2293: v3 2 cells + @@ .
1.65 anton 2294: v3 5 cells dump
1.48 anton 2295: @end example
2296:
2297: @assignment
2298: Write a definition @code{vsum ( addr u -- n )} that computes the sum of
2299: @code{u} cells, with the first of these cells at @code{addr}, the next
2300: one at @code{addr cell+} etc.
2301: @endassignment
2302:
2303: You can also reserve memory without creating a new word:
2304:
2305: @example
1.60 anton 2306: here 10 cells allot .
2307: here .
1.48 anton 2308: @end example
2309:
2310: @code{Here} pushes the start address of the memory area. You should
2311: store it somewhere, or you will have a hard time finding the memory area
2312: again.
2313:
2314: @code{Allot} manages dictionary memory. The dictionary memory contains
2315: the system's data structures for words etc. on Gforth and most other
2316: Forth systems. It is managed like a stack: You can free the memory that
2317: you have just @code{allot}ed with
2318:
2319: @example
2320: -10 cells allot
1.60 anton 2321: here .
1.48 anton 2322: @end example
2323:
2324: Note that you cannot do this if you have created a new word in the
2325: meantime (because then your @code{allot}ed memory is no longer on the
2326: top of the dictionary ``stack'').
2327:
2328: Alternatively, you can use @code{allocate} and @code{free} which allow
2329: freeing memory in any order:
2330:
2331: @example
2332: 10 cells allocate throw .s
2333: 20 cells allocate throw .s
2334: swap
2335: free throw
2336: free throw
2337: @end example
2338:
2339: The @code{throw}s deal with errors (e.g., out of memory).
2340:
1.65 anton 2341: And there is also a
2342: @uref{http://www.complang.tuwien.ac.at/forth/garbage-collection.zip,
2343: garbage collector}, which eliminates the need to @code{free} memory
2344: explicitly.
1.48 anton 2345:
1.66 anton 2346: Reference: @ref{Memory}.
2347:
1.48 anton 2348:
2349: @node Characters and Strings Tutorial, Alignment Tutorial, Memory Tutorial, Tutorial
2350: @section Characters and Strings
1.66 anton 2351: @cindex strings tutorial
2352: @cindex characters tutorial
1.48 anton 2353:
2354: On the stack characters take up a cell, like numbers. In memory they
2355: have their own size (one 8-bit byte on most systems), and therefore
2356: require their own words for memory access:
2357:
2358: @example
2359: create v4
2360: 104 c, 97 c, 108 c, 108 c, 111 c,
1.50 anton 2361: v4 4 chars + c@@ .
1.65 anton 2362: v4 5 chars dump
1.48 anton 2363: @end example
2364:
2365: The preferred representation of strings on the stack is @code{addr
2366: u-count}, where @code{addr} is the address of the first character and
2367: @code{u-count} is the number of characters in the string.
2368:
2369: @example
2370: v4 5 type
2371: @end example
2372:
2373: You get a string constant with
2374:
2375: @example
2376: s" hello, world" .s
2377: type
2378: @end example
2379:
2380: Make sure you have a space between @code{s"} and the string; @code{s"}
2381: is a normal Forth word and must be delimited with white space (try what
2382: happens when you remove the space).
2383:
2384: However, this interpretive use of @code{s"} is quite restricted: the
2385: string exists only until the next call of @code{s"} (some Forth systems
2386: keep more than one of these strings, but usually they still have a
1.62 crook 2387: limited lifetime).
1.48 anton 2388:
2389: @example
2390: s" hello," s" world" .s
2391: type
2392: type
2393: @end example
2394:
1.62 crook 2395: You can also use @code{s"} in a definition, and the resulting
2396: strings then live forever (well, for as long as the definition):
1.48 anton 2397:
2398: @example
2399: : foo s" hello," s" world" ;
2400: foo .s
2401: type
2402: type
2403: @end example
2404:
2405: @assignment
2406: @code{Emit ( c -- )} types @code{c} as character (not a number).
2407: Implement @code{type ( addr u -- )}.
2408: @endassignment
2409:
1.66 anton 2410: Reference: @ref{Memory Blocks}.
2411:
2412:
1.84 pazsan 2413: @node Alignment Tutorial, Files Tutorial, Characters and Strings Tutorial, Tutorial
1.48 anton 2414: @section Alignment
1.66 anton 2415: @cindex alignment tutorial
2416: @cindex memory alignment tutorial
1.48 anton 2417:
2418: On many processors cells have to be aligned in memory, if you want to
2419: access them with @code{@@} and @code{!} (and even if the processor does
1.62 crook 2420: not require alignment, access to aligned cells is faster).
1.48 anton 2421:
2422: @code{Create} aligns @code{here} (i.e., the place where the next
2423: allocation will occur, and that the @code{create}d word points to).
2424: Likewise, the memory produced by @code{allocate} starts at an aligned
2425: address. Adding a number of @code{cells} to an aligned address produces
2426: another aligned address.
2427:
2428: However, address arithmetic involving @code{char+} and @code{chars} can
2429: create an address that is not cell-aligned. @code{Aligned ( addr --
2430: a-addr )} produces the next aligned address:
2431:
2432: @example
1.50 anton 2433: v3 char+ aligned .s @@ .
2434: v3 char+ .s @@ .
1.48 anton 2435: @end example
2436:
2437: Similarly, @code{align} advances @code{here} to the next aligned
2438: address:
2439:
2440: @example
2441: create v5 97 c,
2442: here .
2443: align here .
2444: 1000 ,
2445: @end example
2446:
2447: Note that you should use aligned addresses even if your processor does
2448: not require them, if you want your program to be portable.
2449:
1.66 anton 2450: Reference: @ref{Address arithmetic}.
2451:
1.48 anton 2452:
1.84 pazsan 2453: @node Files Tutorial, Interpretation and Compilation Semantics and Immediacy Tutorial, Alignment Tutorial, Tutorial
2454: @section Files
2455: @cindex files tutorial
2456:
2457: This section gives a short introduction into how to use files inside
2458: Forth. It's broken up into five easy steps:
2459:
2460: @enumerate 1
2461: @item Opened an ASCII text file for input
2462: @item Opened a file for output
2463: @item Read input file until string matched (or some other condition matched)
2464: @item Wrote some lines from input ( modified or not) to output
2465: @item Closed the files.
2466: @end enumerate
2467:
2468: @subsection Open file for input
2469:
2470: @example
2471: s" foo.in" r/o open-file throw Value fd-in
2472: @end example
2473:
2474: @subsection Create file for output
2475:
2476: @example
2477: s" foo.out" w/o create-file throw Value fd-out
2478: @end example
2479:
2480: The available file modes are r/o for read-only access, r/w for
2481: read-write access, and w/o for write-only access. You could open both
2482: files with r/w, too, if you like. All file words return error codes; for
2483: most applications, it's best to pass there error codes with @code{throw}
2484: to the outer error handler.
2485:
2486: If you want words for opening and assigning, define them as follows:
2487:
2488: @example
2489: 0 Value fd-in
2490: 0 Value fd-out
2491: : open-input ( addr u -- ) r/o open-file throw to fd-in ;
2492: : open-output ( addr u -- ) w/o create-file throw to fd-out ;
2493: @end example
2494:
2495: Usage example:
2496:
2497: @example
2498: s" foo.in" open-input
2499: s" foo.out" open-output
2500: @end example
2501:
2502: @subsection Scan file for a particular line
2503:
2504: @example
2505: 256 Constant max-line
2506: Create line-buffer max-line 2 + allot
2507:
2508: : scan-file ( addr u -- )
2509: begin
2510: line-buffer max-line fd-in read-line throw
2511: while
2512: >r 2dup line-buffer r> compare 0=
2513: until
2514: else
2515: drop
2516: then
2517: 2drop ;
2518: @end example
2519:
2520: @code{read-line ( addr u1 fd -- u2 flag ior )} reads up to u1 bytes into
1.94 anton 2521: the buffer at addr, and returns the number of bytes read, a flag that is
2522: false when the end of file is reached, and an error code.
1.84 pazsan 2523:
2524: @code{compare ( addr1 u1 addr2 u2 -- n )} compares two strings and
2525: returns zero if both strings are equal. It returns a positive number if
2526: the first string is lexically greater, a negative if the second string
2527: is lexically greater.
2528:
2529: We haven't seen this loop here; it has two exits. Since the @code{while}
2530: exits with the number of bytes read on the stack, we have to clean up
2531: that separately; that's after the @code{else}.
2532:
2533: Usage example:
2534:
2535: @example
2536: s" The text I search is here" scan-file
2537: @end example
2538:
2539: @subsection Copy input to output
2540:
2541: @example
2542: : copy-file ( -- )
2543: begin
2544: line-buffer max-line fd-in read-line throw
2545: while
2546: line-buffer swap fd-out write-file throw
2547: repeat ;
2548: @end example
2549:
2550: @subsection Close files
2551:
2552: @example
2553: fd-in close-file throw
2554: fd-out close-file throw
2555: @end example
2556:
2557: Likewise, you can put that into definitions, too:
2558:
2559: @example
2560: : close-input ( -- ) fd-in close-file throw ;
2561: : close-output ( -- ) fd-out close-file throw ;
2562: @end example
2563:
2564: @assignment
2565: How could you modify @code{copy-file} so that it copies until a second line is
2566: matched? Can you write a program that extracts a section of a text file,
2567: given the line that starts and the line that terminates that section?
2568: @endassignment
2569:
2570: @node Interpretation and Compilation Semantics and Immediacy Tutorial, Execution Tokens Tutorial, Files Tutorial, Tutorial
1.48 anton 2571: @section Interpretation and Compilation Semantics and Immediacy
1.66 anton 2572: @cindex semantics tutorial
2573: @cindex interpretation semantics tutorial
2574: @cindex compilation semantics tutorial
2575: @cindex immediate, tutorial
1.48 anton 2576:
2577: When a word is compiled, it behaves differently from being interpreted.
2578: E.g., consider @code{+}:
2579:
2580: @example
2581: 1 2 + .
2582: : foo + ;
2583: @end example
2584:
2585: These two behaviours are known as compilation and interpretation
2586: semantics. For normal words (e.g., @code{+}), the compilation semantics
2587: is to append the interpretation semantics to the currently defined word
2588: (@code{foo} in the example above). I.e., when @code{foo} is executed
2589: later, the interpretation semantics of @code{+} (i.e., adding two
2590: numbers) will be performed.
2591:
2592: However, there are words with non-default compilation semantics, e.g.,
2593: the control-flow words like @code{if}. You can use @code{immediate} to
2594: change the compilation semantics of the last defined word to be equal to
2595: the interpretation semantics:
2596:
2597: @example
2598: : [FOO] ( -- )
2599: 5 . ; immediate
2600:
2601: [FOO]
2602: : bar ( -- )
2603: [FOO] ;
2604: bar
2605: see bar
2606: @end example
2607:
2608: Two conventions to mark words with non-default compilation semnatics are
2609: names with brackets (more frequently used) and to write them all in
2610: upper case (less frequently used).
2611:
2612: In Gforth (and many other systems) you can also remove the
2613: interpretation semantics with @code{compile-only} (the compilation
2614: semantics is derived from the original interpretation semantics):
2615:
2616: @example
2617: : flip ( -- )
2618: 6 . ; compile-only \ but not immediate
2619: flip
2620:
2621: : flop ( -- )
2622: flip ;
2623: flop
2624: @end example
2625:
2626: In this example the interpretation semantics of @code{flop} is equal to
2627: the original interpretation semantics of @code{flip}.
2628:
2629: The text interpreter has two states: in interpret state, it performs the
2630: interpretation semantics of words it encounters; in compile state, it
2631: performs the compilation semantics of these words.
2632:
2633: Among other things, @code{:} switches into compile state, and @code{;}
2634: switches back to interpret state. They contain the factors @code{]}
2635: (switch to compile state) and @code{[} (switch to interpret state), that
2636: do nothing but switch the state.
2637:
2638: @example
2639: : xxx ( -- )
2640: [ 5 . ]
2641: ;
2642:
2643: xxx
2644: see xxx
2645: @end example
2646:
2647: These brackets are also the source of the naming convention mentioned
2648: above.
2649:
1.66 anton 2650: Reference: @ref{Interpretation and Compilation Semantics}.
2651:
1.48 anton 2652:
2653: @node Execution Tokens Tutorial, Exceptions Tutorial, Interpretation and Compilation Semantics and Immediacy Tutorial, Tutorial
2654: @section Execution Tokens
1.66 anton 2655: @cindex execution tokens tutorial
2656: @cindex XT tutorial
1.48 anton 2657:
2658: @code{' word} gives you the execution token (XT) of a word. The XT is a
2659: cell representing the interpretation semantics of a word. You can
2660: execute this semantics with @code{execute}:
2661:
2662: @example
2663: ' + .s
2664: 1 2 rot execute .
2665: @end example
2666:
2667: The XT is similar to a function pointer in C. However, parameter
2668: passing through the stack makes it a little more flexible:
2669:
2670: @example
2671: : map-array ( ... addr u xt -- ... )
1.50 anton 2672: \ executes xt ( ... x -- ... ) for every element of the array starting
2673: \ at addr and containing u elements
1.48 anton 2674: @{ xt @}
2675: cells over + swap ?do
1.50 anton 2676: i @@ xt execute
1.48 anton 2677: 1 cells +loop ;
2678:
2679: create a 3 , 4 , 2 , -1 , 4 ,
2680: a 5 ' . map-array .s
2681: 0 a 5 ' + map-array .
2682: s" max-n" environment? drop .s
2683: a 5 ' min map-array .
2684: @end example
2685:
2686: You can use map-array with the XTs of words that consume one element
2687: more than they produce. In theory you can also use it with other XTs,
2688: but the stack effect then depends on the size of the array, which is
2689: hard to understand.
2690:
1.51 pazsan 2691: Since XTs are cell-sized, you can store them in memory and manipulate
2692: them on the stack like other cells. You can also compile the XT into a
1.48 anton 2693: word with @code{compile,}:
2694:
2695: @example
2696: : foo1 ( n1 n2 -- n )
2697: [ ' + compile, ] ;
2698: see foo
2699: @end example
2700:
2701: This is non-standard, because @code{compile,} has no compilation
2702: semantics in the standard, but it works in good Forth systems. For the
2703: broken ones, use
2704:
2705: @example
2706: : [compile,] compile, ; immediate
2707:
2708: : foo1 ( n1 n2 -- n )
2709: [ ' + ] [compile,] ;
2710: see foo
2711: @end example
2712:
2713: @code{'} is a word with default compilation semantics; it parses the
2714: next word when its interpretation semantics are executed, not during
2715: compilation:
2716:
2717: @example
2718: : foo ( -- xt )
2719: ' ;
2720: see foo
2721: : bar ( ... "word" -- ... )
2722: ' execute ;
2723: see bar
1.60 anton 2724: 1 2 bar + .
1.48 anton 2725: @end example
2726:
2727: You often want to parse a word during compilation and compile its XT so
2728: it will be pushed on the stack at run-time. @code{[']} does this:
2729:
2730: @example
2731: : xt-+ ( -- xt )
2732: ['] + ;
2733: see xt-+
2734: 1 2 xt-+ execute .
2735: @end example
2736:
2737: Many programmers tend to see @code{'} and the word it parses as one
2738: unit, and expect it to behave like @code{[']} when compiled, and are
2739: confused by the actual behaviour. If you are, just remember that the
2740: Forth system just takes @code{'} as one unit and has no idea that it is
2741: a parsing word (attempts to convenience programmers in this issue have
2742: usually resulted in even worse pitfalls, see
1.66 anton 2743: @uref{http://www.complang.tuwien.ac.at/papers/ertl98.ps.gz,
2744: @code{State}-smartness---Why it is evil and How to Exorcise it}).
1.48 anton 2745:
2746: Note that the state of the interpreter does not come into play when
1.51 pazsan 2747: creating and executing XTs. I.e., even when you execute @code{'} in
1.48 anton 2748: compile state, it still gives you the interpretation semantics. And
2749: whatever that state is, @code{execute} performs the semantics
1.66 anton 2750: represented by the XT (i.e., for XTs produced with @code{'} the
2751: interpretation semantics).
2752:
2753: Reference: @ref{Tokens for Words}.
1.48 anton 2754:
2755:
2756: @node Exceptions Tutorial, Defining Words Tutorial, Execution Tokens Tutorial, Tutorial
2757: @section Exceptions
1.66 anton 2758: @cindex exceptions tutorial
1.48 anton 2759:
2760: @code{throw ( n -- )} causes an exception unless n is zero.
2761:
2762: @example
2763: 100 throw .s
2764: 0 throw .s
2765: @end example
2766:
2767: @code{catch ( ... xt -- ... n )} behaves similar to @code{execute}, but
2768: it catches exceptions and pushes the number of the exception on the
2769: stack (or 0, if the xt executed without exception). If there was an
2770: exception, the stacks have the same depth as when entering @code{catch}:
2771:
2772: @example
2773: .s
2774: 3 0 ' / catch .s
2775: 3 2 ' / catch .s
2776: @end example
2777:
2778: @assignment
2779: Try the same with @code{execute} instead of @code{catch}.
2780: @endassignment
2781:
2782: @code{Throw} always jumps to the dynamically next enclosing
2783: @code{catch}, even if it has to leave several call levels to achieve
2784: this:
2785:
2786: @example
2787: : foo 100 throw ;
2788: : foo1 foo ." after foo" ;
1.51 pazsan 2789: : bar ['] foo1 catch ;
1.60 anton 2790: bar .
1.48 anton 2791: @end example
2792:
2793: It is often important to restore a value upon leaving a definition, even
2794: if the definition is left through an exception. You can ensure this
2795: like this:
2796:
2797: @example
2798: : ...
2799: save-x
1.51 pazsan 2800: ['] word-changing-x catch ( ... n )
1.48 anton 2801: restore-x
2802: ( ... n ) throw ;
2803: @end example
2804:
1.55 anton 2805: Gforth provides an alternative syntax in addition to @code{catch}:
1.48 anton 2806: @code{try ... recover ... endtry}. If the code between @code{try} and
2807: @code{recover} has an exception, the stack depths are restored, the
2808: exception number is pushed on the stack, and the code between
2809: @code{recover} and @code{endtry} is performed. E.g., the definition for
2810: @code{catch} is
2811:
2812: @example
2813: : catch ( x1 .. xn xt -- y1 .. ym 0 / z1 .. zn error ) \ exception
2814: try
2815: execute 0
2816: recover
2817: nip
2818: endtry ;
2819: @end example
2820:
2821: The equivalent to the restoration code above is
2822:
2823: @example
2824: : ...
2825: save-x
2826: try
1.92 anton 2827: word-changing-x 0
2828: recover endtry
1.48 anton 2829: restore-x
2830: throw ;
2831: @end example
2832:
1.92 anton 2833: This works if @code{word-changing-x} does not change the stack depth,
2834: otherwise you should add some code between @code{recover} and
2835: @code{endtry} to balance the stack.
1.48 anton 2836:
1.66 anton 2837: Reference: @ref{Exception Handling}.
2838:
1.48 anton 2839:
2840: @node Defining Words Tutorial, Arrays and Records Tutorial, Exceptions Tutorial, Tutorial
2841: @section Defining Words
1.66 anton 2842: @cindex defining words tutorial
2843: @cindex does> tutorial
2844: @cindex create...does> tutorial
2845:
2846: @c before semantics?
1.48 anton 2847:
2848: @code{:}, @code{create}, and @code{variable} are definition words: They
2849: define other words. @code{Constant} is another definition word:
2850:
2851: @example
2852: 5 constant foo
2853: foo .
2854: @end example
2855:
2856: You can also use the prefixes @code{2} (double-cell) and @code{f}
2857: (floating point) with @code{variable} and @code{constant}.
2858:
2859: You can also define your own defining words. E.g.:
2860:
2861: @example
2862: : variable ( "name" -- )
2863: create 0 , ;
2864: @end example
2865:
2866: You can also define defining words that create words that do something
2867: other than just producing their address:
2868:
2869: @example
2870: : constant ( n "name" -- )
2871: create ,
2872: does> ( -- n )
1.50 anton 2873: ( addr ) @@ ;
1.48 anton 2874:
2875: 5 constant foo
2876: foo .
2877: @end example
2878:
2879: The definition of @code{constant} above ends at the @code{does>}; i.e.,
2880: @code{does>} replaces @code{;}, but it also does something else: It
2881: changes the last defined word such that it pushes the address of the
2882: body of the word and then performs the code after the @code{does>}
2883: whenever it is called.
2884:
2885: In the example above, @code{constant} uses @code{,} to store 5 into the
2886: body of @code{foo}. When @code{foo} executes, it pushes the address of
2887: the body onto the stack, then (in the code after the @code{does>})
2888: fetches the 5 from there.
2889:
2890: The stack comment near the @code{does>} reflects the stack effect of the
2891: defined word, not the stack effect of the code after the @code{does>}
2892: (the difference is that the code expects the address of the body that
2893: the stack comment does not show).
2894:
2895: You can use these definition words to do factoring in cases that involve
2896: (other) definition words. E.g., a field offset is always added to an
2897: address. Instead of defining
2898:
2899: @example
2900: 2 cells constant offset-field1
2901: @end example
2902:
2903: and using this like
2904:
2905: @example
2906: ( addr ) offset-field1 +
2907: @end example
2908:
2909: you can define a definition word
2910:
2911: @example
2912: : simple-field ( n "name" -- )
2913: create ,
2914: does> ( n1 -- n1+n )
1.50 anton 2915: ( addr ) @@ + ;
1.48 anton 2916: @end example
1.21 crook 2917:
1.48 anton 2918: Definition and use of field offsets now look like this:
1.21 crook 2919:
1.48 anton 2920: @example
2921: 2 cells simple-field field1
1.60 anton 2922: create mystruct 4 cells allot
2923: mystruct .s field1 .s drop
1.48 anton 2924: @end example
1.21 crook 2925:
1.48 anton 2926: If you want to do something with the word without performing the code
2927: after the @code{does>}, you can access the body of a @code{create}d word
2928: with @code{>body ( xt -- addr )}:
1.21 crook 2929:
1.48 anton 2930: @example
2931: : value ( n "name" -- )
2932: create ,
2933: does> ( -- n1 )
1.50 anton 2934: @@ ;
1.48 anton 2935: : to ( n "name" -- )
2936: ' >body ! ;
1.21 crook 2937:
1.48 anton 2938: 5 value foo
2939: foo .
2940: 7 to foo
2941: foo .
2942: @end example
1.21 crook 2943:
1.48 anton 2944: @assignment
2945: Define @code{defer ( "name" -- )}, which creates a word that stores an
2946: XT (at the start the XT of @code{abort}), and upon execution
2947: @code{execute}s the XT. Define @code{is ( xt "name" -- )} that stores
2948: @code{xt} into @code{name}, a word defined with @code{defer}. Indirect
2949: recursion is one application of @code{defer}.
2950: @endassignment
1.29 crook 2951:
1.66 anton 2952: Reference: @ref{User-defined Defining Words}.
2953:
2954:
1.48 anton 2955: @node Arrays and Records Tutorial, POSTPONE Tutorial, Defining Words Tutorial, Tutorial
2956: @section Arrays and Records
1.66 anton 2957: @cindex arrays tutorial
2958: @cindex records tutorial
2959: @cindex structs tutorial
1.29 crook 2960:
1.48 anton 2961: Forth has no standard words for defining data structures such as arrays
2962: and records (structs in C terminology), but you can build them yourself
2963: based on address arithmetic. You can also define words for defining
2964: arrays and records (@pxref{Defining Words Tutorial,, Defining Words}).
1.29 crook 2965:
1.48 anton 2966: One of the first projects a Forth newcomer sets out upon when learning
2967: about defining words is an array defining word (possibly for
2968: n-dimensional arrays). Go ahead and do it, I did it, too; you will
2969: learn something from it. However, don't be disappointed when you later
2970: learn that you have little use for these words (inappropriate use would
2971: be even worse). I have not yet found a set of useful array words yet;
2972: the needs are just too diverse, and named, global arrays (the result of
2973: naive use of defining words) are often not flexible enough (e.g.,
1.66 anton 2974: consider how to pass them as parameters). Another such project is a set
2975: of words to help dealing with strings.
1.29 crook 2976:
1.48 anton 2977: On the other hand, there is a useful set of record words, and it has
2978: been defined in @file{compat/struct.fs}; these words are predefined in
2979: Gforth. They are explained in depth elsewhere in this manual (see
2980: @pxref{Structures}). The @code{simple-field} example above is
2981: simplified variant of fields in this package.
1.21 crook 2982:
2983:
1.48 anton 2984: @node POSTPONE Tutorial, Literal Tutorial, Arrays and Records Tutorial, Tutorial
2985: @section @code{POSTPONE}
1.66 anton 2986: @cindex postpone tutorial
1.21 crook 2987:
1.48 anton 2988: You can compile the compilation semantics (instead of compiling the
2989: interpretation semantics) of a word with @code{POSTPONE}:
1.21 crook 2990:
1.48 anton 2991: @example
2992: : MY-+ ( Compilation: -- ; Run-time of compiled code: n1 n2 -- n )
1.51 pazsan 2993: POSTPONE + ; immediate
1.48 anton 2994: : foo ( n1 n2 -- n )
2995: MY-+ ;
2996: 1 2 foo .
2997: see foo
2998: @end example
1.21 crook 2999:
1.48 anton 3000: During the definition of @code{foo} the text interpreter performs the
3001: compilation semantics of @code{MY-+}, which performs the compilation
3002: semantics of @code{+}, i.e., it compiles @code{+} into @code{foo}.
3003:
3004: This example also displays separate stack comments for the compilation
3005: semantics and for the stack effect of the compiled code. For words with
3006: default compilation semantics these stack effects are usually not
3007: displayed; the stack effect of the compilation semantics is always
3008: @code{( -- )} for these words, the stack effect for the compiled code is
3009: the stack effect of the interpretation semantics.
3010:
3011: Note that the state of the interpreter does not come into play when
3012: performing the compilation semantics in this way. You can also perform
3013: it interpretively, e.g.:
3014:
3015: @example
3016: : foo2 ( n1 n2 -- n )
3017: [ MY-+ ] ;
3018: 1 2 foo .
3019: see foo
3020: @end example
1.21 crook 3021:
1.48 anton 3022: However, there are some broken Forth systems where this does not always
1.62 crook 3023: work, and therefore this practice was been declared non-standard in
1.48 anton 3024: 1999.
3025: @c !! repair.fs
3026:
3027: Here is another example for using @code{POSTPONE}:
1.44 crook 3028:
1.48 anton 3029: @example
3030: : MY-- ( Compilation: -- ; Run-time of compiled code: n1 n2 -- n )
3031: POSTPONE negate POSTPONE + ; immediate compile-only
3032: : bar ( n1 n2 -- n )
3033: MY-- ;
3034: 2 1 bar .
3035: see bar
3036: @end example
1.21 crook 3037:
1.48 anton 3038: You can define @code{ENDIF} in this way:
1.21 crook 3039:
1.48 anton 3040: @example
3041: : ENDIF ( Compilation: orig -- )
3042: POSTPONE then ; immediate
3043: @end example
1.21 crook 3044:
1.48 anton 3045: @assignment
3046: Write @code{MY-2DUP} that has compilation semantics equivalent to
3047: @code{2dup}, but compiles @code{over over}.
3048: @endassignment
1.29 crook 3049:
1.66 anton 3050: @c !! @xref{Macros} for reference
3051:
3052:
1.48 anton 3053: @node Literal Tutorial, Advanced macros Tutorial, POSTPONE Tutorial, Tutorial
3054: @section @code{Literal}
1.66 anton 3055: @cindex literal tutorial
1.29 crook 3056:
1.48 anton 3057: You cannot @code{POSTPONE} numbers:
1.21 crook 3058:
1.48 anton 3059: @example
3060: : [FOO] POSTPONE 500 ; immediate
1.21 crook 3061: @end example
3062:
1.48 anton 3063: Instead, you can use @code{LITERAL (compilation: n --; run-time: -- n )}:
1.29 crook 3064:
1.48 anton 3065: @example
3066: : [FOO] ( compilation: --; run-time: -- n )
3067: 500 POSTPONE literal ; immediate
1.29 crook 3068:
1.60 anton 3069: : flip [FOO] ;
1.48 anton 3070: flip .
3071: see flip
3072: @end example
1.29 crook 3073:
1.48 anton 3074: @code{LITERAL} consumes a number at compile-time (when it's compilation
3075: semantics are executed) and pushes it at run-time (when the code it
3076: compiled is executed). A frequent use of @code{LITERAL} is to compile a
3077: number computed at compile time into the current word:
1.29 crook 3078:
1.48 anton 3079: @example
3080: : bar ( -- n )
3081: [ 2 2 + ] literal ;
3082: see bar
3083: @end example
1.29 crook 3084:
1.48 anton 3085: @assignment
3086: Write @code{]L} which allows writing the example above as @code{: bar (
3087: -- n ) [ 2 2 + ]L ;}
3088: @endassignment
3089:
1.66 anton 3090: @c !! @xref{Macros} for reference
3091:
1.48 anton 3092:
3093: @node Advanced macros Tutorial, Compilation Tokens Tutorial, Literal Tutorial, Tutorial
3094: @section Advanced macros
1.66 anton 3095: @cindex macros, advanced tutorial
3096: @cindex run-time code generation, tutorial
1.48 anton 3097:
1.66 anton 3098: Reconsider @code{map-array} from @ref{Execution Tokens Tutorial,,
3099: Execution Tokens}. It frequently performs @code{execute}, a relatively
3100: expensive operation in some Forth implementations. You can use
1.48 anton 3101: @code{compile,} and @code{POSTPONE} to eliminate these @code{execute}s
3102: and produce a word that contains the word to be performed directly:
3103:
3104: @c use ]] ... [[
3105: @example
3106: : compile-map-array ( compilation: xt -- ; run-time: ... addr u -- ... )
3107: \ at run-time, execute xt ( ... x -- ... ) for each element of the
3108: \ array beginning at addr and containing u elements
3109: @{ xt @}
3110: POSTPONE cells POSTPONE over POSTPONE + POSTPONE swap POSTPONE ?do
1.50 anton 3111: POSTPONE i POSTPONE @@ xt compile,
1.48 anton 3112: 1 cells POSTPONE literal POSTPONE +loop ;
3113:
3114: : sum-array ( addr u -- n )
3115: 0 rot rot [ ' + compile-map-array ] ;
3116: see sum-array
3117: a 5 sum-array .
3118: @end example
3119:
3120: You can use the full power of Forth for generating the code; here's an
3121: example where the code is generated in a loop:
3122:
3123: @example
3124: : compile-vmul-step ( compilation: n --; run-time: n1 addr1 -- n2 addr2 )
3125: \ n2=n1+(addr1)*n, addr2=addr1+cell
1.50 anton 3126: POSTPONE tuck POSTPONE @@
1.48 anton 3127: POSTPONE literal POSTPONE * POSTPONE +
3128: POSTPONE swap POSTPONE cell+ ;
3129:
3130: : compile-vmul ( compilation: addr1 u -- ; run-time: addr2 -- n )
1.51 pazsan 3131: \ n=v1*v2 (inner product), where the v_i are represented as addr_i u
1.48 anton 3132: 0 postpone literal postpone swap
3133: [ ' compile-vmul-step compile-map-array ]
3134: postpone drop ;
3135: see compile-vmul
3136:
3137: : a-vmul ( addr -- n )
1.51 pazsan 3138: \ n=a*v, where v is a vector that's as long as a and starts at addr
1.48 anton 3139: [ a 5 compile-vmul ] ;
3140: see a-vmul
3141: a a-vmul .
3142: @end example
3143:
3144: This example uses @code{compile-map-array} to show off, but you could
1.66 anton 3145: also use @code{map-array} instead (try it now!).
1.48 anton 3146:
3147: You can use this technique for efficient multiplication of large
3148: matrices. In matrix multiplication, you multiply every line of one
3149: matrix with every column of the other matrix. You can generate the code
3150: for one line once, and use it for every column. The only downside of
3151: this technique is that it is cumbersome to recover the memory consumed
3152: by the generated code when you are done (and in more complicated cases
3153: it is not possible portably).
3154:
1.66 anton 3155: @c !! @xref{Macros} for reference
3156:
3157:
1.48 anton 3158: @node Compilation Tokens Tutorial, Wordlists and Search Order Tutorial, Advanced macros Tutorial, Tutorial
3159: @section Compilation Tokens
1.66 anton 3160: @cindex compilation tokens, tutorial
3161: @cindex CT, tutorial
1.48 anton 3162:
3163: This section is Gforth-specific. You can skip it.
3164:
3165: @code{' word compile,} compiles the interpretation semantics. For words
3166: with default compilation semantics this is the same as performing the
3167: compilation semantics. To represent the compilation semantics of other
3168: words (e.g., words like @code{if} that have no interpretation
3169: semantics), Gforth has the concept of a compilation token (CT,
3170: consisting of two cells), and words @code{comp'} and @code{[comp']}.
3171: You can perform the compilation semantics represented by a CT with
3172: @code{execute}:
1.29 crook 3173:
1.48 anton 3174: @example
3175: : foo2 ( n1 n2 -- n )
3176: [ comp' + execute ] ;
3177: see foo
3178: @end example
1.29 crook 3179:
1.48 anton 3180: You can compile the compilation semantics represented by a CT with
3181: @code{postpone,}:
1.30 anton 3182:
1.48 anton 3183: @example
3184: : foo3 ( -- )
3185: [ comp' + postpone, ] ;
3186: see foo3
3187: @end example
1.30 anton 3188:
1.51 pazsan 3189: @code{[ comp' word postpone, ]} is equivalent to @code{POSTPONE word}.
1.48 anton 3190: @code{comp'} is particularly useful for words that have no
3191: interpretation semantics:
1.29 crook 3192:
1.30 anton 3193: @example
1.48 anton 3194: ' if
1.60 anton 3195: comp' if .s 2drop
1.30 anton 3196: @end example
3197:
1.66 anton 3198: Reference: @ref{Tokens for Words}.
3199:
1.29 crook 3200:
1.48 anton 3201: @node Wordlists and Search Order Tutorial, , Compilation Tokens Tutorial, Tutorial
3202: @section Wordlists and Search Order
1.66 anton 3203: @cindex wordlists tutorial
3204: @cindex search order, tutorial
1.48 anton 3205:
3206: The dictionary is not just a memory area that allows you to allocate
3207: memory with @code{allot}, it also contains the Forth words, arranged in
3208: several wordlists. When searching for a word in a wordlist,
3209: conceptually you start searching at the youngest and proceed towards
3210: older words (in reality most systems nowadays use hash-tables); i.e., if
3211: you define a word with the same name as an older word, the new word
3212: shadows the older word.
3213:
3214: Which wordlists are searched in which order is determined by the search
3215: order. You can display the search order with @code{order}. It displays
3216: first the search order, starting with the wordlist searched first, then
3217: it displays the wordlist that will contain newly defined words.
1.21 crook 3218:
1.48 anton 3219: You can create a new, empty wordlist with @code{wordlist ( -- wid )}:
1.21 crook 3220:
1.48 anton 3221: @example
3222: wordlist constant mywords
3223: @end example
1.21 crook 3224:
1.48 anton 3225: @code{Set-current ( wid -- )} sets the wordlist that will contain newly
3226: defined words (the @emph{current} wordlist):
1.21 crook 3227:
1.48 anton 3228: @example
3229: mywords set-current
3230: order
3231: @end example
1.26 crook 3232:
1.48 anton 3233: Gforth does not display a name for the wordlist in @code{mywords}
3234: because this wordlist was created anonymously with @code{wordlist}.
1.21 crook 3235:
1.48 anton 3236: You can get the current wordlist with @code{get-current ( -- wid)}. If
3237: you want to put something into a specific wordlist without overall
3238: effect on the current wordlist, this typically looks like this:
1.21 crook 3239:
1.48 anton 3240: @example
3241: get-current mywords set-current ( wid )
3242: create someword
3243: ( wid ) set-current
3244: @end example
1.21 crook 3245:
1.48 anton 3246: You can write the search order with @code{set-order ( wid1 .. widn n --
3247: )} and read it with @code{get-order ( -- wid1 .. widn n )}. The first
3248: searched wordlist is topmost.
1.21 crook 3249:
1.48 anton 3250: @example
3251: get-order mywords swap 1+ set-order
3252: order
3253: @end example
1.21 crook 3254:
1.48 anton 3255: Yes, the order of wordlists in the output of @code{order} is reversed
3256: from stack comments and the output of @code{.s} and thus unintuitive.
1.21 crook 3257:
1.48 anton 3258: @assignment
3259: Define @code{>order ( wid -- )} with adds @code{wid} as first searched
3260: wordlist to the search order. Define @code{previous ( -- )}, which
3261: removes the first searched wordlist from the search order. Experiment
3262: with boundary conditions (you will see some crashes or situations that
3263: are hard or impossible to leave).
3264: @endassignment
1.21 crook 3265:
1.48 anton 3266: The search order is a powerful foundation for providing features similar
3267: to Modula-2 modules and C++ namespaces. However, trying to modularize
3268: programs in this way has disadvantages for debugging and reuse/factoring
3269: that overcome the advantages in my experience (I don't do huge projects,
1.55 anton 3270: though). These disadvantages are not so clear in other
1.82 anton 3271: languages/programming environments, because these languages are not so
1.48 anton 3272: strong in debugging and reuse.
1.21 crook 3273:
1.66 anton 3274: @c !! example
3275:
3276: Reference: @ref{Word Lists}.
1.21 crook 3277:
1.29 crook 3278: @c ******************************************************************
1.48 anton 3279: @node Introduction, Words, Tutorial, Top
1.29 crook 3280: @comment node-name, next, previous, up
3281: @chapter An Introduction to ANS Forth
3282: @cindex Forth - an introduction
1.21 crook 3283:
1.83 anton 3284: The difference of this chapter from the Tutorial (@pxref{Tutorial}) is
3285: that it is slower-paced in its examples, but uses them to dive deep into
3286: explaining Forth internals (not covered by the Tutorial). Apart from
3287: that, this chapter covers far less material. It is suitable for reading
3288: without using a computer.
3289:
1.29 crook 3290: The primary purpose of this manual is to document Gforth. However, since
3291: Forth is not a widely-known language and there is a lack of up-to-date
3292: teaching material, it seems worthwhile to provide some introductory
1.49 anton 3293: material. For other sources of Forth-related
3294: information, see @ref{Forth-related information}.
1.21 crook 3295:
1.29 crook 3296: The examples in this section should work on any ANS Forth; the
3297: output shown was produced using Gforth. Each example attempts to
3298: reproduce the exact output that Gforth produces. If you try out the
3299: examples (and you should), what you should type is shown @kbd{like this}
3300: and Gforth's response is shown @code{like this}. The single exception is
1.30 anton 3301: that, where the example shows @key{RET} it means that you should
1.29 crook 3302: press the ``carriage return'' key. Unfortunately, some output formats for
3303: this manual cannot show the difference between @kbd{this} and
3304: @code{this} which will make trying out the examples harder (but not
3305: impossible).
1.21 crook 3306:
1.29 crook 3307: Forth is an unusual language. It provides an interactive development
3308: environment which includes both an interpreter and compiler. Forth
3309: programming style encourages you to break a problem down into many
3310: @cindex factoring
3311: small fragments (@dfn{factoring}), and then to develop and test each
3312: fragment interactively. Forth advocates assert that breaking the
3313: edit-compile-test cycle used by conventional programming languages can
3314: lead to great productivity improvements.
1.21 crook 3315:
1.29 crook 3316: @menu
1.67 anton 3317: * Introducing the Text Interpreter::
3318: * Stacks and Postfix notation::
3319: * Your first definition::
3320: * How does that work?::
3321: * Forth is written in Forth::
3322: * Review - elements of a Forth system::
3323: * Where to go next::
3324: * Exercises::
1.29 crook 3325: @end menu
1.21 crook 3326:
1.29 crook 3327: @comment ----------------------------------------------
3328: @node Introducing the Text Interpreter, Stacks and Postfix notation, Introduction, Introduction
3329: @section Introducing the Text Interpreter
3330: @cindex text interpreter
3331: @cindex outer interpreter
1.21 crook 3332:
1.30 anton 3333: @c IMO this is too detailed and the pace is too slow for
3334: @c an introduction. If you know German, take a look at
3335: @c http://www.complang.tuwien.ac.at/anton/lvas/skriptum-stack.html
3336: @c to see how I do it - anton
3337:
1.44 crook 3338: @c nac-> Where I have accepted your comments 100% and modified the text
3339: @c accordingly, I have deleted your comments. Elsewhere I have added a
3340: @c response like this to attempt to rationalise what I have done. Of
3341: @c course, this is a very clumsy mechanism for something that would be
3342: @c done far more efficiently over a beer. Please delete any dialogue
3343: @c you consider closed.
3344:
1.29 crook 3345: When you invoke the Forth image, you will see a startup banner printed
3346: and nothing else (if you have Gforth installed on your system, try
1.30 anton 3347: invoking it now, by typing @kbd{gforth@key{RET}}). Forth is now running
1.29 crook 3348: its command line interpreter, which is called the @dfn{Text Interpreter}
3349: (also known as the @dfn{Outer Interpreter}). (You will learn a lot
1.49 anton 3350: about the text interpreter as you read through this chapter, for more
3351: detail @pxref{The Text Interpreter}).
1.21 crook 3352:
1.29 crook 3353: Although it's not obvious, Forth is actually waiting for your
1.30 anton 3354: input. Type a number and press the @key{RET} key:
1.21 crook 3355:
1.26 crook 3356: @example
1.30 anton 3357: @kbd{45@key{RET}} ok
1.26 crook 3358: @end example
1.21 crook 3359:
1.29 crook 3360: Rather than give you a prompt to invite you to input something, the text
3361: interpreter prints a status message @i{after} it has processed a line
3362: of input. The status message in this case (``@code{ ok}'' followed by
3363: carriage-return) indicates that the text interpreter was able to process
3364: all of your input successfully. Now type something illegal:
3365:
3366: @example
1.30 anton 3367: @kbd{qwer341@key{RET}}
1.29 crook 3368: :1: Undefined word
3369: qwer341
3370: ^^^^^^^
3371: $400D2BA8 Bounce
3372: $400DBDA8 no.extensions
3373: @end example
1.23 crook 3374:
1.29 crook 3375: The exact text, other than the ``Undefined word'' may differ slightly on
3376: your system, but the effect is the same; when the text interpreter
3377: detects an error, it discards any remaining text on a line, resets
1.49 anton 3378: certain internal state and prints an error message. For a detailed description of error messages see @ref{Error
3379: messages}.
1.23 crook 3380:
1.29 crook 3381: The text interpreter waits for you to press carriage-return, and then
3382: processes your input line. Starting at the beginning of the line, it
3383: breaks the line into groups of characters separated by spaces. For each
3384: group of characters in turn, it makes two attempts to do something:
1.23 crook 3385:
1.29 crook 3386: @itemize @bullet
3387: @item
1.44 crook 3388: @cindex name dictionary
1.29 crook 3389: It tries to treat it as a command. It does this by searching a @dfn{name
3390: dictionary}. If the group of characters matches an entry in the name
3391: dictionary, the name dictionary provides the text interpreter with
3392: information that allows the text interpreter perform some actions. In
3393: Forth jargon, we say that the group
3394: @cindex word
3395: @cindex definition
3396: @cindex execution token
3397: @cindex xt
3398: of characters names a @dfn{word}, that the dictionary search returns an
3399: @dfn{execution token (xt)} corresponding to the @dfn{definition} of the
3400: word, and that the text interpreter executes the xt. Often, the terms
3401: @dfn{word} and @dfn{definition} are used interchangeably.
3402: @item
3403: If the text interpreter fails to find a match in the name dictionary, it
3404: tries to treat the group of characters as a number in the current number
3405: base (when you start up Forth, the current number base is base 10). If
3406: the group of characters legitimately represents a number, the text
3407: interpreter pushes the number onto a stack (we'll learn more about that
3408: in the next section).
3409: @end itemize
1.23 crook 3410:
1.29 crook 3411: If the text interpreter is unable to do either of these things with any
3412: group of characters, it discards the group of characters and the rest of
3413: the line, then prints an error message. If the text interpreter reaches
3414: the end of the line without error, it prints the status message ``@code{ ok}''
3415: followed by carriage-return.
1.21 crook 3416:
1.29 crook 3417: This is the simplest command we can give to the text interpreter:
1.23 crook 3418:
3419: @example
1.30 anton 3420: @key{RET} ok
1.23 crook 3421: @end example
1.21 crook 3422:
1.29 crook 3423: The text interpreter did everything we asked it to do (nothing) without
3424: an error, so it said that everything is ``@code{ ok}''. Try a slightly longer
3425: command:
1.21 crook 3426:
1.23 crook 3427: @example
1.30 anton 3428: @kbd{12 dup fred dup@key{RET}}
1.29 crook 3429: :1: Undefined word
3430: 12 dup fred dup
3431: ^^^^
3432: $400D2BA8 Bounce
3433: $400DBDA8 no.extensions
1.23 crook 3434: @end example
1.21 crook 3435:
1.29 crook 3436: When you press the carriage-return key, the text interpreter starts to
3437: work its way along the line:
1.21 crook 3438:
1.29 crook 3439: @itemize @bullet
3440: @item
3441: When it gets to the space after the @code{2}, it takes the group of
3442: characters @code{12} and looks them up in the name
3443: dictionary@footnote{We can't tell if it found them or not, but assume
3444: for now that it did not}. There is no match for this group of characters
3445: in the name dictionary, so it tries to treat them as a number. It is
3446: able to do this successfully, so it puts the number, 12, ``on the stack''
3447: (whatever that means).
3448: @item
3449: The text interpreter resumes scanning the line and gets the next group
3450: of characters, @code{dup}. It looks it up in the name dictionary and
3451: (you'll have to take my word for this) finds it, and executes the word
3452: @code{dup} (whatever that means).
3453: @item
3454: Once again, the text interpreter resumes scanning the line and gets the
3455: group of characters @code{fred}. It looks them up in the name
3456: dictionary, but can't find them. It tries to treat them as a number, but
3457: they don't represent any legal number.
3458: @end itemize
1.21 crook 3459:
1.29 crook 3460: At this point, the text interpreter gives up and prints an error
3461: message. The error message shows exactly how far the text interpreter
3462: got in processing the line. In particular, it shows that the text
3463: interpreter made no attempt to do anything with the final character
3464: group, @code{dup}, even though we have good reason to believe that the
3465: text interpreter would have no problem looking that word up and
3466: executing it a second time.
1.21 crook 3467:
3468:
1.29 crook 3469: @comment ----------------------------------------------
3470: @node Stacks and Postfix notation, Your first definition, Introducing the Text Interpreter, Introduction
3471: @section Stacks, postfix notation and parameter passing
3472: @cindex text interpreter
3473: @cindex outer interpreter
1.21 crook 3474:
1.29 crook 3475: In procedural programming languages (like C and Pascal), the
3476: building-block of programs is the @dfn{function} or @dfn{procedure}. These
3477: functions or procedures are called with @dfn{explicit parameters}. For
3478: example, in C we might write:
1.21 crook 3479:
1.23 crook 3480: @example
1.29 crook 3481: total = total + new_volume(length,height,depth);
1.23 crook 3482: @end example
1.21 crook 3483:
1.23 crook 3484: @noindent
1.29 crook 3485: where new_volume is a function-call to another piece of code, and total,
3486: length, height and depth are all variables. length, height and depth are
3487: parameters to the function-call.
1.21 crook 3488:
1.29 crook 3489: In Forth, the equivalent of the function or procedure is the
3490: @dfn{definition} and parameters are implicitly passed between
3491: definitions using a shared stack that is visible to the
3492: programmer. Although Forth does support variables, the existence of the
3493: stack means that they are used far less often than in most other
3494: programming languages. When the text interpreter encounters a number, it
3495: will place (@dfn{push}) it on the stack. There are several stacks (the
1.30 anton 3496: actual number is implementation-dependent ...) and the particular stack
1.29 crook 3497: used for any operation is implied unambiguously by the operation being
3498: performed. The stack used for all integer operations is called the @dfn{data
3499: stack} and, since this is the stack used most commonly, references to
3500: ``the data stack'' are often abbreviated to ``the stack''.
1.21 crook 3501:
1.29 crook 3502: The stacks have a last-in, first-out (LIFO) organisation. If you type:
1.21 crook 3503:
1.23 crook 3504: @example
1.30 anton 3505: @kbd{1 2 3@key{RET}} ok
1.23 crook 3506: @end example
1.21 crook 3507:
1.29 crook 3508: Then this instructs the text interpreter to placed three numbers on the
3509: (data) stack. An analogy for the behaviour of the stack is to take a
3510: pack of playing cards and deal out the ace (1), 2 and 3 into a pile on
3511: the table. The 3 was the last card onto the pile (``last-in'') and if
3512: you take a card off the pile then, unless you're prepared to fiddle a
3513: bit, the card that you take off will be the 3 (``first-out''). The
3514: number that will be first-out of the stack is called the @dfn{top of
3515: stack}, which
3516: @cindex TOS definition
3517: is often abbreviated to @dfn{TOS}.
1.21 crook 3518:
1.29 crook 3519: To understand how parameters are passed in Forth, consider the
3520: behaviour of the definition @code{+} (pronounced ``plus''). You will not
3521: be surprised to learn that this definition performs addition. More
3522: precisely, it adds two number together and produces a result. Where does
3523: it get the two numbers from? It takes the top two numbers off the
3524: stack. Where does it place the result? On the stack. You can act-out the
3525: behaviour of @code{+} with your playing cards like this:
1.21 crook 3526:
3527: @itemize @bullet
3528: @item
1.29 crook 3529: Pick up two cards from the stack on the table
1.21 crook 3530: @item
1.29 crook 3531: Stare at them intently and ask yourself ``what @i{is} the sum of these two
3532: numbers''
1.21 crook 3533: @item
1.29 crook 3534: Decide that the answer is 5
1.21 crook 3535: @item
1.29 crook 3536: Shuffle the two cards back into the pack and find a 5
1.21 crook 3537: @item
1.29 crook 3538: Put a 5 on the remaining ace that's on the table.
1.21 crook 3539: @end itemize
3540:
1.29 crook 3541: If you don't have a pack of cards handy but you do have Forth running,
3542: you can use the definition @code{.s} to show the current state of the stack,
3543: without affecting the stack. Type:
1.21 crook 3544:
3545: @example
1.30 anton 3546: @kbd{clearstack 1 2 3@key{RET}} ok
3547: @kbd{.s@key{RET}} <3> 1 2 3 ok
1.23 crook 3548: @end example
3549:
1.29 crook 3550: The text interpreter looks up the word @code{clearstack} and executes
3551: it; it tidies up the stack and removes any entries that may have been
3552: left on it by earlier examples. The text interpreter pushes each of the
3553: three numbers in turn onto the stack. Finally, the text interpreter
3554: looks up the word @code{.s} and executes it. The effect of executing
3555: @code{.s} is to print the ``<3>'' (the total number of items on the stack)
3556: followed by a list of all the items on the stack; the item on the far
3557: right-hand side is the TOS.
1.21 crook 3558:
1.29 crook 3559: You can now type:
1.21 crook 3560:
1.29 crook 3561: @example
1.30 anton 3562: @kbd{+ .s@key{RET}} <2> 1 5 ok
1.29 crook 3563: @end example
1.21 crook 3564:
1.29 crook 3565: @noindent
3566: which is correct; there are now 2 items on the stack and the result of
3567: the addition is 5.
1.23 crook 3568:
1.29 crook 3569: If you're playing with cards, try doing a second addition: pick up the
3570: two cards, work out that their sum is 6, shuffle them into the pack,
3571: look for a 6 and place that on the table. You now have just one item on
3572: the stack. What happens if you try to do a third addition? Pick up the
3573: first card, pick up the second card -- ah! There is no second card. This
3574: is called a @dfn{stack underflow} and consitutes an error. If you try to
1.95 anton 3575: do the same thing with Forth it often reports an error (probably a Stack
1.29 crook 3576: Underflow or an Invalid Memory Address error).
1.23 crook 3577:
1.29 crook 3578: The opposite situation to a stack underflow is a @dfn{stack overflow},
3579: which simply accepts that there is a finite amount of storage space
3580: reserved for the stack. To stretch the playing card analogy, if you had
3581: enough packs of cards and you piled the cards up on the table, you would
3582: eventually be unable to add another card; you'd hit the ceiling. Gforth
3583: allows you to set the maximum size of the stacks. In general, the only
3584: time that you will get a stack overflow is because a definition has a
3585: bug in it and is generating data on the stack uncontrollably.
1.23 crook 3586:
1.29 crook 3587: There's one final use for the playing card analogy. If you model your
3588: stack using a pack of playing cards, the maximum number of items on
3589: your stack will be 52 (I assume you didn't use the Joker). The maximum
3590: @i{value} of any item on the stack is 13 (the King). In fact, the only
3591: possible numbers are positive integer numbers 1 through 13; you can't
3592: have (for example) 0 or 27 or 3.52 or -2. If you change the way you
3593: think about some of the cards, you can accommodate different
3594: numbers. For example, you could think of the Jack as representing 0,
3595: the Queen as representing -1 and the King as representing -2. Your
1.45 crook 3596: @i{range} remains unchanged (you can still only represent a total of 13
1.29 crook 3597: numbers) but the numbers that you can represent are -2 through 10.
1.28 crook 3598:
1.29 crook 3599: In that analogy, the limit was the amount of information that a single
3600: stack entry could hold, and Forth has a similar limit. In Forth, the
3601: size of a stack entry is called a @dfn{cell}. The actual size of a cell is
3602: implementation dependent and affects the maximum value that a stack
3603: entry can hold. A Standard Forth provides a cell size of at least
3604: 16-bits, and most desktop systems use a cell size of 32-bits.
1.21 crook 3605:
1.29 crook 3606: Forth does not do any type checking for you, so you are free to
3607: manipulate and combine stack items in any way you wish. A convenient way
3608: of treating stack items is as 2's complement signed integers, and that
3609: is what Standard words like @code{+} do. Therefore you can type:
1.21 crook 3610:
1.29 crook 3611: @example
1.30 anton 3612: @kbd{-5 12 + .s@key{RET}} <1> 7 ok
1.29 crook 3613: @end example
1.21 crook 3614:
1.29 crook 3615: If you use numbers and definitions like @code{+} in order to turn Forth
3616: into a great big pocket calculator, you will realise that it's rather
3617: different from a normal calculator. Rather than typing 2 + 3 = you had
3618: to type 2 3 + (ignore the fact that you had to use @code{.s} to see the
3619: result). The terminology used to describe this difference is to say that
3620: your calculator uses @dfn{Infix Notation} (parameters and operators are
3621: mixed) whilst Forth uses @dfn{Postfix Notation} (parameters and
3622: operators are separate), also called @dfn{Reverse Polish Notation}.
1.21 crook 3623:
1.29 crook 3624: Whilst postfix notation might look confusing to begin with, it has
3625: several important advantages:
1.21 crook 3626:
1.23 crook 3627: @itemize @bullet
3628: @item
1.29 crook 3629: it is unambiguous
1.23 crook 3630: @item
1.29 crook 3631: it is more concise
1.23 crook 3632: @item
1.29 crook 3633: it fits naturally with a stack-based system
1.23 crook 3634: @end itemize
1.21 crook 3635:
1.29 crook 3636: To examine these claims in more detail, consider these sums:
1.21 crook 3637:
1.29 crook 3638: @example
3639: 6 + 5 * 4 =
3640: 4 * 5 + 6 =
3641: @end example
1.21 crook 3642:
1.29 crook 3643: If you're just learning maths or your maths is very rusty, you will
3644: probably come up with the answer 44 for the first and 26 for the
3645: second. If you are a bit of a whizz at maths you will remember the
3646: @i{convention} that multiplication takes precendence over addition, and
3647: you'd come up with the answer 26 both times. To explain the answer 26
3648: to someone who got the answer 44, you'd probably rewrite the first sum
3649: like this:
1.21 crook 3650:
1.29 crook 3651: @example
3652: 6 + (5 * 4) =
3653: @end example
1.21 crook 3654:
1.29 crook 3655: If what you really wanted was to perform the addition before the
3656: multiplication, you would have to use parentheses to force it.
1.21 crook 3657:
1.29 crook 3658: If you did the first two sums on a pocket calculator you would probably
3659: get the right answers, unless you were very cautious and entered them using
3660: these keystroke sequences:
1.21 crook 3661:
1.29 crook 3662: 6 + 5 = * 4 =
3663: 4 * 5 = + 6 =
1.21 crook 3664:
1.29 crook 3665: Postfix notation is unambiguous because the order that the operators
3666: are applied is always explicit; that also means that parentheses are
3667: never required. The operators are @i{active} (the act of quoting the
3668: operator makes the operation occur) which removes the need for ``=''.
1.28 crook 3669:
1.29 crook 3670: The sum 6 + 5 * 4 can be written (in postfix notation) in two
3671: equivalent ways:
1.26 crook 3672:
3673: @example
1.29 crook 3674: 6 5 4 * + or:
3675: 5 4 * 6 +
1.26 crook 3676: @end example
1.23 crook 3677:
1.29 crook 3678: An important thing that you should notice about this notation is that
3679: the @i{order} of the numbers does not change; if you want to subtract
3680: 2 from 10 you type @code{10 2 -}.
1.1 anton 3681:
1.29 crook 3682: The reason that Forth uses postfix notation is very simple to explain: it
3683: makes the implementation extremely simple, and it follows naturally from
3684: using the stack as a mechanism for passing parameters. Another way of
3685: thinking about this is to realise that all Forth definitions are
3686: @i{active}; they execute as they are encountered by the text
3687: interpreter. The result of this is that the syntax of Forth is trivially
3688: simple.
1.1 anton 3689:
3690:
3691:
1.29 crook 3692: @comment ----------------------------------------------
3693: @node Your first definition, How does that work?, Stacks and Postfix notation, Introduction
3694: @section Your first Forth definition
3695: @cindex first definition
1.1 anton 3696:
1.29 crook 3697: Until now, the examples we've seen have been trivial; we've just been
3698: using Forth as a bigger-than-pocket calculator. Also, each calculation
3699: we've shown has been a ``one-off'' -- to repeat it we'd need to type it in
3700: again@footnote{That's not quite true. If you press the up-arrow key on
3701: your keyboard you should be able to scroll back to any earlier command,
3702: edit it and re-enter it.} In this section we'll see how to add new
3703: words to Forth's vocabulary.
1.1 anton 3704:
1.29 crook 3705: The easiest way to create a new word is to use a @dfn{colon
3706: definition}. We'll define a few and try them out before worrying too
3707: much about how they work. Try typing in these examples; be careful to
3708: copy the spaces accurately:
1.1 anton 3709:
1.29 crook 3710: @example
3711: : add-two 2 + . ;
3712: : greet ." Hello and welcome" ;
3713: : demo 5 add-two ;
3714: @end example
1.1 anton 3715:
1.29 crook 3716: @noindent
3717: Now try them out:
1.1 anton 3718:
1.29 crook 3719: @example
1.30 anton 3720: @kbd{greet@key{RET}} Hello and welcome ok
3721: @kbd{greet greet@key{RET}} Hello and welcomeHello and welcome ok
3722: @kbd{4 add-two@key{RET}} 6 ok
3723: @kbd{demo@key{RET}} 7 ok
3724: @kbd{9 greet demo add-two@key{RET}} Hello and welcome7 11 ok
1.29 crook 3725: @end example
1.1 anton 3726:
1.29 crook 3727: The first new thing that we've introduced here is the pair of words
3728: @code{:} and @code{;}. These are used to start and terminate a new
3729: definition, respectively. The first word after the @code{:} is the name
3730: for the new definition.
1.1 anton 3731:
1.29 crook 3732: As you can see from the examples, a definition is built up of words that
3733: have already been defined; Forth makes no distinction between
3734: definitions that existed when you started the system up, and those that
3735: you define yourself.
1.1 anton 3736:
1.29 crook 3737: The examples also introduce the words @code{.} (dot), @code{."}
3738: (dot-quote) and @code{dup} (dewp). Dot takes the value from the top of
3739: the stack and displays it. It's like @code{.s} except that it only
3740: displays the top item of the stack and it is destructive; after it has
3741: executed, the number is no longer on the stack. There is always one
3742: space printed after the number, and no spaces before it. Dot-quote
3743: defines a string (a sequence of characters) that will be printed when
3744: the word is executed. The string can contain any printable characters
3745: except @code{"}. A @code{"} has a special function; it is not a Forth
3746: word but it acts as a delimiter (the way that delimiters work is
3747: described in the next section). Finally, @code{dup} duplicates the value
3748: at the top of the stack. Try typing @code{5 dup .s} to see what it does.
1.1 anton 3749:
1.29 crook 3750: We already know that the text interpreter searches through the
3751: dictionary to locate names. If you've followed the examples earlier, you
3752: will already have a definition called @code{add-two}. Lets try modifying
3753: it by typing in a new definition:
1.1 anton 3754:
1.29 crook 3755: @example
1.30 anton 3756: @kbd{: add-two dup . ." + 2 =" 2 + . ;@key{RET}} redefined add-two ok
1.29 crook 3757: @end example
1.5 anton 3758:
1.29 crook 3759: Forth recognised that we were defining a word that already exists, and
3760: printed a message to warn us of that fact. Let's try out the new
3761: definition:
1.5 anton 3762:
1.29 crook 3763: @example
1.30 anton 3764: @kbd{9 add-two@key{RET}} 9 + 2 =11 ok
1.29 crook 3765: @end example
1.1 anton 3766:
1.29 crook 3767: @noindent
3768: All that we've actually done here, though, is to create a new
3769: definition, with a particular name. The fact that there was already a
3770: definition with the same name did not make any difference to the way
3771: that the new definition was created (except that Forth printed a warning
3772: message). The old definition of add-two still exists (try @code{demo}
3773: again to see that this is true). Any new definition will use the new
3774: definition of @code{add-two}, but old definitions continue to use the
3775: version that already existed at the time that they were @code{compiled}.
1.1 anton 3776:
1.29 crook 3777: Before you go on to the next section, try defining and redefining some
3778: words of your own.
1.1 anton 3779:
1.29 crook 3780: @comment ----------------------------------------------
3781: @node How does that work?, Forth is written in Forth, Your first definition, Introduction
3782: @section How does that work?
3783: @cindex parsing words
1.1 anton 3784:
1.30 anton 3785: @c That's pretty deep (IMO way too deep) for an introduction. - anton
3786:
3787: @c Is it a good idea to talk about the interpretation semantics of a
3788: @c number? We don't have an xt to go along with it. - anton
3789:
3790: @c Now that I have eliminated execution semantics, I wonder if it would not
3791: @c be better to keep them (or add run-time semantics), to make it easier to
3792: @c explain what compilation semantics usually does. - anton
3793:
1.44 crook 3794: @c nac-> I removed the term ``default compilation sematics'' from the
3795: @c introductory chapter. Removing ``execution semantics'' was making
3796: @c everything simpler to explain, then I think the use of this term made
3797: @c everything more complex again. I replaced it with ``default
3798: @c semantics'' (which is used elsewhere in the manual) by which I mean
3799: @c ``a definition that has neither the immediate nor the compile-only
1.83 anton 3800: @c flag set''.
3801:
3802: @c anton: I have eliminated default semantics (except in one place where it
3803: @c means "default interpretation and compilation semantics"), because it
3804: @c makes no sense in the presence of combined words. I reverted to
3805: @c "execution semantics" where necessary.
3806:
3807: @c nac-> I reworded big chunks of the ``how does that work''
1.44 crook 3808: @c section (and, unusually for me, I think I even made it shorter!). See
3809: @c what you think -- I know I have not addressed your primary concern
3810: @c that it is too heavy-going for an introduction. From what I understood
3811: @c of your course notes it looks as though they might be a good framework.
3812: @c Things that I've tried to capture here are some things that came as a
3813: @c great revelation here when I first understood them. Also, I like the
3814: @c fact that a very simple code example shows up almost all of the issues
3815: @c that you need to understand to see how Forth works. That's unique and
3816: @c worthwhile to emphasise.
3817:
1.83 anton 3818: @c anton: I think it's a good idea to present the details, especially those
3819: @c that you found to be a revelation, and probably the tutorial tries to be
3820: @c too superficial and does not get some of the things across that make
3821: @c Forth special. I do believe that most of the time these things should
3822: @c be discussed at the end of a section or in separate sections instead of
3823: @c in the middle of a section (e.g., the stuff you added in "User-defined
3824: @c defining words" leads in a completely different direction from the rest
3825: @c of the section).
3826:
1.29 crook 3827: Now we're going to take another look at the definition of @code{add-two}
3828: from the previous section. From our knowledge of the way that the text
3829: interpreter works, we would have expected this result when we tried to
3830: define @code{add-two}:
1.21 crook 3831:
1.29 crook 3832: @example
1.44 crook 3833: @kbd{: add-two 2 + . ;@key{RET}}
1.29 crook 3834: ^^^^^^^
3835: Error: Undefined word
3836: @end example
1.28 crook 3837:
1.29 crook 3838: The reason that this didn't happen is bound up in the way that @code{:}
3839: works. The word @code{:} does two special things. The first special
3840: thing that it does prevents the text interpreter from ever seeing the
3841: characters @code{add-two}. The text interpreter uses a variable called
3842: @cindex modifying >IN
1.44 crook 3843: @code{>IN} (pronounced ``to-in'') to keep track of where it is in the
1.29 crook 3844: input line. When it encounters the word @code{:} it behaves in exactly
3845: the same way as it does for any other word; it looks it up in the name
3846: dictionary, finds its xt and executes it. When @code{:} executes, it
3847: looks at the input buffer, finds the word @code{add-two} and advances the
3848: value of @code{>IN} to point past it. It then does some other stuff
3849: associated with creating the new definition (including creating an entry
3850: for @code{add-two} in the name dictionary). When the execution of @code{:}
3851: completes, control returns to the text interpreter, which is oblivious
3852: to the fact that it has been tricked into ignoring part of the input
3853: line.
1.21 crook 3854:
1.29 crook 3855: @cindex parsing words
3856: Words like @code{:} -- words that advance the value of @code{>IN} and so
3857: prevent the text interpreter from acting on the whole of the input line
3858: -- are called @dfn{parsing words}.
1.21 crook 3859:
1.29 crook 3860: @cindex @code{state} - effect on the text interpreter
3861: @cindex text interpreter - effect of state
3862: The second special thing that @code{:} does is change the value of a
3863: variable called @code{state}, which affects the way that the text
3864: interpreter behaves. When Gforth starts up, @code{state} has the value
3865: 0, and the text interpreter is said to be @dfn{interpreting}. During a
3866: colon definition (started with @code{:}), @code{state} is set to -1 and
1.44 crook 3867: the text interpreter is said to be @dfn{compiling}.
3868:
3869: In this example, the text interpreter is compiling when it processes the
3870: string ``@code{2 + . ;}''. It still breaks the string down into
3871: character sequences in the same way. However, instead of pushing the
3872: number @code{2} onto the stack, it lays down (@dfn{compiles}) some magic
3873: into the definition of @code{add-two} that will make the number @code{2} get
3874: pushed onto the stack when @code{add-two} is @dfn{executed}. Similarly,
3875: the behaviours of @code{+} and @code{.} are also compiled into the
3876: definition.
3877:
3878: One category of words don't get compiled. These so-called @dfn{immediate
3879: words} get executed (performed @i{now}) regardless of whether the text
3880: interpreter is interpreting or compiling. The word @code{;} is an
3881: immediate word. Rather than being compiled into the definition, it
3882: executes. Its effect is to terminate the current definition, which
3883: includes changing the value of @code{state} back to 0.
3884:
3885: When you execute @code{add-two}, it has a @dfn{run-time effect} that is
3886: exactly the same as if you had typed @code{2 + . @key{RET}} outside of a
3887: definition.
1.28 crook 3888:
1.30 anton 3889: In Forth, every word or number can be described in terms of two
1.29 crook 3890: properties:
1.28 crook 3891:
3892: @itemize @bullet
3893: @item
1.29 crook 3894: @cindex interpretation semantics
1.44 crook 3895: Its @dfn{interpretation semantics} describe how it will behave when the
3896: text interpreter encounters it in @dfn{interpret} state. The
3897: interpretation semantics of a word are represented by an @dfn{execution
3898: token}.
1.28 crook 3899: @item
1.29 crook 3900: @cindex compilation semantics
1.44 crook 3901: Its @dfn{compilation semantics} describe how it will behave when the
3902: text interpreter encounters it in @dfn{compile} state. The compilation
3903: semantics of a word are represented in an implementation-dependent way;
3904: Gforth uses a @dfn{compilation token}.
1.29 crook 3905: @end itemize
3906:
3907: @noindent
3908: Numbers are always treated in a fixed way:
3909:
3910: @itemize @bullet
1.28 crook 3911: @item
1.44 crook 3912: When the number is @dfn{interpreted}, its behaviour is to push the
3913: number onto the stack.
1.28 crook 3914: @item
1.30 anton 3915: When the number is @dfn{compiled}, a piece of code is appended to the
3916: current definition that pushes the number when it runs. (In other words,
3917: the compilation semantics of a number are to postpone its interpretation
3918: semantics until the run-time of the definition that it is being compiled
3919: into.)
1.29 crook 3920: @end itemize
3921:
1.44 crook 3922: Words don't behave in such a regular way, but most have @i{default
3923: semantics} which means that they behave like this:
1.29 crook 3924:
3925: @itemize @bullet
1.28 crook 3926: @item
1.30 anton 3927: The @dfn{interpretation semantics} of the word are to do something useful.
3928: @item
1.29 crook 3929: The @dfn{compilation semantics} of the word are to append its
1.30 anton 3930: @dfn{interpretation semantics} to the current definition (so that its
3931: run-time behaviour is to do something useful).
1.28 crook 3932: @end itemize
3933:
1.30 anton 3934: @cindex immediate words
1.44 crook 3935: The actual behaviour of any particular word can be controlled by using
3936: the words @code{immediate} and @code{compile-only} when the word is
3937: defined. These words set flags in the name dictionary entry of the most
3938: recently defined word, and these flags are retrieved by the text
3939: interpreter when it finds the word in the name dictionary.
3940:
3941: A word that is marked as @dfn{immediate} has compilation semantics that
3942: are identical to its interpretation semantics. In other words, it
3943: behaves like this:
1.29 crook 3944:
3945: @itemize @bullet
3946: @item
1.30 anton 3947: The @dfn{interpretation semantics} of the word are to do something useful.
1.29 crook 3948: @item
1.30 anton 3949: The @dfn{compilation semantics} of the word are to do something useful
3950: (and actually the same thing); i.e., it is executed during compilation.
1.29 crook 3951: @end itemize
1.28 crook 3952:
1.44 crook 3953: Marking a word as @dfn{compile-only} prohibits the text interpreter from
3954: performing the interpretation semantics of the word directly; an attempt
3955: to do so will generate an error. It is never necessary to use
3956: @code{compile-only} (and it is not even part of ANS Forth, though it is
3957: provided by many implementations) but it is good etiquette to apply it
3958: to a word that will not behave correctly (and might have unexpected
3959: side-effects) in interpret state. For example, it is only legal to use
3960: the conditional word @code{IF} within a definition. If you forget this
3961: and try to use it elsewhere, the fact that (in Gforth) it is marked as
3962: @code{compile-only} allows the text interpreter to generate a helpful
3963: error message rather than subjecting you to the consequences of your
3964: folly.
3965:
1.29 crook 3966: This example shows the difference between an immediate and a
3967: non-immediate word:
1.28 crook 3968:
1.29 crook 3969: @example
3970: : show-state state @@ . ;
3971: : show-state-now show-state ; immediate
3972: : word1 show-state ;
3973: : word2 show-state-now ;
1.28 crook 3974: @end example
1.23 crook 3975:
1.29 crook 3976: The word @code{immediate} after the definition of @code{show-state-now}
3977: makes that word an immediate word. These definitions introduce a new
3978: word: @code{@@} (pronounced ``fetch''). This word fetches the value of a
3979: variable, and leaves it on the stack. Therefore, the behaviour of
3980: @code{show-state} is to print a number that represents the current value
3981: of @code{state}.
1.28 crook 3982:
1.29 crook 3983: When you execute @code{word1}, it prints the number 0, indicating that
3984: the system is interpreting. When the text interpreter compiled the
3985: definition of @code{word1}, it encountered @code{show-state} whose
1.30 anton 3986: compilation semantics are to append its interpretation semantics to the
1.29 crook 3987: current definition. When you execute @code{word1}, it performs the
1.30 anton 3988: interpretation semantics of @code{show-state}. At the time that @code{word1}
1.29 crook 3989: (and therefore @code{show-state}) are executed, the system is
3990: interpreting.
1.28 crook 3991:
1.30 anton 3992: When you pressed @key{RET} after entering the definition of @code{word2},
1.29 crook 3993: you should have seen the number -1 printed, followed by ``@code{
3994: ok}''. When the text interpreter compiled the definition of
3995: @code{word2}, it encountered @code{show-state-now}, an immediate word,
1.30 anton 3996: whose compilation semantics are therefore to perform its interpretation
1.29 crook 3997: semantics. It is executed straight away (even before the text
3998: interpreter has moved on to process another group of characters; the
3999: @code{;} in this example). The effect of executing it are to display the
4000: value of @code{state} @i{at the time that the definition of}
4001: @code{word2} @i{is being defined}. Printing -1 demonstrates that the
4002: system is compiling at this time. If you execute @code{word2} it does
4003: nothing at all.
1.28 crook 4004:
1.29 crook 4005: @cindex @code{."}, how it works
4006: Before leaving the subject of immediate words, consider the behaviour of
4007: @code{."} in the definition of @code{greet}, in the previous
4008: section. This word is both a parsing word and an immediate word. Notice
4009: that there is a space between @code{."} and the start of the text
4010: @code{Hello and welcome}, but that there is no space between the last
4011: letter of @code{welcome} and the @code{"} character. The reason for this
4012: is that @code{."} is a Forth word; it must have a space after it so that
4013: the text interpreter can identify it. The @code{"} is not a Forth word;
4014: it is a @dfn{delimiter}. The examples earlier show that, when the string
4015: is displayed, there is neither a space before the @code{H} nor after the
4016: @code{e}. Since @code{."} is an immediate word, it executes at the time
4017: that @code{greet} is defined. When it executes, its behaviour is to
4018: search forward in the input line looking for the delimiter. When it
4019: finds the delimiter, it updates @code{>IN} to point past the
4020: delimiter. It also compiles some magic code into the definition of
4021: @code{greet}; the xt of a run-time routine that prints a text string. It
4022: compiles the string @code{Hello and welcome} into memory so that it is
4023: available to be printed later. When the text interpreter gains control,
4024: the next word it finds in the input stream is @code{;} and so it
4025: terminates the definition of @code{greet}.
1.28 crook 4026:
4027:
4028: @comment ----------------------------------------------
1.29 crook 4029: @node Forth is written in Forth, Review - elements of a Forth system, How does that work?, Introduction
4030: @section Forth is written in Forth
4031: @cindex structure of Forth programs
4032:
4033: When you start up a Forth compiler, a large number of definitions
4034: already exist. In Forth, you develop a new application using bottom-up
4035: programming techniques to create new definitions that are defined in
4036: terms of existing definitions. As you create each definition you can
4037: test and debug it interactively.
4038:
4039: If you have tried out the examples in this section, you will probably
4040: have typed them in by hand; when you leave Gforth, your definitions will
4041: be lost. You can avoid this by using a text editor to enter Forth source
4042: code into a file, and then loading code from the file using
1.49 anton 4043: @code{include} (@pxref{Forth source files}). A Forth source file is
1.29 crook 4044: processed by the text interpreter, just as though you had typed it in by
4045: hand@footnote{Actually, there are some subtle differences -- see
4046: @ref{The Text Interpreter}.}.
4047:
4048: Gforth also supports the traditional Forth alternative to using text
1.49 anton 4049: files for program entry (@pxref{Blocks}).
1.28 crook 4050:
1.29 crook 4051: In common with many, if not most, Forth compilers, most of Gforth is
4052: actually written in Forth. All of the @file{.fs} files in the
4053: installation directory@footnote{For example,
1.30 anton 4054: @file{/usr/local/share/gforth...}} are Forth source files, which you can
1.29 crook 4055: study to see examples of Forth programming.
1.28 crook 4056:
1.29 crook 4057: Gforth maintains a history file that records every line that you type to
4058: the text interpreter. This file is preserved between sessions, and is
4059: used to provide a command-line recall facility. If you enter long
4060: definitions by hand, you can use a text editor to paste them out of the
4061: history file into a Forth source file for reuse at a later time
1.49 anton 4062: (for more information @pxref{Command-line editing}).
1.28 crook 4063:
4064:
4065: @comment ----------------------------------------------
1.29 crook 4066: @node Review - elements of a Forth system, Where to go next, Forth is written in Forth, Introduction
4067: @section Review - elements of a Forth system
4068: @cindex elements of a Forth system
1.28 crook 4069:
1.29 crook 4070: To summarise this chapter:
1.28 crook 4071:
4072: @itemize @bullet
4073: @item
1.29 crook 4074: Forth programs use @dfn{factoring} to break a problem down into small
4075: fragments called @dfn{words} or @dfn{definitions}.
4076: @item
4077: Forth program development is an interactive process.
4078: @item
4079: The main command loop that accepts input, and controls both
4080: interpretation and compilation, is called the @dfn{text interpreter}
4081: (also known as the @dfn{outer interpreter}).
4082: @item
4083: Forth has a very simple syntax, consisting of words and numbers
4084: separated by spaces or carriage-return characters. Any additional syntax
4085: is imposed by @dfn{parsing words}.
4086: @item
4087: Forth uses a stack to pass parameters between words. As a result, it
4088: uses postfix notation.
4089: @item
4090: To use a word that has previously been defined, the text interpreter
4091: searches for the word in the @dfn{name dictionary}.
4092: @item
1.30 anton 4093: Words have @dfn{interpretation semantics} and @dfn{compilation semantics}.
1.28 crook 4094: @item
1.29 crook 4095: The text interpreter uses the value of @code{state} to select between
4096: the use of the @dfn{interpretation semantics} and the @dfn{compilation
4097: semantics} of a word that it encounters.
1.28 crook 4098: @item
1.30 anton 4099: The relationship between the @dfn{interpretation semantics} and
4100: @dfn{compilation semantics} for a word
1.29 crook 4101: depend upon the way in which the word was defined (for example, whether
4102: it is an @dfn{immediate} word).
1.28 crook 4103: @item
1.29 crook 4104: Forth definitions can be implemented in Forth (called @dfn{high-level
4105: definitions}) or in some other way (usually a lower-level language and
4106: as a result often called @dfn{low-level definitions}, @dfn{code
4107: definitions} or @dfn{primitives}).
1.28 crook 4108: @item
1.29 crook 4109: Many Forth systems are implemented mainly in Forth.
1.28 crook 4110: @end itemize
4111:
4112:
1.29 crook 4113: @comment ----------------------------------------------
1.48 anton 4114: @node Where to go next, Exercises, Review - elements of a Forth system, Introduction
1.29 crook 4115: @section Where To Go Next
4116: @cindex where to go next
1.28 crook 4117:
1.29 crook 4118: Amazing as it may seem, if you have read (and understood) this far, you
4119: know almost all the fundamentals about the inner workings of a Forth
4120: system. You certainly know enough to be able to read and understand the
4121: rest of this manual and the ANS Forth document, to learn more about the
4122: facilities that Forth in general and Gforth in particular provide. Even
4123: scarier, you know almost enough to implement your own Forth system.
1.30 anton 4124: However, that's not a good idea just yet... better to try writing some
1.29 crook 4125: programs in Gforth.
1.28 crook 4126:
1.29 crook 4127: Forth has such a rich vocabulary that it can be hard to know where to
4128: start in learning it. This section suggests a few sets of words that are
4129: enough to write small but useful programs. Use the word index in this
4130: document to learn more about each word, then try it out and try to write
4131: small definitions using it. Start by experimenting with these words:
1.28 crook 4132:
4133: @itemize @bullet
4134: @item
1.29 crook 4135: Arithmetic: @code{+ - * / /MOD */ ABS INVERT}
4136: @item
4137: Comparison: @code{MIN MAX =}
4138: @item
4139: Logic: @code{AND OR XOR NOT}
4140: @item
4141: Stack manipulation: @code{DUP DROP SWAP OVER}
1.28 crook 4142: @item
1.29 crook 4143: Loops and decisions: @code{IF ELSE ENDIF ?DO I LOOP}
1.28 crook 4144: @item
1.29 crook 4145: Input/Output: @code{. ." EMIT CR KEY}
1.28 crook 4146: @item
1.29 crook 4147: Defining words: @code{: ; CREATE}
1.28 crook 4148: @item
1.29 crook 4149: Memory allocation words: @code{ALLOT ,}
1.28 crook 4150: @item
1.29 crook 4151: Tools: @code{SEE WORDS .S MARKER}
4152: @end itemize
4153:
4154: When you have mastered those, go on to:
4155:
4156: @itemize @bullet
1.28 crook 4157: @item
1.29 crook 4158: More defining words: @code{VARIABLE CONSTANT VALUE TO CREATE DOES>}
1.28 crook 4159: @item
1.29 crook 4160: Memory access: @code{@@ !}
1.28 crook 4161: @end itemize
1.23 crook 4162:
1.29 crook 4163: When you have mastered these, there's nothing for it but to read through
4164: the whole of this manual and find out what you've missed.
4165:
4166: @comment ----------------------------------------------
1.48 anton 4167: @node Exercises, , Where to go next, Introduction
1.29 crook 4168: @section Exercises
4169: @cindex exercises
4170:
4171: TODO: provide a set of programming excercises linked into the stuff done
4172: already and into other sections of the manual. Provide solutions to all
4173: the exercises in a .fs file in the distribution.
4174:
4175: @c Get some inspiration from Starting Forth and Kelly&Spies.
4176:
4177: @c excercises:
4178: @c 1. take inches and convert to feet and inches.
4179: @c 2. take temperature and convert from fahrenheight to celcius;
4180: @c may need to care about symmetric vs floored??
4181: @c 3. take input line and do character substitution
4182: @c to encipher or decipher
4183: @c 4. as above but work on a file for in and out
4184: @c 5. take input line and convert to pig-latin
4185: @c
4186: @c thing of sets of things to exercise then come up with
4187: @c problems that need those things.
4188:
4189:
1.26 crook 4190: @c ******************************************************************
1.29 crook 4191: @node Words, Error messages, Introduction, Top
1.1 anton 4192: @chapter Forth Words
1.26 crook 4193: @cindex words
1.1 anton 4194:
4195: @menu
4196: * Notation::
1.65 anton 4197: * Case insensitivity::
4198: * Comments::
4199: * Boolean Flags::
1.1 anton 4200: * Arithmetic::
4201: * Stack Manipulation::
1.5 anton 4202: * Memory::
1.1 anton 4203: * Control Structures::
4204: * Defining Words::
1.65 anton 4205: * Interpretation and Compilation Semantics::
1.47 crook 4206: * Tokens for Words::
1.81 anton 4207: * Compiling words::
1.65 anton 4208: * The Text Interpreter::
1.111 anton 4209: * The Input Stream::
1.65 anton 4210: * Word Lists::
4211: * Environmental Queries::
1.12 anton 4212: * Files::
4213: * Blocks::
4214: * Other I/O::
1.78 anton 4215: * Locals::
4216: * Structures::
4217: * Object-oriented Forth::
1.12 anton 4218: * Programming Tools::
4219: * Assembler and Code Words::
4220: * Threading Words::
1.65 anton 4221: * Passing Commands to the OS::
4222: * Keeping track of Time::
4223: * Miscellaneous Words::
1.1 anton 4224: @end menu
4225:
1.65 anton 4226: @node Notation, Case insensitivity, Words, Words
1.1 anton 4227: @section Notation
4228: @cindex notation of glossary entries
4229: @cindex format of glossary entries
4230: @cindex glossary notation format
4231: @cindex word glossary entry format
4232:
4233: The Forth words are described in this section in the glossary notation
1.67 anton 4234: that has become a de-facto standard for Forth texts:
1.1 anton 4235:
4236: @format
1.29 crook 4237: @i{word} @i{Stack effect} @i{wordset} @i{pronunciation}
1.1 anton 4238: @end format
1.29 crook 4239: @i{Description}
1.1 anton 4240:
4241: @table @var
4242: @item word
1.28 crook 4243: The name of the word.
1.1 anton 4244:
4245: @item Stack effect
4246: @cindex stack effect
1.29 crook 4247: The stack effect is written in the notation @code{@i{before} --
4248: @i{after}}, where @i{before} and @i{after} describe the top of
1.1 anton 4249: stack entries before and after the execution of the word. The rest of
4250: the stack is not touched by the word. The top of stack is rightmost,
4251: i.e., a stack sequence is written as it is typed in. Note that Gforth
4252: uses a separate floating point stack, but a unified stack
1.29 crook 4253: notation. Also, return stack effects are not shown in @i{stack
4254: effect}, but in @i{Description}. The name of a stack item describes
1.1 anton 4255: the type and/or the function of the item. See below for a discussion of
4256: the types.
4257:
4258: All words have two stack effects: A compile-time stack effect and a
4259: run-time stack effect. The compile-time stack-effect of most words is
1.29 crook 4260: @i{ -- }. If the compile-time stack-effect of a word deviates from
1.1 anton 4261: this standard behaviour, or the word does other unusual things at
4262: compile time, both stack effects are shown; otherwise only the run-time
4263: stack effect is shown.
4264:
4265: @cindex pronounciation of words
4266: @item pronunciation
4267: How the word is pronounced.
4268:
4269: @cindex wordset
1.67 anton 4270: @cindex environment wordset
1.1 anton 4271: @item wordset
1.21 crook 4272: The ANS Forth standard is divided into several word sets. A standard
4273: system need not support all of them. Therefore, in theory, the fewer
4274: word sets your program uses the more portable it will be. However, we
4275: suspect that most ANS Forth systems on personal machines will feature
1.26 crook 4276: all word sets. Words that are not defined in ANS Forth have
1.21 crook 4277: @code{gforth} or @code{gforth-internal} as word set. @code{gforth}
1.1 anton 4278: describes words that will work in future releases of Gforth;
4279: @code{gforth-internal} words are more volatile. Environmental query
4280: strings are also displayed like words; you can recognize them by the
1.21 crook 4281: @code{environment} in the word set field.
1.1 anton 4282:
4283: @item Description
4284: A description of the behaviour of the word.
4285: @end table
4286:
4287: @cindex types of stack items
4288: @cindex stack item types
4289: The type of a stack item is specified by the character(s) the name
4290: starts with:
4291:
4292: @table @code
4293: @item f
4294: @cindex @code{f}, stack item type
4295: Boolean flags, i.e. @code{false} or @code{true}.
4296: @item c
4297: @cindex @code{c}, stack item type
4298: Char
4299: @item w
4300: @cindex @code{w}, stack item type
4301: Cell, can contain an integer or an address
4302: @item n
4303: @cindex @code{n}, stack item type
4304: signed integer
4305: @item u
4306: @cindex @code{u}, stack item type
4307: unsigned integer
4308: @item d
4309: @cindex @code{d}, stack item type
4310: double sized signed integer
4311: @item ud
4312: @cindex @code{ud}, stack item type
4313: double sized unsigned integer
4314: @item r
4315: @cindex @code{r}, stack item type
4316: Float (on the FP stack)
1.21 crook 4317: @item a-
1.1 anton 4318: @cindex @code{a_}, stack item type
4319: Cell-aligned address
1.21 crook 4320: @item c-
1.1 anton 4321: @cindex @code{c_}, stack item type
4322: Char-aligned address (note that a Char may have two bytes in Windows NT)
1.21 crook 4323: @item f-
1.1 anton 4324: @cindex @code{f_}, stack item type
4325: Float-aligned address
1.21 crook 4326: @item df-
1.1 anton 4327: @cindex @code{df_}, stack item type
4328: Address aligned for IEEE double precision float
1.21 crook 4329: @item sf-
1.1 anton 4330: @cindex @code{sf_}, stack item type
4331: Address aligned for IEEE single precision float
4332: @item xt
4333: @cindex @code{xt}, stack item type
4334: Execution token, same size as Cell
4335: @item wid
4336: @cindex @code{wid}, stack item type
1.21 crook 4337: Word list ID, same size as Cell
1.68 anton 4338: @item ior, wior
4339: @cindex ior type description
4340: @cindex wior type description
4341: I/O result code, cell-sized. In Gforth, you can @code{throw} iors.
1.1 anton 4342: @item f83name
4343: @cindex @code{f83name}, stack item type
4344: Pointer to a name structure
4345: @item "
4346: @cindex @code{"}, stack item type
1.12 anton 4347: string in the input stream (not on the stack). The terminating character
4348: is a blank by default. If it is not a blank, it is shown in @code{<>}
1.1 anton 4349: quotes.
4350: @end table
4351:
1.65 anton 4352: @comment ----------------------------------------------
4353: @node Case insensitivity, Comments, Notation, Words
4354: @section Case insensitivity
4355: @cindex case sensitivity
4356: @cindex upper and lower case
4357:
4358: Gforth is case-insensitive; you can enter definitions and invoke
4359: Standard words using upper, lower or mixed case (however,
4360: @pxref{core-idef, Implementation-defined options, Implementation-defined
4361: options}).
4362:
4363: ANS Forth only @i{requires} implementations to recognise Standard words
4364: when they are typed entirely in upper case. Therefore, a Standard
4365: program must use upper case for all Standard words. You can use whatever
4366: case you like for words that you define, but in a Standard program you
4367: have to use the words in the same case that you defined them.
4368:
4369: Gforth supports case sensitivity through @code{table}s (case-sensitive
4370: wordlists, @pxref{Word Lists}).
4371:
4372: Two people have asked how to convert Gforth to be case-sensitive; while
4373: we think this is a bad idea, you can change all wordlists into tables
4374: like this:
4375:
4376: @example
4377: ' table-find forth-wordlist wordlist-map @ !
4378: @end example
4379:
4380: Note that you now have to type the predefined words in the same case
4381: that we defined them, which are varying. You may want to convert them
4382: to your favourite case before doing this operation (I won't explain how,
4383: because if you are even contemplating doing this, you'd better have
4384: enough knowledge of Forth systems to know this already).
4385:
4386: @node Comments, Boolean Flags, Case insensitivity, Words
1.21 crook 4387: @section Comments
1.26 crook 4388: @cindex comments
1.21 crook 4389:
1.29 crook 4390: Forth supports two styles of comment; the traditional @i{in-line} comment,
4391: @code{(} and its modern cousin, the @i{comment to end of line}; @code{\}.
1.21 crook 4392:
1.44 crook 4393:
1.23 crook 4394: doc-(
1.21 crook 4395: doc-\
1.23 crook 4396: doc-\G
1.21 crook 4397:
1.44 crook 4398:
1.21 crook 4399: @node Boolean Flags, Arithmetic, Comments, Words
4400: @section Boolean Flags
1.26 crook 4401: @cindex Boolean flags
1.21 crook 4402:
4403: A Boolean flag is cell-sized. A cell with all bits clear represents the
4404: flag @code{false} and a flag with all bits set represents the flag
1.26 crook 4405: @code{true}. Words that check a flag (for example, @code{IF}) will treat
1.29 crook 4406: a cell that has @i{any} bit set as @code{true}.
1.67 anton 4407: @c on and off to Memory?
4408: @c true and false to "Bitwise operations" or "Numeric comparison"?
1.44 crook 4409:
1.21 crook 4410: doc-true
4411: doc-false
1.29 crook 4412: doc-on
4413: doc-off
1.21 crook 4414:
1.44 crook 4415:
1.21 crook 4416: @node Arithmetic, Stack Manipulation, Boolean Flags, Words
1.1 anton 4417: @section Arithmetic
4418: @cindex arithmetic words
4419:
4420: @cindex division with potentially negative operands
4421: Forth arithmetic is not checked, i.e., you will not hear about integer
4422: overflow on addition or multiplication, you may hear about division by
4423: zero if you are lucky. The operator is written after the operands, but
4424: the operands are still in the original order. I.e., the infix @code{2-1}
4425: corresponds to @code{2 1 -}. Forth offers a variety of division
4426: operators. If you perform division with potentially negative operands,
4427: you do not want to use @code{/} or @code{/mod} with its undefined
4428: behaviour, but rather @code{fm/mod} or @code{sm/mod} (probably the
4429: former, @pxref{Mixed precision}).
1.26 crook 4430: @comment TODO discuss the different division forms and the std approach
1.1 anton 4431:
4432: @menu
4433: * Single precision::
1.67 anton 4434: * Double precision:: Double-cell integer arithmetic
1.1 anton 4435: * Bitwise operations::
1.67 anton 4436: * Numeric comparison::
1.29 crook 4437: * Mixed precision:: Operations with single and double-cell integers
1.1 anton 4438: * Floating Point::
4439: @end menu
4440:
1.67 anton 4441: @node Single precision, Double precision, Arithmetic, Arithmetic
1.1 anton 4442: @subsection Single precision
4443: @cindex single precision arithmetic words
4444:
1.67 anton 4445: @c !! cell undefined
4446:
4447: By default, numbers in Forth are single-precision integers that are one
1.26 crook 4448: cell in size. They can be signed or unsigned, depending upon how you
1.49 anton 4449: treat them. For the rules used by the text interpreter for recognising
4450: single-precision integers see @ref{Number Conversion}.
1.21 crook 4451:
1.67 anton 4452: These words are all defined for signed operands, but some of them also
4453: work for unsigned numbers: @code{+}, @code{1+}, @code{-}, @code{1-},
4454: @code{*}.
1.44 crook 4455:
1.1 anton 4456: doc-+
1.21 crook 4457: doc-1+
1.1 anton 4458: doc--
1.21 crook 4459: doc-1-
1.1 anton 4460: doc-*
4461: doc-/
4462: doc-mod
4463: doc-/mod
4464: doc-negate
4465: doc-abs
4466: doc-min
4467: doc-max
1.27 crook 4468: doc-floored
1.1 anton 4469:
1.44 crook 4470:
1.67 anton 4471: @node Double precision, Bitwise operations, Single precision, Arithmetic
1.21 crook 4472: @subsection Double precision
4473: @cindex double precision arithmetic words
4474:
1.49 anton 4475: For the rules used by the text interpreter for
4476: recognising double-precision integers, see @ref{Number Conversion}.
1.21 crook 4477:
4478: A double precision number is represented by a cell pair, with the most
1.67 anton 4479: significant cell at the TOS. It is trivial to convert an unsigned single
4480: to a double: simply push a @code{0} onto the TOS. Since numbers are
4481: represented by Gforth using 2's complement arithmetic, converting a
4482: signed single to a (signed) double requires sign-extension across the
4483: most significant cell. This can be achieved using @code{s>d}. The moral
4484: of the story is that you cannot convert a number without knowing whether
4485: it represents an unsigned or a signed number.
1.21 crook 4486:
1.67 anton 4487: These words are all defined for signed operands, but some of them also
4488: work for unsigned numbers: @code{d+}, @code{d-}.
1.44 crook 4489:
1.21 crook 4490: doc-s>d
1.67 anton 4491: doc-d>s
1.21 crook 4492: doc-d+
4493: doc-d-
4494: doc-dnegate
4495: doc-dabs
4496: doc-dmin
4497: doc-dmax
4498:
1.44 crook 4499:
1.67 anton 4500: @node Bitwise operations, Numeric comparison, Double precision, Arithmetic
4501: @subsection Bitwise operations
4502: @cindex bitwise operation words
4503:
4504:
4505: doc-and
4506: doc-or
4507: doc-xor
4508: doc-invert
4509: doc-lshift
4510: doc-rshift
4511: doc-2*
4512: doc-d2*
4513: doc-2/
4514: doc-d2/
4515:
4516:
4517: @node Numeric comparison, Mixed precision, Bitwise operations, Arithmetic
1.21 crook 4518: @subsection Numeric comparison
4519: @cindex numeric comparison words
4520:
1.67 anton 4521: Note that the words that compare for equality (@code{= <> 0= 0<> d= d<>
4522: d0= d0<>}) work for for both signed and unsigned numbers.
1.44 crook 4523:
1.28 crook 4524: doc-<
4525: doc-<=
4526: doc-<>
4527: doc-=
4528: doc->
4529: doc->=
4530:
1.21 crook 4531: doc-0<
1.23 crook 4532: doc-0<=
1.21 crook 4533: doc-0<>
4534: doc-0=
1.23 crook 4535: doc-0>
4536: doc-0>=
1.28 crook 4537:
4538: doc-u<
4539: doc-u<=
1.44 crook 4540: @c u<> and u= exist but are the same as <> and =
1.31 anton 4541: @c doc-u<>
4542: @c doc-u=
1.28 crook 4543: doc-u>
4544: doc-u>=
4545:
4546: doc-within
4547:
4548: doc-d<
4549: doc-d<=
4550: doc-d<>
4551: doc-d=
4552: doc-d>
4553: doc-d>=
1.23 crook 4554:
1.21 crook 4555: doc-d0<
1.23 crook 4556: doc-d0<=
4557: doc-d0<>
1.21 crook 4558: doc-d0=
1.23 crook 4559: doc-d0>
4560: doc-d0>=
4561:
1.21 crook 4562: doc-du<
1.28 crook 4563: doc-du<=
1.44 crook 4564: @c du<> and du= exist but are the same as d<> and d=
1.31 anton 4565: @c doc-du<>
4566: @c doc-du=
1.28 crook 4567: doc-du>
4568: doc-du>=
1.1 anton 4569:
1.44 crook 4570:
1.21 crook 4571: @node Mixed precision, Floating Point, Numeric comparison, Arithmetic
1.1 anton 4572: @subsection Mixed precision
4573: @cindex mixed precision arithmetic words
4574:
1.44 crook 4575:
1.1 anton 4576: doc-m+
4577: doc-*/
4578: doc-*/mod
4579: doc-m*
4580: doc-um*
4581: doc-m*/
4582: doc-um/mod
4583: doc-fm/mod
4584: doc-sm/rem
4585:
1.44 crook 4586:
1.21 crook 4587: @node Floating Point, , Mixed precision, Arithmetic
1.1 anton 4588: @subsection Floating Point
4589: @cindex floating point arithmetic words
4590:
1.49 anton 4591: For the rules used by the text interpreter for
4592: recognising floating-point numbers see @ref{Number Conversion}.
1.1 anton 4593:
1.67 anton 4594: Gforth has a separate floating point stack, but the documentation uses
4595: the unified notation.@footnote{It's easy to generate the separate
4596: notation from that by just separating the floating-point numbers out:
4597: e.g. @code{( n r1 u r2 -- r3 )} becomes @code{( n u -- ) ( F: r1 r2 --
4598: r3 )}.}
1.1 anton 4599:
4600: @cindex floating-point arithmetic, pitfalls
4601: Floating point numbers have a number of unpleasant surprises for the
4602: unwary (e.g., floating point addition is not associative) and even a few
4603: for the wary. You should not use them unless you know what you are doing
4604: or you don't care that the results you get are totally bogus. If you
4605: want to learn about the problems of floating point numbers (and how to
1.66 anton 4606: avoid them), you might start with @cite{David Goldberg,
4607: @uref{http://www.validgh.com/goldberg/paper.ps,What Every Computer
4608: Scientist Should Know About Floating-Point Arithmetic}, ACM Computing
4609: Surveys 23(1):5@minus{}48, March 1991}.
1.1 anton 4610:
1.44 crook 4611:
1.21 crook 4612: doc-d>f
4613: doc-f>d
1.1 anton 4614: doc-f+
4615: doc-f-
4616: doc-f*
4617: doc-f/
4618: doc-fnegate
4619: doc-fabs
4620: doc-fmax
4621: doc-fmin
4622: doc-floor
4623: doc-fround
4624: doc-f**
4625: doc-fsqrt
4626: doc-fexp
4627: doc-fexpm1
4628: doc-fln
4629: doc-flnp1
4630: doc-flog
4631: doc-falog
1.32 anton 4632: doc-f2*
4633: doc-f2/
4634: doc-1/f
4635: doc-precision
4636: doc-set-precision
4637:
4638: @cindex angles in trigonometric operations
4639: @cindex trigonometric operations
4640: Angles in floating point operations are given in radians (a full circle
4641: has 2 pi radians).
4642:
1.1 anton 4643: doc-fsin
4644: doc-fcos
4645: doc-fsincos
4646: doc-ftan
4647: doc-fasin
4648: doc-facos
4649: doc-fatan
4650: doc-fatan2
4651: doc-fsinh
4652: doc-fcosh
4653: doc-ftanh
4654: doc-fasinh
4655: doc-facosh
4656: doc-fatanh
1.21 crook 4657: doc-pi
1.28 crook 4658:
1.32 anton 4659: @cindex equality of floats
4660: @cindex floating-point comparisons
1.31 anton 4661: One particular problem with floating-point arithmetic is that comparison
4662: for equality often fails when you would expect it to succeed. For this
4663: reason approximate equality is often preferred (but you still have to
1.67 anton 4664: know what you are doing). Also note that IEEE NaNs may compare
1.68 anton 4665: differently from what you might expect. The comparison words are:
1.31 anton 4666:
4667: doc-f~rel
4668: doc-f~abs
1.68 anton 4669: doc-f~
1.31 anton 4670: doc-f=
4671: doc-f<>
4672:
4673: doc-f<
4674: doc-f<=
4675: doc-f>
4676: doc-f>=
4677:
1.21 crook 4678: doc-f0<
1.28 crook 4679: doc-f0<=
4680: doc-f0<>
1.21 crook 4681: doc-f0=
1.28 crook 4682: doc-f0>
4683: doc-f0>=
4684:
1.1 anton 4685:
4686: @node Stack Manipulation, Memory, Arithmetic, Words
4687: @section Stack Manipulation
4688: @cindex stack manipulation words
4689:
4690: @cindex floating-point stack in the standard
1.21 crook 4691: Gforth maintains a number of separate stacks:
4692:
1.29 crook 4693: @cindex data stack
4694: @cindex parameter stack
1.21 crook 4695: @itemize @bullet
4696: @item
1.29 crook 4697: A data stack (also known as the @dfn{parameter stack}) -- for
4698: characters, cells, addresses, and double cells.
1.21 crook 4699:
1.29 crook 4700: @cindex floating-point stack
1.21 crook 4701: @item
1.44 crook 4702: A floating point stack -- for holding floating point (FP) numbers.
1.21 crook 4703:
1.29 crook 4704: @cindex return stack
1.21 crook 4705: @item
1.44 crook 4706: A return stack -- for holding the return addresses of colon
1.32 anton 4707: definitions and other (non-FP) data.
1.21 crook 4708:
1.29 crook 4709: @cindex locals stack
1.21 crook 4710: @item
1.44 crook 4711: A locals stack -- for holding local variables.
1.21 crook 4712: @end itemize
4713:
1.1 anton 4714: @menu
4715: * Data stack::
4716: * Floating point stack::
4717: * Return stack::
4718: * Locals stack::
4719: * Stack pointer manipulation::
4720: @end menu
4721:
4722: @node Data stack, Floating point stack, Stack Manipulation, Stack Manipulation
4723: @subsection Data stack
4724: @cindex data stack manipulation words
4725: @cindex stack manipulations words, data stack
4726:
1.44 crook 4727:
1.1 anton 4728: doc-drop
4729: doc-nip
4730: doc-dup
4731: doc-over
4732: doc-tuck
4733: doc-swap
1.21 crook 4734: doc-pick
1.1 anton 4735: doc-rot
4736: doc--rot
4737: doc-?dup
4738: doc-roll
4739: doc-2drop
4740: doc-2nip
4741: doc-2dup
4742: doc-2over
4743: doc-2tuck
4744: doc-2swap
4745: doc-2rot
4746:
1.44 crook 4747:
1.1 anton 4748: @node Floating point stack, Return stack, Data stack, Stack Manipulation
4749: @subsection Floating point stack
4750: @cindex floating-point stack manipulation words
4751: @cindex stack manipulation words, floating-point stack
4752:
1.32 anton 4753: Whilst every sane Forth has a separate floating-point stack, it is not
4754: strictly required; an ANS Forth system could theoretically keep
4755: floating-point numbers on the data stack. As an additional difficulty,
4756: you don't know how many cells a floating-point number takes. It is
4757: reportedly possible to write words in a way that they work also for a
4758: unified stack model, but we do not recommend trying it. Instead, just
4759: say that your program has an environmental dependency on a separate
4760: floating-point stack.
4761:
4762: doc-floating-stack
4763:
1.1 anton 4764: doc-fdrop
4765: doc-fnip
4766: doc-fdup
4767: doc-fover
4768: doc-ftuck
4769: doc-fswap
1.21 crook 4770: doc-fpick
1.1 anton 4771: doc-frot
4772:
1.44 crook 4773:
1.1 anton 4774: @node Return stack, Locals stack, Floating point stack, Stack Manipulation
4775: @subsection Return stack
4776: @cindex return stack manipulation words
4777: @cindex stack manipulation words, return stack
4778:
1.32 anton 4779: @cindex return stack and locals
4780: @cindex locals and return stack
4781: A Forth system is allowed to keep local variables on the
4782: return stack. This is reasonable, as local variables usually eliminate
4783: the need to use the return stack explicitly. So, if you want to produce
4784: a standard compliant program and you are using local variables in a
4785: word, forget about return stack manipulations in that word (refer to the
4786: standard document for the exact rules).
4787:
1.1 anton 4788: doc->r
4789: doc-r>
4790: doc-r@
4791: doc-rdrop
4792: doc-2>r
4793: doc-2r>
4794: doc-2r@
4795: doc-2rdrop
4796:
1.44 crook 4797:
1.1 anton 4798: @node Locals stack, Stack pointer manipulation, Return stack, Stack Manipulation
4799: @subsection Locals stack
4800:
1.78 anton 4801: Gforth uses an extra locals stack. It is described, along with the
4802: reasons for its existence, in @ref{Locals implementation}.
1.21 crook 4803:
1.1 anton 4804: @node Stack pointer manipulation, , Locals stack, Stack Manipulation
4805: @subsection Stack pointer manipulation
4806: @cindex stack pointer manipulation words
4807:
1.44 crook 4808: @c removed s0 r0 l0 -- they are obsolete aliases for sp0 rp0 lp0
1.21 crook 4809: doc-sp0
1.1 anton 4810: doc-sp@
4811: doc-sp!
1.21 crook 4812: doc-fp0
1.1 anton 4813: doc-fp@
4814: doc-fp!
1.21 crook 4815: doc-rp0
1.1 anton 4816: doc-rp@
4817: doc-rp!
1.21 crook 4818: doc-lp0
1.1 anton 4819: doc-lp@
4820: doc-lp!
4821:
1.44 crook 4822:
1.1 anton 4823: @node Memory, Control Structures, Stack Manipulation, Words
4824: @section Memory
1.26 crook 4825: @cindex memory words
1.1 anton 4826:
1.32 anton 4827: @menu
4828: * Memory model::
4829: * Dictionary allocation::
4830: * Heap Allocation::
4831: * Memory Access::
4832: * Address arithmetic::
4833: * Memory Blocks::
4834: @end menu
4835:
1.67 anton 4836: In addition to the standard Forth memory allocation words, there is also
4837: a @uref{http://www.complang.tuwien.ac.at/forth/garbage-collection.zip,
4838: garbage collector}.
4839:
1.32 anton 4840: @node Memory model, Dictionary allocation, Memory, Memory
4841: @subsection ANS Forth and Gforth memory models
4842:
4843: @c The ANS Forth description is a mess (e.g., is the heap part of
4844: @c the dictionary?), so let's not stick to closely with it.
4845:
1.67 anton 4846: ANS Forth considers a Forth system as consisting of several address
4847: spaces, of which only @dfn{data space} is managed and accessible with
4848: the memory words. Memory not necessarily in data space includes the
4849: stacks, the code (called code space) and the headers (called name
4850: space). In Gforth everything is in data space, but the code for the
4851: primitives is usually read-only.
1.32 anton 4852:
4853: Data space is divided into a number of areas: The (data space portion of
4854: the) dictionary@footnote{Sometimes, the term @dfn{dictionary} is used to
4855: refer to the search data structure embodied in word lists and headers,
4856: because it is used for looking up names, just as you would in a
4857: conventional dictionary.}, the heap, and a number of system-allocated
4858: buffers.
4859:
1.68 anton 4860: @cindex address arithmetic restrictions, ANS vs. Gforth
4861: @cindex contiguous regions, ANS vs. Gforth
1.32 anton 4862: In ANS Forth data space is also divided into contiguous regions. You
4863: can only use address arithmetic within a contiguous region, not between
4864: them. Usually each allocation gives you one contiguous region, but the
1.33 anton 4865: dictionary allocation words have additional rules (@pxref{Dictionary
1.32 anton 4866: allocation}).
4867:
4868: Gforth provides one big address space, and address arithmetic can be
4869: performed between any addresses. However, in the dictionary headers or
4870: code are interleaved with data, so almost the only contiguous data space
4871: regions there are those described by ANS Forth as contiguous; but you
4872: can be sure that the dictionary is allocated towards increasing
4873: addresses even between contiguous regions. The memory order of
4874: allocations in the heap is platform-dependent (and possibly different
4875: from one run to the next).
4876:
1.27 crook 4877:
1.32 anton 4878: @node Dictionary allocation, Heap Allocation, Memory model, Memory
4879: @subsection Dictionary allocation
1.27 crook 4880: @cindex reserving data space
4881: @cindex data space - reserving some
4882:
1.32 anton 4883: Dictionary allocation is a stack-oriented allocation scheme, i.e., if
4884: you want to deallocate X, you also deallocate everything
4885: allocated after X.
4886:
1.68 anton 4887: @cindex contiguous regions in dictionary allocation
1.32 anton 4888: The allocations using the words below are contiguous and grow the region
4889: towards increasing addresses. Other words that allocate dictionary
4890: memory of any kind (i.e., defining words including @code{:noname}) end
4891: the contiguous region and start a new one.
4892:
4893: In ANS Forth only @code{create}d words are guaranteed to produce an
4894: address that is the start of the following contiguous region. In
4895: particular, the cell allocated by @code{variable} is not guaranteed to
4896: be contiguous with following @code{allot}ed memory.
4897:
4898: You can deallocate memory by using @code{allot} with a negative argument
4899: (with some restrictions, see @code{allot}). For larger deallocations use
4900: @code{marker}.
1.27 crook 4901:
1.29 crook 4902:
1.27 crook 4903: doc-here
4904: doc-unused
4905: doc-allot
4906: doc-c,
1.29 crook 4907: doc-f,
1.27 crook 4908: doc-,
4909: doc-2,
4910:
1.32 anton 4911: Memory accesses have to be aligned (@pxref{Address arithmetic}). So of
4912: course you should allocate memory in an aligned way, too. I.e., before
4913: allocating allocating a cell, @code{here} must be cell-aligned, etc.
4914: The words below align @code{here} if it is not already. Basically it is
4915: only already aligned for a type, if the last allocation was a multiple
4916: of the size of this type and if @code{here} was aligned for this type
4917: before.
4918:
4919: After freshly @code{create}ing a word, @code{here} is @code{align}ed in
4920: ANS Forth (@code{maxalign}ed in Gforth).
4921:
4922: doc-align
4923: doc-falign
4924: doc-sfalign
4925: doc-dfalign
4926: doc-maxalign
4927: doc-cfalign
4928:
4929:
4930: @node Heap Allocation, Memory Access, Dictionary allocation, Memory
4931: @subsection Heap allocation
4932: @cindex heap allocation
4933: @cindex dynamic allocation of memory
4934: @cindex memory-allocation word set
4935:
1.68 anton 4936: @cindex contiguous regions and heap allocation
1.32 anton 4937: Heap allocation supports deallocation of allocated memory in any
4938: order. Dictionary allocation is not affected by it (i.e., it does not
4939: end a contiguous region). In Gforth, these words are implemented using
4940: the standard C library calls malloc(), free() and resize().
4941:
1.68 anton 4942: The memory region produced by one invocation of @code{allocate} or
4943: @code{resize} is internally contiguous. There is no contiguity between
4944: such a region and any other region (including others allocated from the
4945: heap).
4946:
1.32 anton 4947: doc-allocate
4948: doc-free
4949: doc-resize
4950:
1.27 crook 4951:
1.32 anton 4952: @node Memory Access, Address arithmetic, Heap Allocation, Memory
1.1 anton 4953: @subsection Memory Access
4954: @cindex memory access words
4955:
4956: doc-@
4957: doc-!
4958: doc-+!
4959: doc-c@
4960: doc-c!
4961: doc-2@
4962: doc-2!
4963: doc-f@
4964: doc-f!
4965: doc-sf@
4966: doc-sf!
4967: doc-df@
4968: doc-df!
4969:
1.68 anton 4970:
1.32 anton 4971: @node Address arithmetic, Memory Blocks, Memory Access, Memory
4972: @subsection Address arithmetic
1.1 anton 4973: @cindex address arithmetic words
4974:
1.67 anton 4975: Address arithmetic is the foundation on which you can build data
4976: structures like arrays, records (@pxref{Structures}) and objects
4977: (@pxref{Object-oriented Forth}).
1.32 anton 4978:
1.68 anton 4979: @cindex address unit
4980: @cindex au (address unit)
1.1 anton 4981: ANS Forth does not specify the sizes of the data types. Instead, it
4982: offers a number of words for computing sizes and doing address
1.29 crook 4983: arithmetic. Address arithmetic is performed in terms of address units
4984: (aus); on most systems the address unit is one byte. Note that a
4985: character may have more than one au, so @code{chars} is no noop (on
1.68 anton 4986: platforms where it is a noop, it compiles to nothing).
1.1 anton 4987:
1.67 anton 4988: The basic address arithmetic words are @code{+} and @code{-}. E.g., if
4989: you have the address of a cell, perform @code{1 cells +}, and you will
4990: have the address of the next cell.
4991:
1.68 anton 4992: @cindex contiguous regions and address arithmetic
1.67 anton 4993: In ANS Forth you can perform address arithmetic only within a contiguous
4994: region, i.e., if you have an address into one region, you can only add
4995: and subtract such that the result is still within the region; you can
4996: only subtract or compare addresses from within the same contiguous
4997: region. Reasons: several contiguous regions can be arranged in memory
4998: in any way; on segmented systems addresses may have unusual
4999: representations, such that address arithmetic only works within a
5000: region. Gforth provides a few more guarantees (linear address space,
5001: dictionary grows upwards), but in general I have found it easy to stay
5002: within contiguous regions (exception: computing and comparing to the
5003: address just beyond the end of an array).
5004:
1.1 anton 5005: @cindex alignment of addresses for types
5006: ANS Forth also defines words for aligning addresses for specific
5007: types. Many computers require that accesses to specific data types
5008: must only occur at specific addresses; e.g., that cells may only be
5009: accessed at addresses divisible by 4. Even if a machine allows unaligned
5010: accesses, it can usually perform aligned accesses faster.
5011:
5012: For the performance-conscious: alignment operations are usually only
5013: necessary during the definition of a data structure, not during the
5014: (more frequent) accesses to it.
5015:
5016: ANS Forth defines no words for character-aligning addresses. This is not
5017: an oversight, but reflects the fact that addresses that are not
5018: char-aligned have no use in the standard and therefore will not be
5019: created.
5020:
5021: @cindex @code{CREATE} and alignment
1.29 crook 5022: ANS Forth guarantees that addresses returned by @code{CREATE}d words
1.1 anton 5023: are cell-aligned; in addition, Gforth guarantees that these addresses
5024: are aligned for all purposes.
5025:
1.26 crook 5026: Note that the ANS Forth word @code{char} has nothing to do with address
5027: arithmetic.
1.1 anton 5028:
1.44 crook 5029:
1.1 anton 5030: doc-chars
5031: doc-char+
5032: doc-cells
5033: doc-cell+
5034: doc-cell
5035: doc-aligned
5036: doc-floats
5037: doc-float+
5038: doc-float
5039: doc-faligned
5040: doc-sfloats
5041: doc-sfloat+
5042: doc-sfaligned
5043: doc-dfloats
5044: doc-dfloat+
5045: doc-dfaligned
5046: doc-maxaligned
5047: doc-cfaligned
5048: doc-address-unit-bits
5049:
1.44 crook 5050:
1.32 anton 5051: @node Memory Blocks, , Address arithmetic, Memory
1.1 anton 5052: @subsection Memory Blocks
5053: @cindex memory block words
1.27 crook 5054: @cindex character strings - moving and copying
5055:
1.49 anton 5056: Memory blocks often represent character strings; For ways of storing
5057: character strings in memory see @ref{String Formats}. For other
5058: string-processing words see @ref{Displaying characters and strings}.
1.1 anton 5059:
1.67 anton 5060: A few of these words work on address unit blocks. In that case, you
5061: usually have to insert @code{CHARS} before the word when working on
5062: character strings. Most words work on character blocks, and expect a
5063: char-aligned address.
5064:
5065: When copying characters between overlapping memory regions, use
5066: @code{chars move} or choose carefully between @code{cmove} and
5067: @code{cmove>}.
1.44 crook 5068:
1.1 anton 5069: doc-move
5070: doc-erase
5071: doc-cmove
5072: doc-cmove>
5073: doc-fill
5074: doc-blank
1.21 crook 5075: doc-compare
1.111 anton 5076: doc-str=
5077: doc-str<
5078: doc-string-prefix?
1.21 crook 5079: doc-search
1.27 crook 5080: doc--trailing
5081: doc-/string
1.82 anton 5082: doc-bounds
1.44 crook 5083:
1.111 anton 5084:
1.27 crook 5085: @comment TODO examples
5086:
1.1 anton 5087:
1.26 crook 5088: @node Control Structures, Defining Words, Memory, Words
1.1 anton 5089: @section Control Structures
5090: @cindex control structures
5091:
1.33 anton 5092: Control structures in Forth cannot be used interpretively, only in a
5093: colon definition@footnote{To be precise, they have no interpretation
5094: semantics (@pxref{Interpretation and Compilation Semantics}).}. We do
5095: not like this limitation, but have not seen a satisfying way around it
5096: yet, although many schemes have been proposed.
1.1 anton 5097:
5098: @menu
1.33 anton 5099: * Selection:: IF ... ELSE ... ENDIF
5100: * Simple Loops:: BEGIN ...
1.29 crook 5101: * Counted Loops:: DO
1.67 anton 5102: * Arbitrary control structures::
5103: * Calls and returns::
1.1 anton 5104: * Exception Handling::
5105: @end menu
5106:
5107: @node Selection, Simple Loops, Control Structures, Control Structures
5108: @subsection Selection
5109: @cindex selection control structures
5110: @cindex control structures for selection
5111:
5112: @cindex @code{IF} control structure
5113: @example
1.29 crook 5114: @i{flag}
1.1 anton 5115: IF
1.29 crook 5116: @i{code}
1.1 anton 5117: ENDIF
5118: @end example
1.21 crook 5119: @noindent
1.33 anton 5120:
1.44 crook 5121: If @i{flag} is non-zero (as far as @code{IF} etc. are concerned, a cell
5122: with any bit set represents truth) @i{code} is executed.
1.33 anton 5123:
1.1 anton 5124: @example
1.29 crook 5125: @i{flag}
1.1 anton 5126: IF
1.29 crook 5127: @i{code1}
1.1 anton 5128: ELSE
1.29 crook 5129: @i{code2}
1.1 anton 5130: ENDIF
5131: @end example
5132:
1.44 crook 5133: If @var{flag} is true, @i{code1} is executed, otherwise @i{code2} is
5134: executed.
1.33 anton 5135:
1.1 anton 5136: You can use @code{THEN} instead of @code{ENDIF}. Indeed, @code{THEN} is
5137: standard, and @code{ENDIF} is not, although it is quite popular. We
5138: recommend using @code{ENDIF}, because it is less confusing for people
5139: who also know other languages (and is not prone to reinforcing negative
5140: prejudices against Forth in these people). Adding @code{ENDIF} to a
5141: system that only supplies @code{THEN} is simple:
5142: @example
1.82 anton 5143: : ENDIF POSTPONE then ; immediate
1.1 anton 5144: @end example
5145:
5146: [According to @cite{Webster's New Encyclopedic Dictionary}, @dfn{then
5147: (adv.)} has the following meanings:
5148: @quotation
5149: ... 2b: following next after in order ... 3d: as a necessary consequence
5150: (if you were there, then you saw them).
5151: @end quotation
5152: Forth's @code{THEN} has the meaning 2b, whereas @code{THEN} in Pascal
5153: and many other programming languages has the meaning 3d.]
5154:
1.21 crook 5155: Gforth also provides the words @code{?DUP-IF} and @code{?DUP-0=-IF}, so
1.1 anton 5156: you can avoid using @code{?dup}. Using these alternatives is also more
1.26 crook 5157: efficient than using @code{?dup}. Definitions in ANS Forth
1.1 anton 5158: for @code{ENDIF}, @code{?DUP-IF} and @code{?DUP-0=-IF} are provided in
5159: @file{compat/control.fs}.
5160:
5161: @cindex @code{CASE} control structure
5162: @example
1.29 crook 5163: @i{n}
1.1 anton 5164: CASE
1.29 crook 5165: @i{n1} OF @i{code1} ENDOF
5166: @i{n2} OF @i{code2} ENDOF
1.1 anton 5167: @dots{}
1.68 anton 5168: ( n ) @i{default-code} ( n )
1.1 anton 5169: ENDCASE
5170: @end example
5171:
1.68 anton 5172: Executes the first @i{codei}, where the @i{ni} is equal to @i{n}. If no
5173: @i{ni} matches, the optional @i{default-code} is executed. The optional
5174: default case can be added by simply writing the code after the last
5175: @code{ENDOF}. It may use @i{n}, which is on top of the stack, but must
5176: not consume it.
1.1 anton 5177:
1.69 anton 5178: @progstyle
5179: To keep the code understandable, you should ensure that on all paths
5180: through a selection construct the stack is changed in the same way
5181: (wrt. number and types of stack items consumed and pushed).
5182:
1.1 anton 5183: @node Simple Loops, Counted Loops, Selection, Control Structures
5184: @subsection Simple Loops
5185: @cindex simple loops
5186: @cindex loops without count
5187:
5188: @cindex @code{WHILE} loop
5189: @example
5190: BEGIN
1.29 crook 5191: @i{code1}
5192: @i{flag}
1.1 anton 5193: WHILE
1.29 crook 5194: @i{code2}
1.1 anton 5195: REPEAT
5196: @end example
5197:
1.29 crook 5198: @i{code1} is executed and @i{flag} is computed. If it is true,
5199: @i{code2} is executed and the loop is restarted; If @i{flag} is
1.1 anton 5200: false, execution continues after the @code{REPEAT}.
5201:
5202: @cindex @code{UNTIL} loop
5203: @example
5204: BEGIN
1.29 crook 5205: @i{code}
5206: @i{flag}
1.1 anton 5207: UNTIL
5208: @end example
5209:
1.29 crook 5210: @i{code} is executed. The loop is restarted if @code{flag} is false.
1.1 anton 5211:
1.69 anton 5212: @progstyle
5213: To keep the code understandable, a complete iteration of the loop should
5214: not change the number and types of the items on the stacks.
5215:
1.1 anton 5216: @cindex endless loop
5217: @cindex loops, endless
5218: @example
5219: BEGIN
1.29 crook 5220: @i{code}
1.1 anton 5221: AGAIN
5222: @end example
5223:
5224: This is an endless loop.
5225:
5226: @node Counted Loops, Arbitrary control structures, Simple Loops, Control Structures
5227: @subsection Counted Loops
5228: @cindex counted loops
5229: @cindex loops, counted
5230: @cindex @code{DO} loops
5231:
5232: The basic counted loop is:
5233: @example
1.29 crook 5234: @i{limit} @i{start}
1.1 anton 5235: ?DO
1.29 crook 5236: @i{body}
1.1 anton 5237: LOOP
5238: @end example
5239:
1.29 crook 5240: This performs one iteration for every integer, starting from @i{start}
5241: and up to, but excluding @i{limit}. The counter, or @i{index}, can be
1.21 crook 5242: accessed with @code{i}. For example, the loop:
1.1 anton 5243: @example
5244: 10 0 ?DO
5245: i .
5246: LOOP
5247: @end example
1.21 crook 5248: @noindent
5249: prints @code{0 1 2 3 4 5 6 7 8 9}
5250:
1.1 anton 5251: The index of the innermost loop can be accessed with @code{i}, the index
5252: of the next loop with @code{j}, and the index of the third loop with
5253: @code{k}.
5254:
1.44 crook 5255:
1.1 anton 5256: doc-i
5257: doc-j
5258: doc-k
5259:
1.44 crook 5260:
1.1 anton 5261: The loop control data are kept on the return stack, so there are some
1.21 crook 5262: restrictions on mixing return stack accesses and counted loop words. In
5263: particuler, if you put values on the return stack outside the loop, you
5264: cannot read them inside the loop@footnote{well, not in a way that is
5265: portable.}. If you put values on the return stack within a loop, you
5266: have to remove them before the end of the loop and before accessing the
5267: index of the loop.
1.1 anton 5268:
5269: There are several variations on the counted loop:
5270:
1.21 crook 5271: @itemize @bullet
5272: @item
5273: @code{LEAVE} leaves the innermost counted loop immediately; execution
5274: continues after the associated @code{LOOP} or @code{NEXT}. For example:
5275:
5276: @example
5277: 10 0 ?DO i DUP . 3 = IF LEAVE THEN LOOP
5278: @end example
5279: prints @code{0 1 2 3}
5280:
1.1 anton 5281:
1.21 crook 5282: @item
5283: @code{UNLOOP} prepares for an abnormal loop exit, e.g., via
5284: @code{EXIT}. @code{UNLOOP} removes the loop control parameters from the
5285: return stack so @code{EXIT} can get to its return address. For example:
5286:
5287: @example
5288: : demo 10 0 ?DO i DUP . 3 = IF UNLOOP EXIT THEN LOOP ." Done" ;
5289: @end example
5290: prints @code{0 1 2 3}
5291:
5292:
5293: @item
1.29 crook 5294: If @i{start} is greater than @i{limit}, a @code{?DO} loop is entered
1.1 anton 5295: (and @code{LOOP} iterates until they become equal by wrap-around
5296: arithmetic). This behaviour is usually not what you want. Therefore,
5297: Gforth offers @code{+DO} and @code{U+DO} (as replacements for
1.29 crook 5298: @code{?DO}), which do not enter the loop if @i{start} is greater than
5299: @i{limit}; @code{+DO} is for signed loop parameters, @code{U+DO} for
1.1 anton 5300: unsigned loop parameters.
5301:
1.21 crook 5302: @item
5303: @code{?DO} can be replaced by @code{DO}. @code{DO} always enters
5304: the loop, independent of the loop parameters. Do not use @code{DO}, even
5305: if you know that the loop is entered in any case. Such knowledge tends
5306: to become invalid during maintenance of a program, and then the
5307: @code{DO} will make trouble.
5308:
5309: @item
1.29 crook 5310: @code{LOOP} can be replaced with @code{@i{n} +LOOP}; this updates the
5311: index by @i{n} instead of by 1. The loop is terminated when the border
5312: between @i{limit-1} and @i{limit} is crossed. E.g.:
1.1 anton 5313:
1.21 crook 5314: @example
5315: 4 0 +DO i . 2 +LOOP
5316: @end example
5317: @noindent
5318: prints @code{0 2}
5319:
5320: @example
5321: 4 1 +DO i . 2 +LOOP
5322: @end example
5323: @noindent
5324: prints @code{1 3}
1.1 anton 5325:
1.68 anton 5326: @item
1.1 anton 5327: @cindex negative increment for counted loops
5328: @cindex counted loops with negative increment
1.29 crook 5329: The behaviour of @code{@i{n} +LOOP} is peculiar when @i{n} is negative:
1.1 anton 5330:
1.21 crook 5331: @example
5332: -1 0 ?DO i . -1 +LOOP
5333: @end example
5334: @noindent
5335: prints @code{0 -1}
1.1 anton 5336:
1.21 crook 5337: @example
5338: 0 0 ?DO i . -1 +LOOP
5339: @end example
5340: prints nothing.
1.1 anton 5341:
1.29 crook 5342: Therefore we recommend avoiding @code{@i{n} +LOOP} with negative
5343: @i{n}. One alternative is @code{@i{u} -LOOP}, which reduces the
5344: index by @i{u} each iteration. The loop is terminated when the border
5345: between @i{limit+1} and @i{limit} is crossed. Gforth also provides
1.1 anton 5346: @code{-DO} and @code{U-DO} for down-counting loops. E.g.:
5347:
1.21 crook 5348: @example
5349: -2 0 -DO i . 1 -LOOP
5350: @end example
5351: @noindent
5352: prints @code{0 -1}
1.1 anton 5353:
1.21 crook 5354: @example
5355: -1 0 -DO i . 1 -LOOP
5356: @end example
5357: @noindent
5358: prints @code{0}
5359:
5360: @example
5361: 0 0 -DO i . 1 -LOOP
5362: @end example
5363: @noindent
5364: prints nothing.
1.1 anton 5365:
1.21 crook 5366: @end itemize
1.1 anton 5367:
5368: Unfortunately, @code{+DO}, @code{U+DO}, @code{-DO}, @code{U-DO} and
1.26 crook 5369: @code{-LOOP} are not defined in ANS Forth. However, an implementation
5370: for these words that uses only standard words is provided in
5371: @file{compat/loops.fs}.
1.1 anton 5372:
5373:
5374: @cindex @code{FOR} loops
1.26 crook 5375: Another counted loop is:
1.1 anton 5376: @example
1.29 crook 5377: @i{n}
1.1 anton 5378: FOR
1.29 crook 5379: @i{body}
1.1 anton 5380: NEXT
5381: @end example
5382: This is the preferred loop of native code compiler writers who are too
1.26 crook 5383: lazy to optimize @code{?DO} loops properly. This loop structure is not
1.29 crook 5384: defined in ANS Forth. In Gforth, this loop iterates @i{n+1} times;
5385: @code{i} produces values starting with @i{n} and ending with 0. Other
1.26 crook 5386: Forth systems may behave differently, even if they support @code{FOR}
5387: loops. To avoid problems, don't use @code{FOR} loops.
1.1 anton 5388:
5389: @node Arbitrary control structures, Calls and returns, Counted Loops, Control Structures
5390: @subsection Arbitrary control structures
5391: @cindex control structures, user-defined
5392:
5393: @cindex control-flow stack
5394: ANS Forth permits and supports using control structures in a non-nested
5395: way. Information about incomplete control structures is stored on the
5396: control-flow stack. This stack may be implemented on the Forth data
5397: stack, and this is what we have done in Gforth.
5398:
5399: @cindex @code{orig}, control-flow stack item
5400: @cindex @code{dest}, control-flow stack item
5401: An @i{orig} entry represents an unresolved forward branch, a @i{dest}
5402: entry represents a backward branch target. A few words are the basis for
5403: building any control structure possible (except control structures that
5404: need storage, like calls, coroutines, and backtracking).
5405:
1.44 crook 5406:
1.1 anton 5407: doc-if
5408: doc-ahead
5409: doc-then
5410: doc-begin
5411: doc-until
5412: doc-again
5413: doc-cs-pick
5414: doc-cs-roll
5415:
1.44 crook 5416:
1.21 crook 5417: The Standard words @code{CS-PICK} and @code{CS-ROLL} allow you to
5418: manipulate the control-flow stack in a portable way. Without them, you
5419: would need to know how many stack items are occupied by a control-flow
5420: entry (many systems use one cell. In Gforth they currently take three,
5421: but this may change in the future).
5422:
1.1 anton 5423: Some standard control structure words are built from these words:
5424:
1.44 crook 5425:
1.1 anton 5426: doc-else
5427: doc-while
5428: doc-repeat
5429:
1.44 crook 5430:
5431: @noindent
1.1 anton 5432: Gforth adds some more control-structure words:
5433:
1.44 crook 5434:
1.1 anton 5435: doc-endif
5436: doc-?dup-if
5437: doc-?dup-0=-if
5438:
1.44 crook 5439:
5440: @noindent
1.1 anton 5441: Counted loop words constitute a separate group of words:
5442:
1.44 crook 5443:
1.1 anton 5444: doc-?do
5445: doc-+do
5446: doc-u+do
5447: doc--do
5448: doc-u-do
5449: doc-do
5450: doc-for
5451: doc-loop
5452: doc-+loop
5453: doc--loop
5454: doc-next
5455: doc-leave
5456: doc-?leave
5457: doc-unloop
5458: doc-done
5459:
1.44 crook 5460:
1.21 crook 5461: The standard does not allow using @code{CS-PICK} and @code{CS-ROLL} on
5462: @i{do-sys}. Gforth allows it, but it's your job to ensure that for
1.1 anton 5463: every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path
5464: through the definition (@code{LOOP} etc. compile an @code{UNLOOP} on the
5465: fall-through path). Also, you have to ensure that all @code{LEAVE}s are
5466: resolved (by using one of the loop-ending words or @code{DONE}).
5467:
1.44 crook 5468: @noindent
1.26 crook 5469: Another group of control structure words are:
1.1 anton 5470:
1.44 crook 5471:
1.1 anton 5472: doc-case
5473: doc-endcase
5474: doc-of
5475: doc-endof
5476:
1.44 crook 5477:
1.21 crook 5478: @i{case-sys} and @i{of-sys} cannot be processed using @code{CS-PICK} and
5479: @code{CS-ROLL}.
1.1 anton 5480:
5481: @subsubsection Programming Style
1.47 crook 5482: @cindex control structures programming style
5483: @cindex programming style, arbitrary control structures
1.1 anton 5484:
5485: In order to ensure readability we recommend that you do not create
5486: arbitrary control structures directly, but define new control structure
5487: words for the control structure you want and use these words in your
1.26 crook 5488: program. For example, instead of writing:
1.1 anton 5489:
5490: @example
1.26 crook 5491: BEGIN
1.1 anton 5492: ...
1.26 crook 5493: IF [ 1 CS-ROLL ]
1.1 anton 5494: ...
1.26 crook 5495: AGAIN THEN
1.1 anton 5496: @end example
5497:
1.21 crook 5498: @noindent
1.1 anton 5499: we recommend defining control structure words, e.g.,
5500:
5501: @example
1.26 crook 5502: : WHILE ( DEST -- ORIG DEST )
5503: POSTPONE IF
5504: 1 CS-ROLL ; immediate
5505:
5506: : REPEAT ( orig dest -- )
5507: POSTPONE AGAIN
5508: POSTPONE THEN ; immediate
1.1 anton 5509: @end example
5510:
1.21 crook 5511: @noindent
1.1 anton 5512: and then using these to create the control structure:
5513:
5514: @example
1.26 crook 5515: BEGIN
1.1 anton 5516: ...
1.26 crook 5517: WHILE
1.1 anton 5518: ...
1.26 crook 5519: REPEAT
1.1 anton 5520: @end example
5521:
5522: That's much easier to read, isn't it? Of course, @code{REPEAT} and
5523: @code{WHILE} are predefined, so in this example it would not be
5524: necessary to define them.
5525:
5526: @node Calls and returns, Exception Handling, Arbitrary control structures, Control Structures
5527: @subsection Calls and returns
5528: @cindex calling a definition
5529: @cindex returning from a definition
5530:
1.3 anton 5531: @cindex recursive definitions
5532: A definition can be called simply be writing the name of the definition
1.26 crook 5533: to be called. Normally a definition is invisible during its own
1.3 anton 5534: definition. If you want to write a directly recursive definition, you
1.26 crook 5535: can use @code{recursive} to make the current definition visible, or
5536: @code{recurse} to call the current definition directly.
1.3 anton 5537:
1.44 crook 5538:
1.3 anton 5539: doc-recursive
5540: doc-recurse
5541:
1.44 crook 5542:
1.21 crook 5543: @comment TODO add example of the two recursion methods
1.12 anton 5544: @quotation
5545: @progstyle
5546: I prefer using @code{recursive} to @code{recurse}, because calling the
5547: definition by name is more descriptive (if the name is well-chosen) than
5548: the somewhat cryptic @code{recurse}. E.g., in a quicksort
5549: implementation, it is much better to read (and think) ``now sort the
5550: partitions'' than to read ``now do a recursive call''.
5551: @end quotation
1.3 anton 5552:
1.29 crook 5553: For mutual recursion, use @code{Defer}red words, like this:
1.3 anton 5554:
5555: @example
1.28 crook 5556: Defer foo
1.3 anton 5557:
5558: : bar ( ... -- ... )
5559: ... foo ... ;
5560:
5561: :noname ( ... -- ... )
5562: ... bar ... ;
5563: IS foo
5564: @end example
5565:
1.44 crook 5566: Deferred words are discussed in more detail in @ref{Deferred words}.
1.33 anton 5567:
1.26 crook 5568: The current definition returns control to the calling definition when
1.33 anton 5569: the end of the definition is reached or @code{EXIT} is encountered.
1.1 anton 5570:
5571: doc-exit
5572: doc-;s
5573:
1.44 crook 5574:
1.1 anton 5575: @node Exception Handling, , Calls and returns, Control Structures
5576: @subsection Exception Handling
1.26 crook 5577: @cindex exceptions
1.1 anton 5578:
1.68 anton 5579: @c quit is a very bad idea for error handling,
5580: @c because it does not translate into a THROW
5581: @c it also does not belong into this chapter
5582:
5583: If a word detects an error condition that it cannot handle, it can
5584: @code{throw} an exception. In the simplest case, this will terminate
5585: your program, and report an appropriate error.
1.21 crook 5586:
1.68 anton 5587: doc-throw
1.1 anton 5588:
1.69 anton 5589: @code{Throw} consumes a cell-sized error number on the stack. There are
5590: some predefined error numbers in ANS Forth (see @file{errors.fs}). In
5591: Gforth (and most other systems) you can use the iors produced by various
5592: words as error numbers (e.g., a typical use of @code{allocate} is
5593: @code{allocate throw}). Gforth also provides the word @code{exception}
5594: to define your own error numbers (with decent error reporting); an ANS
5595: Forth version of this word (but without the error messages) is available
5596: in @code{compat/except.fs}. And finally, you can use your own error
1.68 anton 5597: numbers (anything outside the range -4095..0), but won't get nice error
5598: messages, only numbers. For example, try:
5599:
5600: @example
1.69 anton 5601: -10 throw \ ANS defined
5602: -267 throw \ system defined
5603: s" my error" exception throw \ user defined
5604: 7 throw \ arbitrary number
1.68 anton 5605: @end example
5606:
5607: doc---exception-exception
1.1 anton 5608:
1.69 anton 5609: A common idiom to @code{THROW} a specific error if a flag is true is
5610: this:
5611:
5612: @example
5613: @code{( flag ) 0<> @i{errno} and throw}
5614: @end example
5615:
5616: Your program can provide exception handlers to catch exceptions. An
5617: exception handler can be used to correct the problem, or to clean up
5618: some data structures and just throw the exception to the next exception
5619: handler. Note that @code{throw} jumps to the dynamically innermost
5620: exception handler. The system's exception handler is outermost, and just
5621: prints an error and restarts command-line interpretation (or, in batch
5622: mode (i.e., while processing the shell command line), leaves Gforth).
1.1 anton 5623:
1.68 anton 5624: The ANS Forth way to catch exceptions is @code{catch}:
1.1 anton 5625:
1.68 anton 5626: doc-catch
5627:
5628: The most common use of exception handlers is to clean up the state when
5629: an error happens. E.g.,
1.1 anton 5630:
1.26 crook 5631: @example
1.68 anton 5632: base @ >r hex \ actually the hex should be inside foo, or we h
5633: ['] foo catch ( nerror|0 )
5634: r> base !
1.69 anton 5635: ( nerror|0 ) throw \ pass it on
1.26 crook 5636: @end example
1.1 anton 5637:
1.69 anton 5638: A use of @code{catch} for handling the error @code{myerror} might look
5639: like this:
1.44 crook 5640:
1.68 anton 5641: @example
1.69 anton 5642: ['] foo catch
5643: CASE
5644: myerror OF ... ( do something about it ) ENDOF
5645: dup throw \ default: pass other errors on, do nothing on non-errors
5646: ENDCASE
1.68 anton 5647: @end example
1.44 crook 5648:
1.68 anton 5649: Having to wrap the code into a separate word is often cumbersome,
5650: therefore Gforth provides an alternative syntax:
1.1 anton 5651:
5652: @example
1.69 anton 5653: TRY
1.68 anton 5654: @i{code1}
1.69 anton 5655: RECOVER \ optional
1.68 anton 5656: @i{code2} \ optional
1.69 anton 5657: ENDTRY
1.1 anton 5658: @end example
5659:
1.68 anton 5660: This performs @i{Code1}. If @i{code1} completes normally, execution
5661: continues after the @code{endtry}. If @i{Code1} throws, the stacks are
5662: reset to the state during @code{try}, the throw value is pushed on the
5663: data stack, and execution constinues at @i{code2}, and finally falls
1.92 anton 5664: through the @code{endtry} into the following code.
1.26 crook 5665:
1.68 anton 5666: doc-try
5667: doc-recover
5668: doc-endtry
1.26 crook 5669:
1.69 anton 5670: The cleanup example from above in this syntax:
1.26 crook 5671:
1.68 anton 5672: @example
1.69 anton 5673: base @ >r TRY
1.68 anton 5674: hex foo \ now the hex is placed correctly
1.69 anton 5675: 0 \ value for throw
1.92 anton 5676: RECOVER ENDTRY
1.68 anton 5677: r> base ! throw
1.1 anton 5678: @end example
5679:
1.69 anton 5680: And here's the error handling example:
1.1 anton 5681:
1.68 anton 5682: @example
1.69 anton 5683: TRY
1.68 anton 5684: foo
1.69 anton 5685: RECOVER
5686: CASE
5687: myerror OF ... ( do something about it ) ENDOF
5688: throw \ pass other errors on
5689: ENDCASE
5690: ENDTRY
1.68 anton 5691: @end example
1.1 anton 5692:
1.69 anton 5693: @progstyle
5694: As usual, you should ensure that the stack depth is statically known at
5695: the end: either after the @code{throw} for passing on errors, or after
5696: the @code{ENDTRY} (or, if you use @code{catch}, after the end of the
5697: selection construct for handling the error).
5698:
1.68 anton 5699: There are two alternatives to @code{throw}: @code{Abort"} is conditional
5700: and you can provide an error message. @code{Abort} just produces an
5701: ``Aborted'' error.
1.1 anton 5702:
1.68 anton 5703: The problem with these words is that exception handlers cannot
5704: differentiate between different @code{abort"}s; they just look like
5705: @code{-2 throw} to them (the error message cannot be accessed by
5706: standard programs). Similar @code{abort} looks like @code{-1 throw} to
5707: exception handlers.
1.44 crook 5708:
1.68 anton 5709: doc-abort"
1.26 crook 5710: doc-abort
1.29 crook 5711:
5712:
1.44 crook 5713:
1.29 crook 5714: @c -------------------------------------------------------------
1.47 crook 5715: @node Defining Words, Interpretation and Compilation Semantics, Control Structures, Words
1.29 crook 5716: @section Defining Words
5717: @cindex defining words
5718:
1.47 crook 5719: Defining words are used to extend Forth by creating new entries in the dictionary.
5720:
1.29 crook 5721: @menu
1.67 anton 5722: * CREATE::
1.44 crook 5723: * Variables:: Variables and user variables
1.67 anton 5724: * Constants::
1.44 crook 5725: * Values:: Initialised variables
1.67 anton 5726: * Colon Definitions::
1.44 crook 5727: * Anonymous Definitions:: Definitions without names
1.69 anton 5728: * Supplying names:: Passing definition names as strings
1.67 anton 5729: * User-defined Defining Words::
1.44 crook 5730: * Deferred words:: Allow forward references
1.67 anton 5731: * Aliases::
1.29 crook 5732: @end menu
5733:
1.44 crook 5734: @node CREATE, Variables, Defining Words, Defining Words
5735: @subsection @code{CREATE}
1.29 crook 5736: @cindex simple defining words
5737: @cindex defining words, simple
5738:
5739: Defining words are used to create new entries in the dictionary. The
5740: simplest defining word is @code{CREATE}. @code{CREATE} is used like
5741: this:
5742:
5743: @example
5744: CREATE new-word1
5745: @end example
5746:
1.69 anton 5747: @code{CREATE} is a parsing word, i.e., it takes an argument from the
5748: input stream (@code{new-word1} in our example). It generates a
5749: dictionary entry for @code{new-word1}. When @code{new-word1} is
5750: executed, all that it does is leave an address on the stack. The address
5751: represents the value of the data space pointer (@code{HERE}) at the time
5752: that @code{new-word1} was defined. Therefore, @code{CREATE} is a way of
5753: associating a name with the address of a region of memory.
1.29 crook 5754:
1.34 anton 5755: doc-create
5756:
1.69 anton 5757: Note that in ANS Forth guarantees only for @code{create} that its body
5758: is in dictionary data space (i.e., where @code{here}, @code{allot}
5759: etc. work, @pxref{Dictionary allocation}). Also, in ANS Forth only
5760: @code{create}d words can be modified with @code{does>}
5761: (@pxref{User-defined Defining Words}). And in ANS Forth @code{>body}
5762: can only be applied to @code{create}d words.
5763:
1.29 crook 5764: By extending this example to reserve some memory in data space, we end
1.69 anton 5765: up with something like a @i{variable}. Here are two different ways to do
5766: it:
1.29 crook 5767:
5768: @example
5769: CREATE new-word2 1 cells allot \ reserve 1 cell - initial value undefined
5770: CREATE new-word3 4 , \ reserve 1 cell and initialise it (to 4)
5771: @end example
5772:
5773: The variable can be examined and modified using @code{@@} (``fetch'') and
5774: @code{!} (``store'') like this:
5775:
5776: @example
5777: new-word2 @@ . \ get address, fetch from it and display
5778: 1234 new-word2 ! \ new value, get address, store to it
5779: @end example
5780:
1.44 crook 5781: @cindex arrays
5782: A similar mechanism can be used to create arrays. For example, an
5783: 80-character text input buffer:
1.29 crook 5784:
5785: @example
1.44 crook 5786: CREATE text-buf 80 chars allot
5787:
5788: text-buf 0 chars c@@ \ the 1st character (offset 0)
5789: text-buf 3 chars c@@ \ the 4th character (offset 3)
5790: @end example
1.29 crook 5791:
1.44 crook 5792: You can build arbitrarily complex data structures by allocating
1.49 anton 5793: appropriate areas of memory. For further discussions of this, and to
1.66 anton 5794: learn about some Gforth tools that make it easier,
1.49 anton 5795: @xref{Structures}.
1.44 crook 5796:
5797:
5798: @node Variables, Constants, CREATE, Defining Words
5799: @subsection Variables
5800: @cindex variables
5801:
5802: The previous section showed how a sequence of commands could be used to
5803: generate a variable. As a final refinement, the whole code sequence can
5804: be wrapped up in a defining word (pre-empting the subject of the next
5805: section), making it easier to create new variables:
5806:
5807: @example
5808: : myvariableX ( "name" -- a-addr ) CREATE 1 cells allot ;
5809: : myvariable0 ( "name" -- a-addr ) CREATE 0 , ;
5810:
5811: myvariableX foo \ variable foo starts off with an unknown value
5812: myvariable0 joe \ whilst joe is initialised to 0
1.29 crook 5813:
5814: 45 3 * foo ! \ set foo to 135
5815: 1234 joe ! \ set joe to 1234
5816: 3 joe +! \ increment joe by 3.. to 1237
5817: @end example
5818:
5819: Not surprisingly, there is no need to define @code{myvariable}, since
1.44 crook 5820: Forth already has a definition @code{Variable}. ANS Forth does not
1.69 anton 5821: guarantee that a @code{Variable} is initialised when it is created
5822: (i.e., it may behave like @code{myvariableX}). In contrast, Gforth's
5823: @code{Variable} initialises the variable to 0 (i.e., it behaves exactly
5824: like @code{myvariable0}). Forth also provides @code{2Variable} and
1.47 crook 5825: @code{fvariable} for double and floating-point variables, respectively
1.69 anton 5826: -- they are initialised to 0. and 0e in Gforth. If you use a @code{Variable} to
1.47 crook 5827: store a boolean, you can use @code{on} and @code{off} to toggle its
5828: state.
1.29 crook 5829:
1.34 anton 5830: doc-variable
5831: doc-2variable
5832: doc-fvariable
5833:
1.29 crook 5834: @cindex user variables
5835: @cindex user space
5836: The defining word @code{User} behaves in the same way as @code{Variable}.
5837: The difference is that it reserves space in @i{user (data) space} rather
5838: than normal data space. In a Forth system that has a multi-tasker, each
5839: task has its own set of user variables.
5840:
1.34 anton 5841: doc-user
1.67 anton 5842: @c doc-udp
5843: @c doc-uallot
1.34 anton 5844:
1.29 crook 5845: @comment TODO is that stuff about user variables strictly correct? Is it
5846: @comment just terminal tasks that have user variables?
5847: @comment should document tasker.fs (with some examples) elsewhere
5848: @comment in this manual, then expand on user space and user variables.
5849:
1.44 crook 5850: @node Constants, Values, Variables, Defining Words
5851: @subsection Constants
5852: @cindex constants
5853:
5854: @code{Constant} allows you to declare a fixed value and refer to it by
5855: name. For example:
1.29 crook 5856:
5857: @example
5858: 12 Constant INCHES-PER-FOOT
5859: 3E+08 fconstant SPEED-O-LIGHT
5860: @end example
5861:
5862: A @code{Variable} can be both read and written, so its run-time
5863: behaviour is to supply an address through which its current value can be
5864: manipulated. In contrast, the value of a @code{Constant} cannot be
5865: changed once it has been declared@footnote{Well, often it can be -- but
5866: not in a Standard, portable way. It's safer to use a @code{Value} (read
5867: on).} so it's not necessary to supply the address -- it is more
5868: efficient to return the value of the constant directly. That's exactly
5869: what happens; the run-time effect of a constant is to put its value on
1.49 anton 5870: the top of the stack (You can find one
5871: way of implementing @code{Constant} in @ref{User-defined Defining Words}).
1.29 crook 5872:
1.69 anton 5873: Forth also provides @code{2Constant} and @code{fconstant} for defining
1.29 crook 5874: double and floating-point constants, respectively.
5875:
1.34 anton 5876: doc-constant
5877: doc-2constant
5878: doc-fconstant
5879:
5880: @c that's too deep, and it's not necessarily true for all ANS Forths. - anton
1.44 crook 5881: @c nac-> How could that not be true in an ANS Forth? You can't define a
5882: @c constant, use it and then delete the definition of the constant..
1.69 anton 5883:
5884: @c anton->An ANS Forth system can compile a constant to a literal; On
5885: @c decompilation you would see only the number, just as if it had been used
5886: @c in the first place. The word will stay, of course, but it will only be
5887: @c used by the text interpreter (no run-time duties, except when it is
5888: @c POSTPONEd or somesuch).
5889:
5890: @c nac:
1.44 crook 5891: @c I agree that it's rather deep, but IMO it is an important difference
5892: @c relative to other programming languages.. often it's annoying: it
5893: @c certainly changes my programming style relative to C.
5894:
1.69 anton 5895: @c anton: In what way?
5896:
1.29 crook 5897: Constants in Forth behave differently from their equivalents in other
5898: programming languages. In other languages, a constant (such as an EQU in
5899: assembler or a #define in C) only exists at compile-time; in the
5900: executable program the constant has been translated into an absolute
5901: number and, unless you are using a symbolic debugger, it's impossible to
5902: know what abstract thing that number represents. In Forth a constant has
1.44 crook 5903: an entry in the header space and remains there after the code that uses
5904: it has been defined. In fact, it must remain in the dictionary since it
5905: has run-time duties to perform. For example:
1.29 crook 5906:
5907: @example
5908: 12 Constant INCHES-PER-FOOT
5909: : FEET-TO-INCHES ( n1 -- n2 ) INCHES-PER-FOOT * ;
5910: @end example
5911:
5912: @cindex in-lining of constants
5913: When @code{FEET-TO-INCHES} is executed, it will in turn execute the xt
5914: associated with the constant @code{INCHES-PER-FOOT}. If you use
5915: @code{see} to decompile the definition of @code{FEET-TO-INCHES}, you can
5916: see that it makes a call to @code{INCHES-PER-FOOT}. Some Forth compilers
5917: attempt to optimise constants by in-lining them where they are used. You
5918: can force Gforth to in-line a constant like this:
5919:
5920: @example
5921: : FEET-TO-INCHES ( n1 -- n2 ) [ INCHES-PER-FOOT ] LITERAL * ;
5922: @end example
5923:
5924: If you use @code{see} to decompile @i{this} version of
5925: @code{FEET-TO-INCHES}, you can see that @code{INCHES-PER-FOOT} is no
1.49 anton 5926: longer present. To understand how this works, read
5927: @ref{Interpret/Compile states}, and @ref{Literals}.
1.29 crook 5928:
5929: In-lining constants in this way might improve execution time
5930: fractionally, and can ensure that a constant is now only referenced at
5931: compile-time. However, the definition of the constant still remains in
5932: the dictionary. Some Forth compilers provide a mechanism for controlling
5933: a second dictionary for holding transient words such that this second
5934: dictionary can be deleted later in order to recover memory
5935: space. However, there is no standard way of doing this.
5936:
5937:
1.44 crook 5938: @node Values, Colon Definitions, Constants, Defining Words
5939: @subsection Values
5940: @cindex values
1.34 anton 5941:
1.69 anton 5942: A @code{Value} behaves like a @code{Constant}, but it can be changed.
5943: @code{TO} is a parsing word that changes a @code{Values}. In Gforth
5944: (not in ANS Forth) you can access (and change) a @code{value} also with
5945: @code{>body}.
5946:
5947: Here are some
5948: examples:
1.29 crook 5949:
5950: @example
1.69 anton 5951: 12 Value APPLES \ Define APPLES with an initial value of 12
5952: 34 TO APPLES \ Change the value of APPLES. TO is a parsing word
5953: 1 ' APPLES >body +! \ Increment APPLES. Non-standard usage.
5954: APPLES \ puts 35 on the top of the stack.
1.29 crook 5955: @end example
5956:
1.44 crook 5957: doc-value
5958: doc-to
1.29 crook 5959:
1.35 anton 5960:
1.69 anton 5961:
1.44 crook 5962: @node Colon Definitions, Anonymous Definitions, Values, Defining Words
5963: @subsection Colon Definitions
5964: @cindex colon definitions
1.35 anton 5965:
5966: @example
1.44 crook 5967: : name ( ... -- ... )
5968: word1 word2 word3 ;
1.29 crook 5969: @end example
5970:
1.44 crook 5971: @noindent
5972: Creates a word called @code{name} that, upon execution, executes
5973: @code{word1 word2 word3}. @code{name} is a @dfn{(colon) definition}.
1.29 crook 5974:
1.49 anton 5975: The explanation above is somewhat superficial. For simple examples of
5976: colon definitions see @ref{Your first definition}. For an in-depth
1.66 anton 5977: discussion of some of the issues involved, @xref{Interpretation and
1.49 anton 5978: Compilation Semantics}.
1.29 crook 5979:
1.44 crook 5980: doc-:
5981: doc-;
1.1 anton 5982:
1.34 anton 5983:
1.69 anton 5984: @node Anonymous Definitions, Supplying names, Colon Definitions, Defining Words
1.44 crook 5985: @subsection Anonymous Definitions
5986: @cindex colon definitions
5987: @cindex defining words without name
1.34 anton 5988:
1.44 crook 5989: Sometimes you want to define an @dfn{anonymous word}; a word without a
5990: name. You can do this with:
1.1 anton 5991:
1.44 crook 5992: doc-:noname
1.1 anton 5993:
1.44 crook 5994: This leaves the execution token for the word on the stack after the
5995: closing @code{;}. Here's an example in which a deferred word is
5996: initialised with an @code{xt} from an anonymous colon definition:
1.1 anton 5997:
1.29 crook 5998: @example
1.44 crook 5999: Defer deferred
6000: :noname ( ... -- ... )
6001: ... ;
6002: IS deferred
1.29 crook 6003: @end example
1.26 crook 6004:
1.44 crook 6005: @noindent
6006: Gforth provides an alternative way of doing this, using two separate
6007: words:
1.27 crook 6008:
1.44 crook 6009: doc-noname
6010: @cindex execution token of last defined word
1.116 anton 6011: doc-latestxt
1.1 anton 6012:
1.44 crook 6013: @noindent
6014: The previous example can be rewritten using @code{noname} and
1.116 anton 6015: @code{latestxt}:
1.1 anton 6016:
1.26 crook 6017: @example
1.44 crook 6018: Defer deferred
6019: noname : ( ... -- ... )
6020: ... ;
1.116 anton 6021: latestxt IS deferred
1.26 crook 6022: @end example
1.1 anton 6023:
1.29 crook 6024: @noindent
1.44 crook 6025: @code{noname} works with any defining word, not just @code{:}.
6026:
1.116 anton 6027: @code{latestxt} also works when the last word was not defined as
1.71 anton 6028: @code{noname}. It does not work for combined words, though. It also has
6029: the useful property that is is valid as soon as the header for a
6030: definition has been built. Thus:
1.44 crook 6031:
6032: @example
1.116 anton 6033: latestxt . : foo [ latestxt . ] ; ' foo .
1.44 crook 6034: @end example
1.1 anton 6035:
1.44 crook 6036: @noindent
6037: prints 3 numbers; the last two are the same.
1.26 crook 6038:
1.69 anton 6039: @node Supplying names, User-defined Defining Words, Anonymous Definitions, Defining Words
6040: @subsection Supplying the name of a defined word
6041: @cindex names for defined words
6042: @cindex defining words, name given in a string
6043:
6044: By default, a defining word takes the name for the defined word from the
6045: input stream. Sometimes you want to supply the name from a string. You
6046: can do this with:
6047:
6048: doc-nextname
6049:
6050: For example:
6051:
6052: @example
6053: s" foo" nextname create
6054: @end example
6055:
6056: @noindent
6057: is equivalent to:
6058:
6059: @example
6060: create foo
6061: @end example
6062:
6063: @noindent
6064: @code{nextname} works with any defining word.
6065:
1.1 anton 6066:
1.69 anton 6067: @node User-defined Defining Words, Deferred words, Supplying names, Defining Words
1.26 crook 6068: @subsection User-defined Defining Words
6069: @cindex user-defined defining words
6070: @cindex defining words, user-defined
1.1 anton 6071:
1.29 crook 6072: You can create a new defining word by wrapping defining-time code around
6073: an existing defining word and putting the sequence in a colon
1.69 anton 6074: definition.
6075:
6076: @c anton: This example is very complex and leads in a quite different
6077: @c direction from the CREATE-DOES> stuff that follows. It should probably
6078: @c be done elsewhere, or as a subsubsection of this subsection (or as a
6079: @c subsection of Defining Words)
6080:
6081: For example, suppose that you have a word @code{stats} that
1.29 crook 6082: gathers statistics about colon definitions given the @i{xt} of the
6083: definition, and you want every colon definition in your application to
6084: make a call to @code{stats}. You can define and use a new version of
6085: @code{:} like this:
6086:
6087: @example
6088: : stats ( xt -- ) DUP ." (Gathering statistics for " . ." )"
6089: ... ; \ other code
6090:
1.116 anton 6091: : my: : latestxt postpone literal ['] stats compile, ;
1.29 crook 6092:
6093: my: foo + - ;
6094: @end example
6095:
6096: When @code{foo} is defined using @code{my:} these steps occur:
6097:
6098: @itemize @bullet
6099: @item
6100: @code{my:} is executed.
6101: @item
6102: The @code{:} within the definition (the one between @code{my:} and
1.116 anton 6103: @code{latestxt}) is executed, and does just what it always does; it parses
1.29 crook 6104: the input stream for a name, builds a dictionary header for the name
6105: @code{foo} and switches @code{state} from interpret to compile.
6106: @item
1.116 anton 6107: The word @code{latestxt} is executed. It puts the @i{xt} for the word that is
1.29 crook 6108: being defined -- @code{foo} -- onto the stack.
6109: @item
6110: The code that was produced by @code{postpone literal} is executed; this
6111: causes the value on the stack to be compiled as a literal in the code
6112: area of @code{foo}.
6113: @item
6114: The code @code{['] stats} compiles a literal into the definition of
6115: @code{my:}. When @code{compile,} is executed, that literal -- the
6116: execution token for @code{stats} -- is layed down in the code area of
6117: @code{foo} , following the literal@footnote{Strictly speaking, the
6118: mechanism that @code{compile,} uses to convert an @i{xt} into something
6119: in the code area is implementation-dependent. A threaded implementation
6120: might spit out the execution token directly whilst another
6121: implementation might spit out a native code sequence.}.
6122: @item
6123: At this point, the execution of @code{my:} is complete, and control
6124: returns to the text interpreter. The text interpreter is in compile
6125: state, so subsequent text @code{+ -} is compiled into the definition of
6126: @code{foo} and the @code{;} terminates the definition as always.
6127: @end itemize
6128:
6129: You can use @code{see} to decompile a word that was defined using
6130: @code{my:} and see how it is different from a normal @code{:}
6131: definition. For example:
6132:
6133: @example
6134: : bar + - ; \ like foo but using : rather than my:
6135: see bar
6136: : bar
6137: + - ;
6138: see foo
6139: : foo
6140: 107645672 stats + - ;
6141:
6142: \ use ' stats . to show that 107645672 is the xt for stats
6143: @end example
6144:
6145: You can use techniques like this to make new defining words in terms of
6146: @i{any} existing defining word.
1.1 anton 6147:
6148:
1.29 crook 6149: @cindex defining defining words
1.26 crook 6150: @cindex @code{CREATE} ... @code{DOES>}
6151: If you want the words defined with your defining words to behave
6152: differently from words defined with standard defining words, you can
6153: write your defining word like this:
1.1 anton 6154:
6155: @example
1.26 crook 6156: : def-word ( "name" -- )
1.29 crook 6157: CREATE @i{code1}
1.26 crook 6158: DOES> ( ... -- ... )
1.29 crook 6159: @i{code2} ;
1.26 crook 6160:
6161: def-word name
1.1 anton 6162: @end example
6163:
1.29 crook 6164: @cindex child words
6165: This fragment defines a @dfn{defining word} @code{def-word} and then
6166: executes it. When @code{def-word} executes, it @code{CREATE}s a new
6167: word, @code{name}, and executes the code @i{code1}. The code @i{code2}
6168: is not executed at this time. The word @code{name} is sometimes called a
6169: @dfn{child} of @code{def-word}.
6170:
6171: When you execute @code{name}, the address of the body of @code{name} is
6172: put on the data stack and @i{code2} is executed (the address of the body
6173: of @code{name} is the address @code{HERE} returns immediately after the
1.69 anton 6174: @code{CREATE}, i.e., the address a @code{create}d word returns by
6175: default).
6176:
6177: @c anton:
6178: @c www.dictionary.com says:
6179: @c at·a·vism: 1.The reappearance of a characteristic in an organism after
6180: @c several generations of absence, usually caused by the chance
6181: @c recombination of genes. 2.An individual or a part that exhibits
6182: @c atavism. Also called throwback. 3.The return of a trait or recurrence
6183: @c of previous behavior after a period of absence.
6184: @c
6185: @c Doesn't seem to fit.
1.29 crook 6186:
1.69 anton 6187: @c @cindex atavism in child words
1.33 anton 6188: You can use @code{def-word} to define a set of child words that behave
1.69 anton 6189: similarly; they all have a common run-time behaviour determined by
6190: @i{code2}. Typically, the @i{code1} sequence builds a data area in the
6191: body of the child word. The structure of the data is common to all
6192: children of @code{def-word}, but the data values are specific -- and
6193: private -- to each child word. When a child word is executed, the
6194: address of its private data area is passed as a parameter on TOS to be
6195: used and manipulated@footnote{It is legitimate both to read and write to
6196: this data area.} by @i{code2}.
1.29 crook 6197:
6198: The two fragments of code that make up the defining words act (are
6199: executed) at two completely separate times:
1.1 anton 6200:
1.29 crook 6201: @itemize @bullet
6202: @item
6203: At @i{define time}, the defining word executes @i{code1} to generate a
6204: child word
6205: @item
6206: At @i{child execution time}, when a child word is invoked, @i{code2}
6207: is executed, using parameters (data) that are private and specific to
6208: the child word.
6209: @end itemize
6210:
1.44 crook 6211: Another way of understanding the behaviour of @code{def-word} and
6212: @code{name} is to say that, if you make the following definitions:
1.33 anton 6213: @example
6214: : def-word1 ( "name" -- )
6215: CREATE @i{code1} ;
6216:
6217: : action1 ( ... -- ... )
6218: @i{code2} ;
6219:
6220: def-word1 name1
6221: @end example
6222:
1.44 crook 6223: @noindent
6224: Then using @code{name1 action1} is equivalent to using @code{name}.
1.1 anton 6225:
1.29 crook 6226: The classic example is that you can define @code{CONSTANT} in this way:
1.26 crook 6227:
1.1 anton 6228: @example
1.29 crook 6229: : CONSTANT ( w "name" -- )
6230: CREATE ,
1.26 crook 6231: DOES> ( -- w )
6232: @@ ;
1.1 anton 6233: @end example
6234:
1.29 crook 6235: @comment There is a beautiful description of how this works and what
6236: @comment it does in the Forthwrite 100th edition.. as well as an elegant
6237: @comment commentary on the Counting Fruits problem.
6238:
6239: When you create a constant with @code{5 CONSTANT five}, a set of
6240: define-time actions take place; first a new word @code{five} is created,
6241: then the value 5 is laid down in the body of @code{five} with
1.44 crook 6242: @code{,}. When @code{five} is executed, the address of the body is put on
1.29 crook 6243: the stack, and @code{@@} retrieves the value 5. The word @code{five} has
6244: no code of its own; it simply contains a data field and a pointer to the
6245: code that follows @code{DOES>} in its defining word. That makes words
6246: created in this way very compact.
6247:
6248: The final example in this section is intended to remind you that space
6249: reserved in @code{CREATE}d words is @i{data} space and therefore can be
6250: both read and written by a Standard program@footnote{Exercise: use this
6251: example as a starting point for your own implementation of @code{Value}
6252: and @code{TO} -- if you get stuck, investigate the behaviour of @code{'} and
6253: @code{[']}.}:
6254:
6255: @example
6256: : foo ( "name" -- )
6257: CREATE -1 ,
6258: DOES> ( -- )
1.33 anton 6259: @@ . ;
1.29 crook 6260:
6261: foo first-word
6262: foo second-word
6263:
6264: 123 ' first-word >BODY !
6265: @end example
6266:
6267: If @code{first-word} had been a @code{CREATE}d word, we could simply
6268: have executed it to get the address of its data field. However, since it
6269: was defined to have @code{DOES>} actions, its execution semantics are to
6270: perform those @code{DOES>} actions. To get the address of its data field
6271: it's necessary to use @code{'} to get its xt, then @code{>BODY} to
6272: translate the xt into the address of the data field. When you execute
6273: @code{first-word}, it will display @code{123}. When you execute
6274: @code{second-word} it will display @code{-1}.
1.26 crook 6275:
6276: @cindex stack effect of @code{DOES>}-parts
6277: @cindex @code{DOES>}-parts, stack effect
1.29 crook 6278: In the examples above the stack comment after the @code{DOES>} specifies
1.26 crook 6279: the stack effect of the defined words, not the stack effect of the
6280: following code (the following code expects the address of the body on
6281: the top of stack, which is not reflected in the stack comment). This is
6282: the convention that I use and recommend (it clashes a bit with using
6283: locals declarations for stack effect specification, though).
1.1 anton 6284:
1.53 anton 6285: @menu
6286: * CREATE..DOES> applications::
6287: * CREATE..DOES> details::
1.63 anton 6288: * Advanced does> usage example::
1.91 anton 6289: * @code{Const-does>}::
1.53 anton 6290: @end menu
6291:
6292: @node CREATE..DOES> applications, CREATE..DOES> details, User-defined Defining Words, User-defined Defining Words
1.26 crook 6293: @subsubsection Applications of @code{CREATE..DOES>}
6294: @cindex @code{CREATE} ... @code{DOES>}, applications
1.1 anton 6295:
1.26 crook 6296: You may wonder how to use this feature. Here are some usage patterns:
1.1 anton 6297:
1.26 crook 6298: @cindex factoring similar colon definitions
6299: When you see a sequence of code occurring several times, and you can
6300: identify a meaning, you will factor it out as a colon definition. When
6301: you see similar colon definitions, you can factor them using
6302: @code{CREATE..DOES>}. E.g., an assembler usually defines several words
6303: that look very similar:
1.1 anton 6304: @example
1.26 crook 6305: : ori, ( reg-target reg-source n -- )
6306: 0 asm-reg-reg-imm ;
6307: : andi, ( reg-target reg-source n -- )
6308: 1 asm-reg-reg-imm ;
1.1 anton 6309: @end example
6310:
1.26 crook 6311: @noindent
6312: This could be factored with:
6313: @example
6314: : reg-reg-imm ( op-code -- )
6315: CREATE ,
6316: DOES> ( reg-target reg-source n -- )
6317: @@ asm-reg-reg-imm ;
6318:
6319: 0 reg-reg-imm ori,
6320: 1 reg-reg-imm andi,
6321: @end example
1.1 anton 6322:
1.26 crook 6323: @cindex currying
6324: Another view of @code{CREATE..DOES>} is to consider it as a crude way to
6325: supply a part of the parameters for a word (known as @dfn{currying} in
6326: the functional language community). E.g., @code{+} needs two
6327: parameters. Creating versions of @code{+} with one parameter fixed can
6328: be done like this:
1.82 anton 6329:
1.1 anton 6330: @example
1.82 anton 6331: : curry+ ( n1 "name" -- )
1.26 crook 6332: CREATE ,
6333: DOES> ( n2 -- n1+n2 )
6334: @@ + ;
6335:
6336: 3 curry+ 3+
6337: -2 curry+ 2-
1.1 anton 6338: @end example
6339:
1.91 anton 6340:
1.63 anton 6341: @node CREATE..DOES> details, Advanced does> usage example, CREATE..DOES> applications, User-defined Defining Words
1.26 crook 6342: @subsubsection The gory details of @code{CREATE..DOES>}
6343: @cindex @code{CREATE} ... @code{DOES>}, details
1.1 anton 6344:
1.26 crook 6345: doc-does>
1.1 anton 6346:
1.26 crook 6347: @cindex @code{DOES>} in a separate definition
6348: This means that you need not use @code{CREATE} and @code{DOES>} in the
6349: same definition; you can put the @code{DOES>}-part in a separate
1.29 crook 6350: definition. This allows us to, e.g., select among different @code{DOES>}-parts:
1.26 crook 6351: @example
6352: : does1
6353: DOES> ( ... -- ... )
1.44 crook 6354: ... ;
6355:
6356: : does2
6357: DOES> ( ... -- ... )
6358: ... ;
6359:
6360: : def-word ( ... -- ... )
6361: create ...
6362: IF
6363: does1
6364: ELSE
6365: does2
6366: ENDIF ;
6367: @end example
6368:
6369: In this example, the selection of whether to use @code{does1} or
1.69 anton 6370: @code{does2} is made at definition-time; at the time that the child word is
1.44 crook 6371: @code{CREATE}d.
6372:
6373: @cindex @code{DOES>} in interpretation state
6374: In a standard program you can apply a @code{DOES>}-part only if the last
6375: word was defined with @code{CREATE}. In Gforth, the @code{DOES>}-part
6376: will override the behaviour of the last word defined in any case. In a
6377: standard program, you can use @code{DOES>} only in a colon
6378: definition. In Gforth, you can also use it in interpretation state, in a
6379: kind of one-shot mode; for example:
6380: @example
6381: CREATE name ( ... -- ... )
6382: @i{initialization}
6383: DOES>
6384: @i{code} ;
6385: @end example
6386:
6387: @noindent
6388: is equivalent to the standard:
6389: @example
6390: :noname
6391: DOES>
6392: @i{code} ;
6393: CREATE name EXECUTE ( ... -- ... )
6394: @i{initialization}
6395: @end example
6396:
1.53 anton 6397: doc->body
6398:
1.91 anton 6399: @node Advanced does> usage example, @code{Const-does>}, CREATE..DOES> details, User-defined Defining Words
1.63 anton 6400: @subsubsection Advanced does> usage example
6401:
6402: The MIPS disassembler (@file{arch/mips/disasm.fs}) contains many words
6403: for disassembling instructions, that follow a very repetetive scheme:
6404:
6405: @example
6406: :noname @var{disasm-operands} s" @var{inst-name}" type ;
6407: @var{entry-num} cells @var{table} + !
6408: @end example
6409:
6410: Of course, this inspires the idea to factor out the commonalities to
6411: allow a definition like
6412:
6413: @example
6414: @var{disasm-operands} @var{entry-num} @var{table} define-inst @var{inst-name}
6415: @end example
6416:
6417: The parameters @var{disasm-operands} and @var{table} are usually
1.69 anton 6418: correlated. Moreover, before I wrote the disassembler, there already
6419: existed code that defines instructions like this:
1.63 anton 6420:
6421: @example
6422: @var{entry-num} @var{inst-format} @var{inst-name}
6423: @end example
6424:
6425: This code comes from the assembler and resides in
6426: @file{arch/mips/insts.fs}.
6427:
6428: So I had to define the @var{inst-format} words that performed the scheme
6429: above when executed. At first I chose to use run-time code-generation:
6430:
6431: @example
6432: : @var{inst-format} ( entry-num "name" -- ; compiled code: addr w -- )
6433: :noname Postpone @var{disasm-operands}
6434: name Postpone sliteral Postpone type Postpone ;
6435: swap cells @var{table} + ! ;
6436: @end example
6437:
6438: Note that this supplies the other two parameters of the scheme above.
1.44 crook 6439:
1.63 anton 6440: An alternative would have been to write this using
6441: @code{create}/@code{does>}:
6442:
6443: @example
6444: : @var{inst-format} ( entry-num "name" -- )
6445: here name string, ( entry-num c-addr ) \ parse and save "name"
6446: noname create , ( entry-num )
1.116 anton 6447: latestxt swap cells @var{table} + !
1.63 anton 6448: does> ( addr w -- )
6449: \ disassemble instruction w at addr
6450: @@ >r
6451: @var{disasm-operands}
6452: r> count type ;
6453: @end example
6454:
6455: Somehow the first solution is simpler, mainly because it's simpler to
6456: shift a string from definition-time to use-time with @code{sliteral}
6457: than with @code{string,} and friends.
6458:
6459: I wrote a lot of words following this scheme and soon thought about
6460: factoring out the commonalities among them. Note that this uses a
6461: two-level defining word, i.e., a word that defines ordinary defining
6462: words.
6463:
6464: This time a solution involving @code{postpone} and friends seemed more
6465: difficult (try it as an exercise), so I decided to use a
6466: @code{create}/@code{does>} word; since I was already at it, I also used
6467: @code{create}/@code{does>} for the lower level (try using
6468: @code{postpone} etc. as an exercise), resulting in the following
6469: definition:
6470:
6471: @example
6472: : define-format ( disasm-xt table-xt -- )
6473: \ define an instruction format that uses disasm-xt for
6474: \ disassembling and enters the defined instructions into table
6475: \ table-xt
6476: create 2,
6477: does> ( u "inst" -- )
6478: \ defines an anonymous word for disassembling instruction inst,
6479: \ and enters it as u-th entry into table-xt
6480: 2@@ swap here name string, ( u table-xt disasm-xt c-addr ) \ remember string
6481: noname create 2, \ define anonymous word
1.116 anton 6482: execute latestxt swap ! \ enter xt of defined word into table-xt
1.63 anton 6483: does> ( addr w -- )
6484: \ disassemble instruction w at addr
6485: 2@@ >r ( addr w disasm-xt R: c-addr )
6486: execute ( R: c-addr ) \ disassemble operands
6487: r> count type ; \ print name
6488: @end example
6489:
6490: Note that the tables here (in contrast to above) do the @code{cells +}
6491: by themselves (that's why you have to pass an xt). This word is used in
6492: the following way:
6493:
6494: @example
6495: ' @var{disasm-operands} ' @var{table} define-format @var{inst-format}
6496: @end example
6497:
1.71 anton 6498: As shown above, the defined instruction format is then used like this:
6499:
6500: @example
6501: @var{entry-num} @var{inst-format} @var{inst-name}
6502: @end example
6503:
1.63 anton 6504: In terms of currying, this kind of two-level defining word provides the
6505: parameters in three stages: first @var{disasm-operands} and @var{table},
6506: then @var{entry-num} and @var{inst-name}, finally @code{addr w}, i.e.,
6507: the instruction to be disassembled.
6508:
6509: Of course this did not quite fit all the instruction format names used
6510: in @file{insts.fs}, so I had to define a few wrappers that conditioned
6511: the parameters into the right form.
6512:
6513: If you have trouble following this section, don't worry. First, this is
6514: involved and takes time (and probably some playing around) to
6515: understand; second, this is the first two-level
6516: @code{create}/@code{does>} word I have written in seventeen years of
6517: Forth; and if I did not have @file{insts.fs} to start with, I may well
6518: have elected to use just a one-level defining word (with some repeating
6519: of parameters when using the defining word). So it is not necessary to
6520: understand this, but it may improve your understanding of Forth.
1.44 crook 6521:
6522:
1.91 anton 6523: @node @code{Const-does>}, , Advanced does> usage example, User-defined Defining Words
6524: @subsubsection @code{Const-does>}
6525:
6526: A frequent use of @code{create}...@code{does>} is for transferring some
6527: values from definition-time to run-time. Gforth supports this use with
6528:
6529: doc-const-does>
6530:
6531: A typical use of this word is:
6532:
6533: @example
6534: : curry+ ( n1 "name" -- )
6535: 1 0 CONST-DOES> ( n2 -- n1+n2 )
6536: + ;
6537:
6538: 3 curry+ 3+
6539: @end example
6540:
6541: Here the @code{1 0} means that 1 cell and 0 floats are transferred from
6542: definition to run-time.
6543:
6544: The advantages of using @code{const-does>} are:
6545:
6546: @itemize
6547:
6548: @item
6549: You don't have to deal with storing and retrieving the values, i.e.,
6550: your program becomes more writable and readable.
6551:
6552: @item
6553: When using @code{does>}, you have to introduce a @code{@@} that cannot
6554: be optimized away (because you could change the data using
6555: @code{>body}...@code{!}); @code{const-does>} avoids this problem.
6556:
6557: @end itemize
6558:
6559: An ANS Forth implementation of @code{const-does>} is available in
6560: @file{compat/const-does.fs}.
6561:
6562:
1.44 crook 6563: @node Deferred words, Aliases, User-defined Defining Words, Defining Words
6564: @subsection Deferred words
6565: @cindex deferred words
6566:
6567: The defining word @code{Defer} allows you to define a word by name
6568: without defining its behaviour; the definition of its behaviour is
6569: deferred. Here are two situation where this can be useful:
6570:
6571: @itemize @bullet
6572: @item
6573: Where you want to allow the behaviour of a word to be altered later, and
6574: for all precompiled references to the word to change when its behaviour
6575: is changed.
6576: @item
6577: For mutual recursion; @xref{Calls and returns}.
6578: @end itemize
6579:
6580: In the following example, @code{foo} always invokes the version of
6581: @code{greet} that prints ``@code{Good morning}'' whilst @code{bar}
6582: always invokes the version that prints ``@code{Hello}''. There is no way
6583: of getting @code{foo} to use the later version without re-ordering the
6584: source code and recompiling it.
6585:
6586: @example
6587: : greet ." Good morning" ;
6588: : foo ... greet ... ;
6589: : greet ." Hello" ;
6590: : bar ... greet ... ;
6591: @end example
6592:
6593: This problem can be solved by defining @code{greet} as a @code{Defer}red
6594: word. The behaviour of a @code{Defer}red word can be defined and
6595: redefined at any time by using @code{IS} to associate the xt of a
6596: previously-defined word with it. The previous example becomes:
6597:
6598: @example
1.69 anton 6599: Defer greet ( -- )
1.44 crook 6600: : foo ... greet ... ;
6601: : bar ... greet ... ;
1.69 anton 6602: : greet1 ( -- ) ." Good morning" ;
6603: : greet2 ( -- ) ." Hello" ;
1.44 crook 6604: ' greet2 <IS> greet \ make greet behave like greet2
6605: @end example
6606:
1.69 anton 6607: @progstyle
6608: You should write a stack comment for every deferred word, and put only
6609: XTs into deferred words that conform to this stack effect. Otherwise
6610: it's too difficult to use the deferred word.
6611:
1.44 crook 6612: A deferred word can be used to improve the statistics-gathering example
6613: from @ref{User-defined Defining Words}; rather than edit the
6614: application's source code to change every @code{:} to a @code{my:}, do
6615: this:
6616:
6617: @example
6618: : real: : ; \ retain access to the original
6619: defer : \ redefine as a deferred word
1.69 anton 6620: ' my: <IS> : \ use special version of :
1.44 crook 6621: \
6622: \ load application here
6623: \
1.69 anton 6624: ' real: <IS> : \ go back to the original
1.44 crook 6625: @end example
6626:
6627:
6628: One thing to note is that @code{<IS>} consumes its name when it is
6629: executed. If you want to specify the name at compile time, use
6630: @code{[IS]}:
6631:
6632: @example
6633: : set-greet ( xt -- )
6634: [IS] greet ;
6635:
6636: ' greet1 set-greet
6637: @end example
6638:
1.69 anton 6639: A deferred word can only inherit execution semantics from the xt
6640: (because that is all that an xt can represent -- for more discussion of
6641: this @pxref{Tokens for Words}); by default it will have default
6642: interpretation and compilation semantics deriving from this execution
6643: semantics. However, you can change the interpretation and compilation
6644: semantics of the deferred word in the usual ways:
1.44 crook 6645:
6646: @example
6647: : bar .... ; compile-only
6648: Defer fred immediate
6649: Defer jim
6650:
6651: ' bar <IS> jim \ jim has default semantics
6652: ' bar <IS> fred \ fred is immediate
6653: @end example
6654:
6655: doc-defer
6656: doc-<is>
6657: doc-[is]
6658: doc-is
6659: @comment TODO document these: what's defers [is]
6660: doc-what's
6661: doc-defers
6662:
6663: @c Use @code{words-deferred} to see a list of deferred words.
6664:
6665: Definitions in ANS Forth for @code{defer}, @code{<is>} and @code{[is]}
6666: are provided in @file{compat/defer.fs}.
6667:
6668:
1.69 anton 6669: @node Aliases, , Deferred words, Defining Words
1.44 crook 6670: @subsection Aliases
6671: @cindex aliases
1.1 anton 6672:
1.44 crook 6673: The defining word @code{Alias} allows you to define a word by name that
6674: has the same behaviour as some other word. Here are two situation where
6675: this can be useful:
1.1 anton 6676:
1.44 crook 6677: @itemize @bullet
6678: @item
6679: When you want access to a word's definition from a different word list
6680: (for an example of this, see the definition of the @code{Root} word list
6681: in the Gforth source).
6682: @item
6683: When you want to create a synonym; a definition that can be known by
6684: either of two names (for example, @code{THEN} and @code{ENDIF} are
6685: aliases).
6686: @end itemize
1.1 anton 6687:
1.69 anton 6688: Like deferred words, an alias has default compilation and interpretation
6689: semantics at the beginning (not the modifications of the other word),
6690: but you can change them in the usual ways (@code{immediate},
6691: @code{compile-only}). For example:
1.1 anton 6692:
6693: @example
1.44 crook 6694: : foo ... ; immediate
6695:
6696: ' foo Alias bar \ bar is not an immediate word
6697: ' foo Alias fooby immediate \ fooby is an immediate word
1.1 anton 6698: @end example
6699:
1.44 crook 6700: Words that are aliases have the same xt, different headers in the
6701: dictionary, and consequently different name tokens (@pxref{Tokens for
6702: Words}) and possibly different immediate flags. An alias can only have
6703: default or immediate compilation semantics; you can define aliases for
6704: combined words with @code{interpret/compile:} -- see @ref{Combined words}.
1.1 anton 6705:
1.44 crook 6706: doc-alias
1.1 anton 6707:
6708:
1.47 crook 6709: @node Interpretation and Compilation Semantics, Tokens for Words, Defining Words, Words
6710: @section Interpretation and Compilation Semantics
1.26 crook 6711: @cindex semantics, interpretation and compilation
1.1 anton 6712:
1.71 anton 6713: @c !! state and ' are used without explanation
6714: @c example for immediate/compile-only? or is the tutorial enough
6715:
1.26 crook 6716: @cindex interpretation semantics
1.71 anton 6717: The @dfn{interpretation semantics} of a (named) word are what the text
1.26 crook 6718: interpreter does when it encounters the word in interpret state. It also
6719: appears in some other contexts, e.g., the execution token returned by
1.71 anton 6720: @code{' @i{word}} identifies the interpretation semantics of @i{word}
6721: (in other words, @code{' @i{word} execute} is equivalent to
1.29 crook 6722: interpret-state text interpretation of @code{@i{word}}).
1.1 anton 6723:
1.26 crook 6724: @cindex compilation semantics
1.71 anton 6725: The @dfn{compilation semantics} of a (named) word are what the text
6726: interpreter does when it encounters the word in compile state. It also
6727: appears in other contexts, e.g, @code{POSTPONE @i{word}}
6728: compiles@footnote{In standard terminology, ``appends to the current
6729: definition''.} the compilation semantics of @i{word}.
1.1 anton 6730:
1.26 crook 6731: @cindex execution semantics
6732: The standard also talks about @dfn{execution semantics}. They are used
6733: only for defining the interpretation and compilation semantics of many
6734: words. By default, the interpretation semantics of a word are to
6735: @code{execute} its execution semantics, and the compilation semantics of
6736: a word are to @code{compile,} its execution semantics.@footnote{In
6737: standard terminology: The default interpretation semantics are its
6738: execution semantics; the default compilation semantics are to append its
6739: execution semantics to the execution semantics of the current
6740: definition.}
6741:
1.71 anton 6742: Unnamed words (@pxref{Anonymous Definitions}) cannot be encountered by
6743: the text interpreter, ticked, or @code{postpone}d, so they have no
6744: interpretation or compilation semantics. Their behaviour is represented
6745: by their XT (@pxref{Tokens for Words}), and we call it execution
6746: semantics, too.
6747:
1.26 crook 6748: @comment TODO expand, make it co-operate with new sections on text interpreter.
6749:
6750: @cindex immediate words
6751: @cindex compile-only words
6752: You can change the semantics of the most-recently defined word:
6753:
1.44 crook 6754:
1.26 crook 6755: doc-immediate
6756: doc-compile-only
6757: doc-restrict
6758:
1.82 anton 6759: By convention, words with non-default compilation semantics (e.g.,
6760: immediate words) often have names surrounded with brackets (e.g.,
6761: @code{[']}, @pxref{Execution token}).
1.44 crook 6762:
1.26 crook 6763: Note that ticking (@code{'}) a compile-only word gives an error
6764: (``Interpreting a compile-only word'').
1.1 anton 6765:
1.47 crook 6766: @menu
1.67 anton 6767: * Combined words::
1.47 crook 6768: @end menu
1.44 crook 6769:
1.71 anton 6770:
1.48 anton 6771: @node Combined words, , Interpretation and Compilation Semantics, Interpretation and Compilation Semantics
1.44 crook 6772: @subsection Combined Words
6773: @cindex combined words
6774:
6775: Gforth allows you to define @dfn{combined words} -- words that have an
6776: arbitrary combination of interpretation and compilation semantics.
6777:
1.26 crook 6778: doc-interpret/compile:
1.1 anton 6779:
1.26 crook 6780: This feature was introduced for implementing @code{TO} and @code{S"}. I
6781: recommend that you do not define such words, as cute as they may be:
6782: they make it hard to get at both parts of the word in some contexts.
6783: E.g., assume you want to get an execution token for the compilation
6784: part. Instead, define two words, one that embodies the interpretation
6785: part, and one that embodies the compilation part. Once you have done
6786: that, you can define a combined word with @code{interpret/compile:} for
6787: the convenience of your users.
1.1 anton 6788:
1.26 crook 6789: You might try to use this feature to provide an optimizing
6790: implementation of the default compilation semantics of a word. For
6791: example, by defining:
1.1 anton 6792: @example
1.26 crook 6793: :noname
6794: foo bar ;
6795: :noname
6796: POSTPONE foo POSTPONE bar ;
1.29 crook 6797: interpret/compile: opti-foobar
1.1 anton 6798: @end example
1.26 crook 6799:
1.23 crook 6800: @noindent
1.26 crook 6801: as an optimizing version of:
6802:
1.1 anton 6803: @example
1.26 crook 6804: : foobar
6805: foo bar ;
1.1 anton 6806: @end example
6807:
1.26 crook 6808: Unfortunately, this does not work correctly with @code{[compile]},
6809: because @code{[compile]} assumes that the compilation semantics of all
6810: @code{interpret/compile:} words are non-default. I.e., @code{[compile]
1.29 crook 6811: opti-foobar} would compile compilation semantics, whereas
6812: @code{[compile] foobar} would compile interpretation semantics.
1.1 anton 6813:
1.26 crook 6814: @cindex state-smart words (are a bad idea)
1.82 anton 6815: @anchor{state-smartness}
1.29 crook 6816: Some people try to use @dfn{state-smart} words to emulate the feature provided
1.26 crook 6817: by @code{interpret/compile:} (words are state-smart if they check
6818: @code{STATE} during execution). E.g., they would try to code
6819: @code{foobar} like this:
1.1 anton 6820:
1.26 crook 6821: @example
6822: : foobar
6823: STATE @@
6824: IF ( compilation state )
6825: POSTPONE foo POSTPONE bar
6826: ELSE
6827: foo bar
6828: ENDIF ; immediate
6829: @end example
1.1 anton 6830:
1.26 crook 6831: Although this works if @code{foobar} is only processed by the text
6832: interpreter, it does not work in other contexts (like @code{'} or
6833: @code{POSTPONE}). E.g., @code{' foobar} will produce an execution token
6834: for a state-smart word, not for the interpretation semantics of the
6835: original @code{foobar}; when you execute this execution token (directly
6836: with @code{EXECUTE} or indirectly through @code{COMPILE,}) in compile
6837: state, the result will not be what you expected (i.e., it will not
6838: perform @code{foo bar}). State-smart words are a bad idea. Simply don't
6839: write them@footnote{For a more detailed discussion of this topic, see
1.66 anton 6840: M. Anton Ertl,
6841: @cite{@uref{http://www.complang.tuwien.ac.at/papers/ertl98.ps.gz,@code{State}-smartness---Why
6842: it is Evil and How to Exorcise it}}, EuroForth '98.}!
1.1 anton 6843:
1.26 crook 6844: @cindex defining words with arbitrary semantics combinations
6845: It is also possible to write defining words that define words with
6846: arbitrary combinations of interpretation and compilation semantics. In
6847: general, they look like this:
1.1 anton 6848:
1.26 crook 6849: @example
6850: : def-word
6851: create-interpret/compile
1.29 crook 6852: @i{code1}
1.26 crook 6853: interpretation>
1.29 crook 6854: @i{code2}
1.26 crook 6855: <interpretation
6856: compilation>
1.29 crook 6857: @i{code3}
1.26 crook 6858: <compilation ;
6859: @end example
1.1 anton 6860:
1.29 crook 6861: For a @i{word} defined with @code{def-word}, the interpretation
6862: semantics are to push the address of the body of @i{word} and perform
6863: @i{code2}, and the compilation semantics are to push the address of
6864: the body of @i{word} and perform @i{code3}. E.g., @code{constant}
1.26 crook 6865: can also be defined like this (except that the defined constants don't
6866: behave correctly when @code{[compile]}d):
1.1 anton 6867:
1.26 crook 6868: @example
6869: : constant ( n "name" -- )
6870: create-interpret/compile
6871: ,
6872: interpretation> ( -- n )
6873: @@
6874: <interpretation
6875: compilation> ( compilation. -- ; run-time. -- n )
6876: @@ postpone literal
6877: <compilation ;
6878: @end example
1.1 anton 6879:
1.44 crook 6880:
1.26 crook 6881: doc-create-interpret/compile
6882: doc-interpretation>
6883: doc-<interpretation
6884: doc-compilation>
6885: doc-<compilation
1.1 anton 6886:
1.44 crook 6887:
1.29 crook 6888: Words defined with @code{interpret/compile:} and
1.26 crook 6889: @code{create-interpret/compile} have an extended header structure that
6890: differs from other words; however, unless you try to access them with
6891: plain address arithmetic, you should not notice this. Words for
6892: accessing the header structure usually know how to deal with this; e.g.,
1.29 crook 6893: @code{'} @i{word} @code{>body} also gives you the body of a word created
6894: with @code{create-interpret/compile}.
1.1 anton 6895:
1.44 crook 6896:
1.47 crook 6897: @c -------------------------------------------------------------
1.81 anton 6898: @node Tokens for Words, Compiling words, Interpretation and Compilation Semantics, Words
1.47 crook 6899: @section Tokens for Words
6900: @cindex tokens for words
6901:
6902: This section describes the creation and use of tokens that represent
6903: words.
6904:
1.71 anton 6905: @menu
6906: * Execution token:: represents execution/interpretation semantics
6907: * Compilation token:: represents compilation semantics
6908: * Name token:: represents named words
6909: @end menu
1.47 crook 6910:
1.71 anton 6911: @node Execution token, Compilation token, Tokens for Words, Tokens for Words
6912: @subsection Execution token
1.47 crook 6913:
6914: @cindex xt
6915: @cindex execution token
1.71 anton 6916: An @dfn{execution token} (@i{XT}) represents some behaviour of a word.
6917: You can use @code{execute} to invoke this behaviour.
1.47 crook 6918:
1.71 anton 6919: @cindex tick (')
6920: You can use @code{'} to get an execution token that represents the
6921: interpretation semantics of a named word:
1.47 crook 6922:
6923: @example
1.97 anton 6924: 5 ' . ( n xt )
6925: execute ( ) \ execute the xt (i.e., ".")
1.71 anton 6926: @end example
1.47 crook 6927:
1.71 anton 6928: doc-'
6929:
6930: @code{'} parses at run-time; there is also a word @code{[']} that parses
6931: when it is compiled, and compiles the resulting XT:
6932:
6933: @example
6934: : foo ['] . execute ;
6935: 5 foo
6936: : bar ' execute ; \ by contrast,
6937: 5 bar . \ ' parses "." when bar executes
6938: @end example
6939:
6940: doc-[']
6941:
6942: If you want the execution token of @i{word}, write @code{['] @i{word}}
6943: in compiled code and @code{' @i{word}} in interpreted code. Gforth's
6944: @code{'} and @code{[']} behave somewhat unusually by complaining about
6945: compile-only words (because these words have no interpretation
6946: semantics). You might get what you want by using @code{COMP' @i{word}
6947: DROP} or @code{[COMP'] @i{word} DROP} (for details @pxref{Compilation
6948: token}).
6949:
1.116 anton 6950: Another way to get an XT is @code{:noname} or @code{latestxt}
1.71 anton 6951: (@pxref{Anonymous Definitions}). For anonymous words this gives an xt
6952: for the only behaviour the word has (the execution semantics). For
1.116 anton 6953: named words, @code{latestxt} produces an XT for the same behaviour it
1.71 anton 6954: would produce if the word was defined anonymously.
6955:
6956: @example
6957: :noname ." hello" ;
6958: execute
1.47 crook 6959: @end example
6960:
1.71 anton 6961: An XT occupies one cell and can be manipulated like any other cell.
6962:
1.47 crook 6963: @cindex code field address
6964: @cindex CFA
1.71 anton 6965: In ANS Forth the XT is just an abstract data type (i.e., defined by the
6966: operations that produce or consume it). For old hands: In Gforth, the
6967: XT is implemented as a code field address (CFA).
6968:
6969: doc-execute
6970: doc-perform
6971:
6972: @node Compilation token, Name token, Execution token, Tokens for Words
6973: @subsection Compilation token
1.47 crook 6974:
6975: @cindex compilation token
1.71 anton 6976: @cindex CT (compilation token)
6977: Gforth represents the compilation semantics of a named word by a
1.47 crook 6978: @dfn{compilation token} consisting of two cells: @i{w xt}. The top cell
6979: @i{xt} is an execution token. The compilation semantics represented by
6980: the compilation token can be performed with @code{execute}, which
6981: consumes the whole compilation token, with an additional stack effect
6982: determined by the represented compilation semantics.
6983:
6984: At present, the @i{w} part of a compilation token is an execution token,
6985: and the @i{xt} part represents either @code{execute} or
6986: @code{compile,}@footnote{Depending upon the compilation semantics of the
6987: word. If the word has default compilation semantics, the @i{xt} will
6988: represent @code{compile,}. Otherwise (e.g., for immediate words), the
6989: @i{xt} will represent @code{execute}.}. However, don't rely on that
6990: knowledge, unless necessary; future versions of Gforth may introduce
6991: unusual compilation tokens (e.g., a compilation token that represents
6992: the compilation semantics of a literal).
6993:
1.71 anton 6994: You can perform the compilation semantics represented by the compilation
6995: token with @code{execute}. You can compile the compilation semantics
6996: with @code{postpone,}. I.e., @code{COMP' @i{word} postpone,} is
6997: equivalent to @code{postpone @i{word}}.
6998:
6999: doc-[comp']
7000: doc-comp'
7001: doc-postpone,
7002:
7003: @node Name token, , Compilation token, Tokens for Words
7004: @subsection Name token
1.47 crook 7005:
7006: @cindex name token
1.116 anton 7007: Gforth represents named words by the @dfn{name token}, (@i{nt}). Name
7008: token is an abstract data type that occurs as argument or result of the
7009: words below.
7010:
7011: @c !! put this elswhere?
1.47 crook 7012: @cindex name field address
7013: @cindex NFA
1.116 anton 7014: The closest thing to the nt in older Forth systems is the name field
7015: address (NFA), but there are significant differences: in older Forth
7016: systems each word had a unique NFA, LFA, CFA and PFA (in this order, or
7017: LFA, NFA, CFA, PFA) and there were words for getting from one to the
7018: next. In contrast, in Gforth 0@dots{}n nts correspond to one xt; there
7019: is a link field in the structure identified by the name token, but
7020: searching usually uses a hash table external to these structures; the
7021: name in Gforth has a cell-wide count-and-flags field, and the nt is not
7022: implemented as the address of that count field.
1.47 crook 7023:
7024: doc-find-name
1.116 anton 7025: doc-latest
7026: doc->name
1.47 crook 7027: doc-name>int
7028: doc-name?int
7029: doc-name>comp
7030: doc-name>string
1.109 anton 7031: doc-id.
7032: doc-.name
7033: doc-.id
1.47 crook 7034:
1.81 anton 7035: @c ----------------------------------------------------------
7036: @node Compiling words, The Text Interpreter, Tokens for Words, Words
7037: @section Compiling words
7038: @cindex compiling words
7039: @cindex macros
7040:
7041: In contrast to most other languages, Forth has no strict boundary
1.82 anton 7042: between compilation and run-time. E.g., you can run arbitrary code
7043: between defining words (or for computing data used by defining words
7044: like @code{constant}). Moreover, @code{Immediate} (@pxref{Interpretation
7045: and Compilation Semantics} and @code{[}...@code{]} (see below) allow
7046: running arbitrary code while compiling a colon definition (exception:
7047: you must not allot dictionary space).
7048:
7049: @menu
7050: * Literals:: Compiling data values
7051: * Macros:: Compiling words
7052: @end menu
7053:
7054: @node Literals, Macros, Compiling words, Compiling words
7055: @subsection Literals
7056: @cindex Literals
7057:
7058: The simplest and most frequent example is to compute a literal during
7059: compilation. E.g., the following definition prints an array of strings,
7060: one string per line:
7061:
7062: @example
7063: : .strings ( addr u -- ) \ gforth
7064: 2* cells bounds U+DO
7065: cr i 2@@ type
7066: 2 cells +LOOP ;
7067: @end example
1.81 anton 7068:
1.82 anton 7069: With a simple-minded compiler like Gforth's, this computes @code{2
7070: cells} on every loop iteration. You can compute this value once and for
7071: all at compile time and compile it into the definition like this:
7072:
7073: @example
7074: : .strings ( addr u -- ) \ gforth
7075: 2* cells bounds U+DO
7076: cr i 2@@ type
7077: [ 2 cells ] literal +LOOP ;
7078: @end example
7079:
7080: @code{[} switches the text interpreter to interpret state (you will get
7081: an @code{ok} prompt if you type this example interactively and insert a
7082: newline between @code{[} and @code{]}), so it performs the
7083: interpretation semantics of @code{2 cells}; this computes a number.
7084: @code{]} switches the text interpreter back into compile state. It then
7085: performs @code{Literal}'s compilation semantics, which are to compile
7086: this number into the current word. You can decompile the word with
7087: @code{see .strings} to see the effect on the compiled code.
1.81 anton 7088:
1.82 anton 7089: You can also optimize the @code{2* cells} into @code{[ 2 cells ] literal
7090: *} in this way.
1.81 anton 7091:
1.82 anton 7092: doc-[
7093: doc-]
1.81 anton 7094: doc-literal
7095: doc-]L
1.82 anton 7096:
7097: There are also words for compiling other data types than single cells as
7098: literals:
7099:
1.81 anton 7100: doc-2literal
7101: doc-fliteral
1.82 anton 7102: doc-sliteral
7103:
7104: @cindex colon-sys, passing data across @code{:}
7105: @cindex @code{:}, passing data across
7106: You might be tempted to pass data from outside a colon definition to the
7107: inside on the data stack. This does not work, because @code{:} puhes a
7108: colon-sys, making stuff below unaccessible. E.g., this does not work:
7109:
7110: @example
7111: 5 : foo literal ; \ error: "unstructured"
7112: @end example
7113:
7114: Instead, you have to pass the value in some other way, e.g., through a
7115: variable:
7116:
7117: @example
7118: variable temp
7119: 5 temp !
7120: : foo [ temp @@ ] literal ;
7121: @end example
7122:
7123:
7124: @node Macros, , Literals, Compiling words
7125: @subsection Macros
7126: @cindex Macros
7127: @cindex compiling compilation semantics
7128:
7129: @code{Literal} and friends compile data values into the current
7130: definition. You can also write words that compile other words into the
7131: current definition. E.g.,
7132:
7133: @example
7134: : compile-+ ( -- ) \ compiled code: ( n1 n2 -- n )
7135: POSTPONE + ;
7136:
7137: : foo ( n1 n2 -- n )
7138: [ compile-+ ] ;
7139: 1 2 foo .
7140: @end example
7141:
7142: This is equivalent to @code{: foo + ;} (@code{see foo} to check this).
7143: What happens in this example? @code{Postpone} compiles the compilation
7144: semantics of @code{+} into @code{compile-+}; later the text interpreter
7145: executes @code{compile-+} and thus the compilation semantics of +, which
7146: compile (the execution semantics of) @code{+} into
7147: @code{foo}.@footnote{A recent RFI answer requires that compiling words
7148: should only be executed in compile state, so this example is not
7149: guaranteed to work on all standard systems, but on any decent system it
7150: will work.}
7151:
7152: doc-postpone
7153: doc-[compile]
7154:
7155: Compiling words like @code{compile-+} are usually immediate (or similar)
7156: so you do not have to switch to interpret state to execute them;
7157: mopifying the last example accordingly produces:
7158:
7159: @example
7160: : [compile-+] ( compilation: --; interpretation: -- )
7161: \ compiled code: ( n1 n2 -- n )
7162: POSTPONE + ; immediate
7163:
7164: : foo ( n1 n2 -- n )
7165: [compile-+] ;
7166: 1 2 foo .
7167: @end example
7168:
7169: Immediate compiling words are similar to macros in other languages (in
7170: particular, Lisp). The important differences to macros in, e.g., C are:
7171:
7172: @itemize @bullet
7173:
7174: @item
7175: You use the same language for defining and processing macros, not a
7176: separate preprocessing language and processor.
7177:
7178: @item
7179: Consequently, the full power of Forth is available in macro definitions.
7180: E.g., you can perform arbitrarily complex computations, or generate
7181: different code conditionally or in a loop (e.g., @pxref{Advanced macros
7182: Tutorial}). This power is very useful when writing a parser generators
7183: or other code-generating software.
7184:
7185: @item
7186: Macros defined using @code{postpone} etc. deal with the language at a
7187: higher level than strings; name binding happens at macro definition
7188: time, so you can avoid the pitfalls of name collisions that can happen
7189: in C macros. Of course, Forth is a liberal language and also allows to
7190: shoot yourself in the foot with text-interpreted macros like
7191:
7192: @example
7193: : [compile-+] s" +" evaluate ; immediate
7194: @end example
7195:
7196: Apart from binding the name at macro use time, using @code{evaluate}
7197: also makes your definition @code{state}-smart (@pxref{state-smartness}).
7198: @end itemize
7199:
7200: You may want the macro to compile a number into a word. The word to do
7201: it is @code{literal}, but you have to @code{postpone} it, so its
7202: compilation semantics take effect when the macro is executed, not when
7203: it is compiled:
7204:
7205: @example
7206: : [compile-5] ( -- ) \ compiled code: ( -- n )
7207: 5 POSTPONE literal ; immediate
7208:
7209: : foo [compile-5] ;
7210: foo .
7211: @end example
7212:
7213: You may want to pass parameters to a macro, that the macro should
7214: compile into the current definition. If the parameter is a number, then
7215: you can use @code{postpone literal} (similar for other values).
7216:
7217: If you want to pass a word that is to be compiled, the usual way is to
7218: pass an execution token and @code{compile,} it:
7219:
7220: @example
7221: : twice1 ( xt -- ) \ compiled code: ... -- ...
7222: dup compile, compile, ;
7223:
7224: : 2+ ( n1 -- n2 )
7225: [ ' 1+ twice1 ] ;
7226: @end example
7227:
7228: doc-compile,
7229:
7230: An alternative available in Gforth, that allows you to pass compile-only
7231: words as parameters is to use the compilation token (@pxref{Compilation
7232: token}). The same example in this technique:
7233:
7234: @example
7235: : twice ( ... ct -- ... ) \ compiled code: ... -- ...
7236: 2dup 2>r execute 2r> execute ;
7237:
7238: : 2+ ( n1 -- n2 )
7239: [ comp' 1+ twice ] ;
7240: @end example
7241:
7242: In the example above @code{2>r} and @code{2r>} ensure that @code{twice}
7243: works even if the executed compilation semantics has an effect on the
7244: data stack.
7245:
7246: You can also define complete definitions with these words; this provides
7247: an alternative to using @code{does>} (@pxref{User-defined Defining
7248: Words}). E.g., instead of
7249:
7250: @example
7251: : curry+ ( n1 "name" -- )
7252: CREATE ,
7253: DOES> ( n2 -- n1+n2 )
7254: @@ + ;
7255: @end example
7256:
7257: you could define
7258:
7259: @example
7260: : curry+ ( n1 "name" -- )
7261: \ name execution: ( n2 -- n1+n2 )
7262: >r : r> POSTPONE literal POSTPONE + POSTPONE ; ;
1.81 anton 7263:
1.82 anton 7264: -3 curry+ 3-
7265: see 3-
7266: @end example
1.81 anton 7267:
1.82 anton 7268: The sequence @code{>r : r>} is necessary, because @code{:} puts a
7269: colon-sys on the data stack that makes everything below it unaccessible.
1.81 anton 7270:
1.82 anton 7271: This way of writing defining words is sometimes more, sometimes less
7272: convenient than using @code{does>} (@pxref{Advanced does> usage
7273: example}). One advantage of this method is that it can be optimized
7274: better, because the compiler knows that the value compiled with
7275: @code{literal} is fixed, whereas the data associated with a
7276: @code{create}d word can be changed.
1.47 crook 7277:
1.26 crook 7278: @c ----------------------------------------------------------
1.111 anton 7279: @node The Text Interpreter, The Input Stream, Compiling words, Words
1.26 crook 7280: @section The Text Interpreter
7281: @cindex interpreter - outer
7282: @cindex text interpreter
7283: @cindex outer interpreter
1.1 anton 7284:
1.34 anton 7285: @c Should we really describe all these ugly details? IMO the text
7286: @c interpreter should be much cleaner, but that may not be possible within
7287: @c ANS Forth. - anton
1.44 crook 7288: @c nac-> I wanted to explain how it works to show how you can exploit
7289: @c it in your own programs. When I was writing a cross-compiler, figuring out
7290: @c some of these gory details was very helpful to me. None of the textbooks
7291: @c I've seen cover it, and the most modern Forth textbook -- Forth Inc's,
7292: @c seems to positively avoid going into too much detail for some of
7293: @c the internals.
1.34 anton 7294:
1.71 anton 7295: @c anton: ok. I wonder, though, if this is the right place; for some stuff
7296: @c it is; for the ugly details, I would prefer another place. I wonder
7297: @c whether we should have a chapter before "Words" that describes some
7298: @c basic concepts referred to in words, and a chapter after "Words" that
7299: @c describes implementation details.
7300:
1.29 crook 7301: The text interpreter@footnote{This is an expanded version of the
7302: material in @ref{Introducing the Text Interpreter}.} is an endless loop
1.34 anton 7303: that processes input from the current input device. It is also called
7304: the outer interpreter, in contrast to the inner interpreter
7305: (@pxref{Engine}) which executes the compiled Forth code on interpretive
7306: implementations.
1.27 crook 7307:
1.29 crook 7308: @cindex interpret state
7309: @cindex compile state
7310: The text interpreter operates in one of two states: @dfn{interpret
7311: state} and @dfn{compile state}. The current state is defined by the
1.71 anton 7312: aptly-named variable @code{state}.
1.29 crook 7313:
7314: This section starts by describing how the text interpreter behaves when
7315: it is in interpret state, processing input from the user input device --
7316: the keyboard. This is the mode that a Forth system is in after it starts
7317: up.
7318:
7319: @cindex input buffer
7320: @cindex terminal input buffer
7321: The text interpreter works from an area of memory called the @dfn{input
7322: buffer}@footnote{When the text interpreter is processing input from the
7323: keyboard, this area of memory is called the @dfn{terminal input buffer}
7324: (TIB) and is addressed by the (obsolescent) words @code{TIB} and
7325: @code{#TIB}.}, which stores your keyboard input when you press the
1.30 anton 7326: @key{RET} key. Starting at the beginning of the input buffer, it skips
1.29 crook 7327: leading spaces (called @dfn{delimiters}) then parses a string (a
7328: sequence of non-space characters) until it reaches either a space
7329: character or the end of the buffer. Having parsed a string, it makes two
7330: attempts to process it:
1.27 crook 7331:
1.29 crook 7332: @cindex dictionary
1.27 crook 7333: @itemize @bullet
7334: @item
1.29 crook 7335: It looks for the string in a @dfn{dictionary} of definitions. If the
7336: string is found, the string names a @dfn{definition} (also known as a
7337: @dfn{word}) and the dictionary search returns information that allows
7338: the text interpreter to perform the word's @dfn{interpretation
7339: semantics}. In most cases, this simply means that the word will be
7340: executed.
1.27 crook 7341: @item
7342: If the string is not found in the dictionary, the text interpreter
1.29 crook 7343: attempts to treat it as a number, using the rules described in
7344: @ref{Number Conversion}. If the string represents a legal number in the
7345: current radix, the number is pushed onto a parameter stack (the data
7346: stack for integers, the floating-point stack for floating-point
7347: numbers).
7348: @end itemize
7349:
7350: If both attempts fail, or if the word is found in the dictionary but has
7351: no interpretation semantics@footnote{This happens if the word was
7352: defined as @code{COMPILE-ONLY}.} the text interpreter discards the
7353: remainder of the input buffer, issues an error message and waits for
7354: more input. If one of the attempts succeeds, the text interpreter
7355: repeats the parsing process until the whole of the input buffer has been
7356: processed, at which point it prints the status message ``@code{ ok}''
7357: and waits for more input.
7358:
1.71 anton 7359: @c anton: this should be in the input stream subsection (or below it)
7360:
1.29 crook 7361: @cindex parse area
7362: The text interpreter keeps track of its position in the input buffer by
7363: updating a variable called @code{>IN} (pronounced ``to-in''). The value
7364: of @code{>IN} starts out as 0, indicating an offset of 0 from the start
7365: of the input buffer. The region from offset @code{>IN @@} to the end of
7366: the input buffer is called the @dfn{parse area}@footnote{In other words,
7367: the text interpreter processes the contents of the input buffer by
7368: parsing strings from the parse area until the parse area is empty.}.
7369: This example shows how @code{>IN} changes as the text interpreter parses
7370: the input buffer:
7371:
7372: @example
7373: : remaining >IN @@ SOURCE 2 PICK - -ROT + SWAP
7374: CR ." ->" TYPE ." <-" ; IMMEDIATE
7375:
7376: 1 2 3 remaining + remaining .
7377:
7378: : foo 1 2 3 remaining SWAP remaining ;
7379: @end example
7380:
7381: @noindent
7382: The result is:
7383:
7384: @example
7385: ->+ remaining .<-
7386: ->.<-5 ok
7387:
7388: ->SWAP remaining ;-<
7389: ->;<- ok
7390: @end example
7391:
7392: @cindex parsing words
7393: The value of @code{>IN} can also be modified by a word in the input
7394: buffer that is executed by the text interpreter. This means that a word
7395: can ``trick'' the text interpreter into either skipping a section of the
7396: input buffer@footnote{This is how parsing words work.} or into parsing a
7397: section twice. For example:
1.27 crook 7398:
1.29 crook 7399: @example
1.71 anton 7400: : lat ." <<foo>>" ;
7401: : flat ." <<bar>>" >IN DUP @@ 3 - SWAP ! ;
1.29 crook 7402: @end example
7403:
7404: @noindent
7405: When @code{flat} is executed, this output is produced@footnote{Exercise
7406: for the reader: what would happen if the @code{3} were replaced with
7407: @code{4}?}:
7408:
7409: @example
1.71 anton 7410: <<bar>><<foo>>
1.29 crook 7411: @end example
7412:
1.71 anton 7413: This technique can be used to work around some of the interoperability
7414: problems of parsing words. Of course, it's better to avoid parsing
7415: words where possible.
7416:
1.29 crook 7417: @noindent
7418: Two important notes about the behaviour of the text interpreter:
1.27 crook 7419:
7420: @itemize @bullet
7421: @item
7422: It processes each input string to completion before parsing additional
1.29 crook 7423: characters from the input buffer.
7424: @item
7425: It treats the input buffer as a read-only region (and so must your code).
7426: @end itemize
7427:
7428: @noindent
7429: When the text interpreter is in compile state, its behaviour changes in
7430: these ways:
7431:
7432: @itemize @bullet
7433: @item
7434: If a parsed string is found in the dictionary, the text interpreter will
7435: perform the word's @dfn{compilation semantics}. In most cases, this
7436: simply means that the execution semantics of the word will be appended
7437: to the current definition.
1.27 crook 7438: @item
1.29 crook 7439: When a number is encountered, it is compiled into the current definition
7440: (as a literal) rather than being pushed onto a parameter stack.
7441: @item
7442: If an error occurs, @code{state} is modified to put the text interpreter
7443: back into interpret state.
7444: @item
7445: Each time a line is entered from the keyboard, Gforth prints
7446: ``@code{ compiled}'' rather than `` @code{ok}''.
7447: @end itemize
7448:
7449: @cindex text interpreter - input sources
7450: When the text interpreter is using an input device other than the
7451: keyboard, its behaviour changes in these ways:
7452:
7453: @itemize @bullet
7454: @item
7455: When the parse area is empty, the text interpreter attempts to refill
7456: the input buffer from the input source. When the input source is
1.71 anton 7457: exhausted, the input source is set back to the previous input source.
1.29 crook 7458: @item
7459: It doesn't print out ``@code{ ok}'' or ``@code{ compiled}'' messages each
7460: time the parse area is emptied.
7461: @item
7462: If an error occurs, the input source is set back to the user input
7463: device.
1.27 crook 7464: @end itemize
1.21 crook 7465:
1.49 anton 7466: You can read about this in more detail in @ref{Input Sources}.
1.44 crook 7467:
1.26 crook 7468: doc->in
1.27 crook 7469: doc-source
7470:
1.26 crook 7471: doc-tib
7472: doc-#tib
1.1 anton 7473:
1.44 crook 7474:
1.26 crook 7475: @menu
1.67 anton 7476: * Input Sources::
7477: * Number Conversion::
7478: * Interpret/Compile states::
7479: * Interpreter Directives::
1.26 crook 7480: @end menu
1.1 anton 7481:
1.29 crook 7482: @node Input Sources, Number Conversion, The Text Interpreter, The Text Interpreter
7483: @subsection Input Sources
7484: @cindex input sources
7485: @cindex text interpreter - input sources
7486:
1.44 crook 7487: By default, the text interpreter processes input from the user input
1.29 crook 7488: device (the keyboard) when Forth starts up. The text interpreter can
7489: process input from any of these sources:
7490:
7491: @itemize @bullet
7492: @item
7493: The user input device -- the keyboard.
7494: @item
7495: A file, using the words described in @ref{Forth source files}.
7496: @item
7497: A block, using the words described in @ref{Blocks}.
7498: @item
7499: A text string, using @code{evaluate}.
7500: @end itemize
7501:
7502: A program can identify the current input device from the values of
7503: @code{source-id} and @code{blk}.
7504:
1.44 crook 7505:
1.29 crook 7506: doc-source-id
7507: doc-blk
7508:
7509: doc-save-input
7510: doc-restore-input
7511:
7512: doc-evaluate
1.111 anton 7513: doc-query
1.1 anton 7514:
1.29 crook 7515:
1.44 crook 7516:
1.29 crook 7517: @node Number Conversion, Interpret/Compile states, Input Sources, The Text Interpreter
1.26 crook 7518: @subsection Number Conversion
7519: @cindex number conversion
7520: @cindex double-cell numbers, input format
7521: @cindex input format for double-cell numbers
7522: @cindex single-cell numbers, input format
7523: @cindex input format for single-cell numbers
7524: @cindex floating-point numbers, input format
7525: @cindex input format for floating-point numbers
1.1 anton 7526:
1.29 crook 7527: This section describes the rules that the text interpreter uses when it
7528: tries to convert a string into a number.
1.1 anton 7529:
1.26 crook 7530: Let <digit> represent any character that is a legal digit in the current
1.29 crook 7531: number base@footnote{For example, 0-9 when the number base is decimal or
7532: 0-9, A-F when the number base is hexadecimal.}.
1.1 anton 7533:
1.26 crook 7534: Let <decimal digit> represent any character in the range 0-9.
1.1 anton 7535:
1.29 crook 7536: Let @{@i{a b}@} represent the @i{optional} presence of any of the characters
7537: in the braces (@i{a} or @i{b} or neither).
1.1 anton 7538:
1.26 crook 7539: Let * represent any number of instances of the previous character
7540: (including none).
1.1 anton 7541:
1.26 crook 7542: Let any other character represent itself.
1.1 anton 7543:
1.29 crook 7544: @noindent
1.26 crook 7545: Now, the conversion rules are:
1.21 crook 7546:
1.26 crook 7547: @itemize @bullet
7548: @item
7549: A string of the form <digit><digit>* is treated as a single-precision
1.29 crook 7550: (cell-sized) positive integer. Examples are 0 123 6784532 32343212343456 42
1.26 crook 7551: @item
7552: A string of the form -<digit><digit>* is treated as a single-precision
1.29 crook 7553: (cell-sized) negative integer, and is represented using 2's-complement
1.26 crook 7554: arithmetic. Examples are -45 -5681 -0
7555: @item
7556: A string of the form <digit><digit>*.<digit>* is treated as a double-precision
1.29 crook 7557: (double-cell-sized) positive integer. Examples are 3465. 3.465 34.65
7558: (all three of these represent the same number).
1.26 crook 7559: @item
7560: A string of the form -<digit><digit>*.<digit>* is treated as a
1.29 crook 7561: double-precision (double-cell-sized) negative integer, and is
1.26 crook 7562: represented using 2's-complement arithmetic. Examples are -3465. -3.465
1.29 crook 7563: -34.65 (all three of these represent the same number).
1.26 crook 7564: @item
1.29 crook 7565: A string of the form @{+ -@}<decimal digit>@{.@}<decimal digit>*@{e
7566: E@}@{+ -@}<decimal digit><decimal digit>* is treated as a floating-point
1.35 anton 7567: number. Examples are 1e 1e0 1.e 1.e0 +1e+0 (which all represent the same
1.29 crook 7568: number) +12.E-4
1.26 crook 7569: @end itemize
1.1 anton 7570:
1.26 crook 7571: By default, the number base used for integer number conversion is given
1.35 anton 7572: by the contents of the variable @code{base}. Note that a lot of
7573: confusion can result from unexpected values of @code{base}. If you
7574: change @code{base} anywhere, make sure to save the old value and restore
7575: it afterwards. In general I recommend keeping @code{base} decimal, and
7576: using the prefixes described below for the popular non-decimal bases.
1.1 anton 7577:
1.29 crook 7578: doc-dpl
1.26 crook 7579: doc-base
7580: doc-hex
7581: doc-decimal
1.1 anton 7582:
1.44 crook 7583:
1.26 crook 7584: @cindex '-prefix for character strings
7585: @cindex &-prefix for decimal numbers
7586: @cindex %-prefix for binary numbers
7587: @cindex $-prefix for hexadecimal numbers
1.35 anton 7588: Gforth allows you to override the value of @code{base} by using a
1.29 crook 7589: prefix@footnote{Some Forth implementations provide a similar scheme by
7590: implementing @code{$} etc. as parsing words that process the subsequent
7591: number in the input stream and push it onto the stack. For example, see
7592: @cite{Number Conversion and Literals}, by Wil Baden; Forth Dimensions
7593: 20(3) pages 26--27. In such implementations, unlike in Gforth, a space
7594: is required between the prefix and the number.} before the first digit
7595: of an (integer) number. Four prefixes are supported:
1.1 anton 7596:
1.26 crook 7597: @itemize @bullet
7598: @item
1.35 anton 7599: @code{&} -- decimal
1.26 crook 7600: @item
1.35 anton 7601: @code{%} -- binary
1.26 crook 7602: @item
1.35 anton 7603: @code{$} -- hexadecimal
1.26 crook 7604: @item
1.35 anton 7605: @code{'} -- base @code{max-char+1}
1.26 crook 7606: @end itemize
1.1 anton 7607:
1.26 crook 7608: Here are some examples, with the equivalent decimal number shown after
7609: in braces:
1.1 anton 7610:
1.26 crook 7611: -$41 (-65), %1001101 (205), %1001.0001 (145 - a double-precision number),
7612: 'AB (16706; ascii A is 65, ascii B is 66, number is 65*256 + 66),
7613: 'ab (24930; ascii a is 97, ascii B is 98, number is 97*256 + 98),
7614: &905 (905), $abc (2478), $ABC (2478).
1.1 anton 7615:
1.26 crook 7616: @cindex number conversion - traps for the unwary
1.29 crook 7617: @noindent
1.26 crook 7618: Number conversion has a number of traps for the unwary:
1.1 anton 7619:
1.26 crook 7620: @itemize @bullet
7621: @item
7622: You cannot determine the current number base using the code sequence
1.35 anton 7623: @code{base @@ .} -- the number base is always 10 in the current number
7624: base. Instead, use something like @code{base @@ dec.}
1.26 crook 7625: @item
7626: If the number base is set to a value greater than 14 (for example,
7627: hexadecimal), the number 123E4 is ambiguous; the conversion rules allow
7628: it to be intepreted as either a single-precision integer or a
7629: floating-point number (Gforth treats it as an integer). The ambiguity
7630: can be resolved by explicitly stating the sign of the mantissa and/or
7631: exponent: 123E+4 or +123E4 -- if the number base is decimal, no
7632: ambiguity arises; either representation will be treated as a
7633: floating-point number.
7634: @item
1.29 crook 7635: There is a word @code{bin} but it does @i{not} set the number base!
1.26 crook 7636: It is used to specify file types.
7637: @item
1.72 anton 7638: ANS Forth requires the @code{.} of a double-precision number to be the
7639: final character in the string. Gforth allows the @code{.} to be
7640: anywhere after the first digit.
1.26 crook 7641: @item
7642: The number conversion process does not check for overflow.
7643: @item
1.72 anton 7644: In an ANS Forth program @code{base} is required to be decimal when
7645: converting floating-point numbers. In Gforth, number conversion to
7646: floating-point numbers always uses base &10, irrespective of the value
7647: of @code{base}.
1.26 crook 7648: @end itemize
1.1 anton 7649:
1.49 anton 7650: You can read numbers into your programs with the words described in
7651: @ref{Input}.
1.1 anton 7652:
1.82 anton 7653: @node Interpret/Compile states, Interpreter Directives, Number Conversion, The Text Interpreter
1.26 crook 7654: @subsection Interpret/Compile states
7655: @cindex Interpret/Compile states
1.1 anton 7656:
1.29 crook 7657: A standard program is not permitted to change @code{state}
7658: explicitly. However, it can change @code{state} implicitly, using the
7659: words @code{[} and @code{]}. When @code{[} is executed it switches
7660: @code{state} to interpret state, and therefore the text interpreter
7661: starts interpreting. When @code{]} is executed it switches @code{state}
7662: to compile state and therefore the text interpreter starts
1.44 crook 7663: compiling. The most common usage for these words is for switching into
7664: interpret state and back from within a colon definition; this technique
1.49 anton 7665: can be used to compile a literal (for an example, @pxref{Literals}) or
7666: for conditional compilation (for an example, @pxref{Interpreter
7667: Directives}).
1.44 crook 7668:
1.35 anton 7669:
7670: @c This is a bad example: It's non-standard, and it's not necessary.
7671: @c However, I can't think of a good example for switching into compile
7672: @c state when there is no current word (@code{state}-smart words are not a
7673: @c good reason). So maybe we should use an example for switching into
7674: @c interpret @code{state} in a colon def. - anton
1.44 crook 7675: @c nac-> I agree. I started out by putting in the example, then realised
7676: @c that it was non-ANS, so wrote more words around it. I hope this
7677: @c re-written version is acceptable to you. I do want to keep the example
7678: @c as it is helpful for showing what is and what is not portable, particularly
7679: @c where it outlaws a style in common use.
7680:
1.72 anton 7681: @c anton: it's more important to show what's portable. After we have done
1.83 anton 7682: @c that, we can also show what's not. In any case, I have written a
7683: @c section Compiling Words which also deals with [ ].
1.35 anton 7684:
1.95 anton 7685: @c !! The following example does not work in Gforth 0.5.9 or later.
1.29 crook 7686:
1.95 anton 7687: @c @code{[} and @code{]} also give you the ability to switch into compile
7688: @c state and back, but we cannot think of any useful Standard application
7689: @c for this ability. Pre-ANS Forth textbooks have examples like this:
7690:
7691: @c @example
7692: @c : AA ." this is A" ;
7693: @c : BB ." this is B" ;
7694: @c : CC ." this is C" ;
7695:
7696: @c create table ] aa bb cc [
7697:
7698: @c : go ( n -- ) \ n is offset into table.. 0 for 1st entry
7699: @c cells table + @@ execute ;
7700: @c @end example
7701:
7702: @c This example builds a jump table; @code{0 go} will display ``@code{this
7703: @c is A}''. Using @code{[} and @code{]} in this example is equivalent to
7704: @c defining @code{table} like this:
7705:
7706: @c @example
7707: @c create table ' aa COMPILE, ' bb COMPILE, ' cc COMPILE,
7708: @c @end example
7709:
7710: @c The problem with this code is that the definition of @code{table} is not
7711: @c portable -- it @i{compile}s execution tokens into code space. Whilst it
7712: @c @i{may} work on systems where code space and data space co-incide, the
7713: @c Standard only allows data space to be assigned for a @code{CREATE}d
7714: @c word. In addition, the Standard only allows @code{@@} to access data
7715: @c space, whilst this example is using it to access code space. The only
7716: @c portable, Standard way to build this table is to build it in data space,
7717: @c like this:
7718:
7719: @c @example
7720: @c create table ' aa , ' bb , ' cc ,
7721: @c @end example
1.29 crook 7722:
1.95 anton 7723: @c doc-state
1.44 crook 7724:
1.29 crook 7725:
1.82 anton 7726: @node Interpreter Directives, , Interpret/Compile states, The Text Interpreter
1.26 crook 7727: @subsection Interpreter Directives
7728: @cindex interpreter directives
1.72 anton 7729: @cindex conditional compilation
1.1 anton 7730:
1.29 crook 7731: These words are usually used in interpret state; typically to control
7732: which parts of a source file are processed by the text
1.26 crook 7733: interpreter. There are only a few ANS Forth Standard words, but Gforth
7734: supplements these with a rich set of immediate control structure words
7735: to compensate for the fact that the non-immediate versions can only be
1.29 crook 7736: used in compile state (@pxref{Control Structures}). Typical usages:
7737:
7738: @example
1.72 anton 7739: FALSE Constant HAVE-ASSEMBLER
1.29 crook 7740: .
7741: .
1.72 anton 7742: HAVE-ASSEMBLER [IF]
1.29 crook 7743: : ASSEMBLER-FEATURE
7744: ...
7745: ;
7746: [ENDIF]
7747: .
7748: .
7749: : SEE
7750: ... \ general-purpose SEE code
1.72 anton 7751: [ HAVE-ASSEMBLER [IF] ]
1.29 crook 7752: ... \ assembler-specific SEE code
7753: [ [ENDIF] ]
7754: ;
7755: @end example
1.1 anton 7756:
1.44 crook 7757:
1.26 crook 7758: doc-[IF]
7759: doc-[ELSE]
7760: doc-[THEN]
7761: doc-[ENDIF]
1.1 anton 7762:
1.26 crook 7763: doc-[IFDEF]
7764: doc-[IFUNDEF]
1.1 anton 7765:
1.26 crook 7766: doc-[?DO]
7767: doc-[DO]
7768: doc-[FOR]
7769: doc-[LOOP]
7770: doc-[+LOOP]
7771: doc-[NEXT]
1.1 anton 7772:
1.26 crook 7773: doc-[BEGIN]
7774: doc-[UNTIL]
7775: doc-[AGAIN]
7776: doc-[WHILE]
7777: doc-[REPEAT]
1.1 anton 7778:
1.27 crook 7779:
1.26 crook 7780: @c -------------------------------------------------------------
1.111 anton 7781: @node The Input Stream, Word Lists, The Text Interpreter, Words
7782: @section The Input Stream
7783: @cindex input stream
7784:
7785: @c !! integrate this better with the "Text Interpreter" section
7786: The text interpreter reads from the input stream, which can come from
7787: several sources (@pxref{Input Sources}). Some words, in particular
7788: defining words, but also words like @code{'}, read parameters from the
7789: input stream instead of from the stack.
7790:
7791: Such words are called parsing words, because they parse the input
7792: stream. Parsing words are hard to use in other words, because it is
7793: hard to pass program-generated parameters through the input stream.
7794: They also usually have an unintuitive combination of interpretation and
7795: compilation semantics when implemented naively, leading to various
7796: approaches that try to produce a more intuitive behaviour
7797: (@pxref{Combined words}).
7798:
7799: It should be obvious by now that parsing words are a bad idea. If you
7800: want to implement a parsing word for convenience, also provide a factor
7801: of the word that does not parse, but takes the parameters on the stack.
7802: To implement the parsing word on top if it, you can use the following
7803: words:
7804:
7805: @c anton: these belong in the input stream section
7806: doc-parse
7807: doc-parse-word
7808: doc-name
7809: doc-word
7810: doc-\"-parse
7811: doc-refill
7812:
7813: Conversely, if you have the bad luck (or lack of foresight) to have to
7814: deal with parsing words without having such factors, how do you pass a
7815: string that is not in the input stream to it?
7816:
7817: doc-execute-parsing
7818:
7819: If you want to run a parsing word on a file, the following word should
7820: help:
7821:
7822: doc-execute-parsing-file
7823:
7824: @c -------------------------------------------------------------
7825: @node Word Lists, Environmental Queries, The Input Stream, Words
1.26 crook 7826: @section Word Lists
7827: @cindex word lists
1.32 anton 7828: @cindex header space
1.1 anton 7829:
1.36 anton 7830: A wordlist is a list of named words; you can add new words and look up
7831: words by name (and you can remove words in a restricted way with
7832: markers). Every named (and @code{reveal}ed) word is in one wordlist.
7833:
7834: @cindex search order stack
7835: The text interpreter searches the wordlists present in the search order
7836: (a stack of wordlists), from the top to the bottom. Within each
7837: wordlist, the search starts conceptually at the newest word; i.e., if
7838: two words in a wordlist have the same name, the newer word is found.
1.1 anton 7839:
1.26 crook 7840: @cindex compilation word list
1.36 anton 7841: New words are added to the @dfn{compilation wordlist} (aka current
7842: wordlist).
1.1 anton 7843:
1.36 anton 7844: @cindex wid
7845: A word list is identified by a cell-sized word list identifier (@i{wid})
7846: in much the same way as a file is identified by a file handle. The
7847: numerical value of the wid has no (portable) meaning, and might change
7848: from session to session.
1.1 anton 7849:
1.29 crook 7850: The ANS Forth ``Search order'' word set is intended to provide a set of
7851: low-level tools that allow various different schemes to be
1.74 anton 7852: implemented. Gforth also provides @code{vocabulary}, a traditional Forth
1.26 crook 7853: word. @file{compat/vocabulary.fs} provides an implementation in ANS
1.45 crook 7854: Forth.
1.1 anton 7855:
1.27 crook 7856: @comment TODO: locals section refers to here, saying that every word list (aka
7857: @comment vocabulary) has its own methods for searching etc. Need to document that.
1.78 anton 7858: @c anton: but better in a separate subsection on wordlist internals
1.1 anton 7859:
1.45 crook 7860: @comment TODO: document markers, reveal, tables, mappedwordlist
7861:
7862: @comment the gforthman- prefix is used to pick out the true definition of a
1.27 crook 7863: @comment word from the source files, rather than some alias.
1.44 crook 7864:
1.26 crook 7865: doc-forth-wordlist
7866: doc-definitions
7867: doc-get-current
7868: doc-set-current
7869: doc-get-order
1.45 crook 7870: doc---gforthman-set-order
1.26 crook 7871: doc-wordlist
1.30 anton 7872: doc-table
1.79 anton 7873: doc->order
1.36 anton 7874: doc-previous
1.26 crook 7875: doc-also
1.45 crook 7876: doc---gforthman-forth
1.26 crook 7877: doc-only
1.45 crook 7878: doc---gforthman-order
1.15 anton 7879:
1.26 crook 7880: doc-find
7881: doc-search-wordlist
1.15 anton 7882:
1.26 crook 7883: doc-words
7884: doc-vlist
1.44 crook 7885: @c doc-words-deferred
1.1 anton 7886:
1.74 anton 7887: @c doc-mappedwordlist @c map-structure undefined, implemantation-specific
1.26 crook 7888: doc-root
7889: doc-vocabulary
7890: doc-seal
7891: doc-vocs
7892: doc-current
7893: doc-context
1.1 anton 7894:
1.44 crook 7895:
1.26 crook 7896: @menu
1.75 anton 7897: * Vocabularies::
1.67 anton 7898: * Why use word lists?::
1.75 anton 7899: * Word list example::
1.26 crook 7900: @end menu
7901:
1.75 anton 7902: @node Vocabularies, Why use word lists?, Word Lists, Word Lists
7903: @subsection Vocabularies
7904: @cindex Vocabularies, detailed explanation
7905:
7906: Here is an example of creating and using a new wordlist using ANS
7907: Forth words:
7908:
7909: @example
7910: wordlist constant my-new-words-wordlist
7911: : my-new-words get-order nip my-new-words-wordlist swap set-order ;
7912:
7913: \ add it to the search order
7914: also my-new-words
7915:
7916: \ alternatively, add it to the search order and make it
7917: \ the compilation word list
7918: also my-new-words definitions
7919: \ type "order" to see the problem
7920: @end example
7921:
7922: The problem with this example is that @code{order} has no way to
7923: associate the name @code{my-new-words} with the wid of the word list (in
7924: Gforth, @code{order} and @code{vocs} will display @code{???} for a wid
7925: that has no associated name). There is no Standard way of associating a
7926: name with a wid.
7927:
7928: In Gforth, this example can be re-coded using @code{vocabulary}, which
7929: associates a name with a wid:
7930:
7931: @example
7932: vocabulary my-new-words
7933:
7934: \ add it to the search order
7935: also my-new-words
7936:
7937: \ alternatively, add it to the search order and make it
7938: \ the compilation word list
7939: my-new-words definitions
7940: \ type "order" to see that the problem is solved
7941: @end example
7942:
7943:
7944: @node Why use word lists?, Word list example, Vocabularies, Word Lists
1.26 crook 7945: @subsection Why use word lists?
7946: @cindex word lists - why use them?
7947:
1.74 anton 7948: Here are some reasons why people use wordlists:
1.26 crook 7949:
7950: @itemize @bullet
1.74 anton 7951:
7952: @c anton: Gforth's hashing implementation makes the search speed
7953: @c independent from the number of words. But it is linear with the number
7954: @c of wordlists that have to be searched, so in effect using more wordlists
7955: @c actually slows down compilation.
7956:
7957: @c @item
7958: @c To improve compilation speed by reducing the number of header space
7959: @c entries that must be searched. This is achieved by creating a new
7960: @c word list that contains all of the definitions that are used in the
7961: @c definition of a Forth system but which would not usually be used by
7962: @c programs running on that system. That word list would be on the search
7963: @c list when the Forth system was compiled but would be removed from the
7964: @c search list for normal operation. This can be a useful technique for
7965: @c low-performance systems (for example, 8-bit processors in embedded
7966: @c systems) but is unlikely to be necessary in high-performance desktop
7967: @c systems.
7968:
1.26 crook 7969: @item
7970: To prevent a set of words from being used outside the context in which
7971: they are valid. Two classic examples of this are an integrated editor
7972: (all of the edit commands are defined in a separate word list; the
7973: search order is set to the editor word list when the editor is invoked;
7974: the old search order is restored when the editor is terminated) and an
7975: integrated assembler (the op-codes for the machine are defined in a
7976: separate word list which is used when a @code{CODE} word is defined).
1.74 anton 7977:
7978: @item
7979: To organize the words of an application or library into a user-visible
7980: set (in @code{forth-wordlist} or some other common wordlist) and a set
7981: of helper words used just for the implementation (hidden in a separate
1.75 anton 7982: wordlist). This keeps @code{words}' output smaller, separates
7983: implementation and interface, and reduces the chance of name conflicts
7984: within the common wordlist.
1.74 anton 7985:
1.26 crook 7986: @item
7987: To prevent a name-space clash between multiple definitions with the same
7988: name. For example, when building a cross-compiler you might have a word
7989: @code{IF} that generates conditional code for your target system. By
7990: placing this definition in a different word list you can control whether
7991: the host system's @code{IF} or the target system's @code{IF} get used in
7992: any particular context by controlling the order of the word lists on the
7993: search order stack.
1.74 anton 7994:
1.26 crook 7995: @end itemize
1.1 anton 7996:
1.74 anton 7997: The downsides of using wordlists are:
7998:
7999: @itemize
8000:
8001: @item
8002: Debugging becomes more cumbersome.
8003:
8004: @item
8005: Name conflicts worked around with wordlists are still there, and you
8006: have to arrange the search order carefully to get the desired results;
8007: if you forget to do that, you get hard-to-find errors (as in any case
8008: where you read the code differently from the compiler; @code{see} can
1.75 anton 8009: help seeing which of several possible words the name resolves to in such
8010: cases). @code{See} displays just the name of the words, not what
8011: wordlist they belong to, so it might be misleading. Using unique names
8012: is a better approach to avoid name conflicts.
1.74 anton 8013:
8014: @item
8015: You have to explicitly undo any changes to the search order. In many
8016: cases it would be more convenient if this happened implicitly. Gforth
8017: currently does not provide such a feature, but it may do so in the
8018: future.
8019: @end itemize
8020:
8021:
1.75 anton 8022: @node Word list example, , Why use word lists?, Word Lists
8023: @subsection Word list example
8024: @cindex word lists - example
1.1 anton 8025:
1.74 anton 8026: The following example is from the
8027: @uref{http://www.complang.tuwien.ac.at/forth/garbage-collection.zip,
8028: garbage collector} and uses wordlists to separate public words from
8029: helper words:
8030:
8031: @example
8032: get-current ( wid )
8033: vocabulary garbage-collector also garbage-collector definitions
8034: ... \ define helper words
8035: ( wid ) set-current \ restore original (i.e., public) compilation wordlist
8036: ... \ define the public (i.e., API) words
8037: \ they can refer to the helper words
8038: previous \ restore original search order (helper words become invisible)
8039: @end example
8040:
1.26 crook 8041: @c -------------------------------------------------------------
8042: @node Environmental Queries, Files, Word Lists, Words
8043: @section Environmental Queries
8044: @cindex environmental queries
1.21 crook 8045:
1.26 crook 8046: ANS Forth introduced the idea of ``environmental queries'' as a way
8047: for a program running on a system to determine certain characteristics of the system.
8048: The Standard specifies a number of strings that might be recognised by a system.
1.21 crook 8049:
1.32 anton 8050: The Standard requires that the header space used for environmental queries
8051: be distinct from the header space used for definitions.
1.21 crook 8052:
1.26 crook 8053: Typically, environmental queries are supported by creating a set of
1.29 crook 8054: definitions in a word list that is @i{only} used during environmental
1.26 crook 8055: queries; that is what Gforth does. There is no Standard way of adding
8056: definitions to the set of recognised environmental queries, but any
8057: implementation that supports the loading of optional word sets must have
8058: some mechanism for doing this (after loading the word set, the
8059: associated environmental query string must return @code{true}). In
8060: Gforth, the word list used to honour environmental queries can be
8061: manipulated just like any other word list.
1.21 crook 8062:
1.44 crook 8063:
1.26 crook 8064: doc-environment?
8065: doc-environment-wordlist
1.21 crook 8066:
1.26 crook 8067: doc-gforth
8068: doc-os-class
1.21 crook 8069:
1.44 crook 8070:
1.26 crook 8071: Note that, whilst the documentation for (e.g.) @code{gforth} shows it
8072: returning two items on the stack, querying it using @code{environment?}
8073: will return an additional item; the @code{true} flag that shows that the
8074: string was recognised.
1.21 crook 8075:
1.26 crook 8076: @comment TODO Document the standard strings or note where they are documented herein
1.21 crook 8077:
1.26 crook 8078: Here are some examples of using environmental queries:
1.21 crook 8079:
1.26 crook 8080: @example
8081: s" address-unit-bits" environment? 0=
8082: [IF]
8083: cr .( environmental attribute address-units-bits unknown... ) cr
1.75 anton 8084: [ELSE]
8085: drop \ ensure balanced stack effect
1.26 crook 8086: [THEN]
1.21 crook 8087:
1.75 anton 8088: \ this might occur in the prelude of a standard program that uses THROW
8089: s" exception" environment? [IF]
8090: 0= [IF]
8091: : throw abort" exception thrown" ;
8092: [THEN]
8093: [ELSE] \ we don't know, so make sure
8094: : throw abort" exception thrown" ;
8095: [THEN]
1.21 crook 8096:
1.26 crook 8097: s" gforth" environment? [IF] .( Gforth version ) TYPE
8098: [ELSE] .( Not Gforth..) [THEN]
1.75 anton 8099:
8100: \ a program using v*
8101: s" gforth" environment? [IF]
8102: s" 0.5.0" compare 0< [IF] \ v* is a primitive since 0.5.0
8103: : v* ( f_addr1 nstride1 f_addr2 nstride2 ucount -- r )
8104: >r swap 2swap swap 0e r> 0 ?DO
8105: dup f@ over + 2swap dup f@ f* f+ over + 2swap
8106: LOOP
8107: 2drop 2drop ;
8108: [THEN]
8109: [ELSE] \
8110: : v* ( f_addr1 nstride1 f_addr2 nstride2 ucount -- r )
8111: ...
8112: [THEN]
1.26 crook 8113: @end example
1.21 crook 8114:
1.26 crook 8115: Here is an example of adding a definition to the environment word list:
1.21 crook 8116:
1.26 crook 8117: @example
8118: get-current environment-wordlist set-current
8119: true constant block
8120: true constant block-ext
8121: set-current
8122: @end example
1.21 crook 8123:
1.26 crook 8124: You can see what definitions are in the environment word list like this:
1.21 crook 8125:
1.26 crook 8126: @example
1.79 anton 8127: environment-wordlist >order words previous
1.26 crook 8128: @end example
1.21 crook 8129:
8130:
1.26 crook 8131: @c -------------------------------------------------------------
8132: @node Files, Blocks, Environmental Queries, Words
8133: @section Files
1.28 crook 8134: @cindex files
8135: @cindex I/O - file-handling
1.21 crook 8136:
1.26 crook 8137: Gforth provides facilities for accessing files that are stored in the
8138: host operating system's file-system. Files that are processed by Gforth
8139: can be divided into two categories:
1.21 crook 8140:
1.23 crook 8141: @itemize @bullet
8142: @item
1.29 crook 8143: Files that are processed by the Text Interpreter (@dfn{Forth source files}).
1.23 crook 8144: @item
1.29 crook 8145: Files that are processed by some other program (@dfn{general files}).
1.26 crook 8146: @end itemize
8147:
8148: @menu
1.48 anton 8149: * Forth source files::
8150: * General files::
8151: * Search Paths::
1.26 crook 8152: @end menu
8153:
8154: @c -------------------------------------------------------------
8155: @node Forth source files, General files, Files, Files
8156: @subsection Forth source files
8157: @cindex including files
8158: @cindex Forth source files
1.21 crook 8159:
1.26 crook 8160: The simplest way to interpret the contents of a file is to use one of
8161: these two formats:
1.21 crook 8162:
1.26 crook 8163: @example
8164: include mysource.fs
8165: s" mysource.fs" included
8166: @end example
1.21 crook 8167:
1.75 anton 8168: You usually want to include a file only if it is not included already
1.26 crook 8169: (by, say, another source file). In that case, you can use one of these
1.45 crook 8170: three formats:
1.21 crook 8171:
1.26 crook 8172: @example
8173: require mysource.fs
8174: needs mysource.fs
8175: s" mysource.fs" required
8176: @end example
1.21 crook 8177:
1.26 crook 8178: @cindex stack effect of included files
8179: @cindex including files, stack effect
1.45 crook 8180: It is good practice to write your source files such that interpreting them
8181: does not change the stack. Source files designed in this way can be used with
1.26 crook 8182: @code{required} and friends without complications. For example:
1.21 crook 8183:
1.26 crook 8184: @example
1.75 anton 8185: 1024 require foo.fs drop
1.26 crook 8186: @end example
1.21 crook 8187:
1.75 anton 8188: Here you want to pass the argument 1024 (e.g., a buffer size) to
8189: @file{foo.fs}. Interpreting @file{foo.fs} has the stack effect ( n -- n
8190: ), which allows its use with @code{require}. Of course with such
8191: parameters to required files, you have to ensure that the first
8192: @code{require} fits for all uses (i.e., @code{require} it early in the
8193: master load file).
1.44 crook 8194:
1.26 crook 8195: doc-include-file
8196: doc-included
1.28 crook 8197: doc-included?
1.26 crook 8198: doc-include
8199: doc-required
8200: doc-require
8201: doc-needs
1.75 anton 8202: @c doc-init-included-files @c internal
8203: doc-sourcefilename
8204: doc-sourceline#
1.44 crook 8205:
1.26 crook 8206: A definition in ANS Forth for @code{required} is provided in
8207: @file{compat/required.fs}.
1.21 crook 8208:
1.26 crook 8209: @c -------------------------------------------------------------
8210: @node General files, Search Paths, Forth source files, Files
8211: @subsection General files
8212: @cindex general files
8213: @cindex file-handling
1.21 crook 8214:
1.75 anton 8215: Files are opened/created by name and type. The following file access
8216: methods (FAMs) are recognised:
1.44 crook 8217:
1.75 anton 8218: @cindex fam (file access method)
1.26 crook 8219: doc-r/o
8220: doc-r/w
8221: doc-w/o
8222: doc-bin
1.1 anton 8223:
1.44 crook 8224:
1.26 crook 8225: When a file is opened/created, it returns a file identifier,
1.29 crook 8226: @i{wfileid} that is used for all other file commands. All file
8227: commands also return a status value, @i{wior}, that is 0 for a
1.26 crook 8228: successful operation and an implementation-defined non-zero value in the
8229: case of an error.
1.21 crook 8230:
1.44 crook 8231:
1.26 crook 8232: doc-open-file
8233: doc-create-file
1.21 crook 8234:
1.26 crook 8235: doc-close-file
8236: doc-delete-file
8237: doc-rename-file
8238: doc-read-file
8239: doc-read-line
8240: doc-write-file
8241: doc-write-line
8242: doc-emit-file
8243: doc-flush-file
1.21 crook 8244:
1.26 crook 8245: doc-file-status
8246: doc-file-position
8247: doc-reposition-file
8248: doc-file-size
8249: doc-resize-file
1.21 crook 8250:
1.93 anton 8251: doc-slurp-file
8252: doc-slurp-fid
1.112 anton 8253: doc-stdin
8254: doc-stdout
8255: doc-stderr
1.44 crook 8256:
1.26 crook 8257: @c ---------------------------------------------------------
1.48 anton 8258: @node Search Paths, , General files, Files
1.26 crook 8259: @subsection Search Paths
8260: @cindex path for @code{included}
8261: @cindex file search path
8262: @cindex @code{include} search path
8263: @cindex search path for files
1.21 crook 8264:
1.26 crook 8265: If you specify an absolute filename (i.e., a filename starting with
8266: @file{/} or @file{~}, or with @file{:} in the second position (as in
8267: @samp{C:...})) for @code{included} and friends, that file is included
8268: just as you would expect.
1.21 crook 8269:
1.75 anton 8270: If the filename starts with @file{./}, this refers to the directory that
8271: the present file was @code{included} from. This allows files to include
8272: other files relative to their own position (irrespective of the current
8273: working directory or the absolute position). This feature is essential
8274: for libraries consisting of several files, where a file may include
8275: other files from the library. It corresponds to @code{#include "..."}
8276: in C. If the current input source is not a file, @file{.} refers to the
8277: directory of the innermost file being included, or, if there is no file
8278: being included, to the current working directory.
8279:
8280: For relative filenames (not starting with @file{./}), Gforth uses a
8281: search path similar to Forth's search order (@pxref{Word Lists}). It
8282: tries to find the given filename in the directories present in the path,
8283: and includes the first one it finds. There are separate search paths for
8284: Forth source files and general files. If the search path contains the
8285: directory @file{.}, this refers to the directory of the current file, or
8286: the working directory, as if the file had been specified with @file{./}.
1.21 crook 8287:
1.26 crook 8288: Use @file{~+} to refer to the current working directory (as in the
8289: @code{bash}).
1.1 anton 8290:
1.75 anton 8291: @c anton: fold the following subsubsections into this subsection?
1.1 anton 8292:
1.48 anton 8293: @menu
1.75 anton 8294: * Source Search Paths::
1.48 anton 8295: * General Search Paths::
8296: @end menu
8297:
1.26 crook 8298: @c ---------------------------------------------------------
1.75 anton 8299: @node Source Search Paths, General Search Paths, Search Paths, Search Paths
8300: @subsubsection Source Search Paths
8301: @cindex search path control, source files
1.5 anton 8302:
1.26 crook 8303: The search path is initialized when you start Gforth (@pxref{Invoking
1.75 anton 8304: Gforth}). You can display it and change it using @code{fpath} in
8305: combination with the general path handling words.
1.5 anton 8306:
1.75 anton 8307: doc-fpath
8308: @c the functionality of the following words is easily available through
8309: @c fpath and the general path words. The may go away.
8310: @c doc-.fpath
8311: @c doc-fpath+
8312: @c doc-fpath=
8313: @c doc-open-fpath-file
1.44 crook 8314:
8315: @noindent
1.26 crook 8316: Here is an example of using @code{fpath} and @code{require}:
1.5 anton 8317:
1.26 crook 8318: @example
1.75 anton 8319: fpath path= /usr/lib/forth/|./
1.26 crook 8320: require timer.fs
8321: @end example
1.5 anton 8322:
1.75 anton 8323:
1.26 crook 8324: @c ---------------------------------------------------------
1.75 anton 8325: @node General Search Paths, , Source Search Paths, Search Paths
1.26 crook 8326: @subsubsection General Search Paths
1.75 anton 8327: @cindex search path control, source files
1.5 anton 8328:
1.26 crook 8329: Your application may need to search files in several directories, like
8330: @code{included} does. To facilitate this, Gforth allows you to define
8331: and use your own search paths, by providing generic equivalents of the
8332: Forth search path words:
1.5 anton 8333:
1.75 anton 8334: doc-open-path-file
8335: doc-path-allot
8336: doc-clear-path
8337: doc-also-path
1.26 crook 8338: doc-.path
8339: doc-path+
8340: doc-path=
1.5 anton 8341:
1.75 anton 8342: @c anton: better define a word for it, say "path-allot ( ucount -- path-addr )
1.44 crook 8343:
1.75 anton 8344: Here's an example of creating an empty search path:
8345: @c
1.26 crook 8346: @example
1.75 anton 8347: create mypath 500 path-allot \ maximum length 500 chars (is checked)
1.26 crook 8348: @end example
1.5 anton 8349:
1.26 crook 8350: @c -------------------------------------------------------------
8351: @node Blocks, Other I/O, Files, Words
8352: @section Blocks
1.28 crook 8353: @cindex I/O - blocks
8354: @cindex blocks
8355:
8356: When you run Gforth on a modern desk-top computer, it runs under the
8357: control of an operating system which provides certain services. One of
8358: these services is @var{file services}, which allows Forth source code
8359: and data to be stored in files and read into Gforth (@pxref{Files}).
8360:
8361: Traditionally, Forth has been an important programming language on
8362: systems where it has interfaced directly to the underlying hardware with
8363: no intervening operating system. Forth provides a mechanism, called
1.29 crook 8364: @dfn{blocks}, for accessing mass storage on such systems.
1.28 crook 8365:
8366: A block is a 1024-byte data area, which can be used to hold data or
8367: Forth source code. No structure is imposed on the contents of the
8368: block. A block is identified by its number; blocks are numbered
8369: contiguously from 1 to an implementation-defined maximum.
8370:
8371: A typical system that used blocks but no operating system might use a
8372: single floppy-disk drive for mass storage, with the disks formatted to
8373: provide 256-byte sectors. Blocks would be implemented by assigning the
8374: first four sectors of the disk to block 1, the second four sectors to
8375: block 2 and so on, up to the limit of the capacity of the disk. The disk
8376: would not contain any file system information, just the set of blocks.
8377:
1.29 crook 8378: @cindex blocks file
1.28 crook 8379: On systems that do provide file services, blocks are typically
1.29 crook 8380: implemented by storing a sequence of blocks within a single @dfn{blocks
1.28 crook 8381: file}. The size of the blocks file will be an exact multiple of 1024
8382: bytes, corresponding to the number of blocks it contains. This is the
8383: mechanism that Gforth uses.
8384:
1.29 crook 8385: @cindex @file{blocks.fb}
1.75 anton 8386: Only one blocks file can be open at a time. If you use block words without
1.28 crook 8387: having specified a blocks file, Gforth defaults to the blocks file
8388: @file{blocks.fb}. Gforth uses the Forth search path when attempting to
1.75 anton 8389: locate a blocks file (@pxref{Source Search Paths}).
1.28 crook 8390:
1.29 crook 8391: @cindex block buffers
1.28 crook 8392: When you read and write blocks under program control, Gforth uses a
1.29 crook 8393: number of @dfn{block buffers} as intermediate storage. These buffers are
1.28 crook 8394: not used when you use @code{load} to interpret the contents of a block.
8395:
1.75 anton 8396: The behaviour of the block buffers is analagous to that of a cache.
8397: Each block buffer has three states:
1.28 crook 8398:
8399: @itemize @bullet
8400: @item
8401: Unassigned
8402: @item
8403: Assigned-clean
8404: @item
8405: Assigned-dirty
8406: @end itemize
8407:
1.29 crook 8408: Initially, all block buffers are @i{unassigned}. In order to access a
1.28 crook 8409: block, the block (specified by its block number) must be assigned to a
8410: block buffer.
8411:
8412: The assignment of a block to a block buffer is performed by @code{block}
8413: or @code{buffer}. Use @code{block} when you wish to modify the existing
8414: contents of a block. Use @code{buffer} when you don't care about the
8415: existing contents of the block@footnote{The ANS Forth definition of
1.35 anton 8416: @code{buffer} is intended not to cause disk I/O; if the data associated
1.28 crook 8417: with the particular block is already stored in a block buffer due to an
8418: earlier @code{block} command, @code{buffer} will return that block
8419: buffer and the existing contents of the block will be
8420: available. Otherwise, @code{buffer} will simply assign a new, empty
1.29 crook 8421: block buffer for the block.}.
1.28 crook 8422:
1.47 crook 8423: Once a block has been assigned to a block buffer using @code{block} or
1.75 anton 8424: @code{buffer}, that block buffer becomes the @i{current block
8425: buffer}. Data may only be manipulated (read or written) within the
8426: current block buffer.
1.47 crook 8427:
8428: When the contents of the current block buffer has been modified it is
1.48 anton 8429: necessary, @emph{before calling @code{block} or @code{buffer} again}, to
1.75 anton 8430: either abandon the changes (by doing nothing) or mark the block as
8431: changed (assigned-dirty), using @code{update}. Using @code{update} does
8432: not change the blocks file; it simply changes a block buffer's state to
8433: @i{assigned-dirty}. The block will be written implicitly when it's
8434: buffer is needed for another block, or explicitly by @code{flush} or
8435: @code{save-buffers}.
8436:
8437: word @code{Flush} writes all @i{assigned-dirty} blocks back to the
8438: blocks file on disk. Leaving Gforth with @code{bye} also performs a
8439: @code{flush}.
1.28 crook 8440:
1.29 crook 8441: In Gforth, @code{block} and @code{buffer} use a @i{direct-mapped}
1.28 crook 8442: algorithm to assign a block buffer to a block. That means that any
8443: particular block can only be assigned to one specific block buffer,
1.29 crook 8444: called (for the particular operation) the @i{victim buffer}. If the
1.47 crook 8445: victim buffer is @i{unassigned} or @i{assigned-clean} it is allocated to
8446: the new block immediately. If it is @i{assigned-dirty} its current
8447: contents are written back to the blocks file on disk before it is
1.28 crook 8448: allocated to the new block.
8449:
8450: Although no structure is imposed on the contents of a block, it is
8451: traditional to display the contents as 16 lines each of 64 characters. A
8452: block provides a single, continuous stream of input (for example, it
8453: acts as a single parse area) -- there are no end-of-line characters
8454: within a block, and no end-of-file character at the end of a
8455: block. There are two consequences of this:
1.26 crook 8456:
1.28 crook 8457: @itemize @bullet
8458: @item
8459: The last character of one line wraps straight into the first character
8460: of the following line
8461: @item
8462: The word @code{\} -- comment to end of line -- requires special
8463: treatment; in the context of a block it causes all characters until the
8464: end of the current 64-character ``line'' to be ignored.
8465: @end itemize
8466:
8467: In Gforth, when you use @code{block} with a non-existent block number,
1.45 crook 8468: the current blocks file will be extended to the appropriate size and the
1.28 crook 8469: block buffer will be initialised with spaces.
8470:
1.47 crook 8471: Gforth includes a simple block editor (type @code{use blocked.fb 0 list}
8472: for details) but doesn't encourage the use of blocks; the mechanism is
8473: only provided for backward compatibility -- ANS Forth requires blocks to
8474: be available when files are.
1.28 crook 8475:
8476: Common techniques that are used when working with blocks include:
8477:
8478: @itemize @bullet
8479: @item
8480: A screen editor that allows you to edit blocks without leaving the Forth
8481: environment.
8482: @item
8483: Shadow screens; where every code block has an associated block
8484: containing comments (for example: code in odd block numbers, comments in
8485: even block numbers). Typically, the block editor provides a convenient
8486: mechanism to toggle between code and comments.
8487: @item
8488: Load blocks; a single block (typically block 1) contains a number of
8489: @code{thru} commands which @code{load} the whole of the application.
8490: @end itemize
1.26 crook 8491:
1.29 crook 8492: See Frank Sergeant's Pygmy Forth to see just how well blocks can be
8493: integrated into a Forth programming environment.
1.26 crook 8494:
8495: @comment TODO what about errors on open-blocks?
1.44 crook 8496:
1.26 crook 8497: doc-open-blocks
8498: doc-use
1.75 anton 8499: doc-block-offset
1.26 crook 8500: doc-get-block-fid
8501: doc-block-position
1.28 crook 8502:
1.75 anton 8503: doc-list
1.28 crook 8504: doc-scr
8505:
1.45 crook 8506: doc---gforthman-block
1.28 crook 8507: doc-buffer
8508:
1.75 anton 8509: doc-empty-buffers
8510: doc-empty-buffer
1.26 crook 8511: doc-update
1.28 crook 8512: doc-updated?
1.26 crook 8513: doc-save-buffers
1.75 anton 8514: doc-save-buffer
1.26 crook 8515: doc-flush
1.28 crook 8516:
1.26 crook 8517: doc-load
8518: doc-thru
8519: doc-+load
8520: doc-+thru
1.45 crook 8521: doc---gforthman--->
1.26 crook 8522: doc-block-included
8523:
1.44 crook 8524:
1.26 crook 8525: @c -------------------------------------------------------------
1.78 anton 8526: @node Other I/O, Locals, Blocks, Words
1.26 crook 8527: @section Other I/O
1.28 crook 8528: @cindex I/O - keyboard and display
1.26 crook 8529:
8530: @menu
8531: * Simple numeric output:: Predefined formats
8532: * Formatted numeric output:: Formatted (pictured) output
8533: * String Formats:: How Forth stores strings in memory
1.67 anton 8534: * Displaying characters and strings:: Other stuff
1.26 crook 8535: * Input:: Input
1.112 anton 8536: * Pipes:: How to create your own pipes
1.26 crook 8537: @end menu
8538:
8539: @node Simple numeric output, Formatted numeric output, Other I/O, Other I/O
8540: @subsection Simple numeric output
1.28 crook 8541: @cindex numeric output - simple/free-format
1.5 anton 8542:
1.26 crook 8543: The simplest output functions are those that display numbers from the
8544: data or floating-point stacks. Floating-point output is always displayed
8545: using base 10. Numbers displayed from the data stack use the value stored
8546: in @code{base}.
1.5 anton 8547:
1.44 crook 8548:
1.26 crook 8549: doc-.
8550: doc-dec.
8551: doc-hex.
8552: doc-u.
8553: doc-.r
8554: doc-u.r
8555: doc-d.
8556: doc-ud.
8557: doc-d.r
8558: doc-ud.r
8559: doc-f.
8560: doc-fe.
8561: doc-fs.
1.111 anton 8562: doc-f.rdp
1.44 crook 8563:
1.26 crook 8564: Examples of printing the number 1234.5678E23 in the different floating-point output
8565: formats are shown below:
1.5 anton 8566:
8567: @example
1.26 crook 8568: f. 123456779999999000000000000.
8569: fe. 123.456779999999E24
8570: fs. 1.23456779999999E26
1.5 anton 8571: @end example
8572:
8573:
1.26 crook 8574: @node Formatted numeric output, String Formats, Simple numeric output, Other I/O
8575: @subsection Formatted numeric output
1.28 crook 8576: @cindex formatted numeric output
1.26 crook 8577: @cindex pictured numeric output
1.28 crook 8578: @cindex numeric output - formatted
1.26 crook 8579:
1.29 crook 8580: Forth traditionally uses a technique called @dfn{pictured numeric
1.26 crook 8581: output} for formatted printing of integers. In this technique, digits
8582: are extracted from the number (using the current output radix defined by
8583: @code{base}), converted to ASCII codes and appended to a string that is
8584: built in a scratch-pad area of memory (@pxref{core-idef,
8585: Implementation-defined options, Implementation-defined
8586: options}). Arbitrary characters can be appended to the string during the
8587: extraction process. The completed string is specified by an address
8588: and length and can be manipulated (@code{TYPE}ed, copied, modified)
8589: under program control.
1.5 anton 8590:
1.75 anton 8591: All of the integer output words described in the previous section
8592: (@pxref{Simple numeric output}) are implemented in Gforth using pictured
8593: numeric output.
1.5 anton 8594:
1.47 crook 8595: Three important things to remember about pictured numeric output:
1.5 anton 8596:
1.26 crook 8597: @itemize @bullet
8598: @item
1.28 crook 8599: It always operates on double-precision numbers; to display a
1.49 anton 8600: single-precision number, convert it first (for ways of doing this
8601: @pxref{Double precision}).
1.26 crook 8602: @item
1.28 crook 8603: It always treats the double-precision number as though it were
8604: unsigned. The examples below show ways of printing signed numbers.
1.26 crook 8605: @item
8606: The string is built up from right to left; least significant digit first.
8607: @end itemize
1.5 anton 8608:
1.44 crook 8609:
1.26 crook 8610: doc-<#
1.47 crook 8611: doc-<<#
1.26 crook 8612: doc-#
8613: doc-#s
8614: doc-hold
8615: doc-sign
8616: doc-#>
1.47 crook 8617: doc-#>>
1.5 anton 8618:
1.26 crook 8619: doc-represent
1.111 anton 8620: doc-f>str-rdp
8621: doc-f>buf-rdp
1.5 anton 8622:
1.44 crook 8623:
8624: @noindent
1.26 crook 8625: Here are some examples of using pictured numeric output:
1.5 anton 8626:
8627: @example
1.26 crook 8628: : my-u. ( u -- )
8629: \ Simplest use of pns.. behaves like Standard u.
8630: 0 \ convert to unsigned double
1.75 anton 8631: <<# \ start conversion
1.26 crook 8632: #s \ convert all digits
8633: #> \ complete conversion
1.75 anton 8634: TYPE SPACE \ display, with trailing space
8635: #>> ; \ release hold area
1.5 anton 8636:
1.26 crook 8637: : cents-only ( u -- )
8638: 0 \ convert to unsigned double
1.75 anton 8639: <<# \ start conversion
1.26 crook 8640: # # \ convert two least-significant digits
8641: #> \ complete conversion, discard other digits
1.75 anton 8642: TYPE SPACE \ display, with trailing space
8643: #>> ; \ release hold area
1.5 anton 8644:
1.26 crook 8645: : dollars-and-cents ( u -- )
8646: 0 \ convert to unsigned double
1.75 anton 8647: <<# \ start conversion
1.26 crook 8648: # # \ convert two least-significant digits
8649: [char] . hold \ insert decimal point
8650: #s \ convert remaining digits
8651: [char] $ hold \ append currency symbol
8652: #> \ complete conversion
1.75 anton 8653: TYPE SPACE \ display, with trailing space
8654: #>> ; \ release hold area
1.5 anton 8655:
1.26 crook 8656: : my-. ( n -- )
8657: \ handling negatives.. behaves like Standard .
8658: s>d \ convert to signed double
8659: swap over dabs \ leave sign byte followed by unsigned double
1.75 anton 8660: <<# \ start conversion
1.26 crook 8661: #s \ convert all digits
8662: rot sign \ get at sign byte, append "-" if needed
8663: #> \ complete conversion
1.75 anton 8664: TYPE SPACE \ display, with trailing space
8665: #>> ; \ release hold area
1.5 anton 8666:
1.26 crook 8667: : account. ( n -- )
1.75 anton 8668: \ accountants don't like minus signs, they use parentheses
1.26 crook 8669: \ for negative numbers
8670: s>d \ convert to signed double
8671: swap over dabs \ leave sign byte followed by unsigned double
1.75 anton 8672: <<# \ start conversion
1.26 crook 8673: 2 pick \ get copy of sign byte
8674: 0< IF [char] ) hold THEN \ right-most character of output
8675: #s \ convert all digits
8676: rot \ get at sign byte
8677: 0< IF [char] ( hold THEN
8678: #> \ complete conversion
1.75 anton 8679: TYPE SPACE \ display, with trailing space
8680: #>> ; \ release hold area
8681:
1.5 anton 8682: @end example
8683:
1.26 crook 8684: Here are some examples of using these words:
1.5 anton 8685:
8686: @example
1.26 crook 8687: 1 my-u. 1
8688: hex -1 my-u. decimal FFFFFFFF
8689: 1 cents-only 01
8690: 1234 cents-only 34
8691: 2 dollars-and-cents $0.02
8692: 1234 dollars-and-cents $12.34
8693: 123 my-. 123
8694: -123 my. -123
8695: 123 account. 123
8696: -456 account. (456)
1.5 anton 8697: @end example
8698:
8699:
1.26 crook 8700: @node String Formats, Displaying characters and strings, Formatted numeric output, Other I/O
8701: @subsection String Formats
1.27 crook 8702: @cindex strings - see character strings
8703: @cindex character strings - formats
1.28 crook 8704: @cindex I/O - see character strings
1.75 anton 8705: @cindex counted strings
8706:
8707: @c anton: this does not really belong here; maybe the memory section,
8708: @c or the principles chapter
1.26 crook 8709:
1.27 crook 8710: Forth commonly uses two different methods for representing character
8711: strings:
1.26 crook 8712:
8713: @itemize @bullet
8714: @item
8715: @cindex address of counted string
1.45 crook 8716: @cindex counted string
1.29 crook 8717: As a @dfn{counted string}, represented by a @i{c-addr}. The char
8718: addressed by @i{c-addr} contains a character-count, @i{n}, of the
8719: string and the string occupies the subsequent @i{n} char addresses in
1.26 crook 8720: memory.
8721: @item
1.29 crook 8722: As cell pair on the stack; @i{c-addr u}, where @i{u} is the length
8723: of the string in characters, and @i{c-addr} is the address of the
1.26 crook 8724: first byte of the string.
8725: @end itemize
8726:
8727: ANS Forth encourages the use of the second format when representing
1.75 anton 8728: strings.
1.26 crook 8729:
1.44 crook 8730:
1.26 crook 8731: doc-count
8732:
1.44 crook 8733:
1.49 anton 8734: For words that move, copy and search for strings see @ref{Memory
8735: Blocks}. For words that display characters and strings see
8736: @ref{Displaying characters and strings}.
1.26 crook 8737:
8738: @node Displaying characters and strings, Input, String Formats, Other I/O
8739: @subsection Displaying characters and strings
1.27 crook 8740: @cindex characters - compiling and displaying
8741: @cindex character strings - compiling and displaying
1.26 crook 8742:
8743: This section starts with a glossary of Forth words and ends with a set
8744: of examples.
8745:
1.44 crook 8746:
1.26 crook 8747: doc-bl
8748: doc-space
8749: doc-spaces
8750: doc-emit
8751: doc-toupper
8752: doc-."
8753: doc-.(
1.98 anton 8754: doc-.\"
1.26 crook 8755: doc-type
1.44 crook 8756: doc-typewhite
1.26 crook 8757: doc-cr
1.27 crook 8758: @cindex cursor control
1.26 crook 8759: doc-at-xy
8760: doc-page
8761: doc-s"
1.98 anton 8762: doc-s\"
1.26 crook 8763: doc-c"
8764: doc-char
8765: doc-[char]
8766:
1.44 crook 8767:
8768: @noindent
1.26 crook 8769: As an example, consider the following text, stored in a file @file{test.fs}:
1.5 anton 8770:
8771: @example
1.26 crook 8772: .( text-1)
8773: : my-word
8774: ." text-2" cr
8775: .( text-3)
8776: ;
8777:
8778: ." text-4"
8779:
8780: : my-char
8781: [char] ALPHABET emit
8782: char emit
8783: ;
1.5 anton 8784: @end example
8785:
1.26 crook 8786: When you load this code into Gforth, the following output is generated:
1.5 anton 8787:
1.26 crook 8788: @example
1.30 anton 8789: @kbd{include test.fs @key{RET}} text-1text-3text-4 ok
1.26 crook 8790: @end example
1.5 anton 8791:
1.26 crook 8792: @itemize @bullet
8793: @item
8794: Messages @code{text-1} and @code{text-3} are displayed because @code{.(}
8795: is an immediate word; it behaves in the same way whether it is used inside
8796: or outside a colon definition.
8797: @item
8798: Message @code{text-4} is displayed because of Gforth's added interpretation
8799: semantics for @code{."}.
8800: @item
1.29 crook 8801: Message @code{text-2} is @i{not} displayed, because the text interpreter
1.26 crook 8802: performs the compilation semantics for @code{."} within the definition of
8803: @code{my-word}.
8804: @end itemize
1.5 anton 8805:
1.26 crook 8806: Here are some examples of executing @code{my-word} and @code{my-char}:
1.5 anton 8807:
1.26 crook 8808: @example
1.30 anton 8809: @kbd{my-word @key{RET}} text-2
1.26 crook 8810: ok
1.30 anton 8811: @kbd{my-char fred @key{RET}} Af ok
8812: @kbd{my-char jim @key{RET}} Aj ok
1.26 crook 8813: @end example
1.5 anton 8814:
8815: @itemize @bullet
8816: @item
1.26 crook 8817: Message @code{text-2} is displayed because of the run-time behaviour of
8818: @code{."}.
8819: @item
8820: @code{[char]} compiles the ``A'' from ``ALPHABET'' and puts its display code
8821: on the stack at run-time. @code{emit} always displays the character
8822: when @code{my-char} is executed.
8823: @item
8824: @code{char} parses a string at run-time and the second @code{emit} displays
8825: the first character of the string.
1.5 anton 8826: @item
1.26 crook 8827: If you type @code{see my-char} you can see that @code{[char]} discarded
8828: the text ``LPHABET'' and only compiled the display code for ``A'' into the
8829: definition of @code{my-char}.
1.5 anton 8830: @end itemize
8831:
8832:
8833:
1.112 anton 8834: @node Input, Pipes, Displaying characters and strings, Other I/O
1.26 crook 8835: @subsection Input
8836: @cindex input
1.28 crook 8837: @cindex I/O - see input
8838: @cindex parsing a string
1.5 anton 8839:
1.49 anton 8840: For ways of storing character strings in memory see @ref{String Formats}.
1.5 anton 8841:
1.27 crook 8842: @comment TODO examples for >number >float accept key key? pad parse word refill
1.29 crook 8843: @comment then index them
1.27 crook 8844:
1.44 crook 8845:
1.27 crook 8846: doc-key
8847: doc-key?
1.45 crook 8848: doc-ekey
8849: doc-ekey?
8850: doc-ekey>char
1.26 crook 8851: doc->number
8852: doc->float
8853: doc-accept
1.109 anton 8854: doc-edit-line
1.27 crook 8855: doc-pad
8856: @comment obsolescent words..
8857: doc-convert
1.26 crook 8858: doc-expect
1.27 crook 8859: doc-span
1.5 anton 8860:
8861:
1.112 anton 8862: @node Pipes, , Input, Other I/O
8863: @subsection Pipes
8864: @cindex pipes, creating your own
8865:
8866: In addition to using Gforth in pipes created by other processes
8867: (@pxref{Gforth in pipes}), you can create your own pipe with
8868: @code{open-pipe}, and read from or write to it.
8869:
8870: doc-open-pipe
8871: doc-close-pipe
8872:
8873: If you write to a pipe, Gforth can throw a @code{broken-pipe-error}; if
8874: you don't catch this exception, Gforth will catch it and exit, usually
8875: silently (@pxref{Gforth in pipes}). Since you probably do not want
8876: this, you should wrap a @code{catch} or @code{try} block around the code
8877: from @code{open-pipe} to @code{close-pipe}, so you can deal with the
8878: problem yourself, and then return to regular processing.
8879:
8880: doc-broken-pipe-error
8881:
8882:
1.78 anton 8883: @c -------------------------------------------------------------
8884: @node Locals, Structures, Other I/O, Words
8885: @section Locals
8886: @cindex locals
8887:
8888: Local variables can make Forth programming more enjoyable and Forth
8889: programs easier to read. Unfortunately, the locals of ANS Forth are
8890: laden with restrictions. Therefore, we provide not only the ANS Forth
8891: locals wordset, but also our own, more powerful locals wordset (we
8892: implemented the ANS Forth locals wordset through our locals wordset).
1.44 crook 8893:
1.78 anton 8894: The ideas in this section have also been published in M. Anton Ertl,
8895: @cite{@uref{http://www.complang.tuwien.ac.at/papers/ertl94l.ps.gz,
8896: Automatic Scoping of Local Variables}}, EuroForth '94.
1.12 anton 8897:
8898: @menu
1.78 anton 8899: * Gforth locals::
8900: * ANS Forth locals::
1.5 anton 8901: @end menu
8902:
1.78 anton 8903: @node Gforth locals, ANS Forth locals, Locals, Locals
8904: @subsection Gforth locals
8905: @cindex Gforth locals
8906: @cindex locals, Gforth style
1.5 anton 8907:
1.78 anton 8908: Locals can be defined with
1.44 crook 8909:
1.78 anton 8910: @example
8911: @{ local1 local2 ... -- comment @}
8912: @end example
8913: or
8914: @example
8915: @{ local1 local2 ... @}
8916: @end example
1.5 anton 8917:
1.78 anton 8918: E.g.,
8919: @example
8920: : max @{ n1 n2 -- n3 @}
8921: n1 n2 > if
8922: n1
8923: else
8924: n2
8925: endif ;
8926: @end example
1.44 crook 8927:
1.78 anton 8928: The similarity of locals definitions with stack comments is intended. A
8929: locals definition often replaces the stack comment of a word. The order
8930: of the locals corresponds to the order in a stack comment and everything
8931: after the @code{--} is really a comment.
1.77 anton 8932:
1.78 anton 8933: This similarity has one disadvantage: It is too easy to confuse locals
8934: declarations with stack comments, causing bugs and making them hard to
8935: find. However, this problem can be avoided by appropriate coding
8936: conventions: Do not use both notations in the same program. If you do,
8937: they should be distinguished using additional means, e.g. by position.
1.77 anton 8938:
1.78 anton 8939: @cindex types of locals
8940: @cindex locals types
8941: The name of the local may be preceded by a type specifier, e.g.,
8942: @code{F:} for a floating point value:
1.5 anton 8943:
1.78 anton 8944: @example
8945: : CX* @{ F: Ar F: Ai F: Br F: Bi -- Cr Ci @}
8946: \ complex multiplication
8947: Ar Br f* Ai Bi f* f-
8948: Ar Bi f* Ai Br f* f+ ;
8949: @end example
1.44 crook 8950:
1.78 anton 8951: @cindex flavours of locals
8952: @cindex locals flavours
8953: @cindex value-flavoured locals
8954: @cindex variable-flavoured locals
8955: Gforth currently supports cells (@code{W:}, @code{W^}), doubles
8956: (@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters
8957: (@code{C:}, @code{C^}) in two flavours: a value-flavoured local (defined
8958: with @code{W:}, @code{D:} etc.) produces its value and can be changed
8959: with @code{TO}. A variable-flavoured local (defined with @code{W^} etc.)
8960: produces its address (which becomes invalid when the variable's scope is
8961: left). E.g., the standard word @code{emit} can be defined in terms of
8962: @code{type} like this:
1.5 anton 8963:
1.78 anton 8964: @example
8965: : emit @{ C^ char* -- @}
8966: char* 1 type ;
8967: @end example
1.5 anton 8968:
1.78 anton 8969: @cindex default type of locals
8970: @cindex locals, default type
8971: A local without type specifier is a @code{W:} local. Both flavours of
8972: locals are initialized with values from the data or FP stack.
1.44 crook 8973:
1.78 anton 8974: Currently there is no way to define locals with user-defined data
8975: structures, but we are working on it.
1.5 anton 8976:
1.78 anton 8977: Gforth allows defining locals everywhere in a colon definition. This
8978: poses the following questions:
1.5 anton 8979:
1.78 anton 8980: @menu
8981: * Where are locals visible by name?::
8982: * How long do locals live?::
8983: * Locals programming style::
8984: * Locals implementation::
8985: @end menu
1.44 crook 8986:
1.78 anton 8987: @node Where are locals visible by name?, How long do locals live?, Gforth locals, Gforth locals
8988: @subsubsection Where are locals visible by name?
8989: @cindex locals visibility
8990: @cindex visibility of locals
8991: @cindex scope of locals
1.5 anton 8992:
1.78 anton 8993: Basically, the answer is that locals are visible where you would expect
8994: it in block-structured languages, and sometimes a little longer. If you
8995: want to restrict the scope of a local, enclose its definition in
8996: @code{SCOPE}...@code{ENDSCOPE}.
1.5 anton 8997:
8998:
1.78 anton 8999: doc-scope
9000: doc-endscope
1.5 anton 9001:
9002:
1.78 anton 9003: These words behave like control structure words, so you can use them
9004: with @code{CS-PICK} and @code{CS-ROLL} to restrict the scope in
9005: arbitrary ways.
1.77 anton 9006:
1.78 anton 9007: If you want a more exact answer to the visibility question, here's the
9008: basic principle: A local is visible in all places that can only be
9009: reached through the definition of the local@footnote{In compiler
9010: construction terminology, all places dominated by the definition of the
9011: local.}. In other words, it is not visible in places that can be reached
9012: without going through the definition of the local. E.g., locals defined
9013: in @code{IF}...@code{ENDIF} are visible until the @code{ENDIF}, locals
9014: defined in @code{BEGIN}...@code{UNTIL} are visible after the
9015: @code{UNTIL} (until, e.g., a subsequent @code{ENDSCOPE}).
1.77 anton 9016:
1.78 anton 9017: The reasoning behind this solution is: We want to have the locals
9018: visible as long as it is meaningful. The user can always make the
9019: visibility shorter by using explicit scoping. In a place that can
9020: only be reached through the definition of a local, the meaning of a
9021: local name is clear. In other places it is not: How is the local
9022: initialized at the control flow path that does not contain the
9023: definition? Which local is meant, if the same name is defined twice in
9024: two independent control flow paths?
1.77 anton 9025:
1.78 anton 9026: This should be enough detail for nearly all users, so you can skip the
9027: rest of this section. If you really must know all the gory details and
9028: options, read on.
1.77 anton 9029:
1.78 anton 9030: In order to implement this rule, the compiler has to know which places
9031: are unreachable. It knows this automatically after @code{AHEAD},
9032: @code{AGAIN}, @code{EXIT} and @code{LEAVE}; in other cases (e.g., after
9033: most @code{THROW}s), you can use the word @code{UNREACHABLE} to tell the
9034: compiler that the control flow never reaches that place. If
9035: @code{UNREACHABLE} is not used where it could, the only consequence is
9036: that the visibility of some locals is more limited than the rule above
9037: says. If @code{UNREACHABLE} is used where it should not (i.e., if you
9038: lie to the compiler), buggy code will be produced.
1.77 anton 9039:
1.5 anton 9040:
1.78 anton 9041: doc-unreachable
1.5 anton 9042:
1.23 crook 9043:
1.78 anton 9044: Another problem with this rule is that at @code{BEGIN}, the compiler
9045: does not know which locals will be visible on the incoming
9046: back-edge. All problems discussed in the following are due to this
9047: ignorance of the compiler (we discuss the problems using @code{BEGIN}
9048: loops as examples; the discussion also applies to @code{?DO} and other
9049: loops). Perhaps the most insidious example is:
1.26 crook 9050: @example
1.78 anton 9051: AHEAD
9052: BEGIN
9053: x
9054: [ 1 CS-ROLL ] THEN
9055: @{ x @}
9056: ...
9057: UNTIL
1.26 crook 9058: @end example
1.23 crook 9059:
1.78 anton 9060: This should be legal according to the visibility rule. The use of
9061: @code{x} can only be reached through the definition; but that appears
9062: textually below the use.
9063:
9064: From this example it is clear that the visibility rules cannot be fully
9065: implemented without major headaches. Our implementation treats common
9066: cases as advertised and the exceptions are treated in a safe way: The
9067: compiler makes a reasonable guess about the locals visible after a
9068: @code{BEGIN}; if it is too pessimistic, the
9069: user will get a spurious error about the local not being defined; if the
9070: compiler is too optimistic, it will notice this later and issue a
9071: warning. In the case above the compiler would complain about @code{x}
9072: being undefined at its use. You can see from the obscure examples in
9073: this section that it takes quite unusual control structures to get the
9074: compiler into trouble, and even then it will often do fine.
1.23 crook 9075:
1.78 anton 9076: If the @code{BEGIN} is reachable from above, the most optimistic guess
9077: is that all locals visible before the @code{BEGIN} will also be
9078: visible after the @code{BEGIN}. This guess is valid for all loops that
9079: are entered only through the @code{BEGIN}, in particular, for normal
9080: @code{BEGIN}...@code{WHILE}...@code{REPEAT} and
9081: @code{BEGIN}...@code{UNTIL} loops and it is implemented in our
9082: compiler. When the branch to the @code{BEGIN} is finally generated by
9083: @code{AGAIN} or @code{UNTIL}, the compiler checks the guess and
9084: warns the user if it was too optimistic:
1.26 crook 9085: @example
1.78 anton 9086: IF
9087: @{ x @}
9088: BEGIN
9089: \ x ?
9090: [ 1 cs-roll ] THEN
9091: ...
9092: UNTIL
1.26 crook 9093: @end example
1.23 crook 9094:
1.78 anton 9095: Here, @code{x} lives only until the @code{BEGIN}, but the compiler
9096: optimistically assumes that it lives until the @code{THEN}. It notices
9097: this difference when it compiles the @code{UNTIL} and issues a
9098: warning. The user can avoid the warning, and make sure that @code{x}
9099: is not used in the wrong area by using explicit scoping:
9100: @example
9101: IF
9102: SCOPE
9103: @{ x @}
9104: ENDSCOPE
9105: BEGIN
9106: [ 1 cs-roll ] THEN
9107: ...
9108: UNTIL
9109: @end example
1.23 crook 9110:
1.78 anton 9111: Since the guess is optimistic, there will be no spurious error messages
9112: about undefined locals.
1.44 crook 9113:
1.78 anton 9114: If the @code{BEGIN} is not reachable from above (e.g., after
9115: @code{AHEAD} or @code{EXIT}), the compiler cannot even make an
9116: optimistic guess, as the locals visible after the @code{BEGIN} may be
9117: defined later. Therefore, the compiler assumes that no locals are
9118: visible after the @code{BEGIN}. However, the user can use
9119: @code{ASSUME-LIVE} to make the compiler assume that the same locals are
9120: visible at the BEGIN as at the point where the top control-flow stack
9121: item was created.
1.23 crook 9122:
1.44 crook 9123:
1.78 anton 9124: doc-assume-live
1.26 crook 9125:
1.23 crook 9126:
1.78 anton 9127: @noindent
9128: E.g.,
9129: @example
9130: @{ x @}
9131: AHEAD
9132: ASSUME-LIVE
9133: BEGIN
9134: x
9135: [ 1 CS-ROLL ] THEN
9136: ...
9137: UNTIL
9138: @end example
1.44 crook 9139:
1.78 anton 9140: Other cases where the locals are defined before the @code{BEGIN} can be
9141: handled by inserting an appropriate @code{CS-ROLL} before the
9142: @code{ASSUME-LIVE} (and changing the control-flow stack manipulation
9143: behind the @code{ASSUME-LIVE}).
1.23 crook 9144:
1.78 anton 9145: Cases where locals are defined after the @code{BEGIN} (but should be
9146: visible immediately after the @code{BEGIN}) can only be handled by
9147: rearranging the loop. E.g., the ``most insidious'' example above can be
9148: arranged into:
9149: @example
9150: BEGIN
9151: @{ x @}
9152: ... 0=
9153: WHILE
9154: x
9155: REPEAT
9156: @end example
1.44 crook 9157:
1.78 anton 9158: @node How long do locals live?, Locals programming style, Where are locals visible by name?, Gforth locals
9159: @subsubsection How long do locals live?
9160: @cindex locals lifetime
9161: @cindex lifetime of locals
1.23 crook 9162:
1.78 anton 9163: The right answer for the lifetime question would be: A local lives at
9164: least as long as it can be accessed. For a value-flavoured local this
9165: means: until the end of its visibility. However, a variable-flavoured
9166: local could be accessed through its address far beyond its visibility
9167: scope. Ultimately, this would mean that such locals would have to be
9168: garbage collected. Since this entails un-Forth-like implementation
9169: complexities, I adopted the same cowardly solution as some other
9170: languages (e.g., C): The local lives only as long as it is visible;
9171: afterwards its address is invalid (and programs that access it
9172: afterwards are erroneous).
1.23 crook 9173:
1.78 anton 9174: @node Locals programming style, Locals implementation, How long do locals live?, Gforth locals
9175: @subsubsection Locals programming style
9176: @cindex locals programming style
9177: @cindex programming style, locals
1.23 crook 9178:
1.78 anton 9179: The freedom to define locals anywhere has the potential to change
9180: programming styles dramatically. In particular, the need to use the
9181: return stack for intermediate storage vanishes. Moreover, all stack
9182: manipulations (except @code{PICK}s and @code{ROLL}s with run-time
9183: determined arguments) can be eliminated: If the stack items are in the
9184: wrong order, just write a locals definition for all of them; then
9185: write the items in the order you want.
1.23 crook 9186:
1.78 anton 9187: This seems a little far-fetched and eliminating stack manipulations is
9188: unlikely to become a conscious programming objective. Still, the number
9189: of stack manipulations will be reduced dramatically if local variables
9190: are used liberally (e.g., compare @code{max} (@pxref{Gforth locals}) with
9191: a traditional implementation of @code{max}).
1.23 crook 9192:
1.78 anton 9193: This shows one potential benefit of locals: making Forth programs more
9194: readable. Of course, this benefit will only be realized if the
9195: programmers continue to honour the principle of factoring instead of
9196: using the added latitude to make the words longer.
1.23 crook 9197:
1.78 anton 9198: @cindex single-assignment style for locals
9199: Using @code{TO} can and should be avoided. Without @code{TO},
9200: every value-flavoured local has only a single assignment and many
9201: advantages of functional languages apply to Forth. I.e., programs are
9202: easier to analyse, to optimize and to read: It is clear from the
9203: definition what the local stands for, it does not turn into something
9204: different later.
1.23 crook 9205:
1.78 anton 9206: E.g., a definition using @code{TO} might look like this:
9207: @example
9208: : strcmp @{ addr1 u1 addr2 u2 -- n @}
9209: u1 u2 min 0
9210: ?do
9211: addr1 c@@ addr2 c@@ -
9212: ?dup-if
9213: unloop exit
9214: then
9215: addr1 char+ TO addr1
9216: addr2 char+ TO addr2
9217: loop
9218: u1 u2 - ;
1.26 crook 9219: @end example
1.78 anton 9220: Here, @code{TO} is used to update @code{addr1} and @code{addr2} at
9221: every loop iteration. @code{strcmp} is a typical example of the
9222: readability problems of using @code{TO}. When you start reading
9223: @code{strcmp}, you think that @code{addr1} refers to the start of the
9224: string. Only near the end of the loop you realize that it is something
9225: else.
1.23 crook 9226:
1.78 anton 9227: This can be avoided by defining two locals at the start of the loop that
9228: are initialized with the right value for the current iteration.
9229: @example
9230: : strcmp @{ addr1 u1 addr2 u2 -- n @}
9231: addr1 addr2
9232: u1 u2 min 0
9233: ?do @{ s1 s2 @}
9234: s1 c@@ s2 c@@ -
9235: ?dup-if
9236: unloop exit
9237: then
9238: s1 char+ s2 char+
9239: loop
9240: 2drop
9241: u1 u2 - ;
9242: @end example
9243: Here it is clear from the start that @code{s1} has a different value
9244: in every loop iteration.
1.23 crook 9245:
1.78 anton 9246: @node Locals implementation, , Locals programming style, Gforth locals
9247: @subsubsection Locals implementation
9248: @cindex locals implementation
9249: @cindex implementation of locals
1.23 crook 9250:
1.78 anton 9251: @cindex locals stack
9252: Gforth uses an extra locals stack. The most compelling reason for
9253: this is that the return stack is not float-aligned; using an extra stack
9254: also eliminates the problems and restrictions of using the return stack
9255: as locals stack. Like the other stacks, the locals stack grows toward
9256: lower addresses. A few primitives allow an efficient implementation:
9257:
9258:
9259: doc-@local#
9260: doc-f@local#
9261: doc-laddr#
9262: doc-lp+!#
9263: doc-lp!
9264: doc->l
9265: doc-f>l
9266:
9267:
9268: In addition to these primitives, some specializations of these
9269: primitives for commonly occurring inline arguments are provided for
9270: efficiency reasons, e.g., @code{@@local0} as specialization of
9271: @code{@@local#} for the inline argument 0. The following compiling words
9272: compile the right specialized version, or the general version, as
9273: appropriate:
1.23 crook 9274:
1.5 anton 9275:
1.107 dvdkhlng 9276: @c doc-compile-@local
9277: @c doc-compile-f@local
1.78 anton 9278: doc-compile-lp+!
1.5 anton 9279:
9280:
1.78 anton 9281: Combinations of conditional branches and @code{lp+!#} like
9282: @code{?branch-lp+!#} (the locals pointer is only changed if the branch
9283: is taken) are provided for efficiency and correctness in loops.
1.5 anton 9284:
1.78 anton 9285: A special area in the dictionary space is reserved for keeping the
9286: local variable names. @code{@{} switches the dictionary pointer to this
9287: area and @code{@}} switches it back and generates the locals
9288: initializing code. @code{W:} etc.@ are normal defining words. This
9289: special area is cleared at the start of every colon definition.
1.5 anton 9290:
1.78 anton 9291: @cindex word list for defining locals
9292: A special feature of Gforth's dictionary is used to implement the
9293: definition of locals without type specifiers: every word list (aka
9294: vocabulary) has its own methods for searching
9295: etc. (@pxref{Word Lists}). For the present purpose we defined a word list
9296: with a special search method: When it is searched for a word, it
9297: actually creates that word using @code{W:}. @code{@{} changes the search
9298: order to first search the word list containing @code{@}}, @code{W:} etc.,
9299: and then the word list for defining locals without type specifiers.
1.5 anton 9300:
1.78 anton 9301: The lifetime rules support a stack discipline within a colon
9302: definition: The lifetime of a local is either nested with other locals
9303: lifetimes or it does not overlap them.
1.23 crook 9304:
1.78 anton 9305: At @code{BEGIN}, @code{IF}, and @code{AHEAD} no code for locals stack
9306: pointer manipulation is generated. Between control structure words
9307: locals definitions can push locals onto the locals stack. @code{AGAIN}
9308: is the simplest of the other three control flow words. It has to
9309: restore the locals stack depth of the corresponding @code{BEGIN}
9310: before branching. The code looks like this:
9311: @format
9312: @code{lp+!#} current-locals-size @minus{} dest-locals-size
9313: @code{branch} <begin>
9314: @end format
1.26 crook 9315:
1.78 anton 9316: @code{UNTIL} is a little more complicated: If it branches back, it
9317: must adjust the stack just like @code{AGAIN}. But if it falls through,
9318: the locals stack must not be changed. The compiler generates the
9319: following code:
9320: @format
9321: @code{?branch-lp+!#} <begin> current-locals-size @minus{} dest-locals-size
9322: @end format
9323: The locals stack pointer is only adjusted if the branch is taken.
1.44 crook 9324:
1.78 anton 9325: @code{THEN} can produce somewhat inefficient code:
9326: @format
9327: @code{lp+!#} current-locals-size @minus{} orig-locals-size
9328: <orig target>:
9329: @code{lp+!#} orig-locals-size @minus{} new-locals-size
9330: @end format
9331: The second @code{lp+!#} adjusts the locals stack pointer from the
9332: level at the @i{orig} point to the level after the @code{THEN}. The
9333: first @code{lp+!#} adjusts the locals stack pointer from the current
9334: level to the level at the orig point, so the complete effect is an
9335: adjustment from the current level to the right level after the
9336: @code{THEN}.
1.26 crook 9337:
1.78 anton 9338: @cindex locals information on the control-flow stack
9339: @cindex control-flow stack items, locals information
9340: In a conventional Forth implementation a dest control-flow stack entry
9341: is just the target address and an orig entry is just the address to be
9342: patched. Our locals implementation adds a word list to every orig or dest
9343: item. It is the list of locals visible (or assumed visible) at the point
9344: described by the entry. Our implementation also adds a tag to identify
9345: the kind of entry, in particular to differentiate between live and dead
9346: (reachable and unreachable) orig entries.
1.26 crook 9347:
1.78 anton 9348: A few unusual operations have to be performed on locals word lists:
1.44 crook 9349:
1.5 anton 9350:
1.78 anton 9351: doc-common-list
9352: doc-sub-list?
9353: doc-list-size
1.52 anton 9354:
9355:
1.78 anton 9356: Several features of our locals word list implementation make these
9357: operations easy to implement: The locals word lists are organised as
9358: linked lists; the tails of these lists are shared, if the lists
9359: contain some of the same locals; and the address of a name is greater
9360: than the address of the names behind it in the list.
1.5 anton 9361:
1.78 anton 9362: Another important implementation detail is the variable
9363: @code{dead-code}. It is used by @code{BEGIN} and @code{THEN} to
9364: determine if they can be reached directly or only through the branch
9365: that they resolve. @code{dead-code} is set by @code{UNREACHABLE},
9366: @code{AHEAD}, @code{EXIT} etc., and cleared at the start of a colon
9367: definition, by @code{BEGIN} and usually by @code{THEN}.
1.5 anton 9368:
1.78 anton 9369: Counted loops are similar to other loops in most respects, but
9370: @code{LEAVE} requires special attention: It performs basically the same
9371: service as @code{AHEAD}, but it does not create a control-flow stack
9372: entry. Therefore the information has to be stored elsewhere;
9373: traditionally, the information was stored in the target fields of the
9374: branches created by the @code{LEAVE}s, by organizing these fields into a
9375: linked list. Unfortunately, this clever trick does not provide enough
9376: space for storing our extended control flow information. Therefore, we
9377: introduce another stack, the leave stack. It contains the control-flow
9378: stack entries for all unresolved @code{LEAVE}s.
1.44 crook 9379:
1.78 anton 9380: Local names are kept until the end of the colon definition, even if
9381: they are no longer visible in any control-flow path. In a few cases
9382: this may lead to increased space needs for the locals name area, but
9383: usually less than reclaiming this space would cost in code size.
1.5 anton 9384:
1.44 crook 9385:
1.78 anton 9386: @node ANS Forth locals, , Gforth locals, Locals
9387: @subsection ANS Forth locals
9388: @cindex locals, ANS Forth style
1.5 anton 9389:
1.78 anton 9390: The ANS Forth locals wordset does not define a syntax for locals, but
9391: words that make it possible to define various syntaxes. One of the
9392: possible syntaxes is a subset of the syntax we used in the Gforth locals
9393: wordset, i.e.:
1.29 crook 9394:
9395: @example
1.78 anton 9396: @{ local1 local2 ... -- comment @}
9397: @end example
9398: @noindent
9399: or
9400: @example
9401: @{ local1 local2 ... @}
1.29 crook 9402: @end example
9403:
1.78 anton 9404: The order of the locals corresponds to the order in a stack comment. The
9405: restrictions are:
1.5 anton 9406:
1.78 anton 9407: @itemize @bullet
9408: @item
9409: Locals can only be cell-sized values (no type specifiers are allowed).
9410: @item
9411: Locals can be defined only outside control structures.
9412: @item
9413: Locals can interfere with explicit usage of the return stack. For the
9414: exact (and long) rules, see the standard. If you don't use return stack
9415: accessing words in a definition using locals, you will be all right. The
9416: purpose of this rule is to make locals implementation on the return
9417: stack easier.
9418: @item
9419: The whole definition must be in one line.
9420: @end itemize
1.5 anton 9421:
1.78 anton 9422: Locals defined in ANS Forth behave like @code{VALUE}s
9423: (@pxref{Values}). I.e., they are initialized from the stack. Using their
9424: name produces their value. Their value can be changed using @code{TO}.
1.77 anton 9425:
1.78 anton 9426: Since the syntax above is supported by Gforth directly, you need not do
9427: anything to use it. If you want to port a program using this syntax to
9428: another ANS Forth system, use @file{compat/anslocal.fs} to implement the
9429: syntax on the other system.
1.5 anton 9430:
1.78 anton 9431: Note that a syntax shown in the standard, section A.13 looks
9432: similar, but is quite different in having the order of locals
9433: reversed. Beware!
1.5 anton 9434:
1.78 anton 9435: The ANS Forth locals wordset itself consists of one word:
1.5 anton 9436:
1.78 anton 9437: doc-(local)
1.5 anton 9438:
1.78 anton 9439: The ANS Forth locals extension wordset defines a syntax using
9440: @code{locals|}, but it is so awful that we strongly recommend not to use
9441: it. We have implemented this syntax to make porting to Gforth easy, but
9442: do not document it here. The problem with this syntax is that the locals
9443: are defined in an order reversed with respect to the standard stack
9444: comment notation, making programs harder to read, and easier to misread
9445: and miswrite. The only merit of this syntax is that it is easy to
9446: implement using the ANS Forth locals wordset.
1.53 anton 9447:
9448:
1.78 anton 9449: @c ----------------------------------------------------------
9450: @node Structures, Object-oriented Forth, Locals, Words
9451: @section Structures
9452: @cindex structures
9453: @cindex records
1.53 anton 9454:
1.78 anton 9455: This section presents the structure package that comes with Gforth. A
9456: version of the package implemented in ANS Forth is available in
9457: @file{compat/struct.fs}. This package was inspired by a posting on
9458: comp.lang.forth in 1989 (unfortunately I don't remember, by whom;
9459: possibly John Hayes). A version of this section has been published in
9460: M. Anton Ertl,
9461: @uref{http://www.complang.tuwien.ac.at/forth/objects/structs.html, Yet
9462: Another Forth Structures Package}, Forth Dimensions 19(3), pages
9463: 13--16. Marcel Hendrix provided helpful comments.
1.53 anton 9464:
1.78 anton 9465: @menu
9466: * Why explicit structure support?::
9467: * Structure Usage::
9468: * Structure Naming Convention::
9469: * Structure Implementation::
9470: * Structure Glossary::
9471: @end menu
1.55 anton 9472:
1.78 anton 9473: @node Why explicit structure support?, Structure Usage, Structures, Structures
9474: @subsection Why explicit structure support?
1.53 anton 9475:
1.78 anton 9476: @cindex address arithmetic for structures
9477: @cindex structures using address arithmetic
9478: If we want to use a structure containing several fields, we could simply
9479: reserve memory for it, and access the fields using address arithmetic
9480: (@pxref{Address arithmetic}). As an example, consider a structure with
9481: the following fields
1.57 anton 9482:
1.78 anton 9483: @table @code
9484: @item a
9485: is a float
9486: @item b
9487: is a cell
9488: @item c
9489: is a float
9490: @end table
1.57 anton 9491:
1.78 anton 9492: Given the (float-aligned) base address of the structure we get the
9493: address of the field
1.52 anton 9494:
1.78 anton 9495: @table @code
9496: @item a
9497: without doing anything further.
9498: @item b
9499: with @code{float+}
9500: @item c
9501: with @code{float+ cell+ faligned}
9502: @end table
1.52 anton 9503:
1.78 anton 9504: It is easy to see that this can become quite tiring.
1.52 anton 9505:
1.78 anton 9506: Moreover, it is not very readable, because seeing a
9507: @code{cell+} tells us neither which kind of structure is
9508: accessed nor what field is accessed; we have to somehow infer the kind
9509: of structure, and then look up in the documentation, which field of
9510: that structure corresponds to that offset.
1.53 anton 9511:
1.78 anton 9512: Finally, this kind of address arithmetic also causes maintenance
9513: troubles: If you add or delete a field somewhere in the middle of the
9514: structure, you have to find and change all computations for the fields
9515: afterwards.
1.52 anton 9516:
1.78 anton 9517: So, instead of using @code{cell+} and friends directly, how
9518: about storing the offsets in constants:
1.52 anton 9519:
1.78 anton 9520: @example
9521: 0 constant a-offset
9522: 0 float+ constant b-offset
9523: 0 float+ cell+ faligned c-offset
9524: @end example
1.64 pazsan 9525:
1.78 anton 9526: Now we can get the address of field @code{x} with @code{x-offset
9527: +}. This is much better in all respects. Of course, you still
9528: have to change all later offset definitions if you add a field. You can
9529: fix this by declaring the offsets in the following way:
1.57 anton 9530:
1.78 anton 9531: @example
9532: 0 constant a-offset
9533: a-offset float+ constant b-offset
9534: b-offset cell+ faligned constant c-offset
9535: @end example
1.57 anton 9536:
1.78 anton 9537: Since we always use the offsets with @code{+}, we could use a defining
9538: word @code{cfield} that includes the @code{+} in the action of the
9539: defined word:
1.64 pazsan 9540:
1.78 anton 9541: @example
9542: : cfield ( n "name" -- )
9543: create ,
9544: does> ( name execution: addr1 -- addr2 )
9545: @@ + ;
1.64 pazsan 9546:
1.78 anton 9547: 0 cfield a
9548: 0 a float+ cfield b
9549: 0 b cell+ faligned cfield c
9550: @end example
1.64 pazsan 9551:
1.78 anton 9552: Instead of @code{x-offset +}, we now simply write @code{x}.
1.64 pazsan 9553:
1.78 anton 9554: The structure field words now can be used quite nicely. However,
9555: their definition is still a bit cumbersome: We have to repeat the
9556: name, the information about size and alignment is distributed before
9557: and after the field definitions etc. The structure package presented
9558: here addresses these problems.
1.64 pazsan 9559:
1.78 anton 9560: @node Structure Usage, Structure Naming Convention, Why explicit structure support?, Structures
9561: @subsection Structure Usage
9562: @cindex structure usage
1.57 anton 9563:
1.78 anton 9564: @cindex @code{field} usage
9565: @cindex @code{struct} usage
9566: @cindex @code{end-struct} usage
9567: You can define a structure for a (data-less) linked list with:
1.57 anton 9568: @example
1.78 anton 9569: struct
9570: cell% field list-next
9571: end-struct list%
1.57 anton 9572: @end example
9573:
1.78 anton 9574: With the address of the list node on the stack, you can compute the
9575: address of the field that contains the address of the next node with
9576: @code{list-next}. E.g., you can determine the length of a list
9577: with:
1.57 anton 9578:
9579: @example
1.78 anton 9580: : list-length ( list -- n )
9581: \ "list" is a pointer to the first element of a linked list
9582: \ "n" is the length of the list
9583: 0 BEGIN ( list1 n1 )
9584: over
9585: WHILE ( list1 n1 )
9586: 1+ swap list-next @@ swap
9587: REPEAT
9588: nip ;
1.57 anton 9589: @end example
9590:
1.78 anton 9591: You can reserve memory for a list node in the dictionary with
9592: @code{list% %allot}, which leaves the address of the list node on the
9593: stack. For the equivalent allocation on the heap you can use @code{list%
9594: %alloc} (or, for an @code{allocate}-like stack effect (i.e., with ior),
9595: use @code{list% %allocate}). You can get the the size of a list
9596: node with @code{list% %size} and its alignment with @code{list%
9597: %alignment}.
9598:
9599: Note that in ANS Forth the body of a @code{create}d word is
9600: @code{aligned} but not necessarily @code{faligned};
9601: therefore, if you do a:
1.57 anton 9602:
9603: @example
1.78 anton 9604: create @emph{name} foo% %allot drop
1.57 anton 9605: @end example
9606:
1.78 anton 9607: @noindent
9608: then the memory alloted for @code{foo%} is guaranteed to start at the
9609: body of @code{@emph{name}} only if @code{foo%} contains only character,
9610: cell and double fields. Therefore, if your structure contains floats,
9611: better use
1.57 anton 9612:
9613: @example
1.78 anton 9614: foo% %allot constant @emph{name}
1.57 anton 9615: @end example
9616:
1.78 anton 9617: @cindex structures containing structures
9618: You can include a structure @code{foo%} as a field of
9619: another structure, like this:
1.65 anton 9620: @example
1.78 anton 9621: struct
9622: ...
9623: foo% field ...
9624: ...
9625: end-struct ...
1.65 anton 9626: @end example
1.52 anton 9627:
1.78 anton 9628: @cindex structure extension
9629: @cindex extended records
9630: Instead of starting with an empty structure, you can extend an
9631: existing structure. E.g., a plain linked list without data, as defined
9632: above, is hardly useful; You can extend it to a linked list of integers,
9633: like this:@footnote{This feature is also known as @emph{extended
9634: records}. It is the main innovation in the Oberon language; in other
9635: words, adding this feature to Modula-2 led Wirth to create a new
9636: language, write a new compiler etc. Adding this feature to Forth just
9637: required a few lines of code.}
1.52 anton 9638:
1.78 anton 9639: @example
9640: list%
9641: cell% field intlist-int
9642: end-struct intlist%
9643: @end example
1.55 anton 9644:
1.78 anton 9645: @code{intlist%} is a structure with two fields:
9646: @code{list-next} and @code{intlist-int}.
1.55 anton 9647:
1.78 anton 9648: @cindex structures containing arrays
9649: You can specify an array type containing @emph{n} elements of
9650: type @code{foo%} like this:
1.55 anton 9651:
9652: @example
1.78 anton 9653: foo% @emph{n} *
1.56 anton 9654: @end example
1.55 anton 9655:
1.78 anton 9656: You can use this array type in any place where you can use a normal
9657: type, e.g., when defining a @code{field}, or with
9658: @code{%allot}.
9659:
9660: @cindex first field optimization
9661: The first field is at the base address of a structure and the word for
9662: this field (e.g., @code{list-next}) actually does not change the address
9663: on the stack. You may be tempted to leave it away in the interest of
9664: run-time and space efficiency. This is not necessary, because the
9665: structure package optimizes this case: If you compile a first-field
9666: words, no code is generated. So, in the interest of readability and
9667: maintainability you should include the word for the field when accessing
9668: the field.
1.52 anton 9669:
9670:
1.78 anton 9671: @node Structure Naming Convention, Structure Implementation, Structure Usage, Structures
9672: @subsection Structure Naming Convention
9673: @cindex structure naming convention
1.52 anton 9674:
1.78 anton 9675: The field names that come to (my) mind are often quite generic, and,
9676: if used, would cause frequent name clashes. E.g., many structures
9677: probably contain a @code{counter} field. The structure names
9678: that come to (my) mind are often also the logical choice for the names
9679: of words that create such a structure.
1.52 anton 9680:
1.78 anton 9681: Therefore, I have adopted the following naming conventions:
1.52 anton 9682:
1.78 anton 9683: @itemize @bullet
9684: @cindex field naming convention
9685: @item
9686: The names of fields are of the form
9687: @code{@emph{struct}-@emph{field}}, where
9688: @code{@emph{struct}} is the basic name of the structure, and
9689: @code{@emph{field}} is the basic name of the field. You can
9690: think of field words as converting the (address of the)
9691: structure into the (address of the) field.
1.52 anton 9692:
1.78 anton 9693: @cindex structure naming convention
9694: @item
9695: The names of structures are of the form
9696: @code{@emph{struct}%}, where
9697: @code{@emph{struct}} is the basic name of the structure.
9698: @end itemize
1.52 anton 9699:
1.78 anton 9700: This naming convention does not work that well for fields of extended
9701: structures; e.g., the integer list structure has a field
9702: @code{intlist-int}, but has @code{list-next}, not
9703: @code{intlist-next}.
1.53 anton 9704:
1.78 anton 9705: @node Structure Implementation, Structure Glossary, Structure Naming Convention, Structures
9706: @subsection Structure Implementation
9707: @cindex structure implementation
9708: @cindex implementation of structures
1.52 anton 9709:
1.78 anton 9710: The central idea in the implementation is to pass the data about the
9711: structure being built on the stack, not in some global
9712: variable. Everything else falls into place naturally once this design
9713: decision is made.
1.53 anton 9714:
1.78 anton 9715: The type description on the stack is of the form @emph{align
9716: size}. Keeping the size on the top-of-stack makes dealing with arrays
9717: very simple.
1.53 anton 9718:
1.78 anton 9719: @code{field} is a defining word that uses @code{Create}
9720: and @code{DOES>}. The body of the field contains the offset
9721: of the field, and the normal @code{DOES>} action is simply:
1.53 anton 9722:
9723: @example
1.78 anton 9724: @@ +
1.53 anton 9725: @end example
9726:
1.78 anton 9727: @noindent
9728: i.e., add the offset to the address, giving the stack effect
9729: @i{addr1 -- addr2} for a field.
9730:
9731: @cindex first field optimization, implementation
9732: This simple structure is slightly complicated by the optimization
9733: for fields with offset 0, which requires a different
9734: @code{DOES>}-part (because we cannot rely on there being
9735: something on the stack if such a field is invoked during
9736: compilation). Therefore, we put the different @code{DOES>}-parts
9737: in separate words, and decide which one to invoke based on the
9738: offset. For a zero offset, the field is basically a noop; it is
9739: immediate, and therefore no code is generated when it is compiled.
1.53 anton 9740:
1.78 anton 9741: @node Structure Glossary, , Structure Implementation, Structures
9742: @subsection Structure Glossary
9743: @cindex structure glossary
1.53 anton 9744:
1.5 anton 9745:
1.78 anton 9746: doc-%align
9747: doc-%alignment
9748: doc-%alloc
9749: doc-%allocate
9750: doc-%allot
9751: doc-cell%
9752: doc-char%
9753: doc-dfloat%
9754: doc-double%
9755: doc-end-struct
9756: doc-field
9757: doc-float%
9758: doc-naligned
9759: doc-sfloat%
9760: doc-%size
9761: doc-struct
1.54 anton 9762:
9763:
1.26 crook 9764: @c -------------------------------------------------------------
1.78 anton 9765: @node Object-oriented Forth, Programming Tools, Structures, Words
9766: @section Object-oriented Forth
9767:
9768: Gforth comes with three packages for object-oriented programming:
9769: @file{objects.fs}, @file{oof.fs}, and @file{mini-oof.fs}; none of them
9770: is preloaded, so you have to @code{include} them before use. The most
9771: important differences between these packages (and others) are discussed
9772: in @ref{Comparison with other object models}. All packages are written
9773: in ANS Forth and can be used with any other ANS Forth.
1.5 anton 9774:
1.78 anton 9775: @menu
9776: * Why object-oriented programming?::
9777: * Object-Oriented Terminology::
9778: * Objects::
9779: * OOF::
9780: * Mini-OOF::
9781: * Comparison with other object models::
9782: @end menu
1.5 anton 9783:
1.78 anton 9784: @c ----------------------------------------------------------------
9785: @node Why object-oriented programming?, Object-Oriented Terminology, Object-oriented Forth, Object-oriented Forth
9786: @subsection Why object-oriented programming?
9787: @cindex object-oriented programming motivation
9788: @cindex motivation for object-oriented programming
1.44 crook 9789:
1.78 anton 9790: Often we have to deal with several data structures (@emph{objects}),
9791: that have to be treated similarly in some respects, but differently in
9792: others. Graphical objects are the textbook example: circles, triangles,
9793: dinosaurs, icons, and others, and we may want to add more during program
9794: development. We want to apply some operations to any graphical object,
9795: e.g., @code{draw} for displaying it on the screen. However, @code{draw}
9796: has to do something different for every kind of object.
9797: @comment TODO add some other operations eg perimeter, area
9798: @comment and tie in to concrete examples later..
1.5 anton 9799:
1.78 anton 9800: We could implement @code{draw} as a big @code{CASE}
9801: control structure that executes the appropriate code depending on the
9802: kind of object to be drawn. This would be not be very elegant, and,
9803: moreover, we would have to change @code{draw} every time we add
9804: a new kind of graphical object (say, a spaceship).
1.44 crook 9805:
1.78 anton 9806: What we would rather do is: When defining spaceships, we would tell
9807: the system: ``Here's how you @code{draw} a spaceship; you figure
9808: out the rest''.
1.5 anton 9809:
1.78 anton 9810: This is the problem that all systems solve that (rightfully) call
9811: themselves object-oriented; the object-oriented packages presented here
9812: solve this problem (and not much else).
9813: @comment TODO ?list properties of oo systems.. oo vs o-based?
1.44 crook 9814:
1.78 anton 9815: @c ------------------------------------------------------------------------
9816: @node Object-Oriented Terminology, Objects, Why object-oriented programming?, Object-oriented Forth
9817: @subsection Object-Oriented Terminology
9818: @cindex object-oriented terminology
9819: @cindex terminology for object-oriented programming
1.5 anton 9820:
1.78 anton 9821: This section is mainly for reference, so you don't have to understand
9822: all of it right away. The terminology is mainly Smalltalk-inspired. In
9823: short:
1.44 crook 9824:
1.78 anton 9825: @table @emph
9826: @cindex class
9827: @item class
9828: a data structure definition with some extras.
1.5 anton 9829:
1.78 anton 9830: @cindex object
9831: @item object
9832: an instance of the data structure described by the class definition.
1.5 anton 9833:
1.78 anton 9834: @cindex instance variables
9835: @item instance variables
9836: fields of the data structure.
1.5 anton 9837:
1.78 anton 9838: @cindex selector
9839: @cindex method selector
9840: @cindex virtual function
9841: @item selector
9842: (or @emph{method selector}) a word (e.g.,
9843: @code{draw}) that performs an operation on a variety of data
9844: structures (classes). A selector describes @emph{what} operation to
9845: perform. In C++ terminology: a (pure) virtual function.
1.5 anton 9846:
1.78 anton 9847: @cindex method
9848: @item method
9849: the concrete definition that performs the operation
9850: described by the selector for a specific class. A method specifies
9851: @emph{how} the operation is performed for a specific class.
1.5 anton 9852:
1.78 anton 9853: @cindex selector invocation
9854: @cindex message send
9855: @cindex invoking a selector
9856: @item selector invocation
9857: a call of a selector. One argument of the call (the TOS (top-of-stack))
9858: is used for determining which method is used. In Smalltalk terminology:
9859: a message (consisting of the selector and the other arguments) is sent
9860: to the object.
1.5 anton 9861:
1.78 anton 9862: @cindex receiving object
9863: @item receiving object
9864: the object used for determining the method executed by a selector
9865: invocation. In the @file{objects.fs} model, it is the object that is on
9866: the TOS when the selector is invoked. (@emph{Receiving} comes from
9867: the Smalltalk @emph{message} terminology.)
1.5 anton 9868:
1.78 anton 9869: @cindex child class
9870: @cindex parent class
9871: @cindex inheritance
9872: @item child class
9873: a class that has (@emph{inherits}) all properties (instance variables,
9874: selectors, methods) from a @emph{parent class}. In Smalltalk
9875: terminology: The subclass inherits from the superclass. In C++
9876: terminology: The derived class inherits from the base class.
1.5 anton 9877:
1.78 anton 9878: @end table
1.5 anton 9879:
1.78 anton 9880: @c If you wonder about the message sending terminology, it comes from
9881: @c a time when each object had it's own task and objects communicated via
9882: @c message passing; eventually the Smalltalk developers realized that
9883: @c they can do most things through simple (indirect) calls. They kept the
9884: @c terminology.
1.5 anton 9885:
1.78 anton 9886: @c --------------------------------------------------------------
9887: @node Objects, OOF, Object-Oriented Terminology, Object-oriented Forth
9888: @subsection The @file{objects.fs} model
9889: @cindex objects
9890: @cindex object-oriented programming
1.26 crook 9891:
1.78 anton 9892: @cindex @file{objects.fs}
9893: @cindex @file{oof.fs}
1.26 crook 9894:
1.78 anton 9895: This section describes the @file{objects.fs} package. This material also
9896: has been published in M. Anton Ertl,
9897: @cite{@uref{http://www.complang.tuwien.ac.at/forth/objects/objects.html,
9898: Yet Another Forth Objects Package}}, Forth Dimensions 19(2), pages
9899: 37--43.
9900: @c McKewan's and Zsoter's packages
1.26 crook 9901:
1.78 anton 9902: This section assumes that you have read @ref{Structures}.
1.5 anton 9903:
1.78 anton 9904: The techniques on which this model is based have been used to implement
9905: the parser generator, Gray, and have also been used in Gforth for
9906: implementing the various flavours of word lists (hashed or not,
9907: case-sensitive or not, special-purpose word lists for locals etc.).
1.5 anton 9908:
9909:
1.26 crook 9910: @menu
1.78 anton 9911: * Properties of the Objects model::
9912: * Basic Objects Usage::
9913: * The Objects base class::
9914: * Creating objects::
9915: * Object-Oriented Programming Style::
9916: * Class Binding::
9917: * Method conveniences::
9918: * Classes and Scoping::
9919: * Dividing classes::
9920: * Object Interfaces::
9921: * Objects Implementation::
9922: * Objects Glossary::
1.26 crook 9923: @end menu
1.5 anton 9924:
1.78 anton 9925: Marcel Hendrix provided helpful comments on this section.
1.5 anton 9926:
1.78 anton 9927: @node Properties of the Objects model, Basic Objects Usage, Objects, Objects
9928: @subsubsection Properties of the @file{objects.fs} model
9929: @cindex @file{objects.fs} properties
1.5 anton 9930:
1.78 anton 9931: @itemize @bullet
9932: @item
9933: It is straightforward to pass objects on the stack. Passing
9934: selectors on the stack is a little less convenient, but possible.
1.44 crook 9935:
1.78 anton 9936: @item
9937: Objects are just data structures in memory, and are referenced by their
9938: address. You can create words for objects with normal defining words
9939: like @code{constant}. Likewise, there is no difference between instance
9940: variables that contain objects and those that contain other data.
1.5 anton 9941:
1.78 anton 9942: @item
9943: Late binding is efficient and easy to use.
1.44 crook 9944:
1.78 anton 9945: @item
9946: It avoids parsing, and thus avoids problems with state-smartness
9947: and reduced extensibility; for convenience there are a few parsing
9948: words, but they have non-parsing counterparts. There are also a few
9949: defining words that parse. This is hard to avoid, because all standard
9950: defining words parse (except @code{:noname}); however, such
9951: words are not as bad as many other parsing words, because they are not
9952: state-smart.
1.5 anton 9953:
1.78 anton 9954: @item
9955: It does not try to incorporate everything. It does a few things and does
9956: them well (IMO). In particular, this model was not designed to support
9957: information hiding (although it has features that may help); you can use
9958: a separate package for achieving this.
1.5 anton 9959:
1.78 anton 9960: @item
9961: It is layered; you don't have to learn and use all features to use this
9962: model. Only a few features are necessary (@pxref{Basic Objects Usage},
9963: @pxref{The Objects base class}, @pxref{Creating objects}.), the others
9964: are optional and independent of each other.
1.5 anton 9965:
1.78 anton 9966: @item
9967: An implementation in ANS Forth is available.
1.5 anton 9968:
1.78 anton 9969: @end itemize
1.5 anton 9970:
1.44 crook 9971:
1.78 anton 9972: @node Basic Objects Usage, The Objects base class, Properties of the Objects model, Objects
9973: @subsubsection Basic @file{objects.fs} Usage
9974: @cindex basic objects usage
9975: @cindex objects, basic usage
1.5 anton 9976:
1.78 anton 9977: You can define a class for graphical objects like this:
1.44 crook 9978:
1.78 anton 9979: @cindex @code{class} usage
9980: @cindex @code{end-class} usage
9981: @cindex @code{selector} usage
1.5 anton 9982: @example
1.78 anton 9983: object class \ "object" is the parent class
9984: selector draw ( x y graphical -- )
9985: end-class graphical
9986: @end example
9987:
9988: This code defines a class @code{graphical} with an
9989: operation @code{draw}. We can perform the operation
9990: @code{draw} on any @code{graphical} object, e.g.:
9991:
9992: @example
9993: 100 100 t-rex draw
1.26 crook 9994: @end example
1.5 anton 9995:
1.78 anton 9996: @noindent
9997: where @code{t-rex} is a word (say, a constant) that produces a
9998: graphical object.
9999:
10000: @comment TODO add a 2nd operation eg perimeter.. and use for
10001: @comment a concrete example
1.5 anton 10002:
1.78 anton 10003: @cindex abstract class
10004: How do we create a graphical object? With the present definitions,
10005: we cannot create a useful graphical object. The class
10006: @code{graphical} describes graphical objects in general, but not
10007: any concrete graphical object type (C++ users would call it an
10008: @emph{abstract class}); e.g., there is no method for the selector
10009: @code{draw} in the class @code{graphical}.
1.5 anton 10010:
1.78 anton 10011: For concrete graphical objects, we define child classes of the
10012: class @code{graphical}, e.g.:
1.5 anton 10013:
1.78 anton 10014: @cindex @code{overrides} usage
10015: @cindex @code{field} usage in class definition
1.26 crook 10016: @example
1.78 anton 10017: graphical class \ "graphical" is the parent class
10018: cell% field circle-radius
1.5 anton 10019:
1.78 anton 10020: :noname ( x y circle -- )
10021: circle-radius @@ draw-circle ;
10022: overrides draw
1.5 anton 10023:
1.78 anton 10024: :noname ( n-radius circle -- )
10025: circle-radius ! ;
10026: overrides construct
1.5 anton 10027:
1.78 anton 10028: end-class circle
10029: @end example
1.44 crook 10030:
1.78 anton 10031: Here we define a class @code{circle} as a child of @code{graphical},
10032: with field @code{circle-radius} (which behaves just like a field
10033: (@pxref{Structures}); it defines (using @code{overrides}) new methods
10034: for the selectors @code{draw} and @code{construct} (@code{construct} is
10035: defined in @code{object}, the parent class of @code{graphical}).
1.5 anton 10036:
1.78 anton 10037: Now we can create a circle on the heap (i.e.,
10038: @code{allocate}d memory) with:
1.44 crook 10039:
1.78 anton 10040: @cindex @code{heap-new} usage
1.5 anton 10041: @example
1.78 anton 10042: 50 circle heap-new constant my-circle
1.5 anton 10043: @end example
10044:
1.78 anton 10045: @noindent
10046: @code{heap-new} invokes @code{construct}, thus
10047: initializing the field @code{circle-radius} with 50. We can draw
10048: this new circle at (100,100) with:
1.5 anton 10049:
10050: @example
1.78 anton 10051: 100 100 my-circle draw
1.5 anton 10052: @end example
10053:
1.78 anton 10054: @cindex selector invocation, restrictions
10055: @cindex class definition, restrictions
10056: Note: You can only invoke a selector if the object on the TOS
10057: (the receiving object) belongs to the class where the selector was
10058: defined or one of its descendents; e.g., you can invoke
10059: @code{draw} only for objects belonging to @code{graphical}
10060: or its descendents (e.g., @code{circle}). Immediately before
10061: @code{end-class}, the search order has to be the same as
10062: immediately after @code{class}.
10063:
10064: @node The Objects base class, Creating objects, Basic Objects Usage, Objects
10065: @subsubsection The @file{object.fs} base class
10066: @cindex @code{object} class
10067:
10068: When you define a class, you have to specify a parent class. So how do
10069: you start defining classes? There is one class available from the start:
10070: @code{object}. It is ancestor for all classes and so is the
10071: only class that has no parent. It has two selectors: @code{construct}
10072: and @code{print}.
10073:
10074: @node Creating objects, Object-Oriented Programming Style, The Objects base class, Objects
10075: @subsubsection Creating objects
10076: @cindex creating objects
10077: @cindex object creation
10078: @cindex object allocation options
10079:
10080: @cindex @code{heap-new} discussion
10081: @cindex @code{dict-new} discussion
10082: @cindex @code{construct} discussion
10083: You can create and initialize an object of a class on the heap with
10084: @code{heap-new} ( ... class -- object ) and in the dictionary
10085: (allocation with @code{allot}) with @code{dict-new} (
10086: ... class -- object ). Both words invoke @code{construct}, which
10087: consumes the stack items indicated by "..." above.
10088:
10089: @cindex @code{init-object} discussion
10090: @cindex @code{class-inst-size} discussion
10091: If you want to allocate memory for an object yourself, you can get its
10092: alignment and size with @code{class-inst-size 2@@} ( class --
10093: align size ). Once you have memory for an object, you can initialize
10094: it with @code{init-object} ( ... class object -- );
10095: @code{construct} does only a part of the necessary work.
10096:
10097: @node Object-Oriented Programming Style, Class Binding, Creating objects, Objects
10098: @subsubsection Object-Oriented Programming Style
10099: @cindex object-oriented programming style
10100: @cindex programming style, object-oriented
1.5 anton 10101:
1.78 anton 10102: This section is not exhaustive.
1.5 anton 10103:
1.78 anton 10104: @cindex stack effects of selectors
10105: @cindex selectors and stack effects
10106: In general, it is a good idea to ensure that all methods for the
10107: same selector have the same stack effect: when you invoke a selector,
10108: you often have no idea which method will be invoked, so, unless all
10109: methods have the same stack effect, you will not know the stack effect
10110: of the selector invocation.
1.5 anton 10111:
1.78 anton 10112: One exception to this rule is methods for the selector
10113: @code{construct}. We know which method is invoked, because we
10114: specify the class to be constructed at the same place. Actually, I
10115: defined @code{construct} as a selector only to give the users a
10116: convenient way to specify initialization. The way it is used, a
10117: mechanism different from selector invocation would be more natural
10118: (but probably would take more code and more space to explain).
1.5 anton 10119:
1.78 anton 10120: @node Class Binding, Method conveniences, Object-Oriented Programming Style, Objects
10121: @subsubsection Class Binding
10122: @cindex class binding
10123: @cindex early binding
1.5 anton 10124:
1.78 anton 10125: @cindex late binding
10126: Normal selector invocations determine the method at run-time depending
10127: on the class of the receiving object. This run-time selection is called
10128: @i{late binding}.
1.5 anton 10129:
1.78 anton 10130: Sometimes it's preferable to invoke a different method. For example,
10131: you might want to use the simple method for @code{print}ing
10132: @code{object}s instead of the possibly long-winded @code{print} method
10133: of the receiver class. You can achieve this by replacing the invocation
10134: of @code{print} with:
1.5 anton 10135:
1.78 anton 10136: @cindex @code{[bind]} usage
1.5 anton 10137: @example
1.78 anton 10138: [bind] object print
1.5 anton 10139: @end example
10140:
1.78 anton 10141: @noindent
10142: in compiled code or:
10143:
10144: @cindex @code{bind} usage
1.5 anton 10145: @example
1.78 anton 10146: bind object print
1.5 anton 10147: @end example
10148:
1.78 anton 10149: @cindex class binding, alternative to
10150: @noindent
10151: in interpreted code. Alternatively, you can define the method with a
10152: name (e.g., @code{print-object}), and then invoke it through the
10153: name. Class binding is just a (often more convenient) way to achieve
10154: the same effect; it avoids name clutter and allows you to invoke
10155: methods directly without naming them first.
1.5 anton 10156:
1.78 anton 10157: @cindex superclass binding
10158: @cindex parent class binding
10159: A frequent use of class binding is this: When we define a method
10160: for a selector, we often want the method to do what the selector does
10161: in the parent class, and a little more. There is a special word for
10162: this purpose: @code{[parent]}; @code{[parent]
10163: @emph{selector}} is equivalent to @code{[bind] @emph{parent
10164: selector}}, where @code{@emph{parent}} is the parent
10165: class of the current class. E.g., a method definition might look like:
1.44 crook 10166:
1.78 anton 10167: @cindex @code{[parent]} usage
10168: @example
10169: :noname
10170: dup [parent] foo \ do parent's foo on the receiving object
10171: ... \ do some more
10172: ; overrides foo
10173: @end example
1.6 pazsan 10174:
1.78 anton 10175: @cindex class binding as optimization
10176: In @cite{Object-oriented programming in ANS Forth} (Forth Dimensions,
10177: March 1997), Andrew McKewan presents class binding as an optimization
10178: technique. I recommend not using it for this purpose unless you are in
10179: an emergency. Late binding is pretty fast with this model anyway, so the
10180: benefit of using class binding is small; the cost of using class binding
10181: where it is not appropriate is reduced maintainability.
1.44 crook 10182:
1.78 anton 10183: While we are at programming style questions: You should bind
10184: selectors only to ancestor classes of the receiving object. E.g., say,
10185: you know that the receiving object is of class @code{foo} or its
10186: descendents; then you should bind only to @code{foo} and its
10187: ancestors.
1.12 anton 10188:
1.78 anton 10189: @node Method conveniences, Classes and Scoping, Class Binding, Objects
10190: @subsubsection Method conveniences
10191: @cindex method conveniences
1.44 crook 10192:
1.78 anton 10193: In a method you usually access the receiving object pretty often. If
10194: you define the method as a plain colon definition (e.g., with
10195: @code{:noname}), you may have to do a lot of stack
10196: gymnastics. To avoid this, you can define the method with @code{m:
10197: ... ;m}. E.g., you could define the method for
10198: @code{draw}ing a @code{circle} with
1.6 pazsan 10199:
1.78 anton 10200: @cindex @code{this} usage
10201: @cindex @code{m:} usage
10202: @cindex @code{;m} usage
10203: @example
10204: m: ( x y circle -- )
10205: ( x y ) this circle-radius @@ draw-circle ;m
10206: @end example
1.6 pazsan 10207:
1.78 anton 10208: @cindex @code{exit} in @code{m: ... ;m}
10209: @cindex @code{exitm} discussion
10210: @cindex @code{catch} in @code{m: ... ;m}
10211: When this method is executed, the receiver object is removed from the
10212: stack; you can access it with @code{this} (admittedly, in this
10213: example the use of @code{m: ... ;m} offers no advantage). Note
10214: that I specify the stack effect for the whole method (i.e. including
10215: the receiver object), not just for the code between @code{m:}
10216: and @code{;m}. You cannot use @code{exit} in
10217: @code{m:...;m}; instead, use
10218: @code{exitm}.@footnote{Moreover, for any word that calls
10219: @code{catch} and was defined before loading
10220: @code{objects.fs}, you have to redefine it like I redefined
10221: @code{catch}: @code{: catch this >r catch r> to-this ;}}
1.12 anton 10222:
1.78 anton 10223: @cindex @code{inst-var} usage
10224: You will frequently use sequences of the form @code{this
10225: @emph{field}} (in the example above: @code{this
10226: circle-radius}). If you use the field only in this way, you can
10227: define it with @code{inst-var} and eliminate the
10228: @code{this} before the field name. E.g., the @code{circle}
10229: class above could also be defined with:
1.6 pazsan 10230:
1.78 anton 10231: @example
10232: graphical class
10233: cell% inst-var radius
1.6 pazsan 10234:
1.78 anton 10235: m: ( x y circle -- )
10236: radius @@ draw-circle ;m
10237: overrides draw
1.6 pazsan 10238:
1.78 anton 10239: m: ( n-radius circle -- )
10240: radius ! ;m
10241: overrides construct
1.6 pazsan 10242:
1.78 anton 10243: end-class circle
10244: @end example
1.6 pazsan 10245:
1.78 anton 10246: @code{radius} can only be used in @code{circle} and its
10247: descendent classes and inside @code{m:...;m}.
1.6 pazsan 10248:
1.78 anton 10249: @cindex @code{inst-value} usage
10250: You can also define fields with @code{inst-value}, which is
10251: to @code{inst-var} what @code{value} is to
10252: @code{variable}. You can change the value of such a field with
10253: @code{[to-inst]}. E.g., we could also define the class
10254: @code{circle} like this:
1.44 crook 10255:
1.78 anton 10256: @example
10257: graphical class
10258: inst-value radius
1.6 pazsan 10259:
1.78 anton 10260: m: ( x y circle -- )
10261: radius draw-circle ;m
10262: overrides draw
1.44 crook 10263:
1.78 anton 10264: m: ( n-radius circle -- )
10265: [to-inst] radius ;m
10266: overrides construct
1.6 pazsan 10267:
1.78 anton 10268: end-class circle
10269: @end example
1.6 pazsan 10270:
1.78 anton 10271: @c !! :m is easy to confuse with m:. Another name would be better.
1.6 pazsan 10272:
1.78 anton 10273: @c Finally, you can define named methods with @code{:m}. One use of this
10274: @c feature is the definition of words that occur only in one class and are
10275: @c not intended to be overridden, but which still need method context
10276: @c (e.g., for accessing @code{inst-var}s). Another use is for methods that
10277: @c would be bound frequently, if defined anonymously.
1.6 pazsan 10278:
10279:
1.78 anton 10280: @node Classes and Scoping, Dividing classes, Method conveniences, Objects
10281: @subsubsection Classes and Scoping
10282: @cindex classes and scoping
10283: @cindex scoping and classes
1.6 pazsan 10284:
1.78 anton 10285: Inheritance is frequent, unlike structure extension. This exacerbates
10286: the problem with the field name convention (@pxref{Structure Naming
10287: Convention}): One always has to remember in which class the field was
10288: originally defined; changing a part of the class structure would require
10289: changes for renaming in otherwise unaffected code.
1.6 pazsan 10290:
1.78 anton 10291: @cindex @code{inst-var} visibility
10292: @cindex @code{inst-value} visibility
10293: To solve this problem, I added a scoping mechanism (which was not in my
10294: original charter): A field defined with @code{inst-var} (or
10295: @code{inst-value}) is visible only in the class where it is defined and in
10296: the descendent classes of this class. Using such fields only makes
10297: sense in @code{m:}-defined methods in these classes anyway.
1.6 pazsan 10298:
1.78 anton 10299: This scoping mechanism allows us to use the unadorned field name,
10300: because name clashes with unrelated words become much less likely.
1.6 pazsan 10301:
1.78 anton 10302: @cindex @code{protected} discussion
10303: @cindex @code{private} discussion
10304: Once we have this mechanism, we can also use it for controlling the
10305: visibility of other words: All words defined after
10306: @code{protected} are visible only in the current class and its
10307: descendents. @code{public} restores the compilation
10308: (i.e. @code{current}) word list that was in effect before. If you
10309: have several @code{protected}s without an intervening
10310: @code{public} or @code{set-current}, @code{public}
10311: will restore the compilation word list in effect before the first of
10312: these @code{protected}s.
1.6 pazsan 10313:
1.78 anton 10314: @node Dividing classes, Object Interfaces, Classes and Scoping, Objects
10315: @subsubsection Dividing classes
10316: @cindex Dividing classes
10317: @cindex @code{methods}...@code{end-methods}
1.6 pazsan 10318:
1.78 anton 10319: You may want to do the definition of methods separate from the
10320: definition of the class, its selectors, fields, and instance variables,
10321: i.e., separate the implementation from the definition. You can do this
10322: in the following way:
1.6 pazsan 10323:
1.78 anton 10324: @example
10325: graphical class
10326: inst-value radius
10327: end-class circle
1.6 pazsan 10328:
1.78 anton 10329: ... \ do some other stuff
1.6 pazsan 10330:
1.78 anton 10331: circle methods \ now we are ready
1.44 crook 10332:
1.78 anton 10333: m: ( x y circle -- )
10334: radius draw-circle ;m
10335: overrides draw
1.6 pazsan 10336:
1.78 anton 10337: m: ( n-radius circle -- )
10338: [to-inst] radius ;m
10339: overrides construct
1.44 crook 10340:
1.78 anton 10341: end-methods
10342: @end example
1.7 pazsan 10343:
1.78 anton 10344: You can use several @code{methods}...@code{end-methods} sections. The
10345: only things you can do to the class in these sections are: defining
10346: methods, and overriding the class's selectors. You must not define new
10347: selectors or fields.
1.7 pazsan 10348:
1.78 anton 10349: Note that you often have to override a selector before using it. In
10350: particular, you usually have to override @code{construct} with a new
10351: method before you can invoke @code{heap-new} and friends. E.g., you
10352: must not create a circle before the @code{overrides construct} sequence
10353: in the example above.
1.7 pazsan 10354:
1.78 anton 10355: @node Object Interfaces, Objects Implementation, Dividing classes, Objects
10356: @subsubsection Object Interfaces
10357: @cindex object interfaces
10358: @cindex interfaces for objects
1.7 pazsan 10359:
1.78 anton 10360: In this model you can only call selectors defined in the class of the
10361: receiving objects or in one of its ancestors. If you call a selector
10362: with a receiving object that is not in one of these classes, the
10363: result is undefined; if you are lucky, the program crashes
10364: immediately.
1.7 pazsan 10365:
1.78 anton 10366: @cindex selectors common to hardly-related classes
10367: Now consider the case when you want to have a selector (or several)
10368: available in two classes: You would have to add the selector to a
10369: common ancestor class, in the worst case to @code{object}. You
10370: may not want to do this, e.g., because someone else is responsible for
10371: this ancestor class.
1.7 pazsan 10372:
1.78 anton 10373: The solution for this problem is interfaces. An interface is a
10374: collection of selectors. If a class implements an interface, the
10375: selectors become available to the class and its descendents. A class
10376: can implement an unlimited number of interfaces. For the problem
10377: discussed above, we would define an interface for the selector(s), and
10378: both classes would implement the interface.
1.7 pazsan 10379:
1.78 anton 10380: As an example, consider an interface @code{storage} for
10381: writing objects to disk and getting them back, and a class
10382: @code{foo} that implements it. The code would look like this:
1.7 pazsan 10383:
1.78 anton 10384: @cindex @code{interface} usage
10385: @cindex @code{end-interface} usage
10386: @cindex @code{implementation} usage
10387: @example
10388: interface
10389: selector write ( file object -- )
10390: selector read1 ( file object -- )
10391: end-interface storage
1.13 pazsan 10392:
1.78 anton 10393: bar class
10394: storage implementation
1.13 pazsan 10395:
1.78 anton 10396: ... overrides write
10397: ... overrides read1
10398: ...
10399: end-class foo
10400: @end example
1.13 pazsan 10401:
1.78 anton 10402: @noindent
10403: (I would add a word @code{read} @i{( file -- object )} that uses
10404: @code{read1} internally, but that's beyond the point illustrated
10405: here.)
1.13 pazsan 10406:
1.78 anton 10407: Note that you cannot use @code{protected} in an interface; and
10408: of course you cannot define fields.
1.13 pazsan 10409:
1.78 anton 10410: In the Neon model, all selectors are available for all classes;
10411: therefore it does not need interfaces. The price you pay in this model
10412: is slower late binding, and therefore, added complexity to avoid late
10413: binding.
1.13 pazsan 10414:
1.78 anton 10415: @node Objects Implementation, Objects Glossary, Object Interfaces, Objects
10416: @subsubsection @file{objects.fs} Implementation
10417: @cindex @file{objects.fs} implementation
1.13 pazsan 10418:
1.78 anton 10419: @cindex @code{object-map} discussion
10420: An object is a piece of memory, like one of the data structures
10421: described with @code{struct...end-struct}. It has a field
10422: @code{object-map} that points to the method map for the object's
10423: class.
1.13 pazsan 10424:
1.78 anton 10425: @cindex method map
10426: @cindex virtual function table
10427: The @emph{method map}@footnote{This is Self terminology; in C++
10428: terminology: virtual function table.} is an array that contains the
10429: execution tokens (@i{xt}s) of the methods for the object's class. Each
10430: selector contains an offset into a method map.
1.13 pazsan 10431:
1.78 anton 10432: @cindex @code{selector} implementation, class
10433: @code{selector} is a defining word that uses
10434: @code{CREATE} and @code{DOES>}. The body of the
10435: selector contains the offset; the @code{DOES>} action for a
10436: class selector is, basically:
1.8 pazsan 10437:
10438: @example
1.78 anton 10439: ( object addr ) @@ over object-map @@ + @@ execute
1.13 pazsan 10440: @end example
10441:
1.78 anton 10442: Since @code{object-map} is the first field of the object, it
10443: does not generate any code. As you can see, calling a selector has a
10444: small, constant cost.
1.26 crook 10445:
1.78 anton 10446: @cindex @code{current-interface} discussion
10447: @cindex class implementation and representation
10448: A class is basically a @code{struct} combined with a method
10449: map. During the class definition the alignment and size of the class
10450: are passed on the stack, just as with @code{struct}s, so
10451: @code{field} can also be used for defining class
10452: fields. However, passing more items on the stack would be
10453: inconvenient, so @code{class} builds a data structure in memory,
10454: which is accessed through the variable
10455: @code{current-interface}. After its definition is complete, the
10456: class is represented on the stack by a pointer (e.g., as parameter for
10457: a child class definition).
1.26 crook 10458:
1.78 anton 10459: A new class starts off with the alignment and size of its parent,
10460: and a copy of the parent's method map. Defining new fields extends the
10461: size and alignment; likewise, defining new selectors extends the
10462: method map. @code{overrides} just stores a new @i{xt} in the method
10463: map at the offset given by the selector.
1.13 pazsan 10464:
1.78 anton 10465: @cindex class binding, implementation
10466: Class binding just gets the @i{xt} at the offset given by the selector
10467: from the class's method map and @code{compile,}s (in the case of
10468: @code{[bind]}) it.
1.13 pazsan 10469:
1.78 anton 10470: @cindex @code{this} implementation
10471: @cindex @code{catch} and @code{this}
10472: @cindex @code{this} and @code{catch}
10473: I implemented @code{this} as a @code{value}. At the
10474: start of an @code{m:...;m} method the old @code{this} is
10475: stored to the return stack and restored at the end; and the object on
10476: the TOS is stored @code{TO this}. This technique has one
10477: disadvantage: If the user does not leave the method via
10478: @code{;m}, but via @code{throw} or @code{exit},
10479: @code{this} is not restored (and @code{exit} may
10480: crash). To deal with the @code{throw} problem, I have redefined
10481: @code{catch} to save and restore @code{this}; the same
10482: should be done with any word that can catch an exception. As for
10483: @code{exit}, I simply forbid it (as a replacement, there is
10484: @code{exitm}).
1.13 pazsan 10485:
1.78 anton 10486: @cindex @code{inst-var} implementation
10487: @code{inst-var} is just the same as @code{field}, with
10488: a different @code{DOES>} action:
1.13 pazsan 10489: @example
1.78 anton 10490: @@ this +
1.8 pazsan 10491: @end example
1.78 anton 10492: Similar for @code{inst-value}.
1.8 pazsan 10493:
1.78 anton 10494: @cindex class scoping implementation
10495: Each class also has a word list that contains the words defined with
10496: @code{inst-var} and @code{inst-value}, and its protected
10497: words. It also has a pointer to its parent. @code{class} pushes
10498: the word lists of the class and all its ancestors onto the search order stack,
10499: and @code{end-class} drops them.
1.20 pazsan 10500:
1.78 anton 10501: @cindex interface implementation
10502: An interface is like a class without fields, parent and protected
10503: words; i.e., it just has a method map. If a class implements an
10504: interface, its method map contains a pointer to the method map of the
10505: interface. The positive offsets in the map are reserved for class
10506: methods, therefore interface map pointers have negative
10507: offsets. Interfaces have offsets that are unique throughout the
10508: system, unlike class selectors, whose offsets are only unique for the
10509: classes where the selector is available (invokable).
1.20 pazsan 10510:
1.78 anton 10511: This structure means that interface selectors have to perform one
10512: indirection more than class selectors to find their method. Their body
10513: contains the interface map pointer offset in the class method map, and
10514: the method offset in the interface method map. The
10515: @code{does>} action for an interface selector is, basically:
1.20 pazsan 10516:
10517: @example
1.78 anton 10518: ( object selector-body )
10519: 2dup selector-interface @@ ( object selector-body object interface-offset )
10520: swap object-map @@ + @@ ( object selector-body map )
10521: swap selector-offset @@ + @@ execute
1.20 pazsan 10522: @end example
10523:
1.78 anton 10524: where @code{object-map} and @code{selector-offset} are
10525: first fields and generate no code.
1.20 pazsan 10526:
1.78 anton 10527: As a concrete example, consider the following code:
1.20 pazsan 10528:
10529: @example
1.78 anton 10530: interface
10531: selector if1sel1
10532: selector if1sel2
10533: end-interface if1
1.20 pazsan 10534:
1.78 anton 10535: object class
10536: if1 implementation
10537: selector cl1sel1
10538: cell% inst-var cl1iv1
1.20 pazsan 10539:
1.78 anton 10540: ' m1 overrides construct
10541: ' m2 overrides if1sel1
10542: ' m3 overrides if1sel2
10543: ' m4 overrides cl1sel2
10544: end-class cl1
1.20 pazsan 10545:
1.78 anton 10546: create obj1 object dict-new drop
10547: create obj2 cl1 dict-new drop
10548: @end example
1.20 pazsan 10549:
1.78 anton 10550: The data structure created by this code (including the data structure
10551: for @code{object}) is shown in the
10552: @uref{objects-implementation.eps,figure}, assuming a cell size of 4.
10553: @comment TODO add this diagram..
1.20 pazsan 10554:
1.78 anton 10555: @node Objects Glossary, , Objects Implementation, Objects
10556: @subsubsection @file{objects.fs} Glossary
10557: @cindex @file{objects.fs} Glossary
1.20 pazsan 10558:
10559:
1.78 anton 10560: doc---objects-bind
10561: doc---objects-<bind>
10562: doc---objects-bind'
10563: doc---objects-[bind]
10564: doc---objects-class
10565: doc---objects-class->map
10566: doc---objects-class-inst-size
10567: doc---objects-class-override!
1.79 anton 10568: doc---objects-class-previous
10569: doc---objects-class>order
1.78 anton 10570: doc---objects-construct
10571: doc---objects-current'
10572: doc---objects-[current]
10573: doc---objects-current-interface
10574: doc---objects-dict-new
10575: doc---objects-end-class
10576: doc---objects-end-class-noname
10577: doc---objects-end-interface
10578: doc---objects-end-interface-noname
10579: doc---objects-end-methods
10580: doc---objects-exitm
10581: doc---objects-heap-new
10582: doc---objects-implementation
10583: doc---objects-init-object
10584: doc---objects-inst-value
10585: doc---objects-inst-var
10586: doc---objects-interface
10587: doc---objects-m:
10588: doc---objects-:m
10589: doc---objects-;m
10590: doc---objects-method
10591: doc---objects-methods
10592: doc---objects-object
10593: doc---objects-overrides
10594: doc---objects-[parent]
10595: doc---objects-print
10596: doc---objects-protected
10597: doc---objects-public
10598: doc---objects-selector
10599: doc---objects-this
10600: doc---objects-<to-inst>
10601: doc---objects-[to-inst]
10602: doc---objects-to-this
10603: doc---objects-xt-new
1.20 pazsan 10604:
10605:
1.78 anton 10606: @c -------------------------------------------------------------
10607: @node OOF, Mini-OOF, Objects, Object-oriented Forth
10608: @subsection The @file{oof.fs} model
10609: @cindex oof
10610: @cindex object-oriented programming
1.20 pazsan 10611:
1.78 anton 10612: @cindex @file{objects.fs}
10613: @cindex @file{oof.fs}
1.20 pazsan 10614:
1.78 anton 10615: This section describes the @file{oof.fs} package.
1.20 pazsan 10616:
1.78 anton 10617: The package described in this section has been used in bigFORTH since 1991, and
10618: used for two large applications: a chromatographic system used to
10619: create new medicaments, and a graphic user interface library (MINOS).
1.20 pazsan 10620:
1.78 anton 10621: You can find a description (in German) of @file{oof.fs} in @cite{Object
10622: oriented bigFORTH} by Bernd Paysan, published in @cite{Vierte Dimension}
10623: 10(2), 1994.
1.20 pazsan 10624:
1.78 anton 10625: @menu
10626: * Properties of the OOF model::
10627: * Basic OOF Usage::
10628: * The OOF base class::
10629: * Class Declaration::
10630: * Class Implementation::
10631: @end menu
1.20 pazsan 10632:
1.78 anton 10633: @node Properties of the OOF model, Basic OOF Usage, OOF, OOF
10634: @subsubsection Properties of the @file{oof.fs} model
10635: @cindex @file{oof.fs} properties
1.20 pazsan 10636:
1.78 anton 10637: @itemize @bullet
10638: @item
10639: This model combines object oriented programming with information
10640: hiding. It helps you writing large application, where scoping is
10641: necessary, because it provides class-oriented scoping.
1.20 pazsan 10642:
1.78 anton 10643: @item
10644: Named objects, object pointers, and object arrays can be created,
10645: selector invocation uses the ``object selector'' syntax. Selector invocation
10646: to objects and/or selectors on the stack is a bit less convenient, but
10647: possible.
1.44 crook 10648:
1.78 anton 10649: @item
10650: Selector invocation and instance variable usage of the active object is
10651: straightforward, since both make use of the active object.
1.44 crook 10652:
1.78 anton 10653: @item
10654: Late binding is efficient and easy to use.
1.20 pazsan 10655:
1.78 anton 10656: @item
10657: State-smart objects parse selectors. However, extensibility is provided
10658: using a (parsing) selector @code{postpone} and a selector @code{'}.
1.20 pazsan 10659:
1.78 anton 10660: @item
10661: An implementation in ANS Forth is available.
1.20 pazsan 10662:
1.78 anton 10663: @end itemize
1.23 crook 10664:
10665:
1.78 anton 10666: @node Basic OOF Usage, The OOF base class, Properties of the OOF model, OOF
10667: @subsubsection Basic @file{oof.fs} Usage
10668: @cindex @file{oof.fs} usage
1.23 crook 10669:
1.78 anton 10670: This section uses the same example as for @code{objects} (@pxref{Basic Objects Usage}).
1.23 crook 10671:
1.78 anton 10672: You can define a class for graphical objects like this:
1.23 crook 10673:
1.78 anton 10674: @cindex @code{class} usage
10675: @cindex @code{class;} usage
10676: @cindex @code{method} usage
10677: @example
10678: object class graphical \ "object" is the parent class
10679: method draw ( x y graphical -- )
10680: class;
10681: @end example
1.23 crook 10682:
1.78 anton 10683: This code defines a class @code{graphical} with an
10684: operation @code{draw}. We can perform the operation
10685: @code{draw} on any @code{graphical} object, e.g.:
1.23 crook 10686:
1.78 anton 10687: @example
10688: 100 100 t-rex draw
10689: @end example
1.23 crook 10690:
1.78 anton 10691: @noindent
10692: where @code{t-rex} is an object or object pointer, created with e.g.
10693: @code{graphical : t-rex}.
1.23 crook 10694:
1.78 anton 10695: @cindex abstract class
10696: How do we create a graphical object? With the present definitions,
10697: we cannot create a useful graphical object. The class
10698: @code{graphical} describes graphical objects in general, but not
10699: any concrete graphical object type (C++ users would call it an
10700: @emph{abstract class}); e.g., there is no method for the selector
10701: @code{draw} in the class @code{graphical}.
1.23 crook 10702:
1.78 anton 10703: For concrete graphical objects, we define child classes of the
10704: class @code{graphical}, e.g.:
1.23 crook 10705:
1.78 anton 10706: @example
10707: graphical class circle \ "graphical" is the parent class
10708: cell var circle-radius
10709: how:
10710: : draw ( x y -- )
10711: circle-radius @@ draw-circle ;
1.23 crook 10712:
1.78 anton 10713: : init ( n-radius -- (
10714: circle-radius ! ;
10715: class;
10716: @end example
1.1 anton 10717:
1.78 anton 10718: Here we define a class @code{circle} as a child of @code{graphical},
10719: with a field @code{circle-radius}; it defines new methods for the
10720: selectors @code{draw} and @code{init} (@code{init} is defined in
10721: @code{object}, the parent class of @code{graphical}).
1.1 anton 10722:
1.78 anton 10723: Now we can create a circle in the dictionary with:
1.1 anton 10724:
1.78 anton 10725: @example
10726: 50 circle : my-circle
10727: @end example
1.21 crook 10728:
1.78 anton 10729: @noindent
10730: @code{:} invokes @code{init}, thus initializing the field
10731: @code{circle-radius} with 50. We can draw this new circle at (100,100)
10732: with:
1.1 anton 10733:
1.78 anton 10734: @example
10735: 100 100 my-circle draw
10736: @end example
1.1 anton 10737:
1.78 anton 10738: @cindex selector invocation, restrictions
10739: @cindex class definition, restrictions
10740: Note: You can only invoke a selector if the receiving object belongs to
10741: the class where the selector was defined or one of its descendents;
10742: e.g., you can invoke @code{draw} only for objects belonging to
10743: @code{graphical} or its descendents (e.g., @code{circle}). The scoping
10744: mechanism will check if you try to invoke a selector that is not
10745: defined in this class hierarchy, so you'll get an error at compilation
10746: time.
1.1 anton 10747:
10748:
1.78 anton 10749: @node The OOF base class, Class Declaration, Basic OOF Usage, OOF
10750: @subsubsection The @file{oof.fs} base class
10751: @cindex @file{oof.fs} base class
1.1 anton 10752:
1.78 anton 10753: When you define a class, you have to specify a parent class. So how do
10754: you start defining classes? There is one class available from the start:
10755: @code{object}. You have to use it as ancestor for all classes. It is the
10756: only class that has no parent. Classes are also objects, except that
10757: they don't have instance variables; class manipulation such as
10758: inheritance or changing definitions of a class is handled through
10759: selectors of the class @code{object}.
1.1 anton 10760:
1.78 anton 10761: @code{object} provides a number of selectors:
1.1 anton 10762:
1.78 anton 10763: @itemize @bullet
10764: @item
10765: @code{class} for subclassing, @code{definitions} to add definitions
10766: later on, and @code{class?} to get type informations (is the class a
10767: subclass of the class passed on the stack?).
1.1 anton 10768:
1.78 anton 10769: doc---object-class
10770: doc---object-definitions
10771: doc---object-class?
1.1 anton 10772:
10773:
1.26 crook 10774: @item
1.78 anton 10775: @code{init} and @code{dispose} as constructor and destructor of the
10776: object. @code{init} is invocated after the object's memory is allocated,
10777: while @code{dispose} also handles deallocation. Thus if you redefine
10778: @code{dispose}, you have to call the parent's dispose with @code{super
10779: dispose}, too.
10780:
10781: doc---object-init
10782: doc---object-dispose
10783:
1.1 anton 10784:
1.26 crook 10785: @item
1.78 anton 10786: @code{new}, @code{new[]}, @code{:}, @code{ptr}, @code{asptr}, and
10787: @code{[]} to create named and unnamed objects and object arrays or
10788: object pointers.
10789:
10790: doc---object-new
10791: doc---object-new[]
10792: doc---object-:
10793: doc---object-ptr
10794: doc---object-asptr
10795: doc---object-[]
10796:
1.1 anton 10797:
1.26 crook 10798: @item
1.78 anton 10799: @code{::} and @code{super} for explicit scoping. You should use explicit
10800: scoping only for super classes or classes with the same set of instance
10801: variables. Explicitly-scoped selectors use early binding.
1.21 crook 10802:
1.78 anton 10803: doc---object-::
10804: doc---object-super
1.21 crook 10805:
10806:
1.26 crook 10807: @item
1.78 anton 10808: @code{self} to get the address of the object
1.21 crook 10809:
1.78 anton 10810: doc---object-self
1.21 crook 10811:
10812:
1.78 anton 10813: @item
10814: @code{bind}, @code{bound}, @code{link}, and @code{is} to assign object
10815: pointers and instance defers.
1.21 crook 10816:
1.78 anton 10817: doc---object-bind
10818: doc---object-bound
10819: doc---object-link
10820: doc---object-is
1.21 crook 10821:
10822:
1.78 anton 10823: @item
10824: @code{'} to obtain selector tokens, @code{send} to invocate selectors
10825: form the stack, and @code{postpone} to generate selector invocation code.
1.21 crook 10826:
1.78 anton 10827: doc---object-'
10828: doc---object-postpone
1.21 crook 10829:
10830:
1.78 anton 10831: @item
10832: @code{with} and @code{endwith} to select the active object from the
10833: stack, and enable its scope. Using @code{with} and @code{endwith}
10834: also allows you to create code using selector @code{postpone} without being
10835: trapped by the state-smart objects.
1.21 crook 10836:
1.78 anton 10837: doc---object-with
10838: doc---object-endwith
1.21 crook 10839:
10840:
1.78 anton 10841: @end itemize
1.21 crook 10842:
1.78 anton 10843: @node Class Declaration, Class Implementation, The OOF base class, OOF
10844: @subsubsection Class Declaration
10845: @cindex class declaration
1.21 crook 10846:
1.78 anton 10847: @itemize @bullet
10848: @item
10849: Instance variables
1.21 crook 10850:
1.78 anton 10851: doc---oof-var
1.21 crook 10852:
10853:
1.78 anton 10854: @item
10855: Object pointers
1.21 crook 10856:
1.78 anton 10857: doc---oof-ptr
10858: doc---oof-asptr
1.21 crook 10859:
10860:
1.78 anton 10861: @item
10862: Instance defers
1.21 crook 10863:
1.78 anton 10864: doc---oof-defer
1.21 crook 10865:
10866:
1.78 anton 10867: @item
10868: Method selectors
1.21 crook 10869:
1.78 anton 10870: doc---oof-early
10871: doc---oof-method
1.21 crook 10872:
10873:
1.78 anton 10874: @item
10875: Class-wide variables
1.21 crook 10876:
1.78 anton 10877: doc---oof-static
1.21 crook 10878:
10879:
1.78 anton 10880: @item
10881: End declaration
1.1 anton 10882:
1.78 anton 10883: doc---oof-how:
10884: doc---oof-class;
1.21 crook 10885:
10886:
1.78 anton 10887: @end itemize
1.21 crook 10888:
1.78 anton 10889: @c -------------------------------------------------------------
10890: @node Class Implementation, , Class Declaration, OOF
10891: @subsubsection Class Implementation
10892: @cindex class implementation
1.21 crook 10893:
1.78 anton 10894: @c -------------------------------------------------------------
10895: @node Mini-OOF, Comparison with other object models, OOF, Object-oriented Forth
10896: @subsection The @file{mini-oof.fs} model
10897: @cindex mini-oof
1.21 crook 10898:
1.78 anton 10899: Gforth's third object oriented Forth package is a 12-liner. It uses a
1.79 anton 10900: mixture of the @file{objects.fs} and the @file{oof.fs} syntax,
1.78 anton 10901: and reduces to the bare minimum of features. This is based on a posting
10902: of Bernd Paysan in comp.lang.forth.
1.21 crook 10903:
1.78 anton 10904: @menu
10905: * Basic Mini-OOF Usage::
10906: * Mini-OOF Example::
10907: * Mini-OOF Implementation::
10908: @end menu
1.21 crook 10909:
1.78 anton 10910: @c -------------------------------------------------------------
10911: @node Basic Mini-OOF Usage, Mini-OOF Example, Mini-OOF, Mini-OOF
10912: @subsubsection Basic @file{mini-oof.fs} Usage
10913: @cindex mini-oof usage
1.21 crook 10914:
1.78 anton 10915: There is a base class (@code{class}, which allocates one cell for the
10916: object pointer) plus seven other words: to define a method, a variable,
10917: a class; to end a class, to resolve binding, to allocate an object and
10918: to compile a class method.
10919: @comment TODO better description of the last one
1.26 crook 10920:
1.21 crook 10921:
1.78 anton 10922: doc-object
10923: doc-method
10924: doc-var
10925: doc-class
10926: doc-end-class
10927: doc-defines
10928: doc-new
10929: doc-::
1.21 crook 10930:
10931:
10932:
1.78 anton 10933: @c -------------------------------------------------------------
10934: @node Mini-OOF Example, Mini-OOF Implementation, Basic Mini-OOF Usage, Mini-OOF
10935: @subsubsection Mini-OOF Example
10936: @cindex mini-oof example
1.1 anton 10937:
1.78 anton 10938: A short example shows how to use this package. This example, in slightly
10939: extended form, is supplied as @file{moof-exm.fs}
10940: @comment TODO could flesh this out with some comments from the Forthwrite article
1.20 pazsan 10941:
1.26 crook 10942: @example
1.78 anton 10943: object class
10944: method init
10945: method draw
10946: end-class graphical
1.26 crook 10947: @end example
1.20 pazsan 10948:
1.78 anton 10949: This code defines a class @code{graphical} with an
10950: operation @code{draw}. We can perform the operation
10951: @code{draw} on any @code{graphical} object, e.g.:
1.20 pazsan 10952:
1.26 crook 10953: @example
1.78 anton 10954: 100 100 t-rex draw
1.26 crook 10955: @end example
1.12 anton 10956:
1.78 anton 10957: where @code{t-rex} is an object or object pointer, created with e.g.
10958: @code{graphical new Constant t-rex}.
1.12 anton 10959:
1.78 anton 10960: For concrete graphical objects, we define child classes of the
10961: class @code{graphical}, e.g.:
1.12 anton 10962:
1.26 crook 10963: @example
10964: graphical class
1.78 anton 10965: cell var circle-radius
10966: end-class circle \ "graphical" is the parent class
1.12 anton 10967:
1.78 anton 10968: :noname ( x y -- )
10969: circle-radius @@ draw-circle ; circle defines draw
10970: :noname ( r -- )
10971: circle-radius ! ; circle defines init
10972: @end example
1.12 anton 10973:
1.78 anton 10974: There is no implicit init method, so we have to define one. The creation
10975: code of the object now has to call init explicitely.
1.21 crook 10976:
1.78 anton 10977: @example
10978: circle new Constant my-circle
10979: 50 my-circle init
1.12 anton 10980: @end example
10981:
1.78 anton 10982: It is also possible to add a function to create named objects with
10983: automatic call of @code{init}, given that all objects have @code{init}
10984: on the same place:
1.38 anton 10985:
1.78 anton 10986: @example
10987: : new: ( .. o "name" -- )
10988: new dup Constant init ;
10989: 80 circle new: large-circle
10990: @end example
1.12 anton 10991:
1.78 anton 10992: We can draw this new circle at (100,100) with:
1.12 anton 10993:
1.78 anton 10994: @example
10995: 100 100 my-circle draw
10996: @end example
1.12 anton 10997:
1.78 anton 10998: @node Mini-OOF Implementation, , Mini-OOF Example, Mini-OOF
10999: @subsubsection @file{mini-oof.fs} Implementation
1.12 anton 11000:
1.78 anton 11001: Object-oriented systems with late binding typically use a
11002: ``vtable''-approach: the first variable in each object is a pointer to a
11003: table, which contains the methods as function pointers. The vtable
11004: may also contain other information.
1.12 anton 11005:
1.79 anton 11006: So first, let's declare selectors:
1.37 anton 11007:
11008: @example
1.79 anton 11009: : method ( m v "name" -- m' v ) Create over , swap cell+ swap
1.78 anton 11010: DOES> ( ... o -- ... ) @@ over @@ + @@ execute ;
11011: @end example
1.37 anton 11012:
1.79 anton 11013: During selector declaration, the number of selectors and instance
11014: variables is on the stack (in address units). @code{method} creates one
11015: selector and increments the selector number. To execute a selector, it
1.78 anton 11016: takes the object, fetches the vtable pointer, adds the offset, and
1.79 anton 11017: executes the method @i{xt} stored there. Each selector takes the object
11018: it is invoked with as top of stack parameter; it passes the parameters
11019: (including the object) unchanged to the appropriate method which should
1.78 anton 11020: consume that object.
1.37 anton 11021:
1.78 anton 11022: Now, we also have to declare instance variables
1.37 anton 11023:
1.78 anton 11024: @example
1.79 anton 11025: : var ( m v size "name" -- m v' ) Create over , +
1.78 anton 11026: DOES> ( o -- addr ) @@ + ;
1.37 anton 11027: @end example
11028:
1.78 anton 11029: As before, a word is created with the current offset. Instance
11030: variables can have different sizes (cells, floats, doubles, chars), so
11031: all we do is take the size and add it to the offset. If your machine
11032: has alignment restrictions, put the proper @code{aligned} or
11033: @code{faligned} before the variable, to adjust the variable
11034: offset. That's why it is on the top of stack.
1.37 anton 11035:
1.78 anton 11036: We need a starting point (the base object) and some syntactic sugar:
1.37 anton 11037:
1.78 anton 11038: @example
11039: Create object 1 cells , 2 cells ,
1.79 anton 11040: : class ( class -- class selectors vars ) dup 2@@ ;
1.78 anton 11041: @end example
1.12 anton 11042:
1.78 anton 11043: For inheritance, the vtable of the parent object has to be
11044: copied when a new, derived class is declared. This gives all the
11045: methods of the parent class, which can be overridden, though.
1.12 anton 11046:
1.78 anton 11047: @example
1.79 anton 11048: : end-class ( class selectors vars "name" -- )
1.78 anton 11049: Create here >r , dup , 2 cells ?DO ['] noop , 1 cells +LOOP
11050: cell+ dup cell+ r> rot @@ 2 cells /string move ;
11051: @end example
1.12 anton 11052:
1.78 anton 11053: The first line creates the vtable, initialized with
11054: @code{noop}s. The second line is the inheritance mechanism, it
11055: copies the xts from the parent vtable.
1.12 anton 11056:
1.78 anton 11057: We still have no way to define new methods, let's do that now:
1.12 anton 11058:
1.26 crook 11059: @example
1.79 anton 11060: : defines ( xt class "name" -- ) ' >body @@ + ! ;
1.78 anton 11061: @end example
1.12 anton 11062:
1.78 anton 11063: To allocate a new object, we need a word, too:
1.12 anton 11064:
1.78 anton 11065: @example
11066: : new ( class -- o ) here over @@ allot swap over ! ;
1.12 anton 11067: @end example
11068:
1.78 anton 11069: Sometimes derived classes want to access the method of the
11070: parent object. There are two ways to achieve this with Mini-OOF:
11071: first, you could use named words, and second, you could look up the
11072: vtable of the parent object.
1.12 anton 11073:
1.78 anton 11074: @example
11075: : :: ( class "name" -- ) ' >body @@ + @@ compile, ;
11076: @end example
1.12 anton 11077:
11078:
1.78 anton 11079: Nothing can be more confusing than a good example, so here is
11080: one. First let's declare a text object (called
11081: @code{button}), that stores text and position:
1.12 anton 11082:
1.78 anton 11083: @example
11084: object class
11085: cell var text
11086: cell var len
11087: cell var x
11088: cell var y
11089: method init
11090: method draw
11091: end-class button
11092: @end example
1.12 anton 11093:
1.78 anton 11094: @noindent
11095: Now, implement the two methods, @code{draw} and @code{init}:
1.21 crook 11096:
1.26 crook 11097: @example
1.78 anton 11098: :noname ( o -- )
11099: >r r@@ x @@ r@@ y @@ at-xy r@@ text @@ r> len @@ type ;
11100: button defines draw
11101: :noname ( addr u o -- )
11102: >r 0 r@@ x ! 0 r@@ y ! r@@ len ! r> text ! ;
11103: button defines init
1.26 crook 11104: @end example
1.12 anton 11105:
1.78 anton 11106: @noindent
11107: To demonstrate inheritance, we define a class @code{bold-button}, with no
1.79 anton 11108: new data and no new selectors:
1.78 anton 11109:
11110: @example
11111: button class
11112: end-class bold-button
1.12 anton 11113:
1.78 anton 11114: : bold 27 emit ." [1m" ;
11115: : normal 27 emit ." [0m" ;
11116: @end example
1.1 anton 11117:
1.78 anton 11118: @noindent
11119: The class @code{bold-button} has a different draw method to
11120: @code{button}, but the new method is defined in terms of the draw method
11121: for @code{button}:
1.20 pazsan 11122:
1.78 anton 11123: @example
11124: :noname bold [ button :: draw ] normal ; bold-button defines draw
11125: @end example
1.21 crook 11126:
1.78 anton 11127: @noindent
1.79 anton 11128: Finally, create two objects and apply selectors:
1.21 crook 11129:
1.26 crook 11130: @example
1.78 anton 11131: button new Constant foo
11132: s" thin foo" foo init
11133: page
11134: foo draw
11135: bold-button new Constant bar
11136: s" fat bar" bar init
11137: 1 bar y !
11138: bar draw
1.26 crook 11139: @end example
1.21 crook 11140:
11141:
1.78 anton 11142: @node Comparison with other object models, , Mini-OOF, Object-oriented Forth
11143: @subsection Comparison with other object models
11144: @cindex comparison of object models
11145: @cindex object models, comparison
11146:
11147: Many object-oriented Forth extensions have been proposed (@cite{A survey
11148: of object-oriented Forths} (SIGPLAN Notices, April 1996) by Bradford
11149: J. Rodriguez and W. F. S. Poehlman lists 17). This section discusses the
11150: relation of the object models described here to two well-known and two
11151: closely-related (by the use of method maps) models. Andras Zsoter
11152: helped us with this section.
11153:
11154: @cindex Neon model
11155: The most popular model currently seems to be the Neon model (see
11156: @cite{Object-oriented programming in ANS Forth} (Forth Dimensions, March
11157: 1997) by Andrew McKewan) but this model has a number of limitations
11158: @footnote{A longer version of this critique can be
11159: found in @cite{On Standardizing Object-Oriented Forth Extensions} (Forth
11160: Dimensions, May 1997) by Anton Ertl.}:
11161:
11162: @itemize @bullet
11163: @item
11164: It uses a @code{@emph{selector object}} syntax, which makes it unnatural
11165: to pass objects on the stack.
1.21 crook 11166:
1.78 anton 11167: @item
11168: It requires that the selector parses the input stream (at
1.79 anton 11169: compile time); this leads to reduced extensibility and to bugs that are
1.78 anton 11170: hard to find.
1.21 crook 11171:
1.78 anton 11172: @item
1.79 anton 11173: It allows using every selector on every object; this eliminates the
11174: need for interfaces, but makes it harder to create efficient
11175: implementations.
1.78 anton 11176: @end itemize
1.21 crook 11177:
1.78 anton 11178: @cindex Pountain's object-oriented model
11179: Another well-known publication is @cite{Object-Oriented Forth} (Academic
11180: Press, London, 1987) by Dick Pountain. However, it is not really about
11181: object-oriented programming, because it hardly deals with late
11182: binding. Instead, it focuses on features like information hiding and
11183: overloading that are characteristic of modular languages like Ada (83).
1.26 crook 11184:
1.78 anton 11185: @cindex Zsoter's object-oriented model
1.79 anton 11186: In @uref{http://www.forth.org/oopf.html, Does late binding have to be
11187: slow?} (Forth Dimensions 18(1) 1996, pages 31-35) Andras Zsoter
11188: describes a model that makes heavy use of an active object (like
11189: @code{this} in @file{objects.fs}): The active object is not only used
11190: for accessing all fields, but also specifies the receiving object of
11191: every selector invocation; you have to change the active object
11192: explicitly with @code{@{ ... @}}, whereas in @file{objects.fs} it
11193: changes more or less implicitly at @code{m: ... ;m}. Such a change at
11194: the method entry point is unnecessary with Zsoter's model, because the
11195: receiving object is the active object already. On the other hand, the
11196: explicit change is absolutely necessary in that model, because otherwise
11197: no one could ever change the active object. An ANS Forth implementation
11198: of this model is available through
11199: @uref{http://www.forth.org/oopf.html}.
1.21 crook 11200:
1.78 anton 11201: @cindex @file{oof.fs}, differences to other models
11202: The @file{oof.fs} model combines information hiding and overloading
11203: resolution (by keeping names in various word lists) with object-oriented
11204: programming. It sets the active object implicitly on method entry, but
11205: also allows explicit changing (with @code{>o...o>} or with
11206: @code{with...endwith}). It uses parsing and state-smart objects and
11207: classes for resolving overloading and for early binding: the object or
11208: class parses the selector and determines the method from this. If the
11209: selector is not parsed by an object or class, it performs a call to the
11210: selector for the active object (late binding), like Zsoter's model.
11211: Fields are always accessed through the active object. The big
11212: disadvantage of this model is the parsing and the state-smartness, which
11213: reduces extensibility and increases the opportunities for subtle bugs;
11214: essentially, you are only safe if you never tick or @code{postpone} an
11215: object or class (Bernd disagrees, but I (Anton) am not convinced).
1.21 crook 11216:
1.78 anton 11217: @cindex @file{mini-oof.fs}, differences to other models
11218: The @file{mini-oof.fs} model is quite similar to a very stripped-down
11219: version of the @file{objects.fs} model, but syntactically it is a
11220: mixture of the @file{objects.fs} and @file{oof.fs} models.
1.21 crook 11221:
11222:
1.78 anton 11223: @c -------------------------------------------------------------
11224: @node Programming Tools, Assembler and Code Words, Object-oriented Forth, Words
11225: @section Programming Tools
11226: @cindex programming tools
1.21 crook 11227:
1.78 anton 11228: @c !! move this and assembler down below OO stuff.
1.21 crook 11229:
1.78 anton 11230: @menu
11231: * Examining::
11232: * Forgetting words::
11233: * Debugging:: Simple and quick.
11234: * Assertions:: Making your programs self-checking.
11235: * Singlestep Debugger:: Executing your program word by word.
11236: @end menu
1.21 crook 11237:
1.78 anton 11238: @node Examining, Forgetting words, Programming Tools, Programming Tools
11239: @subsection Examining data and code
11240: @cindex examining data and code
11241: @cindex data examination
11242: @cindex code examination
1.44 crook 11243:
1.78 anton 11244: The following words inspect the stack non-destructively:
1.21 crook 11245:
1.78 anton 11246: doc-.s
11247: doc-f.s
1.44 crook 11248:
1.78 anton 11249: There is a word @code{.r} but it does @i{not} display the return stack!
11250: It is used for formatted numeric output (@pxref{Simple numeric output}).
1.21 crook 11251:
1.78 anton 11252: doc-depth
11253: doc-fdepth
11254: doc-clearstack
1.21 crook 11255:
1.78 anton 11256: The following words inspect memory.
1.21 crook 11257:
1.78 anton 11258: doc-?
11259: doc-dump
1.21 crook 11260:
1.78 anton 11261: And finally, @code{see} allows to inspect code:
1.21 crook 11262:
1.78 anton 11263: doc-see
11264: doc-xt-see
1.111 anton 11265: doc-simple-see
11266: doc-simple-see-range
1.21 crook 11267:
1.78 anton 11268: @node Forgetting words, Debugging, Examining, Programming Tools
11269: @subsection Forgetting words
11270: @cindex words, forgetting
11271: @cindex forgeting words
1.21 crook 11272:
1.78 anton 11273: @c anton: other, maybe better places for this subsection: Defining Words;
11274: @c Dictionary allocation. At least a reference should be there.
1.21 crook 11275:
1.78 anton 11276: Forth allows you to forget words (and everything that was alloted in the
11277: dictonary after them) in a LIFO manner.
1.21 crook 11278:
1.78 anton 11279: doc-marker
1.21 crook 11280:
1.78 anton 11281: The most common use of this feature is during progam development: when
11282: you change a source file, forget all the words it defined and load it
11283: again (since you also forget everything defined after the source file
11284: was loaded, you have to reload that, too). Note that effects like
11285: storing to variables and destroyed system words are not undone when you
11286: forget words. With a system like Gforth, that is fast enough at
11287: starting up and compiling, I find it more convenient to exit and restart
11288: Gforth, as this gives me a clean slate.
1.21 crook 11289:
1.78 anton 11290: Here's an example of using @code{marker} at the start of a source file
11291: that you are debugging; it ensures that you only ever have one copy of
11292: the file's definitions compiled at any time:
1.21 crook 11293:
1.78 anton 11294: @example
11295: [IFDEF] my-code
11296: my-code
11297: [ENDIF]
1.26 crook 11298:
1.78 anton 11299: marker my-code
11300: init-included-files
1.21 crook 11301:
1.78 anton 11302: \ .. definitions start here
11303: \ .
11304: \ .
11305: \ end
11306: @end example
1.21 crook 11307:
1.26 crook 11308:
1.78 anton 11309: @node Debugging, Assertions, Forgetting words, Programming Tools
11310: @subsection Debugging
11311: @cindex debugging
1.21 crook 11312:
1.78 anton 11313: Languages with a slow edit/compile/link/test development loop tend to
11314: require sophisticated tracing/stepping debuggers to facilate debugging.
1.21 crook 11315:
1.78 anton 11316: A much better (faster) way in fast-compiling languages is to add
11317: printing code at well-selected places, let the program run, look at
11318: the output, see where things went wrong, add more printing code, etc.,
11319: until the bug is found.
1.21 crook 11320:
1.78 anton 11321: The simple debugging aids provided in @file{debugs.fs}
11322: are meant to support this style of debugging.
1.21 crook 11323:
1.78 anton 11324: The word @code{~~} prints debugging information (by default the source
11325: location and the stack contents). It is easy to insert. If you use Emacs
11326: it is also easy to remove (@kbd{C-x ~} in the Emacs Forth mode to
11327: query-replace them with nothing). The deferred words
1.101 anton 11328: @code{printdebugdata} and @code{.debugline} control the output of
1.78 anton 11329: @code{~~}. The default source location output format works well with
11330: Emacs' compilation mode, so you can step through the program at the
11331: source level using @kbd{C-x `} (the advantage over a stepping debugger
11332: is that you can step in any direction and you know where the crash has
11333: happened or where the strange data has occurred).
1.21 crook 11334:
1.78 anton 11335: doc-~~
11336: doc-printdebugdata
1.101 anton 11337: doc-.debugline
1.21 crook 11338:
1.106 anton 11339: @cindex filenames in @code{~~} output
11340: @code{~~} (and assertions) will usually print the wrong file name if a
11341: marker is executed in the same file after their occurance. They will
11342: print @samp{*somewhere*} as file name if a marker is executed in the
11343: same file before their occurance.
11344:
11345:
1.78 anton 11346: @node Assertions, Singlestep Debugger, Debugging, Programming Tools
11347: @subsection Assertions
11348: @cindex assertions
1.21 crook 11349:
1.78 anton 11350: It is a good idea to make your programs self-checking, especially if you
11351: make an assumption that may become invalid during maintenance (for
11352: example, that a certain field of a data structure is never zero). Gforth
11353: supports @dfn{assertions} for this purpose. They are used like this:
1.21 crook 11354:
11355: @example
1.78 anton 11356: assert( @i{flag} )
1.26 crook 11357: @end example
11358:
1.78 anton 11359: The code between @code{assert(} and @code{)} should compute a flag, that
11360: should be true if everything is alright and false otherwise. It should
11361: not change anything else on the stack. The overall stack effect of the
11362: assertion is @code{( -- )}. E.g.
1.21 crook 11363:
1.26 crook 11364: @example
1.78 anton 11365: assert( 1 1 + 2 = ) \ what we learn in school
11366: assert( dup 0<> ) \ assert that the top of stack is not zero
11367: assert( false ) \ this code should not be reached
1.21 crook 11368: @end example
11369:
1.78 anton 11370: The need for assertions is different at different times. During
11371: debugging, we want more checking, in production we sometimes care more
11372: for speed. Therefore, assertions can be turned off, i.e., the assertion
11373: becomes a comment. Depending on the importance of an assertion and the
11374: time it takes to check it, you may want to turn off some assertions and
11375: keep others turned on. Gforth provides several levels of assertions for
11376: this purpose:
11377:
11378:
11379: doc-assert0(
11380: doc-assert1(
11381: doc-assert2(
11382: doc-assert3(
11383: doc-assert(
11384: doc-)
1.21 crook 11385:
11386:
1.78 anton 11387: The variable @code{assert-level} specifies the highest assertions that
11388: are turned on. I.e., at the default @code{assert-level} of one,
11389: @code{assert0(} and @code{assert1(} assertions perform checking, while
11390: @code{assert2(} and @code{assert3(} assertions are treated as comments.
1.26 crook 11391:
1.78 anton 11392: The value of @code{assert-level} is evaluated at compile-time, not at
11393: run-time. Therefore you cannot turn assertions on or off at run-time;
11394: you have to set the @code{assert-level} appropriately before compiling a
11395: piece of code. You can compile different pieces of code at different
11396: @code{assert-level}s (e.g., a trusted library at level 1 and
11397: newly-written code at level 3).
1.26 crook 11398:
11399:
1.78 anton 11400: doc-assert-level
1.26 crook 11401:
11402:
1.78 anton 11403: If an assertion fails, a message compatible with Emacs' compilation mode
11404: is produced and the execution is aborted (currently with @code{ABORT"}.
11405: If there is interest, we will introduce a special throw code. But if you
11406: intend to @code{catch} a specific condition, using @code{throw} is
11407: probably more appropriate than an assertion).
1.106 anton 11408:
11409: @cindex filenames in assertion output
11410: Assertions (and @code{~~}) will usually print the wrong file name if a
11411: marker is executed in the same file after their occurance. They will
11412: print @samp{*somewhere*} as file name if a marker is executed in the
11413: same file before their occurance.
1.44 crook 11414:
1.78 anton 11415: Definitions in ANS Forth for these assertion words are provided
11416: in @file{compat/assert.fs}.
1.26 crook 11417:
1.44 crook 11418:
1.78 anton 11419: @node Singlestep Debugger, , Assertions, Programming Tools
11420: @subsection Singlestep Debugger
11421: @cindex singlestep Debugger
11422: @cindex debugging Singlestep
1.44 crook 11423:
1.112 anton 11424: The singlestep debugger does not work in this release.
11425:
1.78 anton 11426: When you create a new word there's often the need to check whether it
11427: behaves correctly or not. You can do this by typing @code{dbg
11428: badword}. A debug session might look like this:
1.26 crook 11429:
1.78 anton 11430: @example
11431: : badword 0 DO i . LOOP ; ok
11432: 2 dbg badword
11433: : badword
11434: Scanning code...
1.44 crook 11435:
1.78 anton 11436: Nesting debugger ready!
1.44 crook 11437:
1.78 anton 11438: 400D4738 8049BC4 0 -> [ 2 ] 00002 00000
11439: 400D4740 8049F68 DO -> [ 0 ]
11440: 400D4744 804A0C8 i -> [ 1 ] 00000
11441: 400D4748 400C5E60 . -> 0 [ 0 ]
11442: 400D474C 8049D0C LOOP -> [ 0 ]
11443: 400D4744 804A0C8 i -> [ 1 ] 00001
11444: 400D4748 400C5E60 . -> 1 [ 0 ]
11445: 400D474C 8049D0C LOOP -> [ 0 ]
11446: 400D4758 804B384 ; -> ok
11447: @end example
1.21 crook 11448:
1.78 anton 11449: Each line displayed is one step. You always have to hit return to
11450: execute the next word that is displayed. If you don't want to execute
11451: the next word in a whole, you have to type @kbd{n} for @code{nest}. Here is
11452: an overview what keys are available:
1.44 crook 11453:
1.78 anton 11454: @table @i
1.44 crook 11455:
1.78 anton 11456: @item @key{RET}
11457: Next; Execute the next word.
1.21 crook 11458:
1.78 anton 11459: @item n
11460: Nest; Single step through next word.
1.44 crook 11461:
1.78 anton 11462: @item u
11463: Unnest; Stop debugging and execute rest of word. If we got to this word
11464: with nest, continue debugging with the calling word.
1.44 crook 11465:
1.78 anton 11466: @item d
11467: Done; Stop debugging and execute rest.
1.21 crook 11468:
1.78 anton 11469: @item s
11470: Stop; Abort immediately.
1.44 crook 11471:
1.78 anton 11472: @end table
1.44 crook 11473:
1.78 anton 11474: Debugging large application with this mechanism is very difficult, because
11475: you have to nest very deeply into the program before the interesting part
11476: begins. This takes a lot of time.
1.26 crook 11477:
1.78 anton 11478: To do it more directly put a @code{BREAK:} command into your source code.
11479: When program execution reaches @code{BREAK:} the single step debugger is
11480: invoked and you have all the features described above.
1.44 crook 11481:
1.78 anton 11482: If you have more than one part to debug it is useful to know where the
11483: program has stopped at the moment. You can do this by the
11484: @code{BREAK" string"} command. This behaves like @code{BREAK:} except that
11485: string is typed out when the ``breakpoint'' is reached.
1.44 crook 11486:
1.26 crook 11487:
1.78 anton 11488: doc-dbg
11489: doc-break:
11490: doc-break"
1.44 crook 11491:
11492:
1.26 crook 11493:
1.78 anton 11494: @c -------------------------------------------------------------
11495: @node Assembler and Code Words, Threading Words, Programming Tools, Words
11496: @section Assembler and Code Words
11497: @cindex assembler
11498: @cindex code words
1.44 crook 11499:
1.78 anton 11500: @menu
11501: * Code and ;code::
11502: * Common Assembler:: Assembler Syntax
11503: * Common Disassembler::
11504: * 386 Assembler:: Deviations and special cases
11505: * Alpha Assembler:: Deviations and special cases
11506: * MIPS assembler:: Deviations and special cases
11507: * Other assemblers:: How to write them
11508: @end menu
1.21 crook 11509:
1.78 anton 11510: @node Code and ;code, Common Assembler, Assembler and Code Words, Assembler and Code Words
11511: @subsection @code{Code} and @code{;code}
1.26 crook 11512:
1.78 anton 11513: Gforth provides some words for defining primitives (words written in
11514: machine code), and for defining the machine-code equivalent of
11515: @code{DOES>}-based defining words. However, the machine-independent
11516: nature of Gforth poses a few problems: First of all, Gforth runs on
11517: several architectures, so it can provide no standard assembler. What's
11518: worse is that the register allocation not only depends on the processor,
11519: but also on the @code{gcc} version and options used.
1.44 crook 11520:
1.78 anton 11521: The words that Gforth offers encapsulate some system dependences (e.g.,
11522: the header structure), so a system-independent assembler may be used in
11523: Gforth. If you do not have an assembler, you can compile machine code
11524: directly with @code{,} and @code{c,}@footnote{This isn't portable,
11525: because these words emit stuff in @i{data} space; it works because
11526: Gforth has unified code/data spaces. Assembler isn't likely to be
11527: portable anyway.}.
1.21 crook 11528:
1.44 crook 11529:
1.78 anton 11530: doc-assembler
11531: doc-init-asm
11532: doc-code
11533: doc-end-code
11534: doc-;code
11535: doc-flush-icache
1.44 crook 11536:
1.21 crook 11537:
1.78 anton 11538: If @code{flush-icache} does not work correctly, @code{code} words
11539: etc. will not work (reliably), either.
1.44 crook 11540:
1.78 anton 11541: The typical usage of these @code{code} words can be shown most easily by
11542: analogy to the equivalent high-level defining words:
1.44 crook 11543:
1.78 anton 11544: @example
11545: : foo code foo
11546: <high-level Forth words> <assembler>
11547: ; end-code
11548:
11549: : bar : bar
11550: <high-level Forth words> <high-level Forth words>
11551: CREATE CREATE
11552: <high-level Forth words> <high-level Forth words>
11553: DOES> ;code
11554: <high-level Forth words> <assembler>
11555: ; end-code
11556: @end example
1.21 crook 11557:
1.78 anton 11558: @c anton: the following stuff is also in "Common Assembler", in less detail.
1.44 crook 11559:
1.78 anton 11560: @cindex registers of the inner interpreter
11561: In the assembly code you will want to refer to the inner interpreter's
11562: registers (e.g., the data stack pointer) and you may want to use other
11563: registers for temporary storage. Unfortunately, the register allocation
11564: is installation-dependent.
1.44 crook 11565:
1.78 anton 11566: In particular, @code{ip} (Forth instruction pointer) and @code{rp}
1.100 anton 11567: (return stack pointer) may be in different places in @code{gforth} and
11568: @code{gforth-fast}, or different installations. This means that you
11569: cannot write a @code{NEXT} routine that works reliably on both versions
11570: or different installations; so for doing @code{NEXT}, I recommend
11571: jumping to @code{' noop >code-address}, which contains nothing but a
11572: @code{NEXT}.
1.21 crook 11573:
1.78 anton 11574: For general accesses to the inner interpreter's registers, the easiest
11575: solution is to use explicit register declarations (@pxref{Explicit Reg
11576: Vars, , Variables in Specified Registers, gcc.info, GNU C Manual}) for
11577: all of the inner interpreter's registers: You have to compile Gforth
11578: with @code{-DFORCE_REG} (configure option @code{--enable-force-reg}) and
11579: the appropriate declarations must be present in the @code{machine.h}
11580: file (see @code{mips.h} for an example; you can find a full list of all
11581: declarable register symbols with @code{grep register engine.c}). If you
11582: give explicit registers to all variables that are declared at the
11583: beginning of @code{engine()}, you should be able to use the other
11584: caller-saved registers for temporary storage. Alternatively, you can use
11585: the @code{gcc} option @code{-ffixed-REG} (@pxref{Code Gen Options, ,
11586: Options for Code Generation Conventions, gcc.info, GNU C Manual}) to
11587: reserve a register (however, this restriction on register allocation may
11588: slow Gforth significantly).
1.44 crook 11589:
1.78 anton 11590: If this solution is not viable (e.g., because @code{gcc} does not allow
11591: you to explicitly declare all the registers you need), you have to find
11592: out by looking at the code where the inner interpreter's registers
11593: reside and which registers can be used for temporary storage. You can
11594: get an assembly listing of the engine's code with @code{make engine.s}.
1.44 crook 11595:
1.78 anton 11596: In any case, it is good practice to abstract your assembly code from the
11597: actual register allocation. E.g., if the data stack pointer resides in
11598: register @code{$17}, create an alias for this register called @code{sp},
11599: and use that in your assembly code.
1.21 crook 11600:
1.78 anton 11601: @cindex code words, portable
11602: Another option for implementing normal and defining words efficiently
11603: is to add the desired functionality to the source of Gforth. For normal
11604: words you just have to edit @file{primitives} (@pxref{Automatic
11605: Generation}). Defining words (equivalent to @code{;CODE} words, for fast
11606: defined words) may require changes in @file{engine.c}, @file{kernel.fs},
11607: @file{prims2x.fs}, and possibly @file{cross.fs}.
1.44 crook 11608:
1.78 anton 11609: @node Common Assembler, Common Disassembler, Code and ;code, Assembler and Code Words
11610: @subsection Common Assembler
1.44 crook 11611:
1.78 anton 11612: The assemblers in Gforth generally use a postfix syntax, i.e., the
11613: instruction name follows the operands.
1.21 crook 11614:
1.78 anton 11615: The operands are passed in the usual order (the same that is used in the
11616: manual of the architecture). Since they all are Forth words, they have
11617: to be separated by spaces; you can also use Forth words to compute the
11618: operands.
1.44 crook 11619:
1.78 anton 11620: The instruction names usually end with a @code{,}. This makes it easier
11621: to visually separate instructions if you put several of them on one
11622: line; it also avoids shadowing other Forth words (e.g., @code{and}).
1.21 crook 11623:
1.78 anton 11624: Registers are usually specified by number; e.g., (decimal) @code{11}
11625: specifies registers R11 and F11 on the Alpha architecture (which one,
11626: depends on the instruction). The usual names are also available, e.g.,
11627: @code{s2} for R11 on Alpha.
1.21 crook 11628:
1.78 anton 11629: Control flow is specified similar to normal Forth code (@pxref{Arbitrary
11630: control structures}), with @code{if,}, @code{ahead,}, @code{then,},
11631: @code{begin,}, @code{until,}, @code{again,}, @code{cs-roll},
11632: @code{cs-pick}, @code{else,}, @code{while,}, and @code{repeat,}. The
11633: conditions are specified in a way specific to each assembler.
1.1 anton 11634:
1.78 anton 11635: Note that the register assignments of the Gforth engine can change
11636: between Gforth versions, or even between different compilations of the
11637: same Gforth version (e.g., if you use a different GCC version). So if
11638: you want to refer to Gforth's registers (e.g., the stack pointer or
11639: TOS), I recommend defining your own words for refering to these
11640: registers, and using them later on; then you can easily adapt to a
11641: changed register assignment. The stability of the register assignment
11642: is usually better if you build Gforth with @code{--enable-force-reg}.
1.1 anton 11643:
1.100 anton 11644: The most common use of these registers is to dispatch to the next word
11645: (the @code{next} routine). A portable way to do this is to jump to
11646: @code{' noop >code-address} (of course, this is less efficient than
11647: integrating the @code{next} code and scheduling it well).
1.1 anton 11648:
1.96 anton 11649: Another difference between Gforth version is that the top of stack is
11650: kept in memory in @code{gforth} and, on most platforms, in a register in
11651: @code{gforth-fast}.
11652:
1.78 anton 11653: @node Common Disassembler, 386 Assembler, Common Assembler, Assembler and Code Words
11654: @subsection Common Disassembler
1.1 anton 11655:
1.78 anton 11656: You can disassemble a @code{code} word with @code{see}
11657: (@pxref{Debugging}). You can disassemble a section of memory with
1.1 anton 11658:
1.78 anton 11659: doc-disasm
1.44 crook 11660:
1.78 anton 11661: The disassembler generally produces output that can be fed into the
11662: assembler (i.e., same syntax, etc.). It also includes additional
11663: information in comments. In particular, the address of the instruction
11664: is given in a comment before the instruction.
1.1 anton 11665:
1.78 anton 11666: @code{See} may display more or less than the actual code of the word,
11667: because the recognition of the end of the code is unreliable. You can
11668: use @code{disasm} if it did not display enough. It may display more, if
11669: the code word is not immediately followed by a named word. If you have
1.116 anton 11670: something else there, you can follow the word with @code{align latest ,}
1.78 anton 11671: to ensure that the end is recognized.
1.21 crook 11672:
1.78 anton 11673: @node 386 Assembler, Alpha Assembler, Common Disassembler, Assembler and Code Words
11674: @subsection 386 Assembler
1.44 crook 11675:
1.78 anton 11676: The 386 assembler included in Gforth was written by Bernd Paysan, it's
11677: available under GPL, and originally part of bigFORTH.
1.21 crook 11678:
1.78 anton 11679: The 386 disassembler included in Gforth was written by Andrew McKewan
11680: and is in the public domain.
1.21 crook 11681:
1.91 anton 11682: The disassembler displays code in an Intel-like prefix syntax.
1.21 crook 11683:
1.78 anton 11684: The assembler uses a postfix syntax with reversed parameters.
1.1 anton 11685:
1.78 anton 11686: The assembler includes all instruction of the Athlon, i.e. 486 core
11687: instructions, Pentium and PPro extensions, floating point, MMX, 3Dnow!,
11688: but not ISSE. It's an integrated 16- and 32-bit assembler. Default is 32
11689: bit, you can switch to 16 bit with .86 and back to 32 bit with .386.
1.1 anton 11690:
1.78 anton 11691: There are several prefixes to switch between different operation sizes,
11692: @code{.b} for byte accesses, @code{.w} for word accesses, @code{.d} for
11693: double-word accesses. Addressing modes can be switched with @code{.wa}
11694: for 16 bit addresses, and @code{.da} for 32 bit addresses. You don't
11695: need a prefix for byte register names (@code{AL} et al).
1.1 anton 11696:
1.78 anton 11697: For floating point operations, the prefixes are @code{.fs} (IEEE
11698: single), @code{.fl} (IEEE double), @code{.fx} (extended), @code{.fw}
11699: (word), @code{.fd} (double-word), and @code{.fq} (quad-word).
1.21 crook 11700:
1.78 anton 11701: The MMX opcodes don't have size prefixes, they are spelled out like in
11702: the Intel assembler. Instead of move from and to memory, there are
11703: PLDQ/PLDD and PSTQ/PSTD.
1.21 crook 11704:
1.78 anton 11705: The registers lack the 'e' prefix; even in 32 bit mode, eax is called
11706: ax. Immediate values are indicated by postfixing them with @code{#},
1.91 anton 11707: e.g., @code{3 #}. Here are some examples of addressing modes in various
11708: syntaxes:
1.21 crook 11709:
1.26 crook 11710: @example
1.91 anton 11711: Gforth Intel (NASM) AT&T (gas) Name
11712: .w ax ax %ax register (16 bit)
11713: ax eax %eax register (32 bit)
11714: 3 # offset 3 $3 immediate
11715: 1000 #) byte ptr 1000 1000 displacement
11716: bx ) [ebx] (%ebx) base
11717: 100 di d) 100[edi] 100(%edi) base+displacement
11718: 20 ax *4 i#) 20[eax*4] 20(,%eax,4) (index*scale)+displacement
11719: di ax *4 i) [edi][eax*4] (%edi,%eax,4) base+(index*scale)
11720: 4 bx cx di) 4[ebx][ecx] 4(%ebx,%ecx) base+index+displacement
11721: 12 sp ax *2 di) 12[esp][eax*2] 12(%esp,%eax,2) base+(index*scale)+displacement
11722: @end example
11723:
11724: You can use @code{L)} and @code{LI)} instead of @code{D)} and
11725: @code{DI)} to enforce 32-bit displacement fields (useful for
11726: later patching).
1.21 crook 11727:
1.78 anton 11728: Some example of instructions are:
1.1 anton 11729:
11730: @example
1.78 anton 11731: ax bx mov \ move ebx,eax
11732: 3 # ax mov \ mov eax,3
11733: 100 di ) ax mov \ mov eax,100[edi]
11734: 4 bx cx di) ax mov \ mov eax,4[ebx][ecx]
11735: .w ax bx mov \ mov bx,ax
1.1 anton 11736: @end example
11737:
1.78 anton 11738: The following forms are supported for binary instructions:
1.1 anton 11739:
11740: @example
1.78 anton 11741: <reg> <reg> <inst>
11742: <n> # <reg> <inst>
11743: <mem> <reg> <inst>
11744: <reg> <mem> <inst>
1.1 anton 11745: @end example
11746:
1.78 anton 11747: Immediate to memory is not supported. The shift/rotate syntax is:
1.1 anton 11748:
1.26 crook 11749: @example
1.78 anton 11750: <reg/mem> 1 # shl \ shortens to shift without immediate
11751: <reg/mem> 4 # shl
11752: <reg/mem> cl shl
1.26 crook 11753: @end example
1.1 anton 11754:
1.78 anton 11755: Precede string instructions (@code{movs} etc.) with @code{.b} to get
11756: the byte version.
1.1 anton 11757:
1.78 anton 11758: The control structure words @code{IF} @code{UNTIL} etc. must be preceded
11759: by one of these conditions: @code{vs vc u< u>= 0= 0<> u<= u> 0< 0>= ps
11760: pc < >= <= >}. (Note that most of these words shadow some Forth words
11761: when @code{assembler} is in front of @code{forth} in the search path,
11762: e.g., in @code{code} words). Currently the control structure words use
11763: one stack item, so you have to use @code{roll} instead of @code{cs-roll}
11764: to shuffle them (you can also use @code{swap} etc.).
1.21 crook 11765:
1.78 anton 11766: Here is an example of a @code{code} word (assumes that the stack pointer
11767: is in esi and the TOS is in ebx):
1.21 crook 11768:
1.26 crook 11769: @example
1.78 anton 11770: code my+ ( n1 n2 -- n )
11771: 4 si D) bx add
11772: 4 # si add
11773: Next
11774: end-code
1.26 crook 11775: @end example
1.21 crook 11776:
1.78 anton 11777: @node Alpha Assembler, MIPS assembler, 386 Assembler, Assembler and Code Words
11778: @subsection Alpha Assembler
1.21 crook 11779:
1.78 anton 11780: The Alpha assembler and disassembler were originally written by Bernd
11781: Thallner.
1.26 crook 11782:
1.78 anton 11783: The register names @code{a0}--@code{a5} are not available to avoid
11784: shadowing hex numbers.
1.2 jwilke 11785:
1.78 anton 11786: Immediate forms of arithmetic instructions are distinguished by a
11787: @code{#} just before the @code{,}, e.g., @code{and#,} (note: @code{lda,}
11788: does not count as arithmetic instruction).
1.2 jwilke 11789:
1.78 anton 11790: You have to specify all operands to an instruction, even those that
11791: other assemblers consider optional, e.g., the destination register for
11792: @code{br,}, or the destination register and hint for @code{jmp,}.
1.2 jwilke 11793:
1.78 anton 11794: You can specify conditions for @code{if,} by removing the first @code{b}
11795: and the trailing @code{,} from a branch with a corresponding name; e.g.,
1.2 jwilke 11796:
1.26 crook 11797: @example
1.78 anton 11798: 11 fgt if, \ if F11>0e
11799: ...
11800: endif,
1.26 crook 11801: @end example
1.2 jwilke 11802:
1.78 anton 11803: @code{fbgt,} gives @code{fgt}.
11804:
11805: @node MIPS assembler, Other assemblers, Alpha Assembler, Assembler and Code Words
11806: @subsection MIPS assembler
1.2 jwilke 11807:
1.78 anton 11808: The MIPS assembler was originally written by Christian Pirker.
1.2 jwilke 11809:
1.78 anton 11810: Currently the assembler and disassembler only cover the MIPS-I
11811: architecture (R3000), and don't support FP instructions.
1.2 jwilke 11812:
1.78 anton 11813: The register names @code{$a0}--@code{$a3} are not available to avoid
11814: shadowing hex numbers.
1.2 jwilke 11815:
1.78 anton 11816: Because there is no way to distinguish registers from immediate values,
11817: you have to explicitly use the immediate forms of instructions, i.e.,
11818: @code{addiu,}, not just @code{addu,} (@command{as} does this
11819: implicitly).
1.2 jwilke 11820:
1.78 anton 11821: If the architecture manual specifies several formats for the instruction
11822: (e.g., for @code{jalr,}), you usually have to use the one with more
11823: arguments (i.e., two for @code{jalr,}). When in doubt, see
11824: @code{arch/mips/testasm.fs} for an example of correct use.
1.2 jwilke 11825:
1.78 anton 11826: Branches and jumps in the MIPS architecture have a delay slot. You have
11827: to fill it yourself (the simplest way is to use @code{nop,}), the
11828: assembler does not do it for you (unlike @command{as}). Even
11829: @code{if,}, @code{ahead,}, @code{until,}, @code{again,}, @code{while,},
11830: @code{else,} and @code{repeat,} need a delay slot. Since @code{begin,}
11831: and @code{then,} just specify branch targets, they are not affected.
1.2 jwilke 11832:
1.78 anton 11833: Note that you must not put branches, jumps, or @code{li,} into the delay
11834: slot: @code{li,} may expand to several instructions, and control flow
11835: instructions may not be put into the branch delay slot in any case.
1.2 jwilke 11836:
1.78 anton 11837: For branches the argument specifying the target is a relative address;
11838: You have to add the address of the delay slot to get the absolute
11839: address.
1.1 anton 11840:
1.78 anton 11841: The MIPS architecture also has load delay slots and restrictions on
11842: using @code{mfhi,} and @code{mflo,}; you have to order the instructions
11843: yourself to satisfy these restrictions, the assembler does not do it for
11844: you.
1.1 anton 11845:
1.78 anton 11846: You can specify the conditions for @code{if,} etc. by taking a
11847: conditional branch and leaving away the @code{b} at the start and the
11848: @code{,} at the end. E.g.,
1.1 anton 11849:
1.26 crook 11850: @example
1.78 anton 11851: 4 5 eq if,
11852: ... \ do something if $4 equals $5
11853: then,
1.26 crook 11854: @end example
1.1 anton 11855:
1.78 anton 11856: @node Other assemblers, , MIPS assembler, Assembler and Code Words
11857: @subsection Other assemblers
11858:
11859: If you want to contribute another assembler/disassembler, please contact
1.103 anton 11860: us (@email{anton@@mips.complang.tuwien.ac.at}) to check if we have such
11861: an assembler already. If you are writing them from scratch, please use
11862: a similar syntax style as the one we use (i.e., postfix, commas at the
11863: end of the instruction names, @pxref{Common Assembler}); make the output
11864: of the disassembler be valid input for the assembler, and keep the style
1.78 anton 11865: similar to the style we used.
11866:
11867: Hints on implementation: The most important part is to have a good test
11868: suite that contains all instructions. Once you have that, the rest is
11869: easy. For actual coding you can take a look at
11870: @file{arch/mips/disasm.fs} to get some ideas on how to use data for both
11871: the assembler and disassembler, avoiding redundancy and some potential
11872: bugs. You can also look at that file (and @pxref{Advanced does> usage
11873: example}) to get ideas how to factor a disassembler.
11874:
11875: Start with the disassembler, because it's easier to reuse data from the
11876: disassembler for the assembler than the other way round.
1.1 anton 11877:
1.78 anton 11878: For the assembler, take a look at @file{arch/alpha/asm.fs}, which shows
11879: how simple it can be.
1.1 anton 11880:
1.78 anton 11881: @c -------------------------------------------------------------
11882: @node Threading Words, Passing Commands to the OS, Assembler and Code Words, Words
11883: @section Threading Words
11884: @cindex threading words
1.1 anton 11885:
1.78 anton 11886: @cindex code address
11887: These words provide access to code addresses and other threading stuff
11888: in Gforth (and, possibly, other interpretive Forths). It more or less
11889: abstracts away the differences between direct and indirect threading
11890: (and, for direct threading, the machine dependences). However, at
11891: present this wordset is still incomplete. It is also pretty low-level;
11892: some day it will hopefully be made unnecessary by an internals wordset
11893: that abstracts implementation details away completely.
1.1 anton 11894:
1.78 anton 11895: The terminology used here stems from indirect threaded Forth systems; in
11896: such a system, the XT of a word is represented by the CFA (code field
11897: address) of a word; the CFA points to a cell that contains the code
11898: address. The code address is the address of some machine code that
11899: performs the run-time action of invoking the word (e.g., the
11900: @code{dovar:} routine pushes the address of the body of the word (a
11901: variable) on the stack
11902: ).
1.1 anton 11903:
1.78 anton 11904: @cindex code address
11905: @cindex code field address
11906: In an indirect threaded Forth, you can get the code address of @i{name}
11907: with @code{' @i{name} @@}; in Gforth you can get it with @code{' @i{name}
11908: >code-address}, independent of the threading method.
1.1 anton 11909:
1.78 anton 11910: doc-threading-method
11911: doc->code-address
11912: doc-code-address!
1.1 anton 11913:
1.78 anton 11914: @cindex @code{does>}-handler
11915: @cindex @code{does>}-code
11916: For a word defined with @code{DOES>}, the code address usually points to
11917: a jump instruction (the @dfn{does-handler}) that jumps to the dodoes
11918: routine (in Gforth on some platforms, it can also point to the dodoes
11919: routine itself). What you are typically interested in, though, is
11920: whether a word is a @code{DOES>}-defined word, and what Forth code it
11921: executes; @code{>does-code} tells you that.
1.1 anton 11922:
1.78 anton 11923: doc->does-code
1.1 anton 11924:
1.78 anton 11925: To create a @code{DOES>}-defined word with the following basic words,
11926: you have to set up a @code{DOES>}-handler with @code{does-handler!};
11927: @code{/does-handler} aus behind you have to place your executable Forth
11928: code. Finally you have to create a word and modify its behaviour with
11929: @code{does-handler!}.
1.1 anton 11930:
1.78 anton 11931: doc-does-code!
11932: doc-does-handler!
11933: doc-/does-handler
1.1 anton 11934:
1.78 anton 11935: The code addresses produced by various defining words are produced by
11936: the following words:
1.1 anton 11937:
1.78 anton 11938: doc-docol:
11939: doc-docon:
11940: doc-dovar:
11941: doc-douser:
11942: doc-dodefer:
11943: doc-dofield:
1.1 anton 11944:
1.99 anton 11945: @cindex definer
11946: The following two words generalize @code{>code-address},
11947: @code{>does-code}, @code{code-address!}, and @code{does-code!}:
11948:
11949: doc->definer
11950: doc-definer!
11951:
1.26 crook 11952: @c -------------------------------------------------------------
1.78 anton 11953: @node Passing Commands to the OS, Keeping track of Time, Threading Words, Words
1.21 crook 11954: @section Passing Commands to the Operating System
11955: @cindex operating system - passing commands
11956: @cindex shell commands
11957:
11958: Gforth allows you to pass an arbitrary string to the host operating
11959: system shell (if such a thing exists) for execution.
11960:
1.44 crook 11961:
1.21 crook 11962: doc-sh
11963: doc-system
11964: doc-$?
1.23 crook 11965: doc-getenv
1.21 crook 11966:
1.44 crook 11967:
1.26 crook 11968: @c -------------------------------------------------------------
1.47 crook 11969: @node Keeping track of Time, Miscellaneous Words, Passing Commands to the OS, Words
11970: @section Keeping track of Time
11971: @cindex time-related words
11972:
11973: doc-ms
11974: doc-time&date
1.79 anton 11975: doc-utime
11976: doc-cputime
1.47 crook 11977:
11978:
11979: @c -------------------------------------------------------------
11980: @node Miscellaneous Words, , Keeping track of Time, Words
1.21 crook 11981: @section Miscellaneous Words
11982: @cindex miscellaneous words
11983:
1.29 crook 11984: @comment TODO find homes for these
11985:
1.26 crook 11986: These section lists the ANS Forth words that are not documented
1.21 crook 11987: elsewhere in this manual. Ultimately, they all need proper homes.
11988:
1.68 anton 11989: doc-quit
1.44 crook 11990:
1.26 crook 11991: The following ANS Forth words are not currently supported by Gforth
1.27 crook 11992: (@pxref{ANS conformance}):
1.21 crook 11993:
11994: @code{EDITOR}
11995: @code{EMIT?}
11996: @code{FORGET}
11997:
1.24 anton 11998: @c ******************************************************************
11999: @node Error messages, Tools, Words, Top
12000: @chapter Error messages
12001: @cindex error messages
12002: @cindex backtrace
12003:
12004: A typical Gforth error message looks like this:
12005:
12006: @example
1.86 anton 12007: in file included from \evaluated string/:-1
1.24 anton 12008: in file included from ./yyy.fs:1
12009: ./xxx.fs:4: Invalid memory address
12010: bar
12011: ^^^
1.79 anton 12012: Backtrace:
1.25 anton 12013: $400E664C @@
12014: $400E6664 foo
1.24 anton 12015: @end example
12016:
12017: The message identifying the error is @code{Invalid memory address}. The
12018: error happened when text-interpreting line 4 of the file
12019: @file{./xxx.fs}. This line is given (it contains @code{bar}), and the
12020: word on the line where the error happened, is pointed out (with
12021: @code{^^^}).
12022:
12023: The file containing the error was included in line 1 of @file{./yyy.fs},
12024: and @file{yyy.fs} was included from a non-file (in this case, by giving
12025: @file{yyy.fs} as command-line parameter to Gforth).
12026:
12027: At the end of the error message you find a return stack dump that can be
12028: interpreted as a backtrace (possibly empty). On top you find the top of
12029: the return stack when the @code{throw} happened, and at the bottom you
12030: find the return stack entry just above the return stack of the topmost
12031: text interpreter.
12032:
12033: To the right of most return stack entries you see a guess for the word
12034: that pushed that return stack entry as its return address. This gives a
12035: backtrace. In our case we see that @code{bar} called @code{foo}, and
12036: @code{foo} called @code{@@} (and @code{@@} had an @emph{Invalid memory
12037: address} exception).
12038:
12039: Note that the backtrace is not perfect: We don't know which return stack
12040: entries are return addresses (so we may get false positives); and in
12041: some cases (e.g., for @code{abort"}) we cannot determine from the return
12042: address the word that pushed the return address, so for some return
12043: addresses you see no names in the return stack dump.
1.25 anton 12044:
12045: @cindex @code{catch} and backtraces
12046: The return stack dump represents the return stack at the time when a
12047: specific @code{throw} was executed. In programs that make use of
12048: @code{catch}, it is not necessarily clear which @code{throw} should be
12049: used for the return stack dump (e.g., consider one @code{throw} that
12050: indicates an error, which is caught, and during recovery another error
1.42 anton 12051: happens; which @code{throw} should be used for the stack dump?). Gforth
1.25 anton 12052: presents the return stack dump for the first @code{throw} after the last
12053: executed (not returned-to) @code{catch}; this works well in the usual
12054: case.
12055:
12056: @cindex @code{gforth-fast} and backtraces
12057: @cindex @code{gforth-fast}, difference from @code{gforth}
12058: @cindex backtraces with @code{gforth-fast}
12059: @cindex return stack dump with @code{gforth-fast}
1.79 anton 12060: @code{Gforth} is able to do a return stack dump for throws generated
1.25 anton 12061: from primitives (e.g., invalid memory address, stack empty etc.);
12062: @code{gforth-fast} is only able to do a return stack dump from a
1.96 anton 12063: directly called @code{throw} (including @code{abort} etc.). Given an
1.30 anton 12064: exception caused by a primitive in @code{gforth-fast}, you will
12065: typically see no return stack dump at all; however, if the exception is
12066: caught by @code{catch} (e.g., for restoring some state), and then
12067: @code{throw}n again, the return stack dump will be for the first such
12068: @code{throw}.
1.2 jwilke 12069:
1.5 anton 12070: @c ******************************************************************
1.24 anton 12071: @node Tools, ANS conformance, Error messages, Top
1.1 anton 12072: @chapter Tools
12073:
12074: @menu
12075: * ANS Report:: Report the words used, sorted by wordset.
12076: @end menu
12077:
12078: See also @ref{Emacs and Gforth}.
12079:
12080: @node ANS Report, , Tools, Tools
12081: @section @file{ans-report.fs}: Report the words used, sorted by wordset
12082: @cindex @file{ans-report.fs}
12083: @cindex report the words used in your program
12084: @cindex words used in your program
12085:
12086: If you want to label a Forth program as ANS Forth Program, you must
12087: document which wordsets the program uses; for extension wordsets, it is
12088: helpful to list the words the program requires from these wordsets
12089: (because Forth systems are allowed to provide only some words of them).
12090:
12091: The @file{ans-report.fs} tool makes it easy for you to determine which
12092: words from which wordset and which non-ANS words your application
12093: uses. You simply have to include @file{ans-report.fs} before loading the
12094: program you want to check. After loading your program, you can get the
12095: report with @code{print-ans-report}. A typical use is to run this as
12096: batch job like this:
12097: @example
12098: gforth ans-report.fs myprog.fs -e "print-ans-report bye"
12099: @end example
12100:
12101: The output looks like this (for @file{compat/control.fs}):
12102: @example
12103: The program uses the following words
12104: from CORE :
12105: : POSTPONE THEN ; immediate ?dup IF 0=
12106: from BLOCK-EXT :
12107: \
12108: from FILE :
12109: (
12110: @end example
12111:
12112: @subsection Caveats
12113:
12114: Note that @file{ans-report.fs} just checks which words are used, not whether
12115: they are used in an ANS Forth conforming way!
12116:
12117: Some words are defined in several wordsets in the
12118: standard. @file{ans-report.fs} reports them for only one of the
12119: wordsets, and not necessarily the one you expect. It depends on usage
12120: which wordset is the right one to specify. E.g., if you only use the
12121: compilation semantics of @code{S"}, it is a Core word; if you also use
12122: its interpretation semantics, it is a File word.
12123:
12124: @c ******************************************************************
1.65 anton 12125: @node ANS conformance, Standard vs Extensions, Tools, Top
1.1 anton 12126: @chapter ANS conformance
12127: @cindex ANS conformance of Gforth
12128:
12129: To the best of our knowledge, Gforth is an
12130:
12131: ANS Forth System
12132: @itemize @bullet
12133: @item providing the Core Extensions word set
12134: @item providing the Block word set
12135: @item providing the Block Extensions word set
12136: @item providing the Double-Number word set
12137: @item providing the Double-Number Extensions word set
12138: @item providing the Exception word set
12139: @item providing the Exception Extensions word set
12140: @item providing the Facility word set
1.40 anton 12141: @item providing @code{EKEY}, @code{EKEY>CHAR}, @code{EKEY?}, @code{MS} and @code{TIME&DATE} from the Facility Extensions word set
1.1 anton 12142: @item providing the File Access word set
12143: @item providing the File Access Extensions word set
12144: @item providing the Floating-Point word set
12145: @item providing the Floating-Point Extensions word set
12146: @item providing the Locals word set
12147: @item providing the Locals Extensions word set
12148: @item providing the Memory-Allocation word set
12149: @item providing the Memory-Allocation Extensions word set (that one's easy)
12150: @item providing the Programming-Tools word set
12151: @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
12152: @item providing the Search-Order word set
12153: @item providing the Search-Order Extensions word set
12154: @item providing the String word set
12155: @item providing the String Extensions word set (another easy one)
12156: @end itemize
12157:
1.118 anton 12158: Gforth has the following environmental restrictions:
12159:
12160: @cindex environmental restrictions
12161: @itemize @bullet
12162: @item
12163: While processing the OS command line, if an exception is not caught,
12164: Gforth exits with a non-zero exit code instyead of performing QUIT.
12165:
12166: @item
12167: When an @code{throw} is performed after a @code{query}, Gforth does not
12168: allways restore the input source specification in effect at the
12169: corresponding catch.
12170:
12171: @end itemize
12172:
12173:
1.1 anton 12174: @cindex system documentation
12175: In addition, ANS Forth systems are required to document certain
12176: implementation choices. This chapter tries to meet these
12177: requirements. In many cases it gives a way to ask the system for the
12178: information instead of providing the information directly, in
12179: particular, if the information depends on the processor, the operating
12180: system or the installation options chosen, or if they are likely to
12181: change during the maintenance of Gforth.
12182:
12183: @comment The framework for the rest has been taken from pfe.
12184:
12185: @menu
12186: * The Core Words::
12187: * The optional Block word set::
12188: * The optional Double Number word set::
12189: * The optional Exception word set::
12190: * The optional Facility word set::
12191: * The optional File-Access word set::
12192: * The optional Floating-Point word set::
12193: * The optional Locals word set::
12194: * The optional Memory-Allocation word set::
12195: * The optional Programming-Tools word set::
12196: * The optional Search-Order word set::
12197: @end menu
12198:
12199:
12200: @c =====================================================================
12201: @node The Core Words, The optional Block word set, ANS conformance, ANS conformance
12202: @comment node-name, next, previous, up
12203: @section The Core Words
12204: @c =====================================================================
12205: @cindex core words, system documentation
12206: @cindex system documentation, core words
12207:
12208: @menu
12209: * core-idef:: Implementation Defined Options
12210: * core-ambcond:: Ambiguous Conditions
12211: * core-other:: Other System Documentation
12212: @end menu
12213:
12214: @c ---------------------------------------------------------------------
12215: @node core-idef, core-ambcond, The Core Words, The Core Words
12216: @subsection Implementation Defined Options
12217: @c ---------------------------------------------------------------------
12218: @cindex core words, implementation-defined options
12219: @cindex implementation-defined options, core words
12220:
12221:
12222: @table @i
12223: @item (Cell) aligned addresses:
12224: @cindex cell-aligned addresses
12225: @cindex aligned addresses
12226: processor-dependent. Gforth's alignment words perform natural alignment
12227: (e.g., an address aligned for a datum of size 8 is divisible by
12228: 8). Unaligned accesses usually result in a @code{-23 THROW}.
12229:
12230: @item @code{EMIT} and non-graphic characters:
12231: @cindex @code{EMIT} and non-graphic characters
12232: @cindex non-graphic characters and @code{EMIT}
12233: The character is output using the C library function (actually, macro)
12234: @code{putc}.
12235:
12236: @item character editing of @code{ACCEPT} and @code{EXPECT}:
12237: @cindex character editing of @code{ACCEPT} and @code{EXPECT}
12238: @cindex editing in @code{ACCEPT} and @code{EXPECT}
12239: @cindex @code{ACCEPT}, editing
12240: @cindex @code{EXPECT}, editing
12241: This is modeled on the GNU readline library (@pxref{Readline
12242: Interaction, , Command Line Editing, readline, The GNU Readline
12243: Library}) with Emacs-like key bindings. @kbd{Tab} deviates a little by
12244: producing a full word completion every time you type it (instead of
1.28 crook 12245: producing the common prefix of all completions). @xref{Command-line editing}.
1.1 anton 12246:
12247: @item character set:
12248: @cindex character set
12249: The character set of your computer and display device. Gforth is
12250: 8-bit-clean (but some other component in your system may make trouble).
12251:
12252: @item Character-aligned address requirements:
12253: @cindex character-aligned address requirements
12254: installation-dependent. Currently a character is represented by a C
12255: @code{unsigned char}; in the future we might switch to @code{wchar_t}
12256: (Comments on that requested).
12257:
12258: @item character-set extensions and matching of names:
12259: @cindex character-set extensions and matching of names
1.26 crook 12260: @cindex case-sensitivity for name lookup
12261: @cindex name lookup, case-sensitivity
12262: @cindex locale and case-sensitivity
1.21 crook 12263: Any character except the ASCII NUL character can be used in a
1.1 anton 12264: name. Matching is case-insensitive (except in @code{TABLE}s). The
1.47 crook 12265: matching is performed using the C library function @code{strncasecmp}, whose
1.1 anton 12266: function is probably influenced by the locale. E.g., the @code{C} locale
12267: does not know about accents and umlauts, so they are matched
12268: case-sensitively in that locale. For portability reasons it is best to
12269: write programs such that they work in the @code{C} locale. Then one can
12270: use libraries written by a Polish programmer (who might use words
12271: containing ISO Latin-2 encoded characters) and by a French programmer
12272: (ISO Latin-1) in the same program (of course, @code{WORDS} will produce
12273: funny results for some of the words (which ones, depends on the font you
12274: are using)). Also, the locale you prefer may not be available in other
12275: operating systems. Hopefully, Unicode will solve these problems one day.
12276:
12277: @item conditions under which control characters match a space delimiter:
12278: @cindex space delimiters
12279: @cindex control characters as delimiters
1.117 anton 12280: If @code{word} is called with the space character as a delimiter, all
1.1 anton 12281: white-space characters (as identified by the C macro @code{isspace()})
1.117 anton 12282: are delimiters. @code{Parse}, on the other hand, treats space like other
12283: delimiters. @code{Parse-word}, which is used by the outer
1.1 anton 12284: interpreter (aka text interpreter) by default, treats all white-space
12285: characters as delimiters.
12286:
1.26 crook 12287: @item format of the control-flow stack:
12288: @cindex control-flow stack, format
12289: The data stack is used as control-flow stack. The size of a control-flow
1.1 anton 12290: stack item in cells is given by the constant @code{cs-item-size}. At the
12291: time of this writing, an item consists of a (pointer to a) locals list
12292: (third), an address in the code (second), and a tag for identifying the
12293: item (TOS). The following tags are used: @code{defstart},
12294: @code{live-orig}, @code{dead-orig}, @code{dest}, @code{do-dest},
12295: @code{scopestart}.
12296:
12297: @item conversion of digits > 35
12298: @cindex digits > 35
12299: The characters @code{[\]^_'} are the digits with the decimal value
12300: 36@minus{}41. There is no way to input many of the larger digits.
12301:
12302: @item display after input terminates in @code{ACCEPT} and @code{EXPECT}:
12303: @cindex @code{EXPECT}, display after end of input
12304: @cindex @code{ACCEPT}, display after end of input
12305: The cursor is moved to the end of the entered string. If the input is
12306: terminated using the @kbd{Return} key, a space is typed.
12307:
12308: @item exception abort sequence of @code{ABORT"}:
12309: @cindex exception abort sequence of @code{ABORT"}
12310: @cindex @code{ABORT"}, exception abort sequence
12311: The error string is stored into the variable @code{"error} and a
12312: @code{-2 throw} is performed.
12313:
12314: @item input line terminator:
12315: @cindex input line terminator
12316: @cindex line terminator on input
1.26 crook 12317: @cindex newline character on input
1.1 anton 12318: For interactive input, @kbd{C-m} (CR) and @kbd{C-j} (LF) terminate
12319: lines. One of these characters is typically produced when you type the
12320: @kbd{Enter} or @kbd{Return} key.
12321:
12322: @item maximum size of a counted string:
12323: @cindex maximum size of a counted string
12324: @cindex counted string, maximum size
12325: @code{s" /counted-string" environment? drop .}. Currently 255 characters
1.79 anton 12326: on all platforms, but this may change.
1.1 anton 12327:
12328: @item maximum size of a parsed string:
12329: @cindex maximum size of a parsed string
12330: @cindex parsed string, maximum size
12331: Given by the constant @code{/line}. Currently 255 characters.
12332:
12333: @item maximum size of a definition name, in characters:
12334: @cindex maximum size of a definition name, in characters
12335: @cindex name, maximum length
1.113 anton 12336: MAXU/8
1.1 anton 12337:
12338: @item maximum string length for @code{ENVIRONMENT?}, in characters:
12339: @cindex maximum string length for @code{ENVIRONMENT?}, in characters
12340: @cindex @code{ENVIRONMENT?} string length, maximum
1.113 anton 12341: MAXU/8
1.1 anton 12342:
12343: @item method of selecting the user input device:
12344: @cindex user input device, method of selecting
12345: The user input device is the standard input. There is currently no way to
12346: change it from within Gforth. However, the input can typically be
12347: redirected in the command line that starts Gforth.
12348:
12349: @item method of selecting the user output device:
12350: @cindex user output device, method of selecting
12351: @code{EMIT} and @code{TYPE} output to the file-id stored in the value
1.10 anton 12352: @code{outfile-id} (@code{stdout} by default). Gforth uses unbuffered
12353: output when the user output device is a terminal, otherwise the output
12354: is buffered.
1.1 anton 12355:
12356: @item methods of dictionary compilation:
12357: What are we expected to document here?
12358:
12359: @item number of bits in one address unit:
12360: @cindex number of bits in one address unit
12361: @cindex address unit, size in bits
12362: @code{s" address-units-bits" environment? drop .}. 8 in all current
1.79 anton 12363: platforms.
1.1 anton 12364:
12365: @item number representation and arithmetic:
12366: @cindex number representation and arithmetic
1.79 anton 12367: Processor-dependent. Binary two's complement on all current platforms.
1.1 anton 12368:
12369: @item ranges for integer types:
12370: @cindex ranges for integer types
12371: @cindex integer types, ranges
12372: Installation-dependent. Make environmental queries for @code{MAX-N},
12373: @code{MAX-U}, @code{MAX-D} and @code{MAX-UD}. The lower bounds for
12374: unsigned (and positive) types is 0. The lower bound for signed types on
12375: two's complement and one's complement machines machines can be computed
12376: by adding 1 to the upper bound.
12377:
12378: @item read-only data space regions:
12379: @cindex read-only data space regions
12380: @cindex data-space, read-only regions
12381: The whole Forth data space is writable.
12382:
12383: @item size of buffer at @code{WORD}:
12384: @cindex size of buffer at @code{WORD}
12385: @cindex @code{WORD} buffer size
12386: @code{PAD HERE - .}. 104 characters on 32-bit machines. The buffer is
12387: shared with the pictured numeric output string. If overwriting
12388: @code{PAD} is acceptable, it is as large as the remaining dictionary
12389: space, although only as much can be sensibly used as fits in a counted
12390: string.
12391:
12392: @item size of one cell in address units:
12393: @cindex cell size
12394: @code{1 cells .}.
12395:
12396: @item size of one character in address units:
12397: @cindex char size
1.79 anton 12398: @code{1 chars .}. 1 on all current platforms.
1.1 anton 12399:
12400: @item size of the keyboard terminal buffer:
12401: @cindex size of the keyboard terminal buffer
12402: @cindex terminal buffer, size
12403: Varies. You can determine the size at a specific time using @code{lp@@
12404: tib - .}. It is shared with the locals stack and TIBs of files that
12405: include the current file. You can change the amount of space for TIBs
12406: and locals stack at Gforth startup with the command line option
12407: @code{-l}.
12408:
12409: @item size of the pictured numeric output buffer:
12410: @cindex size of the pictured numeric output buffer
12411: @cindex pictured numeric output buffer, size
12412: @code{PAD HERE - .}. 104 characters on 32-bit machines. The buffer is
12413: shared with @code{WORD}.
12414:
12415: @item size of the scratch area returned by @code{PAD}:
12416: @cindex size of the scratch area returned by @code{PAD}
12417: @cindex @code{PAD} size
12418: The remainder of dictionary space. @code{unused pad here - - .}.
12419:
12420: @item system case-sensitivity characteristics:
12421: @cindex case-sensitivity characteristics
1.26 crook 12422: Dictionary searches are case-insensitive (except in
1.1 anton 12423: @code{TABLE}s). However, as explained above under @i{character-set
12424: extensions}, the matching for non-ASCII characters is determined by the
12425: locale you are using. In the default @code{C} locale all non-ASCII
12426: characters are matched case-sensitively.
12427:
12428: @item system prompt:
12429: @cindex system prompt
12430: @cindex prompt
12431: @code{ ok} in interpret state, @code{ compiled} in compile state.
12432:
12433: @item division rounding:
12434: @cindex division rounding
12435: installation dependent. @code{s" floored" environment? drop .}. We leave
12436: the choice to @code{gcc} (what to use for @code{/}) and to you (whether
12437: to use @code{fm/mod}, @code{sm/rem} or simply @code{/}).
12438:
12439: @item values of @code{STATE} when true:
12440: @cindex @code{STATE} values
12441: -1.
12442:
12443: @item values returned after arithmetic overflow:
12444: On two's complement machines, arithmetic is performed modulo
12445: 2**bits-per-cell for single arithmetic and 4**bits-per-cell for double
12446: arithmetic (with appropriate mapping for signed types). Division by zero
12447: typically results in a @code{-55 throw} (Floating-point unidentified
1.80 anton 12448: fault) or @code{-10 throw} (divide by zero).
1.1 anton 12449:
12450: @item whether the current definition can be found after @t{DOES>}:
12451: @cindex @t{DOES>}, visibility of current definition
12452: No.
12453:
12454: @end table
12455:
12456: @c ---------------------------------------------------------------------
12457: @node core-ambcond, core-other, core-idef, The Core Words
12458: @subsection Ambiguous conditions
12459: @c ---------------------------------------------------------------------
12460: @cindex core words, ambiguous conditions
12461: @cindex ambiguous conditions, core words
12462:
12463: @table @i
12464:
12465: @item a name is neither a word nor a number:
12466: @cindex name not found
1.26 crook 12467: @cindex undefined word
1.80 anton 12468: @code{-13 throw} (Undefined word).
1.1 anton 12469:
12470: @item a definition name exceeds the maximum length allowed:
1.26 crook 12471: @cindex word name too long
1.1 anton 12472: @code{-19 throw} (Word name too long)
12473:
12474: @item addressing a region not inside the various data spaces of the forth system:
12475: @cindex Invalid memory address
1.32 anton 12476: The stacks, code space and header space are accessible. Machine code space is
1.1 anton 12477: typically readable. Accessing other addresses gives results dependent on
12478: the operating system. On decent systems: @code{-9 throw} (Invalid memory
12479: address).
12480:
12481: @item argument type incompatible with parameter:
1.26 crook 12482: @cindex argument type mismatch
1.1 anton 12483: This is usually not caught. Some words perform checks, e.g., the control
12484: flow words, and issue a @code{ABORT"} or @code{-12 THROW} (Argument type
12485: mismatch).
12486:
12487: @item attempting to obtain the execution token of a word with undefined execution semantics:
12488: @cindex Interpreting a compile-only word, for @code{'} etc.
12489: @cindex execution token of words with undefined execution semantics
12490: @code{-14 throw} (Interpreting a compile-only word). In some cases, you
12491: get an execution token for @code{compile-only-error} (which performs a
12492: @code{-14 throw} when executed).
12493:
12494: @item dividing by zero:
12495: @cindex dividing by zero
12496: @cindex floating point unidentified fault, integer division
1.80 anton 12497: On some platforms, this produces a @code{-10 throw} (Division by
1.24 anton 12498: zero); on other systems, this typically results in a @code{-55 throw}
12499: (Floating-point unidentified fault).
1.1 anton 12500:
12501: @item insufficient data stack or return stack space:
12502: @cindex insufficient data stack or return stack space
12503: @cindex stack overflow
1.26 crook 12504: @cindex address alignment exception, stack overflow
1.1 anton 12505: @cindex Invalid memory address, stack overflow
12506: Depending on the operating system, the installation, and the invocation
12507: of Gforth, this is either checked by the memory management hardware, or
1.24 anton 12508: it is not checked. If it is checked, you typically get a @code{-3 throw}
12509: (Stack overflow), @code{-5 throw} (Return stack overflow), or @code{-9
12510: throw} (Invalid memory address) (depending on the platform and how you
12511: achieved the overflow) as soon as the overflow happens. If it is not
12512: checked, overflows typically result in mysterious illegal memory
12513: accesses, producing @code{-9 throw} (Invalid memory address) or
12514: @code{-23 throw} (Address alignment exception); they might also destroy
12515: the internal data structure of @code{ALLOCATE} and friends, resulting in
12516: various errors in these words.
1.1 anton 12517:
12518: @item insufficient space for loop control parameters:
12519: @cindex insufficient space for loop control parameters
1.80 anton 12520: Like other return stack overflows.
1.1 anton 12521:
12522: @item insufficient space in the dictionary:
12523: @cindex insufficient space in the dictionary
12524: @cindex dictionary overflow
1.12 anton 12525: If you try to allot (either directly with @code{allot}, or indirectly
12526: with @code{,}, @code{create} etc.) more memory than available in the
12527: dictionary, you get a @code{-8 throw} (Dictionary overflow). If you try
12528: to access memory beyond the end of the dictionary, the results are
12529: similar to stack overflows.
1.1 anton 12530:
12531: @item interpreting a word with undefined interpretation semantics:
12532: @cindex interpreting a word with undefined interpretation semantics
12533: @cindex Interpreting a compile-only word
12534: For some words, we have defined interpretation semantics. For the
12535: others: @code{-14 throw} (Interpreting a compile-only word).
12536:
12537: @item modifying the contents of the input buffer or a string literal:
12538: @cindex modifying the contents of the input buffer or a string literal
12539: These are located in writable memory and can be modified.
12540:
12541: @item overflow of the pictured numeric output string:
12542: @cindex overflow of the pictured numeric output string
12543: @cindex pictured numeric output string, overflow
1.24 anton 12544: @code{-17 throw} (Pictured numeric ouput string overflow).
1.1 anton 12545:
12546: @item parsed string overflow:
12547: @cindex parsed string overflow
12548: @code{PARSE} cannot overflow. @code{WORD} does not check for overflow.
12549:
12550: @item producing a result out of range:
12551: @cindex result out of range
12552: On two's complement machines, arithmetic is performed modulo
12553: 2**bits-per-cell for single arithmetic and 4**bits-per-cell for double
12554: arithmetic (with appropriate mapping for signed types). Division by zero
1.24 anton 12555: typically results in a @code{-10 throw} (divide by zero) or @code{-55
12556: throw} (floating point unidentified fault). @code{convert} and
12557: @code{>number} currently overflow silently.
1.1 anton 12558:
12559: @item reading from an empty data or return stack:
12560: @cindex stack empty
12561: @cindex stack underflow
1.24 anton 12562: @cindex return stack underflow
1.1 anton 12563: The data stack is checked by the outer (aka text) interpreter after
12564: every word executed. If it has underflowed, a @code{-4 throw} (Stack
12565: underflow) is performed. Apart from that, stacks may be checked or not,
1.24 anton 12566: depending on operating system, installation, and invocation. If they are
12567: caught by a check, they typically result in @code{-4 throw} (Stack
12568: underflow), @code{-6 throw} (Return stack underflow) or @code{-9 throw}
12569: (Invalid memory address), depending on the platform and which stack
12570: underflows and by how much. Note that even if the system uses checking
12571: (through the MMU), your program may have to underflow by a significant
12572: number of stack items to trigger the reaction (the reason for this is
12573: that the MMU, and therefore the checking, works with a page-size
12574: granularity). If there is no checking, the symptoms resulting from an
12575: underflow are similar to those from an overflow. Unbalanced return
1.80 anton 12576: stack errors can result in a variety of symptoms, including @code{-9 throw}
1.24 anton 12577: (Invalid memory address) and Illegal Instruction (typically @code{-260
12578: throw}).
1.1 anton 12579:
12580: @item unexpected end of the input buffer, resulting in an attempt to use a zero-length string as a name:
12581: @cindex unexpected end of the input buffer
12582: @cindex zero-length string as a name
12583: @cindex Attempt to use zero-length string as a name
12584: @code{Create} and its descendants perform a @code{-16 throw} (Attempt to
12585: use zero-length string as a name). Words like @code{'} probably will not
12586: find what they search. Note that it is possible to create zero-length
12587: names with @code{nextname} (should it not?).
12588:
12589: @item @code{>IN} greater than input buffer:
12590: @cindex @code{>IN} greater than input buffer
12591: The next invocation of a parsing word returns a string with length 0.
12592:
12593: @item @code{RECURSE} appears after @code{DOES>}:
12594: @cindex @code{RECURSE} appears after @code{DOES>}
12595: Compiles a recursive call to the defining word, not to the defined word.
12596:
12597: @item argument input source different than current input source for @code{RESTORE-INPUT}:
12598: @cindex argument input source different than current input source for @code{RESTORE-INPUT}
1.26 crook 12599: @cindex argument type mismatch, @code{RESTORE-INPUT}
1.1 anton 12600: @cindex @code{RESTORE-INPUT}, Argument type mismatch
12601: @code{-12 THROW}. Note that, once an input file is closed (e.g., because
12602: the end of the file was reached), its source-id may be
12603: reused. Therefore, restoring an input source specification referencing a
12604: closed file may lead to unpredictable results instead of a @code{-12
12605: THROW}.
12606:
12607: In the future, Gforth may be able to restore input source specifications
12608: from other than the current input source.
12609:
12610: @item data space containing definitions gets de-allocated:
12611: @cindex data space containing definitions gets de-allocated
12612: Deallocation with @code{allot} is not checked. This typically results in
12613: memory access faults or execution of illegal instructions.
12614:
12615: @item data space read/write with incorrect alignment:
12616: @cindex data space read/write with incorrect alignment
12617: @cindex alignment faults
1.26 crook 12618: @cindex address alignment exception
1.1 anton 12619: Processor-dependent. Typically results in a @code{-23 throw} (Address
1.12 anton 12620: alignment exception). Under Linux-Intel on a 486 or later processor with
1.1 anton 12621: alignment turned on, incorrect alignment results in a @code{-9 throw}
12622: (Invalid memory address). There are reportedly some processors with
1.12 anton 12623: alignment restrictions that do not report violations.
1.1 anton 12624:
12625: @item data space pointer not properly aligned, @code{,}, @code{C,}:
12626: @cindex data space pointer not properly aligned, @code{,}, @code{C,}
12627: Like other alignment errors.
12628:
12629: @item less than u+2 stack items (@code{PICK} and @code{ROLL}):
12630: Like other stack underflows.
12631:
12632: @item loop control parameters not available:
12633: @cindex loop control parameters not available
12634: Not checked. The counted loop words simply assume that the top of return
12635: stack items are loop control parameters and behave accordingly.
12636:
12637: @item most recent definition does not have a name (@code{IMMEDIATE}):
12638: @cindex most recent definition does not have a name (@code{IMMEDIATE})
12639: @cindex last word was headerless
12640: @code{abort" last word was headerless"}.
12641:
12642: @item name not defined by @code{VALUE} used by @code{TO}:
12643: @cindex name not defined by @code{VALUE} used by @code{TO}
12644: @cindex @code{TO} on non-@code{VALUE}s
12645: @cindex Invalid name argument, @code{TO}
12646: @code{-32 throw} (Invalid name argument) (unless name is a local or was
12647: defined by @code{CONSTANT}; in the latter case it just changes the constant).
12648:
12649: @item name not found (@code{'}, @code{POSTPONE}, @code{[']}, @code{[COMPILE]}):
12650: @cindex name not found (@code{'}, @code{POSTPONE}, @code{[']}, @code{[COMPILE]})
1.26 crook 12651: @cindex undefined word, @code{'}, @code{POSTPONE}, @code{[']}, @code{[COMPILE]}
1.1 anton 12652: @code{-13 throw} (Undefined word)
12653:
12654: @item parameters are not of the same type (@code{DO}, @code{?DO}, @code{WITHIN}):
12655: @cindex parameters are not of the same type (@code{DO}, @code{?DO}, @code{WITHIN})
12656: Gforth behaves as if they were of the same type. I.e., you can predict
12657: the behaviour by interpreting all parameters as, e.g., signed.
12658:
12659: @item @code{POSTPONE} or @code{[COMPILE]} applied to @code{TO}:
12660: @cindex @code{POSTPONE} or @code{[COMPILE]} applied to @code{TO}
12661: Assume @code{: X POSTPONE TO ; IMMEDIATE}. @code{X} performs the
12662: compilation semantics of @code{TO}.
12663:
12664: @item String longer than a counted string returned by @code{WORD}:
1.26 crook 12665: @cindex string longer than a counted string returned by @code{WORD}
1.1 anton 12666: @cindex @code{WORD}, string overflow
12667: Not checked. The string will be ok, but the count will, of course,
12668: contain only the least significant bits of the length.
12669:
12670: @item u greater than or equal to the number of bits in a cell (@code{LSHIFT}, @code{RSHIFT}):
12671: @cindex @code{LSHIFT}, large shift counts
12672: @cindex @code{RSHIFT}, large shift counts
12673: Processor-dependent. Typical behaviours are returning 0 and using only
12674: the low bits of the shift count.
12675:
12676: @item word not defined via @code{CREATE}:
12677: @cindex @code{>BODY} of non-@code{CREATE}d words
12678: @code{>BODY} produces the PFA of the word no matter how it was defined.
12679:
12680: @cindex @code{DOES>} of non-@code{CREATE}d words
12681: @code{DOES>} changes the execution semantics of the last defined word no
12682: matter how it was defined. E.g., @code{CONSTANT DOES>} is equivalent to
12683: @code{CREATE , DOES>}.
12684:
12685: @item words improperly used outside @code{<#} and @code{#>}:
12686: Not checked. As usual, you can expect memory faults.
12687:
12688: @end table
12689:
12690:
12691: @c ---------------------------------------------------------------------
12692: @node core-other, , core-ambcond, The Core Words
12693: @subsection Other system documentation
12694: @c ---------------------------------------------------------------------
12695: @cindex other system documentation, core words
12696: @cindex core words, other system documentation
12697:
12698: @table @i
12699: @item nonstandard words using @code{PAD}:
12700: @cindex @code{PAD} use by nonstandard words
12701: None.
12702:
12703: @item operator's terminal facilities available:
12704: @cindex operator's terminal facilities available
1.80 anton 12705: After processing the OS's command line, Gforth goes into interactive mode,
1.1 anton 12706: and you can give commands to Gforth interactively. The actual facilities
12707: available depend on how you invoke Gforth.
12708:
12709: @item program data space available:
12710: @cindex program data space available
12711: @cindex data space available
12712: @code{UNUSED .} gives the remaining dictionary space. The total
12713: dictionary space can be specified with the @code{-m} switch
12714: (@pxref{Invoking Gforth}) when Gforth starts up.
12715:
12716: @item return stack space available:
12717: @cindex return stack space available
12718: You can compute the total return stack space in cells with
12719: @code{s" RETURN-STACK-CELLS" environment? drop .}. You can specify it at
12720: startup time with the @code{-r} switch (@pxref{Invoking Gforth}).
12721:
12722: @item stack space available:
12723: @cindex stack space available
12724: You can compute the total data stack space in cells with
12725: @code{s" STACK-CELLS" environment? drop .}. You can specify it at
12726: startup time with the @code{-d} switch (@pxref{Invoking Gforth}).
12727:
12728: @item system dictionary space required, in address units:
12729: @cindex system dictionary space required, in address units
12730: Type @code{here forthstart - .} after startup. At the time of this
12731: writing, this gives 80080 (bytes) on a 32-bit system.
12732: @end table
12733:
12734:
12735: @c =====================================================================
12736: @node The optional Block word set, The optional Double Number word set, The Core Words, ANS conformance
12737: @section The optional Block word set
12738: @c =====================================================================
12739: @cindex system documentation, block words
12740: @cindex block words, system documentation
12741:
12742: @menu
12743: * block-idef:: Implementation Defined Options
12744: * block-ambcond:: Ambiguous Conditions
12745: * block-other:: Other System Documentation
12746: @end menu
12747:
12748:
12749: @c ---------------------------------------------------------------------
12750: @node block-idef, block-ambcond, The optional Block word set, The optional Block word set
12751: @subsection Implementation Defined Options
12752: @c ---------------------------------------------------------------------
12753: @cindex implementation-defined options, block words
12754: @cindex block words, implementation-defined options
12755:
12756: @table @i
12757: @item the format for display by @code{LIST}:
12758: @cindex @code{LIST} display format
12759: First the screen number is displayed, then 16 lines of 64 characters,
12760: each line preceded by the line number.
12761:
12762: @item the length of a line affected by @code{\}:
12763: @cindex length of a line affected by @code{\}
12764: @cindex @code{\}, line length in blocks
12765: 64 characters.
12766: @end table
12767:
12768:
12769: @c ---------------------------------------------------------------------
12770: @node block-ambcond, block-other, block-idef, The optional Block word set
12771: @subsection Ambiguous conditions
12772: @c ---------------------------------------------------------------------
12773: @cindex block words, ambiguous conditions
12774: @cindex ambiguous conditions, block words
12775:
12776: @table @i
12777: @item correct block read was not possible:
12778: @cindex block read not possible
12779: Typically results in a @code{throw} of some OS-derived value (between
12780: -512 and -2048). If the blocks file was just not long enough, blanks are
12781: supplied for the missing portion.
12782:
12783: @item I/O exception in block transfer:
12784: @cindex I/O exception in block transfer
12785: @cindex block transfer, I/O exception
12786: Typically results in a @code{throw} of some OS-derived value (between
12787: -512 and -2048).
12788:
12789: @item invalid block number:
12790: @cindex invalid block number
12791: @cindex block number invalid
12792: @code{-35 throw} (Invalid block number)
12793:
12794: @item a program directly alters the contents of @code{BLK}:
12795: @cindex @code{BLK}, altering @code{BLK}
12796: The input stream is switched to that other block, at the same
12797: position. If the storing to @code{BLK} happens when interpreting
12798: non-block input, the system will get quite confused when the block ends.
12799:
12800: @item no current block buffer for @code{UPDATE}:
12801: @cindex @code{UPDATE}, no current block buffer
12802: @code{UPDATE} has no effect.
12803:
12804: @end table
12805:
12806: @c ---------------------------------------------------------------------
12807: @node block-other, , block-ambcond, The optional Block word set
12808: @subsection Other system documentation
12809: @c ---------------------------------------------------------------------
12810: @cindex other system documentation, block words
12811: @cindex block words, other system documentation
12812:
12813: @table @i
12814: @item any restrictions a multiprogramming system places on the use of buffer addresses:
12815: No restrictions (yet).
12816:
12817: @item the number of blocks available for source and data:
12818: depends on your disk space.
12819:
12820: @end table
12821:
12822:
12823: @c =====================================================================
12824: @node The optional Double Number word set, The optional Exception word set, The optional Block word set, ANS conformance
12825: @section The optional Double Number word set
12826: @c =====================================================================
12827: @cindex system documentation, double words
12828: @cindex double words, system documentation
12829:
12830: @menu
12831: * double-ambcond:: Ambiguous Conditions
12832: @end menu
12833:
12834:
12835: @c ---------------------------------------------------------------------
12836: @node double-ambcond, , The optional Double Number word set, The optional Double Number word set
12837: @subsection Ambiguous conditions
12838: @c ---------------------------------------------------------------------
12839: @cindex double words, ambiguous conditions
12840: @cindex ambiguous conditions, double words
12841:
12842: @table @i
1.29 crook 12843: @item @i{d} outside of range of @i{n} in @code{D>S}:
12844: @cindex @code{D>S}, @i{d} out of range of @i{n}
12845: The least significant cell of @i{d} is produced.
1.1 anton 12846:
12847: @end table
12848:
12849:
12850: @c =====================================================================
12851: @node The optional Exception word set, The optional Facility word set, The optional Double Number word set, ANS conformance
12852: @section The optional Exception word set
12853: @c =====================================================================
12854: @cindex system documentation, exception words
12855: @cindex exception words, system documentation
12856:
12857: @menu
12858: * exception-idef:: Implementation Defined Options
12859: @end menu
12860:
12861:
12862: @c ---------------------------------------------------------------------
12863: @node exception-idef, , The optional Exception word set, The optional Exception word set
12864: @subsection Implementation Defined Options
12865: @c ---------------------------------------------------------------------
12866: @cindex implementation-defined options, exception words
12867: @cindex exception words, implementation-defined options
12868:
12869: @table @i
12870: @item @code{THROW}-codes used in the system:
12871: @cindex @code{THROW}-codes used in the system
12872: The codes -256@minus{}-511 are used for reporting signals. The mapping
1.29 crook 12873: from OS signal numbers to throw codes is -256@minus{}@i{signal}. The
1.1 anton 12874: codes -512@minus{}-2047 are used for OS errors (for file and memory
12875: allocation operations). The mapping from OS error numbers to throw codes
12876: is -512@minus{}@code{errno}. One side effect of this mapping is that
12877: undefined OS errors produce a message with a strange number; e.g.,
12878: @code{-1000 THROW} results in @code{Unknown error 488} on my system.
12879: @end table
12880:
12881: @c =====================================================================
12882: @node The optional Facility word set, The optional File-Access word set, The optional Exception word set, ANS conformance
12883: @section The optional Facility word set
12884: @c =====================================================================
12885: @cindex system documentation, facility words
12886: @cindex facility words, system documentation
12887:
12888: @menu
12889: * facility-idef:: Implementation Defined Options
12890: * facility-ambcond:: Ambiguous Conditions
12891: @end menu
12892:
12893:
12894: @c ---------------------------------------------------------------------
12895: @node facility-idef, facility-ambcond, The optional Facility word set, The optional Facility word set
12896: @subsection Implementation Defined Options
12897: @c ---------------------------------------------------------------------
12898: @cindex implementation-defined options, facility words
12899: @cindex facility words, implementation-defined options
12900:
12901: @table @i
12902: @item encoding of keyboard events (@code{EKEY}):
12903: @cindex keyboard events, encoding in @code{EKEY}
12904: @cindex @code{EKEY}, encoding of keyboard events
1.40 anton 12905: Keys corresponding to ASCII characters are encoded as ASCII characters.
1.41 anton 12906: Other keys are encoded with the constants @code{k-left}, @code{k-right},
12907: @code{k-up}, @code{k-down}, @code{k-home}, @code{k-end}, @code{k1},
12908: @code{k2}, @code{k3}, @code{k4}, @code{k5}, @code{k6}, @code{k7},
12909: @code{k8}, @code{k9}, @code{k10}, @code{k11}, @code{k12}.
1.40 anton 12910:
1.1 anton 12911:
12912: @item duration of a system clock tick:
12913: @cindex duration of a system clock tick
12914: @cindex clock tick duration
12915: System dependent. With respect to @code{MS}, the time is specified in
12916: microseconds. How well the OS and the hardware implement this, is
12917: another question.
12918:
12919: @item repeatability to be expected from the execution of @code{MS}:
12920: @cindex repeatability to be expected from the execution of @code{MS}
12921: @cindex @code{MS}, repeatability to be expected
12922: System dependent. On Unix, a lot depends on load. If the system is
12923: lightly loaded, and the delay is short enough that Gforth does not get
12924: swapped out, the performance should be acceptable. Under MS-DOS and
12925: other single-tasking systems, it should be good.
12926:
12927: @end table
12928:
12929:
12930: @c ---------------------------------------------------------------------
12931: @node facility-ambcond, , facility-idef, The optional Facility word set
12932: @subsection Ambiguous conditions
12933: @c ---------------------------------------------------------------------
12934: @cindex facility words, ambiguous conditions
12935: @cindex ambiguous conditions, facility words
12936:
12937: @table @i
12938: @item @code{AT-XY} can't be performed on user output device:
12939: @cindex @code{AT-XY} can't be performed on user output device
12940: Largely terminal dependent. No range checks are done on the arguments.
12941: No errors are reported. You may see some garbage appearing, you may see
12942: simply nothing happen.
12943:
12944: @end table
12945:
12946:
12947: @c =====================================================================
12948: @node The optional File-Access word set, The optional Floating-Point word set, The optional Facility word set, ANS conformance
12949: @section The optional File-Access word set
12950: @c =====================================================================
12951: @cindex system documentation, file words
12952: @cindex file words, system documentation
12953:
12954: @menu
12955: * file-idef:: Implementation Defined Options
12956: * file-ambcond:: Ambiguous Conditions
12957: @end menu
12958:
12959: @c ---------------------------------------------------------------------
12960: @node file-idef, file-ambcond, The optional File-Access word set, The optional File-Access word set
12961: @subsection Implementation Defined Options
12962: @c ---------------------------------------------------------------------
12963: @cindex implementation-defined options, file words
12964: @cindex file words, implementation-defined options
12965:
12966: @table @i
12967: @item file access methods used:
12968: @cindex file access methods used
12969: @code{R/O}, @code{R/W} and @code{BIN} work as you would
12970: expect. @code{W/O} translates into the C file opening mode @code{w} (or
12971: @code{wb}): The file is cleared, if it exists, and created, if it does
12972: not (with both @code{open-file} and @code{create-file}). Under Unix
12973: @code{create-file} creates a file with 666 permissions modified by your
12974: umask.
12975:
12976: @item file exceptions:
12977: @cindex file exceptions
12978: The file words do not raise exceptions (except, perhaps, memory access
12979: faults when you pass illegal addresses or file-ids).
12980:
12981: @item file line terminator:
12982: @cindex file line terminator
12983: System-dependent. Gforth uses C's newline character as line
12984: terminator. What the actual character code(s) of this are is
12985: system-dependent.
12986:
12987: @item file name format:
12988: @cindex file name format
12989: System dependent. Gforth just uses the file name format of your OS.
12990:
12991: @item information returned by @code{FILE-STATUS}:
12992: @cindex @code{FILE-STATUS}, returned information
12993: @code{FILE-STATUS} returns the most powerful file access mode allowed
12994: for the file: Either @code{R/O}, @code{W/O} or @code{R/W}. If the file
12995: cannot be accessed, @code{R/O BIN} is returned. @code{BIN} is applicable
12996: along with the returned mode.
12997:
12998: @item input file state after an exception when including source:
12999: @cindex exception when including source
13000: All files that are left via the exception are closed.
13001:
1.29 crook 13002: @item @i{ior} values and meaning:
13003: @cindex @i{ior} values and meaning
1.68 anton 13004: @cindex @i{wior} values and meaning
1.29 crook 13005: The @i{ior}s returned by the file and memory allocation words are
1.1 anton 13006: intended as throw codes. They typically are in the range
13007: -512@minus{}-2047 of OS errors. The mapping from OS error numbers to
1.29 crook 13008: @i{ior}s is -512@minus{}@i{errno}.
1.1 anton 13009:
13010: @item maximum depth of file input nesting:
13011: @cindex maximum depth of file input nesting
13012: @cindex file input nesting, maximum depth
13013: limited by the amount of return stack, locals/TIB stack, and the number
13014: of open files available. This should not give you troubles.
13015:
13016: @item maximum size of input line:
13017: @cindex maximum size of input line
13018: @cindex input line size, maximum
13019: @code{/line}. Currently 255.
13020:
13021: @item methods of mapping block ranges to files:
13022: @cindex mapping block ranges to files
13023: @cindex files containing blocks
13024: @cindex blocks in files
13025: By default, blocks are accessed in the file @file{blocks.fb} in the
13026: current working directory. The file can be switched with @code{USE}.
13027:
13028: @item number of string buffers provided by @code{S"}:
13029: @cindex @code{S"}, number of string buffers
13030: 1
13031:
13032: @item size of string buffer used by @code{S"}:
13033: @cindex @code{S"}, size of string buffer
13034: @code{/line}. currently 255.
13035:
13036: @end table
13037:
13038: @c ---------------------------------------------------------------------
13039: @node file-ambcond, , file-idef, The optional File-Access word set
13040: @subsection Ambiguous conditions
13041: @c ---------------------------------------------------------------------
13042: @cindex file words, ambiguous conditions
13043: @cindex ambiguous conditions, file words
13044:
13045: @table @i
13046: @item attempting to position a file outside its boundaries:
13047: @cindex @code{REPOSITION-FILE}, outside the file's boundaries
13048: @code{REPOSITION-FILE} is performed as usual: Afterwards,
13049: @code{FILE-POSITION} returns the value given to @code{REPOSITION-FILE}.
13050:
13051: @item attempting to read from file positions not yet written:
13052: @cindex reading from file positions not yet written
13053: End-of-file, i.e., zero characters are read and no error is reported.
13054:
1.29 crook 13055: @item @i{file-id} is invalid (@code{INCLUDE-FILE}):
13056: @cindex @code{INCLUDE-FILE}, @i{file-id} is invalid
1.1 anton 13057: An appropriate exception may be thrown, but a memory fault or other
13058: problem is more probable.
13059:
1.29 crook 13060: @item I/O exception reading or closing @i{file-id} (@code{INCLUDE-FILE}, @code{INCLUDED}):
13061: @cindex @code{INCLUDE-FILE}, I/O exception reading or closing @i{file-id}
13062: @cindex @code{INCLUDED}, I/O exception reading or closing @i{file-id}
13063: The @i{ior} produced by the operation, that discovered the problem, is
1.1 anton 13064: thrown.
13065:
13066: @item named file cannot be opened (@code{INCLUDED}):
13067: @cindex @code{INCLUDED}, named file cannot be opened
1.29 crook 13068: The @i{ior} produced by @code{open-file} is thrown.
1.1 anton 13069:
13070: @item requesting an unmapped block number:
13071: @cindex unmapped block numbers
13072: There are no unmapped legal block numbers. On some operating systems,
13073: writing a block with a large number may overflow the file system and
13074: have an error message as consequence.
13075:
13076: @item using @code{source-id} when @code{blk} is non-zero:
13077: @cindex @code{SOURCE-ID}, behaviour when @code{BLK} is non-zero
13078: @code{source-id} performs its function. Typically it will give the id of
13079: the source which loaded the block. (Better ideas?)
13080:
13081: @end table
13082:
13083:
13084: @c =====================================================================
13085: @node The optional Floating-Point word set, The optional Locals word set, The optional File-Access word set, ANS conformance
13086: @section The optional Floating-Point word set
13087: @c =====================================================================
13088: @cindex system documentation, floating-point words
13089: @cindex floating-point words, system documentation
13090:
13091: @menu
13092: * floating-idef:: Implementation Defined Options
13093: * floating-ambcond:: Ambiguous Conditions
13094: @end menu
13095:
13096:
13097: @c ---------------------------------------------------------------------
13098: @node floating-idef, floating-ambcond, The optional Floating-Point word set, The optional Floating-Point word set
13099: @subsection Implementation Defined Options
13100: @c ---------------------------------------------------------------------
13101: @cindex implementation-defined options, floating-point words
13102: @cindex floating-point words, implementation-defined options
13103:
13104: @table @i
13105: @item format and range of floating point numbers:
13106: @cindex format and range of floating point numbers
13107: @cindex floating point numbers, format and range
13108: System-dependent; the @code{double} type of C.
13109:
1.29 crook 13110: @item results of @code{REPRESENT} when @i{float} is out of range:
13111: @cindex @code{REPRESENT}, results when @i{float} is out of range
1.1 anton 13112: System dependent; @code{REPRESENT} is implemented using the C library
13113: function @code{ecvt()} and inherits its behaviour in this respect.
13114:
13115: @item rounding or truncation of floating-point numbers:
13116: @cindex rounding of floating-point numbers
13117: @cindex truncation of floating-point numbers
13118: @cindex floating-point numbers, rounding or truncation
13119: System dependent; the rounding behaviour is inherited from the hosting C
13120: compiler. IEEE-FP-based (i.e., most) systems by default round to
13121: nearest, and break ties by rounding to even (i.e., such that the last
13122: bit of the mantissa is 0).
13123:
13124: @item size of floating-point stack:
13125: @cindex floating-point stack size
13126: @code{s" FLOATING-STACK" environment? drop .} gives the total size of
13127: the floating-point stack (in floats). You can specify this on startup
13128: with the command-line option @code{-f} (@pxref{Invoking Gforth}).
13129:
13130: @item width of floating-point stack:
13131: @cindex floating-point stack width
13132: @code{1 floats}.
13133:
13134: @end table
13135:
13136:
13137: @c ---------------------------------------------------------------------
13138: @node floating-ambcond, , floating-idef, The optional Floating-Point word set
13139: @subsection Ambiguous conditions
13140: @c ---------------------------------------------------------------------
13141: @cindex floating-point words, ambiguous conditions
13142: @cindex ambiguous conditions, floating-point words
13143:
13144: @table @i
13145: @item @code{df@@} or @code{df!} used with an address that is not double-float aligned:
13146: @cindex @code{df@@} or @code{df!} used with an address that is not double-float aligned
13147: System-dependent. Typically results in a @code{-23 THROW} like other
13148: alignment violations.
13149:
13150: @item @code{f@@} or @code{f!} used with an address that is not float aligned:
13151: @cindex @code{f@@} used with an address that is not float aligned
13152: @cindex @code{f!} used with an address that is not float aligned
13153: System-dependent. Typically results in a @code{-23 THROW} like other
13154: alignment violations.
13155:
13156: @item floating-point result out of range:
13157: @cindex floating-point result out of range
1.80 anton 13158: System-dependent. Can result in a @code{-43 throw} (floating point
13159: overflow), @code{-54 throw} (floating point underflow), @code{-41 throw}
13160: (floating point inexact result), @code{-55 THROW} (Floating-point
1.1 anton 13161: unidentified fault), or can produce a special value representing, e.g.,
13162: Infinity.
13163:
13164: @item @code{sf@@} or @code{sf!} used with an address that is not single-float aligned:
13165: @cindex @code{sf@@} or @code{sf!} used with an address that is not single-float aligned
13166: System-dependent. Typically results in an alignment fault like other
13167: alignment violations.
13168:
1.35 anton 13169: @item @code{base} is not decimal (@code{REPRESENT}, @code{F.}, @code{FE.}, @code{FS.}):
13170: @cindex @code{base} is not decimal (@code{REPRESENT}, @code{F.}, @code{FE.}, @code{FS.})
1.1 anton 13171: The floating-point number is converted into decimal nonetheless.
13172:
13173: @item Both arguments are equal to zero (@code{FATAN2}):
13174: @cindex @code{FATAN2}, both arguments are equal to zero
13175: System-dependent. @code{FATAN2} is implemented using the C library
13176: function @code{atan2()}.
13177:
1.29 crook 13178: @item Using @code{FTAN} on an argument @i{r1} where cos(@i{r1}) is zero:
13179: @cindex @code{FTAN} on an argument @i{r1} where cos(@i{r1}) is zero
13180: System-dependent. Anyway, typically the cos of @i{r1} will not be zero
1.1 anton 13181: because of small errors and the tan will be a very large (or very small)
13182: but finite number.
13183:
1.29 crook 13184: @item @i{d} cannot be presented precisely as a float in @code{D>F}:
13185: @cindex @code{D>F}, @i{d} cannot be presented precisely as a float
1.1 anton 13186: The result is rounded to the nearest float.
13187:
13188: @item dividing by zero:
13189: @cindex dividing by zero, floating-point
13190: @cindex floating-point dividing by zero
13191: @cindex floating-point unidentified fault, FP divide-by-zero
1.80 anton 13192: Platform-dependent; can produce an Infinity, NaN, @code{-42 throw}
13193: (floating point divide by zero) or @code{-55 throw} (Floating-point
13194: unidentified fault).
1.1 anton 13195:
13196: @item exponent too big for conversion (@code{DF!}, @code{DF@@}, @code{SF!}, @code{SF@@}):
13197: @cindex exponent too big for conversion (@code{DF!}, @code{DF@@}, @code{SF!}, @code{SF@@})
13198: System dependent. On IEEE-FP based systems the number is converted into
13199: an infinity.
13200:
1.29 crook 13201: @item @i{float}<1 (@code{FACOSH}):
13202: @cindex @code{FACOSH}, @i{float}<1
1.1 anton 13203: @cindex floating-point unidentified fault, @code{FACOSH}
1.80 anton 13204: Platform-dependent; on IEEE-FP systems typically produces a NaN.
1.1 anton 13205:
1.29 crook 13206: @item @i{float}=<-1 (@code{FLNP1}):
13207: @cindex @code{FLNP1}, @i{float}=<-1
1.1 anton 13208: @cindex floating-point unidentified fault, @code{FLNP1}
1.80 anton 13209: Platform-dependent; on IEEE-FP systems typically produces a NaN (or a
13210: negative infinity for @i{float}=-1).
1.1 anton 13211:
1.29 crook 13212: @item @i{float}=<0 (@code{FLN}, @code{FLOG}):
13213: @cindex @code{FLN}, @i{float}=<0
13214: @cindex @code{FLOG}, @i{float}=<0
1.1 anton 13215: @cindex floating-point unidentified fault, @code{FLN} or @code{FLOG}
1.80 anton 13216: Platform-dependent; on IEEE-FP systems typically produces a NaN (or a
13217: negative infinity for @i{float}=0).
1.1 anton 13218:
1.29 crook 13219: @item @i{float}<0 (@code{FASINH}, @code{FSQRT}):
13220: @cindex @code{FASINH}, @i{float}<0
13221: @cindex @code{FSQRT}, @i{float}<0
1.1 anton 13222: @cindex floating-point unidentified fault, @code{FASINH} or @code{FSQRT}
1.80 anton 13223: Platform-dependent; for @code{fsqrt} this typically gives a NaN, for
13224: @code{fasinh} some platforms produce a NaN, others a number (bug in the
13225: C library?).
1.1 anton 13226:
1.29 crook 13227: @item |@i{float}|>1 (@code{FACOS}, @code{FASIN}, @code{FATANH}):
13228: @cindex @code{FACOS}, |@i{float}|>1
13229: @cindex @code{FASIN}, |@i{float}|>1
13230: @cindex @code{FATANH}, |@i{float}|>1
1.1 anton 13231: @cindex floating-point unidentified fault, @code{FACOS}, @code{FASIN} or @code{FATANH}
1.80 anton 13232: Platform-dependent; IEEE-FP systems typically produce a NaN.
1.1 anton 13233:
1.29 crook 13234: @item integer part of float cannot be represented by @i{d} in @code{F>D}:
13235: @cindex @code{F>D}, integer part of float cannot be represented by @i{d}
1.1 anton 13236: @cindex floating-point unidentified fault, @code{F>D}
1.80 anton 13237: Platform-dependent; typically, some double number is produced and no
13238: error is reported.
1.1 anton 13239:
13240: @item string larger than pictured numeric output area (@code{f.}, @code{fe.}, @code{fs.}):
13241: @cindex string larger than pictured numeric output area (@code{f.}, @code{fe.}, @code{fs.})
1.80 anton 13242: @code{Precision} characters of the numeric output area are used. If
13243: @code{precision} is too high, these words will smash the data or code
13244: close to @code{here}.
1.1 anton 13245: @end table
13246:
13247: @c =====================================================================
13248: @node The optional Locals word set, The optional Memory-Allocation word set, The optional Floating-Point word set, ANS conformance
13249: @section The optional Locals word set
13250: @c =====================================================================
13251: @cindex system documentation, locals words
13252: @cindex locals words, system documentation
13253:
13254: @menu
13255: * locals-idef:: Implementation Defined Options
13256: * locals-ambcond:: Ambiguous Conditions
13257: @end menu
13258:
13259:
13260: @c ---------------------------------------------------------------------
13261: @node locals-idef, locals-ambcond, The optional Locals word set, The optional Locals word set
13262: @subsection Implementation Defined Options
13263: @c ---------------------------------------------------------------------
13264: @cindex implementation-defined options, locals words
13265: @cindex locals words, implementation-defined options
13266:
13267: @table @i
13268: @item maximum number of locals in a definition:
13269: @cindex maximum number of locals in a definition
13270: @cindex locals, maximum number in a definition
13271: @code{s" #locals" environment? drop .}. Currently 15. This is a lower
13272: bound, e.g., on a 32-bit machine there can be 41 locals of up to 8
13273: characters. The number of locals in a definition is bounded by the size
13274: of locals-buffer, which contains the names of the locals.
13275:
13276: @end table
13277:
13278:
13279: @c ---------------------------------------------------------------------
13280: @node locals-ambcond, , locals-idef, The optional Locals word set
13281: @subsection Ambiguous conditions
13282: @c ---------------------------------------------------------------------
13283: @cindex locals words, ambiguous conditions
13284: @cindex ambiguous conditions, locals words
13285:
13286: @table @i
13287: @item executing a named local in interpretation state:
13288: @cindex local in interpretation state
13289: @cindex Interpreting a compile-only word, for a local
13290: Locals have no interpretation semantics. If you try to perform the
13291: interpretation semantics, you will get a @code{-14 throw} somewhere
13292: (Interpreting a compile-only word). If you perform the compilation
13293: semantics, the locals access will be compiled (irrespective of state).
13294:
1.29 crook 13295: @item @i{name} not defined by @code{VALUE} or @code{(LOCAL)} (@code{TO}):
1.1 anton 13296: @cindex name not defined by @code{VALUE} or @code{(LOCAL)} used by @code{TO}
13297: @cindex @code{TO} on non-@code{VALUE}s and non-locals
13298: @cindex Invalid name argument, @code{TO}
13299: @code{-32 throw} (Invalid name argument)
13300:
13301: @end table
13302:
13303:
13304: @c =====================================================================
13305: @node The optional Memory-Allocation word set, The optional Programming-Tools word set, The optional Locals word set, ANS conformance
13306: @section The optional Memory-Allocation word set
13307: @c =====================================================================
13308: @cindex system documentation, memory-allocation words
13309: @cindex memory-allocation words, system documentation
13310:
13311: @menu
13312: * memory-idef:: Implementation Defined Options
13313: @end menu
13314:
13315:
13316: @c ---------------------------------------------------------------------
13317: @node memory-idef, , The optional Memory-Allocation word set, The optional Memory-Allocation word set
13318: @subsection Implementation Defined Options
13319: @c ---------------------------------------------------------------------
13320: @cindex implementation-defined options, memory-allocation words
13321: @cindex memory-allocation words, implementation-defined options
13322:
13323: @table @i
1.29 crook 13324: @item values and meaning of @i{ior}:
13325: @cindex @i{ior} values and meaning
13326: The @i{ior}s returned by the file and memory allocation words are
1.1 anton 13327: intended as throw codes. They typically are in the range
13328: -512@minus{}-2047 of OS errors. The mapping from OS error numbers to
1.29 crook 13329: @i{ior}s is -512@minus{}@i{errno}.
1.1 anton 13330:
13331: @end table
13332:
13333: @c =====================================================================
13334: @node The optional Programming-Tools word set, The optional Search-Order word set, The optional Memory-Allocation word set, ANS conformance
13335: @section The optional Programming-Tools word set
13336: @c =====================================================================
13337: @cindex system documentation, programming-tools words
13338: @cindex programming-tools words, system documentation
13339:
13340: @menu
13341: * programming-idef:: Implementation Defined Options
13342: * programming-ambcond:: Ambiguous Conditions
13343: @end menu
13344:
13345:
13346: @c ---------------------------------------------------------------------
13347: @node programming-idef, programming-ambcond, The optional Programming-Tools word set, The optional Programming-Tools word set
13348: @subsection Implementation Defined Options
13349: @c ---------------------------------------------------------------------
13350: @cindex implementation-defined options, programming-tools words
13351: @cindex programming-tools words, implementation-defined options
13352:
13353: @table @i
13354: @item ending sequence for input following @code{;CODE} and @code{CODE}:
13355: @cindex @code{;CODE} ending sequence
13356: @cindex @code{CODE} ending sequence
13357: @code{END-CODE}
13358:
13359: @item manner of processing input following @code{;CODE} and @code{CODE}:
13360: @cindex @code{;CODE}, processing input
13361: @cindex @code{CODE}, processing input
13362: The @code{ASSEMBLER} vocabulary is pushed on the search order stack, and
13363: the input is processed by the text interpreter, (starting) in interpret
13364: state.
13365:
13366: @item search order capability for @code{EDITOR} and @code{ASSEMBLER}:
13367: @cindex @code{ASSEMBLER}, search order capability
13368: The ANS Forth search order word set.
13369:
13370: @item source and format of display by @code{SEE}:
13371: @cindex @code{SEE}, source and format of output
1.80 anton 13372: The source for @code{see} is the executable code used by the inner
1.1 anton 13373: interpreter. The current @code{see} tries to output Forth source code
1.80 anton 13374: (and on some platforms, assembly code for primitives) as well as
13375: possible.
1.1 anton 13376:
13377: @end table
13378:
13379: @c ---------------------------------------------------------------------
13380: @node programming-ambcond, , programming-idef, The optional Programming-Tools word set
13381: @subsection Ambiguous conditions
13382: @c ---------------------------------------------------------------------
13383: @cindex programming-tools words, ambiguous conditions
13384: @cindex ambiguous conditions, programming-tools words
13385:
13386: @table @i
13387:
1.21 crook 13388: @item deleting the compilation word list (@code{FORGET}):
13389: @cindex @code{FORGET}, deleting the compilation word list
1.1 anton 13390: Not implemented (yet).
13391:
1.29 crook 13392: @item fewer than @i{u}+1 items on the control-flow stack (@code{CS-PICK}, @code{CS-ROLL}):
13393: @cindex @code{CS-PICK}, fewer than @i{u}+1 items on the control flow-stack
13394: @cindex @code{CS-ROLL}, fewer than @i{u}+1 items on the control flow-stack
1.1 anton 13395: @cindex control-flow stack underflow
13396: This typically results in an @code{abort"} with a descriptive error
13397: message (may change into a @code{-22 throw} (Control structure mismatch)
13398: in the future). You may also get a memory access error. If you are
13399: unlucky, this ambiguous condition is not caught.
13400:
1.29 crook 13401: @item @i{name} can't be found (@code{FORGET}):
13402: @cindex @code{FORGET}, @i{name} can't be found
1.1 anton 13403: Not implemented (yet).
13404:
1.29 crook 13405: @item @i{name} not defined via @code{CREATE}:
13406: @cindex @code{;CODE}, @i{name} not defined via @code{CREATE}
1.1 anton 13407: @code{;CODE} behaves like @code{DOES>} in this respect, i.e., it changes
13408: the execution semantics of the last defined word no matter how it was
13409: defined.
13410:
13411: @item @code{POSTPONE} applied to @code{[IF]}:
13412: @cindex @code{POSTPONE} applied to @code{[IF]}
13413: @cindex @code{[IF]} and @code{POSTPONE}
13414: After defining @code{: X POSTPONE [IF] ; IMMEDIATE}. @code{X} is
13415: equivalent to @code{[IF]}.
13416:
13417: @item reaching the end of the input source before matching @code{[ELSE]} or @code{[THEN]}:
13418: @cindex @code{[IF]}, end of the input source before matching @code{[ELSE]} or @code{[THEN]}
13419: Continue in the same state of conditional compilation in the next outer
13420: input source. Currently there is no warning to the user about this.
13421:
13422: @item removing a needed definition (@code{FORGET}):
13423: @cindex @code{FORGET}, removing a needed definition
13424: Not implemented (yet).
13425:
13426: @end table
13427:
13428:
13429: @c =====================================================================
13430: @node The optional Search-Order word set, , The optional Programming-Tools word set, ANS conformance
13431: @section The optional Search-Order word set
13432: @c =====================================================================
13433: @cindex system documentation, search-order words
13434: @cindex search-order words, system documentation
13435:
13436: @menu
13437: * search-idef:: Implementation Defined Options
13438: * search-ambcond:: Ambiguous Conditions
13439: @end menu
13440:
13441:
13442: @c ---------------------------------------------------------------------
13443: @node search-idef, search-ambcond, The optional Search-Order word set, The optional Search-Order word set
13444: @subsection Implementation Defined Options
13445: @c ---------------------------------------------------------------------
13446: @cindex implementation-defined options, search-order words
13447: @cindex search-order words, implementation-defined options
13448:
13449: @table @i
13450: @item maximum number of word lists in search order:
13451: @cindex maximum number of word lists in search order
13452: @cindex search order, maximum depth
13453: @code{s" wordlists" environment? drop .}. Currently 16.
13454:
13455: @item minimum search order:
13456: @cindex minimum search order
13457: @cindex search order, minimum
13458: @code{root root}.
13459:
13460: @end table
13461:
13462: @c ---------------------------------------------------------------------
13463: @node search-ambcond, , search-idef, The optional Search-Order word set
13464: @subsection Ambiguous conditions
13465: @c ---------------------------------------------------------------------
13466: @cindex search-order words, ambiguous conditions
13467: @cindex ambiguous conditions, search-order words
13468:
13469: @table @i
1.21 crook 13470: @item changing the compilation word list (during compilation):
13471: @cindex changing the compilation word list (during compilation)
13472: @cindex compilation word list, change before definition ends
13473: The word is entered into the word list that was the compilation word list
1.1 anton 13474: at the start of the definition. Any changes to the name field (e.g.,
13475: @code{immediate}) or the code field (e.g., when executing @code{DOES>})
1.116 anton 13476: are applied to the latest defined word (as reported by @code{latest} or
13477: @code{latestxt}), if possible, irrespective of the compilation word list.
1.1 anton 13478:
13479: @item search order empty (@code{previous}):
13480: @cindex @code{previous}, search order empty
1.26 crook 13481: @cindex vocstack empty, @code{previous}
1.1 anton 13482: @code{abort" Vocstack empty"}.
13483:
13484: @item too many word lists in search order (@code{also}):
13485: @cindex @code{also}, too many word lists in search order
1.26 crook 13486: @cindex vocstack full, @code{also}
1.1 anton 13487: @code{abort" Vocstack full"}.
13488:
13489: @end table
13490:
13491: @c ***************************************************************
1.65 anton 13492: @node Standard vs Extensions, Model, ANS conformance, Top
13493: @chapter Should I use Gforth extensions?
13494: @cindex Gforth extensions
13495:
13496: As you read through the rest of this manual, you will see documentation
13497: for @i{Standard} words, and documentation for some appealing Gforth
13498: @i{extensions}. You might ask yourself the question: @i{``Should I
13499: restrict myself to the standard, or should I use the extensions?''}
13500:
13501: The answer depends on the goals you have for the program you are working
13502: on:
13503:
13504: @itemize @bullet
13505:
13506: @item Is it just for yourself or do you want to share it with others?
13507:
13508: @item
13509: If you want to share it, do the others all use Gforth?
13510:
13511: @item
13512: If it is just for yourself, do you want to restrict yourself to Gforth?
13513:
13514: @end itemize
13515:
13516: If restricting the program to Gforth is ok, then there is no reason not
13517: to use extensions. It is still a good idea to keep to the standard
13518: where it is easy, in case you want to reuse these parts in another
13519: program that you want to be portable.
13520:
13521: If you want to be able to port the program to other Forth systems, there
13522: are the following points to consider:
13523:
13524: @itemize @bullet
13525:
13526: @item
13527: Most Forth systems that are being maintained support the ANS Forth
13528: standard. So if your program complies with the standard, it will be
13529: portable among many systems.
13530:
13531: @item
13532: A number of the Gforth extensions can be implemented in ANS Forth using
13533: public-domain files provided in the @file{compat/} directory. These are
13534: mentioned in the text in passing. There is no reason not to use these
13535: extensions, your program will still be ANS Forth compliant; just include
13536: the appropriate compat files with your program.
13537:
13538: @item
13539: The tool @file{ans-report.fs} (@pxref{ANS Report}) makes it easy to
13540: analyse your program and determine what non-Standard words it relies
13541: upon. However, it does not check whether you use standard words in a
13542: non-standard way.
13543:
13544: @item
13545: Some techniques are not standardized by ANS Forth, and are hard or
13546: impossible to implement in a standard way, but can be implemented in
13547: most Forth systems easily, and usually in similar ways (e.g., accessing
13548: word headers). Forth has a rich historical precedent for programmers
13549: taking advantage of implementation-dependent features of their tools
13550: (for example, relying on a knowledge of the dictionary
13551: structure). Sometimes these techniques are necessary to extract every
13552: last bit of performance from the hardware, sometimes they are just a
13553: programming shorthand.
13554:
13555: @item
13556: Does using a Gforth extension save more work than the porting this part
13557: to other Forth systems (if any) will cost?
13558:
13559: @item
13560: Is the additional functionality worth the reduction in portability and
13561: the additional porting problems?
13562:
13563: @end itemize
13564:
13565: In order to perform these consideratios, you need to know what's
13566: standard and what's not. This manual generally states if something is
1.81 anton 13567: non-standard, but the authoritative source is the
13568: @uref{http://www.taygeta.com/forth/dpans.html,standard document}.
1.65 anton 13569: Appendix A of the Standard (@var{Rationale}) provides a valuable insight
13570: into the thought processes of the technical committee.
13571:
13572: Note also that portability between Forth systems is not the only
13573: portability issue; there is also the issue of portability between
13574: different platforms (processor/OS combinations).
13575:
13576: @c ***************************************************************
13577: @node Model, Integrating Gforth, Standard vs Extensions, Top
1.1 anton 13578: @chapter Model
13579:
13580: This chapter has yet to be written. It will contain information, on
13581: which internal structures you can rely.
13582:
13583: @c ***************************************************************
13584: @node Integrating Gforth, Emacs and Gforth, Model, Top
13585: @chapter Integrating Gforth into C programs
13586:
13587: This is not yet implemented.
13588:
13589: Several people like to use Forth as scripting language for applications
13590: that are otherwise written in C, C++, or some other language.
13591:
13592: The Forth system ATLAST provides facilities for embedding it into
13593: applications; unfortunately it has several disadvantages: most
13594: importantly, it is not based on ANS Forth, and it is apparently dead
13595: (i.e., not developed further and not supported). The facilities
1.21 crook 13596: provided by Gforth in this area are inspired by ATLAST's facilities, so
1.1 anton 13597: making the switch should not be hard.
13598:
13599: We also tried to design the interface such that it can easily be
13600: implemented by other Forth systems, so that we may one day arrive at a
13601: standardized interface. Such a standard interface would allow you to
13602: replace the Forth system without having to rewrite C code.
13603:
13604: You embed the Gforth interpreter by linking with the library
13605: @code{libgforth.a} (give the compiler the option @code{-lgforth}). All
13606: global symbols in this library that belong to the interface, have the
13607: prefix @code{forth_}. (Global symbols that are used internally have the
13608: prefix @code{gforth_}).
13609:
13610: You can include the declarations of Forth types and the functions and
13611: variables of the interface with @code{#include <forth.h>}.
13612:
13613: Types.
13614:
13615: Variables.
13616:
13617: Data and FP Stack pointer. Area sizes.
13618:
13619: functions.
13620:
13621: forth_init(imagefile)
13622: forth_evaluate(string) exceptions?
13623: forth_goto(address) (or forth_execute(xt)?)
13624: forth_continue() (a corountining mechanism)
13625:
13626: Adding primitives.
13627:
13628: No checking.
13629:
13630: Signals?
13631:
13632: Accessing the Stacks
13633:
1.26 crook 13634: @c ******************************************************************
1.1 anton 13635: @node Emacs and Gforth, Image Files, Integrating Gforth, Top
13636: @chapter Emacs and Gforth
13637: @cindex Emacs and Gforth
13638:
13639: @cindex @file{gforth.el}
13640: @cindex @file{forth.el}
13641: @cindex Rydqvist, Goran
1.107 dvdkhlng 13642: @cindex Kuehling, David
1.1 anton 13643: @cindex comment editing commands
13644: @cindex @code{\}, editing with Emacs
13645: @cindex debug tracer editing commands
13646: @cindex @code{~~}, removal with Emacs
13647: @cindex Forth mode in Emacs
1.107 dvdkhlng 13648:
1.1 anton 13649: Gforth comes with @file{gforth.el}, an improved version of
13650: @file{forth.el} by Goran Rydqvist (included in the TILE package). The
1.26 crook 13651: improvements are:
13652:
13653: @itemize @bullet
13654: @item
1.107 dvdkhlng 13655: A better handling of indentation.
13656: @item
13657: A custom hilighting engine for Forth-code.
1.26 crook 13658: @item
13659: Comment paragraph filling (@kbd{M-q})
13660: @item
13661: Commenting (@kbd{C-x \}) and uncommenting (@kbd{C-u C-x \}) of regions
13662: @item
13663: Removal of debugging tracers (@kbd{C-x ~}, @pxref{Debugging}).
1.41 anton 13664: @item
13665: Support of the @code{info-lookup} feature for looking up the
13666: documentation of a word.
1.107 dvdkhlng 13667: @item
13668: Support for reading and writing blocks files.
1.26 crook 13669: @end itemize
13670:
1.107 dvdkhlng 13671: To get a basic description of these features, enter Forth mode and
13672: type @kbd{C-h m}.
1.1 anton 13673:
13674: @cindex source location of error or debugging output in Emacs
13675: @cindex error output, finding the source location in Emacs
13676: @cindex debugging output, finding the source location in Emacs
13677: In addition, Gforth supports Emacs quite well: The source code locations
13678: given in error messages, debugging output (from @code{~~}) and failed
13679: assertion messages are in the right format for Emacs' compilation mode
13680: (@pxref{Compilation, , Running Compilations under Emacs, emacs, Emacs
13681: Manual}) so the source location corresponding to an error or other
13682: message is only a few keystrokes away (@kbd{C-x `} for the next error,
13683: @kbd{C-c C-c} for the error under the cursor).
13684:
1.107 dvdkhlng 13685: @cindex viewing the documentation of a word in Emacs
13686: @cindex context-sensitive help
13687: Moreover, for words documented in this manual, you can look up the
13688: glossary entry quickly by using @kbd{C-h TAB}
13689: (@code{info-lookup-symbol}, @pxref{Documentation, ,Documentation
13690: Commands, emacs, Emacs Manual}). This feature requires Emacs 20.3 or
13691: later and does not work for words containing @code{:}.
13692:
13693: @menu
13694: * Installing gforth.el:: Making Emacs aware of Forth.
13695: * Emacs Tags:: Viewing the source of a word in Emacs.
13696: * Hilighting:: Making Forth code look prettier.
13697: * Auto-Indentation:: Customizing auto-indentation.
13698: * Blocks Files:: Reading and writing blocks files.
13699: @end menu
13700:
13701: @c ----------------------------------
1.109 anton 13702: @node Installing gforth.el, Emacs Tags, Emacs and Gforth, Emacs and Gforth
1.107 dvdkhlng 13703: @section Installing gforth.el
13704: @cindex @file{.emacs}
13705: @cindex @file{gforth.el}, installation
13706: To make the features from @file{gforth.el} available in Emacs, add
13707: the following lines to your @file{.emacs} file:
13708:
13709: @example
13710: (autoload 'forth-mode "gforth.el")
13711: (setq auto-mode-alist (cons '("\\.fs\\'" . forth-mode)
13712: auto-mode-alist))
13713: (autoload 'forth-block-mode "gforth.el")
13714: (setq auto-mode-alist (cons '("\\.fb\\'" . forth-block-mode)
13715: auto-mode-alist))
13716: (add-hook 'forth-mode-hook (function (lambda ()
13717: ;; customize variables here:
13718: (setq forth-indent-level 4)
13719: (setq forth-minor-indent-level 2)
13720: (setq forth-hilight-level 3)
13721: ;;; ...
13722: )))
13723: @end example
13724:
13725: @c ----------------------------------
13726: @node Emacs Tags, Hilighting, Installing gforth.el, Emacs and Gforth
13727: @section Emacs Tags
1.1 anton 13728: @cindex @file{TAGS} file
13729: @cindex @file{etags.fs}
13730: @cindex viewing the source of a word in Emacs
1.43 anton 13731: @cindex @code{require}, placement in files
13732: @cindex @code{include}, placement in files
1.107 dvdkhlng 13733: If you @code{require} @file{etags.fs}, a new @file{TAGS} file will be
13734: produced (@pxref{Tags, , Tags Tables, emacs, Emacs Manual}) that
1.1 anton 13735: contains the definitions of all words defined afterwards. You can then
1.107 dvdkhlng 13736: find the source for a word using @kbd{M-.}. Note that Emacs can use
1.1 anton 13737: several tags files at the same time (e.g., one for the Gforth sources
13738: and one for your program, @pxref{Select Tags Table,,Selecting a Tags
13739: Table,emacs, Emacs Manual}). The TAGS file for the preloaded words is
13740: @file{$(datadir)/gforth/$(VERSION)/TAGS} (e.g.,
1.43 anton 13741: @file{/usr/local/share/gforth/0.2.0/TAGS}). To get the best behaviour
13742: with @file{etags.fs}, you should avoid putting definitions both before
13743: and after @code{require} etc., otherwise you will see the same file
13744: visited several times by commands like @code{tags-search}.
1.1 anton 13745:
1.107 dvdkhlng 13746: @c ----------------------------------
13747: @node Hilighting, Auto-Indentation, Emacs Tags, Emacs and Gforth
13748: @section Hilighting
13749: @cindex hilighting Forth code in Emacs
13750: @cindex highlighting Forth code in Emacs
13751: @file{gforth.el} comes with a custom source hilighting engine. When
13752: you open a file in @code{forth-mode}, it will be completely parsed,
13753: assigning faces to keywords, comments, strings etc. While you edit
13754: the file, modified regions get parsed and updated on-the-fly.
13755:
13756: Use the variable `forth-hilight-level' to change the level of
13757: decoration from 0 (no hilighting at all) to 3 (the default). Even if
13758: you set the hilighting level to 0, the parser will still work in the
13759: background, collecting information about whether regions of text are
13760: ``compiled'' or ``interpreted''. Those information are required for
13761: auto-indentation to work properly. Set `forth-disable-parser' to
13762: non-nil if your computer is too slow to handle parsing. This will
13763: have an impact on the smartness of the auto-indentation engine,
13764: though.
13765:
13766: Sometimes Forth sources define new features that should be hilighted,
13767: new control structures, defining-words etc. You can use the variable
13768: `forth-custom-words' to make @code{forth-mode} hilight additional
13769: words and constructs. See the docstring of `forth-words' for details
13770: (in Emacs, type @kbd{C-h v forth-words}).
13771:
13772: `forth-custom-words' is meant to be customized in your
13773: @file{.emacs} file. To customize hilighing in a file-specific manner,
13774: set `forth-local-words' in a local-variables section at the end of
13775: your source file (@pxref{Local Variables in Files,, Variables, emacs, Emacs Manual}).
13776:
13777: Example:
13778: @example
13779: 0 [IF]
13780: Local Variables:
13781: forth-local-words:
13782: ((("t:") definition-starter (font-lock-keyword-face . 1)
13783: "[ \t\n]" t name (font-lock-function-name-face . 3))
13784: ((";t") definition-ender (font-lock-keyword-face . 1)))
13785: End:
13786: [THEN]
13787: @end example
13788:
13789: @c ----------------------------------
13790: @node Auto-Indentation, Blocks Files, Hilighting, Emacs and Gforth
13791: @section Auto-Indentation
13792: @cindex auto-indentation of Forth code in Emacs
13793: @cindex indentation of Forth code in Emacs
13794: @code{forth-mode} automatically tries to indent lines in a smart way,
13795: whenever you type @key{TAB} or break a line with @kbd{C-m}.
13796:
13797: Simple customization can be achieved by setting
13798: `forth-indent-level' and `forth-minor-indent-level' in your
13799: @file{.emacs} file. For historical reasons @file{gforth.el} indents
13800: per default by multiples of 4 columns. To use the more traditional
13801: 3-column indentation, add the following lines to your @file{.emacs}:
13802:
13803: @example
13804: (add-hook 'forth-mode-hook (function (lambda ()
13805: ;; customize variables here:
13806: (setq forth-indent-level 3)
13807: (setq forth-minor-indent-level 1)
13808: )))
13809: @end example
13810:
13811: If you want indentation to recognize non-default words, customize it
13812: by setting `forth-custom-indent-words' in your @file{.emacs}. See the
13813: docstring of `forth-indent-words' for details (in Emacs, type @kbd{C-h
13814: v forth-indent-words}).
13815:
13816: To customize indentation in a file-specific manner, set
13817: `forth-local-indent-words' in a local-variables section at the end of
13818: your source file (@pxref{Local Variables in Files, Variables,,emacs,
13819: Emacs Manual}).
13820:
13821: Example:
13822: @example
13823: 0 [IF]
13824: Local Variables:
13825: forth-local-indent-words:
13826: ((("t:") (0 . 2) (0 . 2))
13827: ((";t") (-2 . 0) (0 . -2)))
13828: End:
13829: [THEN]
13830: @end example
13831:
13832: @c ----------------------------------
1.109 anton 13833: @node Blocks Files, , Auto-Indentation, Emacs and Gforth
1.107 dvdkhlng 13834: @section Blocks Files
13835: @cindex blocks files, use with Emacs
13836: @code{forth-mode} Autodetects blocks files by checking whether the
13837: length of the first line exceeds 1023 characters. It then tries to
13838: convert the file into normal text format. When you save the file, it
13839: will be written to disk as normal stream-source file.
13840:
13841: If you want to write blocks files, use @code{forth-blocks-mode}. It
13842: inherits all the features from @code{forth-mode}, plus some additions:
1.41 anton 13843:
1.107 dvdkhlng 13844: @itemize @bullet
13845: @item
13846: Files are written to disk in blocks file format.
13847: @item
13848: Screen numbers are displayed in the mode line (enumerated beginning
13849: with the value of `forth-block-base')
13850: @item
13851: Warnings are displayed when lines exceed 64 characters.
13852: @item
13853: The beginning of the currently edited block is marked with an
13854: overlay-arrow.
13855: @end itemize
1.41 anton 13856:
1.107 dvdkhlng 13857: There are some restrictions you should be aware of. When you open a
13858: blocks file that contains tabulator or newline characters, these
13859: characters will be translated into spaces when the file is written
13860: back to disk. If tabs or newlines are encountered during blocks file
13861: reading, an error is output to the echo area. So have a look at the
13862: `*Messages*' buffer, when Emacs' bell rings during reading.
1.1 anton 13863:
1.107 dvdkhlng 13864: Please consult the docstring of @code{forth-blocks-mode} for more
13865: information by typing @kbd{C-h v forth-blocks-mode}).
1.1 anton 13866:
1.26 crook 13867: @c ******************************************************************
1.1 anton 13868: @node Image Files, Engine, Emacs and Gforth, Top
13869: @chapter Image Files
1.26 crook 13870: @cindex image file
13871: @cindex @file{.fi} files
1.1 anton 13872: @cindex precompiled Forth code
13873: @cindex dictionary in persistent form
13874: @cindex persistent form of dictionary
13875:
13876: An image file is a file containing an image of the Forth dictionary,
13877: i.e., compiled Forth code and data residing in the dictionary. By
13878: convention, we use the extension @code{.fi} for image files.
13879:
13880: @menu
1.18 anton 13881: * Image Licensing Issues:: Distribution terms for images.
13882: * Image File Background:: Why have image files?
1.67 anton 13883: * Non-Relocatable Image Files:: don't always work.
1.18 anton 13884: * Data-Relocatable Image Files:: are better.
1.67 anton 13885: * Fully Relocatable Image Files:: better yet.
1.18 anton 13886: * Stack and Dictionary Sizes:: Setting the default sizes for an image.
1.29 crook 13887: * Running Image Files:: @code{gforth -i @i{file}} or @i{file}.
1.18 anton 13888: * Modifying the Startup Sequence:: and turnkey applications.
1.1 anton 13889: @end menu
13890:
1.18 anton 13891: @node Image Licensing Issues, Image File Background, Image Files, Image Files
13892: @section Image Licensing Issues
13893: @cindex license for images
13894: @cindex image license
13895:
13896: An image created with @code{gforthmi} (@pxref{gforthmi}) or
13897: @code{savesystem} (@pxref{Non-Relocatable Image Files}) includes the
13898: original image; i.e., according to copyright law it is a derived work of
13899: the original image.
13900:
13901: Since Gforth is distributed under the GNU GPL, the newly created image
13902: falls under the GNU GPL, too. In particular, this means that if you
13903: distribute the image, you have to make all of the sources for the image
1.113 anton 13904: available, including those you wrote. For details see @ref{Copying, ,
1.18 anton 13905: GNU General Public License (Section 3)}.
13906:
13907: If you create an image with @code{cross} (@pxref{cross.fs}), the image
13908: contains only code compiled from the sources you gave it; if none of
13909: these sources is under the GPL, the terms discussed above do not apply
13910: to the image. However, if your image needs an engine (a gforth binary)
13911: that is under the GPL, you should make sure that you distribute both in
13912: a way that is at most a @emph{mere aggregation}, if you don't want the
13913: terms of the GPL to apply to the image.
13914:
13915: @node Image File Background, Non-Relocatable Image Files, Image Licensing Issues, Image Files
1.1 anton 13916: @section Image File Background
13917: @cindex image file background
13918:
1.80 anton 13919: Gforth consists not only of primitives (in the engine), but also of
1.1 anton 13920: definitions written in Forth. Since the Forth compiler itself belongs to
13921: those definitions, it is not possible to start the system with the
1.80 anton 13922: engine and the Forth source alone. Therefore we provide the Forth
1.26 crook 13923: code as an image file in nearly executable form. When Gforth starts up,
13924: a C routine loads the image file into memory, optionally relocates the
13925: addresses, then sets up the memory (stacks etc.) according to
13926: information in the image file, and (finally) starts executing Forth
13927: code.
1.1 anton 13928:
13929: The image file variants represent different compromises between the
13930: goals of making it easy to generate image files and making them
13931: portable.
13932:
13933: @cindex relocation at run-time
1.26 crook 13934: Win32Forth 3.4 and Mitch Bradley's @code{cforth} use relocation at
1.1 anton 13935: run-time. This avoids many of the complications discussed below (image
13936: files are data relocatable without further ado), but costs performance
13937: (one addition per memory access).
13938:
13939: @cindex relocation at load-time
1.26 crook 13940: By contrast, the Gforth loader performs relocation at image load time. The
13941: loader also has to replace tokens that represent primitive calls with the
1.1 anton 13942: appropriate code-field addresses (or code addresses in the case of
13943: direct threading).
13944:
13945: There are three kinds of image files, with different degrees of
13946: relocatability: non-relocatable, data-relocatable, and fully relocatable
13947: image files.
13948:
13949: @cindex image file loader
13950: @cindex relocating loader
13951: @cindex loader for image files
13952: These image file variants have several restrictions in common; they are
13953: caused by the design of the image file loader:
13954:
13955: @itemize @bullet
13956: @item
13957: There is only one segment; in particular, this means, that an image file
13958: cannot represent @code{ALLOCATE}d memory chunks (and pointers to
1.26 crook 13959: them). The contents of the stacks are not represented, either.
1.1 anton 13960:
13961: @item
13962: The only kinds of relocation supported are: adding the same offset to
13963: all cells that represent data addresses; and replacing special tokens
13964: with code addresses or with pieces of machine code.
13965:
13966: If any complex computations involving addresses are performed, the
13967: results cannot be represented in the image file. Several applications that
13968: use such computations come to mind:
13969: @itemize @minus
13970: @item
13971: Hashing addresses (or data structures which contain addresses) for table
13972: lookup. If you use Gforth's @code{table}s or @code{wordlist}s for this
13973: purpose, you will have no problem, because the hash tables are
13974: recomputed automatically when the system is started. If you use your own
13975: hash tables, you will have to do something similar.
13976:
13977: @item
13978: There's a cute implementation of doubly-linked lists that uses
13979: @code{XOR}ed addresses. You could represent such lists as singly-linked
13980: in the image file, and restore the doubly-linked representation on
13981: startup.@footnote{In my opinion, though, you should think thrice before
13982: using a doubly-linked list (whatever implementation).}
13983:
13984: @item
13985: The code addresses of run-time routines like @code{docol:} cannot be
13986: represented in the image file (because their tokens would be replaced by
13987: machine code in direct threaded implementations). As a workaround,
13988: compute these addresses at run-time with @code{>code-address} from the
13989: executions tokens of appropriate words (see the definitions of
1.80 anton 13990: @code{docol:} and friends in @file{kernel/getdoers.fs}).
1.1 anton 13991:
13992: @item
13993: On many architectures addresses are represented in machine code in some
13994: shifted or mangled form. You cannot put @code{CODE} words that contain
13995: absolute addresses in this form in a relocatable image file. Workarounds
13996: are representing the address in some relative form (e.g., relative to
13997: the CFA, which is present in some register), or loading the address from
13998: a place where it is stored in a non-mangled form.
13999: @end itemize
14000: @end itemize
14001:
14002: @node Non-Relocatable Image Files, Data-Relocatable Image Files, Image File Background, Image Files
14003: @section Non-Relocatable Image Files
14004: @cindex non-relocatable image files
1.26 crook 14005: @cindex image file, non-relocatable
1.1 anton 14006:
14007: These files are simple memory dumps of the dictionary. They are specific
14008: to the executable (i.e., @file{gforth} file) they were created
14009: with. What's worse, they are specific to the place on which the
14010: dictionary resided when the image was created. Now, there is no
14011: guarantee that the dictionary will reside at the same place the next
14012: time you start Gforth, so there's no guarantee that a non-relocatable
14013: image will work the next time (Gforth will complain instead of crashing,
14014: though).
14015:
14016: You can create a non-relocatable image file with
14017:
1.44 crook 14018:
1.1 anton 14019: doc-savesystem
14020:
1.44 crook 14021:
1.1 anton 14022: @node Data-Relocatable Image Files, Fully Relocatable Image Files, Non-Relocatable Image Files, Image Files
14023: @section Data-Relocatable Image Files
14024: @cindex data-relocatable image files
1.26 crook 14025: @cindex image file, data-relocatable
1.1 anton 14026:
14027: These files contain relocatable data addresses, but fixed code addresses
14028: (instead of tokens). They are specific to the executable (i.e.,
14029: @file{gforth} file) they were created with. For direct threading on some
14030: architectures (e.g., the i386), data-relocatable images do not work. You
14031: get a data-relocatable image, if you use @file{gforthmi} with a
14032: Gforth binary that is not doubly indirect threaded (@pxref{Fully
14033: Relocatable Image Files}).
14034:
14035: @node Fully Relocatable Image Files, Stack and Dictionary Sizes, Data-Relocatable Image Files, Image Files
14036: @section Fully Relocatable Image Files
14037: @cindex fully relocatable image files
1.26 crook 14038: @cindex image file, fully relocatable
1.1 anton 14039:
14040: @cindex @file{kern*.fi}, relocatability
14041: @cindex @file{gforth.fi}, relocatability
14042: These image files have relocatable data addresses, and tokens for code
14043: addresses. They can be used with different binaries (e.g., with and
14044: without debugging) on the same machine, and even across machines with
14045: the same data formats (byte order, cell size, floating point
14046: format). However, they are usually specific to the version of Gforth
14047: they were created with. The files @file{gforth.fi} and @file{kernl*.fi}
14048: are fully relocatable.
14049:
14050: There are two ways to create a fully relocatable image file:
14051:
14052: @menu
1.29 crook 14053: * gforthmi:: The normal way
1.1 anton 14054: * cross.fs:: The hard way
14055: @end menu
14056:
14057: @node gforthmi, cross.fs, Fully Relocatable Image Files, Fully Relocatable Image Files
14058: @subsection @file{gforthmi}
14059: @cindex @file{comp-i.fs}
14060: @cindex @file{gforthmi}
14061:
14062: You will usually use @file{gforthmi}. If you want to create an
1.29 crook 14063: image @i{file} that contains everything you would load by invoking
14064: Gforth with @code{gforth @i{options}}, you simply say:
1.1 anton 14065: @example
1.29 crook 14066: gforthmi @i{file} @i{options}
1.1 anton 14067: @end example
14068:
14069: E.g., if you want to create an image @file{asm.fi} that has the file
14070: @file{asm.fs} loaded in addition to the usual stuff, you could do it
14071: like this:
14072:
14073: @example
14074: gforthmi asm.fi asm.fs
14075: @end example
14076:
1.27 crook 14077: @file{gforthmi} is implemented as a sh script and works like this: It
14078: produces two non-relocatable images for different addresses and then
14079: compares them. Its output reflects this: first you see the output (if
1.62 crook 14080: any) of the two Gforth invocations that produce the non-relocatable image
1.27 crook 14081: files, then you see the output of the comparing program: It displays the
14082: offset used for data addresses and the offset used for code addresses;
1.1 anton 14083: moreover, for each cell that cannot be represented correctly in the
1.44 crook 14084: image files, it displays a line like this:
1.1 anton 14085:
14086: @example
14087: 78DC BFFFFA50 BFFFFA40
14088: @end example
14089:
14090: This means that at offset $78dc from @code{forthstart}, one input image
14091: contains $bffffa50, and the other contains $bffffa40. Since these cells
14092: cannot be represented correctly in the output image, you should examine
14093: these places in the dictionary and verify that these cells are dead
14094: (i.e., not read before they are written).
1.39 anton 14095:
14096: @cindex --application, @code{gforthmi} option
14097: If you insert the option @code{--application} in front of the image file
14098: name, you will get an image that uses the @code{--appl-image} option
14099: instead of the @code{--image-file} option (@pxref{Invoking
14100: Gforth}). When you execute such an image on Unix (by typing the image
14101: name as command), the Gforth engine will pass all options to the image
14102: instead of trying to interpret them as engine options.
1.1 anton 14103:
1.27 crook 14104: If you type @file{gforthmi} with no arguments, it prints some usage
14105: instructions.
14106:
1.1 anton 14107: @cindex @code{savesystem} during @file{gforthmi}
14108: @cindex @code{bye} during @file{gforthmi}
14109: @cindex doubly indirect threaded code
1.44 crook 14110: @cindex environment variables
14111: @cindex @code{GFORTHD} -- environment variable
14112: @cindex @code{GFORTH} -- environment variable
1.1 anton 14113: @cindex @code{gforth-ditc}
1.29 crook 14114: There are a few wrinkles: After processing the passed @i{options}, the
1.1 anton 14115: words @code{savesystem} and @code{bye} must be visible. A special doubly
14116: indirect threaded version of the @file{gforth} executable is used for
1.62 crook 14117: creating the non-relocatable images; you can pass the exact filename of
1.1 anton 14118: this executable through the environment variable @code{GFORTHD}
14119: (default: @file{gforth-ditc}); if you pass a version that is not doubly
14120: indirect threaded, you will not get a fully relocatable image, but a
1.27 crook 14121: data-relocatable image (because there is no code address offset). The
14122: normal @file{gforth} executable is used for creating the relocatable
14123: image; you can pass the exact filename of this executable through the
14124: environment variable @code{GFORTH}.
1.1 anton 14125:
14126: @node cross.fs, , gforthmi, Fully Relocatable Image Files
14127: @subsection @file{cross.fs}
14128: @cindex @file{cross.fs}
14129: @cindex cross-compiler
14130: @cindex metacompiler
1.47 crook 14131: @cindex target compiler
1.1 anton 14132:
14133: You can also use @code{cross}, a batch compiler that accepts a Forth-like
1.47 crook 14134: programming language (@pxref{Cross Compiler}).
1.1 anton 14135:
1.47 crook 14136: @code{cross} allows you to create image files for machines with
1.1 anton 14137: different data sizes and data formats than the one used for generating
14138: the image file. You can also use it to create an application image that
14139: does not contain a Forth compiler. These features are bought with
14140: restrictions and inconveniences in programming. E.g., addresses have to
14141: be stored in memory with special words (@code{A!}, @code{A,}, etc.) in
14142: order to make the code relocatable.
14143:
14144:
14145: @node Stack and Dictionary Sizes, Running Image Files, Fully Relocatable Image Files, Image Files
14146: @section Stack and Dictionary Sizes
14147: @cindex image file, stack and dictionary sizes
14148: @cindex dictionary size default
14149: @cindex stack size default
14150:
14151: If you invoke Gforth with a command line flag for the size
14152: (@pxref{Invoking Gforth}), the size you specify is stored in the
14153: dictionary. If you save the dictionary with @code{savesystem} or create
14154: an image with @file{gforthmi}, this size will become the default
14155: for the resulting image file. E.g., the following will create a
1.21 crook 14156: fully relocatable version of @file{gforth.fi} with a 1MB dictionary:
1.1 anton 14157:
14158: @example
14159: gforthmi gforth.fi -m 1M
14160: @end example
14161:
14162: In other words, if you want to set the default size for the dictionary
14163: and the stacks of an image, just invoke @file{gforthmi} with the
14164: appropriate options when creating the image.
14165:
14166: @cindex stack size, cache-friendly
14167: Note: For cache-friendly behaviour (i.e., good performance), you should
14168: make the sizes of the stacks modulo, say, 2K, somewhat different. E.g.,
14169: the default stack sizes are: data: 16k (mod 2k=0); fp: 15.5k (mod
14170: 2k=1.5k); return: 15k(mod 2k=1k); locals: 14.5k (mod 2k=0.5k).
14171:
14172: @node Running Image Files, Modifying the Startup Sequence, Stack and Dictionary Sizes, Image Files
14173: @section Running Image Files
14174: @cindex running image files
14175: @cindex invoking image files
14176: @cindex image file invocation
14177:
14178: @cindex -i, invoke image file
14179: @cindex --image file, invoke image file
1.29 crook 14180: You can invoke Gforth with an image file @i{image} instead of the
1.1 anton 14181: default @file{gforth.fi} with the @code{-i} flag (@pxref{Invoking Gforth}):
14182: @example
1.29 crook 14183: gforth -i @i{image}
1.1 anton 14184: @end example
14185:
14186: @cindex executable image file
1.26 crook 14187: @cindex image file, executable
1.1 anton 14188: If your operating system supports starting scripts with a line of the
14189: form @code{#! ...}, you just have to type the image file name to start
14190: Gforth with this image file (note that the file extension @code{.fi} is
1.29 crook 14191: just a convention). I.e., to run Gforth with the image file @i{image},
14192: you can just type @i{image} instead of @code{gforth -i @i{image}}.
1.27 crook 14193: This works because every @code{.fi} file starts with a line of this
14194: format:
14195:
14196: @example
14197: #! /usr/local/bin/gforth-0.4.0 -i
14198: @end example
14199:
14200: The file and pathname for the Gforth engine specified on this line is
14201: the specific Gforth executable that it was built against; i.e. the value
14202: of the environment variable @code{GFORTH} at the time that
14203: @file{gforthmi} was executed.
1.1 anton 14204:
1.27 crook 14205: You can make use of the same shell capability to make a Forth source
14206: file into an executable. For example, if you place this text in a file:
1.26 crook 14207:
14208: @example
14209: #! /usr/local/bin/gforth
14210:
14211: ." Hello, world" CR
14212: bye
14213: @end example
14214:
14215: @noindent
1.27 crook 14216: and then make the file executable (chmod +x in Unix), you can run it
1.26 crook 14217: directly from the command line. The sequence @code{#!} is used in two
14218: ways; firstly, it is recognised as a ``magic sequence'' by the operating
1.29 crook 14219: system@footnote{The Unix kernel actually recognises two types of files:
14220: executable files and files of data, where the data is processed by an
14221: interpreter that is specified on the ``interpreter line'' -- the first
14222: line of the file, starting with the sequence #!. There may be a small
14223: limit (e.g., 32) on the number of characters that may be specified on
14224: the interpreter line.} secondly it is treated as a comment character by
14225: Gforth. Because of the second usage, a space is required between
1.80 anton 14226: @code{#!} and the path to the executable (moreover, some Unixes
14227: require the sequence @code{#! /}).
1.27 crook 14228:
14229: The disadvantage of this latter technique, compared with using
1.80 anton 14230: @file{gforthmi}, is that it is slightly slower; the Forth source code is
14231: compiled on-the-fly, each time the program is invoked.
1.26 crook 14232:
1.1 anton 14233: doc-#!
14234:
1.44 crook 14235:
1.1 anton 14236: @node Modifying the Startup Sequence, , Running Image Files, Image Files
14237: @section Modifying the Startup Sequence
14238: @cindex startup sequence for image file
14239: @cindex image file initialization sequence
14240: @cindex initialization sequence of image file
14241:
1.120 ! anton 14242: You can add your own initialization to the startup sequence of an image
! 14243: through the deferred word @code{'cold}. @code{'cold} is invoked just
! 14244: before the image-specific command line processing (i.e., loading files
! 14245: and evaluating (@code{-e}) strings) starts.
1.1 anton 14246:
14247: A sequence for adding your initialization usually looks like this:
14248:
14249: @example
14250: :noname
14251: Defers 'cold \ do other initialization stuff (e.g., rehashing wordlists)
14252: ... \ your stuff
14253: ; IS 'cold
14254: @end example
14255:
14256: @cindex turnkey image files
1.26 crook 14257: @cindex image file, turnkey applications
1.1 anton 14258: You can make a turnkey image by letting @code{'cold} execute a word
14259: (your turnkey application) that never returns; instead, it exits Gforth
14260: via @code{bye} or @code{throw}.
14261:
14262: @cindex command-line arguments, access
14263: @cindex arguments on the command line, access
14264: You can access the (image-specific) command-line arguments through the
1.26 crook 14265: variables @code{argc} and @code{argv}. @code{arg} provides convenient
1.1 anton 14266: access to @code{argv}.
14267:
1.26 crook 14268: If @code{'cold} exits normally, Gforth processes the command-line
14269: arguments as files to be loaded and strings to be evaluated. Therefore,
14270: @code{'cold} should remove the arguments it has used in this case.
14271:
1.44 crook 14272:
14273:
1.26 crook 14274: doc-'cold
1.1 anton 14275: doc-argc
14276: doc-argv
14277: doc-arg
14278:
14279:
1.44 crook 14280:
1.1 anton 14281: @c ******************************************************************
1.113 anton 14282: @node Engine, Cross Compiler, Image Files, Top
1.1 anton 14283: @chapter Engine
14284: @cindex engine
14285: @cindex virtual machine
14286:
1.26 crook 14287: Reading this chapter is not necessary for programming with Gforth. It
1.1 anton 14288: may be helpful for finding your way in the Gforth sources.
14289:
1.109 anton 14290: The ideas in this section have also been published in the following
14291: papers: Bernd Paysan, @cite{ANS fig/GNU/??? Forth} (in German),
14292: Forth-Tagung '93; M. Anton Ertl,
14293: @cite{@uref{http://www.complang.tuwien.ac.at/papers/ertl93.ps.Z, A
14294: Portable Forth Engine}}, EuroForth '93; M. Anton Ertl,
14295: @cite{@uref{http://www.complang.tuwien.ac.at/papers/ertl02.ps.gz,
14296: Threaded code variations and optimizations (extended version)}},
14297: Forth-Tagung '02.
1.1 anton 14298:
14299: @menu
14300: * Portability::
14301: * Threading::
14302: * Primitives::
14303: * Performance::
14304: @end menu
14305:
14306: @node Portability, Threading, Engine, Engine
14307: @section Portability
14308: @cindex engine portability
14309:
1.26 crook 14310: An important goal of the Gforth Project is availability across a wide
14311: range of personal machines. fig-Forth, and, to a lesser extent, F83,
14312: achieved this goal by manually coding the engine in assembly language
14313: for several then-popular processors. This approach is very
14314: labor-intensive and the results are short-lived due to progress in
14315: computer architecture.
1.1 anton 14316:
14317: @cindex C, using C for the engine
14318: Others have avoided this problem by coding in C, e.g., Mitch Bradley
14319: (cforth), Mikael Patel (TILE) and Dirk Zoller (pfe). This approach is
14320: particularly popular for UNIX-based Forths due to the large variety of
14321: architectures of UNIX machines. Unfortunately an implementation in C
14322: does not mix well with the goals of efficiency and with using
14323: traditional techniques: Indirect or direct threading cannot be expressed
14324: in C, and switch threading, the fastest technique available in C, is
14325: significantly slower. Another problem with C is that it is very
14326: cumbersome to express double integer arithmetic.
14327:
14328: @cindex GNU C for the engine
14329: @cindex long long
14330: Fortunately, there is a portable language that does not have these
14331: limitations: GNU C, the version of C processed by the GNU C compiler
14332: (@pxref{C Extensions, , Extensions to the C Language Family, gcc.info,
14333: GNU C Manual}). Its labels as values feature (@pxref{Labels as Values, ,
14334: Labels as Values, gcc.info, GNU C Manual}) makes direct and indirect
14335: threading possible, its @code{long long} type (@pxref{Long Long, ,
14336: Double-Word Integers, gcc.info, GNU C Manual}) corresponds to Forth's
1.109 anton 14337: double numbers on many systems. GNU C is freely available on all
1.1 anton 14338: important (and many unimportant) UNIX machines, VMS, 80386s running
14339: MS-DOS, the Amiga, and the Atari ST, so a Forth written in GNU C can run
14340: on all these machines.
14341:
14342: Writing in a portable language has the reputation of producing code that
14343: is slower than assembly. For our Forth engine we repeatedly looked at
14344: the code produced by the compiler and eliminated most compiler-induced
14345: inefficiencies by appropriate changes in the source code.
14346:
14347: @cindex explicit register declarations
14348: @cindex --enable-force-reg, configuration flag
14349: @cindex -DFORCE_REG
14350: However, register allocation cannot be portably influenced by the
14351: programmer, leading to some inefficiencies on register-starved
14352: machines. We use explicit register declarations (@pxref{Explicit Reg
14353: Vars, , Variables in Specified Registers, gcc.info, GNU C Manual}) to
14354: improve the speed on some machines. They are turned on by using the
14355: configuration flag @code{--enable-force-reg} (@code{gcc} switch
14356: @code{-DFORCE_REG}). Unfortunately, this feature not only depends on the
14357: machine, but also on the compiler version: On some machines some
14358: compiler versions produce incorrect code when certain explicit register
14359: declarations are used. So by default @code{-DFORCE_REG} is not used.
14360:
14361: @node Threading, Primitives, Portability, Engine
14362: @section Threading
14363: @cindex inner interpreter implementation
14364: @cindex threaded code implementation
14365:
14366: @cindex labels as values
14367: GNU C's labels as values extension (available since @code{gcc-2.0},
14368: @pxref{Labels as Values, , Labels as Values, gcc.info, GNU C Manual})
1.29 crook 14369: makes it possible to take the address of @i{label} by writing
14370: @code{&&@i{label}}. This address can then be used in a statement like
14371: @code{goto *@i{address}}. I.e., @code{goto *&&x} is the same as
1.1 anton 14372: @code{goto x}.
14373:
1.26 crook 14374: @cindex @code{NEXT}, indirect threaded
1.1 anton 14375: @cindex indirect threaded inner interpreter
14376: @cindex inner interpreter, indirect threaded
1.26 crook 14377: With this feature an indirect threaded @code{NEXT} looks like:
1.1 anton 14378: @example
14379: cfa = *ip++;
14380: ca = *cfa;
14381: goto *ca;
14382: @end example
14383: @cindex instruction pointer
14384: For those unfamiliar with the names: @code{ip} is the Forth instruction
14385: pointer; the @code{cfa} (code-field address) corresponds to ANS Forths
14386: execution token and points to the code field of the next word to be
14387: executed; The @code{ca} (code address) fetched from there points to some
14388: executable code, e.g., a primitive or the colon definition handler
14389: @code{docol}.
14390:
1.26 crook 14391: @cindex @code{NEXT}, direct threaded
1.1 anton 14392: @cindex direct threaded inner interpreter
14393: @cindex inner interpreter, direct threaded
14394: Direct threading is even simpler:
14395: @example
14396: ca = *ip++;
14397: goto *ca;
14398: @end example
14399:
14400: Of course we have packaged the whole thing neatly in macros called
1.26 crook 14401: @code{NEXT} and @code{NEXT1} (the part of @code{NEXT} after fetching the cfa).
1.1 anton 14402:
14403: @menu
14404: * Scheduling::
14405: * Direct or Indirect Threaded?::
1.109 anton 14406: * Dynamic Superinstructions::
1.1 anton 14407: * DOES>::
14408: @end menu
14409:
14410: @node Scheduling, Direct or Indirect Threaded?, Threading, Threading
14411: @subsection Scheduling
14412: @cindex inner interpreter optimization
14413:
14414: There is a little complication: Pipelined and superscalar processors,
14415: i.e., RISC and some modern CISC machines can process independent
14416: instructions while waiting for the results of an instruction. The
14417: compiler usually reorders (schedules) the instructions in a way that
14418: achieves good usage of these delay slots. However, on our first tries
14419: the compiler did not do well on scheduling primitives. E.g., for
14420: @code{+} implemented as
14421: @example
14422: n=sp[0]+sp[1];
14423: sp++;
14424: sp[0]=n;
14425: NEXT;
14426: @end example
1.81 anton 14427: the @code{NEXT} comes strictly after the other code, i.e., there is
14428: nearly no scheduling. After a little thought the problem becomes clear:
14429: The compiler cannot know that @code{sp} and @code{ip} point to different
1.21 crook 14430: addresses (and the version of @code{gcc} we used would not know it even
14431: if it was possible), so it could not move the load of the cfa above the
14432: store to the TOS. Indeed the pointers could be the same, if code on or
14433: very near the top of stack were executed. In the interest of speed we
14434: chose to forbid this probably unused ``feature'' and helped the compiler
1.81 anton 14435: in scheduling: @code{NEXT} is divided into several parts:
14436: @code{NEXT_P0}, @code{NEXT_P1} and @code{NEXT_P2}). @code{+} now looks
14437: like:
1.1 anton 14438: @example
1.81 anton 14439: NEXT_P0;
1.1 anton 14440: n=sp[0]+sp[1];
14441: sp++;
14442: NEXT_P1;
14443: sp[0]=n;
14444: NEXT_P2;
14445: @end example
14446:
1.81 anton 14447: There are various schemes that distribute the different operations of
14448: NEXT between these parts in several ways; in general, different schemes
14449: perform best on different processors. We use a scheme for most
14450: architectures that performs well for most processors of this
1.109 anton 14451: architecture; in the future we may switch to benchmarking and chosing
1.81 anton 14452: the scheme on installation time.
14453:
1.1 anton 14454:
1.109 anton 14455: @node Direct or Indirect Threaded?, Dynamic Superinstructions, Scheduling, Threading
1.1 anton 14456: @subsection Direct or Indirect Threaded?
14457: @cindex threading, direct or indirect?
14458:
1.109 anton 14459: Threaded forth code consists of references to primitives (simple machine
14460: code routines like @code{+}) and to non-primitives (e.g., colon
14461: definitions, variables, constants); for a specific class of
14462: non-primitives (e.g., variables) there is one code routine (e.g.,
14463: @code{dovar}), but each variable needs a separate reference to its data.
14464:
14465: Traditionally Forth has been implemented as indirect threaded code,
14466: because this allows to use only one cell to reference a non-primitive
14467: (basically you point to the data, and find the code address there).
14468:
14469: @cindex primitive-centric threaded code
14470: However, threaded code in Gforth (since 0.6.0) uses two cells for
14471: non-primitives, one for the code address, and one for the data address;
14472: the data pointer is an immediate argument for the virtual machine
14473: instruction represented by the code address. We call this
14474: @emph{primitive-centric} threaded code, because all code addresses point
14475: to simple primitives. E.g., for a variable, the code address is for
14476: @code{lit} (also used for integer literals like @code{99}).
14477:
14478: Primitive-centric threaded code allows us to use (faster) direct
14479: threading as dispatch method, completely portably (direct threaded code
14480: in Gforth before 0.6.0 required architecture-specific code). It also
14481: eliminates the performance problems related to I-cache consistency that
14482: 386 implementations have with direct threaded code, and allows
14483: additional optimizations.
14484:
14485: @cindex hybrid direct/indirect threaded code
14486: There is a catch, however: the @var{xt} parameter of @code{execute} can
14487: occupy only one cell, so how do we pass non-primitives with their code
14488: @emph{and} data addresses to them? Our answer is to use indirect
14489: threaded dispatch for @code{execute} and other words that use a
14490: single-cell xt. So, normal threaded code in colon definitions uses
14491: direct threading, and @code{execute} and similar words, which dispatch
14492: to xts on the data stack, use indirect threaded code. We call this
14493: @emph{hybrid direct/indirect} threaded code.
14494:
14495: @cindex engines, gforth vs. gforth-fast vs. gforth-itc
14496: @cindex gforth engine
14497: @cindex gforth-fast engine
14498: The engines @command{gforth} and @command{gforth-fast} use hybrid
14499: direct/indirect threaded code. This means that with these engines you
14500: cannot use @code{,} to compile an xt. Instead, you have to use
14501: @code{compile,}.
14502:
14503: @cindex gforth-itc engine
1.115 anton 14504: If you want to compile xts with @code{,}, use @command{gforth-itc}.
14505: This engine uses plain old indirect threaded code. It still compiles in
14506: a primitive-centric style, so you cannot use @code{compile,} instead of
1.109 anton 14507: @code{,} (e.g., for producing tables of xts with @code{] word1 word2
1.115 anton 14508: ... [}). If you want to do that, you have to use @command{gforth-itc}
1.109 anton 14509: and execute @code{' , is compile,}. Your program can check if it is
14510: running on a hybrid direct/indirect threaded engine or a pure indirect
14511: threaded engine with @code{threading-method} (@pxref{Threading Words}).
14512:
14513:
14514: @node Dynamic Superinstructions, DOES>, Direct or Indirect Threaded?, Threading
14515: @subsection Dynamic Superinstructions
14516: @cindex Dynamic superinstructions with replication
14517: @cindex Superinstructions
14518: @cindex Replication
14519:
14520: The engines @command{gforth} and @command{gforth-fast} use another
14521: optimization: Dynamic superinstructions with replication. As an
14522: example, consider the following colon definition:
14523:
14524: @example
14525: : squared ( n1 -- n2 )
14526: dup * ;
14527: @end example
14528:
14529: Gforth compiles this into the threaded code sequence
14530:
14531: @example
14532: dup
14533: *
14534: ;s
14535: @end example
14536:
14537: In normal direct threaded code there is a code address occupying one
14538: cell for each of these primitives. Each code address points to a
14539: machine code routine, and the interpreter jumps to this machine code in
14540: order to execute the primitive. The routines for these three
14541: primitives are (in @command{gforth-fast} on the 386):
14542:
14543: @example
14544: Code dup
14545: ( $804B950 ) add esi , # -4 \ $83 $C6 $FC
14546: ( $804B953 ) add ebx , # 4 \ $83 $C3 $4
14547: ( $804B956 ) mov dword ptr 4 [esi] , ecx \ $89 $4E $4
14548: ( $804B959 ) jmp dword ptr FC [ebx] \ $FF $63 $FC
14549: end-code
14550: Code *
14551: ( $804ACC4 ) mov eax , dword ptr 4 [esi] \ $8B $46 $4
14552: ( $804ACC7 ) add esi , # 4 \ $83 $C6 $4
14553: ( $804ACCA ) add ebx , # 4 \ $83 $C3 $4
14554: ( $804ACCD ) imul ecx , eax \ $F $AF $C8
14555: ( $804ACD0 ) jmp dword ptr FC [ebx] \ $FF $63 $FC
14556: end-code
14557: Code ;s
14558: ( $804A693 ) mov eax , dword ptr [edi] \ $8B $7
14559: ( $804A695 ) add edi , # 4 \ $83 $C7 $4
14560: ( $804A698 ) lea ebx , dword ptr 4 [eax] \ $8D $58 $4
14561: ( $804A69B ) jmp dword ptr FC [ebx] \ $FF $63 $FC
14562: end-code
14563: @end example
14564:
14565: With dynamic superinstructions and replication the compiler does not
14566: just lay down the threaded code, but also copies the machine code
14567: fragments, usually without the jump at the end.
14568:
14569: @example
14570: ( $4057D27D ) add esi , # -4 \ $83 $C6 $FC
14571: ( $4057D280 ) add ebx , # 4 \ $83 $C3 $4
14572: ( $4057D283 ) mov dword ptr 4 [esi] , ecx \ $89 $4E $4
14573: ( $4057D286 ) mov eax , dword ptr 4 [esi] \ $8B $46 $4
14574: ( $4057D289 ) add esi , # 4 \ $83 $C6 $4
14575: ( $4057D28C ) add ebx , # 4 \ $83 $C3 $4
14576: ( $4057D28F ) imul ecx , eax \ $F $AF $C8
14577: ( $4057D292 ) mov eax , dword ptr [edi] \ $8B $7
14578: ( $4057D294 ) add edi , # 4 \ $83 $C7 $4
14579: ( $4057D297 ) lea ebx , dword ptr 4 [eax] \ $8D $58 $4
14580: ( $4057D29A ) jmp dword ptr FC [ebx] \ $FF $63 $FC
14581: @end example
14582:
14583: Only when a threaded-code control-flow change happens (e.g., in
14584: @code{;s}), the jump is appended. This optimization eliminates many of
14585: these jumps and makes the rest much more predictable. The speedup
14586: depends on the processor and the application; on the Athlon and Pentium
14587: III this optimization typically produces a speedup by a factor of 2.
14588:
14589: The code addresses in the direct-threaded code are set to point to the
14590: appropriate points in the copied machine code, in this example like
14591: this:
1.1 anton 14592:
1.109 anton 14593: @example
14594: primitive code address
14595: dup $4057D27D
14596: * $4057D286
14597: ;s $4057D292
14598: @end example
14599:
14600: Thus there can be threaded-code jumps to any place in this piece of
14601: code. This also simplifies decompilation quite a bit.
14602:
14603: @cindex --no-dynamic command-line option
14604: @cindex --no-super command-line option
14605: You can disable this optimization with @option{--no-dynamic}. You can
14606: use the copying without eliminating the jumps (i.e., dynamic
14607: replication, but without superinstructions) with @option{--no-super};
14608: this gives the branch prediction benefit alone; the effect on
1.110 anton 14609: performance depends on the CPU; on the Athlon and Pentium III the
14610: speedup is a little less than for dynamic superinstructions with
14611: replication.
14612:
14613: @cindex patching threaded code
14614: One use of these options is if you want to patch the threaded code.
14615: With superinstructions, many of the dispatch jumps are eliminated, so
14616: patching often has no effect. These options preserve all the dispatch
14617: jumps.
1.109 anton 14618:
14619: @cindex --dynamic command-line option
1.110 anton 14620: On some machines dynamic superinstructions are disabled by default,
14621: because it is unsafe on these machines. However, if you feel
14622: adventurous, you can enable it with @option{--dynamic}.
1.109 anton 14623:
14624: @node DOES>, , Dynamic Superinstructions, Threading
1.1 anton 14625: @subsection DOES>
14626: @cindex @code{DOES>} implementation
14627:
1.26 crook 14628: @cindex @code{dodoes} routine
14629: @cindex @code{DOES>}-code
1.1 anton 14630: One of the most complex parts of a Forth engine is @code{dodoes}, i.e.,
14631: the chunk of code executed by every word defined by a
1.109 anton 14632: @code{CREATE}...@code{DOES>} pair; actually with primitive-centric code,
14633: this is only needed if the xt of the word is @code{execute}d. The main
14634: problem here is: How to find the Forth code to be executed, i.e. the
14635: code after the @code{DOES>} (the @code{DOES>}-code)? There are two
14636: solutions:
1.1 anton 14637:
1.21 crook 14638: In fig-Forth the code field points directly to the @code{dodoes} and the
1.109 anton 14639: @code{DOES>}-code address is stored in the cell after the code address
14640: (i.e. at @code{@i{CFA} cell+}). It may seem that this solution is
14641: illegal in the Forth-79 and all later standards, because in fig-Forth
14642: this address lies in the body (which is illegal in these
14643: standards). However, by making the code field larger for all words this
14644: solution becomes legal again. We use this approach. Leaving a cell
14645: unused in most words is a bit wasteful, but on the machines we are
14646: targeting this is hardly a problem.
14647:
1.1 anton 14648:
14649: @node Primitives, Performance, Threading, Engine
14650: @section Primitives
14651: @cindex primitives, implementation
14652: @cindex virtual machine instructions, implementation
14653:
14654: @menu
14655: * Automatic Generation::
14656: * TOS Optimization::
14657: * Produced code::
14658: @end menu
14659:
14660: @node Automatic Generation, TOS Optimization, Primitives, Primitives
14661: @subsection Automatic Generation
14662: @cindex primitives, automatic generation
14663:
14664: @cindex @file{prims2x.fs}
1.109 anton 14665:
1.1 anton 14666: Since the primitives are implemented in a portable language, there is no
14667: longer any need to minimize the number of primitives. On the contrary,
14668: having many primitives has an advantage: speed. In order to reduce the
14669: number of errors in primitives and to make programming them easier, we
1.109 anton 14670: provide a tool, the primitive generator (@file{prims2x.fs} aka Vmgen,
14671: @pxref{Top, Vmgen, Introduction, vmgen, Vmgen}), that automatically
14672: generates most (and sometimes all) of the C code for a primitive from
14673: the stack effect notation. The source for a primitive has the following
14674: form:
1.1 anton 14675:
14676: @cindex primitive source format
14677: @format
1.58 anton 14678: @i{Forth-name} ( @i{stack-effect} ) @i{category} [@i{pronounc.}]
1.29 crook 14679: [@code{""}@i{glossary entry}@code{""}]
14680: @i{C code}
1.1 anton 14681: [@code{:}
1.29 crook 14682: @i{Forth code}]
1.1 anton 14683: @end format
14684:
14685: The items in brackets are optional. The category and glossary fields
14686: are there for generating the documentation, the Forth code is there
14687: for manual implementations on machines without GNU C. E.g., the source
14688: for the primitive @code{+} is:
14689: @example
1.58 anton 14690: + ( n1 n2 -- n ) core plus
1.1 anton 14691: n = n1+n2;
14692: @end example
14693:
14694: This looks like a specification, but in fact @code{n = n1+n2} is C
14695: code. Our primitive generation tool extracts a lot of information from
14696: the stack effect notations@footnote{We use a one-stack notation, even
14697: though we have separate data and floating-point stacks; The separate
14698: notation can be generated easily from the unified notation.}: The number
14699: of items popped from and pushed on the stack, their type, and by what
14700: name they are referred to in the C code. It then generates a C code
14701: prelude and postlude for each primitive. The final C code for @code{+}
14702: looks like this:
14703:
14704: @example
1.46 pazsan 14705: I_plus: /* + ( n1 n2 -- n ) */ /* label, stack effect */
1.1 anton 14706: /* */ /* documentation */
1.81 anton 14707: NAME("+") /* debugging output (with -DDEBUG) */
1.1 anton 14708: @{
14709: DEF_CA /* definition of variable ca (indirect threading) */
14710: Cell n1; /* definitions of variables */
14711: Cell n2;
14712: Cell n;
1.81 anton 14713: NEXT_P0; /* NEXT part 0 */
1.1 anton 14714: n1 = (Cell) sp[1]; /* input */
14715: n2 = (Cell) TOS;
14716: sp += 1; /* stack adjustment */
14717: @{
14718: n = n1+n2; /* C code taken from the source */
14719: @}
14720: NEXT_P1; /* NEXT part 1 */
14721: TOS = (Cell)n; /* output */
14722: NEXT_P2; /* NEXT part 2 */
14723: @}
14724: @end example
14725:
14726: This looks long and inefficient, but the GNU C compiler optimizes quite
14727: well and produces optimal code for @code{+} on, e.g., the R3000 and the
14728: HP RISC machines: Defining the @code{n}s does not produce any code, and
14729: using them as intermediate storage also adds no cost.
14730:
1.26 crook 14731: There are also other optimizations that are not illustrated by this
14732: example: assignments between simple variables are usually for free (copy
1.1 anton 14733: propagation). If one of the stack items is not used by the primitive
14734: (e.g. in @code{drop}), the compiler eliminates the load from the stack
14735: (dead code elimination). On the other hand, there are some things that
14736: the compiler does not do, therefore they are performed by
14737: @file{prims2x.fs}: The compiler does not optimize code away that stores
14738: a stack item to the place where it just came from (e.g., @code{over}).
14739:
14740: While programming a primitive is usually easy, there are a few cases
14741: where the programmer has to take the actions of the generator into
14742: account, most notably @code{?dup}, but also words that do not (always)
1.26 crook 14743: fall through to @code{NEXT}.
1.109 anton 14744:
14745: For more information
1.1 anton 14746:
14747: @node TOS Optimization, Produced code, Automatic Generation, Primitives
14748: @subsection TOS Optimization
14749: @cindex TOS optimization for primitives
14750: @cindex primitives, keeping the TOS in a register
14751:
14752: An important optimization for stack machine emulators, e.g., Forth
14753: engines, is keeping one or more of the top stack items in
1.29 crook 14754: registers. If a word has the stack effect @i{in1}...@i{inx} @code{--}
14755: @i{out1}...@i{outy}, keeping the top @i{n} items in registers
1.1 anton 14756: @itemize @bullet
14757: @item
1.29 crook 14758: is better than keeping @i{n-1} items, if @i{x>=n} and @i{y>=n},
1.1 anton 14759: due to fewer loads from and stores to the stack.
1.29 crook 14760: @item is slower than keeping @i{n-1} items, if @i{x<>y} and @i{x<n} and
14761: @i{y<n}, due to additional moves between registers.
1.1 anton 14762: @end itemize
14763:
14764: @cindex -DUSE_TOS
14765: @cindex -DUSE_NO_TOS
14766: In particular, keeping one item in a register is never a disadvantage,
14767: if there are enough registers. Keeping two items in registers is a
14768: disadvantage for frequent words like @code{?branch}, constants,
14769: variables, literals and @code{i}. Therefore our generator only produces
14770: code that keeps zero or one items in registers. The generated C code
14771: covers both cases; the selection between these alternatives is made at
14772: C-compile time using the switch @code{-DUSE_TOS}. @code{TOS} in the C
14773: code for @code{+} is just a simple variable name in the one-item case,
14774: otherwise it is a macro that expands into @code{sp[0]}. Note that the
14775: GNU C compiler tries to keep simple variables like @code{TOS} in
14776: registers, and it usually succeeds, if there are enough registers.
14777:
14778: @cindex -DUSE_FTOS
14779: @cindex -DUSE_NO_FTOS
14780: The primitive generator performs the TOS optimization for the
14781: floating-point stack, too (@code{-DUSE_FTOS}). For floating-point
14782: operations the benefit of this optimization is even larger:
14783: floating-point operations take quite long on most processors, but can be
14784: performed in parallel with other operations as long as their results are
14785: not used. If the FP-TOS is kept in a register, this works. If
14786: it is kept on the stack, i.e., in memory, the store into memory has to
14787: wait for the result of the floating-point operation, lengthening the
14788: execution time of the primitive considerably.
14789:
14790: The TOS optimization makes the automatic generation of primitives a
14791: bit more complicated. Just replacing all occurrences of @code{sp[0]} by
14792: @code{TOS} is not sufficient. There are some special cases to
14793: consider:
14794: @itemize @bullet
14795: @item In the case of @code{dup ( w -- w w )} the generator must not
14796: eliminate the store to the original location of the item on the stack,
14797: if the TOS optimization is turned on.
14798: @item Primitives with stack effects of the form @code{--}
1.29 crook 14799: @i{out1}...@i{outy} must store the TOS to the stack at the start.
14800: Likewise, primitives with the stack effect @i{in1}...@i{inx} @code{--}
1.1 anton 14801: must load the TOS from the stack at the end. But for the null stack
14802: effect @code{--} no stores or loads should be generated.
14803: @end itemize
14804:
14805: @node Produced code, , TOS Optimization, Primitives
14806: @subsection Produced code
14807: @cindex primitives, assembly code listing
14808:
14809: @cindex @file{engine.s}
14810: To see what assembly code is produced for the primitives on your machine
14811: with your compiler and your flag settings, type @code{make engine.s} and
1.81 anton 14812: look at the resulting file @file{engine.s}. Alternatively, you can also
14813: disassemble the code of primitives with @code{see} on some architectures.
1.1 anton 14814:
14815: @node Performance, , Primitives, Engine
14816: @section Performance
14817: @cindex performance of some Forth interpreters
14818: @cindex engine performance
14819: @cindex benchmarking Forth systems
14820: @cindex Gforth performance
14821:
14822: On RISCs the Gforth engine is very close to optimal; i.e., it is usually
1.112 anton 14823: impossible to write a significantly faster threaded-code engine.
1.1 anton 14824:
14825: On register-starved machines like the 386 architecture processors
14826: improvements are possible, because @code{gcc} does not utilize the
14827: registers as well as a human, even with explicit register declarations;
14828: e.g., Bernd Beuster wrote a Forth system fragment in assembly language
14829: and hand-tuned it for the 486; this system is 1.19 times faster on the
14830: Sieve benchmark on a 486DX2/66 than Gforth compiled with
1.40 anton 14831: @code{gcc-2.6.3} with @code{-DFORCE_REG}. The situation has improved
14832: with gcc-2.95 and gforth-0.4.9; now the most important virtual machine
14833: registers fit in real registers (and we can even afford to use the TOS
14834: optimization), resulting in a speedup of 1.14 on the sieve over the
1.112 anton 14835: earlier results. And dynamic superinstructions provide another speedup
14836: (but only around a factor 1.2 on the 486).
1.1 anton 14837:
14838: @cindex Win32Forth performance
14839: @cindex NT Forth performance
14840: @cindex eforth performance
14841: @cindex ThisForth performance
14842: @cindex PFE performance
14843: @cindex TILE performance
1.81 anton 14844: The potential advantage of assembly language implementations is not
1.112 anton 14845: necessarily realized in complete Forth systems: We compared Gforth-0.5.9
1.81 anton 14846: (direct threaded, compiled with @code{gcc-2.95.1} and
14847: @code{-DFORCE_REG}) with Win32Forth 1.2093 (newer versions are
14848: reportedly much faster), LMI's NT Forth (Beta, May 1994) and Eforth
14849: (with and without peephole (aka pinhole) optimization of the threaded
14850: code); all these systems were written in assembly language. We also
14851: compared Gforth with three systems written in C: PFE-0.9.14 (compiled
14852: with @code{gcc-2.6.3} with the default configuration for Linux:
14853: @code{-O2 -fomit-frame-pointer -DUSE_REGS -DUNROLL_NEXT}), ThisForth
14854: Beta (compiled with @code{gcc-2.6.3 -O3 -fomit-frame-pointer}; ThisForth
14855: employs peephole optimization of the threaded code) and TILE (compiled
14856: with @code{make opt}). We benchmarked Gforth, PFE, ThisForth and TILE on
14857: a 486DX2/66 under Linux. Kenneth O'Heskin kindly provided the results
14858: for Win32Forth and NT Forth on a 486DX2/66 with similar memory
14859: performance under Windows NT. Marcel Hendrix ported Eforth to Linux,
14860: then extended it to run the benchmarks, added the peephole optimizer,
14861: ran the benchmarks and reported the results.
1.40 anton 14862:
1.1 anton 14863: We used four small benchmarks: the ubiquitous Sieve; bubble-sorting and
14864: matrix multiplication come from the Stanford integer benchmarks and have
14865: been translated into Forth by Martin Fraeman; we used the versions
14866: included in the TILE Forth package, but with bigger data set sizes; and
14867: a recursive Fibonacci number computation for benchmarking calling
14868: performance. The following table shows the time taken for the benchmarks
14869: scaled by the time taken by Gforth (in other words, it shows the speedup
14870: factor that Gforth achieved over the other systems).
14871:
14872: @example
1.112 anton 14873: relative Win32- NT eforth This-
14874: time Gforth Forth Forth eforth +opt PFE Forth TILE
14875: sieve 1.00 2.16 1.78 2.16 1.32 2.46 4.96 13.37
14876: bubble 1.00 1.93 2.07 2.18 1.29 2.21 5.70
14877: matmul 1.00 1.92 1.76 1.90 0.96 2.06 5.32
14878: fib 1.00 2.32 2.03 1.86 1.31 2.64 4.55 6.54
1.1 anton 14879: @end example
14880:
1.26 crook 14881: You may be quite surprised by the good performance of Gforth when
14882: compared with systems written in assembly language. One important reason
14883: for the disappointing performance of these other systems is probably
14884: that they are not written optimally for the 486 (e.g., they use the
14885: @code{lods} instruction). In addition, Win32Forth uses a comfortable,
14886: but costly method for relocating the Forth image: like @code{cforth}, it
14887: computes the actual addresses at run time, resulting in two address
14888: computations per @code{NEXT} (@pxref{Image File Background}).
14889:
1.1 anton 14890: The speedup of Gforth over PFE, ThisForth and TILE can be easily
14891: explained with the self-imposed restriction of the latter systems to
14892: standard C, which makes efficient threading impossible (however, the
1.4 anton 14893: measured implementation of PFE uses a GNU C extension: @pxref{Global Reg
1.1 anton 14894: Vars, , Defining Global Register Variables, gcc.info, GNU C Manual}).
14895: Moreover, current C compilers have a hard time optimizing other aspects
14896: of the ThisForth and the TILE source.
14897:
1.26 crook 14898: The performance of Gforth on 386 architecture processors varies widely
14899: with the version of @code{gcc} used. E.g., @code{gcc-2.5.8} failed to
14900: allocate any of the virtual machine registers into real machine
14901: registers by itself and would not work correctly with explicit register
1.112 anton 14902: declarations, giving a significantly slower engine (on a 486DX2/66
14903: running the Sieve) than the one measured above.
1.1 anton 14904:
1.26 crook 14905: Note that there have been several releases of Win32Forth since the
14906: release presented here, so the results presented above may have little
1.40 anton 14907: predictive value for the performance of Win32Forth today (results for
14908: the current release on an i486DX2/66 are welcome).
1.1 anton 14909:
14910: @cindex @file{Benchres}
1.66 anton 14911: In
14912: @cite{@uref{http://www.complang.tuwien.ac.at/papers/ertl&maierhofer95.ps.gz,
14913: Translating Forth to Efficient C}} by M. Anton Ertl and Martin
1.1 anton 14914: Maierhofer (presented at EuroForth '95), an indirect threaded version of
1.66 anton 14915: Gforth is compared with Win32Forth, NT Forth, PFE, ThisForth, and
14916: several native code systems; that version of Gforth is slower on a 486
1.112 anton 14917: than the version used here. You can find a newer version of these
14918: measurements at
1.47 crook 14919: @uref{http://www.complang.tuwien.ac.at/forth/performance.html}. You can
1.1 anton 14920: find numbers for Gforth on various machines in @file{Benchres}.
14921:
1.26 crook 14922: @c ******************************************************************
1.113 anton 14923: @c @node Binding to System Library, Cross Compiler, Engine, Top
14924: @c @chapter Binding to System Library
1.13 pazsan 14925:
1.113 anton 14926: @c ****************************************************************
14927: @node Cross Compiler, Bugs, Engine, Top
1.14 pazsan 14928: @chapter Cross Compiler
1.47 crook 14929: @cindex @file{cross.fs}
14930: @cindex cross-compiler
14931: @cindex metacompiler
14932: @cindex target compiler
1.13 pazsan 14933:
1.46 pazsan 14934: The cross compiler is used to bootstrap a Forth kernel. Since Gforth is
14935: mostly written in Forth, including crucial parts like the outer
14936: interpreter and compiler, it needs compiled Forth code to get
14937: started. The cross compiler allows to create new images for other
14938: architectures, even running under another Forth system.
1.13 pazsan 14939:
14940: @menu
1.67 anton 14941: * Using the Cross Compiler::
14942: * How the Cross Compiler Works::
1.13 pazsan 14943: @end menu
14944:
1.21 crook 14945: @node Using the Cross Compiler, How the Cross Compiler Works, Cross Compiler, Cross Compiler
1.14 pazsan 14946: @section Using the Cross Compiler
1.46 pazsan 14947:
14948: The cross compiler uses a language that resembles Forth, but isn't. The
14949: main difference is that you can execute Forth code after definition,
14950: while you usually can't execute the code compiled by cross, because the
14951: code you are compiling is typically for a different computer than the
14952: one you are compiling on.
14953:
1.81 anton 14954: @c anton: This chapter is somewhat different from waht I would expect: I
14955: @c would expect an explanation of the cross language and how to create an
14956: @c application image with it. The section explains some aspects of
14957: @c creating a Gforth kernel.
14958:
1.46 pazsan 14959: The Makefile is already set up to allow you to create kernels for new
14960: architectures with a simple make command. The generic kernels using the
14961: GCC compiled virtual machine are created in the normal build process
14962: with @code{make}. To create a embedded Gforth executable for e.g. the
14963: 8086 processor (running on a DOS machine), type
14964:
14965: @example
14966: make kernl-8086.fi
14967: @end example
14968:
14969: This will use the machine description from the @file{arch/8086}
14970: directory to create a new kernel. A machine file may look like that:
14971:
14972: @example
14973: \ Parameter for target systems 06oct92py
14974:
14975: 4 Constant cell \ cell size in bytes
14976: 2 Constant cell<< \ cell shift to bytes
14977: 5 Constant cell>bit \ cell shift to bits
14978: 8 Constant bits/char \ bits per character
14979: 8 Constant bits/byte \ bits per byte [default: 8]
14980: 8 Constant float \ bytes per float
14981: 8 Constant /maxalign \ maximum alignment in bytes
14982: false Constant bigendian \ byte order
14983: ( true=big, false=little )
14984:
14985: include machpc.fs \ feature list
14986: @end example
14987:
14988: This part is obligatory for the cross compiler itself, the feature list
14989: is used by the kernel to conditionally compile some features in and out,
14990: depending on whether the target supports these features.
14991:
14992: There are some optional features, if you define your own primitives,
14993: have an assembler, or need special, nonstandard preparation to make the
1.81 anton 14994: boot process work. @code{asm-include} includes an assembler,
1.46 pazsan 14995: @code{prims-include} includes primitives, and @code{>boot} prepares for
14996: booting.
14997:
14998: @example
14999: : asm-include ." Include assembler" cr
15000: s" arch/8086/asm.fs" included ;
15001:
15002: : prims-include ." Include primitives" cr
15003: s" arch/8086/prim.fs" included ;
15004:
15005: : >boot ." Prepare booting" cr
15006: s" ' boot >body into-forth 1+ !" evaluate ;
15007: @end example
15008:
15009: These words are used as sort of macro during the cross compilation in
1.81 anton 15010: the file @file{kernel/main.fs}. Instead of using these macros, it would
1.46 pazsan 15011: be possible --- but more complicated --- to write a new kernel project
15012: file, too.
15013:
15014: @file{kernel/main.fs} expects the machine description file name on the
15015: stack; the cross compiler itself (@file{cross.fs}) assumes that either
15016: @code{mach-file} leaves a counted string on the stack, or
15017: @code{machine-file} leaves an address, count pair of the filename on the
15018: stack.
15019:
15020: The feature list is typically controlled using @code{SetValue}, generic
15021: files that are used by several projects can use @code{DefaultValue}
15022: instead. Both functions work like @code{Value}, when the value isn't
15023: defined, but @code{SetValue} works like @code{to} if the value is
15024: defined, and @code{DefaultValue} doesn't set anything, if the value is
15025: defined.
15026:
15027: @example
15028: \ generic mach file for pc gforth 03sep97jaw
15029:
15030: true DefaultValue NIL \ relocating
15031:
15032: >ENVIRON
15033:
15034: true DefaultValue file \ controls the presence of the
15035: \ file access wordset
15036: true DefaultValue OS \ flag to indicate a operating system
15037:
15038: true DefaultValue prims \ true: primitives are c-code
15039:
15040: true DefaultValue floating \ floating point wordset is present
15041:
15042: true DefaultValue glocals \ gforth locals are present
15043: \ will be loaded
15044: true DefaultValue dcomps \ double number comparisons
15045:
15046: true DefaultValue hash \ hashing primitives are loaded/present
15047:
15048: true DefaultValue xconds \ used together with glocals,
15049: \ special conditionals supporting gforths'
15050: \ local variables
15051: true DefaultValue header \ save a header information
15052:
15053: true DefaultValue backtrace \ enables backtrace code
15054:
15055: false DefaultValue ec
15056: false DefaultValue crlf
15057:
15058: cell 2 = [IF] &32 [ELSE] &256 [THEN] KB DefaultValue kernel-size
15059:
15060: &16 KB DefaultValue stack-size
15061: &15 KB &512 + DefaultValue fstack-size
15062: &15 KB DefaultValue rstack-size
15063: &14 KB &512 + DefaultValue lstack-size
15064: @end example
1.13 pazsan 15065:
1.48 anton 15066: @node How the Cross Compiler Works, , Using the Cross Compiler, Cross Compiler
1.14 pazsan 15067: @section How the Cross Compiler Works
1.13 pazsan 15068:
15069: @node Bugs, Origin, Cross Compiler, Top
1.21 crook 15070: @appendix Bugs
1.1 anton 15071: @cindex bug reporting
15072:
1.21 crook 15073: Known bugs are described in the file @file{BUGS} in the Gforth distribution.
1.1 anton 15074:
1.103 anton 15075: If you find a bug, please submit a bug report through
15076: @uref{https://savannah.gnu.org/bugs/?func=addbug&group=gforth}.
1.21 crook 15077:
15078: @itemize @bullet
15079: @item
1.81 anton 15080: A program (or a sequence of keyboard commands) that reproduces the bug.
15081: @item
15082: A description of what you think constitutes the buggy behaviour.
15083: @item
1.21 crook 15084: The Gforth version used (it is announced at the start of an
15085: interactive Gforth session).
15086: @item
15087: The machine and operating system (on Unix
15088: systems @code{uname -a} will report this information).
15089: @item
1.81 anton 15090: The installation options (you can find the configure options at the
15091: start of @file{config.status}) and configuration (@code{configure}
15092: output or @file{config.cache}).
1.21 crook 15093: @item
15094: A complete list of changes (if any) you (or your installer) have made to the
15095: Gforth sources.
15096: @end itemize
1.1 anton 15097:
15098: For a thorough guide on reporting bugs read @ref{Bug Reporting, , How
15099: to Report Bugs, gcc.info, GNU C Manual}.
15100:
15101:
1.21 crook 15102: @node Origin, Forth-related information, Bugs, Top
15103: @appendix Authors and Ancestors of Gforth
1.1 anton 15104:
15105: @section Authors and Contributors
15106: @cindex authors of Gforth
15107: @cindex contributors to Gforth
15108:
15109: The Gforth project was started in mid-1992 by Bernd Paysan and Anton
1.81 anton 15110: Ertl. The third major author was Jens Wilke. Neal Crook contributed a
15111: lot to the manual. Assemblers and disassemblers were contributed by
15112: Andrew McKewan, Christian Pirker, and Bernd Thallner. Lennart Benschop
15113: (who was one of Gforth's first users, in mid-1993) and Stuart Ramsden
15114: inspired us with their continuous feedback. Lennart Benshop contributed
1.1 anton 15115: @file{glosgen.fs}, while Stuart Ramsden has been working on automatic
15116: support for calling C libraries. Helpful comments also came from Paul
15117: Kleinrubatscher, Christian Pirker, Dirk Zoller, Marcel Hendrix, John
1.113 anton 15118: Wavrik, Barrie Stott, Marc de Groot, Jorge Acerada, Bruce Hoyt, Robert
15119: Epprecht, Dennis Ruffer and David N. Williams. Since the release of
15120: Gforth-0.2.1 there were also helpful comments from many others; thank
15121: you all, sorry for not listing you here (but digging through my mailbox
15122: to extract your names is on my to-do list).
1.1 anton 15123:
15124: Gforth also owes a lot to the authors of the tools we used (GCC, CVS,
15125: and autoconf, among others), and to the creators of the Internet: Gforth
1.21 crook 15126: was developed across the Internet, and its authors did not meet
1.20 pazsan 15127: physically for the first 4 years of development.
1.1 anton 15128:
15129: @section Pedigree
1.26 crook 15130: @cindex pedigree of Gforth
1.1 anton 15131:
1.81 anton 15132: Gforth descends from bigFORTH (1993) and fig-Forth. Of course, a
15133: significant part of the design of Gforth was prescribed by ANS Forth.
1.1 anton 15134:
1.20 pazsan 15135: Bernd Paysan wrote bigFORTH, a descendent from TurboForth, an unreleased
1.1 anton 15136: 32 bit native code version of VolksForth for the Atari ST, written
15137: mostly by Dietrich Weineck.
15138:
1.81 anton 15139: VolksForth was written by Klaus Schleisiek, Bernd Pennemann, Georg
15140: Rehfeld and Dietrich Weineck for the C64 (called UltraForth there) in
15141: the mid-80s and ported to the Atari ST in 1986. It descends from F83.
1.1 anton 15142:
15143: Henry Laxen and Mike Perry wrote F83 as a model implementation of the
15144: Forth-83 standard. !! Pedigree? When?
15145:
15146: A team led by Bill Ragsdale implemented fig-Forth on many processors in
15147: 1979. Robert Selzer and Bill Ragsdale developed the original
15148: implementation of fig-Forth for the 6502 based on microForth.
15149:
15150: The principal architect of microForth was Dean Sanderson. microForth was
15151: FORTH, Inc.'s first off-the-shelf product. It was developed in 1976 for
15152: the 1802, and subsequently implemented on the 8080, the 6800 and the
15153: Z80.
15154:
15155: All earlier Forth systems were custom-made, usually by Charles Moore,
15156: who discovered (as he puts it) Forth during the late 60s. The first full
15157: Forth existed in 1971.
15158:
1.81 anton 15159: A part of the information in this section comes from
15160: @cite{@uref{http://www.forth.com/Content/History/History1.htm,The
15161: Evolution of Forth}} by Elizabeth D. Rather, Donald R. Colburn and
15162: Charles H. Moore, presented at the HOPL-II conference and preprinted in
15163: SIGPLAN Notices 28(3), 1993. You can find more historical and
15164: genealogical information about Forth there.
1.1 anton 15165:
1.81 anton 15166: @c ------------------------------------------------------------------
1.113 anton 15167: @node Forth-related information, Licenses, Origin, Top
1.21 crook 15168: @appendix Other Forth-related information
15169: @cindex Forth-related information
15170:
1.81 anton 15171: @c anton: I threw most of this stuff out, because it can be found through
15172: @c the FAQ and the FAQ is more likely to be up-to-date.
1.21 crook 15173:
15174: @cindex comp.lang.forth
15175: @cindex frequently asked questions
1.81 anton 15176: There is an active news group (comp.lang.forth) discussing Forth
15177: (including Gforth) and Forth-related issues. Its
15178: @uref{http://www.complang.tuwien.ac.at/forth/faq/faq-general-2.html,FAQs}
15179: (frequently asked questions and their answers) contains a lot of
15180: information on Forth. You should read it before posting to
15181: comp.lang.forth.
1.21 crook 15182:
1.81 anton 15183: The ANS Forth standard is most usable in its
15184: @uref{http://www.taygeta.com/forth/dpans.html, HTML form}.
1.21 crook 15185:
1.113 anton 15186: @c ---------------------------------------------------
15187: @node Licenses, Word Index, Forth-related information, Top
15188: @appendix Licenses
15189:
15190: @menu
15191: * GNU Free Documentation License:: License for copying this manual.
15192: * Copying:: GPL (for copying this software).
15193: @end menu
15194:
15195: @include fdl.texi
15196:
15197: @include gpl.texi
15198:
15199:
15200:
1.81 anton 15201: @c ------------------------------------------------------------------
1.113 anton 15202: @node Word Index, Concept Index, Licenses, Top
1.1 anton 15203: @unnumbered Word Index
15204:
1.26 crook 15205: This index is a list of Forth words that have ``glossary'' entries
15206: within this manual. Each word is listed with its stack effect and
15207: wordset.
1.1 anton 15208:
15209: @printindex fn
15210:
1.81 anton 15211: @c anton: the name index seems superfluous given the word and concept indices.
15212:
15213: @c @node Name Index, Concept Index, Word Index, Top
15214: @c @unnumbered Name Index
1.41 anton 15215:
1.81 anton 15216: @c This index is a list of Forth words that have ``glossary'' entries
15217: @c within this manual.
1.41 anton 15218:
1.81 anton 15219: @c @printindex ky
1.41 anton 15220:
1.113 anton 15221: @c -------------------------------------------------------
1.81 anton 15222: @node Concept Index, , Word Index, Top
1.1 anton 15223: @unnumbered Concept and Word Index
15224:
1.26 crook 15225: Not all entries listed in this index are present verbatim in the
15226: text. This index also duplicates, in abbreviated form, all of the words
15227: listed in the Word Index (only the names are listed for the words here).
1.1 anton 15228:
15229: @printindex cp
15230:
15231: @bye
1.81 anton 15232:
15233:
1.1 anton 15234:
FreeBSD-CVSweb <freebsd-cvsweb@FreeBSD.org>