1: \input texinfo @c -*-texinfo-*-
2: @comment The source is gforth.ds, from which gforth.texi is generated
3: @comment %**start of header (This is for running Texinfo on a region.)
4: @setfilename gforth.info
5: @settitle GNU Forth Manual
6: @comment @setchapternewpage odd
7: @comment %**end of header (This is for running Texinfo on a region.)
8:
9: @ifinfo
10: This file documents GNU Forth 0.0
11:
12: Copyright @copyright{} 1994 GNU Forth Development Group
13:
14: Permission is granted to make and distribute verbatim copies of
15: this manual provided the copyright notice and this permission notice
16: are preserved on all copies.
17:
18: @ignore
19: Permission is granted to process this file through TeX and print the
20: results, provided the printed document carries a copying permission
21: notice identical to this one except for the removal of this paragraph
22: (this paragraph not being relevant to the printed manual).
23:
24: @end ignore
25: Permission is granted to copy and distribute modified versions of this
26: manual under the conditions for verbatim copying, provided also that the
27: sections entitled "Distribution" and "General Public License" are
28: included exactly as in the original, and provided that the entire
29: resulting derived work is distributed under the terms of a permission
30: notice identical to this one.
31:
32: Permission is granted to copy and distribute translations of this manual
33: into another language, under the above conditions for modified versions,
34: except that the sections entitled "Distribution" and "General Public
35: License" may be included in a translation approved by the author instead
36: of in the original English.
37: @end ifinfo
38:
39: @titlepage
40: @sp 10
41: @center @titlefont{GNU Forth Manual}
42: @sp 2
43: @center for version 0.0
44: @sp 2
45: @center Anton Ertl
46:
47: @comment The following two commands start the copyright page.
48: @page
49: @vskip 0pt plus 1filll
50: Copyright @copyright{} 1994 GNU Forth Development Group
51:
52: @comment !! Published by ... or You can get a copy of this manual ...
53:
54: Permission is granted to make and distribute verbatim copies of
55: this manual provided the copyright notice and this permission notice
56: are preserved on all copies.
57:
58: Permission is granted to copy and distribute modified versions of this
59: manual under the conditions for verbatim copying, provided also that the
60: sections entitled "Distribution" and "General Public License" are
61: included exactly as in the original, and provided that the entire
62: resulting derived work is distributed under the terms of a permission
63: notice identical to this one.
64:
65: Permission is granted to copy and distribute translations of this manual
66: into another language, under the above conditions for modified versions,
67: except that the sections entitled "Distribution" and "General Public
68: License" may be included in a translation approved by the author instead
69: of in the original English.
70: @end titlepage
71:
72:
73: @node Top, License, (dir), (dir)
74: @ifinfo
75: GNU Forth is a free implementation of ANS Forth available on many
76: personal machines. This manual corresponds to version 0.0.
77: @end ifinfo
78:
79: @menu
80: * License::
81: * Goals:: About the GNU Forth Project
82: * Other Books:: Things you might want to read
83: * Invocation:: Starting GNU Forth
84: * Words:: Forth words available in GNU Forth
85: * ANS conformance:: Implementation-defined options etc.
86: * Model:: The abstract machine of GNU Forth
87: * Emacs and GForth:: The GForth Mode
88: * Internals:: Implementation details
89: * Bugs:: How to report them
90: * Pedigree:: Ancestors of GNU Forth
91: * Word Index:: An item for each Forth word
92: * Node Index:: An item for each node
93: @end menu
94:
95: @node License, Goals, Top, Top
96: @unnumbered License
97: !! Insert GPL here
98:
99: @iftex
100: @unnumbered Preface
101: This manual documents GNU Forth. The reader is expected to know
102: Forth. This manual is primarily a reference manual. @xref{Other Books}
103: for introductory material.
104: @end iftex
105:
106: @node Goals, Other Books, License, Top
107: @comment node-name, next, previous, up
108: @chapter Goals of GNU Forth
109: @cindex Goals
110: The goal of the GNU Forth Project is to develop a standard model for
111: ANSI Forth. This can be split into several subgoals:
112:
113: @itemize @bullet
114: @item
115: GNU Forth should conform to the ANSI Forth standard.
116: @item
117: It should be a model, i.e. it should define all the
118: implementation-dependent things.
119: @item
120: It should become standard, i.e. widely accepted and used. This goal
121: is the most difficult one.
122: @end itemize
123:
124: To achieve these goals GNU Forth should be
125: @itemize @bullet
126: @item
127: Similar to previous models (fig-Forth, F83)
128: @item
129: Powerful. It should provide for all the things that are considered
130: necessary today and even some that are not yet considered necessary.
131: @item
132: Efficient. It should not get the reputation of being exceptionally
133: slow.
134: @item
135: Free.
136: @item
137: Available on many machines/easy to port.
138: @end itemize
139:
140: Have we achieved these goals? GNU Forth conforms to the ANS Forth
141: standard; it may be considered a model, but we have not yet documented
142: which parts of the model are stable and which parts we are likely to
143: change; it certainly has not yet become a de facto standard. It has some
144: similarities and some differences to previous models; It has some
145: powerful features, but not yet everything that we envisioned; on RISCs
146: it is as fast as interpreters programmed in assembly, on
147: register-starved machines it is not so fast, but still faster than any
148: other C-based interpretive implementation; it is free and available on
149: many machines.
150:
151: @node Other Books, Invocation, Goals, Top
152: @chapter Other books on ANS Forth
153:
154: As the standard is relatively new, there are not many books out yet. It
155: is not recommended to learn Forth by using GNU Forth and a book that is
156: not written for ANS Forth, as you will not know your mistakes from the
157: deviations of the book.
158:
159: There is, of course, the standard, the definite reference if you want to
160: write ANS Forth programs. It will be available in printed form from
161: Global Engineering Documents !! somtime in spring or summer 1994. If you
162: are lucky, you can still get dpANS6 (the draft that was approved as
163: standard) by aftp from ftp.uu.net:/vendor/minerva/x3j14.
164:
165: @cite{Forth: The new model} by Jack Woehr (!! Publisher) is an
166: introductory book based on a draft version of the standard. It does not
167: cover the whole standard. It also contains interesting background
168: information (Jack Woehr was in the ANS Forth Technical Committe). It is
169: not appropriate for complete newbies, but programmers experienced in
170: other languages should find it ok.
171:
172: @node Invocation, Words, Other Books, Top
173: @chapter Invocation
174:
175: You will usually just say @code{gforth}. In many other cases the default
176: GNU Forth image will be invoked like this:
177:
178: @example
179: gforth [files] [-e forth-code]
180: @end example
181:
182: executing the contents of the files and the Forth code in the order they
183: are given.
184:
185: In general, the command line looks like this:
186:
187: @example
188: gforth [initialization options] [image-specific options]
189: @end example
190:
191: The initialization options must come before the rest of the command
192: line. They are:
193:
194: @table @code
195: @item --image-file @var{file}
196: Loads the Forth image @var{file} instead of the default
197: @file{gforth.fi}.
198:
199: @item --path @var{path}
200: Uses @var{path} for searching the image file and Forth source code
201: files instead of the default in the environment variable
202: @code{GFORTHPATH} or the path specified at installation time (typically
203: @file{/usr/local/lib/gforth:.}). A path is given as a @code{:}-separated
204: list.
205:
206: @item --dictionary-size @var{size}
207: @item -m @var{size}
208: Allocate @var{size} space for the Forth dictionary space instead of
209: using the default specified in the image (typically 256K). The
210: @var{size} specification consists of an integer and a unit (e.g.,
211: @code{4M}). The unit can be one of @code{b} (bytes), @code{e} (element
212: size, in this case Cells), @code{k} (kilobytes), and @code{M}
213: (Megabytes). If no unit is specified, @code{e} is used.
214:
215: @item --data-stack-size @var{size}
216: @item -d @var{size}
217: Allocate @var{size} space for the data stack instead of using the
218: default specified in the image (typically 16K).
219:
220: @item --return-stack-size @var{size}
221: @item -r @var{size}
222: Allocate @var{size} space for the return stack instead of using the
223: default specified in the image (typically 16K).
224:
225: @item --fp-stack-size @var{size}
226: @item -f @var{size}
227: Allocate @var{size} space for the floating point stack instead of
228: using the default specified in the image (typically 16K). In this case
229: the unit specifier @code{e} refers to floating point numbers.
230:
231: @item --locals-stack-size @var{size}
232: @item -l @var{size}
233: Allocate @var{size} space for the locals stack instead of using the
234: default specified in the image (typically 16K).
235:
236: @end table
237:
238: As explained above, the image-specific command-line arguments for the
239: default image @file{gforth.fi} consist of a sequence of filenames and
240: @code{-e @var{forth-code}} options that are interpreted in the seqence
241: in which they are given. The @code{-e @var{forth-code}} or
242: @code{--evaluate @var{forth-code}} option evaluates the forth
243: code. This option takes only one argument; if you want to evaluate more
244: Forth words, you have to quote them or use several @code{-e}s. To exit
245: after processing the command line (instead of entering interactive mode)
246: append @code{-e bye} to the command line.
247:
248: Not yet implemented:
249: On startup the system first executes the system initialization file
250: (unless the option @code{--no-init-file} is given; note that the system
251: resulting from using this option may not be ANS Forth conformant). Then
252: the user initialization file @file{.gforth.fs} is executed, unless the
253: option @code{--no-rc} is given; this file is first searched in @file{.},
254: then in @file{~}, then in the normal path (see above).
255:
256: @node Words, ANS conformance, Invocation, Top
257: @chapter Forth Words
258:
259: @menu
260: * Notation::
261: * Arithmetic::
262: * Stack Manipulation::
263: * Memory access::
264: * Control Structures::
265: * Locals::
266: * Defining Words::
267: * Wordlists::
268: * Files::
269: * Blocks::
270: * Other I/O::
271: * Programming Tools::
272: * Threading Words::
273: @end menu
274:
275: @node Notation, Arithmetic, Words, Words
276: @section Notation
277:
278: The Forth words are described in this section in the glossary notation
279: that has become a de-facto standard for Forth texts, i.e.
280:
281: @format
282: @var{word} @var{Stack effect} @var{wordset} @var{pronunciation}
283: @end format
284: @var{Description}
285:
286: @table @var
287: @item word
288: The name of the word. BTW, GNU Forth is case insensitive, so you can
289: type the words in in lower case (However, @pxref{core-idef}).
290:
291: @item Stack effect
292: The stack effect is written in the notation @code{@var{before} --
293: @var{after}}, where @var{before} and @var{after} describe the top of
294: stack entries before and after the execution of the word. The rest of
295: the stack is not touched by the word. The top of stack is rightmost,
296: i.e., a stack sequence is written as it is typed in. Note that GNU Forth
297: uses a separate floating point stack, but a unified stack
298: notation. Also, return stack effects are not shown in @var{stack
299: effect}, but in @var{Description}. The name of a stack item describes
300: the type and/or the function of the item. See below for a discussion of
301: the types.
302:
303: @item pronunciation
304: How the word is pronounced
305:
306: @item wordset
307: The ANS Forth standard is divided into several wordsets. A standard
308: system need not support all of them. So, the fewer wordsets your program
309: uses the more portable it will be in theory. However, we suspect that
310: most ANS Forth systems on personal machines will feature all
311: wordsets. Words that are not defined in the ANS standard have
312: @code{gforth} as wordset.
313:
314: @item Description
315: A description of the behaviour of the word.
316: @end table
317:
318: The type of a stack item is specified by the character(s) the name
319: starts with:
320:
321: @table @code
322: @item f
323: Bool, i.e. @code{false} or @code{true}.
324: @item c
325: Char
326: @item w
327: Cell, can contain an integer or an address
328: @item n
329: signed integer
330: @item u
331: unsigned integer
332: @item d
333: double sized signed integer
334: @item ud
335: double sized unsigned integer
336: @item r
337: Float
338: @item a_
339: Cell-aligned address
340: @item c_
341: Char-aligned address (note that a Char is two bytes in Windows NT)
342: @item f_
343: Float-aligned address
344: @item df_
345: Address aligned for IEEE double precision float
346: @item sf_
347: Address aligned for IEEE single precision float
348: @item xt
349: Execution token, same size as Cell
350: @item wid
351: Wordlist ID, same size as Cell
352: @item f83name
353: Pointer to a name structure
354: @end table
355:
356: @node Arithmetic, Stack Manipulation, Notation, Words
357: @section Arithmetic
358: Forth arithmetic is not checked, i.e., you will not hear about integer
359: overflow on addition or multiplication, you may hear about division by
360: zero if you are lucky. The operator is written after the operands, but
361: the operands are still in the original order. I.e., the infix @code{2-1}
362: corresponds to @code{2 1 -}. Forth offers a variety of division
363: operators. If you perform division with potentially negative operands,
364: you do not want to use @code{/} or @code{/mod} with its undefined
365: behaviour, but rather @code{fm/mod} or @code{sm/mod} (probably the
366: former, @pxref{Mixed precision}).
367:
368: @menu
369: * Single precision::
370: * Bitwise operations::
371: * Mixed precision:: operations with single and double-cell integers
372: * Double precision:: Double-cell integer arithmetic
373: * Floating Point::
374: @end menu
375:
376: @node Single precision, Bitwise operations, Arithmetic, Arithmetic
377: @subsection Single precision
378: doc-+
379: doc--
380: doc-*
381: doc-/
382: doc-mod
383: doc-/mod
384: doc-negate
385: doc-abs
386: doc-min
387: doc-max
388:
389: @node Bitwise operations, Mixed precision, Single precision, Arithmetic
390: @subsection Bitwise operations
391: doc-and
392: doc-or
393: doc-xor
394: doc-invert
395: doc-2*
396: doc-2/
397:
398: @node Mixed precision, Double precision, Bitwise operations, Arithmetic
399: @subsection Mixed precision
400: doc-m+
401: doc-*/
402: doc-*/mod
403: doc-m*
404: doc-um*
405: doc-m*/
406: doc-um/mod
407: doc-fm/mod
408: doc-sm/rem
409:
410: @node Double precision, Floating Point, Mixed precision, Arithmetic
411: @subsection Double precision
412: doc-d+
413: doc-d-
414: doc-dnegate
415: doc-dabs
416: doc-dmin
417: doc-dmax
418:
419: @node Floating Point, , Double precision, Arithmetic
420: @subsection Floating Point
421:
422: Angles in floating point operations are given in radians (a full circle
423: has 2 pi radians). Note, that gforth has a separate floating point
424: stack, but we use the unified notation.
425:
426: Floating point numbers have a number of unpleasant surprises for the
427: unwary (e.g., floating point addition is not associative) and even a few
428: for the wary. You should not use them unless you know what you are doing
429: or you don't care that the results you get are totally bogus. If you
430: want to learn about the problems of floating point numbers (and how to
431: avoid them), you might start with @cite{David Goldberg, What Every
432: Computer Scientist Should Know About Floating-Point Arithmetic, ACM
433: Computing Surveys 23(1):5@minus{}48, March 1991}.
434:
435: doc-f+
436: doc-f-
437: doc-f*
438: doc-f/
439: doc-fnegate
440: doc-fabs
441: doc-fmax
442: doc-fmin
443: doc-floor
444: doc-fround
445: doc-f**
446: doc-fsqrt
447: doc-fexp
448: doc-fexpm1
449: doc-fln
450: doc-flnp1
451: doc-flog
452: doc-falog
453: doc-fsin
454: doc-fcos
455: doc-fsincos
456: doc-ftan
457: doc-fasin
458: doc-facos
459: doc-fatan
460: doc-fatan2
461: doc-fsinh
462: doc-fcosh
463: doc-ftanh
464: doc-fasinh
465: doc-facosh
466: doc-fatanh
467:
468: @node Stack Manipulation, Memory access, Arithmetic, Words
469: @section Stack Manipulation
470:
471: gforth has a data stack (aka parameter stack) for characters, cells,
472: addresses, and double cells, a floating point stack for floating point
473: numbers, a return stack for storing the return addresses of colon
474: definitions and other data, and a locals stack for storing local
475: variables. Note that while every sane Forth has a separate floating
476: point stack, this is not strictly required; an ANS Forth system could
477: theoretically keep floating point numbers on the data stack. As an
478: additional difficulty, you don't know how many cells a floating point
479: number takes. It is reportedly possible to write words in a way that
480: they work also for a unified stack model, but we do not recommend trying
481: it. Instead, just say that your program has an environmental dependency
482: on a separate FP stack.
483:
484: Also, a Forth system is allowed to keep the local variables on the
485: return stack. This is reasonable, as local variables usually eliminate
486: the need to use the return stack explicitly. So, if you want to produce
487: a standard complying program and if you are using local variables in a
488: word, forget about return stack manipulations in that word (see the
489: standard document for the exact rules).
490:
491: @menu
492: * Data stack::
493: * Floating point stack::
494: * Return stack::
495: * Locals stack::
496: * Stack pointer manipulation::
497: @end menu
498:
499: @node Data stack, Floating point stack, Stack Manipulation, Stack Manipulation
500: @subsection Data stack
501: doc-drop
502: doc-nip
503: doc-dup
504: doc-over
505: doc-tuck
506: doc-swap
507: doc-rot
508: doc--rot
509: doc-?dup
510: doc-pick
511: doc-roll
512: doc-2drop
513: doc-2nip
514: doc-2dup
515: doc-2over
516: doc-2tuck
517: doc-2swap
518: doc-2rot
519:
520: @node Floating point stack, Return stack, Data stack, Stack Manipulation
521: @subsection Floating point stack
522: doc-fdrop
523: doc-fnip
524: doc-fdup
525: doc-fover
526: doc-ftuck
527: doc-fswap
528: doc-frot
529:
530: @node Return stack, Locals stack, Floating point stack, Stack Manipulation
531: @subsection Return stack
532: doc->r
533: doc-r>
534: doc-r@
535: doc-rdrop
536: doc-2>r
537: doc-2r>
538: doc-2r@
539: doc-2rdrop
540:
541: @node Locals stack, Stack pointer manipulation, Return stack, Stack Manipulation
542: @subsection Locals stack
543:
544: @node Stack pointer manipulation, , Locals stack, Stack Manipulation
545: @subsection Stack pointer manipulation
546: doc-sp@
547: doc-sp!
548: doc-fp@
549: doc-fp!
550: doc-rp@
551: doc-rp!
552: doc-lp@
553: doc-lp!
554:
555: @node Memory access, Control Structures, Stack Manipulation, Words
556: @section Memory access
557:
558: @menu
559: * Stack-Memory transfers::
560: * Address arithmetic::
561: * Memory block access::
562: @end menu
563:
564: @node Stack-Memory transfers, Address arithmetic, Memory access, Memory access
565: @subsection Stack-Memory transfers
566:
567: doc-@
568: doc-!
569: doc-+!
570: doc-c@
571: doc-c!
572: doc-2@
573: doc-2!
574: doc-f@
575: doc-f!
576: doc-sf@
577: doc-sf!
578: doc-df@
579: doc-df!
580:
581: @node Address arithmetic, Memory block access, Stack-Memory transfers, Memory access
582: @subsection Address arithmetic
583:
584: ANS Forth does not specify the sizes of the data types. Instead, it
585: offers a number of words for computing sizes and doing address
586: arithmetic. Basically, address arithmetic is performed in terms of
587: address units (aus); on most systems the address unit is one byte. Note
588: that a character may have more than one au, so @code{chars} is no noop
589: (on systems where it is a noop, it compiles to nothing).
590:
591: ANS Forth also defines words for aligning addresses for specific
592: addresses. Many computers require that accesses to specific data types
593: must only occur at specific addresses; e.g., that cells may only be
594: accessed at addresses divisible by 4. Even if a machine allows unaligned
595: accesses, it can usually perform aligned accesses faster.
596:
597: For the performance-concious: alignment operations are usually only
598: necessary during the definition of a data structure, not during the
599: (more frequent) accesses to it.
600:
601: ANS Forth defines no words for character-aligning addresses. This is not
602: an oversight, but reflects the fact that addresses that are not
603: char-aligned have no use in the standard and therefore will not be
604: created.
605:
606: The standard guarantees that addresses returned by @code{CREATE}d words
607: are cell-aligned; in addition, gforth guarantees that these addresses
608: are aligned for all purposes.
609:
610: Note that the standard defines a word @code{char}, which has nothing to
611: do with address arithmetic.
612:
613: doc-chars
614: doc-char+
615: doc-cells
616: doc-cell+
617: doc-align
618: doc-aligned
619: doc-floats
620: doc-float+
621: doc-falign
622: doc-faligned
623: doc-sfloats
624: doc-sfloat+
625: doc-sfalign
626: doc-sfaligned
627: doc-dfloats
628: doc-dfloat+
629: doc-dfalign
630: doc-dfaligned
631: doc-maxalign
632: doc-maxaligned
633: doc-cfalign
634: doc-cfaligned
635: doc-address-unit-bits
636:
637: @node Memory block access, , Address arithmetic, Memory access
638: @subsection Memory block access
639:
640: doc-move
641: doc-erase
642:
643: While the previous words work on address units, the rest works on
644: characters.
645:
646: doc-cmove
647: doc-cmove>
648: doc-fill
649: doc-blank
650:
651: @node Control Structures, Locals, Memory access, Words
652: @section Control Structures
653:
654: Control structures in Forth cannot be used in interpret state, only in
655: compile state, i.e., in a colon definition. We do not like this
656: limitation, but have not seen a satisfying way around it yet, although
657: many schemes have been proposed.
658:
659: @menu
660: * Selection::
661: * Simple Loops::
662: * Counted Loops::
663: * Arbitrary control structures::
664: * Calls and returns::
665: * Exception Handling::
666: @end menu
667:
668: @node Selection, Simple Loops, Control Structures, Control Structures
669: @subsection Selection
670:
671: @example
672: @var{flag}
673: IF
674: @var{code}
675: ENDIF
676: @end example
677: or
678: @example
679: @var{flag}
680: IF
681: @var{code1}
682: ELSE
683: @var{code2}
684: ENDIF
685: @end example
686:
687: You can use @code{THEN} instead of @code{ENDIF}. Indeed, @code{THEN} is
688: standard, and @code{ENDIF} is not, although it is quite popular. We
689: recommend using @code{ENDIF}, because it is less confusing for people
690: who also know other languages (and is not prone to reinforcing negative
691: prejudices against Forth in these people). Adding @code{ENDIF} to a
692: system that only supplies @code{THEN} is simple:
693: @example
694: : endif POSTPONE then ; immediate
695: @end example
696:
697: [According to @cite{Webster's New Encyclopedic Dictionary}, @dfn{then
698: (adv.)} has the following meanings:
699: @quotation
700: ... 2b: following next after in order ... 3d: as a necessary consequence
701: (if you were there, then you saw them).
702: @end quotation
703: Forth's @code{THEN} has the meaning 2b, whereas @code{THEN} in Pascal
704: and many other programming languages has the meaning 3d.]
705:
706: We also provide the words @code{?dup-if} and @code{?dup-0=-if}, so you
707: can avoid using @code{?dup}.
708:
709: @example
710: @var{n}
711: CASE
712: @var{n1} OF @var{code1} ENDOF
713: @var{n2} OF @var{code2} ENDOF
714: @dots{}
715: ENDCASE
716: @end example
717:
718: Executes the first @var{codei}, where the @var{ni} is equal to
719: @var{n}. A default case can be added by simply writing the code after
720: the last @code{ENDOF}. It may use @var{n}, which is on top of the stack,
721: but must not consume it.
722:
723: @node Simple Loops, Counted Loops, Selection, Control Structures
724: @subsection Simple Loops
725:
726: @example
727: BEGIN
728: @var{code1}
729: @var{flag}
730: WHILE
731: @var{code2}
732: REPEAT
733: @end example
734:
735: @var{code1} is executed and @var{flag} is computed. If it is true,
736: @var{code2} is executed and the loop is restarted; If @var{flag} is false, execution continues after the @code{REPEAT}.
737:
738: @example
739: BEGIN
740: @var{code}
741: @var{flag}
742: UNTIL
743: @end example
744:
745: @var{code} is executed. The loop is restarted if @code{flag} is false.
746:
747: @example
748: BEGIN
749: @var{code}
750: AGAIN
751: @end example
752:
753: This is an endless loop.
754:
755: @node Counted Loops, Arbitrary control structures, Simple Loops, Control Structures
756: @subsection Counted Loops
757:
758: The basic counted loop is:
759: @example
760: @var{limit} @var{start}
761: ?DO
762: @var{body}
763: LOOP
764: @end example
765:
766: This performs one iteration for every integer, starting from @var{start}
767: and up to, but excluding @var{limit}. The counter, aka index, can be
768: accessed with @code{i}. E.g., the loop
769: @example
770: 10 0 ?DO
771: i .
772: LOOP
773: @end example
774: prints
775: @example
776: 0 1 2 3 4 5 6 7 8 9
777: @end example
778: The index of the innermost loop can be accessed with @code{i}, the index
779: of the next loop with @code{j}, and the index of the third loop with
780: @code{k}.
781:
782: The loop control data are kept on the return stack, so there are some
783: restrictions on mixing return stack accesses and counted loop
784: words. E.g., if you put values on the return stack outside the loop, you
785: cannot read them inside the loop. If you put values on the return stack
786: within a loop, you have to remove them before the end of the loop and
787: before accessing the index of the loop.
788:
789: There are several variations on the counted loop:
790:
791: @code{LEAVE} leaves the innermost counted loop immediately.
792:
793: @code{LOOP} can be replaced with @code{@var{n} +LOOP}; this updates the
794: index by @var{n} instead of by 1. The loop is terminated when the border
795: between @var{limit-1} and @var{limit} is crossed. E.g.:
796:
797: @code{4 0 ?DO i . 2 +LOOP} prints @code{0 2}
798:
799: @code{4 1 ?DO i . 2 +LOOP} prints @code{1 3}
800:
801: The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative:
802:
803: @code{-1 0 ?DO i . -1 +LOOP} prints @code{0 -1}
804:
805: @code{ 0 0 ?DO i . -1 +LOOP} prints nothing
806:
807: Therefore we recommend avoiding using @code{@var{n} +LOOP} with negative
808: @var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative
809: case behaves symmetrical to the positive case:
810:
811: @code{-2 0 ?DO i . -1 S+LOOP} prints @code{0 -1}
812:
813: @code{-1 0 ?DO i . -1 S+LOOP} prints @code{0}
814:
815: @code{ 0 0 ?DO i . -1 S+LOOP} prints nothing
816:
817: The loop is terminated when the border between @var{limit@minus{}sgn(n)} and
818: @var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS
819: Forth standard.
820:
821: @code{?DO} can be replaced by @code{DO}. @code{DO} enters the loop even
822: when the start and the limit value are equal. We do not recommend using
823: @code{DO}. It will just give you maintenance troubles.
824:
825: @code{UNLOOP} is used to prepare for an abnormal loop exit, e.g., via
826: @code{EXIT}. @code{UNLOOP} removes the loop control parameters from the
827: return stack so @code{EXIT} can get to its return address.
828:
829: Another counted loop is
830: @example
831: @var{n}
832: FOR
833: @var{body}
834: NEXT
835: @end example
836: This is the preferred loop of native code compiler writers who are too
837: lazy to optimize @code{?DO} loops properly. In GNU Forth, this loop
838: iterates @var{n+1} times; @code{i} produces values starting with @var{n}
839: and ending with 0. Other Forth systems may behave differently, even if
840: they support @code{FOR} loops.
841:
842: @node Arbitrary control structures, Calls and returns, Counted Loops, Control Structures
843: @subsection Arbitrary control structures
844:
845: ANS Forth permits and supports using control structures in a non-nested
846: way. Information about incomplete control structures is stored on the
847: control-flow stack. This stack may be implemented on the Forth data
848: stack, and this is what we have done in gforth.
849:
850: An @i{orig} entry represents an unresolved forward branch, a @i{dest}
851: entry represents a backward branch target. A few words are the basis for
852: building any control structure possible (except control structures that
853: need storage, like calls, coroutines, and backtracking).
854:
855: doc-if
856: doc-ahead
857: doc-then
858: doc-begin
859: doc-until
860: doc-again
861: doc-cs-pick
862: doc-cs-roll
863:
864: On many systems control-flow stack items take one word, in gforth they
865: currently take three (this may change in the future). Therefore it is a
866: really good idea to manipulate the control flow stack with
867: @code{cs-pick} and @code{cs-roll}, not with data stack manipulation
868: words.
869:
870: Some standard control structure words are built from these words:
871:
872: doc-else
873: doc-while
874: doc-repeat
875:
876: Counted loop words constitute a separate group of words:
877:
878: doc-?do
879: doc-do
880: doc-for
881: doc-loop
882: doc-s+loop
883: doc-+loop
884: doc-next
885: doc-leave
886: doc-?leave
887: doc-unloop
888: doc-done
889:
890: The standard does not allow using @code{cs-pick} and @code{cs-roll} on
891: @i{do-sys}. Our system allows it, but it's your job to ensure that for
892: every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path
893: through the definition (@code{LOOP} etc. compile an @code{UNLOOP} on the
894: fall-through path). Also, you have to ensure that all @code{LEAVE}s are
895: resolved (by using one of the loop-ending words or @code{DONE}).
896:
897: Another group of control structure words are
898:
899: doc-case
900: doc-endcase
901: doc-of
902: doc-endof
903:
904: @i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and
905: @code{cs-roll}.
906:
907: @subsubsection Programming Style
908:
909: In order to ensure readability we recommend that you do not create
910: arbitrary control structures directly, but define new control structure
911: words for the control structure you want and use these words in your
912: program.
913:
914: E.g., instead of writing
915:
916: @example
917: begin
918: ...
919: if [ 1 cs-roll ]
920: ...
921: again then
922: @end example
923:
924: we recommend defining control structure words, e.g.,
925:
926: @example
927: : while ( dest -- orig dest )
928: POSTPONE if
929: 1 cs-roll ; immediate
930:
931: : repeat ( orig dest -- )
932: POSTPONE again
933: POSTPONE then ; immediate
934: @end example
935:
936: and then using these to create the control structure:
937:
938: @example
939: begin
940: ...
941: while
942: ...
943: repeat
944: @end example
945:
946: That's much easier to read, isn't it? Of course, @code{BEGIN} and
947: @code{WHILE} are predefined, so in this example it would not be
948: necessary to define them.
949:
950: @node Calls and returns, Exception Handling, Arbitrary control structures, Control Structures
951: @subsection Calls and returns
952:
953: A definition can be called simply be writing the name of the
954: definition. When the end of the definition is reached, it returns. An earlier return can be forced using
955:
956: doc-exit
957:
958: Don't forget to clean up the return stack and @code{UNLOOP} any
959: outstanding @code{?DO}...@code{LOOP}s before @code{EXIT}ing. The
960: primitive compiled by @code{EXIT} is
961:
962: doc-;s
963:
964: @node Exception Handling, , Calls and returns, Control Structures
965: @subsection Exception Handling
966:
967: doc-catch
968: doc-throw
969:
970: @node Locals, Defining Words, Control Structures, Words
971: @section Locals
972:
973: Local variables can make Forth programming more enjoyable and Forth
974: programs easier to read. Unfortunately, the locals of ANS Forth are
975: laden with restrictions. Therefore, we provide not only the ANS Forth
976: locals wordset, but also our own, more powerful locals wordset (we
977: implemented the ANS Forth locals wordset through our locals wordset).
978:
979: @menu
980: * gforth locals::
981: * ANS Forth locals::
982: @end menu
983:
984: @node gforth locals, ANS Forth locals, Locals, Locals
985: @subsection gforth locals
986:
987: Locals can be defined with
988:
989: @example
990: @{ local1 local2 ... -- comment @}
991: @end example
992: or
993: @example
994: @{ local1 local2 ... @}
995: @end example
996:
997: E.g.,
998: @example
999: : max @{ n1 n2 -- n3 @}
1000: n1 n2 > if
1001: n1
1002: else
1003: n2
1004: endif ;
1005: @end example
1006:
1007: The similarity of locals definitions with stack comments is intended. A
1008: locals definition often replaces the stack comment of a word. The order
1009: of the locals corresponds to the order in a stack comment and everything
1010: after the @code{--} is really a comment.
1011:
1012: This similarity has one disadvantage: It is too easy to confuse locals
1013: declarations with stack comments, causing bugs and making them hard to
1014: find. However, this problem can be avoided by appropriate coding
1015: conventions: Do not use both notations in the same program. If you do,
1016: they should be distinguished using additional means, e.g. by position.
1017:
1018: The name of the local may be preceded by a type specifier, e.g.,
1019: @code{F:} for a floating point value:
1020:
1021: @example
1022: : CX* @{ F: Ar F: Ai F: Br F: Bi -- Cr Ci @}
1023: \ complex multiplication
1024: Ar Br f* Ai Bi f* f-
1025: Ar Bi f* Ai Br f* f+ ;
1026: @end example
1027:
1028: GNU Forth currently supports cells (@code{W:}, @code{W^}), doubles
1029: (@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters
1030: (@code{C:}, @code{C^}) in two flavours: a value-flavoured local (defined
1031: with @code{W:}, @code{D:} etc.) produces its value and can be changed
1032: with @code{TO}. A variable-flavoured local (defined with @code{W^} etc.)
1033: produces its address (which becomes invalid when the variable's scope is
1034: left). E.g., the standard word @code{emit} can be defined in therms of
1035: @code{type} like this:
1036:
1037: @example
1038: : emit @{ C^ char* -- @}
1039: char* 1 type ;
1040: @end example
1041:
1042: A local without type specifier is a @code{W:} local. Both flavours of
1043: locals are initialized with values from the data or FP stack.
1044:
1045: Currently there is no way to define locals with user-defined data
1046: structures, but we are working on it.
1047:
1048: GNU Forth allows defining locals everywhere in a colon definition. This
1049: poses the following questions:
1050:
1051: @menu
1052: * Where are locals visible by name?::
1053: * How long do locals live?::
1054: * Programming Style::
1055: * Implementation::
1056: @end menu
1057:
1058: @node Where are locals visible by name?, How long do locals live?, gforth locals, gforth locals
1059: @subsubsection Where are locals visible by name?
1060:
1061: Basically, the answer is that locals are visible where you would expect
1062: it in block-structured languages, and sometimes a little longer. If you
1063: want to restrict the scope of a local, enclose its definition in
1064: @code{SCOPE}...@code{ENDSCOPE}.
1065:
1066: doc-scope
1067: doc-endscope
1068:
1069: These words behave like control structure words, so you can use them
1070: with @code{CS-PICK} and @code{CS-ROLL} to restrict the scope in
1071: arbitrary ways.
1072:
1073: If you want a more exact answer to the visibility question, here's the
1074: basic principle: A local is visible in all places that can only be
1075: reached through the definition of the local@footnote{In compiler
1076: construction terminology, all places dominated by the definition of the
1077: local.}. In other words, it is not visible in places that can be reached
1078: without going through the definition of the local. E.g., locals defined
1079: in @code{IF}...@code{ENDIF} are visible until the @code{ENDIF}, locals
1080: defined in @code{BEGIN}...@code{UNTIL} are visible after the
1081: @code{UNTIL} (until, e.g., a subsequent @code{ENDSCOPE}).
1082:
1083: The reasoning behind this solution is: We want to have the locals
1084: visible as long as it is meaningful. The user can always make the
1085: visibility shorter by using explicit scoping. In a place that can
1086: only be reached through the definition of a local, the meaning of a
1087: local name is clear. In other places it is not: How is the local
1088: initialized at the control flow path that does not contain the
1089: definition? Which local is meant, if the same name is defined twice in
1090: two independent control flow paths?
1091:
1092: This should be enough detail for nearly all users, so you can skip the
1093: rest of this section. If you relly must know all the gory details and
1094: options, read on.
1095:
1096: In order to implement this rule, the compiler has to know which places
1097: are unreachable. It knows this automatically after @code{AHEAD},
1098: @code{AGAIN}, @code{EXIT} and @code{LEAVE}; in other cases (e.g., after
1099: most @code{THROW}s), you can use the word @code{UNREACHABLE} to tell the
1100: compiler that the control flow never reaches that place. If
1101: @code{UNREACHABLE} is not used where it could, the only consequence is
1102: that the visibility of some locals is more limited than the rule above
1103: says. If @code{UNREACHABLE} is used where it should not (i.e., if you
1104: lie to the compiler), buggy code will be produced.
1105:
1106: Another problem with this rule is that at @code{BEGIN}, the compiler
1107: does not know which locals will be visible on the incoming
1108: back-edge. All problems discussed in the following are due to this
1109: ignorance of the compiler (we discuss the problems using @code{BEGIN}
1110: loops as examples; the discussion also applies to @code{?DO} and other
1111: loops). Perhaps the most insidious example is:
1112: @example
1113: AHEAD
1114: BEGIN
1115: x
1116: [ 1 CS-ROLL ] THEN
1117: @{ x @}
1118: ...
1119: UNTIL
1120: @end example
1121:
1122: This should be legal according to the visibility rule. The use of
1123: @code{x} can only be reached through the definition; but that appears
1124: textually below the use.
1125:
1126: From this example it is clear that the visibility rules cannot be fully
1127: implemented without major headaches. Our implementation treats common
1128: cases as advertised and the exceptions are treated in a safe way: The
1129: compiler makes a reasonable guess about the locals visible after a
1130: @code{BEGIN}; if it is too pessimistic, the
1131: user will get a spurious error about the local not being defined; if the
1132: compiler is too optimistic, it will notice this later and issue a
1133: warning. In the case above the compiler would complain about @code{x}
1134: being undefined at its use. You can see from the obscure examples in
1135: this section that it takes quite unusual control structures to get the
1136: compiler into trouble, and even then it will often do fine.
1137:
1138: If the @code{BEGIN} is reachable from above, the most optimistic guess
1139: is that all locals visible before the @code{BEGIN} will also be
1140: visible after the @code{BEGIN}. This guess is valid for all loops that
1141: are entered only through the @code{BEGIN}, in particular, for normal
1142: @code{BEGIN}...@code{WHILE}...@code{REPEAT} and
1143: @code{BEGIN}...@code{UNTIL} loops and it is implemented in our
1144: compiler. When the branch to the @code{BEGIN} is finally generated by
1145: @code{AGAIN} or @code{UNTIL}, the compiler checks the guess and
1146: warns the user if it was too optimisitic:
1147: @example
1148: IF
1149: @{ x @}
1150: BEGIN
1151: \ x ?
1152: [ 1 cs-roll ] THEN
1153: ...
1154: UNTIL
1155: @end example
1156:
1157: Here, @code{x} lives only until the @code{BEGIN}, but the compiler
1158: optimistically assumes that it lives until the @code{THEN}. It notices
1159: this difference when it compiles the @code{UNTIL} and issues a
1160: warning. The user can avoid the warning, and make sure that @code{x}
1161: is not used in the wrong area by using explicit scoping:
1162: @example
1163: IF
1164: SCOPE
1165: @{ x @}
1166: ENDSCOPE
1167: BEGIN
1168: [ 1 cs-roll ] THEN
1169: ...
1170: UNTIL
1171: @end example
1172:
1173: Since the guess is optimistic, there will be no spurious error messages
1174: about undefined locals.
1175:
1176: If the @code{BEGIN} is not reachable from above (e.g., after
1177: @code{AHEAD} or @code{EXIT}), the compiler cannot even make an
1178: optimistic guess, as the locals visible after the @code{BEGIN} may be
1179: defined later. Therefore, the compiler assumes that no locals are
1180: visible after the @code{BEGIN}. However, the useer can use
1181: @code{ASSUME-LIVE} to make the compiler assume that the same locals are
1182: visible at the BEGIN as at the point where the item was created.
1183:
1184: doc-assume-live
1185:
1186: E.g.,
1187: @example
1188: @{ x @}
1189: AHEAD
1190: ASSUME-LIVE
1191: BEGIN
1192: x
1193: [ 1 CS-ROLL ] THEN
1194: ...
1195: UNTIL
1196: @end example
1197:
1198: Other cases where the locals are defined before the @code{BEGIN} can be
1199: handled by inserting an appropriate @code{CS-ROLL} before the
1200: @code{ASSUME-LIVE} (and changing the control-flow stack manipulation
1201: behind the @code{ASSUME-LIVE}).
1202:
1203: Cases where locals are defined after the @code{BEGIN} (but should be
1204: visible immediately after the @code{BEGIN}) can only be handled by
1205: rearranging the loop. E.g., the ``most insidious'' example above can be
1206: arranged into:
1207: @example
1208: BEGIN
1209: @{ x @}
1210: ... 0=
1211: WHILE
1212: x
1213: REPEAT
1214: @end example
1215:
1216: @node How long do locals live?, Programming Style, Where are locals visible by name?, gforth locals
1217: @subsubsection How long do locals live?
1218:
1219: The right answer for the lifetime question would be: A local lives at
1220: least as long as it can be accessed. For a value-flavoured local this
1221: means: until the end of its visibility. However, a variable-flavoured
1222: local could be accessed through its address far beyond its visibility
1223: scope. Ultimately, this would mean that such locals would have to be
1224: garbage collected. Since this entails un-Forth-like implementation
1225: complexities, I adopted the same cowardly solution as some other
1226: languages (e.g., C): The local lives only as long as it is visible;
1227: afterwards its address is invalid (and programs that access it
1228: afterwards are erroneous).
1229:
1230: @node Programming Style, Implementation, How long do locals live?, gforth locals
1231: @subsubsection Programming Style
1232:
1233: The freedom to define locals anywhere has the potential to change
1234: programming styles dramatically. In particular, the need to use the
1235: return stack for intermediate storage vanishes. Moreover, all stack
1236: manipulations (except @code{PICK}s and @code{ROLL}s with run-time
1237: determined arguments) can be eliminated: If the stack items are in the
1238: wrong order, just write a locals definition for all of them; then
1239: write the items in the order you want.
1240:
1241: This seems a little far-fetched and eliminating stack manipulations is
1242: unlikely to become a conscious programming objective. Still, the number
1243: of stack manipulations will be reduced dramatically if local variables
1244: are used liberally (e.g., compare @code{max} in @ref{gforth locals} with
1245: a traditional implementation of @code{max}).
1246:
1247: This shows one potential benefit of locals: making Forth programs more
1248: readable. Of course, this benefit will only be realized if the
1249: programmers continue to honour the principle of factoring instead of
1250: using the added latitude to make the words longer.
1251:
1252: Using @code{TO} can and should be avoided. Without @code{TO},
1253: every value-flavoured local has only a single assignment and many
1254: advantages of functional languages apply to Forth. I.e., programs are
1255: easier to analyse, to optimize and to read: It is clear from the
1256: definition what the local stands for, it does not turn into something
1257: different later.
1258:
1259: E.g., a definition using @code{TO} might look like this:
1260: @example
1261: : strcmp @{ addr1 u1 addr2 u2 -- n @}
1262: u1 u2 min 0
1263: ?do
1264: addr1 c@ addr2 c@ - ?dup
1265: if
1266: unloop exit
1267: then
1268: addr1 char+ TO addr1
1269: addr2 char+ TO addr2
1270: loop
1271: u1 u2 - ;
1272: @end example
1273: Here, @code{TO} is used to update @code{addr1} and @code{addr2} at
1274: every loop iteration. @code{strcmp} is a typical example of the
1275: readability problems of using @code{TO}. When you start reading
1276: @code{strcmp}, you think that @code{addr1} refers to the start of the
1277: string. Only near the end of the loop you realize that it is something
1278: else.
1279:
1280: This can be avoided by defining two locals at the start of the loop that
1281: are initialized with the right value for the current iteration.
1282: @example
1283: : strcmp @{ addr1 u1 addr2 u2 -- n @}
1284: addr1 addr2
1285: u1 u2 min 0
1286: ?do @{ s1 s2 @}
1287: s1 c@ s2 c@ - ?dup
1288: if
1289: unloop exit
1290: then
1291: s1 char+ s2 char+
1292: loop
1293: 2drop
1294: u1 u2 - ;
1295: @end example
1296: Here it is clear from the start that @code{s1} has a different value
1297: in every loop iteration.
1298:
1299: @node Implementation, , Programming Style, gforth locals
1300: @subsubsection Implementation
1301:
1302: GNU Forth uses an extra locals stack. The most compelling reason for
1303: this is that the return stack is not float-aligned; using an extra stack
1304: also eliminates the problems and restrictions of using the return stack
1305: as locals stack. Like the other stacks, the locals stack grows toward
1306: lower addresses. A few primitives allow an efficient implementation:
1307:
1308: doc-@local#
1309: doc-f@local#
1310: doc-laddr#
1311: doc-lp+!#
1312: doc-lp!
1313: doc->l
1314: doc-f>l
1315:
1316: In addition to these primitives, some specializations of these
1317: primitives for commonly occurring inline arguments are provided for
1318: efficiency reasons, e.g., @code{@@local0} as specialization of
1319: @code{@@local#} for the inline argument 0. The following compiling words
1320: compile the right specialized version, or the general version, as
1321: appropriate:
1322:
1323: doc-compile-@local
1324: doc-compile-f@local
1325: doc-compile-lp+!
1326:
1327: Combinations of conditional branches and @code{lp+!#} like
1328: @code{?branch-lp+!#} (the locals pointer is only changed if the branch
1329: is taken) are provided for efficiency and correctness in loops.
1330:
1331: A special area in the dictionary space is reserved for keeping the
1332: local variable names. @code{@{} switches the dictionary pointer to this
1333: area and @code{@}} switches it back and generates the locals
1334: initializing code. @code{W:} etc.@ are normal defining words. This
1335: special area is cleared at the start of every colon definition.
1336:
1337: A special feature of GNU Forths dictionary is used to implement the
1338: definition of locals without type specifiers: every wordlist (aka
1339: vocabulary) has its own methods for searching
1340: etc. (@pxref{Wordlists}). For the present purpose we defined a wordlist
1341: with a special search method: When it is searched for a word, it
1342: actually creates that word using @code{W:}. @code{@{} changes the search
1343: order to first search the wordlist containing @code{@}}, @code{W:} etc.,
1344: and then the wordlist for defining locals without type specifiers.
1345:
1346: The lifetime rules support a stack discipline within a colon
1347: definition: The lifetime of a local is either nested with other locals
1348: lifetimes or it does not overlap them.
1349:
1350: At @code{BEGIN}, @code{IF}, and @code{AHEAD} no code for locals stack
1351: pointer manipulation is generated. Between control structure words
1352: locals definitions can push locals onto the locals stack. @code{AGAIN}
1353: is the simplest of the other three control flow words. It has to
1354: restore the locals stack depth of the corresponding @code{BEGIN}
1355: before branching. The code looks like this:
1356: @format
1357: @code{lp+!#} current-locals-size @minus{} dest-locals-size
1358: @code{branch} <begin>
1359: @end format
1360:
1361: @code{UNTIL} is a little more complicated: If it branches back, it
1362: must adjust the stack just like @code{AGAIN}. But if it falls through,
1363: the locals stack must not be changed. The compiler generates the
1364: following code:
1365: @format
1366: @code{?branch-lp+!#} <begin> current-locals-size @minus{} dest-locals-size
1367: @end format
1368: The locals stack pointer is only adjusted if the branch is taken.
1369:
1370: @code{THEN} can produce somewhat inefficient code:
1371: @format
1372: @code{lp+!#} current-locals-size @minus{} orig-locals-size
1373: <orig target>:
1374: @code{lp+!#} orig-locals-size @minus{} new-locals-size
1375: @end format
1376: The second @code{lp+!#} adjusts the locals stack pointer from the
1377: level at the @var{orig} point to the level after the @code{THEN}. The
1378: first @code{lp+!#} adjusts the locals stack pointer from the current
1379: level to the level at the orig point, so the complete effect is an
1380: adjustment from the current level to the right level after the
1381: @code{THEN}.
1382:
1383: In a conventional Forth implementation a dest control-flow stack entry
1384: is just the target address and an orig entry is just the address to be
1385: patched. Our locals implementation adds a wordlist to every orig or dest
1386: item. It is the list of locals visible (or assumed visible) at the point
1387: described by the entry. Our implementation also adds a tag to identify
1388: the kind of entry, in particular to differentiate between live and dead
1389: (reachable and unreachable) orig entries.
1390:
1391: A few unusual operations have to be performed on locals wordlists:
1392:
1393: doc-common-list
1394: doc-sub-list?
1395: doc-list-size
1396:
1397: Several features of our locals wordlist implementation make these
1398: operations easy to implement: The locals wordlists are organised as
1399: linked lists; the tails of these lists are shared, if the lists
1400: contain some of the same locals; and the address of a name is greater
1401: than the address of the names behind it in the list.
1402:
1403: Another important implementation detail is the variable
1404: @code{dead-code}. It is used by @code{BEGIN} and @code{THEN} to
1405: determine if they can be reached directly or only through the branch
1406: that they resolve. @code{dead-code} is set by @code{UNREACHABLE},
1407: @code{AHEAD}, @code{EXIT} etc., and cleared at the start of a colon
1408: definition, by @code{BEGIN} and usually by @code{THEN}.
1409:
1410: Counted loops are similar to other loops in most respects, but
1411: @code{LEAVE} requires special attention: It performs basically the same
1412: service as @code{AHEAD}, but it does not create a control-flow stack
1413: entry. Therefore the information has to be stored elsewhere;
1414: traditionally, the information was stored in the target fields of the
1415: branches created by the @code{LEAVE}s, by organizing these fields into a
1416: linked list. Unfortunately, this clever trick does not provide enough
1417: space for storing our extended control flow information. Therefore, we
1418: introduce another stack, the leave stack. It contains the control-flow
1419: stack entries for all unresolved @code{LEAVE}s.
1420:
1421: Local names are kept until the end of the colon definition, even if
1422: they are no longer visible in any control-flow path. In a few cases
1423: this may lead to increased space needs for the locals name area, but
1424: usually less than reclaiming this space would cost in code size.
1425:
1426:
1427: @node ANS Forth locals, , gforth locals, Locals
1428: @subsection ANS Forth locals
1429:
1430: The ANS Forth locals wordset does not define a syntax for locals, but
1431: words that make it possible to define various syntaxes. One of the
1432: possible syntaxes is a subset of the syntax we used in the gforth locals
1433: wordset, i.e.:
1434:
1435: @example
1436: @{ local1 local2 ... -- comment @}
1437: @end example
1438: or
1439: @example
1440: @{ local1 local2 ... @}
1441: @end example
1442:
1443: The order of the locals corresponds to the order in a stack comment. The
1444: restrictions are:
1445:
1446: @itemize @bullet
1447: @item
1448: Locals can only be cell-sized values (no type specifers are allowed).
1449: @item
1450: Locals can be defined only outside control structures.
1451: @item
1452: Locals can interfere with explicit usage of the return stack. For the
1453: exact (and long) rules, see the standard. If you don't use return stack
1454: accessing words in a definition using locals, you will we all right. The
1455: purpose of this rule is to make locals implementation on the return
1456: stack easier.
1457: @item
1458: The whole definition must be in one line.
1459: @end itemize
1460:
1461: Locals defined in this way behave like @code{VALUE}s
1462: (@xref{Values}). I.e., they are initialized from the stack. Using their
1463: name produces their value. Their value can be changed using @code{TO}.
1464:
1465: Since this syntax is supported by gforth directly, you need not do
1466: anything to use it. If you want to port a program using this syntax to
1467: another ANS Forth system, use @file{anslocal.fs} to implement the syntax
1468: on the other system.
1469:
1470: Note that a syntax shown in the standard, section A.13 looks
1471: similar, but is quite different in having the order of locals
1472: reversed. Beware!
1473:
1474: The ANS Forth locals wordset itself consists of the following word
1475:
1476: doc-(local)
1477:
1478: The ANS Forth locals extension wordset defines a syntax, but it is so
1479: awful that we strongly recommend not to use it. We have implemented this
1480: syntax to make porting to gforth easy, but do not document it here. The
1481: problem with this syntax is that the locals are defined in an order
1482: reversed with respect to the standard stack comment notation, making
1483: programs harder to read, and easier to misread and miswrite. The only
1484: merit of this syntax is that it is easy to implement using the ANS Forth
1485: locals wordset.
1486:
1487: @node Defining Words, Wordlists, Locals, Words
1488: @section Defining Words
1489:
1490: @menu
1491: * Values::
1492: @end menu
1493:
1494: @node Values, , Defining Words, Defining Words
1495: @subsection Values
1496:
1497: @node Wordlists, Files, Defining Words, Words
1498: @section Wordlists
1499:
1500: @node Files, Blocks, Wordlists, Words
1501: @section Files
1502:
1503: @node Blocks, Other I/O, Files, Words
1504: @section Blocks
1505:
1506: @node Other I/O, Programming Tools, Blocks, Words
1507: @section Other I/O
1508:
1509: @node Programming Tools, Threading Words, Other I/O, Words
1510: @section Programming Tools
1511:
1512: @menu
1513: * Debugging:: Simple and quick.
1514: * Assertions:: Making your programs self-checking.
1515: @end menu
1516:
1517: @node Debugging, Assertions, Programming Tools, Programming Tools
1518: @subsection Debugging
1519:
1520: The simple debugging aids provided in @file{debugging.fs}
1521: are meant to support a different style of debugging than the
1522: tracing/stepping debuggers used in languages with long turn-around
1523: times.
1524:
1525: A much better (faster) way in fast-compilig languages is to add
1526: printing code at well-selected places, let the program run, look at
1527: the output, see where things went wrong, add more printing code, etc.,
1528: until the bug is found.
1529:
1530: The word @code{~~} is easy to insert. It just prints debugging
1531: information (by default the source location and the stack contents). It
1532: is also easy to remove (@kbd{C-x ~} in the Emacs Forth mode to
1533: query-replace them with nothing). The deferred words
1534: @code{printdebugdata} and @code{printdebugline} control the output of
1535: @code{~~}. The default source location output format works well with
1536: Emacs' compilation mode, so you can step through the program at the
1537: source level using @kbd{C-x `} (the advantage over a stepping debugger
1538: is that you can step in any direction and you know where the crash has
1539: happened or where the strange data has occurred).
1540:
1541: Note that the default actions clobber the contents of the pictured
1542: numeric output string, so you should not use @code{~~}, e.g., between
1543: @code{<#} and @code{#>}.
1544:
1545: doc-~~
1546: doc-printdebugdata
1547: doc-printdebugline
1548:
1549: @node Assertions, , Debugging, Programming Tools
1550: @subsection Assertions
1551:
1552: It is a good idea to make your programs self-checking, in particular, if
1553: you use an assumption (e.g., that a certain field of a data structure is
1554: never zero) that may become wrong during maintenance. GForth supports
1555: assertions for this purpose. They are used like this:
1556:
1557: @example
1558: assert( @var{flag} )
1559: @end example
1560:
1561: The code between @code{assert(} and @code{)} should compute a flag, that
1562: should be true if everything is alright and false otherwise. It should
1563: not change anything else on the stack. The overall stack effect of the
1564: assertion is @code{( -- )}. E.g.
1565:
1566: @example
1567: assert( 1 1 + 2 = ) \ what we learn in school
1568: assert( dup 0<> ) \ assert that the top of stack is not zero
1569: assert( false ) \ this code should not be reached
1570: @end example
1571:
1572: The need for assertions is different at different times. During
1573: debugging, we want more checking, in production we sometimes care more
1574: for speed. Therefore, assertions can be turned off, i.e., the assertion
1575: becomes a comment. Depending on the importance of an assertion and the
1576: time it takes to check it, you may want to turn off some assertions and
1577: keep others turned on. GForth provides several levels of assertions for
1578: this purpose:
1579:
1580: doc-assert0(
1581: doc-assert1(
1582: doc-assert2(
1583: doc-assert3(
1584: doc-assert(
1585: doc-)
1586:
1587: @code{Assert(} is the same as @code{assert1(}. The variable
1588: @code{assert-level} specifies the highest assertions that are turned
1589: on. I.e., at the default @code{assert-level} of one, @code{assert0(} and
1590: @code{assert1(} assertions perform checking, while @code{assert2(} and
1591: @code{assert3(} assertions are treated as comments.
1592:
1593: Note that the @code{assert-level} is evaluated at compile-time, not at
1594: run-time. I.e., you cannot turn assertions on or off at run-time, you
1595: have to set the @code{assert-level} appropriately before compiling a
1596: piece of code. You can compile several pieces of code at several
1597: @code{assert-level}s (e.g., a trusted library at level 1 and newly
1598: written code at level 3).
1599:
1600: doc-assert-level
1601:
1602: If an assertion fails, a message compatible with Emacs' compilation mode
1603: is produced and the execution is aborted (currently with @code{ABORT"}.
1604: If there is interest, we will introduce a special throw code. But if you
1605: intend to @code{catch} a specific condition, using @code{throw} is
1606: probably more appropriate than an assertion).
1607:
1608: @node Threading Words, , Programming Tools, Words
1609: @section Threading Words
1610:
1611: These words provide access to code addresses and other threading stuff
1612: in gforth (and, possibly, other interpretive Forths). It more or less
1613: abstracts away the differences between direct and indirect threading
1614: (and, for direct threading, the machine dependences). However, at
1615: present this wordset is still inclomplete. It is also pretty low-level;
1616: some day it will hopefully be made unnecessary by an internals words set
1617: that abstracts implementation details away completely.
1618:
1619: doc->code-address
1620: doc->does-code
1621: doc-code-address!
1622: doc-does-code!
1623: doc-does-handler!
1624: doc-/does-handler
1625:
1626:
1627:
1628: @node ANS conformance, Model, Words, Top
1629: @chapter ANS conformance
1630:
1631: To the best of our knowledge, gforth is an
1632:
1633: ANS Forth System
1634: @itemize
1635: @item providing the Core Extensions word set
1636: @item providing the Block word set
1637: @item providing the Block Extensions word set
1638: @item providing the Double-Number word set
1639: @item providing the Double-Number Extensions word set
1640: @item providing the Exception word set
1641: @item providing the Exception Extensions word set
1642: @item providing the Facility word set
1643: @item providing @code{MS} and @code{TIME&DATE} from the Facility Extensions word set
1644: @item providing the File Access word set
1645: @item providing the File Access Extensions word set
1646: @item providing the Floating-Point word set
1647: @item providing the Floating-Point Extensions word set
1648: @item providing the Locals word set
1649: @item providing the Locals Extensions word set
1650: @item providing the Memory-Allocation word set
1651: @item providing the Memory-Allocation Extensions word set (that one's easy)
1652: @item providing the Programming-Tools word set
1653: @item providing @code{AHEAD}, @code{BYE}, @code{CS-PICK}, @code{CS-ROLL}, @code{STATE}, @code{[ELSE]}, @code{[IF]}, @code{[THEN]} from the Programming-Tools Extensions word set
1654: @item providing the Search-Order word set
1655: @item providing the Search-Order Extensions word set
1656: @item providing the String word set
1657: @item providing the String Extensions word set (another easy one)
1658: @end itemize
1659:
1660: In addition, ANS Forth systems are required to document certain
1661: implementation choices. This chapter tries to meet these
1662: requirements. In many cases it gives a way to ask the system for the
1663: information instead of providing the information directly, in
1664: particular, if the information depends on the processor, the operating
1665: system or the installation options chosen, or if they are likely to
1666: change during the maintenance of gforth.
1667:
1668: @comment The framework for the rest has been taken from pfe.
1669:
1670: @menu
1671: * The Core Words::
1672: * The optional Block word set::
1673: * The optional Double Number word set::
1674: * The optional Exception word set::
1675: * The optional Facility word set::
1676: * The optional File-Access word set::
1677: * The optional Floating-Point word set::
1678: * The optional Locals word set::
1679: * The optional Memory-Allocation word set::
1680: * The optional Programming-Tools word set::
1681: * The optional Search-Order word set::
1682: @end menu
1683:
1684:
1685: @c =====================================================================
1686: @node The Core Words, The optional Block word set, ANS conformance, ANS conformance
1687: @comment node-name, next, previous, up
1688: @section The Core Words
1689: @c =====================================================================
1690:
1691: @menu
1692: * core-idef:: Implementation Defined Options
1693: * core-ambcond:: Ambiguous Conditions
1694: * core-other:: Other System Documentation
1695: @end menu
1696:
1697: @c ---------------------------------------------------------------------
1698: @node core-idef, core-ambcond, The Core Words, The Core Words
1699: @subsection Implementation Defined Options
1700: @c ---------------------------------------------------------------------
1701:
1702: @table @i
1703:
1704: @item (Cell) aligned addresses:
1705: processor-dependent. Gforths alignment words perform natural alignment
1706: (e.g., an address aligned for a datum of size 8 is divisible by
1707: 8). Unaligned accesses usually result in a @code{-23 THROW}.
1708:
1709: @item @code{EMIT} and non-graphic characters:
1710: The character is output using the C library function (actually, macro)
1711: @code{putchar}.
1712:
1713: @item character editing of @code{ACCEPT} and @code{EXPECT}:
1714: This is modeled on the GNU readline library (@pxref{Readline
1715: Interaction, , Command Line Editing, readline, The GNU Readline
1716: Library}) with Emacs-like key bindings. @kbd{Tab} deviates a little by
1717: producing a full word completion every time you type it (instead of
1718: producing the common prefix of all completions).
1719:
1720: @item character set:
1721: The character set of your computer and display device. Gforth is
1722: 8-bit-clean (but some other component in your system may make trouble).
1723:
1724: @item Character-aligned address requirements:
1725: installation-dependent. Currently a character is represented by a C
1726: @code{unsigned char}; in the future we might switch to @code{wchar_t}
1727: (Comments on that requested).
1728:
1729: @item character-set extensions and matching of names:
1730: Any character except 0 can be used in a name. Matching is
1731: case-insensitive. The matching is performed using the C function
1732: @code{strncasecmp}, whose function is probably influenced by the
1733: locale. E.g., the @code{C} locale does not know about accents and
1734: umlauts, so they are matched case-sensitively in that locale. For
1735: portability reasons it is best to write programs such that they work in
1736: the @code{C} locale. Then one can use libraries written by a Polish
1737: programmer (who might use words containing ISO Latin-2 encoded
1738: characters) and by a French programmer (ISO Latin-1) in the same program
1739: (of course, @code{WORDS} will produce funny results for some of the
1740: words (which ones, depends on the font you are using)). Also, the locale
1741: you prefer may not be available in other operating systems. Hopefully,
1742: Unicode will solve these problems one day.
1743:
1744: @item conditions under which control characters match a space delimiter:
1745: If @code{WORD} is called with the space character as a delimiter, all
1746: white-space characters (as identified by the C macro @code{isspace()})
1747: are delimiters. @code{PARSE}, on the other hand, treats space like other
1748: delimiters. @code{PARSE-WORD} treats space like @code{WORD}, but behaves
1749: like @code{PARSE} otherwise. @code{(NAME)}, which is used by the outer
1750: interpreter (aka text interpreter) by default, treats all white-space
1751: characters as delimiters.
1752:
1753: @item format of the control flow stack:
1754: The data stack is used as control flow stack. The size of a control flow
1755: stack item in cells is given by the constant @code{cs-item-size}. At the
1756: time of this writing, an item consists of a (pointer to a) locals list
1757: (third), an address in the code (second), and a tag for identifying the
1758: item (TOS). The following tags are used: @code{defstart},
1759: @code{live-orig}, @code{dead-orig}, @code{dest}, @code{do-dest},
1760: @code{scopestart}.
1761:
1762: @item conversion of digits > 35
1763: The characters @code{[\]^_'} are the digits with the decimal value
1764: 36@minus{}41. There is no way to input many of the larger digits.
1765:
1766: @item display after input terminates in @code{ACCEPT} and @code{EXPECT}:
1767: The cursor is moved to the end of the entered string. If the input is
1768: terminated using the @kbd{Return} key, a space is typed.
1769:
1770: @item exception abort sequence of @code{ABORT"}:
1771: The error string is stored into the variable @code{"error} and a
1772: @code{-2 throw} is performed.
1773:
1774: @item input line terminator:
1775: For interactive input, @kbd{C-m} and @kbd{C-j} terminate lines. One of
1776: these characters is typically produced when you type the @kbd{Enter} or
1777: @kbd{Return} key.
1778:
1779: @item maximum size of a counted string:
1780: @code{s" /counted-string" environment? drop .}. Currently 255 characters
1781: on all ports, but this may change.
1782:
1783: @item maximum size of a parsed string:
1784: Given by the constant @code{/line}. Currently 255 characters.
1785:
1786: @item maximum size of a definition name, in characters:
1787: 31
1788:
1789: @item maximum string length for @code{ENVIRONMENT?}, in characters:
1790: 31
1791:
1792: @item method of selecting the user input device:
1793: The user input device is the standard input. There is current no way to
1794: change it from within gforth. However, the input can typically be
1795: redirected in the command line that starts gforth.
1796:
1797: @item method of selecting the user output device:
1798: The user output device is the standard output. It cannot be redirected
1799: from within gforth, but typically from the command line that starts
1800: gforth. Gforth uses buffered output, so output on a terminal does not
1801: become visible before the next newline or buffer overflow. Output on
1802: non-terminals is invisible until the buffer overflows.
1803:
1804: @item methods of dictionary compilation:
1805: Waht are we expected to document here?
1806:
1807: @item number of bits in one address unit:
1808: @code{s" address-units-bits" environment? drop .}. 8 in all current
1809: ports.
1810:
1811: @item number representation and arithmetic:
1812: Processor-dependent. Binary two's complement on all current ports.
1813:
1814: @item ranges for integer types:
1815: Installation-dependent. Make environmental queries for @code{MAX-N},
1816: @code{MAX-U}, @code{MAX-D} and @code{MAX-UD}. The lower bounds for
1817: unsigned (and positive) types is 0. The lower bound for signed types on
1818: two's complement and one's complement machines machines can be computed
1819: by adding 1 to the upper bound.
1820:
1821: @item read-only data space regions:
1822: The whole Forth data space is writable.
1823:
1824: @item size of buffer at @code{WORD}:
1825: @code{PAD HERE - .}. 104 characters on 32-bit machines. The buffer is
1826: shared with the pictured numeric output string. If overwriting
1827: @code{PAD} is acceptable, it is as large as the remaining dictionary
1828: space, although only as much can be sensibly used as fits in a counted
1829: string.
1830:
1831: @item size of one cell in address units:
1832: @code{1 cells .}.
1833:
1834: @item size of one character in address units:
1835: @code{1 chars .}. 1 on all current ports.
1836:
1837: @item size of the keyboard terminal buffer:
1838: Varies. You can determine the size at a specific time using @code{lp@
1839: tib - .}. It is shared with the locals stack and TIBs of files that
1840: include the current file. You can change the amount of space for TIBs
1841: and locals stack at gforth startup with the command line option
1842: @code{-l}.
1843:
1844: @item size of the pictured numeric output buffer:
1845: @code{PAD HERE - .}. 104 characters on 32-bit machines. The buffer is
1846: shared with @code{WORD}.
1847:
1848: @item size of the scratch area returned by @code{PAD}:
1849: The remainder of dictionary space. You can even use the unused part of
1850: the data stack space. The current size can be computed with @code{sp@
1851: pad - .}.
1852:
1853: @item system case-sensitivity characteristics:
1854: Dictionary searches are case insensitive. However, as explained above
1855: under @i{character-set extensions}, the matching for non-ASCII
1856: characters is determined by the locale you are using. In the default
1857: @code{C} locale all non-ASCII characters are matched case-sensitively.
1858:
1859: @item system prompt:
1860: @code{ ok} in interpret state, @code{ compiled} in compile state.
1861:
1862: @item division rounding:
1863: installation dependent. @code{s" floored" environment? drop .}. We leave
1864: the choice to gcc (what to use for @code{/}) and to you (whether to use
1865: @code{fm/mod}, @code{sm/rem} or simply @code{/}).
1866:
1867: @item values of @code{STATE} when true:
1868: -1.
1869:
1870: @item values returned after arithmetic overflow:
1871: On two's complement machines, arithmetic is performed modulo
1872: 2**bits-per-cell for single arithmetic and 4**bits-per-cell for double
1873: arithmetic (with appropriate mapping for signed types). Division by zero
1874: typically results in a @code{-55 throw} (floatingpoint unidentified
1875: fault), although a @code{-10 throw} (divide by zero) would be more
1876: appropriate.
1877:
1878: @item whether the current definition can be found after @t{DOES>}:
1879: No.
1880:
1881: @end table
1882:
1883: @c ---------------------------------------------------------------------
1884: @node core-ambcond, core-other, core-idef, The Core Words
1885: @subsection Ambiguous conditions
1886: @c ---------------------------------------------------------------------
1887:
1888: @table @i
1889:
1890: @item a name is neither a word nor a number:
1891: @code{-13 throw} (Undefined word)
1892:
1893: @item a definition name exceeds the maximum length allowed:
1894: @code{-19 throw} (Word name too long)
1895:
1896: @item addressing a region not inside the various data spaces of the forth system:
1897: The stacks, code space and name space are accessible. Machine code space is
1898: typically readable. Accessing other addresses gives results dependent on
1899: the operating system. On decent systems: @code{-9 throw} (Invalid memory
1900: address).
1901:
1902: @item argument type incompatible with parameter:
1903: This is usually not caught. Some words perform checks, e.g., the control
1904: flow words, and issue a @code{ABORT"} or @code{-12 THROW} (Argument type
1905: mismatch).
1906:
1907: @item attempting to obtain the execution token of a word with undefined execution semantics:
1908: You get an execution token representing the compilation semantics
1909: instead.
1910:
1911: @item dividing by zero:
1912: typically results in a @code{-55 throw} (floating point unidentified
1913: fault), although a @code{-10 throw} (divide by zero) would be more
1914: appropriate.
1915:
1916: @item insufficient data stack or return stack space:
1917: Not checked. This typically results in mysterious illegal memory
1918: accesses, producing @code{-9 throw} (Invalid memory address) or
1919: @code{-23 throw} (Address alignment exception).
1920:
1921: @item insufficient space for loop control parameters:
1922: like other return stack overflows.
1923:
1924: @item insufficient space in the dictionary:
1925: Not checked. Similar results as stack overflows. However, typically the
1926: error appears at a different place when one inserts or removes code.
1927:
1928: @item interpreting a word with undefined interpretation semantics:
1929: For some words, we defined interpretation semantics. For the others:
1930: @code{-14 throw} (Interpreting a compile-only word). Note that this is
1931: checked only by the outer (aka text) interpreter; if the word is
1932: @code{execute}d in some other way, it will typically perform it's
1933: compilation semantics even in interpret state. (We could change @code{'}
1934: and relatives not to give the xt of such words, but we think that would
1935: be too restrictive).
1936:
1937: @item modifying the contents of the input buffer or a string literal:
1938: These are located in writable memory and can be modified.
1939:
1940: @item overflow of the pictured numeric output string:
1941: Not checked.
1942:
1943: @item parsed string overflow:
1944: @code{PARSE} cannot overflow. @code{WORD} does not check for overflow.
1945:
1946: @item producing a result out of range:
1947: On two's complement machines, arithmetic is performed modulo
1948: 2**bits-per-cell for single arithmetic and 4**bits-per-cell for double
1949: arithmetic (with appropriate mapping for signed types). Division by zero
1950: typically results in a @code{-55 throw} (floatingpoint unidentified
1951: fault), although a @code{-10 throw} (divide by zero) would be more
1952: appropriate. @code{convert} and @code{>number} currently overflow
1953: silently.
1954:
1955: @item reading from an empty data or return stack:
1956: The data stack is checked by the outer (aka text) interpreter after
1957: every word executed. If it has underflowed, a @code{-4 throw} (Stack
1958: underflow) is performed. Apart from that, the stacks are not checked and
1959: underflows can result in similar behaviour as overflows (of adjacent
1960: stacks).
1961:
1962: @item unexepected end of the input buffer, resulting in an attempt to use a zero-length string as a name:
1963: @code{Create} and its descendants perform a @code{-16 throw} (Attempt to
1964: use zero-length string as a name). Words like @code{'} probably will not
1965: find what they search. Note that it is possible to create zero-length
1966: names with @code{nextname} (should it not?).
1967:
1968: @item @code{>IN} greater than input buffer:
1969: The next invocation of a parsing word returns a string wih length 0.
1970:
1971: @item @code{RECURSE} appears after @code{DOES>}:
1972: Compiles a recursive call to the defining word not to the defined word.
1973:
1974: @item argument input source different than current input source for @code{RESTORE-INPUT}:
1975: !!???If the argument input source is a valid input source then it gets
1976: restored. Otherwise causes @code{-12 THROW} which unless caught issues
1977: the message "argument type mismatch" and aborts.
1978:
1979: @item data space containing definitions gets de-allocated:
1980: Deallocation with @code{allot} is not checked. This typically resuls in
1981: memory access faults or execution of illegal instructions.
1982:
1983: @item data space read/write with incorrect alignment:
1984: Processor-dependent. Typically results in a @code{-23 throw} (Address
1985: alignment exception). Under Linux on a 486 or later processor with
1986: alignment turned on, incorrect alignment results in a @code{-9 throw}
1987: (Invalid memory address). There are reportedly some processors with
1988: alignment restrictions that do not report them.
1989:
1990: @item data space pointer not properly aligned, @code{,}, @code{C,}:
1991: Like other alignment errors.
1992:
1993: @item less than u+2 stack items (@code{PICK} and @code{ROLL}):
1994: Not checked. May cause an illegal memory access.
1995:
1996: @item loop control parameters not available:
1997: Not checked. The counted loop words simply assume that the top of return
1998: stack items are loop control parameters and behave accordingly.
1999:
2000: @item most recent definition does not have a name (@code{IMMEDIATE}):
2001: @code{abort" last word was headerless"}.
2002:
2003: @item name not defined by @code{VALUE} used by @code{TO}:
2004: @code{-32 throw} (Invalid name argument)
2005:
2006: @item name not found (@code{'}, @code{POSTPONE}, @code{[']}, @code{[COMPILE]}):
2007: @code{-13 throw} (Undefined word)
2008:
2009: @item parameters are not of the same type (@code{DO}, @code{?DO}, @code{WITHIN}):
2010: Gforth behaves as if they were of the same type. I.e., you can predict
2011: the behaviour by interpreting all parameters as, e.g., signed.
2012:
2013: @item @code{POSTPONE} or @code{[COMPILE]} applied to @code{TO}:
2014: Assume @code{: X POSTPONE TO ; IMMEDIATE}. @code{X} is equivalent to
2015: @code{TO}.
2016:
2017: @item String longer than a counted string returned by @code{WORD}:
2018: Not checked. The string will be ok, but the count will, of course,
2019: contain only the least significant bits of the length.
2020:
2021: @item u greater than or equal to the number of bits in a cell (@code{LSHIFT}, @code{RSHIFT}):
2022: Processor-dependent. Typical behaviours are returning 0 and using only
2023: the low bits of the shift count.
2024:
2025: @item word not defined via @code{CREATE}:
2026: @code{>BODY} produces the PFA of the word no matter how it was defined.
2027:
2028: @code{DOES>} changes the execution semantics of the last defined word no
2029: matter how it was defined. E.g., @code{CONSTANT DOES>} is equivalent to
2030: @code{CREATE , DOES>}.
2031:
2032: @item words improperly used outside @code{<#} and @code{#>}:
2033: Not checked. As usual, you can expect memory faults.
2034:
2035: @end table
2036:
2037:
2038: @c ---------------------------------------------------------------------
2039: @node core-other, , core-ambcond, The Core Words
2040: @subsection Other system documentation
2041: @c ---------------------------------------------------------------------
2042:
2043: @table @i
2044:
2045: @item nonstandard words using @code{PAD}:
2046: None.
2047:
2048: @item operator's terminal facilities available:
2049: !!??
2050:
2051: @item program data space available:
2052: @code{sp@ here - .} gives the space remaining for dictionary and data
2053: stack together.
2054:
2055: @item return stack space available:
2056: !!??
2057:
2058: @item stack space available:
2059: @code{sp@ here - .} gives the space remaining for dictionary and data
2060: stack together.
2061:
2062: @item system dictionary space required, in address units:
2063: Type @code{here forthstart - .} after startup. At the time of this
2064: writing, this gives 70108 (bytes) on a 32-bit system.
2065: @end table
2066:
2067:
2068: @c =====================================================================
2069: @node The optional Block word set, The optional Double Number word set, The Core Words, ANS conformance
2070: @section The optional Block word set
2071: @c =====================================================================
2072:
2073: @menu
2074: * block-idef:: Implementation Defined Options
2075: * block-ambcond:: Ambiguous Conditions
2076: * block-other:: Other System Documentation
2077: @end menu
2078:
2079:
2080: @c ---------------------------------------------------------------------
2081: @node block-idef, block-ambcond, The optional Block word set, The optional Block word set
2082: @subsection Implementation Defined Options
2083: @c ---------------------------------------------------------------------
2084:
2085: @table @i
2086:
2087: @item the format for display by @code{LIST}:
2088: First the screen number is displayed, then 16 lines of 64 characters,
2089: each line preceded by the line number.
2090:
2091: @item the length of a line affected by @code{\}:
2092: 64 characters.
2093: @end table
2094:
2095:
2096: @c ---------------------------------------------------------------------
2097: @node block-ambcond, block-other, block-idef, The optional Block word set
2098: @subsection Ambiguous conditions
2099: @c ---------------------------------------------------------------------
2100:
2101: @table @i
2102:
2103: @item correct block read was not possible:
2104: Typically results in a @code{throw} of some OS-derived value (between
2105: -512 and -2048). If the blocks file was just not long enough, blanks are
2106: supplied for the missing portion.
2107:
2108: @item I/O exception in block transfer:
2109: Typically results in a @code{throw} of some OS-derived value (between
2110: -512 and -2048).
2111:
2112: @item invalid block number:
2113: @code{-35 throw} (Invalid block number)
2114:
2115: @item a program directly alters the contents of @code{BLK}:
2116: The input stream is switched to that other block, at the same
2117: position. If the storing to @code{BLK} happens when interpreting
2118: non-block input, the system will get quite confused when the block ends.
2119:
2120: @item no current block buffer for @code{UPDATE}:
2121: @code{UPDATE} has no effect.
2122:
2123: @end table
2124:
2125:
2126: @c ---------------------------------------------------------------------
2127: @node block-other, , block-ambcond, The optional Block word set
2128: @subsection Other system documentation
2129: @c ---------------------------------------------------------------------
2130:
2131: @table @i
2132:
2133: @item any restrictions a multiprogramming system places on the use of buffer addresses:
2134: No restrictions (yet).
2135:
2136: @item the number of blocks available for source and data:
2137: depends on your disk space.
2138:
2139: @end table
2140:
2141:
2142: @c =====================================================================
2143: @node The optional Double Number word set, The optional Exception word set, The optional Block word set, ANS conformance
2144: @section The optional Double Number word set
2145: @c =====================================================================
2146:
2147: @menu
2148: * double-ambcond:: Ambiguous Conditions
2149: @end menu
2150:
2151:
2152: @c ---------------------------------------------------------------------
2153: @node double-ambcond, , The optional Double Number word set, The optional Double Number word set
2154: @subsection Ambiguous conditions
2155: @c ---------------------------------------------------------------------
2156:
2157: @table @i
2158:
2159: @item @var{d} outside of range of @var{n} in @code{D>S}:
2160: The least significant cell of @var{d} is produced.
2161:
2162: @end table
2163:
2164:
2165: @c =====================================================================
2166: @node The optional Exception word set, The optional Facility word set, The optional Double Number word set, ANS conformance
2167: @section The optional Exception word set
2168: @c =====================================================================
2169:
2170: @menu
2171: * exception-idef:: Implementation Defined Options
2172: @end menu
2173:
2174:
2175: @c ---------------------------------------------------------------------
2176: @node exception-idef, , The optional Exception word set, The optional Exception word set
2177: @subsection Implementation Defined Options
2178: @c ---------------------------------------------------------------------
2179:
2180: @table @i
2181: @item @code{THROW}-codes used in the system:
2182: The codes -256@minus{}-511 are used for reporting signals (see
2183: @file{errore.fs}). The codes -512@minus{}-2047 are used for OS errors
2184: (for file and memory allocation operations). The mapping from OS error
2185: numbers to throw code is -512@minus{}@var{errno}. One side effect of
2186: this mapping is that undefined OS errors produce a message with a
2187: strange number; e.g., @code{-1000 THROW} results in @code{Unknown error
2188: 488} on my system.
2189: @end table
2190:
2191: @c =====================================================================
2192: @node The optional Facility word set, The optional File-Access word set, The optional Exception word set, ANS conformance
2193: @section The optional Facility word set
2194: @c =====================================================================
2195:
2196: @menu
2197: * facility-idef:: Implementation Defined Options
2198: * facility-ambcond:: Ambiguous Conditions
2199: @end menu
2200:
2201:
2202: @c ---------------------------------------------------------------------
2203: @node facility-idef, facility-ambcond, The optional Facility word set, The optional Facility word set
2204: @subsection Implementation Defined Options
2205: @c ---------------------------------------------------------------------
2206:
2207: @table @i
2208:
2209: @item encoding of keyboard events (@code{EKEY}):
2210: Not yet implemeted.
2211:
2212: @item duration of a system clock tick
2213: System dependent. With respect to @code{MS}, the time is specified in
2214: microseconds. How well the OS and the hardware implement this, is
2215: another question.
2216:
2217: @item repeatability to be expected from the execution of @code{MS}:
2218: System dependent. On Unix, a lot depends on load. If the system is
2219: lightly loaded, and the delay is short enough that gforth does not get
2220: swapped out, the performance should be acceptable. Under MS-DOS and
2221: other single-tasking systems, it should be good.
2222:
2223: @end table
2224:
2225:
2226: @c ---------------------------------------------------------------------
2227: @node facility-ambcond, , facility-idef, The optional Facility word set
2228: @subsection Ambiguous conditions
2229: @c ---------------------------------------------------------------------
2230:
2231: @table @i
2232:
2233: @item @code{AT-XY} can't be performed on user output device:
2234: Largely terminal dependant. No range checks are done on the arguments.
2235: No errors are reported. You may see some garbage appearing, you may see
2236: simply nothing happen.
2237:
2238: @end table
2239:
2240:
2241: @c =====================================================================
2242: @node The optional File-Access word set, The optional Floating-Point word set, The optional Facility word set, ANS conformance
2243: @section The optional File-Access word set
2244: @c =====================================================================
2245:
2246: @menu
2247: * file-idef:: Implementation Defined Options
2248: * file-ambcond:: Ambiguous Conditions
2249: @end menu
2250:
2251:
2252: @c ---------------------------------------------------------------------
2253: @node file-idef, file-ambcond, The optional File-Access word set, The optional File-Access word set
2254: @subsection Implementation Defined Options
2255: @c ---------------------------------------------------------------------
2256:
2257: @table @i
2258:
2259: @item File access methods used:
2260: @code{R/O}, @code{R/W} and @code{BIN} work as you would
2261: expect. @code{W/O} translates into the C file opening mode @code{w} (or
2262: @code{wb}): The file is cleared, if it exists, and created, if it does
2263: not (both with @code{open-file} and @code{create-file}). Under Unix
2264: @code{create-file} creates a file with 666 permissions modified by your
2265: umask.
2266:
2267: @item file exceptions:
2268: The file words do not raise exceptions (except, perhaps, memory access
2269: faults when you pass illegal addresses or file-ids).
2270:
2271: @item file line terminator:
2272: System-dependent. Gforth uses C's newline character as line
2273: terminator. What the actual character code(s) of this are is
2274: system-dependent.
2275:
2276: @item file name format
2277: System dependent. Gforth just uses the file name format of your OS.
2278:
2279: @item information returned by @code{FILE-STATUS}:
2280: @code{FILE-STATUS} returns the most powerful file access mode allowed
2281: for the file: Either @code{R/O}, @code{W/O} or @code{R/W}. If the file
2282: cannot be accessed, @code{R/O BIN} is returned. @code{BIN} is applicable
2283: along with the retured mode.
2284:
2285: @item input file state after an exception when including source:
2286: All files that are left via the exception are closed.
2287:
2288: @item @var{ior} values and meaning:
2289: The @var{ior}s returned by the file and memory allocation words are
2290: intended as throw codes. They typically are in the range
2291: -512@minus{}-2047 of OS errors. The mapping from OS error numbers to
2292: @var{ior}s is -512@minus{}@var{errno}.
2293:
2294: @item maximum depth of file input nesting:
2295: limited by the amount of return stack, locals/TIB stack, and the number
2296: of open files available. This should not give you troubles.
2297:
2298: @item maximum size of input line:
2299: @code{/line}. Currently 255.
2300:
2301: @item methods of mapping block ranges to files:
2302: Currently, the block words automatically access the file
2303: @file{blocks.fb} in the currend working directory. More sophisticated
2304: methods could be implemented if there is demand (and a volunteer).
2305:
2306: @item number of string buffers provided by @code{S"}:
2307: 1
2308:
2309: @item size of string buffer used by @code{S"}:
2310: @code{/line}. currently 255.
2311:
2312: @end table
2313:
2314: @c ---------------------------------------------------------------------
2315: @node file-ambcond, , file-idef, The optional File-Access word set
2316: @subsection Ambiguous conditions
2317: @c ---------------------------------------------------------------------
2318:
2319: @table @i
2320:
2321: @item attempting to position a file outside it's boundaries:
2322: @code{REPOSITION-FILE} is performed as usual: Afterwards,
2323: @code{FILE-POSITION} returns the value given to @code{REPOSITION-FILE}.
2324:
2325: @item attempting to read from file positions not yet written:
2326: End-of-file, i.e., zero characters are read and no error is reported.
2327:
2328: @item @var{file-id} is invalid (@code{INCLUDE-FILE}):
2329: An appropriate exception may be thrown, but a memory fault or other
2330: problem is more probable.
2331:
2332: @item I/O exception reading or closing @var{file-id} (@code{include-file}, @code{included}):
2333: The @var{ior} produced by the operation, that discovered the problem, is
2334: thrown.
2335:
2336: @item named file cannot be opened (@code{included}):
2337: The @var{ior} produced by @code{open-file} is thrown.
2338:
2339: @item requesting an unmapped block number:
2340: There are no unmapped legal block numbers. On some operating systems,
2341: writing a block with a large number may overflow the file system and
2342: have an error message as consequence.
2343:
2344: @item using @code{source-id} when @code{blk} is non-zero:
2345: @code{source-id} performs its function. Typically it will give the id of
2346: the source which loaded the block. (Better ideas?)
2347:
2348: @end table
2349:
2350:
2351: @c =====================================================================
2352: @node The optional Floating-Point word set, The optional Locals word set, The optional File-Access word set, ANS conformance
2353: @section The optional Floating-Point word set
2354: @c =====================================================================
2355:
2356: @menu
2357: * floating-idef:: Implementation Defined Options
2358: * floating-ambcond:: Ambiguous Conditions
2359: @end menu
2360:
2361:
2362: @c ---------------------------------------------------------------------
2363: @node floating-idef, floating-ambcond, The optional Floating-Point word set, The optional Floating-Point word set
2364: @subsection Implementation Defined Options
2365: @c ---------------------------------------------------------------------
2366:
2367: @table @i
2368:
2369: @item format and range of floating point numbers:
2370: System-dependent; the @code{double} type of C.
2371:
2372: @item results of @code{REPRESENT} when @var{float} is out of range:
2373: System dependent; @code{REPRESENT} is implemented using the C library
2374: function @code{ecvt()} and inherits its behaviour in this respect.
2375:
2376: @item rounding or truncation of floating-point numbers:
2377: What's the question?!!
2378:
2379: @item size of floating-point stack:
2380: @code{s" FLOATING-STACK" environment? drop .}. Can be changed at startup
2381: with the command-line option @code{-f}.
2382:
2383: @item width of floating-point stack:
2384: @code{1 floats}.
2385:
2386: @end table
2387:
2388:
2389: @c ---------------------------------------------------------------------
2390: @node floating-ambcond, , floating-idef, The optional Floating-Point word set
2391: @subsection Ambiguous conditions
2392: @c ---------------------------------------------------------------------
2393:
2394: @table @i
2395:
2396: @item @code{df@@} or @code{df!} used with an address that is not double-float aligned:
2397: System-dependent. Typically results in an alignment fault like other
2398: alignment violations.
2399:
2400: @item @code{f@@} or @code{f!} used with an address that is not float aligned:
2401: System-dependent. Typically results in an alignment fault like other
2402: alignment violations.
2403:
2404: @item Floating-point result out of range:
2405: System-dependent. Can result in a @code{-55 THROW} (Floating-point
2406: unidentified fault), or can produce a special value representing, e.g.,
2407: Infinity.
2408:
2409: @item @code{sf@@} or @code{sf!} used with an address that is not single-float aligned:
2410: System-dependent. Typically results in an alignment fault like other
2411: alignment violations.
2412:
2413: @item BASE is not decimal (@code{REPRESENT}, @code{F.}, @code{FE.}, @code{FS.}):
2414: The floating-point number is converted into decimal nonetheless.
2415:
2416: @item Both arguments are equal to zero (@code{FATAN2}):
2417: System-dependent. @code{FATAN2} is implemented using the C library
2418: function @code{atan2()}.
2419:
2420: @item Using ftan on an argument @var{r1} where cos(@var{r1}) is zero:
2421: System-dependent. Anyway, typically the cos of @var{r1} will not be zero
2422: because of small errors and the tan will be a very large (or very small)
2423: but finite number.
2424:
2425: @item @var{d} cannot be presented precisely as a float in @code{D>F}:
2426: The result is rounded to the nearest float.
2427:
2428: @item dividing by zero:
2429: @code{-55 throw} (Floating-point unidentified fault)
2430:
2431: @item exponent too big for conversion (@code{DF!}, @code{DF@@}, @code{SF!}, @code{SF@@}):
2432: System dependent. On IEEE-FP based systems the number is converted into
2433: an infinity.
2434:
2435: @item @var{float}<1 (@code{facosh}):
2436: @code{-55 throw} (Floating-point unidentified fault)
2437:
2438: @item @var{float}=<-1 (@code{flnp1}):
2439: @code{-55 throw} (Floating-point unidentified fault). On IEEE-FP systems
2440: negative infinity is typically produced for @var{float}=-1.
2441:
2442: @item @var{float}=<0 (@code{fln}, @code{flog}):
2443: @code{-55 throw} (Floating-point unidentified fault). On IEEE-FP systems
2444: negative infinity is typically produced for @var{float}=0.
2445:
2446: @item @var{float}<0 (@code{fasinh}, @code{fsqrt}):
2447: @code{-55 throw} (Floating-point unidentified fault). @code{fasinh}
2448: produces values for these inputs on my Linux box (Bug in the C library?)
2449:
2450: @item |@var{float}|>1 (@code{facos}, @code{fasin}, @code{fatanh}):
2451: @code{-55 throw} (Floating-point unidentified fault).
2452:
2453: @item integer part of float cannot be represented by @var{d} in @code{f>d}:
2454: @code{-55 throw} (Floating-point unidentified fault).
2455:
2456: @item string larger than pictured numeric output area (@code{f.}, @code{fe.}, @code{fs.}):
2457: This does not happen.
2458: @end table
2459:
2460:
2461:
2462: @c =====================================================================
2463: @node The optional Locals word set, The optional Memory-Allocation word set, The optional Floating-Point word set, ANS conformance
2464: @section The optional Locals word set
2465: @c =====================================================================
2466:
2467: @menu
2468: * locals-idef:: Implementation Defined Options
2469: * locals-ambcond:: Ambiguous Conditions
2470: @end menu
2471:
2472:
2473: @c ---------------------------------------------------------------------
2474: @node locals-idef, locals-ambcond, The optional Locals word set, The optional Locals word set
2475: @subsection Implementation Defined Options
2476: @c ---------------------------------------------------------------------
2477:
2478: @table @i
2479:
2480: @item maximum number of locals in a definition:
2481: @code{s" #locals" environment? drop .}. Currently 15. This is a lower
2482: bound, e.g., on a 32-bit machine there can be 41 locals of up to 8
2483: characters. The number of locals in a definition is bounded by the size
2484: of locals-buffer, which contains the names of the locals.
2485:
2486: @end table
2487:
2488:
2489: @c ---------------------------------------------------------------------
2490: @node locals-ambcond, , locals-idef, The optional Locals word set
2491: @subsection Ambiguous conditions
2492: @c ---------------------------------------------------------------------
2493:
2494: @table @i
2495:
2496: @item executing a named local in interpretation state:
2497: @code{-14 throw} (Interpreting a compile-only word).
2498:
2499: @item @var{name} not defined by @code{VALUE} or @code{(LOCAL)} (@code{TO}):
2500: @code{-32 throw} (Invalid name argument)
2501:
2502: @end table
2503:
2504:
2505: @c =====================================================================
2506: @node The optional Memory-Allocation word set, The optional Programming-Tools word set, The optional Locals word set, ANS conformance
2507: @section The optional Memory-Allocation word set
2508: @c =====================================================================
2509:
2510: @menu
2511: * memory-idef:: Implementation Defined Options
2512: @end menu
2513:
2514:
2515: @c ---------------------------------------------------------------------
2516: @node memory-idef, , The optional Memory-Allocation word set, The optional Memory-Allocation word set
2517: @subsection Implementation Defined Options
2518: @c ---------------------------------------------------------------------
2519:
2520: @table @i
2521:
2522: @item values and meaning of @var{ior}:
2523: The @var{ior}s returned by the file and memory allocation words are
2524: intended as throw codes. They typically are in the range
2525: -512@minus{}-2047 of OS errors. The mapping from OS error numbers to
2526: @var{ior}s is -512@minus{}@var{errno}.
2527:
2528: @end table
2529:
2530: @c =====================================================================
2531: @node The optional Programming-Tools word set, The optional Search-Order word set, The optional Memory-Allocation word set, ANS conformance
2532: @section The optional Programming-Tools word set
2533: @c =====================================================================
2534:
2535: @menu
2536: * programming-idef:: Implementation Defined Options
2537: * programming-ambcond:: Ambiguous Conditions
2538: @end menu
2539:
2540:
2541: @c ---------------------------------------------------------------------
2542: @node programming-idef, programming-ambcond, The optional Programming-Tools word set, The optional Programming-Tools word set
2543: @subsection Implementation Defined Options
2544: @c ---------------------------------------------------------------------
2545:
2546: @table @i
2547:
2548: @item ending sequence for input following @code{;code} and @code{code}:
2549: Not implemented (yet).
2550:
2551: @item manner of processing input following @code{;code} and @code{code}:
2552: Not implemented (yet).
2553:
2554: @item search order capability for @code{EDITOR} and @code{ASSEMBLER}:
2555: Not implemented (yet). If they were implemented, they would use the
2556: search order wordset.
2557:
2558: @item source and format of display by @code{SEE}:
2559: The source for @code{see} is the intermediate code used by the inner
2560: interpreter. The current @code{see} tries to output Forth source code
2561: as well as possible.
2562:
2563: @end table
2564:
2565: @c ---------------------------------------------------------------------
2566: @node programming-ambcond, , programming-idef, The optional Programming-Tools word set
2567: @subsection Ambiguous conditions
2568: @c ---------------------------------------------------------------------
2569:
2570: @table @i
2571:
2572: @item deleting the compilation wordlist (@code{FORGET}):
2573: Not implemented (yet).
2574:
2575: @item fewer than @var{u}+1 items on the control flow stack (@code{CS-PICK}, @code{CS-ROLL}):
2576: This typically results in an @code{abort"} with a descriptive error
2577: message (may change into a @code{-22 throw} (Control structure mismatch)
2578: in the future). You may also get a memory access error. If you are
2579: unlucky, this ambiguous condition is not caught.
2580:
2581: @item @var{name} can't be found (@code{forget}):
2582: Not implemented (yet).
2583:
2584: @item @var{name} not defined via @code{CREATE}:
2585: @code{;code} is not implemented (yet). If it were, it would behave like
2586: @code{DOES>} in this respect, i.e., change the execution semantics of
2587: the last defined word no matter how it was defined.
2588:
2589: @item @code{POSTPONE} applied to @code{[IF]}:
2590: After defining @code{: X POSTPONE [IF] ; IMMEDIATE}. @code{X} is
2591: equivalent to @code{[IF]}.
2592:
2593: @item reaching the end of the input source before matching @code{[ELSE]} or @code{[THEN]}:
2594: Continue in the same state of conditional compilation in the next outer
2595: input source. Currently there is no warning to the user about this.
2596:
2597: @item removing a needed definition (@code{FORGET}):
2598: Not implemented (yet).
2599:
2600: @end table
2601:
2602:
2603: @c =====================================================================
2604: @node The optional Search-Order word set, , The optional Programming-Tools word set, ANS conformance
2605: @section The optional Search-Order word set
2606: @c =====================================================================
2607:
2608: @menu
2609: * search-idef:: Implementation Defined Options
2610: * search-ambcond:: Ambiguous Conditions
2611: @end menu
2612:
2613:
2614: @c ---------------------------------------------------------------------
2615: @node search-idef, search-ambcond, The optional Search-Order word set, The optional Search-Order word set
2616: @subsection Implementation Defined Options
2617: @c ---------------------------------------------------------------------
2618:
2619: @table @i
2620:
2621: @item maximum number of word lists in search order:
2622: @code{s" wordlists" environment? drop .}. Currently 16.
2623:
2624: @item minimum search order:
2625: @code{root root}.
2626:
2627: @end table
2628:
2629: @c ---------------------------------------------------------------------
2630: @node search-ambcond, , search-idef, The optional Search-Order word set
2631: @subsection Ambiguous conditions
2632: @c ---------------------------------------------------------------------
2633:
2634: @table @i
2635:
2636: @item changing the compilation wordlist (during compilation):
2637: The definition is put into the wordlist that is the compilation wordlist
2638: when @code{REVEAL} is executed (by @code{;}, @code{DOES>},
2639: @code{RECURSIVE}, etc.).
2640:
2641: @item search order empty (@code{previous}):
2642: @code{abort" Vocstack empty"}.
2643:
2644: @item too many word lists in search order (@code{also}):
2645: @code{abort" Vocstack full"}.
2646:
2647: @end table
2648:
2649:
2650: @node Model, Emacs and GForth, ANS conformance, Top
2651: @chapter Model
2652:
2653: @node Emacs and GForth, Internals, Model, Top
2654: @chapter Emacs and GForth
2655:
2656: GForth comes with @file{gforth.el}, an improved version of
2657: @file{forth.el} by Goran Rydqvist (icluded in the TILE package). The
2658: improvements are a better (but still not perfect) handling of
2659: indentation. I have also added comment paragraph filling (@kbd{M-q}),
2660: commenting (@kbd{C-x \}) and uncommenting (@kbd{C-u C-x \}) regions and
2661: removing debugging tracers (@kbd{C-x ~}, @pxref{Debugging}). I left the
2662: stuff I do not use alone, even though some of it only makes sense for
2663: TILE. To get a description of these features, enter Forth mode and type
2664: @kbd{C-h m}.
2665:
2666: In addition, GForth supports Emacs quite well: The source code locations
2667: given in error messages, debugging output (from @code{~~}) and failed
2668: assertion messages are in the right format for Emacs' compilation mode
2669: (@pxref{Compilation, , Running Compilations under Emacs, emacs, Emacs
2670: Manual}) so the source location corresponding to an error or other
2671: message is only a few keystrokes away (@kbd{C-x `} for the next error,
2672: @kbd{C-c C-c} for the error under the cursor).
2673:
2674: Also, if you @code{include} @file{etags.fs}, a new @file{TAGS} file
2675: (@pxref{Tags, , Tags Tables, emacs, Emacs Manual}) will be produced that
2676: contains the definitions of all words defined afterwards. You can then
2677: find the source for a word using @kbd{M-.}. Note that emacs can use
2678: several tags files at the same time (e.g., one for the gforth sources
2679: and one for your program).
2680:
2681: To get all these benefits, add the following lines to your @file{.emacs}
2682: file:
2683:
2684: @example
2685: (autoload 'forth-mode "gforth.el")
2686: (setq auto-mode-alist (cons '("\\.fs\\'" . forth-mode) auto-mode-alist))
2687: @end example
2688:
2689: @node Internals, Bugs, Emacs and GForth, Top
2690: @chapter Internals
2691:
2692: Reading this section is not necessary for programming with gforth. It
2693: should be helpful for finding your way in the gforth sources.
2694:
2695: @menu
2696: * Portability::
2697: * Threading::
2698: * Primitives::
2699: * System Architecture::
2700: @end menu
2701:
2702: @node Portability, Threading, Internals, Internals
2703: @section Portability
2704:
2705: One of the main goals of the effort is availability across a wide range
2706: of personal machines. fig-Forth, and, to a lesser extent, F83, achieved
2707: this goal by manually coding the engine in assembly language for several
2708: then-popular processors. This approach is very labor-intensive and the
2709: results are short-lived due to progress in computer architecture.
2710:
2711: Others have avoided this problem by coding in C, e.g., Mitch Bradley
2712: (cforth), Mikael Patel (TILE) and Dirk Zoller (pfe). This approach is
2713: particularly popular for UNIX-based Forths due to the large variety of
2714: architectures of UNIX machines. Unfortunately an implementation in C
2715: does not mix well with the goals of efficiency and with using
2716: traditional techniques: Indirect or direct threading cannot be expressed
2717: in C, and switch threading, the fastest technique available in C, is
2718: significantly slower. Another problem with C is that it's very
2719: cumbersome to express double integer arithmetic.
2720:
2721: Fortunately, there is a portable language that does not have these
2722: limitations: GNU C, the version of C processed by the GNU C compiler
2723: (@pxref{C Extensions, , Extensions to the C Language Family, gcc.info,
2724: GNU C Manual}). Its labels as values feature (@pxref{Labels as Values, ,
2725: Labels as Values, gcc.info, GNU C Manual}) makes direct and indirect
2726: threading possible, its @code{long long} type (@pxref{Long Long, ,
2727: Double-Word Integers, gcc.info, GNU C Manual}) corresponds to Forths
2728: double numbers. GNU C is available for free on all important (and many
2729: unimportant) UNIX machines, VMS, 80386s running MS-DOS, the Amiga, and
2730: the Atari ST, so a Forth written in GNU C can run on all these
2731: machines@footnote{Due to Apple's look-and-feel lawsuit it is not
2732: available on the Mac (@pxref{Boycott, , Protect Your Freedom---Fight
2733: ``Look And Feel'', gcc.info, GNU C Manual}).}.
2734:
2735: Writing in a portable language has the reputation of producing code that
2736: is slower than assembly. For our Forth engine we repeatedly looked at
2737: the code produced by the compiler and eliminated most compiler-induced
2738: inefficiencies by appropriate changes in the source-code.
2739:
2740: However, register allocation cannot be portably influenced by the
2741: programmer, leading to some inefficiencies on register-starved
2742: machines. We use explicit register declarations (@pxref{Explicit Reg
2743: Vars, , Variables in Specified Registers, gcc.info, GNU C Manual}) to
2744: improve the speed on some machines. They are turned on by using the
2745: @code{gcc} switch @code{-DFORCE_REG}. Unfortunately, this feature not
2746: only depends on the machine, but also on the compiler version: On some
2747: machines some compiler versions produce incorrect code when certain
2748: explicit register declarations are used. So by default
2749: @code{-DFORCE_REG} is not used.
2750:
2751: @node Threading, Primitives, Portability, Internals
2752: @section Threading
2753:
2754: GNU C's labels as values extension (available since @code{gcc-2.0},
2755: @pxref{Labels as Values, , Labels as Values, gcc.info, GNU C Manual})
2756: makes it possible to take the address of @var{label} by writing
2757: @code{&&@var{label}}. This address can then be used in a statement like
2758: @code{goto *@var{address}}. I.e., @code{goto *&&x} is the same as
2759: @code{goto x}.
2760:
2761: With this feature an indirect threaded NEXT looks like:
2762: @example
2763: cfa = *ip++;
2764: ca = *cfa;
2765: goto *ca;
2766: @end example
2767: For those unfamiliar with the names: @code{ip} is the Forth instruction
2768: pointer; the @code{cfa} (code-field address) corresponds to ANS Forths
2769: execution token and points to the code field of the next word to be
2770: executed; The @code{ca} (code address) fetched from there points to some
2771: executable code, e.g., a primitive or the colon definition handler
2772: @code{docol}.
2773:
2774: Direct threading is even simpler:
2775: @example
2776: ca = *ip++;
2777: goto *ca;
2778: @end example
2779:
2780: Of course we have packaged the whole thing neatly in macros called
2781: @code{NEXT} and @code{NEXT1} (the part of NEXT after fetching the cfa).
2782:
2783: @menu
2784: * Scheduling::
2785: * Direct or Indirect Threaded?::
2786: * DOES>::
2787: @end menu
2788:
2789: @node Scheduling, Direct or Indirect Threaded?, Threading, Threading
2790: @subsection Scheduling
2791:
2792: There is a little complication: Pipelined and superscalar processors,
2793: i.e., RISC and some modern CISC machines can process independent
2794: instructions while waiting for the results of an instruction. The
2795: compiler usually reorders (schedules) the instructions in a way that
2796: achieves good usage of these delay slots. However, on our first tries
2797: the compiler did not do well on scheduling primitives. E.g., for
2798: @code{+} implemented as
2799: @example
2800: n=sp[0]+sp[1];
2801: sp++;
2802: sp[0]=n;
2803: NEXT;
2804: @end example
2805: the NEXT comes strictly after the other code, i.e., there is nearly no
2806: scheduling. After a little thought the problem becomes clear: The
2807: compiler cannot know that sp and ip point to different addresses (and
2808: the version of @code{gcc} we used would not know it even if it was
2809: possible), so it could not move the load of the cfa above the store to
2810: the TOS. Indeed the pointers could be the same, if code on or very near
2811: the top of stack were executed. In the interest of speed we chose to
2812: forbid this probably unused ``feature'' and helped the compiler in
2813: scheduling: NEXT is divided into the loading part (@code{NEXT_P1}) and
2814: the goto part (@code{NEXT_P2}). @code{+} now looks like:
2815: @example
2816: n=sp[0]+sp[1];
2817: sp++;
2818: NEXT_P1;
2819: sp[0]=n;
2820: NEXT_P2;
2821: @end example
2822: This can be scheduled optimally by the compiler.
2823:
2824: This division can be turned off with the switch @code{-DCISC_NEXT}. This
2825: switch is on by default on machines that do not profit from scheduling
2826: (e.g., the 80386), in order to preserve registers.
2827:
2828: @node Direct or Indirect Threaded?, DOES>, Scheduling, Threading
2829: @subsection Direct or Indirect Threaded?
2830:
2831: Both! After packaging the nasty details in macro definitions we
2832: realized that we could switch between direct and indirect threading by
2833: simply setting a compilation flag (@code{-DDIRECT_THREADED}) and
2834: defining a few machine-specific macros for the direct-threading case.
2835: On the Forth level we also offer access words that hide the
2836: differences between the threading methods (@pxref{Threading Words}).
2837:
2838: Indirect threading is implemented completely
2839: machine-independently. Direct threading needs routines for creating
2840: jumps to the executable code (e.g. to docol or dodoes). These routines
2841: are inherently machine-dependent, but they do not amount to many source
2842: lines. I.e., even porting direct threading to a new machine is a small
2843: effort.
2844:
2845: @node DOES>, , Direct or Indirect Threaded?, Threading
2846: @subsection DOES>
2847: One of the most complex parts of a Forth engine is @code{dodoes}, i.e.,
2848: the chunk of code executed by every word defined by a
2849: @code{CREATE}...@code{DOES>} pair. The main problem here is: How to find
2850: the Forth code to be executed, i.e. the code after the @code{DOES>} (the
2851: DOES-code)? There are two solutions:
2852:
2853: In fig-Forth the code field points directly to the dodoes and the
2854: DOES-code address is stored in the cell after the code address
2855: (i.e. at cfa cell+). It may seem that this solution is illegal in the
2856: Forth-79 and all later standards, because in fig-Forth this address
2857: lies in the body (which is illegal in these standards). However, by
2858: making the code field larger for all words this solution becomes legal
2859: again. We use this approach for the indirect threaded version. Leaving
2860: a cell unused in most words is a bit wasteful, but on the machines we
2861: are targetting this is hardly a problem. The other reason for having a
2862: code field size of two cells is to avoid having different image files
2863: for direct and indirect threaded systems (@pxref{System Architecture}).
2864:
2865: The other approach is that the code field points or jumps to the cell
2866: after @code{DOES}. In this variant there is a jump to @code{dodoes} at
2867: this address. @code{dodoes} can then get the DOES-code address by
2868: computing the code address, i.e., the address of the jump to dodoes,
2869: and add the length of that jump field. A variant of this is to have a
2870: call to @code{dodoes} after the @code{DOES>}; then the return address
2871: (which can be found in the return register on RISCs) is the DOES-code
2872: address. Since the two cells available in the code field are usually
2873: used up by the jump to the code address in direct threading, we use
2874: this approach for direct threading. We did not want to add another
2875: cell to the code field.
2876:
2877: @node Primitives, System Architecture, Threading, Internals
2878: @section Primitives
2879:
2880: @menu
2881: * Automatic Generation::
2882: * TOS Optimization::
2883: * Produced code::
2884: @end menu
2885:
2886: @node Automatic Generation, TOS Optimization, Primitives, Primitives
2887: @subsection Automatic Generation
2888:
2889: Since the primitives are implemented in a portable language, there is no
2890: longer any need to minimize the number of primitives. On the contrary,
2891: having many primitives is an advantage: speed. In order to reduce the
2892: number of errors in primitives and to make programming them easier, we
2893: provide a tool, the primitive generator (@file{prims2x.fs}), that
2894: automatically generates most (and sometimes all) of the C code for a
2895: primitive from the stack effect notation. The source for a primitive
2896: has the following form:
2897:
2898: @format
2899: @var{Forth-name} @var{stack-effect} @var{category} [@var{pronounc.}]
2900: [@code{""}@var{glossary entry}@code{""}]
2901: @var{C code}
2902: [@code{:}
2903: @var{Forth code}]
2904: @end format
2905:
2906: The items in brackets are optional. The category and glossary fields
2907: are there for generating the documentation, the Forth code is there
2908: for manual implementations on machines without GNU C. E.g., the source
2909: for the primitive @code{+} is:
2910: @example
2911: + n1 n2 -- n core plus
2912: n = n1+n2;
2913: @end example
2914:
2915: This looks like a specification, but in fact @code{n = n1+n2} is C
2916: code. Our primitive generation tool extracts a lot of information from
2917: the stack effect notations@footnote{We use a one-stack notation, even
2918: though we have separate data and floating-point stacks; The separate
2919: notation can be generated easily from the unified notation.}: The number
2920: of items popped from and pushed on the stack, their type, and by what
2921: name they are referred to in the C code. It then generates a C code
2922: prelude and postlude for each primitive. The final C code for @code{+}
2923: looks like this:
2924:
2925: @example
2926: I_plus: /* + ( n1 n2 -- n ) */ /* label, stack effect */
2927: /* */ /* documentation */
2928: @{
2929: DEF_CA /* definition of variable ca (indirect threading) */
2930: Cell n1; /* definitions of variables */
2931: Cell n2;
2932: Cell n;
2933: n1 = (Cell) sp[1]; /* input */
2934: n2 = (Cell) TOS;
2935: sp += 1; /* stack adjustment */
2936: NAME("+") /* debugging output (with -DDEBUG) */
2937: @{
2938: n = n1+n2; /* C code taken from the source */
2939: @}
2940: NEXT_P1; /* NEXT part 1 */
2941: TOS = (Cell)n; /* output */
2942: NEXT_P2; /* NEXT part 2 */
2943: @}
2944: @end example
2945:
2946: This looks long and inefficient, but the GNU C compiler optimizes quite
2947: well and produces optimal code for @code{+} on, e.g., the R3000 and the
2948: HP RISC machines: Defining the @code{n}s does not produce any code, and
2949: using them as intermediate storage also adds no cost.
2950:
2951: There are also other optimizations, that are not illustrated by this
2952: example: Assignments between simple variables are usually for free (copy
2953: propagation). If one of the stack items is not used by the primitive
2954: (e.g. in @code{drop}), the compiler eliminates the load from the stack
2955: (dead code elimination). On the other hand, there are some things that
2956: the compiler does not do, therefore they are performed by
2957: @file{prims2x.fs}: The compiler does not optimize code away that stores
2958: a stack item to the place where it just came from (e.g., @code{over}).
2959:
2960: While programming a primitive is usually easy, there are a few cases
2961: where the programmer has to take the actions of the generator into
2962: account, most notably @code{?dup}, but also words that do not (always)
2963: fall through to NEXT.
2964:
2965: @node TOS Optimization, Produced code, Automatic Generation, Primitives
2966: @subsection TOS Optimization
2967:
2968: An important optimization for stack machine emulators, e.g., Forth
2969: engines, is keeping one or more of the top stack items in
2970: registers. If a word has the stack effect @var{in1}...@var{inx} @code{--}
2971: @var{out1}...@var{outy}, keeping the top @var{n} items in registers
2972: @itemize
2973: @item
2974: is better than keeping @var{n-1} items, if @var{x>=n} and @var{y>=n},
2975: due to fewer loads from and stores to the stack.
2976: @item is slower than keeping @var{n-1} items, if @var{x<>y} and @var{x<n} and
2977: @var{y<n}, due to additional moves between registers.
2978: @end itemize
2979:
2980: In particular, keeping one item in a register is never a disadvantage,
2981: if there are enough registers. Keeping two items in registers is a
2982: disadvantage for frequent words like @code{?branch}, constants,
2983: variables, literals and @code{i}. Therefore our generator only produces
2984: code that keeps zero or one items in registers. The generated C code
2985: covers both cases; the selection between these alternatives is made at
2986: C-compile time using the switch @code{-DUSE_TOS}. @code{TOS} in the C
2987: code for @code{+} is just a simple variable name in the one-item case,
2988: otherwise it is a macro that expands into @code{sp[0]}. Note that the
2989: GNU C compiler tries to keep simple variables like @code{TOS} in
2990: registers, and it usually succeeds, if there are enough registers.
2991:
2992: The primitive generator performs the TOS optimization for the
2993: floating-point stack, too (@code{-DUSE_FTOS}). For floating-point
2994: operations the benefit of this optimization is even larger:
2995: floating-point operations take quite long on most processors, but can be
2996: performed in parallel with other operations as long as their results are
2997: not used. If the FP-TOS is kept in a register, this works. If
2998: it is kept on the stack, i.e., in memory, the store into memory has to
2999: wait for the result of the floating-point operation, lengthening the
3000: execution time of the primitive considerably.
3001:
3002: The TOS optimization makes the automatic generation of primitives a
3003: bit more complicated. Just replacing all occurrences of @code{sp[0]} by
3004: @code{TOS} is not sufficient. There are some special cases to
3005: consider:
3006: @itemize
3007: @item In the case of @code{dup ( w -- w w )} the generator must not
3008: eliminate the store to the original location of the item on the stack,
3009: if the TOS optimization is turned on.
3010: @item Primitives with stack effects of the form @code{--}
3011: @var{out1}...@var{outy} must store the TOS to the stack at the start.
3012: Likewise, primitives with the stack effect @var{in1}...@var{inx} @code{--}
3013: must load the TOS from the stack at the end. But for the null stack
3014: effect @code{--} no stores or loads should be generated.
3015: @end itemize
3016:
3017: @node Produced code, , TOS Optimization, Primitives
3018: @subsection Produced code
3019:
3020: To see what assembly code is produced for the primitives on your machine
3021: with your compiler and your flag settings, type @code{make engine.s} and
3022: look at the resulting file @file{engine.s}.
3023:
3024: @node System Architecture, , Primitives, Internals
3025: @section System Architecture
3026:
3027: Our Forth system consists not only of primitives, but also of
3028: definitions written in Forth. Since the Forth compiler itself belongs
3029: to those definitions, it is not possible to start the system with the
3030: primitives and the Forth source alone. Therefore we provide the Forth
3031: code as an image file in nearly executable form. At the start of the
3032: system a C routine loads the image file into memory, sets up the
3033: memory (stacks etc.) according to information in the image file, and
3034: starts executing Forth code.
3035:
3036: The image file format is a compromise between the goals of making it
3037: easy to generate image files and making them portable. The easiest way
3038: to generate an image file is to just generate a memory dump. However,
3039: this kind of image file cannot be used on a different machine, or on
3040: the next version of the engine on the same machine, it even might not
3041: work with the same engine compiled by a different version of the C
3042: compiler. We would like to have as few versions of the image file as
3043: possible, because we do not want to distribute many versions of the
3044: same image file, and to make it easy for the users to use their image
3045: files on many machines. We currently need to create a different image
3046: file for machines with different cell sizes and different byte order
3047: (little- or big-endian)@footnote{We consider adding information to the
3048: image file that enables the loader to change the byte order.}.
3049:
3050: Forth code that is going to end up in a portable image file has to
3051: comply to some restrictions: addresses have to be stored in memory with
3052: special words (@code{A!}, @code{A,}, etc.) in order to make the code
3053: relocatable. Cells, floats, etc., have to be stored at the natural
3054: alignment boundaries@footnote{E.g., store floats (8 bytes) at an address
3055: dividable by~8. This happens automatically in our system when you use
3056: the ANS Forth alignment words.}, in order to avoid alignment faults on
3057: machines with stricter alignment. The image file is produced by a
3058: metacompiler (@file{cross.fs}).
3059:
3060: So, unlike the image file of Mitch Bradleys @code{cforth}, our image
3061: file is not directly executable, but has to undergo some manipulations
3062: during loading. Address relocation is performed at image load-time, not
3063: at run-time. The loader also has to replace tokens standing for
3064: primitive calls with the appropriate code-field addresses (or code
3065: addresses in the case of direct threading).
3066:
3067: @node Bugs, Pedigree, Internals, Top
3068: @chapter Bugs
3069:
3070: @node Pedigree, Word Index, Bugs, Top
3071: @chapter Pedigree
3072:
3073: @node Word Index, Node Index, Pedigree, Top
3074: @chapter Word Index
3075:
3076: @node Node Index, , Word Index, Top
3077: @chapter Node Index
3078:
3079: @contents
3080: @bye
3081:
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