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Sat Jun 3 14:24:42 1995 UTC (28 years, 10 months ago) by
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replaced Bernds solution for the path searching problem with mine.
Use different register declarations for Intel depending on compiler version
Updated bug list
added GNU-convention make target check
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:
413: The outer (aka text) interpreter converts numbers containing a dot into
414: a double precision number. Note that only numbers with the dot as last
415: character are standard-conforming.
416:
417: doc-d+
418: doc-d-
419: doc-dnegate
420: doc-dabs
421: doc-dmin
422: doc-dmax
423:
424: @node Floating Point, , Double precision, Arithmetic
425: @subsection Floating Point
426:
427: The format of floating point numbers recognized by the outer (aka text)
428: interpreter is: a signed decimal number, possibly containing a decimal
429: point (@code{.}), followed by @code{E} or @code{e}, optionally followed
430: by a signed integer (the exponent). E.g., @code{1e} ist the same as
431: @code{+1.0e+1}. Note that a number without @code{e}
432: is not interpreted as floating-point number, but as double (if the
433: number contains a @code{.}) or single precision integer. Also,
434: conversions between string and floating point numbers always use base
435: 10, irrespective of the value of @code{BASE}. If @code{BASE} contains a
436: value greater then 14, the @code{E} may be interpreted as digit and the
437: number will be interpreted as integer, unless it has a signed exponent
438: (both @code{+} and @code{-} are allowed as signs).
439:
440: Angles in floating point operations are given in radians (a full circle
441: has 2 pi radians). Note, that gforth has a separate floating point
442: stack, but we use the unified notation.
443:
444: Floating point numbers have a number of unpleasant surprises for the
445: unwary (e.g., floating point addition is not associative) and even a few
446: for the wary. You should not use them unless you know what you are doing
447: or you don't care that the results you get are totally bogus. If you
448: want to learn about the problems of floating point numbers (and how to
449: avoid them), you might start with @cite{David Goldberg, What Every
450: Computer Scientist Should Know About Floating-Point Arithmetic, ACM
451: Computing Surveys 23(1):5@minus{}48, March 1991}.
452:
453: doc-f+
454: doc-f-
455: doc-f*
456: doc-f/
457: doc-fnegate
458: doc-fabs
459: doc-fmax
460: doc-fmin
461: doc-floor
462: doc-fround
463: doc-f**
464: doc-fsqrt
465: doc-fexp
466: doc-fexpm1
467: doc-fln
468: doc-flnp1
469: doc-flog
470: doc-falog
471: doc-fsin
472: doc-fcos
473: doc-fsincos
474: doc-ftan
475: doc-fasin
476: doc-facos
477: doc-fatan
478: doc-fatan2
479: doc-fsinh
480: doc-fcosh
481: doc-ftanh
482: doc-fasinh
483: doc-facosh
484: doc-fatanh
485:
486: @node Stack Manipulation, Memory access, Arithmetic, Words
487: @section Stack Manipulation
488:
489: gforth has a data stack (aka parameter stack) for characters, cells,
490: addresses, and double cells, a floating point stack for floating point
491: numbers, a return stack for storing the return addresses of colon
492: definitions and other data, and a locals stack for storing local
493: variables. Note that while every sane Forth has a separate floating
494: point stack, this is not strictly required; an ANS Forth system could
495: theoretically keep floating point numbers on the data stack. As an
496: additional difficulty, you don't know how many cells a floating point
497: number takes. It is reportedly possible to write words in a way that
498: they work also for a unified stack model, but we do not recommend trying
499: it. Instead, just say that your program has an environmental dependency
500: on a separate FP stack.
501:
502: Also, a Forth system is allowed to keep the local variables on the
503: return stack. This is reasonable, as local variables usually eliminate
504: the need to use the return stack explicitly. So, if you want to produce
505: a standard complying program and if you are using local variables in a
506: word, forget about return stack manipulations in that word (see the
507: standard document for the exact rules).
508:
509: @menu
510: * Data stack::
511: * Floating point stack::
512: * Return stack::
513: * Locals stack::
514: * Stack pointer manipulation::
515: @end menu
516:
517: @node Data stack, Floating point stack, Stack Manipulation, Stack Manipulation
518: @subsection Data stack
519: doc-drop
520: doc-nip
521: doc-dup
522: doc-over
523: doc-tuck
524: doc-swap
525: doc-rot
526: doc--rot
527: doc-?dup
528: doc-pick
529: doc-roll
530: doc-2drop
531: doc-2nip
532: doc-2dup
533: doc-2over
534: doc-2tuck
535: doc-2swap
536: doc-2rot
537:
538: @node Floating point stack, Return stack, Data stack, Stack Manipulation
539: @subsection Floating point stack
540: doc-fdrop
541: doc-fnip
542: doc-fdup
543: doc-fover
544: doc-ftuck
545: doc-fswap
546: doc-frot
547:
548: @node Return stack, Locals stack, Floating point stack, Stack Manipulation
549: @subsection Return stack
550: doc->r
551: doc-r>
552: doc-r@
553: doc-rdrop
554: doc-2>r
555: doc-2r>
556: doc-2r@
557: doc-2rdrop
558:
559: @node Locals stack, Stack pointer manipulation, Return stack, Stack Manipulation
560: @subsection Locals stack
561:
562: @node Stack pointer manipulation, , Locals stack, Stack Manipulation
563: @subsection Stack pointer manipulation
564: doc-sp@
565: doc-sp!
566: doc-fp@
567: doc-fp!
568: doc-rp@
569: doc-rp!
570: doc-lp@
571: doc-lp!
572:
573: @node Memory access, Control Structures, Stack Manipulation, Words
574: @section Memory access
575:
576: @menu
577: * Stack-Memory transfers::
578: * Address arithmetic::
579: * Memory block access::
580: @end menu
581:
582: @node Stack-Memory transfers, Address arithmetic, Memory access, Memory access
583: @subsection Stack-Memory transfers
584:
585: doc-@
586: doc-!
587: doc-+!
588: doc-c@
589: doc-c!
590: doc-2@
591: doc-2!
592: doc-f@
593: doc-f!
594: doc-sf@
595: doc-sf!
596: doc-df@
597: doc-df!
598:
599: @node Address arithmetic, Memory block access, Stack-Memory transfers, Memory access
600: @subsection Address arithmetic
601:
602: ANS Forth does not specify the sizes of the data types. Instead, it
603: offers a number of words for computing sizes and doing address
604: arithmetic. Basically, address arithmetic is performed in terms of
605: address units (aus); on most systems the address unit is one byte. Note
606: that a character may have more than one au, so @code{chars} is no noop
607: (on systems where it is a noop, it compiles to nothing).
608:
609: ANS Forth also defines words for aligning addresses for specific
610: addresses. Many computers require that accesses to specific data types
611: must only occur at specific addresses; e.g., that cells may only be
612: accessed at addresses divisible by 4. Even if a machine allows unaligned
613: accesses, it can usually perform aligned accesses faster.
614:
615: For the performance-concious: alignment operations are usually only
616: necessary during the definition of a data structure, not during the
617: (more frequent) accesses to it.
618:
619: ANS Forth defines no words for character-aligning addresses. This is not
620: an oversight, but reflects the fact that addresses that are not
621: char-aligned have no use in the standard and therefore will not be
622: created.
623:
624: The standard guarantees that addresses returned by @code{CREATE}d words
625: are cell-aligned; in addition, gforth guarantees that these addresses
626: are aligned for all purposes.
627:
628: Note that the standard defines a word @code{char}, which has nothing to
629: do with address arithmetic.
630:
631: doc-chars
632: doc-char+
633: doc-cells
634: doc-cell+
635: doc-align
636: doc-aligned
637: doc-floats
638: doc-float+
639: doc-falign
640: doc-faligned
641: doc-sfloats
642: doc-sfloat+
643: doc-sfalign
644: doc-sfaligned
645: doc-dfloats
646: doc-dfloat+
647: doc-dfalign
648: doc-dfaligned
649: doc-maxalign
650: doc-maxaligned
651: doc-cfalign
652: doc-cfaligned
653: doc-address-unit-bits
654:
655: @node Memory block access, , Address arithmetic, Memory access
656: @subsection Memory block access
657:
658: doc-move
659: doc-erase
660:
661: While the previous words work on address units, the rest works on
662: characters.
663:
664: doc-cmove
665: doc-cmove>
666: doc-fill
667: doc-blank
668:
669: @node Control Structures, Locals, Memory access, Words
670: @section Control Structures
671:
672: Control structures in Forth cannot be used in interpret state, only in
673: compile state, i.e., in a colon definition. We do not like this
674: limitation, but have not seen a satisfying way around it yet, although
675: many schemes have been proposed.
676:
677: @menu
678: * Selection::
679: * Simple Loops::
680: * Counted Loops::
681: * Arbitrary control structures::
682: * Calls and returns::
683: * Exception Handling::
684: @end menu
685:
686: @node Selection, Simple Loops, Control Structures, Control Structures
687: @subsection Selection
688:
689: @example
690: @var{flag}
691: IF
692: @var{code}
693: ENDIF
694: @end example
695: or
696: @example
697: @var{flag}
698: IF
699: @var{code1}
700: ELSE
701: @var{code2}
702: ENDIF
703: @end example
704:
705: You can use @code{THEN} instead of @code{ENDIF}. Indeed, @code{THEN} is
706: standard, and @code{ENDIF} is not, although it is quite popular. We
707: recommend using @code{ENDIF}, because it is less confusing for people
708: who also know other languages (and is not prone to reinforcing negative
709: prejudices against Forth in these people). Adding @code{ENDIF} to a
710: system that only supplies @code{THEN} is simple:
711: @example
712: : endif POSTPONE then ; immediate
713: @end example
714:
715: [According to @cite{Webster's New Encyclopedic Dictionary}, @dfn{then
716: (adv.)} has the following meanings:
717: @quotation
718: ... 2b: following next after in order ... 3d: as a necessary consequence
719: (if you were there, then you saw them).
720: @end quotation
721: Forth's @code{THEN} has the meaning 2b, whereas @code{THEN} in Pascal
722: and many other programming languages has the meaning 3d.]
723:
724: We also provide the words @code{?dup-if} and @code{?dup-0=-if}, so you
725: can avoid using @code{?dup}.
726:
727: @example
728: @var{n}
729: CASE
730: @var{n1} OF @var{code1} ENDOF
731: @var{n2} OF @var{code2} ENDOF
732: @dots{}
733: ENDCASE
734: @end example
735:
736: Executes the first @var{codei}, where the @var{ni} is equal to
737: @var{n}. A default case can be added by simply writing the code after
738: the last @code{ENDOF}. It may use @var{n}, which is on top of the stack,
739: but must not consume it.
740:
741: @node Simple Loops, Counted Loops, Selection, Control Structures
742: @subsection Simple Loops
743:
744: @example
745: BEGIN
746: @var{code1}
747: @var{flag}
748: WHILE
749: @var{code2}
750: REPEAT
751: @end example
752:
753: @var{code1} is executed and @var{flag} is computed. If it is true,
754: @var{code2} is executed and the loop is restarted; If @var{flag} is false, execution continues after the @code{REPEAT}.
755:
756: @example
757: BEGIN
758: @var{code}
759: @var{flag}
760: UNTIL
761: @end example
762:
763: @var{code} is executed. The loop is restarted if @code{flag} is false.
764:
765: @example
766: BEGIN
767: @var{code}
768: AGAIN
769: @end example
770:
771: This is an endless loop.
772:
773: @node Counted Loops, Arbitrary control structures, Simple Loops, Control Structures
774: @subsection Counted Loops
775:
776: The basic counted loop is:
777: @example
778: @var{limit} @var{start}
779: ?DO
780: @var{body}
781: LOOP
782: @end example
783:
784: This performs one iteration for every integer, starting from @var{start}
785: and up to, but excluding @var{limit}. The counter, aka index, can be
786: accessed with @code{i}. E.g., the loop
787: @example
788: 10 0 ?DO
789: i .
790: LOOP
791: @end example
792: prints
793: @example
794: 0 1 2 3 4 5 6 7 8 9
795: @end example
796: The index of the innermost loop can be accessed with @code{i}, the index
797: of the next loop with @code{j}, and the index of the third loop with
798: @code{k}.
799:
800: The loop control data are kept on the return stack, so there are some
801: restrictions on mixing return stack accesses and counted loop
802: words. E.g., if you put values on the return stack outside the loop, you
803: cannot read them inside the loop. If you put values on the return stack
804: within a loop, you have to remove them before the end of the loop and
805: before accessing the index of the loop.
806:
807: There are several variations on the counted loop:
808:
809: @code{LEAVE} leaves the innermost counted loop immediately.
810:
811: @code{LOOP} can be replaced with @code{@var{n} +LOOP}; this updates the
812: index by @var{n} instead of by 1. The loop is terminated when the border
813: between @var{limit-1} and @var{limit} is crossed. E.g.:
814:
815: @code{4 0 ?DO i . 2 +LOOP} prints @code{0 2}
816:
817: @code{4 1 ?DO i . 2 +LOOP} prints @code{1 3}
818:
819: The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative:
820:
821: @code{-1 0 ?DO i . -1 +LOOP} prints @code{0 -1}
822:
823: @code{ 0 0 ?DO i . -1 +LOOP} prints nothing
824:
825: Therefore we recommend avoiding using @code{@var{n} +LOOP} with negative
826: @var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative
827: case behaves symmetrical to the positive case:
828:
829: @code{-2 0 ?DO i . -1 S+LOOP} prints @code{0 -1}
830:
831: @code{-1 0 ?DO i . -1 S+LOOP} prints @code{0}
832:
833: @code{ 0 0 ?DO i . -1 S+LOOP} prints nothing
834:
835: The loop is terminated when the border between @var{limit@minus{}sgn(n)} and
836: @var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS
837: Forth standard.
838:
839: @code{?DO} can be replaced by @code{DO}. @code{DO} enters the loop even
840: when the start and the limit value are equal. We do not recommend using
841: @code{DO}. It will just give you maintenance troubles.
842:
843: @code{UNLOOP} is used to prepare for an abnormal loop exit, e.g., via
844: @code{EXIT}. @code{UNLOOP} removes the loop control parameters from the
845: return stack so @code{EXIT} can get to its return address.
846:
847: Another counted loop is
848: @example
849: @var{n}
850: FOR
851: @var{body}
852: NEXT
853: @end example
854: This is the preferred loop of native code compiler writers who are too
855: lazy to optimize @code{?DO} loops properly. In GNU Forth, this loop
856: iterates @var{n+1} times; @code{i} produces values starting with @var{n}
857: and ending with 0. Other Forth systems may behave differently, even if
858: they support @code{FOR} loops.
859:
860: @node Arbitrary control structures, Calls and returns, Counted Loops, Control Structures
861: @subsection Arbitrary control structures
862:
863: ANS Forth permits and supports using control structures in a non-nested
864: way. Information about incomplete control structures is stored on the
865: control-flow stack. This stack may be implemented on the Forth data
866: stack, and this is what we have done in gforth.
867:
868: An @i{orig} entry represents an unresolved forward branch, a @i{dest}
869: entry represents a backward branch target. A few words are the basis for
870: building any control structure possible (except control structures that
871: need storage, like calls, coroutines, and backtracking).
872:
873: doc-if
874: doc-ahead
875: doc-then
876: doc-begin
877: doc-until
878: doc-again
879: doc-cs-pick
880: doc-cs-roll
881:
882: On many systems control-flow stack items take one word, in gforth they
883: currently take three (this may change in the future). Therefore it is a
884: really good idea to manipulate the control flow stack with
885: @code{cs-pick} and @code{cs-roll}, not with data stack manipulation
886: words.
887:
888: Some standard control structure words are built from these words:
889:
890: doc-else
891: doc-while
892: doc-repeat
893:
894: Counted loop words constitute a separate group of words:
895:
896: doc-?do
897: doc-do
898: doc-for
899: doc-loop
900: doc-s+loop
901: doc-+loop
902: doc-next
903: doc-leave
904: doc-?leave
905: doc-unloop
906: doc-done
907:
908: The standard does not allow using @code{cs-pick} and @code{cs-roll} on
909: @i{do-sys}. Our system allows it, but it's your job to ensure that for
910: every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path
911: through the definition (@code{LOOP} etc. compile an @code{UNLOOP} on the
912: fall-through path). Also, you have to ensure that all @code{LEAVE}s are
913: resolved (by using one of the loop-ending words or @code{DONE}).
914:
915: Another group of control structure words are
916:
917: doc-case
918: doc-endcase
919: doc-of
920: doc-endof
921:
922: @i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and
923: @code{cs-roll}.
924:
925: @subsubsection Programming Style
926:
927: In order to ensure readability we recommend that you do not create
928: arbitrary control structures directly, but define new control structure
929: words for the control structure you want and use these words in your
930: program.
931:
932: E.g., instead of writing
933:
934: @example
935: begin
936: ...
937: if [ 1 cs-roll ]
938: ...
939: again then
940: @end example
941:
942: we recommend defining control structure words, e.g.,
943:
944: @example
945: : while ( dest -- orig dest )
946: POSTPONE if
947: 1 cs-roll ; immediate
948:
949: : repeat ( orig dest -- )
950: POSTPONE again
951: POSTPONE then ; immediate
952: @end example
953:
954: and then using these to create the control structure:
955:
956: @example
957: begin
958: ...
959: while
960: ...
961: repeat
962: @end example
963:
964: That's much easier to read, isn't it? Of course, @code{BEGIN} and
965: @code{WHILE} are predefined, so in this example it would not be
966: necessary to define them.
967:
968: @node Calls and returns, Exception Handling, Arbitrary control structures, Control Structures
969: @subsection Calls and returns
970:
971: A definition can be called simply be writing the name of the
972: definition. When the end of the definition is reached, it returns. An earlier return can be forced using
973:
974: doc-exit
975:
976: Don't forget to clean up the return stack and @code{UNLOOP} any
977: outstanding @code{?DO}...@code{LOOP}s before @code{EXIT}ing. The
978: primitive compiled by @code{EXIT} is
979:
980: doc-;s
981:
982: @node Exception Handling, , Calls and returns, Control Structures
983: @subsection Exception Handling
984:
985: doc-catch
986: doc-throw
987:
988: @node Locals, Defining Words, Control Structures, Words
989: @section Locals
990:
991: Local variables can make Forth programming more enjoyable and Forth
992: programs easier to read. Unfortunately, the locals of ANS Forth are
993: laden with restrictions. Therefore, we provide not only the ANS Forth
994: locals wordset, but also our own, more powerful locals wordset (we
995: implemented the ANS Forth locals wordset through our locals wordset).
996:
997: @menu
998: * gforth locals::
999: * ANS Forth locals::
1000: @end menu
1001:
1002: @node gforth locals, ANS Forth locals, Locals, Locals
1003: @subsection gforth locals
1004:
1005: Locals can be defined with
1006:
1007: @example
1008: @{ local1 local2 ... -- comment @}
1009: @end example
1010: or
1011: @example
1012: @{ local1 local2 ... @}
1013: @end example
1014:
1015: E.g.,
1016: @example
1017: : max @{ n1 n2 -- n3 @}
1018: n1 n2 > if
1019: n1
1020: else
1021: n2
1022: endif ;
1023: @end example
1024:
1025: The similarity of locals definitions with stack comments is intended. A
1026: locals definition often replaces the stack comment of a word. The order
1027: of the locals corresponds to the order in a stack comment and everything
1028: after the @code{--} is really a comment.
1029:
1030: This similarity has one disadvantage: It is too easy to confuse locals
1031: declarations with stack comments, causing bugs and making them hard to
1032: find. However, this problem can be avoided by appropriate coding
1033: conventions: Do not use both notations in the same program. If you do,
1034: they should be distinguished using additional means, e.g. by position.
1035:
1036: The name of the local may be preceded by a type specifier, e.g.,
1037: @code{F:} for a floating point value:
1038:
1039: @example
1040: : CX* @{ F: Ar F: Ai F: Br F: Bi -- Cr Ci @}
1041: \ complex multiplication
1042: Ar Br f* Ai Bi f* f-
1043: Ar Bi f* Ai Br f* f+ ;
1044: @end example
1045:
1046: GNU Forth currently supports cells (@code{W:}, @code{W^}), doubles
1047: (@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters
1048: (@code{C:}, @code{C^}) in two flavours: a value-flavoured local (defined
1049: with @code{W:}, @code{D:} etc.) produces its value and can be changed
1050: with @code{TO}. A variable-flavoured local (defined with @code{W^} etc.)
1051: produces its address (which becomes invalid when the variable's scope is
1052: left). E.g., the standard word @code{emit} can be defined in therms of
1053: @code{type} like this:
1054:
1055: @example
1056: : emit @{ C^ char* -- @}
1057: char* 1 type ;
1058: @end example
1059:
1060: A local without type specifier is a @code{W:} local. Both flavours of
1061: locals are initialized with values from the data or FP stack.
1062:
1063: Currently there is no way to define locals with user-defined data
1064: structures, but we are working on it.
1065:
1066: GNU Forth allows defining locals everywhere in a colon definition. This
1067: poses the following questions:
1068:
1069: @menu
1070: * Where are locals visible by name?::
1071: * How long do locals live?::
1072: * Programming Style::
1073: * Implementation::
1074: @end menu
1075:
1076: @node Where are locals visible by name?, How long do locals live?, gforth locals, gforth locals
1077: @subsubsection Where are locals visible by name?
1078:
1079: Basically, the answer is that locals are visible where you would expect
1080: it in block-structured languages, and sometimes a little longer. If you
1081: want to restrict the scope of a local, enclose its definition in
1082: @code{SCOPE}...@code{ENDSCOPE}.
1083:
1084: doc-scope
1085: doc-endscope
1086:
1087: These words behave like control structure words, so you can use them
1088: with @code{CS-PICK} and @code{CS-ROLL} to restrict the scope in
1089: arbitrary ways.
1090:
1091: If you want a more exact answer to the visibility question, here's the
1092: basic principle: A local is visible in all places that can only be
1093: reached through the definition of the local@footnote{In compiler
1094: construction terminology, all places dominated by the definition of the
1095: local.}. In other words, it is not visible in places that can be reached
1096: without going through the definition of the local. E.g., locals defined
1097: in @code{IF}...@code{ENDIF} are visible until the @code{ENDIF}, locals
1098: defined in @code{BEGIN}...@code{UNTIL} are visible after the
1099: @code{UNTIL} (until, e.g., a subsequent @code{ENDSCOPE}).
1100:
1101: The reasoning behind this solution is: We want to have the locals
1102: visible as long as it is meaningful. The user can always make the
1103: visibility shorter by using explicit scoping. In a place that can
1104: only be reached through the definition of a local, the meaning of a
1105: local name is clear. In other places it is not: How is the local
1106: initialized at the control flow path that does not contain the
1107: definition? Which local is meant, if the same name is defined twice in
1108: two independent control flow paths?
1109:
1110: This should be enough detail for nearly all users, so you can skip the
1111: rest of this section. If you relly must know all the gory details and
1112: options, read on.
1113:
1114: In order to implement this rule, the compiler has to know which places
1115: are unreachable. It knows this automatically after @code{AHEAD},
1116: @code{AGAIN}, @code{EXIT} and @code{LEAVE}; in other cases (e.g., after
1117: most @code{THROW}s), you can use the word @code{UNREACHABLE} to tell the
1118: compiler that the control flow never reaches that place. If
1119: @code{UNREACHABLE} is not used where it could, the only consequence is
1120: that the visibility of some locals is more limited than the rule above
1121: says. If @code{UNREACHABLE} is used where it should not (i.e., if you
1122: lie to the compiler), buggy code will be produced.
1123:
1124: Another problem with this rule is that at @code{BEGIN}, the compiler
1125: does not know which locals will be visible on the incoming
1126: back-edge. All problems discussed in the following are due to this
1127: ignorance of the compiler (we discuss the problems using @code{BEGIN}
1128: loops as examples; the discussion also applies to @code{?DO} and other
1129: loops). Perhaps the most insidious example is:
1130: @example
1131: AHEAD
1132: BEGIN
1133: x
1134: [ 1 CS-ROLL ] THEN
1135: @{ x @}
1136: ...
1137: UNTIL
1138: @end example
1139:
1140: This should be legal according to the visibility rule. The use of
1141: @code{x} can only be reached through the definition; but that appears
1142: textually below the use.
1143:
1144: From this example it is clear that the visibility rules cannot be fully
1145: implemented without major headaches. Our implementation treats common
1146: cases as advertised and the exceptions are treated in a safe way: The
1147: compiler makes a reasonable guess about the locals visible after a
1148: @code{BEGIN}; if it is too pessimistic, the
1149: user will get a spurious error about the local not being defined; if the
1150: compiler is too optimistic, it will notice this later and issue a
1151: warning. In the case above the compiler would complain about @code{x}
1152: being undefined at its use. You can see from the obscure examples in
1153: this section that it takes quite unusual control structures to get the
1154: compiler into trouble, and even then it will often do fine.
1155:
1156: If the @code{BEGIN} is reachable from above, the most optimistic guess
1157: is that all locals visible before the @code{BEGIN} will also be
1158: visible after the @code{BEGIN}. This guess is valid for all loops that
1159: are entered only through the @code{BEGIN}, in particular, for normal
1160: @code{BEGIN}...@code{WHILE}...@code{REPEAT} and
1161: @code{BEGIN}...@code{UNTIL} loops and it is implemented in our
1162: compiler. When the branch to the @code{BEGIN} is finally generated by
1163: @code{AGAIN} or @code{UNTIL}, the compiler checks the guess and
1164: warns the user if it was too optimisitic:
1165: @example
1166: IF
1167: @{ x @}
1168: BEGIN
1169: \ x ?
1170: [ 1 cs-roll ] THEN
1171: ...
1172: UNTIL
1173: @end example
1174:
1175: Here, @code{x} lives only until the @code{BEGIN}, but the compiler
1176: optimistically assumes that it lives until the @code{THEN}. It notices
1177: this difference when it compiles the @code{UNTIL} and issues a
1178: warning. The user can avoid the warning, and make sure that @code{x}
1179: is not used in the wrong area by using explicit scoping:
1180: @example
1181: IF
1182: SCOPE
1183: @{ x @}
1184: ENDSCOPE
1185: BEGIN
1186: [ 1 cs-roll ] THEN
1187: ...
1188: UNTIL
1189: @end example
1190:
1191: Since the guess is optimistic, there will be no spurious error messages
1192: about undefined locals.
1193:
1194: If the @code{BEGIN} is not reachable from above (e.g., after
1195: @code{AHEAD} or @code{EXIT}), the compiler cannot even make an
1196: optimistic guess, as the locals visible after the @code{BEGIN} may be
1197: defined later. Therefore, the compiler assumes that no locals are
1198: visible after the @code{BEGIN}. However, the useer can use
1199: @code{ASSUME-LIVE} to make the compiler assume that the same locals are
1200: visible at the BEGIN as at the point where the item was created.
1201:
1202: doc-assume-live
1203:
1204: E.g.,
1205: @example
1206: @{ x @}
1207: AHEAD
1208: ASSUME-LIVE
1209: BEGIN
1210: x
1211: [ 1 CS-ROLL ] THEN
1212: ...
1213: UNTIL
1214: @end example
1215:
1216: Other cases where the locals are defined before the @code{BEGIN} can be
1217: handled by inserting an appropriate @code{CS-ROLL} before the
1218: @code{ASSUME-LIVE} (and changing the control-flow stack manipulation
1219: behind the @code{ASSUME-LIVE}).
1220:
1221: Cases where locals are defined after the @code{BEGIN} (but should be
1222: visible immediately after the @code{BEGIN}) can only be handled by
1223: rearranging the loop. E.g., the ``most insidious'' example above can be
1224: arranged into:
1225: @example
1226: BEGIN
1227: @{ x @}
1228: ... 0=
1229: WHILE
1230: x
1231: REPEAT
1232: @end example
1233:
1234: @node How long do locals live?, Programming Style, Where are locals visible by name?, gforth locals
1235: @subsubsection How long do locals live?
1236:
1237: The right answer for the lifetime question would be: A local lives at
1238: least as long as it can be accessed. For a value-flavoured local this
1239: means: until the end of its visibility. However, a variable-flavoured
1240: local could be accessed through its address far beyond its visibility
1241: scope. Ultimately, this would mean that such locals would have to be
1242: garbage collected. Since this entails un-Forth-like implementation
1243: complexities, I adopted the same cowardly solution as some other
1244: languages (e.g., C): The local lives only as long as it is visible;
1245: afterwards its address is invalid (and programs that access it
1246: afterwards are erroneous).
1247:
1248: @node Programming Style, Implementation, How long do locals live?, gforth locals
1249: @subsubsection Programming Style
1250:
1251: The freedom to define locals anywhere has the potential to change
1252: programming styles dramatically. In particular, the need to use the
1253: return stack for intermediate storage vanishes. Moreover, all stack
1254: manipulations (except @code{PICK}s and @code{ROLL}s with run-time
1255: determined arguments) can be eliminated: If the stack items are in the
1256: wrong order, just write a locals definition for all of them; then
1257: write the items in the order you want.
1258:
1259: This seems a little far-fetched and eliminating stack manipulations is
1260: unlikely to become a conscious programming objective. Still, the number
1261: of stack manipulations will be reduced dramatically if local variables
1262: are used liberally (e.g., compare @code{max} in @ref{gforth locals} with
1263: a traditional implementation of @code{max}).
1264:
1265: This shows one potential benefit of locals: making Forth programs more
1266: readable. Of course, this benefit will only be realized if the
1267: programmers continue to honour the principle of factoring instead of
1268: using the added latitude to make the words longer.
1269:
1270: Using @code{TO} can and should be avoided. Without @code{TO},
1271: every value-flavoured local has only a single assignment and many
1272: advantages of functional languages apply to Forth. I.e., programs are
1273: easier to analyse, to optimize and to read: It is clear from the
1274: definition what the local stands for, it does not turn into something
1275: different later.
1276:
1277: E.g., a definition using @code{TO} might look like this:
1278: @example
1279: : strcmp @{ addr1 u1 addr2 u2 -- n @}
1280: u1 u2 min 0
1281: ?do
1282: addr1 c@ addr2 c@ - ?dup
1283: if
1284: unloop exit
1285: then
1286: addr1 char+ TO addr1
1287: addr2 char+ TO addr2
1288: loop
1289: u1 u2 - ;
1290: @end example
1291: Here, @code{TO} is used to update @code{addr1} and @code{addr2} at
1292: every loop iteration. @code{strcmp} is a typical example of the
1293: readability problems of using @code{TO}. When you start reading
1294: @code{strcmp}, you think that @code{addr1} refers to the start of the
1295: string. Only near the end of the loop you realize that it is something
1296: else.
1297:
1298: This can be avoided by defining two locals at the start of the loop that
1299: are initialized with the right value for the current iteration.
1300: @example
1301: : strcmp @{ addr1 u1 addr2 u2 -- n @}
1302: addr1 addr2
1303: u1 u2 min 0
1304: ?do @{ s1 s2 @}
1305: s1 c@ s2 c@ - ?dup
1306: if
1307: unloop exit
1308: then
1309: s1 char+ s2 char+
1310: loop
1311: 2drop
1312: u1 u2 - ;
1313: @end example
1314: Here it is clear from the start that @code{s1} has a different value
1315: in every loop iteration.
1316:
1317: @node Implementation, , Programming Style, gforth locals
1318: @subsubsection Implementation
1319:
1320: GNU Forth uses an extra locals stack. The most compelling reason for
1321: this is that the return stack is not float-aligned; using an extra stack
1322: also eliminates the problems and restrictions of using the return stack
1323: as locals stack. Like the other stacks, the locals stack grows toward
1324: lower addresses. A few primitives allow an efficient implementation:
1325:
1326: doc-@local#
1327: doc-f@local#
1328: doc-laddr#
1329: doc-lp+!#
1330: doc-lp!
1331: doc->l
1332: doc-f>l
1333:
1334: In addition to these primitives, some specializations of these
1335: primitives for commonly occurring inline arguments are provided for
1336: efficiency reasons, e.g., @code{@@local0} as specialization of
1337: @code{@@local#} for the inline argument 0. The following compiling words
1338: compile the right specialized version, or the general version, as
1339: appropriate:
1340:
1341: doc-compile-@local
1342: doc-compile-f@local
1343: doc-compile-lp+!
1344:
1345: Combinations of conditional branches and @code{lp+!#} like
1346: @code{?branch-lp+!#} (the locals pointer is only changed if the branch
1347: is taken) are provided for efficiency and correctness in loops.
1348:
1349: A special area in the dictionary space is reserved for keeping the
1350: local variable names. @code{@{} switches the dictionary pointer to this
1351: area and @code{@}} switches it back and generates the locals
1352: initializing code. @code{W:} etc.@ are normal defining words. This
1353: special area is cleared at the start of every colon definition.
1354:
1355: A special feature of GNU Forths dictionary is used to implement the
1356: definition of locals without type specifiers: every wordlist (aka
1357: vocabulary) has its own methods for searching
1358: etc. (@pxref{Wordlists}). For the present purpose we defined a wordlist
1359: with a special search method: When it is searched for a word, it
1360: actually creates that word using @code{W:}. @code{@{} changes the search
1361: order to first search the wordlist containing @code{@}}, @code{W:} etc.,
1362: and then the wordlist for defining locals without type specifiers.
1363:
1364: The lifetime rules support a stack discipline within a colon
1365: definition: The lifetime of a local is either nested with other locals
1366: lifetimes or it does not overlap them.
1367:
1368: At @code{BEGIN}, @code{IF}, and @code{AHEAD} no code for locals stack
1369: pointer manipulation is generated. Between control structure words
1370: locals definitions can push locals onto the locals stack. @code{AGAIN}
1371: is the simplest of the other three control flow words. It has to
1372: restore the locals stack depth of the corresponding @code{BEGIN}
1373: before branching. The code looks like this:
1374: @format
1375: @code{lp+!#} current-locals-size @minus{} dest-locals-size
1376: @code{branch} <begin>
1377: @end format
1378:
1379: @code{UNTIL} is a little more complicated: If it branches back, it
1380: must adjust the stack just like @code{AGAIN}. But if it falls through,
1381: the locals stack must not be changed. The compiler generates the
1382: following code:
1383: @format
1384: @code{?branch-lp+!#} <begin> current-locals-size @minus{} dest-locals-size
1385: @end format
1386: The locals stack pointer is only adjusted if the branch is taken.
1387:
1388: @code{THEN} can produce somewhat inefficient code:
1389: @format
1390: @code{lp+!#} current-locals-size @minus{} orig-locals-size
1391: <orig target>:
1392: @code{lp+!#} orig-locals-size @minus{} new-locals-size
1393: @end format
1394: The second @code{lp+!#} adjusts the locals stack pointer from the
1395: level at the @var{orig} point to the level after the @code{THEN}. The
1396: first @code{lp+!#} adjusts the locals stack pointer from the current
1397: level to the level at the orig point, so the complete effect is an
1398: adjustment from the current level to the right level after the
1399: @code{THEN}.
1400:
1401: In a conventional Forth implementation a dest control-flow stack entry
1402: is just the target address and an orig entry is just the address to be
1403: patched. Our locals implementation adds a wordlist to every orig or dest
1404: item. It is the list of locals visible (or assumed visible) at the point
1405: described by the entry. Our implementation also adds a tag to identify
1406: the kind of entry, in particular to differentiate between live and dead
1407: (reachable and unreachable) orig entries.
1408:
1409: A few unusual operations have to be performed on locals wordlists:
1410:
1411: doc-common-list
1412: doc-sub-list?
1413: doc-list-size
1414:
1415: Several features of our locals wordlist implementation make these
1416: operations easy to implement: The locals wordlists are organised as
1417: linked lists; the tails of these lists are shared, if the lists
1418: contain some of the same locals; and the address of a name is greater
1419: than the address of the names behind it in the list.
1420:
1421: Another important implementation detail is the variable
1422: @code{dead-code}. It is used by @code{BEGIN} and @code{THEN} to
1423: determine if they can be reached directly or only through the branch
1424: that they resolve. @code{dead-code} is set by @code{UNREACHABLE},
1425: @code{AHEAD}, @code{EXIT} etc., and cleared at the start of a colon
1426: definition, by @code{BEGIN} and usually by @code{THEN}.
1427:
1428: Counted loops are similar to other loops in most respects, but
1429: @code{LEAVE} requires special attention: It performs basically the same
1430: service as @code{AHEAD}, but it does not create a control-flow stack
1431: entry. Therefore the information has to be stored elsewhere;
1432: traditionally, the information was stored in the target fields of the
1433: branches created by the @code{LEAVE}s, by organizing these fields into a
1434: linked list. Unfortunately, this clever trick does not provide enough
1435: space for storing our extended control flow information. Therefore, we
1436: introduce another stack, the leave stack. It contains the control-flow
1437: stack entries for all unresolved @code{LEAVE}s.
1438:
1439: Local names are kept until the end of the colon definition, even if
1440: they are no longer visible in any control-flow path. In a few cases
1441: this may lead to increased space needs for the locals name area, but
1442: usually less than reclaiming this space would cost in code size.
1443:
1444:
1445: @node ANS Forth locals, , gforth locals, Locals
1446: @subsection ANS Forth locals
1447:
1448: The ANS Forth locals wordset does not define a syntax for locals, but
1449: words that make it possible to define various syntaxes. One of the
1450: possible syntaxes is a subset of the syntax we used in the gforth locals
1451: wordset, i.e.:
1452:
1453: @example
1454: @{ local1 local2 ... -- comment @}
1455: @end example
1456: or
1457: @example
1458: @{ local1 local2 ... @}
1459: @end example
1460:
1461: The order of the locals corresponds to the order in a stack comment. The
1462: restrictions are:
1463:
1464: @itemize @bullet
1465: @item
1466: Locals can only be cell-sized values (no type specifers are allowed).
1467: @item
1468: Locals can be defined only outside control structures.
1469: @item
1470: Locals can interfere with explicit usage of the return stack. For the
1471: exact (and long) rules, see the standard. If you don't use return stack
1472: accessing words in a definition using locals, you will we all right. The
1473: purpose of this rule is to make locals implementation on the return
1474: stack easier.
1475: @item
1476: The whole definition must be in one line.
1477: @end itemize
1478:
1479: Locals defined in this way behave like @code{VALUE}s
1480: (@xref{Values}). I.e., they are initialized from the stack. Using their
1481: name produces their value. Their value can be changed using @code{TO}.
1482:
1483: Since this syntax is supported by gforth directly, you need not do
1484: anything to use it. If you want to port a program using this syntax to
1485: another ANS Forth system, use @file{anslocal.fs} to implement the syntax
1486: on the other system.
1487:
1488: Note that a syntax shown in the standard, section A.13 looks
1489: similar, but is quite different in having the order of locals
1490: reversed. Beware!
1491:
1492: The ANS Forth locals wordset itself consists of the following word
1493:
1494: doc-(local)
1495:
1496: The ANS Forth locals extension wordset defines a syntax, but it is so
1497: awful that we strongly recommend not to use it. We have implemented this
1498: syntax to make porting to gforth easy, but do not document it here. The
1499: problem with this syntax is that the locals are defined in an order
1500: reversed with respect to the standard stack comment notation, making
1501: programs harder to read, and easier to misread and miswrite. The only
1502: merit of this syntax is that it is easy to implement using the ANS Forth
1503: locals wordset.
1504:
1505: @node Defining Words, Wordlists, Locals, Words
1506: @section Defining Words
1507:
1508: @menu
1509: * Values::
1510: @end menu
1511:
1512: @node Values, , Defining Words, Defining Words
1513: @subsection Values
1514:
1515: @node Wordlists, Files, Defining Words, Words
1516: @section Wordlists
1517:
1518: @node Files, Blocks, Wordlists, Words
1519: @section Files
1520:
1521: @node Blocks, Other I/O, Files, Words
1522: @section Blocks
1523:
1524: @node Other I/O, Programming Tools, Blocks, Words
1525: @section Other I/O
1526:
1527: @node Programming Tools, Threading Words, Other I/O, Words
1528: @section Programming Tools
1529:
1530: @menu
1531: * Debugging:: Simple and quick.
1532: * Assertions:: Making your programs self-checking.
1533: @end menu
1534:
1535: @node Debugging, Assertions, Programming Tools, Programming Tools
1536: @subsection Debugging
1537:
1538: The simple debugging aids provided in @file{debugging.fs}
1539: are meant to support a different style of debugging than the
1540: tracing/stepping debuggers used in languages with long turn-around
1541: times.
1542:
1543: A much better (faster) way in fast-compilig languages is to add
1544: printing code at well-selected places, let the program run, look at
1545: the output, see where things went wrong, add more printing code, etc.,
1546: until the bug is found.
1547:
1548: The word @code{~~} is easy to insert. It just prints debugging
1549: information (by default the source location and the stack contents). It
1550: is also easy to remove (@kbd{C-x ~} in the Emacs Forth mode to
1551: query-replace them with nothing). The deferred words
1552: @code{printdebugdata} and @code{printdebugline} control the output of
1553: @code{~~}. The default source location output format works well with
1554: Emacs' compilation mode, so you can step through the program at the
1555: source level using @kbd{C-x `} (the advantage over a stepping debugger
1556: is that you can step in any direction and you know where the crash has
1557: happened or where the strange data has occurred).
1558:
1559: Note that the default actions clobber the contents of the pictured
1560: numeric output string, so you should not use @code{~~}, e.g., between
1561: @code{<#} and @code{#>}.
1562:
1563: doc-~~
1564: doc-printdebugdata
1565: doc-printdebugline
1566:
1567: @node Assertions, , Debugging, Programming Tools
1568: @subsection Assertions
1569:
1570: It is a good idea to make your programs self-checking, in particular, if
1571: you use an assumption (e.g., that a certain field of a data structure is
1572: never zero) that may become wrong during maintenance. GForth supports
1573: assertions for this purpose. They are used like this:
1574:
1575: @example
1576: assert( @var{flag} )
1577: @end example
1578:
1579: The code between @code{assert(} and @code{)} should compute a flag, that
1580: should be true if everything is alright and false otherwise. It should
1581: not change anything else on the stack. The overall stack effect of the
1582: assertion is @code{( -- )}. E.g.
1583:
1584: @example
1585: assert( 1 1 + 2 = ) \ what we learn in school
1586: assert( dup 0<> ) \ assert that the top of stack is not zero
1587: assert( false ) \ this code should not be reached
1588: @end example
1589:
1590: The need for assertions is different at different times. During
1591: debugging, we want more checking, in production we sometimes care more
1592: for speed. Therefore, assertions can be turned off, i.e., the assertion
1593: becomes a comment. Depending on the importance of an assertion and the
1594: time it takes to check it, you may want to turn off some assertions and
1595: keep others turned on. GForth provides several levels of assertions for
1596: this purpose:
1597:
1598: doc-assert0(
1599: doc-assert1(
1600: doc-assert2(
1601: doc-assert3(
1602: doc-assert(
1603: doc-)
1604:
1605: @code{Assert(} is the same as @code{assert1(}. The variable
1606: @code{assert-level} specifies the highest assertions that are turned
1607: on. I.e., at the default @code{assert-level} of one, @code{assert0(} and
1608: @code{assert1(} assertions perform checking, while @code{assert2(} and
1609: @code{assert3(} assertions are treated as comments.
1610:
1611: Note that the @code{assert-level} is evaluated at compile-time, not at
1612: run-time. I.e., you cannot turn assertions on or off at run-time, you
1613: have to set the @code{assert-level} appropriately before compiling a
1614: piece of code. You can compile several pieces of code at several
1615: @code{assert-level}s (e.g., a trusted library at level 1 and newly
1616: written code at level 3).
1617:
1618: doc-assert-level
1619:
1620: If an assertion fails, a message compatible with Emacs' compilation mode
1621: is produced and the execution is aborted (currently with @code{ABORT"}.
1622: If there is interest, we will introduce a special throw code. But if you
1623: intend to @code{catch} a specific condition, using @code{throw} is
1624: probably more appropriate than an assertion).
1625:
1626: @node Threading Words, , Programming Tools, Words
1627: @section Threading Words
1628:
1629: These words provide access to code addresses and other threading stuff
1630: in gforth (and, possibly, other interpretive Forths). It more or less
1631: abstracts away the differences between direct and indirect threading
1632: (and, for direct threading, the machine dependences). However, at
1633: present this wordset is still inclomplete. It is also pretty low-level;
1634: some day it will hopefully be made unnecessary by an internals words set
1635: that abstracts implementation details away completely.
1636:
1637: doc->code-address
1638: doc->does-code
1639: doc-code-address!
1640: doc-does-code!
1641: doc-does-handler!
1642: doc-/does-handler
1643:
1644:
1645:
1646: @node ANS conformance, Model, Words, Top
1647: @chapter ANS conformance
1648:
1649: To the best of our knowledge, gforth is an
1650:
1651: ANS Forth System
1652: @itemize
1653: @item providing the Core Extensions word set
1654: @item providing the Block word set
1655: @item providing the Block Extensions word set
1656: @item providing the Double-Number word set
1657: @item providing the Double-Number Extensions word set
1658: @item providing the Exception word set
1659: @item providing the Exception Extensions word set
1660: @item providing the Facility word set
1661: @item providing @code{MS} and @code{TIME&DATE} from the Facility Extensions word set
1662: @item providing the File Access word set
1663: @item providing the File Access Extensions word set
1664: @item providing the Floating-Point word set
1665: @item providing the Floating-Point Extensions word set
1666: @item providing the Locals word set
1667: @item providing the Locals Extensions word set
1668: @item providing the Memory-Allocation word set
1669: @item providing the Memory-Allocation Extensions word set (that one's easy)
1670: @item providing the Programming-Tools word set
1671: @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
1672: @item providing the Search-Order word set
1673: @item providing the Search-Order Extensions word set
1674: @item providing the String word set
1675: @item providing the String Extensions word set (another easy one)
1676: @end itemize
1677:
1678: In addition, ANS Forth systems are required to document certain
1679: implementation choices. This chapter tries to meet these
1680: requirements. In many cases it gives a way to ask the system for the
1681: information instead of providing the information directly, in
1682: particular, if the information depends on the processor, the operating
1683: system or the installation options chosen, or if they are likely to
1684: change during the maintenance of gforth.
1685:
1686: @comment The framework for the rest has been taken from pfe.
1687:
1688: @menu
1689: * The Core Words::
1690: * The optional Block word set::
1691: * The optional Double Number word set::
1692: * The optional Exception word set::
1693: * The optional Facility word set::
1694: * The optional File-Access word set::
1695: * The optional Floating-Point word set::
1696: * The optional Locals word set::
1697: * The optional Memory-Allocation word set::
1698: * The optional Programming-Tools word set::
1699: * The optional Search-Order word set::
1700: @end menu
1701:
1702:
1703: @c =====================================================================
1704: @node The Core Words, The optional Block word set, ANS conformance, ANS conformance
1705: @comment node-name, next, previous, up
1706: @section The Core Words
1707: @c =====================================================================
1708:
1709: @menu
1710: * core-idef:: Implementation Defined Options
1711: * core-ambcond:: Ambiguous Conditions
1712: * core-other:: Other System Documentation
1713: @end menu
1714:
1715: @c ---------------------------------------------------------------------
1716: @node core-idef, core-ambcond, The Core Words, The Core Words
1717: @subsection Implementation Defined Options
1718: @c ---------------------------------------------------------------------
1719:
1720: @table @i
1721:
1722: @item (Cell) aligned addresses:
1723: processor-dependent. Gforths alignment words perform natural alignment
1724: (e.g., an address aligned for a datum of size 8 is divisible by
1725: 8). Unaligned accesses usually result in a @code{-23 THROW}.
1726:
1727: @item @code{EMIT} and non-graphic characters:
1728: The character is output using the C library function (actually, macro)
1729: @code{putchar}.
1730:
1731: @item character editing of @code{ACCEPT} and @code{EXPECT}:
1732: This is modeled on the GNU readline library (@pxref{Readline
1733: Interaction, , Command Line Editing, readline, The GNU Readline
1734: Library}) with Emacs-like key bindings. @kbd{Tab} deviates a little by
1735: producing a full word completion every time you type it (instead of
1736: producing the common prefix of all completions).
1737:
1738: @item character set:
1739: The character set of your computer and display device. Gforth is
1740: 8-bit-clean (but some other component in your system may make trouble).
1741:
1742: @item Character-aligned address requirements:
1743: installation-dependent. Currently a character is represented by a C
1744: @code{unsigned char}; in the future we might switch to @code{wchar_t}
1745: (Comments on that requested).
1746:
1747: @item character-set extensions and matching of names:
1748: Any character except 0 can be used in a name. Matching is
1749: case-insensitive. The matching is performed using the C function
1750: @code{strncasecmp}, whose function is probably influenced by the
1751: locale. E.g., the @code{C} locale does not know about accents and
1752: umlauts, so they are matched case-sensitively in that locale. For
1753: portability reasons it is best to write programs such that they work in
1754: the @code{C} locale. Then one can use libraries written by a Polish
1755: programmer (who might use words containing ISO Latin-2 encoded
1756: characters) and by a French programmer (ISO Latin-1) in the same program
1757: (of course, @code{WORDS} will produce funny results for some of the
1758: words (which ones, depends on the font you are using)). Also, the locale
1759: you prefer may not be available in other operating systems. Hopefully,
1760: Unicode will solve these problems one day.
1761:
1762: @item conditions under which control characters match a space delimiter:
1763: If @code{WORD} is called with the space character as a delimiter, all
1764: white-space characters (as identified by the C macro @code{isspace()})
1765: are delimiters. @code{PARSE}, on the other hand, treats space like other
1766: delimiters. @code{PARSE-WORD} treats space like @code{WORD}, but behaves
1767: like @code{PARSE} otherwise. @code{(NAME)}, which is used by the outer
1768: interpreter (aka text interpreter) by default, treats all white-space
1769: characters as delimiters.
1770:
1771: @item format of the control flow stack:
1772: The data stack is used as control flow stack. The size of a control flow
1773: stack item in cells is given by the constant @code{cs-item-size}. At the
1774: time of this writing, an item consists of a (pointer to a) locals list
1775: (third), an address in the code (second), and a tag for identifying the
1776: item (TOS). The following tags are used: @code{defstart},
1777: @code{live-orig}, @code{dead-orig}, @code{dest}, @code{do-dest},
1778: @code{scopestart}.
1779:
1780: @item conversion of digits > 35
1781: The characters @code{[\]^_'} are the digits with the decimal value
1782: 36@minus{}41. There is no way to input many of the larger digits.
1783:
1784: @item display after input terminates in @code{ACCEPT} and @code{EXPECT}:
1785: The cursor is moved to the end of the entered string. If the input is
1786: terminated using the @kbd{Return} key, a space is typed.
1787:
1788: @item exception abort sequence of @code{ABORT"}:
1789: The error string is stored into the variable @code{"error} and a
1790: @code{-2 throw} is performed.
1791:
1792: @item input line terminator:
1793: For interactive input, @kbd{C-m} and @kbd{C-j} terminate lines. One of
1794: these characters is typically produced when you type the @kbd{Enter} or
1795: @kbd{Return} key.
1796:
1797: @item maximum size of a counted string:
1798: @code{s" /counted-string" environment? drop .}. Currently 255 characters
1799: on all ports, but this may change.
1800:
1801: @item maximum size of a parsed string:
1802: Given by the constant @code{/line}. Currently 255 characters.
1803:
1804: @item maximum size of a definition name, in characters:
1805: 31
1806:
1807: @item maximum string length for @code{ENVIRONMENT?}, in characters:
1808: 31
1809:
1810: @item method of selecting the user input device:
1811: The user input device is the standard input. There is current no way to
1812: change it from within gforth. However, the input can typically be
1813: redirected in the command line that starts gforth.
1814:
1815: @item method of selecting the user output device:
1816: The user output device is the standard output. It cannot be redirected
1817: from within gforth, but typically from the command line that starts
1818: gforth. Gforth uses buffered output, so output on a terminal does not
1819: become visible before the next newline or buffer overflow. Output on
1820: non-terminals is invisible until the buffer overflows.
1821:
1822: @item methods of dictionary compilation:
1823: Waht are we expected to document here?
1824:
1825: @item number of bits in one address unit:
1826: @code{s" address-units-bits" environment? drop .}. 8 in all current
1827: ports.
1828:
1829: @item number representation and arithmetic:
1830: Processor-dependent. Binary two's complement on all current ports.
1831:
1832: @item ranges for integer types:
1833: Installation-dependent. Make environmental queries for @code{MAX-N},
1834: @code{MAX-U}, @code{MAX-D} and @code{MAX-UD}. The lower bounds for
1835: unsigned (and positive) types is 0. The lower bound for signed types on
1836: two's complement and one's complement machines machines can be computed
1837: by adding 1 to the upper bound.
1838:
1839: @item read-only data space regions:
1840: The whole Forth data space is writable.
1841:
1842: @item size of buffer at @code{WORD}:
1843: @code{PAD HERE - .}. 104 characters on 32-bit machines. The buffer is
1844: shared with the pictured numeric output string. If overwriting
1845: @code{PAD} is acceptable, it is as large as the remaining dictionary
1846: space, although only as much can be sensibly used as fits in a counted
1847: string.
1848:
1849: @item size of one cell in address units:
1850: @code{1 cells .}.
1851:
1852: @item size of one character in address units:
1853: @code{1 chars .}. 1 on all current ports.
1854:
1855: @item size of the keyboard terminal buffer:
1856: Varies. You can determine the size at a specific time using @code{lp@
1857: tib - .}. It is shared with the locals stack and TIBs of files that
1858: include the current file. You can change the amount of space for TIBs
1859: and locals stack at gforth startup with the command line option
1860: @code{-l}.
1861:
1862: @item size of the pictured numeric output buffer:
1863: @code{PAD HERE - .}. 104 characters on 32-bit machines. The buffer is
1864: shared with @code{WORD}.
1865:
1866: @item size of the scratch area returned by @code{PAD}:
1867: The remainder of dictionary space. You can even use the unused part of
1868: the data stack space. The current size can be computed with @code{sp@
1869: pad - .}.
1870:
1871: @item system case-sensitivity characteristics:
1872: Dictionary searches are case insensitive. However, as explained above
1873: under @i{character-set extensions}, the matching for non-ASCII
1874: characters is determined by the locale you are using. In the default
1875: @code{C} locale all non-ASCII characters are matched case-sensitively.
1876:
1877: @item system prompt:
1878: @code{ ok} in interpret state, @code{ compiled} in compile state.
1879:
1880: @item division rounding:
1881: installation dependent. @code{s" floored" environment? drop .}. We leave
1882: the choice to gcc (what to use for @code{/}) and to you (whether to use
1883: @code{fm/mod}, @code{sm/rem} or simply @code{/}).
1884:
1885: @item values of @code{STATE} when true:
1886: -1.
1887:
1888: @item values returned after arithmetic overflow:
1889: On two's complement machines, arithmetic is performed modulo
1890: 2**bits-per-cell for single arithmetic and 4**bits-per-cell for double
1891: arithmetic (with appropriate mapping for signed types). Division by zero
1892: typically results in a @code{-55 throw} (floatingpoint unidentified
1893: fault), although a @code{-10 throw} (divide by zero) would be more
1894: appropriate.
1895:
1896: @item whether the current definition can be found after @t{DOES>}:
1897: No.
1898:
1899: @end table
1900:
1901: @c ---------------------------------------------------------------------
1902: @node core-ambcond, core-other, core-idef, The Core Words
1903: @subsection Ambiguous conditions
1904: @c ---------------------------------------------------------------------
1905:
1906: @table @i
1907:
1908: @item a name is neither a word nor a number:
1909: @code{-13 throw} (Undefined word)
1910:
1911: @item a definition name exceeds the maximum length allowed:
1912: @code{-19 throw} (Word name too long)
1913:
1914: @item addressing a region not inside the various data spaces of the forth system:
1915: The stacks, code space and name space are accessible. Machine code space is
1916: typically readable. Accessing other addresses gives results dependent on
1917: the operating system. On decent systems: @code{-9 throw} (Invalid memory
1918: address).
1919:
1920: @item argument type incompatible with parameter:
1921: This is usually not caught. Some words perform checks, e.g., the control
1922: flow words, and issue a @code{ABORT"} or @code{-12 THROW} (Argument type
1923: mismatch).
1924:
1925: @item attempting to obtain the execution token of a word with undefined execution semantics:
1926: You get an execution token representing the compilation semantics
1927: instead.
1928:
1929: @item dividing by zero:
1930: typically results in a @code{-55 throw} (floating point unidentified
1931: fault), although a @code{-10 throw} (divide by zero) would be more
1932: appropriate.
1933:
1934: @item insufficient data stack or return stack space:
1935: Not checked. This typically results in mysterious illegal memory
1936: accesses, producing @code{-9 throw} (Invalid memory address) or
1937: @code{-23 throw} (Address alignment exception).
1938:
1939: @item insufficient space for loop control parameters:
1940: like other return stack overflows.
1941:
1942: @item insufficient space in the dictionary:
1943: Not checked. Similar results as stack overflows. However, typically the
1944: error appears at a different place when one inserts or removes code.
1945:
1946: @item interpreting a word with undefined interpretation semantics:
1947: For some words, we defined interpretation semantics. For the others:
1948: @code{-14 throw} (Interpreting a compile-only word). Note that this is
1949: checked only by the outer (aka text) interpreter; if the word is
1950: @code{execute}d in some other way, it will typically perform it's
1951: compilation semantics even in interpret state. (We could change @code{'}
1952: and relatives not to give the xt of such words, but we think that would
1953: be too restrictive).
1954:
1955: @item modifying the contents of the input buffer or a string literal:
1956: These are located in writable memory and can be modified.
1957:
1958: @item overflow of the pictured numeric output string:
1959: Not checked.
1960:
1961: @item parsed string overflow:
1962: @code{PARSE} cannot overflow. @code{WORD} does not check for overflow.
1963:
1964: @item producing a result out of range:
1965: On two's complement machines, arithmetic is performed modulo
1966: 2**bits-per-cell for single arithmetic and 4**bits-per-cell for double
1967: arithmetic (with appropriate mapping for signed types). Division by zero
1968: typically results in a @code{-55 throw} (floatingpoint unidentified
1969: fault), although a @code{-10 throw} (divide by zero) would be more
1970: appropriate. @code{convert} and @code{>number} currently overflow
1971: silently.
1972:
1973: @item reading from an empty data or return stack:
1974: The data stack is checked by the outer (aka text) interpreter after
1975: every word executed. If it has underflowed, a @code{-4 throw} (Stack
1976: underflow) is performed. Apart from that, the stacks are not checked and
1977: underflows can result in similar behaviour as overflows (of adjacent
1978: stacks).
1979:
1980: @item unexepected end of the input buffer, resulting in an attempt to use a zero-length string as a name:
1981: @code{Create} and its descendants perform a @code{-16 throw} (Attempt to
1982: use zero-length string as a name). Words like @code{'} probably will not
1983: find what they search. Note that it is possible to create zero-length
1984: names with @code{nextname} (should it not?).
1985:
1986: @item @code{>IN} greater than input buffer:
1987: The next invocation of a parsing word returns a string wih length 0.
1988:
1989: @item @code{RECURSE} appears after @code{DOES>}:
1990: Compiles a recursive call to the defining word not to the defined word.
1991:
1992: @item argument input source different than current input source for @code{RESTORE-INPUT}:
1993: !!???If the argument input source is a valid input source then it gets
1994: restored. Otherwise causes @code{-12 THROW} which unless caught issues
1995: the message "argument type mismatch" and aborts.
1996:
1997: @item data space containing definitions gets de-allocated:
1998: Deallocation with @code{allot} is not checked. This typically resuls in
1999: memory access faults or execution of illegal instructions.
2000:
2001: @item data space read/write with incorrect alignment:
2002: Processor-dependent. Typically results in a @code{-23 throw} (Address
2003: alignment exception). Under Linux on a 486 or later processor with
2004: alignment turned on, incorrect alignment results in a @code{-9 throw}
2005: (Invalid memory address). There are reportedly some processors with
2006: alignment restrictions that do not report them.
2007:
2008: @item data space pointer not properly aligned, @code{,}, @code{C,}:
2009: Like other alignment errors.
2010:
2011: @item less than u+2 stack items (@code{PICK} and @code{ROLL}):
2012: Not checked. May cause an illegal memory access.
2013:
2014: @item loop control parameters not available:
2015: Not checked. The counted loop words simply assume that the top of return
2016: stack items are loop control parameters and behave accordingly.
2017:
2018: @item most recent definition does not have a name (@code{IMMEDIATE}):
2019: @code{abort" last word was headerless"}.
2020:
2021: @item name not defined by @code{VALUE} used by @code{TO}:
2022: @code{-32 throw} (Invalid name argument)
2023:
2024: @item name not found (@code{'}, @code{POSTPONE}, @code{[']}, @code{[COMPILE]}):
2025: @code{-13 throw} (Undefined word)
2026:
2027: @item parameters are not of the same type (@code{DO}, @code{?DO}, @code{WITHIN}):
2028: Gforth behaves as if they were of the same type. I.e., you can predict
2029: the behaviour by interpreting all parameters as, e.g., signed.
2030:
2031: @item @code{POSTPONE} or @code{[COMPILE]} applied to @code{TO}:
2032: Assume @code{: X POSTPONE TO ; IMMEDIATE}. @code{X} is equivalent to
2033: @code{TO}.
2034:
2035: @item String longer than a counted string returned by @code{WORD}:
2036: Not checked. The string will be ok, but the count will, of course,
2037: contain only the least significant bits of the length.
2038:
2039: @item u greater than or equal to the number of bits in a cell (@code{LSHIFT}, @code{RSHIFT}):
2040: Processor-dependent. Typical behaviours are returning 0 and using only
2041: the low bits of the shift count.
2042:
2043: @item word not defined via @code{CREATE}:
2044: @code{>BODY} produces the PFA of the word no matter how it was defined.
2045:
2046: @code{DOES>} changes the execution semantics of the last defined word no
2047: matter how it was defined. E.g., @code{CONSTANT DOES>} is equivalent to
2048: @code{CREATE , DOES>}.
2049:
2050: @item words improperly used outside @code{<#} and @code{#>}:
2051: Not checked. As usual, you can expect memory faults.
2052:
2053: @end table
2054:
2055:
2056: @c ---------------------------------------------------------------------
2057: @node core-other, , core-ambcond, The Core Words
2058: @subsection Other system documentation
2059: @c ---------------------------------------------------------------------
2060:
2061: @table @i
2062:
2063: @item nonstandard words using @code{PAD}:
2064: None.
2065:
2066: @item operator's terminal facilities available:
2067: !!??
2068:
2069: @item program data space available:
2070: @code{sp@ here - .} gives the space remaining for dictionary and data
2071: stack together.
2072:
2073: @item return stack space available:
2074: !!??
2075:
2076: @item stack space available:
2077: @code{sp@ here - .} gives the space remaining for dictionary and data
2078: stack together.
2079:
2080: @item system dictionary space required, in address units:
2081: Type @code{here forthstart - .} after startup. At the time of this
2082: writing, this gives 70108 (bytes) on a 32-bit system.
2083: @end table
2084:
2085:
2086: @c =====================================================================
2087: @node The optional Block word set, The optional Double Number word set, The Core Words, ANS conformance
2088: @section The optional Block word set
2089: @c =====================================================================
2090:
2091: @menu
2092: * block-idef:: Implementation Defined Options
2093: * block-ambcond:: Ambiguous Conditions
2094: * block-other:: Other System Documentation
2095: @end menu
2096:
2097:
2098: @c ---------------------------------------------------------------------
2099: @node block-idef, block-ambcond, The optional Block word set, The optional Block word set
2100: @subsection Implementation Defined Options
2101: @c ---------------------------------------------------------------------
2102:
2103: @table @i
2104:
2105: @item the format for display by @code{LIST}:
2106: First the screen number is displayed, then 16 lines of 64 characters,
2107: each line preceded by the line number.
2108:
2109: @item the length of a line affected by @code{\}:
2110: 64 characters.
2111: @end table
2112:
2113:
2114: @c ---------------------------------------------------------------------
2115: @node block-ambcond, block-other, block-idef, The optional Block word set
2116: @subsection Ambiguous conditions
2117: @c ---------------------------------------------------------------------
2118:
2119: @table @i
2120:
2121: @item correct block read was not possible:
2122: Typically results in a @code{throw} of some OS-derived value (between
2123: -512 and -2048). If the blocks file was just not long enough, blanks are
2124: supplied for the missing portion.
2125:
2126: @item I/O exception in block transfer:
2127: Typically results in a @code{throw} of some OS-derived value (between
2128: -512 and -2048).
2129:
2130: @item invalid block number:
2131: @code{-35 throw} (Invalid block number)
2132:
2133: @item a program directly alters the contents of @code{BLK}:
2134: The input stream is switched to that other block, at the same
2135: position. If the storing to @code{BLK} happens when interpreting
2136: non-block input, the system will get quite confused when the block ends.
2137:
2138: @item no current block buffer for @code{UPDATE}:
2139: @code{UPDATE} has no effect.
2140:
2141: @end table
2142:
2143:
2144: @c ---------------------------------------------------------------------
2145: @node block-other, , block-ambcond, The optional Block word set
2146: @subsection Other system documentation
2147: @c ---------------------------------------------------------------------
2148:
2149: @table @i
2150:
2151: @item any restrictions a multiprogramming system places on the use of buffer addresses:
2152: No restrictions (yet).
2153:
2154: @item the number of blocks available for source and data:
2155: depends on your disk space.
2156:
2157: @end table
2158:
2159:
2160: @c =====================================================================
2161: @node The optional Double Number word set, The optional Exception word set, The optional Block word set, ANS conformance
2162: @section The optional Double Number word set
2163: @c =====================================================================
2164:
2165: @menu
2166: * double-ambcond:: Ambiguous Conditions
2167: @end menu
2168:
2169:
2170: @c ---------------------------------------------------------------------
2171: @node double-ambcond, , The optional Double Number word set, The optional Double Number word set
2172: @subsection Ambiguous conditions
2173: @c ---------------------------------------------------------------------
2174:
2175: @table @i
2176:
2177: @item @var{d} outside of range of @var{n} in @code{D>S}:
2178: The least significant cell of @var{d} is produced.
2179:
2180: @end table
2181:
2182:
2183: @c =====================================================================
2184: @node The optional Exception word set, The optional Facility word set, The optional Double Number word set, ANS conformance
2185: @section The optional Exception word set
2186: @c =====================================================================
2187:
2188: @menu
2189: * exception-idef:: Implementation Defined Options
2190: @end menu
2191:
2192:
2193: @c ---------------------------------------------------------------------
2194: @node exception-idef, , The optional Exception word set, The optional Exception word set
2195: @subsection Implementation Defined Options
2196: @c ---------------------------------------------------------------------
2197:
2198: @table @i
2199: @item @code{THROW}-codes used in the system:
2200: The codes -256@minus{}-511 are used for reporting signals (see
2201: @file{errore.fs}). The codes -512@minus{}-2047 are used for OS errors
2202: (for file and memory allocation operations). The mapping from OS error
2203: numbers to throw code is -512@minus{}@var{errno}. One side effect of
2204: this mapping is that undefined OS errors produce a message with a
2205: strange number; e.g., @code{-1000 THROW} results in @code{Unknown error
2206: 488} on my system.
2207: @end table
2208:
2209: @c =====================================================================
2210: @node The optional Facility word set, The optional File-Access word set, The optional Exception word set, ANS conformance
2211: @section The optional Facility word set
2212: @c =====================================================================
2213:
2214: @menu
2215: * facility-idef:: Implementation Defined Options
2216: * facility-ambcond:: Ambiguous Conditions
2217: @end menu
2218:
2219:
2220: @c ---------------------------------------------------------------------
2221: @node facility-idef, facility-ambcond, The optional Facility word set, The optional Facility word set
2222: @subsection Implementation Defined Options
2223: @c ---------------------------------------------------------------------
2224:
2225: @table @i
2226:
2227: @item encoding of keyboard events (@code{EKEY}):
2228: Not yet implemeted.
2229:
2230: @item duration of a system clock tick
2231: System dependent. With respect to @code{MS}, the time is specified in
2232: microseconds. How well the OS and the hardware implement this, is
2233: another question.
2234:
2235: @item repeatability to be expected from the execution of @code{MS}:
2236: System dependent. On Unix, a lot depends on load. If the system is
2237: lightly loaded, and the delay is short enough that gforth does not get
2238: swapped out, the performance should be acceptable. Under MS-DOS and
2239: other single-tasking systems, it should be good.
2240:
2241: @end table
2242:
2243:
2244: @c ---------------------------------------------------------------------
2245: @node facility-ambcond, , facility-idef, The optional Facility word set
2246: @subsection Ambiguous conditions
2247: @c ---------------------------------------------------------------------
2248:
2249: @table @i
2250:
2251: @item @code{AT-XY} can't be performed on user output device:
2252: Largely terminal dependant. No range checks are done on the arguments.
2253: No errors are reported. You may see some garbage appearing, you may see
2254: simply nothing happen.
2255:
2256: @end table
2257:
2258:
2259: @c =====================================================================
2260: @node The optional File-Access word set, The optional Floating-Point word set, The optional Facility word set, ANS conformance
2261: @section The optional File-Access word set
2262: @c =====================================================================
2263:
2264: @menu
2265: * file-idef:: Implementation Defined Options
2266: * file-ambcond:: Ambiguous Conditions
2267: @end menu
2268:
2269:
2270: @c ---------------------------------------------------------------------
2271: @node file-idef, file-ambcond, The optional File-Access word set, The optional File-Access word set
2272: @subsection Implementation Defined Options
2273: @c ---------------------------------------------------------------------
2274:
2275: @table @i
2276:
2277: @item File access methods used:
2278: @code{R/O}, @code{R/W} and @code{BIN} work as you would
2279: expect. @code{W/O} translates into the C file opening mode @code{w} (or
2280: @code{wb}): The file is cleared, if it exists, and created, if it does
2281: not (both with @code{open-file} and @code{create-file}). Under Unix
2282: @code{create-file} creates a file with 666 permissions modified by your
2283: umask.
2284:
2285: @item file exceptions:
2286: The file words do not raise exceptions (except, perhaps, memory access
2287: faults when you pass illegal addresses or file-ids).
2288:
2289: @item file line terminator:
2290: System-dependent. Gforth uses C's newline character as line
2291: terminator. What the actual character code(s) of this are is
2292: system-dependent.
2293:
2294: @item file name format
2295: System dependent. Gforth just uses the file name format of your OS.
2296:
2297: @item information returned by @code{FILE-STATUS}:
2298: @code{FILE-STATUS} returns the most powerful file access mode allowed
2299: for the file: Either @code{R/O}, @code{W/O} or @code{R/W}. If the file
2300: cannot be accessed, @code{R/O BIN} is returned. @code{BIN} is applicable
2301: along with the retured mode.
2302:
2303: @item input file state after an exception when including source:
2304: All files that are left via the exception are closed.
2305:
2306: @item @var{ior} values and meaning:
2307: The @var{ior}s returned by the file and memory allocation words are
2308: intended as throw codes. They typically are in the range
2309: -512@minus{}-2047 of OS errors. The mapping from OS error numbers to
2310: @var{ior}s is -512@minus{}@var{errno}.
2311:
2312: @item maximum depth of file input nesting:
2313: limited by the amount of return stack, locals/TIB stack, and the number
2314: of open files available. This should not give you troubles.
2315:
2316: @item maximum size of input line:
2317: @code{/line}. Currently 255.
2318:
2319: @item methods of mapping block ranges to files:
2320: Currently, the block words automatically access the file
2321: @file{blocks.fb} in the currend working directory. More sophisticated
2322: methods could be implemented if there is demand (and a volunteer).
2323:
2324: @item number of string buffers provided by @code{S"}:
2325: 1
2326:
2327: @item size of string buffer used by @code{S"}:
2328: @code{/line}. currently 255.
2329:
2330: @end table
2331:
2332: @c ---------------------------------------------------------------------
2333: @node file-ambcond, , file-idef, The optional File-Access word set
2334: @subsection Ambiguous conditions
2335: @c ---------------------------------------------------------------------
2336:
2337: @table @i
2338:
2339: @item attempting to position a file outside it's boundaries:
2340: @code{REPOSITION-FILE} is performed as usual: Afterwards,
2341: @code{FILE-POSITION} returns the value given to @code{REPOSITION-FILE}.
2342:
2343: @item attempting to read from file positions not yet written:
2344: End-of-file, i.e., zero characters are read and no error is reported.
2345:
2346: @item @var{file-id} is invalid (@code{INCLUDE-FILE}):
2347: An appropriate exception may be thrown, but a memory fault or other
2348: problem is more probable.
2349:
2350: @item I/O exception reading or closing @var{file-id} (@code{include-file}, @code{included}):
2351: The @var{ior} produced by the operation, that discovered the problem, is
2352: thrown.
2353:
2354: @item named file cannot be opened (@code{included}):
2355: The @var{ior} produced by @code{open-file} is thrown.
2356:
2357: @item requesting an unmapped block number:
2358: There are no unmapped legal block numbers. On some operating systems,
2359: writing a block with a large number may overflow the file system and
2360: have an error message as consequence.
2361:
2362: @item using @code{source-id} when @code{blk} is non-zero:
2363: @code{source-id} performs its function. Typically it will give the id of
2364: the source which loaded the block. (Better ideas?)
2365:
2366: @end table
2367:
2368:
2369: @c =====================================================================
2370: @node The optional Floating-Point word set, The optional Locals word set, The optional File-Access word set, ANS conformance
2371: @section The optional Floating-Point word set
2372: @c =====================================================================
2373:
2374: @menu
2375: * floating-idef:: Implementation Defined Options
2376: * floating-ambcond:: Ambiguous Conditions
2377: @end menu
2378:
2379:
2380: @c ---------------------------------------------------------------------
2381: @node floating-idef, floating-ambcond, The optional Floating-Point word set, The optional Floating-Point word set
2382: @subsection Implementation Defined Options
2383: @c ---------------------------------------------------------------------
2384:
2385: @table @i
2386:
2387: @item format and range of floating point numbers:
2388: System-dependent; the @code{double} type of C.
2389:
2390: @item results of @code{REPRESENT} when @var{float} is out of range:
2391: System dependent; @code{REPRESENT} is implemented using the C library
2392: function @code{ecvt()} and inherits its behaviour in this respect.
2393:
2394: @item rounding or truncation of floating-point numbers:
2395: What's the question?!!
2396:
2397: @item size of floating-point stack:
2398: @code{s" FLOATING-STACK" environment? drop .}. Can be changed at startup
2399: with the command-line option @code{-f}.
2400:
2401: @item width of floating-point stack:
2402: @code{1 floats}.
2403:
2404: @end table
2405:
2406:
2407: @c ---------------------------------------------------------------------
2408: @node floating-ambcond, , floating-idef, The optional Floating-Point word set
2409: @subsection Ambiguous conditions
2410: @c ---------------------------------------------------------------------
2411:
2412: @table @i
2413:
2414: @item @code{df@@} or @code{df!} used with an address that is not double-float aligned:
2415: System-dependent. Typically results in an alignment fault like other
2416: alignment violations.
2417:
2418: @item @code{f@@} or @code{f!} used with an address that is not float aligned:
2419: System-dependent. Typically results in an alignment fault like other
2420: alignment violations.
2421:
2422: @item Floating-point result out of range:
2423: System-dependent. Can result in a @code{-55 THROW} (Floating-point
2424: unidentified fault), or can produce a special value representing, e.g.,
2425: Infinity.
2426:
2427: @item @code{sf@@} or @code{sf!} used with an address that is not single-float aligned:
2428: System-dependent. Typically results in an alignment fault like other
2429: alignment violations.
2430:
2431: @item BASE is not decimal (@code{REPRESENT}, @code{F.}, @code{FE.}, @code{FS.}):
2432: The floating-point number is converted into decimal nonetheless.
2433:
2434: @item Both arguments are equal to zero (@code{FATAN2}):
2435: System-dependent. @code{FATAN2} is implemented using the C library
2436: function @code{atan2()}.
2437:
2438: @item Using ftan on an argument @var{r1} where cos(@var{r1}) is zero:
2439: System-dependent. Anyway, typically the cos of @var{r1} will not be zero
2440: because of small errors and the tan will be a very large (or very small)
2441: but finite number.
2442:
2443: @item @var{d} cannot be presented precisely as a float in @code{D>F}:
2444: The result is rounded to the nearest float.
2445:
2446: @item dividing by zero:
2447: @code{-55 throw} (Floating-point unidentified fault)
2448:
2449: @item exponent too big for conversion (@code{DF!}, @code{DF@@}, @code{SF!}, @code{SF@@}):
2450: System dependent. On IEEE-FP based systems the number is converted into
2451: an infinity.
2452:
2453: @item @var{float}<1 (@code{facosh}):
2454: @code{-55 throw} (Floating-point unidentified fault)
2455:
2456: @item @var{float}=<-1 (@code{flnp1}):
2457: @code{-55 throw} (Floating-point unidentified fault). On IEEE-FP systems
2458: negative infinity is typically produced for @var{float}=-1.
2459:
2460: @item @var{float}=<0 (@code{fln}, @code{flog}):
2461: @code{-55 throw} (Floating-point unidentified fault). On IEEE-FP systems
2462: negative infinity is typically produced for @var{float}=0.
2463:
2464: @item @var{float}<0 (@code{fasinh}, @code{fsqrt}):
2465: @code{-55 throw} (Floating-point unidentified fault). @code{fasinh}
2466: produces values for these inputs on my Linux box (Bug in the C library?)
2467:
2468: @item |@var{float}|>1 (@code{facos}, @code{fasin}, @code{fatanh}):
2469: @code{-55 throw} (Floating-point unidentified fault).
2470:
2471: @item integer part of float cannot be represented by @var{d} in @code{f>d}:
2472: @code{-55 throw} (Floating-point unidentified fault).
2473:
2474: @item string larger than pictured numeric output area (@code{f.}, @code{fe.}, @code{fs.}):
2475: This does not happen.
2476: @end table
2477:
2478:
2479:
2480: @c =====================================================================
2481: @node The optional Locals word set, The optional Memory-Allocation word set, The optional Floating-Point word set, ANS conformance
2482: @section The optional Locals word set
2483: @c =====================================================================
2484:
2485: @menu
2486: * locals-idef:: Implementation Defined Options
2487: * locals-ambcond:: Ambiguous Conditions
2488: @end menu
2489:
2490:
2491: @c ---------------------------------------------------------------------
2492: @node locals-idef, locals-ambcond, The optional Locals word set, The optional Locals word set
2493: @subsection Implementation Defined Options
2494: @c ---------------------------------------------------------------------
2495:
2496: @table @i
2497:
2498: @item maximum number of locals in a definition:
2499: @code{s" #locals" environment? drop .}. Currently 15. This is a lower
2500: bound, e.g., on a 32-bit machine there can be 41 locals of up to 8
2501: characters. The number of locals in a definition is bounded by the size
2502: of locals-buffer, which contains the names of the locals.
2503:
2504: @end table
2505:
2506:
2507: @c ---------------------------------------------------------------------
2508: @node locals-ambcond, , locals-idef, The optional Locals word set
2509: @subsection Ambiguous conditions
2510: @c ---------------------------------------------------------------------
2511:
2512: @table @i
2513:
2514: @item executing a named local in interpretation state:
2515: @code{-14 throw} (Interpreting a compile-only word).
2516:
2517: @item @var{name} not defined by @code{VALUE} or @code{(LOCAL)} (@code{TO}):
2518: @code{-32 throw} (Invalid name argument)
2519:
2520: @end table
2521:
2522:
2523: @c =====================================================================
2524: @node The optional Memory-Allocation word set, The optional Programming-Tools word set, The optional Locals word set, ANS conformance
2525: @section The optional Memory-Allocation word set
2526: @c =====================================================================
2527:
2528: @menu
2529: * memory-idef:: Implementation Defined Options
2530: @end menu
2531:
2532:
2533: @c ---------------------------------------------------------------------
2534: @node memory-idef, , The optional Memory-Allocation word set, The optional Memory-Allocation word set
2535: @subsection Implementation Defined Options
2536: @c ---------------------------------------------------------------------
2537:
2538: @table @i
2539:
2540: @item values and meaning of @var{ior}:
2541: The @var{ior}s returned by the file and memory allocation words are
2542: intended as throw codes. They typically are in the range
2543: -512@minus{}-2047 of OS errors. The mapping from OS error numbers to
2544: @var{ior}s is -512@minus{}@var{errno}.
2545:
2546: @end table
2547:
2548: @c =====================================================================
2549: @node The optional Programming-Tools word set, The optional Search-Order word set, The optional Memory-Allocation word set, ANS conformance
2550: @section The optional Programming-Tools word set
2551: @c =====================================================================
2552:
2553: @menu
2554: * programming-idef:: Implementation Defined Options
2555: * programming-ambcond:: Ambiguous Conditions
2556: @end menu
2557:
2558:
2559: @c ---------------------------------------------------------------------
2560: @node programming-idef, programming-ambcond, The optional Programming-Tools word set, The optional Programming-Tools word set
2561: @subsection Implementation Defined Options
2562: @c ---------------------------------------------------------------------
2563:
2564: @table @i
2565:
2566: @item ending sequence for input following @code{;code} and @code{code}:
2567: Not implemented (yet).
2568:
2569: @item manner of processing input following @code{;code} and @code{code}:
2570: Not implemented (yet).
2571:
2572: @item search order capability for @code{EDITOR} and @code{ASSEMBLER}:
2573: Not implemented (yet). If they were implemented, they would use the
2574: search order wordset.
2575:
2576: @item source and format of display by @code{SEE}:
2577: The source for @code{see} is the intermediate code used by the inner
2578: interpreter. The current @code{see} tries to output Forth source code
2579: as well as possible.
2580:
2581: @end table
2582:
2583: @c ---------------------------------------------------------------------
2584: @node programming-ambcond, , programming-idef, The optional Programming-Tools word set
2585: @subsection Ambiguous conditions
2586: @c ---------------------------------------------------------------------
2587:
2588: @table @i
2589:
2590: @item deleting the compilation wordlist (@code{FORGET}):
2591: Not implemented (yet).
2592:
2593: @item fewer than @var{u}+1 items on the control flow stack (@code{CS-PICK}, @code{CS-ROLL}):
2594: This typically results in an @code{abort"} with a descriptive error
2595: message (may change into a @code{-22 throw} (Control structure mismatch)
2596: in the future). You may also get a memory access error. If you are
2597: unlucky, this ambiguous condition is not caught.
2598:
2599: @item @var{name} can't be found (@code{forget}):
2600: Not implemented (yet).
2601:
2602: @item @var{name} not defined via @code{CREATE}:
2603: @code{;code} is not implemented (yet). If it were, it would behave like
2604: @code{DOES>} in this respect, i.e., change the execution semantics of
2605: the last defined word no matter how it was defined.
2606:
2607: @item @code{POSTPONE} applied to @code{[IF]}:
2608: After defining @code{: X POSTPONE [IF] ; IMMEDIATE}. @code{X} is
2609: equivalent to @code{[IF]}.
2610:
2611: @item reaching the end of the input source before matching @code{[ELSE]} or @code{[THEN]}:
2612: Continue in the same state of conditional compilation in the next outer
2613: input source. Currently there is no warning to the user about this.
2614:
2615: @item removing a needed definition (@code{FORGET}):
2616: Not implemented (yet).
2617:
2618: @end table
2619:
2620:
2621: @c =====================================================================
2622: @node The optional Search-Order word set, , The optional Programming-Tools word set, ANS conformance
2623: @section The optional Search-Order word set
2624: @c =====================================================================
2625:
2626: @menu
2627: * search-idef:: Implementation Defined Options
2628: * search-ambcond:: Ambiguous Conditions
2629: @end menu
2630:
2631:
2632: @c ---------------------------------------------------------------------
2633: @node search-idef, search-ambcond, The optional Search-Order word set, The optional Search-Order word set
2634: @subsection Implementation Defined Options
2635: @c ---------------------------------------------------------------------
2636:
2637: @table @i
2638:
2639: @item maximum number of word lists in search order:
2640: @code{s" wordlists" environment? drop .}. Currently 16.
2641:
2642: @item minimum search order:
2643: @code{root root}.
2644:
2645: @end table
2646:
2647: @c ---------------------------------------------------------------------
2648: @node search-ambcond, , search-idef, The optional Search-Order word set
2649: @subsection Ambiguous conditions
2650: @c ---------------------------------------------------------------------
2651:
2652: @table @i
2653:
2654: @item changing the compilation wordlist (during compilation):
2655: The definition is put into the wordlist that is the compilation wordlist
2656: when @code{REVEAL} is executed (by @code{;}, @code{DOES>},
2657: @code{RECURSIVE}, etc.).
2658:
2659: @item search order empty (@code{previous}):
2660: @code{abort" Vocstack empty"}.
2661:
2662: @item too many word lists in search order (@code{also}):
2663: @code{abort" Vocstack full"}.
2664:
2665: @end table
2666:
2667:
2668: @node Model, Emacs and GForth, ANS conformance, Top
2669: @chapter Model
2670:
2671: @node Emacs and GForth, Internals, Model, Top
2672: @chapter Emacs and GForth
2673:
2674: GForth comes with @file{gforth.el}, an improved version of
2675: @file{forth.el} by Goran Rydqvist (icluded in the TILE package). The
2676: improvements are a better (but still not perfect) handling of
2677: indentation. I have also added comment paragraph filling (@kbd{M-q}),
2678: commenting (@kbd{C-x \}) and uncommenting (@kbd{C-u C-x \}) regions and
2679: removing debugging tracers (@kbd{C-x ~}, @pxref{Debugging}). I left the
2680: stuff I do not use alone, even though some of it only makes sense for
2681: TILE. To get a description of these features, enter Forth mode and type
2682: @kbd{C-h m}.
2683:
2684: In addition, GForth supports Emacs quite well: The source code locations
2685: given in error messages, debugging output (from @code{~~}) and failed
2686: assertion messages are in the right format for Emacs' compilation mode
2687: (@pxref{Compilation, , Running Compilations under Emacs, emacs, Emacs
2688: Manual}) so the source location corresponding to an error or other
2689: message is only a few keystrokes away (@kbd{C-x `} for the next error,
2690: @kbd{C-c C-c} for the error under the cursor).
2691:
2692: Also, if you @code{include} @file{etags.fs}, a new @file{TAGS} file
2693: (@pxref{Tags, , Tags Tables, emacs, Emacs Manual}) will be produced that
2694: contains the definitions of all words defined afterwards. You can then
2695: find the source for a word using @kbd{M-.}. Note that emacs can use
2696: several tags files at the same time (e.g., one for the gforth sources
2697: and one for your program).
2698:
2699: To get all these benefits, add the following lines to your @file{.emacs}
2700: file:
2701:
2702: @example
2703: (autoload 'forth-mode "gforth.el")
2704: (setq auto-mode-alist (cons '("\\.fs\\'" . forth-mode) auto-mode-alist))
2705: @end example
2706:
2707: @node Internals, Bugs, Emacs and GForth, Top
2708: @chapter Internals
2709:
2710: Reading this section is not necessary for programming with gforth. It
2711: should be helpful for finding your way in the gforth sources.
2712:
2713: @menu
2714: * Portability::
2715: * Threading::
2716: * Primitives::
2717: * System Architecture::
2718: @end menu
2719:
2720: @node Portability, Threading, Internals, Internals
2721: @section Portability
2722:
2723: One of the main goals of the effort is availability across a wide range
2724: of personal machines. fig-Forth, and, to a lesser extent, F83, achieved
2725: this goal by manually coding the engine in assembly language for several
2726: then-popular processors. This approach is very labor-intensive and the
2727: results are short-lived due to progress in computer architecture.
2728:
2729: Others have avoided this problem by coding in C, e.g., Mitch Bradley
2730: (cforth), Mikael Patel (TILE) and Dirk Zoller (pfe). This approach is
2731: particularly popular for UNIX-based Forths due to the large variety of
2732: architectures of UNIX machines. Unfortunately an implementation in C
2733: does not mix well with the goals of efficiency and with using
2734: traditional techniques: Indirect or direct threading cannot be expressed
2735: in C, and switch threading, the fastest technique available in C, is
2736: significantly slower. Another problem with C is that it's very
2737: cumbersome to express double integer arithmetic.
2738:
2739: Fortunately, there is a portable language that does not have these
2740: limitations: GNU C, the version of C processed by the GNU C compiler
2741: (@pxref{C Extensions, , Extensions to the C Language Family, gcc.info,
2742: GNU C Manual}). Its labels as values feature (@pxref{Labels as Values, ,
2743: Labels as Values, gcc.info, GNU C Manual}) makes direct and indirect
2744: threading possible, its @code{long long} type (@pxref{Long Long, ,
2745: Double-Word Integers, gcc.info, GNU C Manual}) corresponds to Forths
2746: double numbers. GNU C is available for free on all important (and many
2747: unimportant) UNIX machines, VMS, 80386s running MS-DOS, the Amiga, and
2748: the Atari ST, so a Forth written in GNU C can run on all these
2749: machines@footnote{Due to Apple's look-and-feel lawsuit it is not
2750: available on the Mac (@pxref{Boycott, , Protect Your Freedom---Fight
2751: ``Look And Feel'', gcc.info, GNU C Manual}).}.
2752:
2753: Writing in a portable language has the reputation of producing code that
2754: is slower than assembly. For our Forth engine we repeatedly looked at
2755: the code produced by the compiler and eliminated most compiler-induced
2756: inefficiencies by appropriate changes in the source-code.
2757:
2758: However, register allocation cannot be portably influenced by the
2759: programmer, leading to some inefficiencies on register-starved
2760: machines. We use explicit register declarations (@pxref{Explicit Reg
2761: Vars, , Variables in Specified Registers, gcc.info, GNU C Manual}) to
2762: improve the speed on some machines. They are turned on by using the
2763: @code{gcc} switch @code{-DFORCE_REG}. Unfortunately, this feature not
2764: only depends on the machine, but also on the compiler version: On some
2765: machines some compiler versions produce incorrect code when certain
2766: explicit register declarations are used. So by default
2767: @code{-DFORCE_REG} is not used.
2768:
2769: @node Threading, Primitives, Portability, Internals
2770: @section Threading
2771:
2772: GNU C's labels as values extension (available since @code{gcc-2.0},
2773: @pxref{Labels as Values, , Labels as Values, gcc.info, GNU C Manual})
2774: makes it possible to take the address of @var{label} by writing
2775: @code{&&@var{label}}. This address can then be used in a statement like
2776: @code{goto *@var{address}}. I.e., @code{goto *&&x} is the same as
2777: @code{goto x}.
2778:
2779: With this feature an indirect threaded NEXT looks like:
2780: @example
2781: cfa = *ip++;
2782: ca = *cfa;
2783: goto *ca;
2784: @end example
2785: For those unfamiliar with the names: @code{ip} is the Forth instruction
2786: pointer; the @code{cfa} (code-field address) corresponds to ANS Forths
2787: execution token and points to the code field of the next word to be
2788: executed; The @code{ca} (code address) fetched from there points to some
2789: executable code, e.g., a primitive or the colon definition handler
2790: @code{docol}.
2791:
2792: Direct threading is even simpler:
2793: @example
2794: ca = *ip++;
2795: goto *ca;
2796: @end example
2797:
2798: Of course we have packaged the whole thing neatly in macros called
2799: @code{NEXT} and @code{NEXT1} (the part of NEXT after fetching the cfa).
2800:
2801: @menu
2802: * Scheduling::
2803: * Direct or Indirect Threaded?::
2804: * DOES>::
2805: @end menu
2806:
2807: @node Scheduling, Direct or Indirect Threaded?, Threading, Threading
2808: @subsection Scheduling
2809:
2810: There is a little complication: Pipelined and superscalar processors,
2811: i.e., RISC and some modern CISC machines can process independent
2812: instructions while waiting for the results of an instruction. The
2813: compiler usually reorders (schedules) the instructions in a way that
2814: achieves good usage of these delay slots. However, on our first tries
2815: the compiler did not do well on scheduling primitives. E.g., for
2816: @code{+} implemented as
2817: @example
2818: n=sp[0]+sp[1];
2819: sp++;
2820: sp[0]=n;
2821: NEXT;
2822: @end example
2823: the NEXT comes strictly after the other code, i.e., there is nearly no
2824: scheduling. After a little thought the problem becomes clear: The
2825: compiler cannot know that sp and ip point to different addresses (and
2826: the version of @code{gcc} we used would not know it even if it was
2827: possible), so it could not move the load of the cfa above the store to
2828: the TOS. Indeed the pointers could be the same, if code on or very near
2829: the top of stack were executed. In the interest of speed we chose to
2830: forbid this probably unused ``feature'' and helped the compiler in
2831: scheduling: NEXT is divided into the loading part (@code{NEXT_P1}) and
2832: the goto part (@code{NEXT_P2}). @code{+} now looks like:
2833: @example
2834: n=sp[0]+sp[1];
2835: sp++;
2836: NEXT_P1;
2837: sp[0]=n;
2838: NEXT_P2;
2839: @end example
2840: This can be scheduled optimally by the compiler.
2841:
2842: This division can be turned off with the switch @code{-DCISC_NEXT}. This
2843: switch is on by default on machines that do not profit from scheduling
2844: (e.g., the 80386), in order to preserve registers.
2845:
2846: @node Direct or Indirect Threaded?, DOES>, Scheduling, Threading
2847: @subsection Direct or Indirect Threaded?
2848:
2849: Both! After packaging the nasty details in macro definitions we
2850: realized that we could switch between direct and indirect threading by
2851: simply setting a compilation flag (@code{-DDIRECT_THREADED}) and
2852: defining a few machine-specific macros for the direct-threading case.
2853: On the Forth level we also offer access words that hide the
2854: differences between the threading methods (@pxref{Threading Words}).
2855:
2856: Indirect threading is implemented completely
2857: machine-independently. Direct threading needs routines for creating
2858: jumps to the executable code (e.g. to docol or dodoes). These routines
2859: are inherently machine-dependent, but they do not amount to many source
2860: lines. I.e., even porting direct threading to a new machine is a small
2861: effort.
2862:
2863: @node DOES>, , Direct or Indirect Threaded?, Threading
2864: @subsection DOES>
2865: One of the most complex parts of a Forth engine is @code{dodoes}, i.e.,
2866: the chunk of code executed by every word defined by a
2867: @code{CREATE}...@code{DOES>} pair. The main problem here is: How to find
2868: the Forth code to be executed, i.e. the code after the @code{DOES>} (the
2869: DOES-code)? There are two solutions:
2870:
2871: In fig-Forth the code field points directly to the dodoes and the
2872: DOES-code address is stored in the cell after the code address
2873: (i.e. at cfa cell+). It may seem that this solution is illegal in the
2874: Forth-79 and all later standards, because in fig-Forth this address
2875: lies in the body (which is illegal in these standards). However, by
2876: making the code field larger for all words this solution becomes legal
2877: again. We use this approach for the indirect threaded version. Leaving
2878: a cell unused in most words is a bit wasteful, but on the machines we
2879: are targetting this is hardly a problem. The other reason for having a
2880: code field size of two cells is to avoid having different image files
2881: for direct and indirect threaded systems (@pxref{System Architecture}).
2882:
2883: The other approach is that the code field points or jumps to the cell
2884: after @code{DOES}. In this variant there is a jump to @code{dodoes} at
2885: this address. @code{dodoes} can then get the DOES-code address by
2886: computing the code address, i.e., the address of the jump to dodoes,
2887: and add the length of that jump field. A variant of this is to have a
2888: call to @code{dodoes} after the @code{DOES>}; then the return address
2889: (which can be found in the return register on RISCs) is the DOES-code
2890: address. Since the two cells available in the code field are usually
2891: used up by the jump to the code address in direct threading, we use
2892: this approach for direct threading. We did not want to add another
2893: cell to the code field.
2894:
2895: @node Primitives, System Architecture, Threading, Internals
2896: @section Primitives
2897:
2898: @menu
2899: * Automatic Generation::
2900: * TOS Optimization::
2901: * Produced code::
2902: @end menu
2903:
2904: @node Automatic Generation, TOS Optimization, Primitives, Primitives
2905: @subsection Automatic Generation
2906:
2907: Since the primitives are implemented in a portable language, there is no
2908: longer any need to minimize the number of primitives. On the contrary,
2909: having many primitives is an advantage: speed. In order to reduce the
2910: number of errors in primitives and to make programming them easier, we
2911: provide a tool, the primitive generator (@file{prims2x.fs}), that
2912: automatically generates most (and sometimes all) of the C code for a
2913: primitive from the stack effect notation. The source for a primitive
2914: has the following form:
2915:
2916: @format
2917: @var{Forth-name} @var{stack-effect} @var{category} [@var{pronounc.}]
2918: [@code{""}@var{glossary entry}@code{""}]
2919: @var{C code}
2920: [@code{:}
2921: @var{Forth code}]
2922: @end format
2923:
2924: The items in brackets are optional. The category and glossary fields
2925: are there for generating the documentation, the Forth code is there
2926: for manual implementations on machines without GNU C. E.g., the source
2927: for the primitive @code{+} is:
2928: @example
2929: + n1 n2 -- n core plus
2930: n = n1+n2;
2931: @end example
2932:
2933: This looks like a specification, but in fact @code{n = n1+n2} is C
2934: code. Our primitive generation tool extracts a lot of information from
2935: the stack effect notations@footnote{We use a one-stack notation, even
2936: though we have separate data and floating-point stacks; The separate
2937: notation can be generated easily from the unified notation.}: The number
2938: of items popped from and pushed on the stack, their type, and by what
2939: name they are referred to in the C code. It then generates a C code
2940: prelude and postlude for each primitive. The final C code for @code{+}
2941: looks like this:
2942:
2943: @example
2944: I_plus: /* + ( n1 n2 -- n ) */ /* label, stack effect */
2945: /* */ /* documentation */
2946: @{
2947: DEF_CA /* definition of variable ca (indirect threading) */
2948: Cell n1; /* definitions of variables */
2949: Cell n2;
2950: Cell n;
2951: n1 = (Cell) sp[1]; /* input */
2952: n2 = (Cell) TOS;
2953: sp += 1; /* stack adjustment */
2954: NAME("+") /* debugging output (with -DDEBUG) */
2955: @{
2956: n = n1+n2; /* C code taken from the source */
2957: @}
2958: NEXT_P1; /* NEXT part 1 */
2959: TOS = (Cell)n; /* output */
2960: NEXT_P2; /* NEXT part 2 */
2961: @}
2962: @end example
2963:
2964: This looks long and inefficient, but the GNU C compiler optimizes quite
2965: well and produces optimal code for @code{+} on, e.g., the R3000 and the
2966: HP RISC machines: Defining the @code{n}s does not produce any code, and
2967: using them as intermediate storage also adds no cost.
2968:
2969: There are also other optimizations, that are not illustrated by this
2970: example: Assignments between simple variables are usually for free (copy
2971: propagation). If one of the stack items is not used by the primitive
2972: (e.g. in @code{drop}), the compiler eliminates the load from the stack
2973: (dead code elimination). On the other hand, there are some things that
2974: the compiler does not do, therefore they are performed by
2975: @file{prims2x.fs}: The compiler does not optimize code away that stores
2976: a stack item to the place where it just came from (e.g., @code{over}).
2977:
2978: While programming a primitive is usually easy, there are a few cases
2979: where the programmer has to take the actions of the generator into
2980: account, most notably @code{?dup}, but also words that do not (always)
2981: fall through to NEXT.
2982:
2983: @node TOS Optimization, Produced code, Automatic Generation, Primitives
2984: @subsection TOS Optimization
2985:
2986: An important optimization for stack machine emulators, e.g., Forth
2987: engines, is keeping one or more of the top stack items in
2988: registers. If a word has the stack effect @var{in1}...@var{inx} @code{--}
2989: @var{out1}...@var{outy}, keeping the top @var{n} items in registers
2990: @itemize
2991: @item
2992: is better than keeping @var{n-1} items, if @var{x>=n} and @var{y>=n},
2993: due to fewer loads from and stores to the stack.
2994: @item is slower than keeping @var{n-1} items, if @var{x<>y} and @var{x<n} and
2995: @var{y<n}, due to additional moves between registers.
2996: @end itemize
2997:
2998: In particular, keeping one item in a register is never a disadvantage,
2999: if there are enough registers. Keeping two items in registers is a
3000: disadvantage for frequent words like @code{?branch}, constants,
3001: variables, literals and @code{i}. Therefore our generator only produces
3002: code that keeps zero or one items in registers. The generated C code
3003: covers both cases; the selection between these alternatives is made at
3004: C-compile time using the switch @code{-DUSE_TOS}. @code{TOS} in the C
3005: code for @code{+} is just a simple variable name in the one-item case,
3006: otherwise it is a macro that expands into @code{sp[0]}. Note that the
3007: GNU C compiler tries to keep simple variables like @code{TOS} in
3008: registers, and it usually succeeds, if there are enough registers.
3009:
3010: The primitive generator performs the TOS optimization for the
3011: floating-point stack, too (@code{-DUSE_FTOS}). For floating-point
3012: operations the benefit of this optimization is even larger:
3013: floating-point operations take quite long on most processors, but can be
3014: performed in parallel with other operations as long as their results are
3015: not used. If the FP-TOS is kept in a register, this works. If
3016: it is kept on the stack, i.e., in memory, the store into memory has to
3017: wait for the result of the floating-point operation, lengthening the
3018: execution time of the primitive considerably.
3019:
3020: The TOS optimization makes the automatic generation of primitives a
3021: bit more complicated. Just replacing all occurrences of @code{sp[0]} by
3022: @code{TOS} is not sufficient. There are some special cases to
3023: consider:
3024: @itemize
3025: @item In the case of @code{dup ( w -- w w )} the generator must not
3026: eliminate the store to the original location of the item on the stack,
3027: if the TOS optimization is turned on.
3028: @item Primitives with stack effects of the form @code{--}
3029: @var{out1}...@var{outy} must store the TOS to the stack at the start.
3030: Likewise, primitives with the stack effect @var{in1}...@var{inx} @code{--}
3031: must load the TOS from the stack at the end. But for the null stack
3032: effect @code{--} no stores or loads should be generated.
3033: @end itemize
3034:
3035: @node Produced code, , TOS Optimization, Primitives
3036: @subsection Produced code
3037:
3038: To see what assembly code is produced for the primitives on your machine
3039: with your compiler and your flag settings, type @code{make engine.s} and
3040: look at the resulting file @file{engine.s}.
3041:
3042: @node System Architecture, , Primitives, Internals
3043: @section System Architecture
3044:
3045: Our Forth system consists not only of primitives, but also of
3046: definitions written in Forth. Since the Forth compiler itself belongs
3047: to those definitions, it is not possible to start the system with the
3048: primitives and the Forth source alone. Therefore we provide the Forth
3049: code as an image file in nearly executable form. At the start of the
3050: system a C routine loads the image file into memory, sets up the
3051: memory (stacks etc.) according to information in the image file, and
3052: starts executing Forth code.
3053:
3054: The image file format is a compromise between the goals of making it
3055: easy to generate image files and making them portable. The easiest way
3056: to generate an image file is to just generate a memory dump. However,
3057: this kind of image file cannot be used on a different machine, or on
3058: the next version of the engine on the same machine, it even might not
3059: work with the same engine compiled by a different version of the C
3060: compiler. We would like to have as few versions of the image file as
3061: possible, because we do not want to distribute many versions of the
3062: same image file, and to make it easy for the users to use their image
3063: files on many machines. We currently need to create a different image
3064: file for machines with different cell sizes and different byte order
3065: (little- or big-endian)@footnote{We consider adding information to the
3066: image file that enables the loader to change the byte order.}.
3067:
3068: Forth code that is going to end up in a portable image file has to
3069: comply to some restrictions: addresses have to be stored in memory with
3070: special words (@code{A!}, @code{A,}, etc.) in order to make the code
3071: relocatable. Cells, floats, etc., have to be stored at the natural
3072: alignment boundaries@footnote{E.g., store floats (8 bytes) at an address
3073: dividable by~8. This happens automatically in our system when you use
3074: the ANS Forth alignment words.}, in order to avoid alignment faults on
3075: machines with stricter alignment. The image file is produced by a
3076: metacompiler (@file{cross.fs}).
3077:
3078: So, unlike the image file of Mitch Bradleys @code{cforth}, our image
3079: file is not directly executable, but has to undergo some manipulations
3080: during loading. Address relocation is performed at image load-time, not
3081: at run-time. The loader also has to replace tokens standing for
3082: primitive calls with the appropriate code-field addresses (or code
3083: addresses in the case of direct threading).
3084:
3085: @node Bugs, Pedigree, Internals, Top
3086: @chapter Bugs
3087:
3088: @node Pedigree, Word Index, Bugs, Top
3089: @chapter Pedigree
3090:
3091: @node Word Index, Node Index, Pedigree, Top
3092: @chapter Word Index
3093:
3094: @node Node Index, , Word Index, Top
3095: @chapter Node Index
3096:
3097: @contents
3098: @bye
3099:
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