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.
290:
291: @item Stack effect
292: The stack effect is written in the notation @code{@var{before} --
293: @var{after}}, where @var{before} and @var{after} describe the top of
294: stack entries before and after the execution of the word. The rest of
295: the stack is not touched by the word. The top of stack is rightmost,
296: i.e., a stack sequence is written as it is typed in. Note that GNU Forth
297: uses a separate floating point stack, but a unified stack
298: notation. Also, return stack effects are not shown in @var{stack
299: effect}, but in @var{Description}. The name of a stack item describes
300: the type and/or the function of the item. See below for a discussion of
301: the types.
302:
303: @item pronunciation
304: How the word is pronounced
305:
306: @item wordset
307: The ANS Forth standard is divided into several wordsets. A standard
308: system need not support all of them. So, the fewer wordsets your program
309: uses the more portable it will be in theory. However, we suspect that
310: most ANS Forth systems on personal machines will feature all
311: wordsets. Words that are not defined in the ANS standard have
312: @code{gforth} as wordset.
313:
314: @item Description
315: A description of the behaviour of the word.
316: @end table
317:
318: The type of a stack item is specified by the character(s) the name
319: starts with:
320:
321: @table @code
322: @item f
323: Bool, i.e. @code{false} or @code{true}.
324: @item c
325: Char
326: @item w
327: Cell, can contain an integer or an address
328: @item n
329: signed integer
330: @item u
331: unsigned integer
332: @item d
333: double sized signed integer
334: @item ud
335: double sized unsigned integer
336: @item r
337: Float
338: @item a_
339: Cell-aligned address
340: @item c_
341: Char-aligned address (note that a Char is two bytes in Windows NT)
342: @item f_
343: Float-aligned address
344: @item df_
345: Address aligned for IEEE double precision float
346: @item sf_
347: Address aligned for IEEE single precision float
348: @item xt
349: Execution token, same size as Cell
350: @item wid
351: Wordlist ID, same size as Cell
352: @item f83name
353: Pointer to a name structure
354: @end table
355:
356: @node Arithmetic, Stack Manipulation, Notation, Words
357: @section Arithmetic
358: Forth arithmetic is not checked, i.e., you will not hear about integer
359: overflow on addition or multiplication, you may hear about division by
360: zero if you are lucky. The operator is written after the operands, but
361: the operands are still in the original order. I.e., the infix @code{2-1}
362: corresponds to @code{2 1 -}. Forth offers a variety of division
363: operators. If you perform division with potentially negative operands,
364: you do not want to use @code{/} or @code{/mod} with its undefined
365: behaviour, but rather @code{fm/mod} or @code{sm/mod} (probably the
366: former, @pxref{Mixed precision}).
367:
368: @menu
369: * Single precision::
370: * Bitwise operations::
371: * Mixed precision:: operations with single and double-cell integers
372: * Double precision:: Double-cell integer arithmetic
373: * Floating Point::
374: @end menu
375:
376: @node Single precision, Bitwise operations, Arithmetic, Arithmetic
377: @subsection Single precision
378: doc-+
379: doc--
380: doc-*
381: doc-/
382: doc-mod
383: doc-/mod
384: doc-negate
385: doc-abs
386: doc-min
387: doc-max
388:
389: @node Bitwise operations, Mixed precision, Single precision, Arithmetic
390: @subsection Bitwise operations
391: doc-and
392: doc-or
393: doc-xor
394: doc-invert
395: doc-2*
396: doc-2/
397:
398: @node Mixed precision, Double precision, Bitwise operations, Arithmetic
399: @subsection Mixed precision
400: doc-m+
401: doc-*/
402: doc-*/mod
403: doc-m*
404: doc-um*
405: doc-m*/
406: doc-um/mod
407: doc-fm/mod
408: doc-sm/rem
409:
410: @node Double precision, Floating Point, Mixed precision, Arithmetic
411: @subsection Double precision
412: doc-d+
413: doc-d-
414: doc-dnegate
415: doc-dabs
416: doc-dmin
417: doc-dmax
418:
419: @node Floating Point, , Double precision, Arithmetic
420: @subsection Floating Point
421:
422: Angles in floating point operations are given in radians (a full circle
423: has 2 pi radians). Note, that gforth has a separate floating point
424: stack, but we use the unified notation.
425:
426: Floating point numbers have a number of unpleasant surprises for the
427: unwary (e.g., floating point addition is not associative) and even a few
428: for the wary. You should not use them unless you know what you are doing
429: or you don't care that the results you get are totally bogus. If you
430: want to learn about the problems of floating point numbers (and how to
431: avoid them), you might start with @cite{David (?) Goldberg, What Every
432: Computer Scientist Should Know About Floating-Point Arithmetic, ACM
433: Computing Surveys 23(1):5@minus{}48, March 1991}.
434:
435: doc-f+
436: doc-f-
437: doc-f*
438: doc-f/
439: doc-fnegate
440: doc-fabs
441: doc-fmax
442: doc-fmin
443: doc-floor
444: doc-fround
445: doc-f**
446: doc-fsqrt
447: doc-fexp
448: doc-fexpm1
449: doc-fln
450: doc-flnp1
451: doc-flog
452: doc-falog
453: doc-fsin
454: doc-fcos
455: doc-fsincos
456: doc-ftan
457: doc-fasin
458: doc-facos
459: doc-fatan
460: doc-fatan2
461: doc-fsinh
462: doc-fcosh
463: doc-ftanh
464: doc-fasinh
465: doc-facosh
466: doc-fatanh
467:
468: @node Stack Manipulation, Memory access, Arithmetic, Words
469: @section Stack Manipulation
470:
471: gforth has a data stack (aka parameter stack) for characters, cells,
472: addresses, and double cells, a floating point stack for floating point
473: numbers, a return stack for storing the return addresses of colon
474: definitions and other data, and a locals stack for storing local
475: variables. Note that while every sane Forth has a separate floating
476: point stack, this is not strictly required; an ANS Forth system could
477: theoretically keep floating point numbers on the data stack. As an
478: additional difficulty, you don't know how many cells a floating point
479: number takes. It is reportedly possible to write words in a way that
480: they work also for a unified stack model, but we do not recommend trying
481: it. Instead, just say that your program has an environmental dependency
482: on a separate FP stack.
483:
484: Also, a Forth system is allowed to keep the local variables on the
485: return stack. This is reasonable, as local variables usually eliminate
486: the need to use the return stack explicitly. So, if you want to produce
487: a standard complying program and if you are using local variables in a
488: word, forget about return stack manipulations in that word (see the
489: standard document for the exact rules).
490:
491: @menu
492: * Data stack::
493: * Floating point stack::
494: * Return stack::
495: * Locals stack::
496: * Stack pointer manipulation::
497: @end menu
498:
499: @node Data stack, Floating point stack, Stack Manipulation, Stack Manipulation
500: @subsection Data stack
501: doc-drop
502: doc-nip
503: doc-dup
504: doc-over
505: doc-tuck
506: doc-swap
507: doc-rot
508: doc--rot
509: doc-?dup
510: doc-pick
511: doc-roll
512: doc-2drop
513: doc-2nip
514: doc-2dup
515: doc-2over
516: doc-2tuck
517: doc-2swap
518: doc-2rot
519:
520: @node Floating point stack, Return stack, Data stack, Stack Manipulation
521: @subsection Floating point stack
522: doc-fdrop
523: doc-fnip
524: doc-fdup
525: doc-fover
526: doc-ftuck
527: doc-fswap
528: doc-frot
529:
530: @node Return stack, Locals stack, Floating point stack, Stack Manipulation
531: @subsection Return stack
532: doc->r
533: doc-r>
534: doc-r@
535: doc-rdrop
536: doc-2>r
537: doc-2r>
538: doc-2r@
539: doc-2rdrop
540:
541: @node Locals stack, Stack pointer manipulation, Return stack, Stack Manipulation
542: @subsection Locals stack
543:
544: @node Stack pointer manipulation, , Locals stack, Stack Manipulation
545: @subsection Stack pointer manipulation
546: doc-sp@
547: doc-sp!
548: doc-fp@
549: doc-fp!
550: doc-rp@
551: doc-rp!
552: doc-lp@
553: doc-lp!
554:
555: @node Memory access, Control Structures, Stack Manipulation, Words
556: @section Memory access
557:
558: @menu
559: * Stack-Memory transfers::
560: * Address arithmetic::
561: * Memory block access::
562: @end menu
563:
564: @node Stack-Memory transfers, Address arithmetic, Memory access, Memory access
565: @subsection Stack-Memory transfers
566:
567: doc-@
568: doc-!
569: doc-+!
570: doc-c@
571: doc-c!
572: doc-2@
573: doc-2!
574: doc-f@
575: doc-f!
576: doc-sf@
577: doc-sf!
578: doc-df@
579: doc-df!
580:
581: @node Address arithmetic, Memory block access, Stack-Memory transfers, Memory access
582: @subsection Address arithmetic
583:
584: ANS Forth does not specify the sizes of the data types. Instead, it
585: offers a number of words for computing sizes and doing address
586: arithmetic. Basically, address arithmetic is performed in terms of
587: address units (aus); on most systems the address unit is one byte. Note
588: that a character may have more than one au, so @code{chars} is no noop
589: (on systems where it is a noop, it compiles to nothing).
590:
591: ANS Forth also defines words for aligning addresses for specific
592: addresses. Many computers require that accesses to specific data types
593: must only occur at specific addresses; e.g., that cells may only be
594: accessed at addresses divisible by 4. Even if a machine allows unaligned
595: accesses, it can usually perform aligned accesses faster.
596:
597: For the performance-concious: alignment operations are usually only
598: necessary during the definition of a data structure, not during the
599: (more frequent) accesses to it.
600:
601: ANS Forth defines no words for character-aligning addresses. This is not
602: an oversight, but reflects the fact that addresses that are not
603: char-aligned have no use in the standard and therefore will not be
604: created.
605:
606: The standard guarantees that addresses returned by @code{CREATE}d words
607: are cell-aligned; in addition, gforth guarantees that these addresses
608: are aligned for all purposes.
609:
610: Note that the standard defines a word @code{char}, which has nothing to
611: do with address arithmetic.
612:
613: doc-chars
614: doc-char+
615: doc-cells
616: doc-cell+
617: doc-align
618: doc-aligned
619: doc-floats
620: doc-float+
621: doc-falign
622: doc-faligned
623: doc-sfloats
624: doc-sfloat+
625: doc-sfalign
626: doc-sfaligned
627: doc-dfloats
628: doc-dfloat+
629: doc-dfalign
630: doc-dfaligned
631: doc-address-unit-bits
632:
633: @node Memory block access, , Address arithmetic, Memory access
634: @subsection Memory block access
635:
636: doc-move
637: doc-erase
638:
639: While the previous words work on address units, the rest works on
640: characters.
641:
642: doc-cmove
643: doc-cmove>
644: doc-fill
645: doc-blank
646:
647: @node Control Structures, Locals, Memory access, Words
648: @section Control Structures
649:
650: Control structures in Forth cannot be used in interpret state, only in
651: compile state, i.e., in a colon definition. We do not like this
652: limitation, but have not seen a satisfying way around it yet, although
653: many schemes have been proposed.
654:
655: @menu
656: * Selection::
657: * Simple Loops::
658: * Counted Loops::
659: * Arbitrary control structures::
660: * Calls and returns::
661: * Exception Handling::
662: @end menu
663:
664: @node Selection, Simple Loops, Control Structures, Control Structures
665: @subsection Selection
666:
667: @example
668: @var{flag}
669: IF
670: @var{code}
671: ENDIF
672: @end example
673: or
674: @example
675: @var{flag}
676: IF
677: @var{code1}
678: ELSE
679: @var{code2}
680: ENDIF
681: @end example
682:
683: You can use @code{THEN} instead of @code{ENDIF}. Indeed, @code{THEN} is
684: standard, and @code{ENDIF} is not, although it is quite popular. We
685: recommend using @code{ENDIF}, because it is less confusing for people
686: who also know other languages (and is not prone to reinforcing negative
687: prejudices against Forth in these people). Adding @code{ENDIF} to a
688: system that only supplies @code{THEN} is simple:
689: @example
690: : endif POSTPONE then ; immediate
691: @end example
692:
693: [According to @cite{Webster's New Encyclopedic Dictionary}, @dfn{then
694: (adv.)} has the following meanings:
695: @quotation
696: ... 2b: following next after in order ... 3d: as a necessary consequence
697: (if you were there, then you saw them).
698: @end quotation
699: Forth's @code{THEN} has the meaning 2b, whereas @code{THEN} in Pascal
700: and many other programming languages has the meaning 3d.]
701:
702: We also provide the words @code{?dup-if} and @code{?dup-0=-if}, so you
703: can avoid using @code{?dup}.
704:
705: @example
706: @var{n}
707: CASE
708: @var{n1} OF @var{code1} ENDOF
709: @var{n2} OF @var{code2} ENDOF
710: @dots{}
711: ENDCASE
712: @end example
713:
714: Executes the first @var{codei}, where the @var{ni} is equal to
715: @var{n}. A default case can be added by simply writing the code after
716: the last @code{ENDOF}. It may use @var{n}, which is on top of the stack,
717: but must not consume it.
718:
719: @node Simple Loops, Counted Loops, Selection, Control Structures
720: @subsection Simple Loops
721:
722: @example
723: BEGIN
724: @var{code1}
725: @var{flag}
726: WHILE
727: @var{code2}
728: REPEAT
729: @end example
730:
731: @var{code1} is executed and @var{flag} is computed. If it is true,
732: @var{code2} is executed and the loop is restarted; If @var{flag} is false, execution continues after the @code{REPEAT}.
733:
734: @example
735: BEGIN
736: @var{code}
737: @var{flag}
738: UNTIL
739: @end example
740:
741: @var{code} is executed. The loop is restarted if @code{flag} is false.
742:
743: @example
744: BEGIN
745: @var{code}
746: AGAIN
747: @end example
748:
749: This is an endless loop.
750:
751: @node Counted Loops, Arbitrary control structures, Simple Loops, Control Structures
752: @subsection Counted Loops
753:
754: The basic counted loop is:
755: @example
756: @var{limit} @var{start}
757: ?DO
758: @var{body}
759: LOOP
760: @end example
761:
762: This performs one iteration for every integer, starting from @var{start}
763: and up to, but excluding @var{limit}. The counter, aka index, can be
764: accessed with @code{i}. E.g., the loop
765: @example
766: 10 0 ?DO
767: i .
768: LOOP
769: @end example
770: prints
771: @example
772: 0 1 2 3 4 5 6 7 8 9
773: @end example
774: The index of the innermost loop can be accessed with @code{i}, the index
775: of the next loop with @code{j}, and the index of the third loop with
776: @code{k}.
777:
778: The loop control data are kept on the return stack, so there are some
779: restrictions on mixing return stack accesses and counted loop
780: words. E.g., if you put values on the return stack outside the loop, you
781: cannot read them inside the loop. If you put values on the return stack
782: within a loop, you have to remove them before the end of the loop and
783: before accessing the index of the loop.
784:
785: There are several variations on the counted loop:
786:
787: @code{LEAVE} leaves the innermost counted loop immediately.
788:
789: @code{LOOP} can be replaced with @code{@var{n} +LOOP}; this updates the
790: index by @var{n} instead of by 1. The loop is terminated when the border
791: between @var{limit-1} and @var{limit} is crossed. E.g.:
792:
793: @code{4 0 ?DO i . 2 +LOOP} prints @code{0 2}
794:
795: @code{4 1 ?DO i . 2 +LOOP} prints @code{1 3}
796:
797: The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative:
798:
799: @code{-1 0 ?DO i . -1 +LOOP} prints @code{0 -1}
800:
801: @code{ 0 0 ?DO i . -1 +LOOP} prints nothing
802:
803: Therefore we recommend avoiding using @code{@var{n} +LOOP} with negative
804: @var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative
805: case behaves symmetrical to the positive case:
806:
807: @code{-2 0 ?DO i . -1 S+LOOP} prints @code{0 -1}
808:
809: @code{-1 0 ?DO i . -1 S+LOOP} prints @code{0}
810:
811: @code{ 0 0 ?DO i . -1 S+LOOP} prints nothing
812:
813: The loop is terminated when the border between @var{limit@minus{}sgn(n)} and
814: @var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS
815: Forth standard.
816:
817: @code{?DO} can be replaced by @code{DO}. @code{DO} enters the loop even
818: when the start and the limit value are equal. We do not recommend using
819: @code{DO}. It will just give you maintenance troubles.
820:
821: @code{UNLOOP} is used to prepare for an abnormal loop exit, e.g., via
822: @code{EXIT}. @code{UNLOOP} removes the loop control parameters from the
823: return stack so @code{EXIT} can get to its return address.
824:
825: Another counted loop is
826: @example
827: @var{n}
828: FOR
829: @var{body}
830: NEXT
831: @end example
832: This is the preferred loop of native code compiler writers who are too
833: lazy to optimize @code{?DO} loops properly. In GNU Forth, this loop
834: iterates @var{n+1} times; @code{i} produces values starting with @var{n}
835: and ending with 0. Other Forth systems may behave differently, even if
836: they support @code{FOR} loops.
837:
838: @node Arbitrary control structures, Calls and returns, Counted Loops, Control Structures
839: @subsection Arbitrary control structures
840:
841: ANS Forth permits and supports using control structures in a non-nested
842: way. Information about incomplete control structures is stored on the
843: control-flow stack. This stack may be implemented on the Forth data
844: stack, and this is what we have done in gforth.
845:
846: An @i{orig} entry represents an unresolved forward branch, a @i{dest}
847: entry represents a backward branch target. A few words are the basis for
848: building any control structure possible (except control structures that
849: need storage, like calls, coroutines, and backtracking).
850:
851: doc-if
852: doc-ahead
853: doc-then
854: doc-begin
855: doc-until
856: doc-again
857: doc-cs-pick
858: doc-cs-roll
859:
860: On many systems control-flow stack items take one word, in gforth they
861: currently take three (this may change in the future). Therefore it is a
862: really good idea to manipulate the control flow stack with
863: @code{cs-pick} and @code{cs-roll}, not with data stack manipulation
864: words.
865:
866: Some standard control structure words are built from these words:
867:
868: doc-else
869: doc-while
870: doc-repeat
871:
872: Counted loop words constitute a separate group of words:
873:
874: doc-?do
875: doc-do
876: doc-for
877: doc-loop
878: doc-s+loop
879: doc-+loop
880: doc-next
881: doc-leave
882: doc-?leave
883: doc-unloop
884: doc-undo
885:
886: The standard does not allow using @code{cs-pick} and @code{cs-roll} on
887: @i{do-sys}. Our system allows it, but it's your job to ensure that for
888: every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path
889: through the definition (@code{LOOP} etc. compile an @code{UNLOOP} on the
890: fall-through path). Also, you have to ensure that all @code{LEAVE}s are
891: resolved (by using one of the loop-ending words or @code{DONE}).
892:
893: Another group of control structure words are
894:
895: doc-case
896: doc-endcase
897: doc-of
898: doc-endof
899:
900: @i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and
901: @code{cs-roll}.
902:
903: @subsubsection Programming Style
904:
905: In order to ensure readability we recommend that you do not create
906: arbitrary control structures directly, but define new control structure
907: words for the control structure you want and use these words in your
908: program.
909:
910: E.g., instead of writing
911:
912: @example
913: begin
914: ...
915: if [ 1 cs-roll ]
916: ...
917: again then
918: @end example
919:
920: we recommend defining control structure words, e.g.,
921:
922: @example
923: : while ( dest -- orig dest )
924: POSTPONE if
925: 1 cs-roll ; immediate
926:
927: : repeat ( orig dest -- )
928: POSTPONE again
929: POSTPONE then ; immediate
930: @end example
931:
932: and then using these to create the control structure:
933:
934: @example
935: begin
936: ...
937: while
938: ...
939: repeat
940: @end example
941:
942: That's much easier to read, isn't it? Of course, @code{BEGIN} and
943: @code{WHILE} are predefined, so in this example it would not be
944: necessary to define them.
945:
946: @node Calls and returns, Exception Handling, Arbitrary control structures, Control Structures
947: @subsection Calls and returns
948:
949: A definition can be called simply be writing the name of the
950: definition. When the end of the definition is reached, it returns. An earlier return can be forced using
951:
952: doc-exit
953:
954: Don't forget to clean up the return stack and @code{UNLOOP} any
955: outstanding @code{?DO}...@code{LOOP}s before @code{EXIT}ing. The
956: primitive compiled by @code{EXIT} is
957:
958: doc-;s
959:
960: @node Exception Handling, , Calls and returns, Control Structures
961: @subsection Exception Handling
962:
963: doc-catch
964: doc-throw
965:
966: @node Locals, Defining Words, Control Structures, Words
967: @section Locals
968:
969: Local variables can make Forth programming more enjoyable and Forth
970: programs easier to read. Unfortunately, the locals of ANS Forth are
971: laden with restrictions. Therefore, we provide not only the ANS Forth
972: locals wordset, but also our own, more powerful locals wordset (we
973: implemented the ANS Forth locals wordset through our locals wordset).
974:
975: @menu
976: * gforth locals::
977: * ANS Forth locals::
978: @end menu
979:
980: @node gforth locals, ANS Forth locals, Locals, Locals
981: @subsection gforth locals
982:
983: Locals can be defined with
984:
985: @example
986: @{ local1 local2 ... -- comment @}
987: @end example
988: or
989: @example
990: @{ local1 local2 ... @}
991: @end example
992:
993: E.g.,
994: @example
995: : max @{ n1 n2 -- n3 @}
996: n1 n2 > if
997: n1
998: else
999: n2
1000: endif ;
1001: @end example
1002:
1003: The similarity of locals definitions with stack comments is intended. A
1004: locals definition often replaces the stack comment of a word. The order
1005: of the locals corresponds to the order in a stack comment and everything
1006: after the @code{--} is really a comment.
1007:
1008: This similarity has one disadvantage: It is too easy to confuse locals
1009: declarations with stack comments, causing bugs and making them hard to
1010: find. However, this problem can be avoided by appropriate coding
1011: conventions: Do not use both notations in the same program. If you do,
1012: they should be distinguished using additional means, e.g. by position.
1013:
1014: The name of the local may be preceded by a type specifier, e.g.,
1015: @code{F:} for a floating point value:
1016:
1017: @example
1018: : CX* @{ F: Ar F: Ai F: Br F: Bi -- Cr Ci @}
1019: \ complex multiplication
1020: Ar Br f* Ai Bi f* f-
1021: Ar Bi f* Ai Br f* f+ ;
1022: @end example
1023:
1024: GNU Forth currently supports cells (@code{W:}, @code{W^}), doubles
1025: (@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters
1026: (@code{C:}, @code{C^}) in two flavours: a value-flavoured local (defined
1027: with @code{W:}, @code{D:} etc.) produces its value and can be changed
1028: with @code{TO}. A variable-flavoured local (defined with @code{W^} etc.)
1029: produces its address (which becomes invalid when the variable's scope is
1030: left). E.g., the standard word @code{emit} can be defined in therms of
1031: @code{type} like this:
1032:
1033: @example
1034: : emit @{ C^ char* -- @}
1035: char* 1 type ;
1036: @end example
1037:
1038: A local without type specifier is a @code{W:} local. Both flavours of
1039: locals are initialized with values from the data or FP stack.
1040:
1041: Currently there is no way to define locals with user-defined data
1042: structures, but we are working on it.
1043:
1044: GNU Forth allows defining locals everywhere in a colon definition. This
1045: poses the following questions:
1046:
1047: @menu
1048: * Where are locals visible by name?::
1049: * How long do locals live? ::
1050: * Programming Style::
1051: * Implementation::
1052: @end menu
1053:
1054: @node Where are locals visible by name?, How long do locals live?, gforth locals, gforth locals
1055: @subsubsection Where are locals visible by name?
1056:
1057: Basically, the answer is that locals are visible where you would expect
1058: it in block-structured languages, and sometimes a little longer. If you
1059: want to restrict the scope of a local, enclose its definition in
1060: @code{SCOPE}...@code{ENDSCOPE}.
1061:
1062: doc-scope
1063: doc-endscope
1064:
1065: These words behave like control structure words, so you can use them
1066: with @code{CS-PICK} and @code{CS-ROLL} to restrict the scope in
1067: arbitrary ways.
1068:
1069: If you want a more exact answer to the visibility question, here's the
1070: basic principle: A local is visible in all places that can only be
1071: reached through the definition of the local@footnote{In compiler
1072: construction terminology, all places dominated by the definition of the
1073: local.}. In other words, it is not visible in places that can be reached
1074: without going through the definition of the local. E.g., locals defined
1075: in @code{IF}...@code{ENDIF} are visible until the @code{ENDIF}, locals
1076: defined in @code{BEGIN}...@code{UNTIL} are visible after the
1077: @code{UNTIL} (until, e.g., a subsequent @code{ENDSCOPE}).
1078:
1079: The reasoning behind this solution is: We want to have the locals
1080: visible as long as it is meaningful. The user can always make the
1081: visibility shorter by using explicit scoping. In a place that can
1082: only be reached through the definition of a local, the meaning of a
1083: local name is clear. In other places it is not: How is the local
1084: initialized at the control flow path that does not contain the
1085: definition? Which local is meant, if the same name is defined twice in
1086: two independent control flow paths?
1087:
1088: This should be enough detail for nearly all users, so you can skip the
1089: rest of this section. If you relly must know all the gory details and
1090: options, read on.
1091:
1092: In order to implement this rule, the compiler has to know which places
1093: are unreachable. It knows this automatically after @code{AHEAD},
1094: @code{AGAIN}, @code{EXIT} and @code{LEAVE}; in other cases (e.g., after
1095: most @code{THROW}s), you can use the word @code{UNREACHABLE} to tell the
1096: compiler that the control flow never reaches that place. If
1097: @code{UNREACHABLE} is not used where it could, the only consequence is
1098: that the visibility of some locals is more limited than the rule above
1099: says. If @code{UNREACHABLE} is used where it should not (i.e., if you
1100: lie to the compiler), buggy code will be produced.
1101:
1102: Another problem with this rule is that at @code{BEGIN}, the compiler
1103: does not know which locals will be visible on the incoming
1104: back-edge. All problems discussed in the following are due to this
1105: ignorance of the compiler (we discuss the problems using @code{BEGIN}
1106: loops as examples; the discussion also applies to @code{?DO} and other
1107: loops). Perhaps the most insidious example is:
1108: @example
1109: AHEAD
1110: BEGIN
1111: x
1112: [ 1 CS-ROLL ] THEN
1113: @{ x @}
1114: ...
1115: UNTIL
1116: @end example
1117:
1118: This should be legal according to the visibility rule. The use of
1119: @code{x} can only be reached through the definition; but that appears
1120: textually below the use.
1121:
1122: From this example it is clear that the visibility rules cannot be fully
1123: implemented without major headaches. Our implementation treats common
1124: cases as advertised and the exceptions are treated in a safe way: The
1125: compiler makes a reasonable guess about the locals visible after a
1126: @code{BEGIN}; if it is too pessimistic, the
1127: user will get a spurious error about the local not being defined; if the
1128: compiler is too optimistic, it will notice this later and issue a
1129: warning. In the case above the compiler would complain about @code{x}
1130: being undefined at its use. You can see from the obscure examples in
1131: this section that it takes quite unusual control structures to get the
1132: compiler into trouble, and even then it will often do fine.
1133:
1134: If the @code{BEGIN} is reachable from above, the most optimistic guess
1135: is that all locals visible before the @code{BEGIN} will also be
1136: visible after the @code{BEGIN}. This guess is valid for all loops that
1137: are entered only through the @code{BEGIN}, in particular, for normal
1138: @code{BEGIN}...@code{WHILE}...@code{REPEAT} and
1139: @code{BEGIN}...@code{UNTIL} loops and it is implemented in our
1140: compiler. When the branch to the @code{BEGIN} is finally generated by
1141: @code{AGAIN} or @code{UNTIL}, the compiler checks the guess and
1142: warns the user if it was too optimisitic:
1143: @example
1144: IF
1145: @{ x @}
1146: BEGIN
1147: \ x ?
1148: [ 1 cs-roll ] THEN
1149: ...
1150: UNTIL
1151: @end example
1152:
1153: Here, @code{x} lives only until the @code{BEGIN}, but the compiler
1154: optimistically assumes that it lives until the @code{THEN}. It notices
1155: this difference when it compiles the @code{UNTIL} and issues a
1156: warning. The user can avoid the warning, and make sure that @code{x}
1157: is not used in the wrong area by using explicit scoping:
1158: @example
1159: IF
1160: SCOPE
1161: @{ x @}
1162: ENDSCOPE
1163: BEGIN
1164: [ 1 cs-roll ] THEN
1165: ...
1166: UNTIL
1167: @end example
1168:
1169: Since the guess is optimistic, there will be no spurious error messages
1170: about undefined locals.
1171:
1172: If the @code{BEGIN} is not reachable from above (e.g., after
1173: @code{AHEAD} or @code{EXIT}), the compiler cannot even make an
1174: optimistic guess, as the locals visible after the @code{BEGIN} may be
1175: defined later. Therefore, the compiler assumes that no locals are
1176: visible after the @code{BEGIN}. However, the useer can use
1177: @code{ASSUME-LIVE} to make the compiler assume that the same locals are
1178: visible at the BEGIN as at the point where the item was created.
1179:
1180: doc-assume-live
1181:
1182: E.g.,
1183: @example
1184: @{ x @}
1185: AHEAD
1186: ASSUME-LIVE
1187: BEGIN
1188: x
1189: [ 1 CS-ROLL ] THEN
1190: ...
1191: UNTIL
1192: @end example
1193:
1194: Other cases where the locals are defined before the @code{BEGIN} can be
1195: handled by inserting an appropriate @code{CS-ROLL} before the
1196: @code{ASSUME-LIVE} (and changing the control-flow stack manipulation
1197: behind the @code{ASSUME-LIVE}).
1198:
1199: Cases where locals are defined after the @code{BEGIN} (but should be
1200: visible immediately after the @code{BEGIN}) can only be handled by
1201: rearranging the loop. E.g., the ``most insidious'' example above can be
1202: arranged into:
1203: @example
1204: BEGIN
1205: @{ x @}
1206: ... 0=
1207: WHILE
1208: x
1209: REPEAT
1210: @end example
1211:
1212: @node How long do locals live?, Programming Style, Where are locals visible by name?, gforth locals
1213: @subsubsection How long do locals live?
1214:
1215: The right answer for the lifetime question would be: A local lives at
1216: least as long as it can be accessed. For a value-flavoured local this
1217: means: until the end of its visibility. However, a variable-flavoured
1218: local could be accessed through its address far beyond its visibility
1219: scope. Ultimately, this would mean that such locals would have to be
1220: garbage collected. Since this entails un-Forth-like implementation
1221: complexities, I adopted the same cowardly solution as some other
1222: languages (e.g., C): The local lives only as long as it is visible;
1223: afterwards its address is invalid (and programs that access it
1224: afterwards are erroneous).
1225:
1226: @node Programming Style, Implementation, How long do locals live?, gforth locals
1227: @subsubsection Programming Style
1228:
1229: The freedom to define locals anywhere has the potential to change
1230: programming styles dramatically. In particular, the need to use the
1231: return stack for intermediate storage vanishes. Moreover, all stack
1232: manipulations (except @code{PICK}s and @code{ROLL}s with run-time
1233: determined arguments) can be eliminated: If the stack items are in the
1234: wrong order, just write a locals definition for all of them; then
1235: write the items in the order you want.
1236:
1237: This seems a little far-fetched and eliminating stack manipulations is
1238: unlikely to become a conscious programming objective. Still, the number
1239: of stack manipulations will be reduced dramatically if local variables
1240: are used liberally (e.g., compare @code{max} in @ref{gforth locals} with
1241: a traditional implementation of @code{max}).
1242:
1243: This shows one potential benefit of locals: making Forth programs more
1244: readable. Of course, this benefit will only be realized if the
1245: programmers continue to honour the principle of factoring instead of
1246: using the added latitude to make the words longer.
1247:
1248: Using @code{TO} can and should be avoided. Without @code{TO},
1249: every value-flavoured local has only a single assignment and many
1250: advantages of functional languages apply to Forth. I.e., programs are
1251: easier to analyse, to optimize and to read: It is clear from the
1252: definition what the local stands for, it does not turn into something
1253: different later.
1254:
1255: E.g., a definition using @code{TO} might look like this:
1256: @example
1257: : strcmp @{ addr1 u1 addr2 u2 -- n @}
1258: u1 u2 min 0
1259: ?do
1260: addr1 c@ addr2 c@ - ?dup
1261: if
1262: unloop exit
1263: then
1264: addr1 char+ TO addr1
1265: addr2 char+ TO addr2
1266: loop
1267: u1 u2 - ;
1268: @end example
1269: Here, @code{TO} is used to update @code{addr1} and @code{addr2} at
1270: every loop iteration. @code{strcmp} is a typical example of the
1271: readability problems of using @code{TO}. When you start reading
1272: @code{strcmp}, you think that @code{addr1} refers to the start of the
1273: string. Only near the end of the loop you realize that it is something
1274: else.
1275:
1276: This can be avoided by defining two locals at the start of the loop that
1277: are initialized with the right value for the current iteration.
1278: @example
1279: : strcmp @{ addr1 u1 addr2 u2 -- n @}
1280: addr1 addr2
1281: u1 u2 min 0
1282: ?do @{ s1 s2 @}
1283: s1 c@ s2 c@ - ?dup
1284: if
1285: unloop exit
1286: then
1287: s1 char+ s2 char+
1288: loop
1289: 2drop
1290: u1 u2 - ;
1291: @end example
1292: Here it is clear from the start that @code{s1} has a different value
1293: in every loop iteration.
1294:
1295: @node Implementation, , Programming Style, gforth locals
1296: @subsubsection Implementation
1297:
1298: GNU Forth uses an extra locals stack. The most compelling reason for
1299: this is that the return stack is not float-aligned; using an extra stack
1300: also eliminates the problems and restrictions of using the return stack
1301: as locals stack. Like the other stacks, the locals stack grows toward
1302: lower addresses. A few primitives allow an efficient implementation:
1303:
1304: doc-@local#
1305: doc-f@local#
1306: doc-laddr#
1307: doc-lp+!#
1308: doc-lp!
1309: doc->l
1310: doc-f>l
1311:
1312: In addition to these primitives, some specializations of these
1313: primitives for commonly occurring inline arguments are provided for
1314: efficiency reasons, e.g., @code{@@local0} as specialization of
1315: @code{@@local#} for the inline argument 0. The following compiling words
1316: compile the right specialized version, or the general version, as
1317: appropriate:
1318:
1319: doc-compile-@@local
1320: doc-compile-f@@local
1321: doc-compile-lp+!
1322:
1323: Combinations of conditional branches and @code{lp+!#} like
1324: @code{?branch-lp+!#} (the locals pointer is only changed if the branch
1325: is taken) are provided for efficiency and correctness in loops.
1326:
1327: A special area in the dictionary space is reserved for keeping the
1328: local variable names. @code{@{} switches the dictionary pointer to this
1329: area and @code{@}} switches it back and generates the locals
1330: initializing code. @code{W:} etc.@ are normal defining words. This
1331: special area is cleared at the start of every colon definition.
1332:
1333: A special feature of GNU Forths dictionary is used to implement the
1334: definition of locals without type specifiers: every wordlist (aka
1335: vocabulary) has its own methods for searching
1336: etc. (@pxref{Wordlists}). For the present purpose we defined a wordlist
1337: with a special search method: When it is searched for a word, it
1338: actually creates that word using @code{W:}. @code{@{} changes the search
1339: order to first search the wordlist containing @code{@}}, @code{W:} etc.,
1340: and then the wordlist for defining locals without type specifiers.
1341:
1342: The lifetime rules support a stack discipline within a colon
1343: definition: The lifetime of a local is either nested with other locals
1344: lifetimes or it does not overlap them.
1345:
1346: At @code{BEGIN}, @code{IF}, and @code{AHEAD} no code for locals stack
1347: pointer manipulation is generated. Between control structure words
1348: locals definitions can push locals onto the locals stack. @code{AGAIN}
1349: is the simplest of the other three control flow words. It has to
1350: restore the locals stack depth of the corresponding @code{BEGIN}
1351: before branching. The code looks like this:
1352: @format
1353: @code{lp+!#} current-locals-size @minus{} dest-locals-size
1354: @code{branch} <begin>
1355: @end format
1356:
1357: @code{UNTIL} is a little more complicated: If it branches back, it
1358: must adjust the stack just like @code{AGAIN}. But if it falls through,
1359: the locals stack must not be changed. The compiler generates the
1360: following code:
1361: @format
1362: @code{?branch-lp+!#} <begin> current-locals-size @minus{} dest-locals-size
1363: @end format
1364: The locals stack pointer is only adjusted if the branch is taken.
1365:
1366: @code{THEN} can produce somewhat inefficient code:
1367: @format
1368: @code{lp+!#} current-locals-size @minus{} orig-locals-size
1369: <orig target>:
1370: @code{lp+!#} orig-locals-size @minus{} new-locals-size
1371: @end format
1372: The second @code{lp+!#} adjusts the locals stack pointer from the
1373: level at the @var{orig} point to the level after the @code{THEN}. The
1374: first @code{lp+!#} adjusts the locals stack pointer from the current
1375: level to the level at the orig point, so the complete effect is an
1376: adjustment from the current level to the right level after the
1377: @code{THEN}.
1378:
1379: In a conventional Forth implementation a dest control-flow stack entry
1380: is just the target address and an orig entry is just the address to be
1381: patched. Our locals implementation adds a wordlist to every orig or dest
1382: item. It is the list of locals visible (or assumed visible) at the point
1383: described by the entry. Our implementation also adds a tag to identify
1384: the kind of entry, in particular to differentiate between live and dead
1385: (reachable and unreachable) orig entries.
1386:
1387: A few unusual operations have to be performed on locals wordlists:
1388:
1389: doc-common-list
1390: doc-sub-list?
1391: doc-list-size
1392:
1393: Several features of our locals wordlist implementation make these
1394: operations easy to implement: The locals wordlists are organised as
1395: linked lists; the tails of these lists are shared, if the lists
1396: contain some of the same locals; and the address of a name is greater
1397: than the address of the names behind it in the list.
1398:
1399: Another important implementation detail is the variable
1400: @code{dead-code}. It is used by @code{BEGIN} and @code{THEN} to
1401: determine if they can be reached directly or only through the branch
1402: that they resolve. @code{dead-code} is set by @code{UNREACHABLE},
1403: @code{AHEAD}, @code{EXIT} etc., and cleared at the start of a colon
1404: definition, by @code{BEGIN} and usually by @code{THEN}.
1405:
1406: Counted loops are similar to other loops in most respects, but
1407: @code{LEAVE} requires special attention: It performs basically the same
1408: service as @code{AHEAD}, but it does not create a control-flow stack
1409: entry. Therefore the information has to be stored elsewhere;
1410: traditionally, the information was stored in the target fields of the
1411: branches created by the @code{LEAVE}s, by organizing these fields into a
1412: linked list. Unfortunately, this clever trick does not provide enough
1413: space for storing our extended control flow information. Therefore, we
1414: introduce another stack, the leave stack. It contains the control-flow
1415: stack entries for all unresolved @code{LEAVE}s.
1416:
1417: Local names are kept until the end of the colon definition, even if
1418: they are no longer visible in any control-flow path. In a few cases
1419: this may lead to increased space needs for the locals name area, but
1420: usually less than reclaiming this space would cost in code size.
1421:
1422:
1423: @node ANS Forth locals, , gforth locals, Locals
1424: @subsection ANS Forth locals
1425:
1426: The ANS Forth locals wordset does not define a syntax for locals, but
1427: words that make it possible to define various syntaxes. One of the
1428: possible syntaxes is a subset of the syntax we used in the gforth locals
1429: wordset, i.e.:
1430:
1431: @example
1432: @{ local1 local2 ... -- comment @}
1433: @end example
1434: or
1435: @example
1436: @{ local1 local2 ... @}
1437: @end example
1438:
1439: The order of the locals corresponds to the order in a stack comment. The
1440: restrictions are:
1441:
1442: @itemize @bullet
1443: @item
1444: Locals can only be cell-sized values (no type specifers are allowed).
1445: @item
1446: Locals can be defined only outside control structures.
1447: @item
1448: Locals can interfere with explicit usage of the return stack. For the
1449: exact (and long) rules, see the standard. If you don't use return stack
1450: accessing words in a definition using locals, you will we all right. The
1451: purpose of this rule is to make locals implementation on the return
1452: stack easier.
1453: @item
1454: The whole definition must be in one line.
1455: @end itemize
1456:
1457: Locals defined in this way behave like @code{VALUE}s
1458: (@xref{Values}). I.e., they are initialized from the stack. Using their
1459: name produces their value. Their value can be changed using @code{TO}.
1460:
1461: Since this syntax is supported by gforth directly, you need not do
1462: anything to use it. If you want to port a program using this syntax to
1463: another ANS Forth system, use @file{anslocal.fs} to implement the syntax
1464: on the other system.
1465:
1466: Note that a syntax shown in the standard, section A.13 looks
1467: similar, but is quite different in having the order of locals
1468: reversed. Beware!
1469:
1470: The ANS Forth locals wordset itself consists of the following word
1471:
1472: doc-(local)
1473:
1474: The ANS Forth locals extension wordset defines a syntax, but it is so
1475: awful that we strongly recommend not to use it. We have implemented this
1476: syntax to make porting to gforth easy, but do not document it here. The
1477: problem with this syntax is that the locals are defined in an order
1478: reversed with respect to the standard stack comment notation, making
1479: programs harder to read, and easier to misread and miswrite. The only
1480: merit of this syntax is that it is easy to implement using the ANS Forth
1481: locals wordset.
1482:
1483: @node Defining Words, Wordlists, Locals, Words
1484: @section Defining Words
1485:
1486: @node Values, , Defining Words, Defining Words
1487: @subsection Values
1488:
1489: @node Wordlists, Files, Defining Words, Words
1490: @section Wordlists
1491:
1492: @node Files, Blocks, Wordlists, Words
1493: @section Files
1494:
1495: @node Blocks, Other I/O, Files, Words
1496: @section Blocks
1497:
1498: @node Other I/O, Programming Tools, Blocks, Words
1499: @section Other I/O
1500:
1501: @node Programming Tools, Threading Words, Other I/O, Words
1502: @section Programming Tools
1503:
1504: @menu
1505: * Debugging:: Simple and quick.
1506: * Assertions:: Making your programs self-checking.
1507: @end menu
1508:
1509: @node Debugging, Assertions, Programming Tools, Programming Tools
1510: @subsection Debugging
1511:
1512: The simple debugging aids provided in @file{debugging.fs}
1513: are meant to support a different style of debugging than the
1514: tracing/stepping debuggers used in languages with long turn-around
1515: times.
1516:
1517: A much better (faster) way in fast-compilig languages is to add
1518: printing code at well-selected places, let the program run, look at
1519: the output, see where things went wrong, add more printing code, etc.,
1520: until the bug is found.
1521:
1522: The word @code{~~} is easy to insert. It just prints debugging
1523: information (by default the source location and the stack contents). It
1524: is also easy to remove (@kbd{C-x ~} in the Emacs Forth mode to
1525: query-replace them with nothing). The deferred words
1526: @code{printdebugdata} and @code{printdebugline} control the output of
1527: @code{~~}. The default source location output format works well with
1528: Emacs' compilation mode, so you can step through the program at the
1529: source level using @kbd{C-x `} (the advantage over a stepping debugger
1530: is that you can step in any direction and you know where the crash has
1531: happened or where the strange data has occurred).
1532:
1533: Note that the default actions clobber the contents of the pictured
1534: numeric output string, so you should not use @code{~~}, e.g., between
1535: @code{<#} and @code{#>}.
1536:
1537: doc-~~
1538: doc-printdebugdata
1539: doc-printdebugline
1540:
1541: @node Assertions, , Debugging, Programming Tools
1542: @subsection Assertions
1543:
1544: It is a good idea to make your programs self-checking, in particular, if
1545: you use an assumption (e.g., that a certain field of a data structure is
1546: never zero) that may become wrong during maintenance. GForth supports
1547: assertions for this purpose. They are used like this:
1548:
1549: @example
1550: assert( @var{flag} )
1551: @end example
1552:
1553: The code between @code{assert(} and @code{)} should compute a flag, that
1554: should be true if everything is alright and false otherwise. It should
1555: not change anything else on the stack. The overall stack effect of the
1556: assertion is @code{( -- )}. E.g.
1557:
1558: @example
1559: assert( 1 1 + 2 = ) \ what we learn in school
1560: assert( dup 0<> ) \ assert that the top of stack is not zero
1561: assert( false ) \ this code should not be reached
1562: @end example
1563:
1564: The need for assertions is different at different times. During
1565: debugging, we want more checking, in production we sometimes care more
1566: for speed. Therefore, assertions can be turned off, i.e., the assertion
1567: becomes a comment. Depending on the importance of an assertion and the
1568: time it takes to check it, you may want to turn off some assertions and
1569: keep others turned on. GForth provides several levels of assertions for
1570: this purpose:
1571:
1572: doc-assert0(
1573: doc-assert1(
1574: doc-assert2(
1575: doc-assert3(
1576: doc-assert(
1577: doc-)
1578:
1579: @code{Assert(} is the same as @code{assert1(}. The variable
1580: @code{assert-level} specifies the highest assertions that are turned
1581: on. I.e., at the default @code{assert-level} of one, @code{assert0(} and
1582: @code{assert1(} assertions perform checking, while @code{assert2(} and
1583: @code{assert3(} assertions are treated as comments.
1584:
1585: Note that the @code{assert-level} is evaluated at compile-time, not at
1586: run-time. I.e., you cannot turn assertions on or off at run-time, you
1587: have to set the @code{assert-level} appropriately before compiling a
1588: piece of code. You can compile several pieces of code at several
1589: @code{assert-level}s (e.g., a trusted library at level 1 and newly
1590: written code at level 3).
1591:
1592: doc-assert-level
1593:
1594: If an assertion fails, a message compatible with Emacs' compilation mode
1595: is produced and the execution is aborted (currently with @code{ABORT"}.
1596: If there is interest, we will introduce a special throw code. But if you
1597: intend to @code{catch} a specific condition, using @code{throw} is
1598: probably more appropriate than an assertion).
1599:
1600: @node Threading Words, , Programming Tools, Words
1601: @section Threading Words
1602:
1603: These words provide access to code addresses and other threading stuff
1604: in gforth (and, possibly, other interpretive Forths). It more or less
1605: abstracts away the differences between direct and indirect threading
1606: (and, for direct threading, the machine dependences). However, at
1607: present this wordset is still inclomplete. It is also pretty low-level;
1608: some day it will hopefully be made unnecessary by an internals words set
1609: that abstracts implementation details away completely.
1610:
1611: doc->code-address
1612: doc->does-code
1613: doc-code-address!
1614: doc-does-code!
1615: doc-does-handler!
1616: doc-/does-handler
1617:
1618: @node ANS conformance, Model, Words, Top
1619: @chapter ANS conformance
1620:
1621: @node Model, Emacs and GForth, ANS conformance, Top
1622: @chapter Model
1623:
1624: @node Emacs and GForth, Internals, Model, Top
1625: @chapter Emacs and GForth
1626:
1627: GForth comes with @file{gforth.el}, an improved version of
1628: @file{forth.el} by Goran Rydqvist (icluded in the TILE package). The
1629: improvements are a better (but still not perfect) handling of
1630: indentation. I have also added comment paragraph filling (@kbd{M-q}),
1631: commenting (@kbd{C-x \}) and uncommenting (@kbd{C-u C-x \}) regions and
1632: removing debugging tracers (@kbd{C-x ~}, @pxref{Debugging}). I left the
1633: stuff I do not use alone, even though some of it only makes sense for
1634: TILE. To get a description of these features, enter Forth mode and type
1635: @kbd{C-h m}.
1636:
1637: In addition, GForth supports Emacs quite well: The source code locations
1638: given in error messages, debugging output (from @code{~~}) and failed
1639: assertion messages are in the right format for Emacs' compilation mode
1640: (@pxref{Compilation, , Running Compilations under Emacs, emacs, Emacs
1641: Manual}) so the source location corresponding to an error or other
1642: message is only a few keystrokes away (@kbd{C-x `} for the next error,
1643: @kbd{C-c C-c} for the error under the cursor).
1644:
1645: Also, if you @code{include} @file{etags.fs}, a new @file{TAGS} file
1646: (@pxref{Tags, , Tags Tables, emacs, Emacs Manual}) will be produced that
1647: contains the definitions of all words defined afterwards. You can then
1648: find the source for a word using @kbd{M-.}. Note that emacs can use
1649: several tags files at the same time (e.g., one for the gforth sources
1650: and one for your program).
1651:
1652: To get all these benefits, add the following lines to your @file{.emacs}
1653: file:
1654:
1655: @example
1656: (autoload 'forth-mode "gforth.el")
1657: (setq auto-mode-alist (cons '("\\.fs\\'" . forth-mode) auto-mode-alist))
1658: @end example
1659:
1660: @node Internals, Bugs, Emacs and GForth, Top
1661: @chapter Internals
1662:
1663: Reading this section is not necessary for programming with gforth. It
1664: should be helpful for finding your way in the gforth sources.
1665:
1666: @menu
1667: * Portability::
1668: * Threading::
1669: * Primitives::
1670: * System Architecture::
1671: @end menu
1672:
1673: @node Portability, Threading, Internals, Internals
1674: @section Portability
1675:
1676: One of the main goals of the effort is availability across a wide range
1677: of personal machines. fig-Forth, and, to a lesser extent, F83, achieved
1678: this goal by manually coding the engine in assembly language for several
1679: then-popular processors. This approach is very labor-intensive and the
1680: results are short-lived due to progress in computer architecture.
1681:
1682: Others have avoided this problem by coding in C, e.g., Mitch Bradley
1683: (cforth), Mikael Patel (TILE) and Dirk Zoller (pfe). This approach is
1684: particularly popular for UNIX-based Forths due to the large variety of
1685: architectures of UNIX machines. Unfortunately an implementation in C
1686: does not mix well with the goals of efficiency and with using
1687: traditional techniques: Indirect or direct threading cannot be expressed
1688: in C, and switch threading, the fastest technique available in C, is
1689: significantly slower. Another problem with C is that it's very
1690: cumbersome to express double integer arithmetic.
1691:
1692: Fortunately, there is a portable language that does not have these
1693: limitations: GNU C, the version of C processed by the GNU C compiler
1694: (@pxref{C Extensions, , Extensions to the C Language Family, gcc.info,
1695: GNU C Manual}). Its labels as values feature (@pxref{Labels as Values, ,
1696: Labels as Values, gcc.info, GNU C Manual}) makes direct and indirect
1697: threading possible, its @code{long long} type (@pxref{Long Long, ,
1698: Double-Word Integers, gcc.info, GNU C Manual}) corresponds to Forths
1699: double numbers. GNU C is available for free on all important (and many
1700: unimportant) UNIX machines, VMS, 80386s running MS-DOS, the Amiga, and
1701: the Atari ST, so a Forth written in GNU C can run on all these
1702: machines@footnote{Due to Apple's look-and-feel lawsuit it is not
1703: available on the Mac (@pxref{Boycott, , Protect Your Freedom---Fight
1704: ``Look And Feel'', gcc.info, GNU C Manual}).}.
1705:
1706: Writing in a portable language has the reputation of producing code that
1707: is slower than assembly. For our Forth engine we repeatedly looked at
1708: the code produced by the compiler and eliminated most compiler-induced
1709: inefficiencies by appropriate changes in the source-code.
1710:
1711: However, register allocation cannot be portably influenced by the
1712: programmer, leading to some inefficiencies on register-starved
1713: machines. We use explicit register declarations (@pxref{Explicit Reg
1714: Vars, , Variables in Specified Registers, gcc.info, GNU C Manual}) to
1715: improve the speed on some machines. They are turned on by using the
1716: @code{gcc} switch @code{-DFORCE_REG}. Unfortunately, this feature not
1717: only depends on the machine, but also on the compiler version: On some
1718: machines some compiler versions produce incorrect code when certain
1719: explicit register declarations are used. So by default
1720: @code{-DFORCE_REG} is not used.
1721:
1722: @node Threading, Primitives, Portability, Internals
1723: @section Threading
1724:
1725: GNU C's labels as values extension (available since @code{gcc-2.0},
1726: @pxref{Labels as Values, , Labels as Values, gcc.info, GNU C Manual})
1727: makes it possible to take the address of @var{label} by writing
1728: @code{&&@var{label}}. This address can then be used in a statement like
1729: @code{goto *@var{address}}. I.e., @code{goto *&&x} is the same as
1730: @code{goto x}.
1731:
1732: With this feature an indirect threaded NEXT looks like:
1733: @example
1734: cfa = *ip++;
1735: ca = *cfa;
1736: goto *ca;
1737: @end example
1738: For those unfamiliar with the names: @code{ip} is the Forth instruction
1739: pointer; the @code{cfa} (code-field address) corresponds to ANS Forths
1740: execution token and points to the code field of the next word to be
1741: executed; The @code{ca} (code address) fetched from there points to some
1742: executable code, e.g., a primitive or the colon definition handler
1743: @code{docol}.
1744:
1745: Direct threading is even simpler:
1746: @example
1747: ca = *ip++;
1748: goto *ca;
1749: @end example
1750:
1751: Of course we have packaged the whole thing neatly in macros called
1752: @code{NEXT} and @code{NEXT1} (the part of NEXT after fetching the cfa).
1753:
1754: @menu
1755: * Scheduling::
1756: * Direct or Indirect Threaded?::
1757: * DOES>::
1758: @end menu
1759:
1760: @node Scheduling, Direct or Indirect Threaded?, Threading, Threading
1761: @subsection Scheduling
1762:
1763: There is a little complication: Pipelined and superscalar processors,
1764: i.e., RISC and some modern CISC machines can process independent
1765: instructions while waiting for the results of an instruction. The
1766: compiler usually reorders (schedules) the instructions in a way that
1767: achieves good usage of these delay slots. However, on our first tries
1768: the compiler did not do well on scheduling primitives. E.g., for
1769: @code{+} implemented as
1770: @example
1771: n=sp[0]+sp[1];
1772: sp++;
1773: sp[0]=n;
1774: NEXT;
1775: @end example
1776: the NEXT comes strictly after the other code, i.e., there is nearly no
1777: scheduling. After a little thought the problem becomes clear: The
1778: compiler cannot know that sp and ip point to different addresses (and
1779: the version of @code{gcc} we used would not know it even if it was
1780: possible), so it could not move the load of the cfa above the store to
1781: the TOS. Indeed the pointers could be the same, if code on or very near
1782: the top of stack were executed. In the interest of speed we chose to
1783: forbid this probably unused ``feature'' and helped the compiler in
1784: scheduling: NEXT is divided into the loading part (@code{NEXT_P1}) and
1785: the goto part (@code{NEXT_P2}). @code{+} now looks like:
1786: @example
1787: n=sp[0]+sp[1];
1788: sp++;
1789: NEXT_P1;
1790: sp[0]=n;
1791: NEXT_P2;
1792: @end example
1793: This can be scheduled optimally by the compiler.
1794:
1795: This division can be turned off with the switch @code{-DCISC_NEXT}. This
1796: switch is on by default on machines that do not profit from scheduling
1797: (e.g., the 80386), in order to preserve registers.
1798:
1799: @node Direct or Indirect Threaded?, DOES>, Scheduling, Threading
1800: @subsection Direct or Indirect Threaded?
1801:
1802: Both! After packaging the nasty details in macro definitions we
1803: realized that we could switch between direct and indirect threading by
1804: simply setting a compilation flag (@code{-DDIRECT_THREADED}) and
1805: defining a few machine-specific macros for the direct-threading case.
1806: On the Forth level we also offer access words that hide the
1807: differences between the threading methods (@pxref{Threading Words}).
1808:
1809: Indirect threading is implemented completely
1810: machine-independently. Direct threading needs routines for creating
1811: jumps to the executable code (e.g. to docol or dodoes). These routines
1812: are inherently machine-dependent, but they do not amount to many source
1813: lines. I.e., even porting direct threading to a new machine is a small
1814: effort.
1815:
1816: @node DOES>, , Direct or Indirect Threaded?, Threading
1817: @subsection DOES>
1818: One of the most complex parts of a Forth engine is @code{dodoes}, i.e.,
1819: the chunk of code executed by every word defined by a
1820: @code{CREATE}...@code{DOES>} pair. The main problem here is: How to find
1821: the Forth code to be executed, i.e. the code after the @code{DOES>} (the
1822: DOES-code)? There are two solutions:
1823:
1824: In fig-Forth the code field points directly to the dodoes and the
1825: DOES-code address is stored in the cell after the code address
1826: (i.e. at cfa cell+). It may seem that this solution is illegal in the
1827: Forth-79 and all later standards, because in fig-Forth this address
1828: lies in the body (which is illegal in these standards). However, by
1829: making the code field larger for all words this solution becomes legal
1830: again. We use this approach for the indirect threaded version. Leaving
1831: a cell unused in most words is a bit wasteful, but on the machines we
1832: are targetting this is hardly a problem. The other reason for having a
1833: code field size of two cells is to avoid having different image files
1834: for direct and indirect threaded systems (@pxref{System Architecture}).
1835:
1836: The other approach is that the code field points or jumps to the cell
1837: after @code{DOES}. In this variant there is a jump to @code{dodoes} at
1838: this address. @code{dodoes} can then get the DOES-code address by
1839: computing the code address, i.e., the address of the jump to dodoes,
1840: and add the length of that jump field. A variant of this is to have a
1841: call to @code{dodoes} after the @code{DOES>}; then the return address
1842: (which can be found in the return register on RISCs) is the DOES-code
1843: address. Since the two cells available in the code field are usually
1844: used up by the jump to the code address in direct threading, we use
1845: this approach for direct threading. We did not want to add another
1846: cell to the code field.
1847:
1848: @node Primitives, System Architecture, Threading, Internals
1849: @section Primitives
1850:
1851: @menu
1852: * Automatic Generation::
1853: * TOS Optimization::
1854: * Produced code::
1855: @end menu
1856:
1857: @node Automatic Generation, TOS Optimization, Primitives, Primitives
1858: @subsection Automatic Generation
1859:
1860: Since the primitives are implemented in a portable language, there is no
1861: longer any need to minimize the number of primitives. On the contrary,
1862: having many primitives is an advantage: speed. In order to reduce the
1863: number of errors in primitives and to make programming them easier, we
1864: provide a tool, the primitive generator (@file{prims2x.fs}), that
1865: automatically generates most (and sometimes all) of the C code for a
1866: primitive from the stack effect notation. The source for a primitive
1867: has the following form:
1868:
1869: @format
1870: @var{Forth-name} @var{stack-effect} @var{category} [@var{pronounc.}]
1871: [@code{""}@var{glossary entry}@code{""}]
1872: @var{C code}
1873: [@code{:}
1874: @var{Forth code}]
1875: @end format
1876:
1877: The items in brackets are optional. The category and glossary fields
1878: are there for generating the documentation, the Forth code is there
1879: for manual implementations on machines without GNU C. E.g., the source
1880: for the primitive @code{+} is:
1881: @example
1882: + n1 n2 -- n core plus
1883: n = n1+n2;
1884: @end example
1885:
1886: This looks like a specification, but in fact @code{n = n1+n2} is C
1887: code. Our primitive generation tool extracts a lot of information from
1888: the stack effect notations@footnote{We use a one-stack notation, even
1889: though we have separate data and floating-point stacks; The separate
1890: notation can be generated easily from the unified notation.}: The number
1891: of items popped from and pushed on the stack, their type, and by what
1892: name they are referred to in the C code. It then generates a C code
1893: prelude and postlude for each primitive. The final C code for @code{+}
1894: looks like this:
1895:
1896: @example
1897: I_plus: /* + ( n1 n2 -- n ) */ /* label, stack effect */
1898: /* */ /* documentation */
1899: @{
1900: DEF_CA /* definition of variable ca (indirect threading) */
1901: Cell n1; /* definitions of variables */
1902: Cell n2;
1903: Cell n;
1904: n1 = (Cell) sp[1]; /* input */
1905: n2 = (Cell) TOS;
1906: sp += 1; /* stack adjustment */
1907: NAME("+") /* debugging output (with -DDEBUG) */
1908: @{
1909: n = n1+n2; /* C code taken from the source */
1910: @}
1911: NEXT_P1; /* NEXT part 1 */
1912: TOS = (Cell)n; /* output */
1913: NEXT_P2; /* NEXT part 2 */
1914: @}
1915: @end example
1916:
1917: This looks long and inefficient, but the GNU C compiler optimizes quite
1918: well and produces optimal code for @code{+} on, e.g., the R3000 and the
1919: HP RISC machines: Defining the @code{n}s does not produce any code, and
1920: using them as intermediate storage also adds no cost.
1921:
1922: There are also other optimizations, that are not illustrated by this
1923: example: Assignments between simple variables are usually for free (copy
1924: propagation). If one of the stack items is not used by the primitive
1925: (e.g. in @code{drop}), the compiler eliminates the load from the stack
1926: (dead code elimination). On the other hand, there are some things that
1927: the compiler does not do, therefore they are performed by
1928: @file{prims2x.fs}: The compiler does not optimize code away that stores
1929: a stack item to the place where it just came from (e.g., @code{over}).
1930:
1931: While programming a primitive is usually easy, there are a few cases
1932: where the programmer has to take the actions of the generator into
1933: account, most notably @code{?dup}, but also words that do not (always)
1934: fall through to NEXT.
1935:
1936: @node TOS Optimization, Produced code, Automatic Generation, Primitives
1937: @subsection TOS Optimization
1938:
1939: An important optimization for stack machine emulators, e.g., Forth
1940: engines, is keeping one or more of the top stack items in
1941: registers. If a word has the stack effect @var{in1}...@var{inx} @code{--}
1942: @var{out1}...@var{outy}, keeping the top @var{n} items in registers
1943: @itemize
1944: @item
1945: is better than keeping @var{n-1} items, if @var{x>=n} and @var{y>=n},
1946: due to fewer loads from and stores to the stack.
1947: @item is slower than keeping @var{n-1} items, if @var{x<>y} and @var{x<n} and
1948: @var{y<n}, due to additional moves between registers.
1949: @end itemize
1950:
1951: In particular, keeping one item in a register is never a disadvantage,
1952: if there are enough registers. Keeping two items in registers is a
1953: disadvantage for frequent words like @code{?branch}, constants,
1954: variables, literals and @code{i}. Therefore our generator only produces
1955: code that keeps zero or one items in registers. The generated C code
1956: covers both cases; the selection between these alternatives is made at
1957: C-compile time using the switch @code{-DUSE_TOS}. @code{TOS} in the C
1958: code for @code{+} is just a simple variable name in the one-item case,
1959: otherwise it is a macro that expands into @code{sp[0]}. Note that the
1960: GNU C compiler tries to keep simple variables like @code{TOS} in
1961: registers, and it usually succeeds, if there are enough registers.
1962:
1963: The primitive generator performs the TOS optimization for the
1964: floating-point stack, too (@code{-DUSE_FTOS}). For floating-point
1965: operations the benefit of this optimization is even larger:
1966: floating-point operations take quite long on most processors, but can be
1967: performed in parallel with other operations as long as their results are
1968: not used. If the FP-TOS is kept in a register, this works. If
1969: it is kept on the stack, i.e., in memory, the store into memory has to
1970: wait for the result of the floating-point operation, lengthening the
1971: execution time of the primitive considerably.
1972:
1973: The TOS optimization makes the automatic generation of primitives a
1974: bit more complicated. Just replacing all occurrences of @code{sp[0]} by
1975: @code{TOS} is not sufficient. There are some special cases to
1976: consider:
1977: @itemize
1978: @item In the case of @code{dup ( w -- w w )} the generator must not
1979: eliminate the store to the original location of the item on the stack,
1980: if the TOS optimization is turned on.
1981: @item Primitives with stack effects of the form @code{--}
1982: @var{out1}...@var{outy} must store the TOS to the stack at the start.
1983: Likewise, primitives with the stack effect @var{in1}...@var{inx} @code{--}
1984: must load the TOS from the stack at the end. But for the null stack
1985: effect @code{--} no stores or loads should be generated.
1986: @end itemize
1987:
1988: @node Produced code, , TOS Optimization, Primitives
1989: @subsection Produced code
1990:
1991: To see what assembly code is produced for the primitives on your machine
1992: with your compiler and your flag settings, type @code{make engine.s} and
1993: look at the resulting file @file{engine.s}.
1994:
1995: @node System Architecture, , Primitives, Internals
1996: @section System Architecture
1997:
1998: Our Forth system consists not only of primitives, but also of
1999: definitions written in Forth. Since the Forth compiler itself belongs
2000: to those definitions, it is not possible to start the system with the
2001: primitives and the Forth source alone. Therefore we provide the Forth
2002: code as an image file in nearly executable form. At the start of the
2003: system a C routine loads the image file into memory, sets up the
2004: memory (stacks etc.) according to information in the image file, and
2005: starts executing Forth code.
2006:
2007: The image file format is a compromise between the goals of making it
2008: easy to generate image files and making them portable. The easiest way
2009: to generate an image file is to just generate a memory dump. However,
2010: this kind of image file cannot be used on a different machine, or on
2011: the next version of the engine on the same machine, it even might not
2012: work with the same engine compiled by a different version of the C
2013: compiler. We would like to have as few versions of the image file as
2014: possible, because we do not want to distribute many versions of the
2015: same image file, and to make it easy for the users to use their image
2016: files on many machines. We currently need to create a different image
2017: file for machines with different cell sizes and different byte order
2018: (little- or big-endian)@footnote{We consider adding information to the
2019: image file that enables the loader to change the byte order.}.
2020:
2021: Forth code that is going to end up in a portable image file has to
2022: comply to some restrictions: addresses have to be stored in memory with
2023: special words (@code{A!}, @code{A,}, etc.) in order to make the code
2024: relocatable. Cells, floats, etc., have to be stored at the natural
2025: alignment boundaries@footnote{E.g., store floats (8 bytes) at an address
2026: dividable by~8. This happens automatically in our system when you use
2027: the ANS Forth alignment words.}, in order to avoid alignment faults on
2028: machines with stricter alignment. The image file is produced by a
2029: metacompiler (@file{cross.fs}).
2030:
2031: So, unlike the image file of Mitch Bradleys @code{cforth}, our image
2032: file is not directly executable, but has to undergo some manipulations
2033: during loading. Address relocation is performed at image load-time, not
2034: at run-time. The loader also has to replace tokens standing for
2035: primitive calls with the appropriate code-field addresses (or code
2036: addresses in the case of direct threading).
2037:
2038: @node Bugs, Pedigree, Internals, Top
2039: @chapter Bugs
2040:
2041: @node Pedigree, Word Index, Bugs, Top
2042: @chapter Pedigree
2043:
2044: @node Word Index, Node Index, Pedigree, Top
2045: @chapter Word Index
2046:
2047: @node Node Index, , Word Index, Top
2048: @chapter Node Index
2049:
2050: @contents
2051: @bye
2052:
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