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-maxalign
632: doc-maxaligned
633: doc-cfalign
634: doc-cfaligned
635: doc-address-unit-bits
636:
637: @node Memory block access, , Address arithmetic, Memory access
638: @subsection Memory block access
639:
640: doc-move
641: doc-erase
642:
643: While the previous words work on address units, the rest works on
644: characters.
645:
646: doc-cmove
647: doc-cmove>
648: doc-fill
649: doc-blank
650:
651: @node Control Structures, Locals, Memory access, Words
652: @section Control Structures
653:
654: Control structures in Forth cannot be used in interpret state, only in
655: compile state, i.e., in a colon definition. We do not like this
656: limitation, but have not seen a satisfying way around it yet, although
657: many schemes have been proposed.
658:
659: @menu
660: * Selection::
661: * Simple Loops::
662: * Counted Loops::
663: * Arbitrary control structures::
664: * Calls and returns::
665: * Exception Handling::
666: @end menu
667:
668: @node Selection, Simple Loops, Control Structures, Control Structures
669: @subsection Selection
670:
671: @example
672: @var{flag}
673: IF
674: @var{code}
675: ENDIF
676: @end example
677: or
678: @example
679: @var{flag}
680: IF
681: @var{code1}
682: ELSE
683: @var{code2}
684: ENDIF
685: @end example
686:
687: You can use @code{THEN} instead of @code{ENDIF}. Indeed, @code{THEN} is
688: standard, and @code{ENDIF} is not, although it is quite popular. We
689: recommend using @code{ENDIF}, because it is less confusing for people
690: who also know other languages (and is not prone to reinforcing negative
691: prejudices against Forth in these people). Adding @code{ENDIF} to a
692: system that only supplies @code{THEN} is simple:
693: @example
694: : endif POSTPONE then ; immediate
695: @end example
696:
697: [According to @cite{Webster's New Encyclopedic Dictionary}, @dfn{then
698: (adv.)} has the following meanings:
699: @quotation
700: ... 2b: following next after in order ... 3d: as a necessary consequence
701: (if you were there, then you saw them).
702: @end quotation
703: Forth's @code{THEN} has the meaning 2b, whereas @code{THEN} in Pascal
704: and many other programming languages has the meaning 3d.]
705:
706: We also provide the words @code{?dup-if} and @code{?dup-0=-if}, so you
707: can avoid using @code{?dup}.
708:
709: @example
710: @var{n}
711: CASE
712: @var{n1} OF @var{code1} ENDOF
713: @var{n2} OF @var{code2} ENDOF
714: @dots{}
715: ENDCASE
716: @end example
717:
718: Executes the first @var{codei}, where the @var{ni} is equal to
719: @var{n}. A default case can be added by simply writing the code after
720: the last @code{ENDOF}. It may use @var{n}, which is on top of the stack,
721: but must not consume it.
722:
723: @node Simple Loops, Counted Loops, Selection, Control Structures
724: @subsection Simple Loops
725:
726: @example
727: BEGIN
728: @var{code1}
729: @var{flag}
730: WHILE
731: @var{code2}
732: REPEAT
733: @end example
734:
735: @var{code1} is executed and @var{flag} is computed. If it is true,
736: @var{code2} is executed and the loop is restarted; If @var{flag} is false, execution continues after the @code{REPEAT}.
737:
738: @example
739: BEGIN
740: @var{code}
741: @var{flag}
742: UNTIL
743: @end example
744:
745: @var{code} is executed. The loop is restarted if @code{flag} is false.
746:
747: @example
748: BEGIN
749: @var{code}
750: AGAIN
751: @end example
752:
753: This is an endless loop.
754:
755: @node Counted Loops, Arbitrary control structures, Simple Loops, Control Structures
756: @subsection Counted Loops
757:
758: The basic counted loop is:
759: @example
760: @var{limit} @var{start}
761: ?DO
762: @var{body}
763: LOOP
764: @end example
765:
766: This performs one iteration for every integer, starting from @var{start}
767: and up to, but excluding @var{limit}. The counter, aka index, can be
768: accessed with @code{i}. E.g., the loop
769: @example
770: 10 0 ?DO
771: i .
772: LOOP
773: @end example
774: prints
775: @example
776: 0 1 2 3 4 5 6 7 8 9
777: @end example
778: The index of the innermost loop can be accessed with @code{i}, the index
779: of the next loop with @code{j}, and the index of the third loop with
780: @code{k}.
781:
782: The loop control data are kept on the return stack, so there are some
783: restrictions on mixing return stack accesses and counted loop
784: words. E.g., if you put values on the return stack outside the loop, you
785: cannot read them inside the loop. If you put values on the return stack
786: within a loop, you have to remove them before the end of the loop and
787: before accessing the index of the loop.
788:
789: There are several variations on the counted loop:
790:
791: @code{LEAVE} leaves the innermost counted loop immediately.
792:
793: @code{LOOP} can be replaced with @code{@var{n} +LOOP}; this updates the
794: index by @var{n} instead of by 1. The loop is terminated when the border
795: between @var{limit-1} and @var{limit} is crossed. E.g.:
796:
797: @code{4 0 ?DO i . 2 +LOOP} prints @code{0 2}
798:
799: @code{4 1 ?DO i . 2 +LOOP} prints @code{1 3}
800:
801: The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative:
802:
803: @code{-1 0 ?DO i . -1 +LOOP} prints @code{0 -1}
804:
805: @code{ 0 0 ?DO i . -1 +LOOP} prints nothing
806:
807: Therefore we recommend avoiding using @code{@var{n} +LOOP} with negative
808: @var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative
809: case behaves symmetrical to the positive case:
810:
811: @code{-2 0 ?DO i . -1 S+LOOP} prints @code{0 -1}
812:
813: @code{-1 0 ?DO i . -1 S+LOOP} prints @code{0}
814:
815: @code{ 0 0 ?DO i . -1 S+LOOP} prints nothing
816:
817: The loop is terminated when the border between @var{limit@minus{}sgn(n)} and
818: @var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS
819: Forth standard.
820:
821: @code{?DO} can be replaced by @code{DO}. @code{DO} enters the loop even
822: when the start and the limit value are equal. We do not recommend using
823: @code{DO}. It will just give you maintenance troubles.
824:
825: @code{UNLOOP} is used to prepare for an abnormal loop exit, e.g., via
826: @code{EXIT}. @code{UNLOOP} removes the loop control parameters from the
827: return stack so @code{EXIT} can get to its return address.
828:
829: Another counted loop is
830: @example
831: @var{n}
832: FOR
833: @var{body}
834: NEXT
835: @end example
836: This is the preferred loop of native code compiler writers who are too
837: lazy to optimize @code{?DO} loops properly. In GNU Forth, this loop
838: iterates @var{n+1} times; @code{i} produces values starting with @var{n}
839: and ending with 0. Other Forth systems may behave differently, even if
840: they support @code{FOR} loops.
841:
842: @node Arbitrary control structures, Calls and returns, Counted Loops, Control Structures
843: @subsection Arbitrary control structures
844:
845: ANS Forth permits and supports using control structures in a non-nested
846: way. Information about incomplete control structures is stored on the
847: control-flow stack. This stack may be implemented on the Forth data
848: stack, and this is what we have done in gforth.
849:
850: An @i{orig} entry represents an unresolved forward branch, a @i{dest}
851: entry represents a backward branch target. A few words are the basis for
852: building any control structure possible (except control structures that
853: need storage, like calls, coroutines, and backtracking).
854:
855: doc-if
856: doc-ahead
857: doc-then
858: doc-begin
859: doc-until
860: doc-again
861: doc-cs-pick
862: doc-cs-roll
863:
864: On many systems control-flow stack items take one word, in gforth they
865: currently take three (this may change in the future). Therefore it is a
866: really good idea to manipulate the control flow stack with
867: @code{cs-pick} and @code{cs-roll}, not with data stack manipulation
868: words.
869:
870: Some standard control structure words are built from these words:
871:
872: doc-else
873: doc-while
874: doc-repeat
875:
876: Counted loop words constitute a separate group of words:
877:
878: doc-?do
879: doc-do
880: doc-for
881: doc-loop
882: doc-s+loop
883: doc-+loop
884: doc-next
885: doc-leave
886: doc-?leave
887: doc-unloop
888: doc-done
889:
890: The standard does not allow using @code{cs-pick} and @code{cs-roll} on
891: @i{do-sys}. Our system allows it, but it's your job to ensure that for
892: every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path
893: through the definition (@code{LOOP} etc. compile an @code{UNLOOP} on the
894: fall-through path). Also, you have to ensure that all @code{LEAVE}s are
895: resolved (by using one of the loop-ending words or @code{DONE}).
896:
897: Another group of control structure words are
898:
899: doc-case
900: doc-endcase
901: doc-of
902: doc-endof
903:
904: @i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and
905: @code{cs-roll}.
906:
907: @subsubsection Programming Style
908:
909: In order to ensure readability we recommend that you do not create
910: arbitrary control structures directly, but define new control structure
911: words for the control structure you want and use these words in your
912: program.
913:
914: E.g., instead of writing
915:
916: @example
917: begin
918: ...
919: if [ 1 cs-roll ]
920: ...
921: again then
922: @end example
923:
924: we recommend defining control structure words, e.g.,
925:
926: @example
927: : while ( dest -- orig dest )
928: POSTPONE if
929: 1 cs-roll ; immediate
930:
931: : repeat ( orig dest -- )
932: POSTPONE again
933: POSTPONE then ; immediate
934: @end example
935:
936: and then using these to create the control structure:
937:
938: @example
939: begin
940: ...
941: while
942: ...
943: repeat
944: @end example
945:
946: That's much easier to read, isn't it? Of course, @code{BEGIN} and
947: @code{WHILE} are predefined, so in this example it would not be
948: necessary to define them.
949:
950: @node Calls and returns, Exception Handling, Arbitrary control structures, Control Structures
951: @subsection Calls and returns
952:
953: A definition can be called simply be writing the name of the
954: definition. When the end of the definition is reached, it returns. An earlier return can be forced using
955:
956: doc-exit
957:
958: Don't forget to clean up the return stack and @code{UNLOOP} any
959: outstanding @code{?DO}...@code{LOOP}s before @code{EXIT}ing. The
960: primitive compiled by @code{EXIT} is
961:
962: doc-;s
963:
964: @node Exception Handling, , Calls and returns, Control Structures
965: @subsection Exception Handling
966:
967: doc-catch
968: doc-throw
969:
970: @node Locals, Defining Words, Control Structures, Words
971: @section Locals
972:
973: Local variables can make Forth programming more enjoyable and Forth
974: programs easier to read. Unfortunately, the locals of ANS Forth are
975: laden with restrictions. Therefore, we provide not only the ANS Forth
976: locals wordset, but also our own, more powerful locals wordset (we
977: implemented the ANS Forth locals wordset through our locals wordset).
978:
979: @menu
980: * gforth locals::
981: * ANS Forth locals::
982: @end menu
983:
984: @node gforth locals, ANS Forth locals, Locals, Locals
985: @subsection gforth locals
986:
987: Locals can be defined with
988:
989: @example
990: @{ local1 local2 ... -- comment @}
991: @end example
992: or
993: @example
994: @{ local1 local2 ... @}
995: @end example
996:
997: E.g.,
998: @example
999: : max @{ n1 n2 -- n3 @}
1000: n1 n2 > if
1001: n1
1002: else
1003: n2
1004: endif ;
1005: @end example
1006:
1007: The similarity of locals definitions with stack comments is intended. A
1008: locals definition often replaces the stack comment of a word. The order
1009: of the locals corresponds to the order in a stack comment and everything
1010: after the @code{--} is really a comment.
1011:
1012: This similarity has one disadvantage: It is too easy to confuse locals
1013: declarations with stack comments, causing bugs and making them hard to
1014: find. However, this problem can be avoided by appropriate coding
1015: conventions: Do not use both notations in the same program. If you do,
1016: they should be distinguished using additional means, e.g. by position.
1017:
1018: The name of the local may be preceded by a type specifier, e.g.,
1019: @code{F:} for a floating point value:
1020:
1021: @example
1022: : CX* @{ F: Ar F: Ai F: Br F: Bi -- Cr Ci @}
1023: \ complex multiplication
1024: Ar Br f* Ai Bi f* f-
1025: Ar Bi f* Ai Br f* f+ ;
1026: @end example
1027:
1028: GNU Forth currently supports cells (@code{W:}, @code{W^}), doubles
1029: (@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters
1030: (@code{C:}, @code{C^}) in two flavours: a value-flavoured local (defined
1031: with @code{W:}, @code{D:} etc.) produces its value and can be changed
1032: with @code{TO}. A variable-flavoured local (defined with @code{W^} etc.)
1033: produces its address (which becomes invalid when the variable's scope is
1034: left). E.g., the standard word @code{emit} can be defined in therms of
1035: @code{type} like this:
1036:
1037: @example
1038: : emit @{ C^ char* -- @}
1039: char* 1 type ;
1040: @end example
1041:
1042: A local without type specifier is a @code{W:} local. Both flavours of
1043: locals are initialized with values from the data or FP stack.
1044:
1045: Currently there is no way to define locals with user-defined data
1046: structures, but we are working on it.
1047:
1048: GNU Forth allows defining locals everywhere in a colon definition. This
1049: poses the following questions:
1050:
1051: @menu
1052: * Where are locals visible by name?::
1053: * How long do locals live? ::
1054: * Programming Style::
1055: * Implementation::
1056: @end menu
1057:
1058: @node Where are locals visible by name?, How long do locals live?, gforth locals, gforth locals
1059: @subsubsection Where are locals visible by name?
1060:
1061: Basically, the answer is that locals are visible where you would expect
1062: it in block-structured languages, and sometimes a little longer. If you
1063: want to restrict the scope of a local, enclose its definition in
1064: @code{SCOPE}...@code{ENDSCOPE}.
1065:
1066: doc-scope
1067: doc-endscope
1068:
1069: These words behave like control structure words, so you can use them
1070: with @code{CS-PICK} and @code{CS-ROLL} to restrict the scope in
1071: arbitrary ways.
1072:
1073: If you want a more exact answer to the visibility question, here's the
1074: basic principle: A local is visible in all places that can only be
1075: reached through the definition of the local@footnote{In compiler
1076: construction terminology, all places dominated by the definition of the
1077: local.}. In other words, it is not visible in places that can be reached
1078: without going through the definition of the local. E.g., locals defined
1079: in @code{IF}...@code{ENDIF} are visible until the @code{ENDIF}, locals
1080: defined in @code{BEGIN}...@code{UNTIL} are visible after the
1081: @code{UNTIL} (until, e.g., a subsequent @code{ENDSCOPE}).
1082:
1083: The reasoning behind this solution is: We want to have the locals
1084: visible as long as it is meaningful. The user can always make the
1085: visibility shorter by using explicit scoping. In a place that can
1086: only be reached through the definition of a local, the meaning of a
1087: local name is clear. In other places it is not: How is the local
1088: initialized at the control flow path that does not contain the
1089: definition? Which local is meant, if the same name is defined twice in
1090: two independent control flow paths?
1091:
1092: This should be enough detail for nearly all users, so you can skip the
1093: rest of this section. If you relly must know all the gory details and
1094: options, read on.
1095:
1096: In order to implement this rule, the compiler has to know which places
1097: are unreachable. It knows this automatically after @code{AHEAD},
1098: @code{AGAIN}, @code{EXIT} and @code{LEAVE}; in other cases (e.g., after
1099: most @code{THROW}s), you can use the word @code{UNREACHABLE} to tell the
1100: compiler that the control flow never reaches that place. If
1101: @code{UNREACHABLE} is not used where it could, the only consequence is
1102: that the visibility of some locals is more limited than the rule above
1103: says. If @code{UNREACHABLE} is used where it should not (i.e., if you
1104: lie to the compiler), buggy code will be produced.
1105:
1106: Another problem with this rule is that at @code{BEGIN}, the compiler
1107: does not know which locals will be visible on the incoming
1108: back-edge. All problems discussed in the following are due to this
1109: ignorance of the compiler (we discuss the problems using @code{BEGIN}
1110: loops as examples; the discussion also applies to @code{?DO} and other
1111: loops). Perhaps the most insidious example is:
1112: @example
1113: AHEAD
1114: BEGIN
1115: x
1116: [ 1 CS-ROLL ] THEN
1117: @{ x @}
1118: ...
1119: UNTIL
1120: @end example
1121:
1122: This should be legal according to the visibility rule. The use of
1123: @code{x} can only be reached through the definition; but that appears
1124: textually below the use.
1125:
1126: From this example it is clear that the visibility rules cannot be fully
1127: implemented without major headaches. Our implementation treats common
1128: cases as advertised and the exceptions are treated in a safe way: The
1129: compiler makes a reasonable guess about the locals visible after a
1130: @code{BEGIN}; if it is too pessimistic, the
1131: user will get a spurious error about the local not being defined; if the
1132: compiler is too optimistic, it will notice this later and issue a
1133: warning. In the case above the compiler would complain about @code{x}
1134: being undefined at its use. You can see from the obscure examples in
1135: this section that it takes quite unusual control structures to get the
1136: compiler into trouble, and even then it will often do fine.
1137:
1138: If the @code{BEGIN} is reachable from above, the most optimistic guess
1139: is that all locals visible before the @code{BEGIN} will also be
1140: visible after the @code{BEGIN}. This guess is valid for all loops that
1141: are entered only through the @code{BEGIN}, in particular, for normal
1142: @code{BEGIN}...@code{WHILE}...@code{REPEAT} and
1143: @code{BEGIN}...@code{UNTIL} loops and it is implemented in our
1144: compiler. When the branch to the @code{BEGIN} is finally generated by
1145: @code{AGAIN} or @code{UNTIL}, the compiler checks the guess and
1146: warns the user if it was too optimisitic:
1147: @example
1148: IF
1149: @{ x @}
1150: BEGIN
1151: \ x ?
1152: [ 1 cs-roll ] THEN
1153: ...
1154: UNTIL
1155: @end example
1156:
1157: Here, @code{x} lives only until the @code{BEGIN}, but the compiler
1158: optimistically assumes that it lives until the @code{THEN}. It notices
1159: this difference when it compiles the @code{UNTIL} and issues a
1160: warning. The user can avoid the warning, and make sure that @code{x}
1161: is not used in the wrong area by using explicit scoping:
1162: @example
1163: IF
1164: SCOPE
1165: @{ x @}
1166: ENDSCOPE
1167: BEGIN
1168: [ 1 cs-roll ] THEN
1169: ...
1170: UNTIL
1171: @end example
1172:
1173: Since the guess is optimistic, there will be no spurious error messages
1174: about undefined locals.
1175:
1176: If the @code{BEGIN} is not reachable from above (e.g., after
1177: @code{AHEAD} or @code{EXIT}), the compiler cannot even make an
1178: optimistic guess, as the locals visible after the @code{BEGIN} may be
1179: defined later. Therefore, the compiler assumes that no locals are
1180: visible after the @code{BEGIN}. However, the useer can use
1181: @code{ASSUME-LIVE} to make the compiler assume that the same locals are
1182: visible at the BEGIN as at the point where the item was created.
1183:
1184: doc-assume-live
1185:
1186: E.g.,
1187: @example
1188: @{ x @}
1189: AHEAD
1190: ASSUME-LIVE
1191: BEGIN
1192: x
1193: [ 1 CS-ROLL ] THEN
1194: ...
1195: UNTIL
1196: @end example
1197:
1198: Other cases where the locals are defined before the @code{BEGIN} can be
1199: handled by inserting an appropriate @code{CS-ROLL} before the
1200: @code{ASSUME-LIVE} (and changing the control-flow stack manipulation
1201: behind the @code{ASSUME-LIVE}).
1202:
1203: Cases where locals are defined after the @code{BEGIN} (but should be
1204: visible immediately after the @code{BEGIN}) can only be handled by
1205: rearranging the loop. E.g., the ``most insidious'' example above can be
1206: arranged into:
1207: @example
1208: BEGIN
1209: @{ x @}
1210: ... 0=
1211: WHILE
1212: x
1213: REPEAT
1214: @end example
1215:
1216: @node How long do locals live?, Programming Style, Where are locals visible by name?, gforth locals
1217: @subsubsection How long do locals live?
1218:
1219: The right answer for the lifetime question would be: A local lives at
1220: least as long as it can be accessed. For a value-flavoured local this
1221: means: until the end of its visibility. However, a variable-flavoured
1222: local could be accessed through its address far beyond its visibility
1223: scope. Ultimately, this would mean that such locals would have to be
1224: garbage collected. Since this entails un-Forth-like implementation
1225: complexities, I adopted the same cowardly solution as some other
1226: languages (e.g., C): The local lives only as long as it is visible;
1227: afterwards its address is invalid (and programs that access it
1228: afterwards are erroneous).
1229:
1230: @node Programming Style, Implementation, How long do locals live?, gforth locals
1231: @subsubsection Programming Style
1232:
1233: The freedom to define locals anywhere has the potential to change
1234: programming styles dramatically. In particular, the need to use the
1235: return stack for intermediate storage vanishes. Moreover, all stack
1236: manipulations (except @code{PICK}s and @code{ROLL}s with run-time
1237: determined arguments) can be eliminated: If the stack items are in the
1238: wrong order, just write a locals definition for all of them; then
1239: write the items in the order you want.
1240:
1241: This seems a little far-fetched and eliminating stack manipulations is
1242: unlikely to become a conscious programming objective. Still, the number
1243: of stack manipulations will be reduced dramatically if local variables
1244: are used liberally (e.g., compare @code{max} in @ref{gforth locals} with
1245: a traditional implementation of @code{max}).
1246:
1247: This shows one potential benefit of locals: making Forth programs more
1248: readable. Of course, this benefit will only be realized if the
1249: programmers continue to honour the principle of factoring instead of
1250: using the added latitude to make the words longer.
1251:
1252: Using @code{TO} can and should be avoided. Without @code{TO},
1253: every value-flavoured local has only a single assignment and many
1254: advantages of functional languages apply to Forth. I.e., programs are
1255: easier to analyse, to optimize and to read: It is clear from the
1256: definition what the local stands for, it does not turn into something
1257: different later.
1258:
1259: E.g., a definition using @code{TO} might look like this:
1260: @example
1261: : strcmp @{ addr1 u1 addr2 u2 -- n @}
1262: u1 u2 min 0
1263: ?do
1264: addr1 c@ addr2 c@ - ?dup
1265: if
1266: unloop exit
1267: then
1268: addr1 char+ TO addr1
1269: addr2 char+ TO addr2
1270: loop
1271: u1 u2 - ;
1272: @end example
1273: Here, @code{TO} is used to update @code{addr1} and @code{addr2} at
1274: every loop iteration. @code{strcmp} is a typical example of the
1275: readability problems of using @code{TO}. When you start reading
1276: @code{strcmp}, you think that @code{addr1} refers to the start of the
1277: string. Only near the end of the loop you realize that it is something
1278: else.
1279:
1280: This can be avoided by defining two locals at the start of the loop that
1281: are initialized with the right value for the current iteration.
1282: @example
1283: : strcmp @{ addr1 u1 addr2 u2 -- n @}
1284: addr1 addr2
1285: u1 u2 min 0
1286: ?do @{ s1 s2 @}
1287: s1 c@ s2 c@ - ?dup
1288: if
1289: unloop exit
1290: then
1291: s1 char+ s2 char+
1292: loop
1293: 2drop
1294: u1 u2 - ;
1295: @end example
1296: Here it is clear from the start that @code{s1} has a different value
1297: in every loop iteration.
1298:
1299: @node Implementation, , Programming Style, gforth locals
1300: @subsubsection Implementation
1301:
1302: GNU Forth uses an extra locals stack. The most compelling reason for
1303: this is that the return stack is not float-aligned; using an extra stack
1304: also eliminates the problems and restrictions of using the return stack
1305: as locals stack. Like the other stacks, the locals stack grows toward
1306: lower addresses. A few primitives allow an efficient implementation:
1307:
1308: doc-@local#
1309: doc-f@local#
1310: doc-laddr#
1311: doc-lp+!#
1312: doc-lp!
1313: doc->l
1314: doc-f>l
1315:
1316: In addition to these primitives, some specializations of these
1317: primitives for commonly occurring inline arguments are provided for
1318: efficiency reasons, e.g., @code{@@local0} as specialization of
1319: @code{@@local#} for the inline argument 0. The following compiling words
1320: compile the right specialized version, or the general version, as
1321: appropriate:
1322:
1323: doc-compile-@local
1324: doc-compile-f@local
1325: doc-compile-lp+!
1326:
1327: Combinations of conditional branches and @code{lp+!#} like
1328: @code{?branch-lp+!#} (the locals pointer is only changed if the branch
1329: is taken) are provided for efficiency and correctness in loops.
1330:
1331: A special area in the dictionary space is reserved for keeping the
1332: local variable names. @code{@{} switches the dictionary pointer to this
1333: area and @code{@}} switches it back and generates the locals
1334: initializing code. @code{W:} etc.@ are normal defining words. This
1335: special area is cleared at the start of every colon definition.
1336:
1337: A special feature of GNU Forths dictionary is used to implement the
1338: definition of locals without type specifiers: every wordlist (aka
1339: vocabulary) has its own methods for searching
1340: etc. (@pxref{Wordlists}). For the present purpose we defined a wordlist
1341: with a special search method: When it is searched for a word, it
1342: actually creates that word using @code{W:}. @code{@{} changes the search
1343: order to first search the wordlist containing @code{@}}, @code{W:} etc.,
1344: and then the wordlist for defining locals without type specifiers.
1345:
1346: The lifetime rules support a stack discipline within a colon
1347: definition: The lifetime of a local is either nested with other locals
1348: lifetimes or it does not overlap them.
1349:
1350: At @code{BEGIN}, @code{IF}, and @code{AHEAD} no code for locals stack
1351: pointer manipulation is generated. Between control structure words
1352: locals definitions can push locals onto the locals stack. @code{AGAIN}
1353: is the simplest of the other three control flow words. It has to
1354: restore the locals stack depth of the corresponding @code{BEGIN}
1355: before branching. The code looks like this:
1356: @format
1357: @code{lp+!#} current-locals-size @minus{} dest-locals-size
1358: @code{branch} <begin>
1359: @end format
1360:
1361: @code{UNTIL} is a little more complicated: If it branches back, it
1362: must adjust the stack just like @code{AGAIN}. But if it falls through,
1363: the locals stack must not be changed. The compiler generates the
1364: following code:
1365: @format
1366: @code{?branch-lp+!#} <begin> current-locals-size @minus{} dest-locals-size
1367: @end format
1368: The locals stack pointer is only adjusted if the branch is taken.
1369:
1370: @code{THEN} can produce somewhat inefficient code:
1371: @format
1372: @code{lp+!#} current-locals-size @minus{} orig-locals-size
1373: <orig target>:
1374: @code{lp+!#} orig-locals-size @minus{} new-locals-size
1375: @end format
1376: The second @code{lp+!#} adjusts the locals stack pointer from the
1377: level at the @var{orig} point to the level after the @code{THEN}. The
1378: first @code{lp+!#} adjusts the locals stack pointer from the current
1379: level to the level at the orig point, so the complete effect is an
1380: adjustment from the current level to the right level after the
1381: @code{THEN}.
1382:
1383: In a conventional Forth implementation a dest control-flow stack entry
1384: is just the target address and an orig entry is just the address to be
1385: patched. Our locals implementation adds a wordlist to every orig or dest
1386: item. It is the list of locals visible (or assumed visible) at the point
1387: described by the entry. Our implementation also adds a tag to identify
1388: the kind of entry, in particular to differentiate between live and dead
1389: (reachable and unreachable) orig entries.
1390:
1391: A few unusual operations have to be performed on locals wordlists:
1392:
1393: doc-common-list
1394: doc-sub-list?
1395: doc-list-size
1396:
1397: Several features of our locals wordlist implementation make these
1398: operations easy to implement: The locals wordlists are organised as
1399: linked lists; the tails of these lists are shared, if the lists
1400: contain some of the same locals; and the address of a name is greater
1401: than the address of the names behind it in the list.
1402:
1403: Another important implementation detail is the variable
1404: @code{dead-code}. It is used by @code{BEGIN} and @code{THEN} to
1405: determine if they can be reached directly or only through the branch
1406: that they resolve. @code{dead-code} is set by @code{UNREACHABLE},
1407: @code{AHEAD}, @code{EXIT} etc., and cleared at the start of a colon
1408: definition, by @code{BEGIN} and usually by @code{THEN}.
1409:
1410: Counted loops are similar to other loops in most respects, but
1411: @code{LEAVE} requires special attention: It performs basically the same
1412: service as @code{AHEAD}, but it does not create a control-flow stack
1413: entry. Therefore the information has to be stored elsewhere;
1414: traditionally, the information was stored in the target fields of the
1415: branches created by the @code{LEAVE}s, by organizing these fields into a
1416: linked list. Unfortunately, this clever trick does not provide enough
1417: space for storing our extended control flow information. Therefore, we
1418: introduce another stack, the leave stack. It contains the control-flow
1419: stack entries for all unresolved @code{LEAVE}s.
1420:
1421: Local names are kept until the end of the colon definition, even if
1422: they are no longer visible in any control-flow path. In a few cases
1423: this may lead to increased space needs for the locals name area, but
1424: usually less than reclaiming this space would cost in code size.
1425:
1426:
1427: @node ANS Forth locals, , gforth locals, Locals
1428: @subsection ANS Forth locals
1429:
1430: The ANS Forth locals wordset does not define a syntax for locals, but
1431: words that make it possible to define various syntaxes. One of the
1432: possible syntaxes is a subset of the syntax we used in the gforth locals
1433: wordset, i.e.:
1434:
1435: @example
1436: @{ local1 local2 ... -- comment @}
1437: @end example
1438: or
1439: @example
1440: @{ local1 local2 ... @}
1441: @end example
1442:
1443: The order of the locals corresponds to the order in a stack comment. The
1444: restrictions are:
1445:
1446: @itemize @bullet
1447: @item
1448: Locals can only be cell-sized values (no type specifers are allowed).
1449: @item
1450: Locals can be defined only outside control structures.
1451: @item
1452: Locals can interfere with explicit usage of the return stack. For the
1453: exact (and long) rules, see the standard. If you don't use return stack
1454: accessing words in a definition using locals, you will we all right. The
1455: purpose of this rule is to make locals implementation on the return
1456: stack easier.
1457: @item
1458: The whole definition must be in one line.
1459: @end itemize
1460:
1461: Locals defined in this way behave like @code{VALUE}s
1462: (@xref{Values}). I.e., they are initialized from the stack. Using their
1463: name produces their value. Their value can be changed using @code{TO}.
1464:
1465: Since this syntax is supported by gforth directly, you need not do
1466: anything to use it. If you want to port a program using this syntax to
1467: another ANS Forth system, use @file{anslocal.fs} to implement the syntax
1468: on the other system.
1469:
1470: Note that a syntax shown in the standard, section A.13 looks
1471: similar, but is quite different in having the order of locals
1472: reversed. Beware!
1473:
1474: The ANS Forth locals wordset itself consists of the following word
1475:
1476: doc-(local)
1477:
1478: The ANS Forth locals extension wordset defines a syntax, but it is so
1479: awful that we strongly recommend not to use it. We have implemented this
1480: syntax to make porting to gforth easy, but do not document it here. The
1481: problem with this syntax is that the locals are defined in an order
1482: reversed with respect to the standard stack comment notation, making
1483: programs harder to read, and easier to misread and miswrite. The only
1484: merit of this syntax is that it is easy to implement using the ANS Forth
1485: locals wordset.
1486:
1487: @node Defining Words, Wordlists, Locals, Words
1488: @section Defining Words
1489:
1490: @node Values, , Defining Words, Defining Words
1491: @subsection Values
1492:
1493: @node Wordlists, Files, Defining Words, Words
1494: @section Wordlists
1495:
1496: @node Files, Blocks, Wordlists, Words
1497: @section Files
1498:
1499: @node Blocks, Other I/O, Files, Words
1500: @section Blocks
1501:
1502: @node Other I/O, Programming Tools, Blocks, Words
1503: @section Other I/O
1504:
1505: @node Programming Tools, Threading Words, Other I/O, Words
1506: @section Programming Tools
1507:
1508: @menu
1509: * Debugging:: Simple and quick.
1510: * Assertions:: Making your programs self-checking.
1511: @end menu
1512:
1513: @node Debugging, Assertions, Programming Tools, Programming Tools
1514: @subsection Debugging
1515:
1516: The simple debugging aids provided in @file{debugging.fs}
1517: are meant to support a different style of debugging than the
1518: tracing/stepping debuggers used in languages with long turn-around
1519: times.
1520:
1521: A much better (faster) way in fast-compilig languages is to add
1522: printing code at well-selected places, let the program run, look at
1523: the output, see where things went wrong, add more printing code, etc.,
1524: until the bug is found.
1525:
1526: The word @code{~~} is easy to insert. It just prints debugging
1527: information (by default the source location and the stack contents). It
1528: is also easy to remove (@kbd{C-x ~} in the Emacs Forth mode to
1529: query-replace them with nothing). The deferred words
1530: @code{printdebugdata} and @code{printdebugline} control the output of
1531: @code{~~}. The default source location output format works well with
1532: Emacs' compilation mode, so you can step through the program at the
1533: source level using @kbd{C-x `} (the advantage over a stepping debugger
1534: is that you can step in any direction and you know where the crash has
1535: happened or where the strange data has occurred).
1536:
1537: Note that the default actions clobber the contents of the pictured
1538: numeric output string, so you should not use @code{~~}, e.g., between
1539: @code{<#} and @code{#>}.
1540:
1541: doc-~~
1542: doc-printdebugdata
1543: doc-printdebugline
1544:
1545: @node Assertions, , Debugging, Programming Tools
1546: @subsection Assertions
1547:
1548: It is a good idea to make your programs self-checking, in particular, if
1549: you use an assumption (e.g., that a certain field of a data structure is
1550: never zero) that may become wrong during maintenance. GForth supports
1551: assertions for this purpose. They are used like this:
1552:
1553: @example
1554: assert( @var{flag} )
1555: @end example
1556:
1557: The code between @code{assert(} and @code{)} should compute a flag, that
1558: should be true if everything is alright and false otherwise. It should
1559: not change anything else on the stack. The overall stack effect of the
1560: assertion is @code{( -- )}. E.g.
1561:
1562: @example
1563: assert( 1 1 + 2 = ) \ what we learn in school
1564: assert( dup 0<> ) \ assert that the top of stack is not zero
1565: assert( false ) \ this code should not be reached
1566: @end example
1567:
1568: The need for assertions is different at different times. During
1569: debugging, we want more checking, in production we sometimes care more
1570: for speed. Therefore, assertions can be turned off, i.e., the assertion
1571: becomes a comment. Depending on the importance of an assertion and the
1572: time it takes to check it, you may want to turn off some assertions and
1573: keep others turned on. GForth provides several levels of assertions for
1574: this purpose:
1575:
1576: doc-assert0(
1577: doc-assert1(
1578: doc-assert2(
1579: doc-assert3(
1580: doc-assert(
1581: doc-)
1582:
1583: @code{Assert(} is the same as @code{assert1(}. The variable
1584: @code{assert-level} specifies the highest assertions that are turned
1585: on. I.e., at the default @code{assert-level} of one, @code{assert0(} and
1586: @code{assert1(} assertions perform checking, while @code{assert2(} and
1587: @code{assert3(} assertions are treated as comments.
1588:
1589: Note that the @code{assert-level} is evaluated at compile-time, not at
1590: run-time. I.e., you cannot turn assertions on or off at run-time, you
1591: have to set the @code{assert-level} appropriately before compiling a
1592: piece of code. You can compile several pieces of code at several
1593: @code{assert-level}s (e.g., a trusted library at level 1 and newly
1594: written code at level 3).
1595:
1596: doc-assert-level
1597:
1598: If an assertion fails, a message compatible with Emacs' compilation mode
1599: is produced and the execution is aborted (currently with @code{ABORT"}.
1600: If there is interest, we will introduce a special throw code. But if you
1601: intend to @code{catch} a specific condition, using @code{throw} is
1602: probably more appropriate than an assertion).
1603:
1604: @node Threading Words, , Programming Tools, Words
1605: @section Threading Words
1606:
1607: These words provide access to code addresses and other threading stuff
1608: in gforth (and, possibly, other interpretive Forths). It more or less
1609: abstracts away the differences between direct and indirect threading
1610: (and, for direct threading, the machine dependences). However, at
1611: present this wordset is still inclomplete. It is also pretty low-level;
1612: some day it will hopefully be made unnecessary by an internals words set
1613: that abstracts implementation details away completely.
1614:
1615: doc->code-address
1616: doc->does-code
1617: doc-code-address!
1618: doc-does-code!
1619: doc-does-handler!
1620: doc-/does-handler
1621:
1622: @node ANS conformance, Model, Words, Top
1623: @chapter ANS conformance
1624:
1625:
1626:
1627:
1628: @node Model, Emacs and GForth, ANS conformance, Top
1629: @chapter Model
1630:
1631: @node Emacs and GForth, Internals, Model, Top
1632: @chapter Emacs and GForth
1633:
1634: GForth comes with @file{gforth.el}, an improved version of
1635: @file{forth.el} by Goran Rydqvist (icluded in the TILE package). The
1636: improvements are a better (but still not perfect) handling of
1637: indentation. I have also added comment paragraph filling (@kbd{M-q}),
1638: commenting (@kbd{C-x \}) and uncommenting (@kbd{C-u C-x \}) regions and
1639: removing debugging tracers (@kbd{C-x ~}, @pxref{Debugging}). I left the
1640: stuff I do not use alone, even though some of it only makes sense for
1641: TILE. To get a description of these features, enter Forth mode and type
1642: @kbd{C-h m}.
1643:
1644: In addition, GForth supports Emacs quite well: The source code locations
1645: given in error messages, debugging output (from @code{~~}) and failed
1646: assertion messages are in the right format for Emacs' compilation mode
1647: (@pxref{Compilation, , Running Compilations under Emacs, emacs, Emacs
1648: Manual}) so the source location corresponding to an error or other
1649: message is only a few keystrokes away (@kbd{C-x `} for the next error,
1650: @kbd{C-c C-c} for the error under the cursor).
1651:
1652: Also, if you @code{include} @file{etags.fs}, a new @file{TAGS} file
1653: (@pxref{Tags, , Tags Tables, emacs, Emacs Manual}) will be produced that
1654: contains the definitions of all words defined afterwards. You can then
1655: find the source for a word using @kbd{M-.}. Note that emacs can use
1656: several tags files at the same time (e.g., one for the gforth sources
1657: and one for your program).
1658:
1659: To get all these benefits, add the following lines to your @file{.emacs}
1660: file:
1661:
1662: @example
1663: (autoload 'forth-mode "gforth.el")
1664: (setq auto-mode-alist (cons '("\\.fs\\'" . forth-mode) auto-mode-alist))
1665: @end example
1666:
1667: @node Internals, Bugs, Emacs and GForth, Top
1668: @chapter Internals
1669:
1670: Reading this section is not necessary for programming with gforth. It
1671: should be helpful for finding your way in the gforth sources.
1672:
1673: @menu
1674: * Portability::
1675: * Threading::
1676: * Primitives::
1677: * System Architecture::
1678: @end menu
1679:
1680: @node Portability, Threading, Internals, Internals
1681: @section Portability
1682:
1683: One of the main goals of the effort is availability across a wide range
1684: of personal machines. fig-Forth, and, to a lesser extent, F83, achieved
1685: this goal by manually coding the engine in assembly language for several
1686: then-popular processors. This approach is very labor-intensive and the
1687: results are short-lived due to progress in computer architecture.
1688:
1689: Others have avoided this problem by coding in C, e.g., Mitch Bradley
1690: (cforth), Mikael Patel (TILE) and Dirk Zoller (pfe). This approach is
1691: particularly popular for UNIX-based Forths due to the large variety of
1692: architectures of UNIX machines. Unfortunately an implementation in C
1693: does not mix well with the goals of efficiency and with using
1694: traditional techniques: Indirect or direct threading cannot be expressed
1695: in C, and switch threading, the fastest technique available in C, is
1696: significantly slower. Another problem with C is that it's very
1697: cumbersome to express double integer arithmetic.
1698:
1699: Fortunately, there is a portable language that does not have these
1700: limitations: GNU C, the version of C processed by the GNU C compiler
1701: (@pxref{C Extensions, , Extensions to the C Language Family, gcc.info,
1702: GNU C Manual}). Its labels as values feature (@pxref{Labels as Values, ,
1703: Labels as Values, gcc.info, GNU C Manual}) makes direct and indirect
1704: threading possible, its @code{long long} type (@pxref{Long Long, ,
1705: Double-Word Integers, gcc.info, GNU C Manual}) corresponds to Forths
1706: double numbers. GNU C is available for free on all important (and many
1707: unimportant) UNIX machines, VMS, 80386s running MS-DOS, the Amiga, and
1708: the Atari ST, so a Forth written in GNU C can run on all these
1709: machines@footnote{Due to Apple's look-and-feel lawsuit it is not
1710: available on the Mac (@pxref{Boycott, , Protect Your Freedom---Fight
1711: ``Look And Feel'', gcc.info, GNU C Manual}).}.
1712:
1713: Writing in a portable language has the reputation of producing code that
1714: is slower than assembly. For our Forth engine we repeatedly looked at
1715: the code produced by the compiler and eliminated most compiler-induced
1716: inefficiencies by appropriate changes in the source-code.
1717:
1718: However, register allocation cannot be portably influenced by the
1719: programmer, leading to some inefficiencies on register-starved
1720: machines. We use explicit register declarations (@pxref{Explicit Reg
1721: Vars, , Variables in Specified Registers, gcc.info, GNU C Manual}) to
1722: improve the speed on some machines. They are turned on by using the
1723: @code{gcc} switch @code{-DFORCE_REG}. Unfortunately, this feature not
1724: only depends on the machine, but also on the compiler version: On some
1725: machines some compiler versions produce incorrect code when certain
1726: explicit register declarations are used. So by default
1727: @code{-DFORCE_REG} is not used.
1728:
1729: @node Threading, Primitives, Portability, Internals
1730: @section Threading
1731:
1732: GNU C's labels as values extension (available since @code{gcc-2.0},
1733: @pxref{Labels as Values, , Labels as Values, gcc.info, GNU C Manual})
1734: makes it possible to take the address of @var{label} by writing
1735: @code{&&@var{label}}. This address can then be used in a statement like
1736: @code{goto *@var{address}}. I.e., @code{goto *&&x} is the same as
1737: @code{goto x}.
1738:
1739: With this feature an indirect threaded NEXT looks like:
1740: @example
1741: cfa = *ip++;
1742: ca = *cfa;
1743: goto *ca;
1744: @end example
1745: For those unfamiliar with the names: @code{ip} is the Forth instruction
1746: pointer; the @code{cfa} (code-field address) corresponds to ANS Forths
1747: execution token and points to the code field of the next word to be
1748: executed; The @code{ca} (code address) fetched from there points to some
1749: executable code, e.g., a primitive or the colon definition handler
1750: @code{docol}.
1751:
1752: Direct threading is even simpler:
1753: @example
1754: ca = *ip++;
1755: goto *ca;
1756: @end example
1757:
1758: Of course we have packaged the whole thing neatly in macros called
1759: @code{NEXT} and @code{NEXT1} (the part of NEXT after fetching the cfa).
1760:
1761: @menu
1762: * Scheduling::
1763: * Direct or Indirect Threaded?::
1764: * DOES>::
1765: @end menu
1766:
1767: @node Scheduling, Direct or Indirect Threaded?, Threading, Threading
1768: @subsection Scheduling
1769:
1770: There is a little complication: Pipelined and superscalar processors,
1771: i.e., RISC and some modern CISC machines can process independent
1772: instructions while waiting for the results of an instruction. The
1773: compiler usually reorders (schedules) the instructions in a way that
1774: achieves good usage of these delay slots. However, on our first tries
1775: the compiler did not do well on scheduling primitives. E.g., for
1776: @code{+} implemented as
1777: @example
1778: n=sp[0]+sp[1];
1779: sp++;
1780: sp[0]=n;
1781: NEXT;
1782: @end example
1783: the NEXT comes strictly after the other code, i.e., there is nearly no
1784: scheduling. After a little thought the problem becomes clear: The
1785: compiler cannot know that sp and ip point to different addresses (and
1786: the version of @code{gcc} we used would not know it even if it was
1787: possible), so it could not move the load of the cfa above the store to
1788: the TOS. Indeed the pointers could be the same, if code on or very near
1789: the top of stack were executed. In the interest of speed we chose to
1790: forbid this probably unused ``feature'' and helped the compiler in
1791: scheduling: NEXT is divided into the loading part (@code{NEXT_P1}) and
1792: the goto part (@code{NEXT_P2}). @code{+} now looks like:
1793: @example
1794: n=sp[0]+sp[1];
1795: sp++;
1796: NEXT_P1;
1797: sp[0]=n;
1798: NEXT_P2;
1799: @end example
1800: This can be scheduled optimally by the compiler.
1801:
1802: This division can be turned off with the switch @code{-DCISC_NEXT}. This
1803: switch is on by default on machines that do not profit from scheduling
1804: (e.g., the 80386), in order to preserve registers.
1805:
1806: @node Direct or Indirect Threaded?, DOES>, Scheduling, Threading
1807: @subsection Direct or Indirect Threaded?
1808:
1809: Both! After packaging the nasty details in macro definitions we
1810: realized that we could switch between direct and indirect threading by
1811: simply setting a compilation flag (@code{-DDIRECT_THREADED}) and
1812: defining a few machine-specific macros for the direct-threading case.
1813: On the Forth level we also offer access words that hide the
1814: differences between the threading methods (@pxref{Threading Words}).
1815:
1816: Indirect threading is implemented completely
1817: machine-independently. Direct threading needs routines for creating
1818: jumps to the executable code (e.g. to docol or dodoes). These routines
1819: are inherently machine-dependent, but they do not amount to many source
1820: lines. I.e., even porting direct threading to a new machine is a small
1821: effort.
1822:
1823: @node DOES>, , Direct or Indirect Threaded?, Threading
1824: @subsection DOES>
1825: One of the most complex parts of a Forth engine is @code{dodoes}, i.e.,
1826: the chunk of code executed by every word defined by a
1827: @code{CREATE}...@code{DOES>} pair. The main problem here is: How to find
1828: the Forth code to be executed, i.e. the code after the @code{DOES>} (the
1829: DOES-code)? There are two solutions:
1830:
1831: In fig-Forth the code field points directly to the dodoes and the
1832: DOES-code address is stored in the cell after the code address
1833: (i.e. at cfa cell+). It may seem that this solution is illegal in the
1834: Forth-79 and all later standards, because in fig-Forth this address
1835: lies in the body (which is illegal in these standards). However, by
1836: making the code field larger for all words this solution becomes legal
1837: again. We use this approach for the indirect threaded version. Leaving
1838: a cell unused in most words is a bit wasteful, but on the machines we
1839: are targetting this is hardly a problem. The other reason for having a
1840: code field size of two cells is to avoid having different image files
1841: for direct and indirect threaded systems (@pxref{System Architecture}).
1842:
1843: The other approach is that the code field points or jumps to the cell
1844: after @code{DOES}. In this variant there is a jump to @code{dodoes} at
1845: this address. @code{dodoes} can then get the DOES-code address by
1846: computing the code address, i.e., the address of the jump to dodoes,
1847: and add the length of that jump field. A variant of this is to have a
1848: call to @code{dodoes} after the @code{DOES>}; then the return address
1849: (which can be found in the return register on RISCs) is the DOES-code
1850: address. Since the two cells available in the code field are usually
1851: used up by the jump to the code address in direct threading, we use
1852: this approach for direct threading. We did not want to add another
1853: cell to the code field.
1854:
1855: @node Primitives, System Architecture, Threading, Internals
1856: @section Primitives
1857:
1858: @menu
1859: * Automatic Generation::
1860: * TOS Optimization::
1861: * Produced code::
1862: @end menu
1863:
1864: @node Automatic Generation, TOS Optimization, Primitives, Primitives
1865: @subsection Automatic Generation
1866:
1867: Since the primitives are implemented in a portable language, there is no
1868: longer any need to minimize the number of primitives. On the contrary,
1869: having many primitives is an advantage: speed. In order to reduce the
1870: number of errors in primitives and to make programming them easier, we
1871: provide a tool, the primitive generator (@file{prims2x.fs}), that
1872: automatically generates most (and sometimes all) of the C code for a
1873: primitive from the stack effect notation. The source for a primitive
1874: has the following form:
1875:
1876: @format
1877: @var{Forth-name} @var{stack-effect} @var{category} [@var{pronounc.}]
1878: [@code{""}@var{glossary entry}@code{""}]
1879: @var{C code}
1880: [@code{:}
1881: @var{Forth code}]
1882: @end format
1883:
1884: The items in brackets are optional. The category and glossary fields
1885: are there for generating the documentation, the Forth code is there
1886: for manual implementations on machines without GNU C. E.g., the source
1887: for the primitive @code{+} is:
1888: @example
1889: + n1 n2 -- n core plus
1890: n = n1+n2;
1891: @end example
1892:
1893: This looks like a specification, but in fact @code{n = n1+n2} is C
1894: code. Our primitive generation tool extracts a lot of information from
1895: the stack effect notations@footnote{We use a one-stack notation, even
1896: though we have separate data and floating-point stacks; The separate
1897: notation can be generated easily from the unified notation.}: The number
1898: of items popped from and pushed on the stack, their type, and by what
1899: name they are referred to in the C code. It then generates a C code
1900: prelude and postlude for each primitive. The final C code for @code{+}
1901: looks like this:
1902:
1903: @example
1904: I_plus: /* + ( n1 n2 -- n ) */ /* label, stack effect */
1905: /* */ /* documentation */
1906: @{
1907: DEF_CA /* definition of variable ca (indirect threading) */
1908: Cell n1; /* definitions of variables */
1909: Cell n2;
1910: Cell n;
1911: n1 = (Cell) sp[1]; /* input */
1912: n2 = (Cell) TOS;
1913: sp += 1; /* stack adjustment */
1914: NAME("+") /* debugging output (with -DDEBUG) */
1915: @{
1916: n = n1+n2; /* C code taken from the source */
1917: @}
1918: NEXT_P1; /* NEXT part 1 */
1919: TOS = (Cell)n; /* output */
1920: NEXT_P2; /* NEXT part 2 */
1921: @}
1922: @end example
1923:
1924: This looks long and inefficient, but the GNU C compiler optimizes quite
1925: well and produces optimal code for @code{+} on, e.g., the R3000 and the
1926: HP RISC machines: Defining the @code{n}s does not produce any code, and
1927: using them as intermediate storage also adds no cost.
1928:
1929: There are also other optimizations, that are not illustrated by this
1930: example: Assignments between simple variables are usually for free (copy
1931: propagation). If one of the stack items is not used by the primitive
1932: (e.g. in @code{drop}), the compiler eliminates the load from the stack
1933: (dead code elimination). On the other hand, there are some things that
1934: the compiler does not do, therefore they are performed by
1935: @file{prims2x.fs}: The compiler does not optimize code away that stores
1936: a stack item to the place where it just came from (e.g., @code{over}).
1937:
1938: While programming a primitive is usually easy, there are a few cases
1939: where the programmer has to take the actions of the generator into
1940: account, most notably @code{?dup}, but also words that do not (always)
1941: fall through to NEXT.
1942:
1943: @node TOS Optimization, Produced code, Automatic Generation, Primitives
1944: @subsection TOS Optimization
1945:
1946: An important optimization for stack machine emulators, e.g., Forth
1947: engines, is keeping one or more of the top stack items in
1948: registers. If a word has the stack effect @var{in1}...@var{inx} @code{--}
1949: @var{out1}...@var{outy}, keeping the top @var{n} items in registers
1950: @itemize
1951: @item
1952: is better than keeping @var{n-1} items, if @var{x>=n} and @var{y>=n},
1953: due to fewer loads from and stores to the stack.
1954: @item is slower than keeping @var{n-1} items, if @var{x<>y} and @var{x<n} and
1955: @var{y<n}, due to additional moves between registers.
1956: @end itemize
1957:
1958: In particular, keeping one item in a register is never a disadvantage,
1959: if there are enough registers. Keeping two items in registers is a
1960: disadvantage for frequent words like @code{?branch}, constants,
1961: variables, literals and @code{i}. Therefore our generator only produces
1962: code that keeps zero or one items in registers. The generated C code
1963: covers both cases; the selection between these alternatives is made at
1964: C-compile time using the switch @code{-DUSE_TOS}. @code{TOS} in the C
1965: code for @code{+} is just a simple variable name in the one-item case,
1966: otherwise it is a macro that expands into @code{sp[0]}. Note that the
1967: GNU C compiler tries to keep simple variables like @code{TOS} in
1968: registers, and it usually succeeds, if there are enough registers.
1969:
1970: The primitive generator performs the TOS optimization for the
1971: floating-point stack, too (@code{-DUSE_FTOS}). For floating-point
1972: operations the benefit of this optimization is even larger:
1973: floating-point operations take quite long on most processors, but can be
1974: performed in parallel with other operations as long as their results are
1975: not used. If the FP-TOS is kept in a register, this works. If
1976: it is kept on the stack, i.e., in memory, the store into memory has to
1977: wait for the result of the floating-point operation, lengthening the
1978: execution time of the primitive considerably.
1979:
1980: The TOS optimization makes the automatic generation of primitives a
1981: bit more complicated. Just replacing all occurrences of @code{sp[0]} by
1982: @code{TOS} is not sufficient. There are some special cases to
1983: consider:
1984: @itemize
1985: @item In the case of @code{dup ( w -- w w )} the generator must not
1986: eliminate the store to the original location of the item on the stack,
1987: if the TOS optimization is turned on.
1988: @item Primitives with stack effects of the form @code{--}
1989: @var{out1}...@var{outy} must store the TOS to the stack at the start.
1990: Likewise, primitives with the stack effect @var{in1}...@var{inx} @code{--}
1991: must load the TOS from the stack at the end. But for the null stack
1992: effect @code{--} no stores or loads should be generated.
1993: @end itemize
1994:
1995: @node Produced code, , TOS Optimization, Primitives
1996: @subsection Produced code
1997:
1998: To see what assembly code is produced for the primitives on your machine
1999: with your compiler and your flag settings, type @code{make engine.s} and
2000: look at the resulting file @file{engine.s}.
2001:
2002: @node System Architecture, , Primitives, Internals
2003: @section System Architecture
2004:
2005: Our Forth system consists not only of primitives, but also of
2006: definitions written in Forth. Since the Forth compiler itself belongs
2007: to those definitions, it is not possible to start the system with the
2008: primitives and the Forth source alone. Therefore we provide the Forth
2009: code as an image file in nearly executable form. At the start of the
2010: system a C routine loads the image file into memory, sets up the
2011: memory (stacks etc.) according to information in the image file, and
2012: starts executing Forth code.
2013:
2014: The image file format is a compromise between the goals of making it
2015: easy to generate image files and making them portable. The easiest way
2016: to generate an image file is to just generate a memory dump. However,
2017: this kind of image file cannot be used on a different machine, or on
2018: the next version of the engine on the same machine, it even might not
2019: work with the same engine compiled by a different version of the C
2020: compiler. We would like to have as few versions of the image file as
2021: possible, because we do not want to distribute many versions of the
2022: same image file, and to make it easy for the users to use their image
2023: files on many machines. We currently need to create a different image
2024: file for machines with different cell sizes and different byte order
2025: (little- or big-endian)@footnote{We consider adding information to the
2026: image file that enables the loader to change the byte order.}.
2027:
2028: Forth code that is going to end up in a portable image file has to
2029: comply to some restrictions: addresses have to be stored in memory with
2030: special words (@code{A!}, @code{A,}, etc.) in order to make the code
2031: relocatable. Cells, floats, etc., have to be stored at the natural
2032: alignment boundaries@footnote{E.g., store floats (8 bytes) at an address
2033: dividable by~8. This happens automatically in our system when you use
2034: the ANS Forth alignment words.}, in order to avoid alignment faults on
2035: machines with stricter alignment. The image file is produced by a
2036: metacompiler (@file{cross.fs}).
2037:
2038: So, unlike the image file of Mitch Bradleys @code{cforth}, our image
2039: file is not directly executable, but has to undergo some manipulations
2040: during loading. Address relocation is performed at image load-time, not
2041: at run-time. The loader also has to replace tokens standing for
2042: primitive calls with the appropriate code-field addresses (or code
2043: addresses in the case of direct threading).
2044:
2045: @node Bugs, Pedigree, Internals, Top
2046: @chapter Bugs
2047:
2048: @node Pedigree, Word Index, Bugs, Top
2049: @chapter Pedigree
2050:
2051: @node Word Index, Node Index, Pedigree, Top
2052: @chapter Word Index
2053:
2054: @node Node Index, , Word Index, Top
2055: @chapter Node Index
2056:
2057: @contents
2058: @bye
2059:
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