Annotation of gforth/gforth.ds, revision 1.8
1.1 anton 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.)
1.4 anton 4: @setfilename gforth.info
1.1 anton 5: @settitle GNU Forth Manual
1.4 anton 6: @comment @setchapternewpage odd
1.1 anton 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:
1.4 anton 18: @ignore
1.1 anton 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:
1.4 anton 24: @end ignore
1.1 anton 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
1.4 anton 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
1.1 anton 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:
1.4 anton 256: @node Words, ANS conformance, Invocation, Top
1.1 anton 257: @chapter Forth Words
258:
259: @menu
1.4 anton 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::
1.1 anton 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:
1.4 anton 281: @format
1.1 anton 282: @var{word} @var{Stack effect} @var{wordset} @var{pronunciation}
1.4 anton 283: @end format
1.1 anton 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:
1.4 anton 318: The type of a stack item is specified by the character(s) the name
319: starts with:
1.1 anton 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:
1.4 anton 356: @node Arithmetic, Stack Manipulation, Notation, Words
1.1 anton 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
1.4 anton 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
1.1 anton 375:
1.4 anton 376: @node Single precision, Bitwise operations, Arithmetic, Arithmetic
1.1 anton 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:
1.4 anton 389: @node Bitwise operations, Mixed precision, Single precision, Arithmetic
1.1 anton 390: @subsection Bitwise operations
391: doc-and
392: doc-or
393: doc-xor
394: doc-invert
395: doc-2*
396: doc-2/
397:
1.4 anton 398: @node Mixed precision, Double precision, Bitwise operations, Arithmetic
1.1 anton 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:
1.4 anton 410: @node Double precision, Floating Point, Mixed precision, Arithmetic
1.1 anton 411: @subsection Double precision
412: doc-d+
413: doc-d-
414: doc-dnegate
415: doc-dabs
416: doc-dmin
417: doc-dmax
418:
1.4 anton 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
1.6 anton 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}.
1.4 anton 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
1.6 anton 452: doc-falog
1.4 anton 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
1.1 anton 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
1.4 anton 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
1.1 anton 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:
1.4 anton 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
1.1 anton 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:
1.4 anton 520: @node Floating point stack, Return stack, Data stack, Stack Manipulation
1.1 anton 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:
1.4 anton 530: @node Return stack, Locals stack, Floating point stack, Stack Manipulation
1.1 anton 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:
1.4 anton 541: @node Locals stack, Stack pointer manipulation, Return stack, Stack Manipulation
1.1 anton 542: @subsection Locals stack
543:
1.4 anton 544: @node Stack pointer manipulation, , Locals stack, Stack Manipulation
1.1 anton 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:
1.4 anton 555: @node Memory access, Control Structures, Stack Manipulation, Words
1.1 anton 556: @section Memory access
557:
1.4 anton 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
1.1 anton 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:
1.4 anton 581: @node Address arithmetic, Memory block access, Stack-Memory transfers, Memory access
1.1 anton 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: doc-chars
611: doc-char+
612: doc-cells
613: doc-cell+
614: doc-align
615: doc-aligned
616: doc-floats
617: doc-float+
618: doc-falign
619: doc-faligned
620: doc-sfloats
621: doc-sfloat+
622: doc-sfalign
623: doc-sfaligned
624: doc-dfloats
625: doc-dfloat+
626: doc-dfalign
627: doc-dfaligned
628: doc-address-unit-bits
629:
1.4 anton 630: @node Memory block access, , Address arithmetic, Memory access
1.1 anton 631: @subsection Memory block access
632:
633: doc-move
634: doc-erase
635:
636: While the previous words work on address units, the rest works on
637: characters.
638:
639: doc-cmove
640: doc-cmove>
641: doc-fill
642: doc-blank
643:
1.4 anton 644: @node Control Structures, Locals, Memory access, Words
1.1 anton 645: @section Control Structures
646:
647: Control structures in Forth cannot be used in interpret state, only in
648: compile state, i.e., in a colon definition. We do not like this
649: limitation, but have not seen a satisfying way around it yet, although
650: many schemes have been proposed.
651:
1.4 anton 652: @menu
653: * Selection::
654: * Simple Loops::
655: * Counted Loops::
656: * Arbitrary control structures::
657: * Calls and returns::
658: * Exception Handling::
659: @end menu
660:
661: @node Selection, Simple Loops, Control Structures, Control Structures
1.1 anton 662: @subsection Selection
663:
664: @example
665: @var{flag}
666: IF
667: @var{code}
668: ENDIF
669: @end example
670: or
671: @example
672: @var{flag}
673: IF
674: @var{code1}
675: ELSE
676: @var{code2}
677: ENDIF
678: @end example
679:
1.4 anton 680: You can use @code{THEN} instead of @code{ENDIF}. Indeed, @code{THEN} is
1.1 anton 681: standard, and @code{ENDIF} is not, although it is quite popular. We
682: recommend using @code{ENDIF}, because it is less confusing for people
683: who also know other languages (and is not prone to reinforcing negative
684: prejudices against Forth in these people). Adding @code{ENDIF} to a
685: system that only supplies @code{THEN} is simple:
686: @example
687: : endif POSTPONE then ; immediate
688: @end example
689:
690: [According to @cite{Webster's New Encyclopedic Dictionary}, @dfn{then
691: (adv.)} has the following meanings:
692: @quotation
693: ... 2b: following next after in order ... 3d: as a necessary consequence
694: (if you were there, then you saw them).
695: @end quotation
696: Forth's @code{THEN} has the meaning 2b, whereas @code{THEN} in Pascal
697: and many other programming languages has the meaning 3d.]
698:
699: We also provide the words @code{?dup-if} and @code{?dup-0=-if}, so you
700: can avoid using @code{?dup}.
701:
702: @example
703: @var{n}
704: CASE
705: @var{n1} OF @var{code1} ENDOF
706: @var{n2} OF @var{code2} ENDOF
1.4 anton 707: @dots{}
1.1 anton 708: ENDCASE
709: @end example
710:
711: Executes the first @var{codei}, where the @var{ni} is equal to
712: @var{n}. A default case can be added by simply writing the code after
713: the last @code{ENDOF}. It may use @var{n}, which is on top of the stack,
714: but must not consume it.
715:
1.4 anton 716: @node Simple Loops, Counted Loops, Selection, Control Structures
1.1 anton 717: @subsection Simple Loops
718:
719: @example
720: BEGIN
721: @var{code1}
722: @var{flag}
723: WHILE
724: @var{code2}
725: REPEAT
726: @end example
727:
728: @var{code1} is executed and @var{flag} is computed. If it is true,
729: @var{code2} is executed and the loop is restarted; If @var{flag} is false, execution continues after the @code{REPEAT}.
730:
731: @example
732: BEGIN
733: @var{code}
734: @var{flag}
735: UNTIL
736: @end example
737:
738: @var{code} is executed. The loop is restarted if @code{flag} is false.
739:
740: @example
741: BEGIN
742: @var{code}
743: AGAIN
744: @end example
745:
746: This is an endless loop.
747:
1.4 anton 748: @node Counted Loops, Arbitrary control structures, Simple Loops, Control Structures
1.1 anton 749: @subsection Counted Loops
750:
751: The basic counted loop is:
752: @example
753: @var{limit} @var{start}
754: ?DO
755: @var{body}
756: LOOP
757: @end example
758:
759: This performs one iteration for every integer, starting from @var{start}
760: and up to, but excluding @var{limit}. The counter, aka index, can be
761: accessed with @code{i}. E.g., the loop
762: @example
763: 10 0 ?DO
764: i .
765: LOOP
766: @end example
767: prints
768: @example
769: 0 1 2 3 4 5 6 7 8 9
770: @end example
771: The index of the innermost loop can be accessed with @code{i}, the index
772: of the next loop with @code{j}, and the index of the third loop with
773: @code{k}.
774:
775: The loop control data are kept on the return stack, so there are some
776: restrictions on mixing return stack accesses and counted loop
777: words. E.g., if you put values on the return stack outside the loop, you
778: cannot read them inside the loop. If you put values on the return stack
779: within a loop, you have to remove them before the end of the loop and
780: before accessing the index of the loop.
781:
782: There are several variations on the counted loop:
783:
784: @code{LEAVE} leaves the innermost counted loop immediately.
785:
786: @code{LOOP} can be replaced with @code{@var{n} +LOOP}; this updates the
787: index by @var{n} instead of by 1. The loop is terminated when the border
788: between @var{limit-1} and @var{limit} is crossed. E.g.:
789:
1.2 anton 790: @code{4 0 ?DO i . 2 +LOOP} prints @code{0 2}
1.1 anton 791:
1.2 anton 792: @code{4 1 ?DO i . 2 +LOOP} prints @code{1 3}
1.1 anton 793:
794: The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative:
795:
1.2 anton 796: @code{-1 0 ?DO i . -1 +LOOP} prints @code{0 -1}
1.1 anton 797:
1.2 anton 798: @code{ 0 0 ?DO i . -1 +LOOP} prints nothing
1.1 anton 799:
800: Therefore we recommend avoiding using @code{@var{n} +LOOP} with negative
801: @var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative
802: case behaves symmetrical to the positive case:
803:
1.7 pazsan 804: @code{-2 0 ?DO i . -1 S+LOOP} prints @code{0 -1}
1.1 anton 805:
1.7 pazsan 806: @code{-1 0 ?DO i . -1 S+LOOP} prints @code{0}
1.1 anton 807:
1.7 pazsan 808: @code{ 0 0 ?DO i . -1 S+LOOP} prints nothing
1.1 anton 809:
1.2 anton 810: The loop is terminated when the border between @var{limit@minus{}sgn(n)} and
1.1 anton 811: @var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS
812: Forth standard.
813:
814: @code{?DO} can be replaced by @code{DO}. @code{DO} enters the loop even
815: when the start and the limit value are equal. We do not recommend using
816: @code{DO}. It will just give you maintenance troubles.
817:
818: @code{UNLOOP} is used to prepare for an abnormal loop exit, e.g., via
819: @code{EXIT}. @code{UNLOOP} removes the loop control parameters from the
820: return stack so @code{EXIT} can get to its return address.
821:
822: Another counted loop is
823: @example
824: @var{n}
825: FOR
826: @var{body}
827: NEXT
828: @end example
829: This is the preferred loop of native code compiler writers who are too
830: lazy to optimize @code{?DO} loops properly. In GNU Forth, this loop
831: iterates @var{n+1} times; @code{i} produces values starting with @var{n}
832: and ending with 0. Other Forth systems may behave differently, even if
833: they support @code{FOR} loops.
834:
1.4 anton 835: @node Arbitrary control structures, Calls and returns, Counted Loops, Control Structures
1.2 anton 836: @subsection Arbitrary control structures
837:
838: ANS Forth permits and supports using control structures in a non-nested
839: way. Information about incomplete control structures is stored on the
840: control-flow stack. This stack may be implemented on the Forth data
841: stack, and this is what we have done in gforth.
842:
843: An @i{orig} entry represents an unresolved forward branch, a @i{dest}
844: entry represents a backward branch target. A few words are the basis for
845: building any control structure possible (except control structures that
846: need storage, like calls, coroutines, and backtracking).
847:
1.3 anton 848: doc-if
849: doc-ahead
850: doc-then
851: doc-begin
852: doc-until
853: doc-again
854: doc-cs-pick
855: doc-cs-roll
1.2 anton 856:
857: On many systems control-flow stack items take one word, in gforth they
858: currently take three (this may change in the future). Therefore it is a
859: really good idea to manipulate the control flow stack with
860: @code{cs-pick} and @code{cs-roll}, not with data stack manipulation
861: words.
862:
863: Some standard control structure words are built from these words:
864:
1.3 anton 865: doc-else
866: doc-while
867: doc-repeat
1.2 anton 868:
869: Counted loop words constitute a separate group of words:
870:
1.3 anton 871: doc-?do
872: doc-do
873: doc-for
874: doc-loop
875: doc-s+loop
876: doc-+loop
877: doc-next
878: doc-leave
879: doc-?leave
880: doc-unloop
881: doc-undo
1.2 anton 882:
883: The standard does not allow using @code{cs-pick} and @code{cs-roll} on
884: @i{do-sys}. Our system allows it, but it's your job to ensure that for
885: every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path
1.3 anton 886: through the definition (@code{LOOP} etc. compile an @code{UNLOOP} on the
887: fall-through path). Also, you have to ensure that all @code{LEAVE}s are
1.7 pazsan 888: resolved (by using one of the loop-ending words or @code{DONE}).
1.2 anton 889:
890: Another group of control structure words are
891:
1.3 anton 892: doc-case
893: doc-endcase
894: doc-of
895: doc-endof
1.2 anton 896:
897: @i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and
898: @code{cs-roll}.
899:
1.3 anton 900: @subsubsection Programming Style
901:
902: In order to ensure readability we recommend that you do not create
903: arbitrary control structures directly, but define new control structure
904: words for the control structure you want and use these words in your
905: program.
906:
907: E.g., instead of writing
908:
909: @example
910: begin
911: ...
912: if [ 1 cs-roll ]
913: ...
914: again then
915: @end example
916:
917: we recommend defining control structure words, e.g.,
918:
919: @example
920: : while ( dest -- orig dest )
921: POSTPONE if
922: 1 cs-roll ; immediate
923:
924: : repeat ( orig dest -- )
925: POSTPONE again
926: POSTPONE then ; immediate
927: @end example
928:
929: and then using these to create the control structure:
930:
931: @example
932: begin
933: ...
934: while
935: ...
936: repeat
937: @end example
938:
939: That's much easier to read, isn't it? Of course, @code{BEGIN} and
940: @code{WHILE} are predefined, so in this example it would not be
941: necessary to define them.
942:
1.4 anton 943: @node Calls and returns, Exception Handling, Arbitrary control structures, Control Structures
1.3 anton 944: @subsection Calls and returns
945:
946: A definition can be called simply be writing the name of the
947: definition. When the end of the definition is reached, it returns. An earlier return can be forced using
948:
949: doc-exit
950:
951: Don't forget to clean up the return stack and @code{UNLOOP} any
952: outstanding @code{?DO}...@code{LOOP}s before @code{EXIT}ing. The
953: primitive compiled by @code{EXIT} is
954:
955: doc-;s
956:
1.4 anton 957: @node Exception Handling, , Calls and returns, Control Structures
1.3 anton 958: @subsection Exception Handling
959:
960: doc-catch
961: doc-throw
962:
1.4 anton 963: @node Locals, Defining Words, Control Structures, Words
1.1 anton 964: @section Locals
965:
1.2 anton 966: Local variables can make Forth programming more enjoyable and Forth
967: programs easier to read. Unfortunately, the locals of ANS Forth are
968: laden with restrictions. Therefore, we provide not only the ANS Forth
969: locals wordset, but also our own, more powerful locals wordset (we
970: implemented the ANS Forth locals wordset through our locals wordset).
971:
972: @menu
1.4 anton 973: * gforth locals::
974: * ANS Forth locals::
1.2 anton 975: @end menu
976:
1.4 anton 977: @node gforth locals, ANS Forth locals, Locals, Locals
1.2 anton 978: @subsection gforth locals
979:
980: Locals can be defined with
981:
982: @example
983: @{ local1 local2 ... -- comment @}
984: @end example
985: or
986: @example
987: @{ local1 local2 ... @}
988: @end example
989:
990: E.g.,
991: @example
992: : max @{ n1 n2 -- n3 @}
993: n1 n2 > if
994: n1
995: else
996: n2
997: endif ;
998: @end example
999:
1000: The similarity of locals definitions with stack comments is intended. A
1001: locals definition often replaces the stack comment of a word. The order
1002: of the locals corresponds to the order in a stack comment and everything
1003: after the @code{--} is really a comment.
1004:
1005: This similarity has one disadvantage: It is too easy to confuse locals
1006: declarations with stack comments, causing bugs and making them hard to
1007: find. However, this problem can be avoided by appropriate coding
1008: conventions: Do not use both notations in the same program. If you do,
1009: they should be distinguished using additional means, e.g. by position.
1010:
1011: The name of the local may be preceded by a type specifier, e.g.,
1012: @code{F:} for a floating point value:
1013:
1014: @example
1015: : CX* @{ F: Ar F: Ai F: Br F: Bi -- Cr Ci @}
1016: \ complex multiplication
1017: Ar Br f* Ai Bi f* f-
1018: Ar Bi f* Ai Br f* f+ ;
1019: @end example
1020:
1021: GNU Forth currently supports cells (@code{W:}, @code{W^}), doubles
1022: (@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters
1023: (@code{C:}, @code{C^}) in two flavours: a value-flavoured local (defined
1024: with @code{W:}, @code{D:} etc.) produces its value and can be changed
1025: with @code{TO}. A variable-flavoured local (defined with @code{W^} etc.)
1026: produces its address (which becomes invalid when the variable's scope is
1027: left). E.g., the standard word @code{emit} can be defined in therms of
1028: @code{type} like this:
1029:
1030: @example
1031: : emit @{ C^ char* -- @}
1032: char* 1 type ;
1033: @end example
1034:
1035: A local without type specifier is a @code{W:} local. Both flavours of
1036: locals are initialized with values from the data or FP stack.
1037:
1038: Currently there is no way to define locals with user-defined data
1039: structures, but we are working on it.
1040:
1.7 pazsan 1041: GNU Forth allows defining locals everywhere in a colon definition. This
1042: poses the following questions:
1.2 anton 1043:
1.4 anton 1044: @menu
1045: * Where are locals visible by name?::
1046: * How long do locals live? ::
1047: * Programming Style::
1048: * Implementation::
1049: @end menu
1050:
1051: @node Where are locals visible by name?, How long do locals live?, gforth locals, gforth locals
1.2 anton 1052: @subsubsection Where are locals visible by name?
1053:
1054: Basically, the answer is that locals are visible where you would expect
1055: it in block-structured languages, and sometimes a little longer. If you
1056: want to restrict the scope of a local, enclose its definition in
1057: @code{SCOPE}...@code{ENDSCOPE}.
1058:
1059: doc-scope
1060: doc-endscope
1061:
1062: These words behave like control structure words, so you can use them
1063: with @code{CS-PICK} and @code{CS-ROLL} to restrict the scope in
1064: arbitrary ways.
1065:
1066: If you want a more exact answer to the visibility question, here's the
1067: basic principle: A local is visible in all places that can only be
1068: reached through the definition of the local@footnote{In compiler
1069: construction terminology, all places dominated by the definition of the
1070: local.}. In other words, it is not visible in places that can be reached
1071: without going through the definition of the local. E.g., locals defined
1072: in @code{IF}...@code{ENDIF} are visible until the @code{ENDIF}, locals
1073: defined in @code{BEGIN}...@code{UNTIL} are visible after the
1074: @code{UNTIL} (until, e.g., a subsequent @code{ENDSCOPE}).
1075:
1076: The reasoning behind this solution is: We want to have the locals
1077: visible as long as it is meaningful. The user can always make the
1078: visibility shorter by using explicit scoping. In a place that can
1079: only be reached through the definition of a local, the meaning of a
1080: local name is clear. In other places it is not: How is the local
1081: initialized at the control flow path that does not contain the
1082: definition? Which local is meant, if the same name is defined twice in
1083: two independent control flow paths?
1084:
1085: This should be enough detail for nearly all users, so you can skip the
1086: rest of this section. If you relly must know all the gory details and
1087: options, read on.
1088:
1089: In order to implement this rule, the compiler has to know which places
1090: are unreachable. It knows this automatically after @code{AHEAD},
1091: @code{AGAIN}, @code{EXIT} and @code{LEAVE}; in other cases (e.g., after
1092: most @code{THROW}s), you can use the word @code{UNREACHABLE} to tell the
1093: compiler that the control flow never reaches that place. If
1094: @code{UNREACHABLE} is not used where it could, the only consequence is
1095: that the visibility of some locals is more limited than the rule above
1096: says. If @code{UNREACHABLE} is used where it should not (i.e., if you
1097: lie to the compiler), buggy code will be produced.
1098:
1099: Another problem with this rule is that at @code{BEGIN}, the compiler
1.3 anton 1100: does not know which locals will be visible on the incoming
1101: back-edge. All problems discussed in the following are due to this
1102: ignorance of the compiler (we discuss the problems using @code{BEGIN}
1103: loops as examples; the discussion also applies to @code{?DO} and other
1.2 anton 1104: loops). Perhaps the most insidious example is:
1105: @example
1106: AHEAD
1107: BEGIN
1108: x
1109: [ 1 CS-ROLL ] THEN
1.4 anton 1110: @{ x @}
1.2 anton 1111: ...
1112: UNTIL
1113: @end example
1114:
1115: This should be legal according to the visibility rule. The use of
1116: @code{x} can only be reached through the definition; but that appears
1117: textually below the use.
1118:
1119: From this example it is clear that the visibility rules cannot be fully
1120: implemented without major headaches. Our implementation treats common
1121: cases as advertised and the exceptions are treated in a safe way: The
1122: compiler makes a reasonable guess about the locals visible after a
1123: @code{BEGIN}; if it is too pessimistic, the
1124: user will get a spurious error about the local not being defined; if the
1125: compiler is too optimistic, it will notice this later and issue a
1126: warning. In the case above the compiler would complain about @code{x}
1127: being undefined at its use. You can see from the obscure examples in
1128: this section that it takes quite unusual control structures to get the
1129: compiler into trouble, and even then it will often do fine.
1130:
1131: If the @code{BEGIN} is reachable from above, the most optimistic guess
1132: is that all locals visible before the @code{BEGIN} will also be
1133: visible after the @code{BEGIN}. This guess is valid for all loops that
1134: are entered only through the @code{BEGIN}, in particular, for normal
1135: @code{BEGIN}...@code{WHILE}...@code{REPEAT} and
1136: @code{BEGIN}...@code{UNTIL} loops and it is implemented in our
1137: compiler. When the branch to the @code{BEGIN} is finally generated by
1138: @code{AGAIN} or @code{UNTIL}, the compiler checks the guess and
1139: warns the user if it was too optimisitic:
1140: @example
1141: IF
1.4 anton 1142: @{ x @}
1.2 anton 1143: BEGIN
1144: \ x ?
1145: [ 1 cs-roll ] THEN
1146: ...
1147: UNTIL
1148: @end example
1149:
1150: Here, @code{x} lives only until the @code{BEGIN}, but the compiler
1151: optimistically assumes that it lives until the @code{THEN}. It notices
1152: this difference when it compiles the @code{UNTIL} and issues a
1153: warning. The user can avoid the warning, and make sure that @code{x}
1154: is not used in the wrong area by using explicit scoping:
1155: @example
1156: IF
1157: SCOPE
1.4 anton 1158: @{ x @}
1.2 anton 1159: ENDSCOPE
1160: BEGIN
1161: [ 1 cs-roll ] THEN
1162: ...
1163: UNTIL
1164: @end example
1165:
1166: Since the guess is optimistic, there will be no spurious error messages
1167: about undefined locals.
1168:
1169: If the @code{BEGIN} is not reachable from above (e.g., after
1170: @code{AHEAD} or @code{EXIT}), the compiler cannot even make an
1171: optimistic guess, as the locals visible after the @code{BEGIN} may be
1172: defined later. Therefore, the compiler assumes that no locals are
1173: visible after the @code{BEGIN}. However, the useer can use
1174: @code{ASSUME-LIVE} to make the compiler assume that the same locals are
1175: visible at the BEGIN as at the point where the item was created.
1176:
1177: doc-assume-live
1178:
1179: E.g.,
1180: @example
1.4 anton 1181: @{ x @}
1.2 anton 1182: AHEAD
1183: ASSUME-LIVE
1184: BEGIN
1185: x
1186: [ 1 CS-ROLL ] THEN
1187: ...
1188: UNTIL
1189: @end example
1190:
1191: Other cases where the locals are defined before the @code{BEGIN} can be
1192: handled by inserting an appropriate @code{CS-ROLL} before the
1193: @code{ASSUME-LIVE} (and changing the control-flow stack manipulation
1194: behind the @code{ASSUME-LIVE}).
1195:
1196: Cases where locals are defined after the @code{BEGIN} (but should be
1197: visible immediately after the @code{BEGIN}) can only be handled by
1198: rearranging the loop. E.g., the ``most insidious'' example above can be
1199: arranged into:
1200: @example
1201: BEGIN
1.4 anton 1202: @{ x @}
1.2 anton 1203: ... 0=
1204: WHILE
1205: x
1206: REPEAT
1207: @end example
1208:
1.4 anton 1209: @node How long do locals live?, Programming Style, Where are locals visible by name?, gforth locals
1.2 anton 1210: @subsubsection How long do locals live?
1211:
1212: The right answer for the lifetime question would be: A local lives at
1213: least as long as it can be accessed. For a value-flavoured local this
1214: means: until the end of its visibility. However, a variable-flavoured
1215: local could be accessed through its address far beyond its visibility
1216: scope. Ultimately, this would mean that such locals would have to be
1217: garbage collected. Since this entails un-Forth-like implementation
1218: complexities, I adopted the same cowardly solution as some other
1219: languages (e.g., C): The local lives only as long as it is visible;
1220: afterwards its address is invalid (and programs that access it
1221: afterwards are erroneous).
1222:
1.4 anton 1223: @node Programming Style, Implementation, How long do locals live?, gforth locals
1.2 anton 1224: @subsubsection Programming Style
1225:
1226: The freedom to define locals anywhere has the potential to change
1227: programming styles dramatically. In particular, the need to use the
1228: return stack for intermediate storage vanishes. Moreover, all stack
1229: manipulations (except @code{PICK}s and @code{ROLL}s with run-time
1230: determined arguments) can be eliminated: If the stack items are in the
1231: wrong order, just write a locals definition for all of them; then
1232: write the items in the order you want.
1233:
1234: This seems a little far-fetched and eliminating stack manipulations is
1.4 anton 1235: unlikely to become a conscious programming objective. Still, the number
1236: of stack manipulations will be reduced dramatically if local variables
1237: are used liberally (e.g., compare @code{max} in @ref{gforth locals} with
1238: a traditional implementation of @code{max}).
1.2 anton 1239:
1240: This shows one potential benefit of locals: making Forth programs more
1241: readable. Of course, this benefit will only be realized if the
1242: programmers continue to honour the principle of factoring instead of
1243: using the added latitude to make the words longer.
1244:
1245: Using @code{TO} can and should be avoided. Without @code{TO},
1246: every value-flavoured local has only a single assignment and many
1247: advantages of functional languages apply to Forth. I.e., programs are
1248: easier to analyse, to optimize and to read: It is clear from the
1249: definition what the local stands for, it does not turn into something
1250: different later.
1251:
1252: E.g., a definition using @code{TO} might look like this:
1253: @example
1254: : strcmp @{ addr1 u1 addr2 u2 -- n @}
1255: u1 u2 min 0
1256: ?do
1257: addr1 c@ addr2 c@ - ?dup
1258: if
1259: unloop exit
1260: then
1261: addr1 char+ TO addr1
1262: addr2 char+ TO addr2
1263: loop
1264: u1 u2 - ;
1265: @end example
1266: Here, @code{TO} is used to update @code{addr1} and @code{addr2} at
1267: every loop iteration. @code{strcmp} is a typical example of the
1268: readability problems of using @code{TO}. When you start reading
1269: @code{strcmp}, you think that @code{addr1} refers to the start of the
1270: string. Only near the end of the loop you realize that it is something
1271: else.
1272:
1273: This can be avoided by defining two locals at the start of the loop that
1274: are initialized with the right value for the current iteration.
1275: @example
1276: : strcmp @{ addr1 u1 addr2 u2 -- n @}
1277: addr1 addr2
1278: u1 u2 min 0
1279: ?do @{ s1 s2 @}
1280: s1 c@ s2 c@ - ?dup
1281: if
1282: unloop exit
1283: then
1284: s1 char+ s2 char+
1285: loop
1286: 2drop
1287: u1 u2 - ;
1288: @end example
1289: Here it is clear from the start that @code{s1} has a different value
1290: in every loop iteration.
1291:
1.4 anton 1292: @node Implementation, , Programming Style, gforth locals
1.2 anton 1293: @subsubsection Implementation
1294:
1295: GNU Forth uses an extra locals stack. The most compelling reason for
1296: this is that the return stack is not float-aligned; using an extra stack
1297: also eliminates the problems and restrictions of using the return stack
1298: as locals stack. Like the other stacks, the locals stack grows toward
1299: lower addresses. A few primitives allow an efficient implementation:
1300:
1301: doc-@local#
1302: doc-f@local#
1303: doc-laddr#
1304: doc-lp+!#
1305: doc-lp!
1306: doc->l
1307: doc-f>l
1308:
1309: In addition to these primitives, some specializations of these
1310: primitives for commonly occurring inline arguments are provided for
1311: efficiency reasons, e.g., @code{@@local0} as specialization of
1312: @code{@@local#} for the inline argument 0. The following compiling words
1313: compile the right specialized version, or the general version, as
1314: appropriate:
1315:
1316: doc-compile-@@local
1317: doc-compile-f@@local
1318: doc-compile-lp+!
1319:
1320: Combinations of conditional branches and @code{lp+!#} like
1321: @code{?branch-lp+!#} (the locals pointer is only changed if the branch
1322: is taken) are provided for efficiency and correctness in loops.
1323:
1324: A special area in the dictionary space is reserved for keeping the
1325: local variable names. @code{@{} switches the dictionary pointer to this
1326: area and @code{@}} switches it back and generates the locals
1327: initializing code. @code{W:} etc.@ are normal defining words. This
1328: special area is cleared at the start of every colon definition.
1329:
1330: A special feature of GNU Forths dictionary is used to implement the
1331: definition of locals without type specifiers: every wordlist (aka
1332: vocabulary) has its own methods for searching
1.4 anton 1333: etc. (@pxref{Wordlists}). For the present purpose we defined a wordlist
1.2 anton 1334: with a special search method: When it is searched for a word, it
1335: actually creates that word using @code{W:}. @code{@{} changes the search
1336: order to first search the wordlist containing @code{@}}, @code{W:} etc.,
1337: and then the wordlist for defining locals without type specifiers.
1338:
1339: The lifetime rules support a stack discipline within a colon
1340: definition: The lifetime of a local is either nested with other locals
1341: lifetimes or it does not overlap them.
1342:
1343: At @code{BEGIN}, @code{IF}, and @code{AHEAD} no code for locals stack
1344: pointer manipulation is generated. Between control structure words
1345: locals definitions can push locals onto the locals stack. @code{AGAIN}
1346: is the simplest of the other three control flow words. It has to
1347: restore the locals stack depth of the corresponding @code{BEGIN}
1348: before branching. The code looks like this:
1349: @format
1350: @code{lp+!#} current-locals-size @minus{} dest-locals-size
1351: @code{branch} <begin>
1352: @end format
1353:
1354: @code{UNTIL} is a little more complicated: If it branches back, it
1355: must adjust the stack just like @code{AGAIN}. But if it falls through,
1356: the locals stack must not be changed. The compiler generates the
1357: following code:
1358: @format
1359: @code{?branch-lp+!#} <begin> current-locals-size @minus{} dest-locals-size
1360: @end format
1361: The locals stack pointer is only adjusted if the branch is taken.
1362:
1363: @code{THEN} can produce somewhat inefficient code:
1364: @format
1365: @code{lp+!#} current-locals-size @minus{} orig-locals-size
1366: <orig target>:
1367: @code{lp+!#} orig-locals-size @minus{} new-locals-size
1368: @end format
1369: The second @code{lp+!#} adjusts the locals stack pointer from the
1.4 anton 1370: level at the @var{orig} point to the level after the @code{THEN}. The
1.2 anton 1371: first @code{lp+!#} adjusts the locals stack pointer from the current
1372: level to the level at the orig point, so the complete effect is an
1373: adjustment from the current level to the right level after the
1374: @code{THEN}.
1375:
1376: In a conventional Forth implementation a dest control-flow stack entry
1377: is just the target address and an orig entry is just the address to be
1378: patched. Our locals implementation adds a wordlist to every orig or dest
1379: item. It is the list of locals visible (or assumed visible) at the point
1380: described by the entry. Our implementation also adds a tag to identify
1381: the kind of entry, in particular to differentiate between live and dead
1382: (reachable and unreachable) orig entries.
1383:
1384: A few unusual operations have to be performed on locals wordlists:
1385:
1386: doc-common-list
1387: doc-sub-list?
1388: doc-list-size
1389:
1390: Several features of our locals wordlist implementation make these
1391: operations easy to implement: The locals wordlists are organised as
1392: linked lists; the tails of these lists are shared, if the lists
1393: contain some of the same locals; and the address of a name is greater
1394: than the address of the names behind it in the list.
1395:
1396: Another important implementation detail is the variable
1397: @code{dead-code}. It is used by @code{BEGIN} and @code{THEN} to
1398: determine if they can be reached directly or only through the branch
1399: that they resolve. @code{dead-code} is set by @code{UNREACHABLE},
1400: @code{AHEAD}, @code{EXIT} etc., and cleared at the start of a colon
1401: definition, by @code{BEGIN} and usually by @code{THEN}.
1402:
1403: Counted loops are similar to other loops in most respects, but
1404: @code{LEAVE} requires special attention: It performs basically the same
1405: service as @code{AHEAD}, but it does not create a control-flow stack
1406: entry. Therefore the information has to be stored elsewhere;
1407: traditionally, the information was stored in the target fields of the
1408: branches created by the @code{LEAVE}s, by organizing these fields into a
1409: linked list. Unfortunately, this clever trick does not provide enough
1410: space for storing our extended control flow information. Therefore, we
1411: introduce another stack, the leave stack. It contains the control-flow
1412: stack entries for all unresolved @code{LEAVE}s.
1413:
1414: Local names are kept until the end of the colon definition, even if
1415: they are no longer visible in any control-flow path. In a few cases
1416: this may lead to increased space needs for the locals name area, but
1417: usually less than reclaiming this space would cost in code size.
1418:
1419:
1.4 anton 1420: @node ANS Forth locals, , gforth locals, Locals
1.2 anton 1421: @subsection ANS Forth locals
1422:
1423: The ANS Forth locals wordset does not define a syntax for locals, but
1424: words that make it possible to define various syntaxes. One of the
1425: possible syntaxes is a subset of the syntax we used in the gforth locals
1426: wordset, i.e.:
1427:
1428: @example
1429: @{ local1 local2 ... -- comment @}
1430: @end example
1431: or
1432: @example
1433: @{ local1 local2 ... @}
1434: @end example
1435:
1436: The order of the locals corresponds to the order in a stack comment. The
1437: restrictions are:
1.1 anton 1438:
1.2 anton 1439: @itemize @bullet
1440: @item
1441: Locals can only be cell-sized values (no type specifers are allowed).
1442: @item
1443: Locals can be defined only outside control structures.
1444: @item
1445: Locals can interfere with explicit usage of the return stack. For the
1446: exact (and long) rules, see the standard. If you don't use return stack
1447: accessing words in a definition using locals, you will we all right. The
1448: purpose of this rule is to make locals implementation on the return
1449: stack easier.
1450: @item
1451: The whole definition must be in one line.
1452: @end itemize
1453:
1454: Locals defined in this way behave like @code{VALUE}s
1.4 anton 1455: (@xref{Values}). I.e., they are initialized from the stack. Using their
1.2 anton 1456: name produces their value. Their value can be changed using @code{TO}.
1457:
1458: Since this syntax is supported by gforth directly, you need not do
1459: anything to use it. If you want to port a program using this syntax to
1460: another ANS Forth system, use @file{anslocal.fs} to implement the syntax
1461: on the other system.
1462:
1463: Note that a syntax shown in the standard, section A.13 looks
1464: similar, but is quite different in having the order of locals
1465: reversed. Beware!
1466:
1467: The ANS Forth locals wordset itself consists of the following word
1468:
1469: doc-(local)
1470:
1471: The ANS Forth locals extension wordset defines a syntax, but it is so
1472: awful that we strongly recommend not to use it. We have implemented this
1473: syntax to make porting to gforth easy, but do not document it here. The
1474: problem with this syntax is that the locals are defined in an order
1475: reversed with respect to the standard stack comment notation, making
1476: programs harder to read, and easier to misread and miswrite. The only
1477: merit of this syntax is that it is easy to implement using the ANS Forth
1478: locals wordset.
1.3 anton 1479:
1.4 anton 1480: @node Defining Words, Wordlists, Locals, Words
1481: @section Defining Words
1482:
1483: @node Values, , Defining Words, Defining Words
1484: @subsection Values
1485:
1486: @node Wordlists, Files, Defining Words, Words
1487: @section Wordlists
1488:
1489: @node Files, Blocks, Wordlists, Words
1490: @section Files
1491:
1492: @node Blocks, Other I/O, Files, Words
1493: @section Blocks
1494:
1495: @node Other I/O, Programming Tools, Blocks, Words
1496: @section Other I/O
1497:
1498: @node Programming Tools, Threading Words, Other I/O, Words
1499: @section Programming Tools
1500:
1.5 anton 1501: @menu
1502: * Debugging:: Simple and quick.
1503: * Assertions:: Making your programs self-checking.
1504: @end menu
1505:
1506: @node Debugging, Assertions, Programming Tools, Programming Tools
1.4 anton 1507: @subsection Debugging
1508:
1509: The simple debugging aids provided in @file{debugging.fs}
1510: are meant to support a different style of debugging than the
1511: tracing/stepping debuggers used in languages with long turn-around
1512: times.
1513:
1514: A much better (faster) way in fast-compilig languages is to add
1515: printing code at well-selected places, let the program run, look at
1516: the output, see where things went wrong, add more printing code, etc.,
1517: until the bug is found.
1518:
1519: The word @code{~~} is easy to insert. It just prints debugging
1520: information (by default the source location and the stack contents). It
1521: is also easy to remove (@kbd{C-x ~} in the Emacs Forth mode to
1522: query-replace them with nothing). The deferred words
1523: @code{printdebugdata} and @code{printdebugline} control the output of
1524: @code{~~}. The default source location output format works well with
1525: Emacs' compilation mode, so you can step through the program at the
1.5 anton 1526: source level using @kbd{C-x `} (the advantage over a stepping debugger
1527: is that you can step in any direction and you know where the crash has
1528: happened or where the strange data has occurred).
1.4 anton 1529:
1530: Note that the default actions clobber the contents of the pictured
1531: numeric output string, so you should not use @code{~~}, e.g., between
1532: @code{<#} and @code{#>}.
1533:
1534: doc-~~
1535: doc-printdebugdata
1536: doc-printdebugline
1537:
1.5 anton 1538: @node Assertions, , Debugging, Programming Tools
1.4 anton 1539: @subsection Assertions
1540:
1.5 anton 1541: It is a good idea to make your programs self-checking, in particular, if
1542: you use an assumption (e.g., that a certain field of a data structure is
1543: never zero) that may become wrong during maintenance. GForth supports
1544: assertions for this purpose. They are used like this:
1545:
1546: @example
1547: assert( @var{flag} )
1548: @end example
1549:
1550: The code between @code{assert(} and @code{)} should compute a flag, that
1551: should be true if everything is alright and false otherwise. It should
1552: not change anything else on the stack. The overall stack effect of the
1553: assertion is @code{( -- )}. E.g.
1554:
1555: @example
1556: assert( 1 1 + 2 = ) \ what we learn in school
1557: assert( dup 0<> ) \ assert that the top of stack is not zero
1558: assert( false ) \ this code should not be reached
1559: @end example
1560:
1561: The need for assertions is different at different times. During
1562: debugging, we want more checking, in production we sometimes care more
1563: for speed. Therefore, assertions can be turned off, i.e., the assertion
1564: becomes a comment. Depending on the importance of an assertion and the
1565: time it takes to check it, you may want to turn off some assertions and
1566: keep others turned on. GForth provides several levels of assertions for
1567: this purpose:
1568:
1569: doc-assert0(
1570: doc-assert1(
1571: doc-assert2(
1572: doc-assert3(
1573: doc-assert(
1574: doc-)
1575:
1576: @code{Assert(} is the same as @code{assert1(}. The variable
1577: @code{assert-level} specifies the highest assertions that are turned
1578: on. I.e., at the default @code{assert-level} of one, @code{assert0(} and
1579: @code{assert1(} assertions perform checking, while @code{assert2(} and
1580: @code{assert3(} assertions are treated as comments.
1581:
1582: Note that the @code{assert-level} is evaluated at compile-time, not at
1583: run-time. I.e., you cannot turn assertions on or off at run-time, you
1584: have to set the @code{assert-level} appropriately before compiling a
1585: piece of code. You can compile several pieces of code at several
1586: @code{assert-level}s (e.g., a trusted library at level 1 and newly
1587: written code at level 3).
1588:
1589: doc-assert-level
1590:
1591: If an assertion fails, a message compatible with Emacs' compilation mode
1592: is produced and the execution is aborted (currently with @code{ABORT"}.
1593: If there is interest, we will introduce a special throw code. But if you
1594: intend to @code{catch} a specific condition, using @code{throw} is
1595: probably more appropriate than an assertion).
1596:
1.4 anton 1597: @node Threading Words, , Programming Tools, Words
1598: @section Threading Words
1599:
1600: These words provide access to code addresses and other threading stuff
1601: in gforth (and, possibly, other interpretive Forths). It more or less
1602: abstracts away the differences between direct and indirect threading
1603: (and, for direct threading, the machine dependences). However, at
1604: present this wordset is still inclomplete. It is also pretty low-level;
1605: some day it will hopefully be made unnecessary by an internals words set
1606: that abstracts implementation details away completely.
1607:
1608: doc->code-address
1609: doc->does-code
1610: doc-code-address!
1611: doc-does-code!
1612: doc-does-handler!
1613: doc-/does-handler
1614:
1615: @node ANS conformance, Model, Words, Top
1616: @chapter ANS conformance
1617:
1618: @node Model, Emacs and GForth, ANS conformance, Top
1619: @chapter Model
1620:
1621: @node Emacs and GForth, Internals, Model, Top
1622: @chapter Emacs and GForth
1623:
1624: GForth comes with @file{gforth.el}, an improved version of
1625: @file{forth.el} by Goran Rydqvist (icluded in the TILE package). The
1626: improvements are a better (but still not perfect) handling of
1627: indentation. I have also added comment paragraph filling (@kbd{M-q}),
1.8 ! anton 1628: commenting (@kbd{C-x \}) and uncommenting (@kbd{C-u C-x \}) regions and
! 1629: removing debugging tracers (@kbd{C-x ~}, @pxref{Debugging}). I left the
! 1630: stuff I do not use alone, even though some of it only makes sense for
! 1631: TILE. To get a description of these features, enter Forth mode and type
! 1632: @kbd{C-h m}.
1.4 anton 1633:
1634: In addition, GForth supports Emacs quite well: The source code locations
1635: given in error messages, debugging output (from @code{~~}) and failed
1636: assertion messages are in the right format for Emacs' compilation mode
1637: (@pxref{Compilation, , Running Compilations under Emacs, emacs, Emacs
1638: Manual}) so the source location corresponding to an error or other
1639: message is only a few keystrokes away (@kbd{C-x `} for the next error,
1640: @kbd{C-c C-c} for the error under the cursor).
1641:
1642: Also, if you @code{include} @file{etags.fs}, a new @file{TAGS} file
1643: (@pxref{Tags, , Tags Tables, emacs, Emacs Manual}) will be produced that
1644: contains the definitions of all words defined afterwards. You can then
1645: find the source for a word using @kbd{M-.}. Note that emacs can use
1646: several tags files at the same time (e.g., one for the gforth sources
1647: and one for your program).
1648:
1649: To get all these benefits, add the following lines to your @file{.emacs}
1650: file:
1651:
1652: @example
1653: (autoload 'forth-mode "gforth.el")
1654: (setq auto-mode-alist (cons '("\\.fs\\'" . forth-mode) auto-mode-alist))
1655: @end example
1656:
1657: @node Internals, Bugs, Emacs and GForth, Top
1.3 anton 1658: @chapter Internals
1659:
1660: Reading this section is not necessary for programming with gforth. It
1661: should be helpful for finding your way in the gforth sources.
1662:
1.4 anton 1663: @menu
1664: * Portability::
1665: * Threading::
1666: * Primitives::
1667: * System Architecture::
1668: @end menu
1669:
1670: @node Portability, Threading, Internals, Internals
1.3 anton 1671: @section Portability
1672:
1673: One of the main goals of the effort is availability across a wide range
1674: of personal machines. fig-Forth, and, to a lesser extent, F83, achieved
1675: this goal by manually coding the engine in assembly language for several
1676: then-popular processors. This approach is very labor-intensive and the
1677: results are short-lived due to progress in computer architecture.
1678:
1679: Others have avoided this problem by coding in C, e.g., Mitch Bradley
1680: (cforth), Mikael Patel (TILE) and Dirk Zoller (pfe). This approach is
1681: particularly popular for UNIX-based Forths due to the large variety of
1682: architectures of UNIX machines. Unfortunately an implementation in C
1683: does not mix well with the goals of efficiency and with using
1684: traditional techniques: Indirect or direct threading cannot be expressed
1685: in C, and switch threading, the fastest technique available in C, is
1686: significantly slower. Another problem with C is that it's very
1687: cumbersome to express double integer arithmetic.
1688:
1689: Fortunately, there is a portable language that does not have these
1690: limitations: GNU C, the version of C processed by the GNU C compiler
1691: (@pxref{C Extensions, , Extensions to the C Language Family, gcc.info,
1692: GNU C Manual}). Its labels as values feature (@pxref{Labels as Values, ,
1693: Labels as Values, gcc.info, GNU C Manual}) makes direct and indirect
1694: threading possible, its @code{long long} type (@pxref{Long Long, ,
1695: Double-Word Integers, gcc.info, GNU C Manual}) corresponds to Forths
1696: double numbers. GNU C is available for free on all important (and many
1697: unimportant) UNIX machines, VMS, 80386s running MS-DOS, the Amiga, and
1698: the Atari ST, so a Forth written in GNU C can run on all these
1699: machines@footnote{Due to Apple's look-and-feel lawsuit it is not
1.5 anton 1700: available on the Mac (@pxref{Boycott, , Protect Your Freedom---Fight
1.3 anton 1701: ``Look And Feel'', gcc.info, GNU C Manual}).}.
1702:
1703: Writing in a portable language has the reputation of producing code that
1704: is slower than assembly. For our Forth engine we repeatedly looked at
1705: the code produced by the compiler and eliminated most compiler-induced
1706: inefficiencies by appropriate changes in the source-code.
1707:
1708: However, register allocation cannot be portably influenced by the
1709: programmer, leading to some inefficiencies on register-starved
1710: machines. We use explicit register declarations (@pxref{Explicit Reg
1711: Vars, , Variables in Specified Registers, gcc.info, GNU C Manual}) to
1712: improve the speed on some machines. They are turned on by using the
1713: @code{gcc} switch @code{-DFORCE_REG}. Unfortunately, this feature not
1714: only depends on the machine, but also on the compiler version: On some
1715: machines some compiler versions produce incorrect code when certain
1716: explicit register declarations are used. So by default
1717: @code{-DFORCE_REG} is not used.
1718:
1.4 anton 1719: @node Threading, Primitives, Portability, Internals
1.3 anton 1720: @section Threading
1721:
1722: GNU C's labels as values extension (available since @code{gcc-2.0},
1723: @pxref{Labels as Values, , Labels as Values, gcc.info, GNU C Manual})
1724: makes it possible to take the address of @var{label} by writing
1725: @code{&&@var{label}}. This address can then be used in a statement like
1726: @code{goto *@var{address}}. I.e., @code{goto *&&x} is the same as
1727: @code{goto x}.
1728:
1729: With this feature an indirect threaded NEXT looks like:
1730: @example
1731: cfa = *ip++;
1732: ca = *cfa;
1733: goto *ca;
1734: @end example
1735: For those unfamiliar with the names: @code{ip} is the Forth instruction
1736: pointer; the @code{cfa} (code-field address) corresponds to ANS Forths
1737: execution token and points to the code field of the next word to be
1738: executed; The @code{ca} (code address) fetched from there points to some
1739: executable code, e.g., a primitive or the colon definition handler
1740: @code{docol}.
1741:
1742: Direct threading is even simpler:
1743: @example
1744: ca = *ip++;
1745: goto *ca;
1746: @end example
1747:
1748: Of course we have packaged the whole thing neatly in macros called
1749: @code{NEXT} and @code{NEXT1} (the part of NEXT after fetching the cfa).
1750:
1.4 anton 1751: @menu
1752: * Scheduling::
1753: * Direct or Indirect Threaded?::
1754: * DOES>::
1755: @end menu
1756:
1757: @node Scheduling, Direct or Indirect Threaded?, Threading, Threading
1.3 anton 1758: @subsection Scheduling
1759:
1760: There is a little complication: Pipelined and superscalar processors,
1761: i.e., RISC and some modern CISC machines can process independent
1762: instructions while waiting for the results of an instruction. The
1763: compiler usually reorders (schedules) the instructions in a way that
1764: achieves good usage of these delay slots. However, on our first tries
1765: the compiler did not do well on scheduling primitives. E.g., for
1766: @code{+} implemented as
1767: @example
1768: n=sp[0]+sp[1];
1769: sp++;
1770: sp[0]=n;
1771: NEXT;
1772: @end example
1773: the NEXT comes strictly after the other code, i.e., there is nearly no
1774: scheduling. After a little thought the problem becomes clear: The
1775: compiler cannot know that sp and ip point to different addresses (and
1.4 anton 1776: the version of @code{gcc} we used would not know it even if it was
1777: possible), so it could not move the load of the cfa above the store to
1778: the TOS. Indeed the pointers could be the same, if code on or very near
1779: the top of stack were executed. In the interest of speed we chose to
1780: forbid this probably unused ``feature'' and helped the compiler in
1781: scheduling: NEXT is divided into the loading part (@code{NEXT_P1}) and
1782: the goto part (@code{NEXT_P2}). @code{+} now looks like:
1.3 anton 1783: @example
1784: n=sp[0]+sp[1];
1785: sp++;
1786: NEXT_P1;
1787: sp[0]=n;
1788: NEXT_P2;
1789: @end example
1.4 anton 1790: This can be scheduled optimally by the compiler.
1.3 anton 1791:
1792: This division can be turned off with the switch @code{-DCISC_NEXT}. This
1793: switch is on by default on machines that do not profit from scheduling
1794: (e.g., the 80386), in order to preserve registers.
1795:
1.4 anton 1796: @node Direct or Indirect Threaded?, DOES>, Scheduling, Threading
1.3 anton 1797: @subsection Direct or Indirect Threaded?
1798:
1799: Both! After packaging the nasty details in macro definitions we
1800: realized that we could switch between direct and indirect threading by
1801: simply setting a compilation flag (@code{-DDIRECT_THREADED}) and
1802: defining a few machine-specific macros for the direct-threading case.
1803: On the Forth level we also offer access words that hide the
1804: differences between the threading methods (@pxref{Threading Words}).
1805:
1806: Indirect threading is implemented completely
1807: machine-independently. Direct threading needs routines for creating
1808: jumps to the executable code (e.g. to docol or dodoes). These routines
1809: are inherently machine-dependent, but they do not amount to many source
1810: lines. I.e., even porting direct threading to a new machine is a small
1811: effort.
1812:
1.4 anton 1813: @node DOES>, , Direct or Indirect Threaded?, Threading
1.3 anton 1814: @subsection DOES>
1815: One of the most complex parts of a Forth engine is @code{dodoes}, i.e.,
1816: the chunk of code executed by every word defined by a
1817: @code{CREATE}...@code{DOES>} pair. The main problem here is: How to find
1818: the Forth code to be executed, i.e. the code after the @code{DOES>} (the
1819: DOES-code)? There are two solutions:
1820:
1821: In fig-Forth the code field points directly to the dodoes and the
1822: DOES-code address is stored in the cell after the code address
1823: (i.e. at cfa cell+). It may seem that this solution is illegal in the
1824: Forth-79 and all later standards, because in fig-Forth this address
1825: lies in the body (which is illegal in these standards). However, by
1826: making the code field larger for all words this solution becomes legal
1827: again. We use this approach for the indirect threaded version. Leaving
1828: a cell unused in most words is a bit wasteful, but on the machines we
1829: are targetting this is hardly a problem. The other reason for having a
1830: code field size of two cells is to avoid having different image files
1.4 anton 1831: for direct and indirect threaded systems (@pxref{System Architecture}).
1.3 anton 1832:
1833: The other approach is that the code field points or jumps to the cell
1834: after @code{DOES}. In this variant there is a jump to @code{dodoes} at
1835: this address. @code{dodoes} can then get the DOES-code address by
1836: computing the code address, i.e., the address of the jump to dodoes,
1837: and add the length of that jump field. A variant of this is to have a
1838: call to @code{dodoes} after the @code{DOES>}; then the return address
1839: (which can be found in the return register on RISCs) is the DOES-code
1840: address. Since the two cells available in the code field are usually
1841: used up by the jump to the code address in direct threading, we use
1842: this approach for direct threading. We did not want to add another
1843: cell to the code field.
1844:
1.4 anton 1845: @node Primitives, System Architecture, Threading, Internals
1.3 anton 1846: @section Primitives
1847:
1.4 anton 1848: @menu
1849: * Automatic Generation::
1850: * TOS Optimization::
1851: * Produced code::
1852: @end menu
1853:
1854: @node Automatic Generation, TOS Optimization, Primitives, Primitives
1.3 anton 1855: @subsection Automatic Generation
1856:
1857: Since the primitives are implemented in a portable language, there is no
1858: longer any need to minimize the number of primitives. On the contrary,
1859: having many primitives is an advantage: speed. In order to reduce the
1860: number of errors in primitives and to make programming them easier, we
1861: provide a tool, the primitive generator (@file{prims2x.fs}), that
1862: automatically generates most (and sometimes all) of the C code for a
1863: primitive from the stack effect notation. The source for a primitive
1864: has the following form:
1865:
1866: @format
1867: @var{Forth-name} @var{stack-effect} @var{category} [@var{pronounc.}]
1868: [@code{""}@var{glossary entry}@code{""}]
1869: @var{C code}
1870: [@code{:}
1871: @var{Forth code}]
1872: @end format
1873:
1874: The items in brackets are optional. The category and glossary fields
1875: are there for generating the documentation, the Forth code is there
1876: for manual implementations on machines without GNU C. E.g., the source
1877: for the primitive @code{+} is:
1878: @example
1879: + n1 n2 -- n core plus
1880: n = n1+n2;
1881: @end example
1882:
1883: This looks like a specification, but in fact @code{n = n1+n2} is C
1884: code. Our primitive generation tool extracts a lot of information from
1885: the stack effect notations@footnote{We use a one-stack notation, even
1886: though we have separate data and floating-point stacks; The separate
1887: notation can be generated easily from the unified notation.}: The number
1888: of items popped from and pushed on the stack, their type, and by what
1889: name they are referred to in the C code. It then generates a C code
1890: prelude and postlude for each primitive. The final C code for @code{+}
1891: looks like this:
1892:
1893: @example
1894: I_plus: /* + ( n1 n2 -- n ) */ /* label, stack effect */
1895: /* */ /* documentation */
1.4 anton 1896: @{
1.3 anton 1897: DEF_CA /* definition of variable ca (indirect threading) */
1898: Cell n1; /* definitions of variables */
1899: Cell n2;
1900: Cell n;
1901: n1 = (Cell) sp[1]; /* input */
1902: n2 = (Cell) TOS;
1903: sp += 1; /* stack adjustment */
1904: NAME("+") /* debugging output (with -DDEBUG) */
1.4 anton 1905: @{
1.3 anton 1906: n = n1+n2; /* C code taken from the source */
1.4 anton 1907: @}
1.3 anton 1908: NEXT_P1; /* NEXT part 1 */
1909: TOS = (Cell)n; /* output */
1910: NEXT_P2; /* NEXT part 2 */
1.4 anton 1911: @}
1.3 anton 1912: @end example
1913:
1914: This looks long and inefficient, but the GNU C compiler optimizes quite
1915: well and produces optimal code for @code{+} on, e.g., the R3000 and the
1916: HP RISC machines: Defining the @code{n}s does not produce any code, and
1917: using them as intermediate storage also adds no cost.
1918:
1919: There are also other optimizations, that are not illustrated by this
1920: example: Assignments between simple variables are usually for free (copy
1921: propagation). If one of the stack items is not used by the primitive
1922: (e.g. in @code{drop}), the compiler eliminates the load from the stack
1923: (dead code elimination). On the other hand, there are some things that
1924: the compiler does not do, therefore they are performed by
1925: @file{prims2x.fs}: The compiler does not optimize code away that stores
1926: a stack item to the place where it just came from (e.g., @code{over}).
1927:
1928: While programming a primitive is usually easy, there are a few cases
1929: where the programmer has to take the actions of the generator into
1930: account, most notably @code{?dup}, but also words that do not (always)
1931: fall through to NEXT.
1932:
1.4 anton 1933: @node TOS Optimization, Produced code, Automatic Generation, Primitives
1.3 anton 1934: @subsection TOS Optimization
1935:
1936: An important optimization for stack machine emulators, e.g., Forth
1937: engines, is keeping one or more of the top stack items in
1.4 anton 1938: registers. If a word has the stack effect @var{in1}...@var{inx} @code{--}
1939: @var{out1}...@var{outy}, keeping the top @var{n} items in registers
1.3 anton 1940: @itemize
1941: @item
1942: is better than keeping @var{n-1} items, if @var{x>=n} and @var{y>=n},
1943: due to fewer loads from and stores to the stack.
1944: @item is slower than keeping @var{n-1} items, if @var{x<>y} and @var{x<n} and
1945: @var{y<n}, due to additional moves between registers.
1946: @end itemize
1947:
1948: In particular, keeping one item in a register is never a disadvantage,
1949: if there are enough registers. Keeping two items in registers is a
1950: disadvantage for frequent words like @code{?branch}, constants,
1951: variables, literals and @code{i}. Therefore our generator only produces
1952: code that keeps zero or one items in registers. The generated C code
1953: covers both cases; the selection between these alternatives is made at
1954: C-compile time using the switch @code{-DUSE_TOS}. @code{TOS} in the C
1955: code for @code{+} is just a simple variable name in the one-item case,
1956: otherwise it is a macro that expands into @code{sp[0]}. Note that the
1957: GNU C compiler tries to keep simple variables like @code{TOS} in
1958: registers, and it usually succeeds, if there are enough registers.
1959:
1960: The primitive generator performs the TOS optimization for the
1961: floating-point stack, too (@code{-DUSE_FTOS}). For floating-point
1962: operations the benefit of this optimization is even larger:
1963: floating-point operations take quite long on most processors, but can be
1964: performed in parallel with other operations as long as their results are
1965: not used. If the FP-TOS is kept in a register, this works. If
1966: it is kept on the stack, i.e., in memory, the store into memory has to
1967: wait for the result of the floating-point operation, lengthening the
1968: execution time of the primitive considerably.
1969:
1970: The TOS optimization makes the automatic generation of primitives a
1971: bit more complicated. Just replacing all occurrences of @code{sp[0]} by
1972: @code{TOS} is not sufficient. There are some special cases to
1973: consider:
1974: @itemize
1975: @item In the case of @code{dup ( w -- w w )} the generator must not
1976: eliminate the store to the original location of the item on the stack,
1977: if the TOS optimization is turned on.
1.4 anton 1978: @item Primitives with stack effects of the form @code{--}
1979: @var{out1}...@var{outy} must store the TOS to the stack at the start.
1980: Likewise, primitives with the stack effect @var{in1}...@var{inx} @code{--}
1.3 anton 1981: must load the TOS from the stack at the end. But for the null stack
1982: effect @code{--} no stores or loads should be generated.
1983: @end itemize
1984:
1.4 anton 1985: @node Produced code, , TOS Optimization, Primitives
1.3 anton 1986: @subsection Produced code
1987:
1988: To see what assembly code is produced for the primitives on your machine
1989: with your compiler and your flag settings, type @code{make engine.s} and
1.4 anton 1990: look at the resulting file @file{engine.s}.
1.3 anton 1991:
1.4 anton 1992: @node System Architecture, , Primitives, Internals
1.3 anton 1993: @section System Architecture
1994:
1995: Our Forth system consists not only of primitives, but also of
1996: definitions written in Forth. Since the Forth compiler itself belongs
1997: to those definitions, it is not possible to start the system with the
1998: primitives and the Forth source alone. Therefore we provide the Forth
1999: code as an image file in nearly executable form. At the start of the
2000: system a C routine loads the image file into memory, sets up the
2001: memory (stacks etc.) according to information in the image file, and
2002: starts executing Forth code.
2003:
2004: The image file format is a compromise between the goals of making it
2005: easy to generate image files and making them portable. The easiest way
2006: to generate an image file is to just generate a memory dump. However,
2007: this kind of image file cannot be used on a different machine, or on
2008: the next version of the engine on the same machine, it even might not
2009: work with the same engine compiled by a different version of the C
2010: compiler. We would like to have as few versions of the image file as
2011: possible, because we do not want to distribute many versions of the
2012: same image file, and to make it easy for the users to use their image
2013: files on many machines. We currently need to create a different image
2014: file for machines with different cell sizes and different byte order
2015: (little- or big-endian)@footnote{We consider adding information to the
2016: image file that enables the loader to change the byte order.}.
2017:
2018: Forth code that is going to end up in a portable image file has to
1.4 anton 2019: comply to some restrictions: addresses have to be stored in memory with
2020: special words (@code{A!}, @code{A,}, etc.) in order to make the code
2021: relocatable. Cells, floats, etc., have to be stored at the natural
2022: alignment boundaries@footnote{E.g., store floats (8 bytes) at an address
2023: dividable by~8. This happens automatically in our system when you use
2024: the ANS Forth alignment words.}, in order to avoid alignment faults on
2025: machines with stricter alignment. The image file is produced by a
2026: metacompiler (@file{cross.fs}).
1.3 anton 2027:
2028: So, unlike the image file of Mitch Bradleys @code{cforth}, our image
2029: file is not directly executable, but has to undergo some manipulations
2030: during loading. Address relocation is performed at image load-time, not
2031: at run-time. The loader also has to replace tokens standing for
2032: primitive calls with the appropriate code-field addresses (or code
2033: addresses in the case of direct threading).
1.4 anton 2034:
2035: @node Bugs, Pedigree, Internals, Top
2036: @chapter Bugs
2037:
2038: @node Pedigree, Word Index, Bugs, Top
2039: @chapter Pedigree
2040:
2041: @node Word Index, Node Index, Pedigree, Top
2042: @chapter Word Index
2043:
2044: @node Node Index, , Word Index, Top
2045: @chapter Node Index
1.1 anton 2046:
2047: @contents
2048: @bye
2049:
FreeBSD-CVSweb <freebsd-cvsweb@FreeBSD.org>