Annotation of gforth/gforth.ds, revision 1.4
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
! 431: avoid them), you might start with @cite{Goldberg, What every computer
! 432: scientist should know about floating-point numbers, Computing Surveys
! 433: ?}.
! 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-fsin
! 453: doc-fcos
! 454: doc-fsincos
! 455: doc-ftan
! 456: doc-fasin
! 457: doc-facos
! 458: doc-fatan
! 459: doc-fatan2
! 460: doc-fsinh
! 461: doc-fcosh
! 462: doc-ftanh
! 463: doc-fasinh
! 464: doc-facosh
! 465: doc-fatanh
! 466:
! 467: @node Stack Manipulation, Memory access, Arithmetic, Words
1.1 anton 468: @section Stack Manipulation
469:
470: gforth has a data stack (aka parameter stack) for characters, cells,
471: addresses, and double cells, a floating point stack for floating point
472: numbers, a return stack for storing the return addresses of colon
473: definitions and other data, and a locals stack for storing local
474: variables. Note that while every sane Forth has a separate floating
475: point stack, this is not strictly required; an ANS Forth system could
476: theoretically keep floating point numbers on the data stack. As an
477: additional difficulty, you don't know how many cells a floating point
478: number takes. It is reportedly possible to write words in a way that
479: they work also for a unified stack model, but we do not recommend trying
1.4 ! anton 480: it. Instead, just say that your program has an environmental dependency
! 481: on a separate FP stack.
! 482:
! 483: Also, a Forth system is allowed to keep the local variables on the
1.1 anton 484: return stack. This is reasonable, as local variables usually eliminate
485: the need to use the return stack explicitly. So, if you want to produce
486: a standard complying program and if you are using local variables in a
487: word, forget about return stack manipulations in that word (see the
488: standard document for the exact rules).
489:
1.4 ! anton 490: @menu
! 491: * Data stack::
! 492: * Floating point stack::
! 493: * Return stack::
! 494: * Locals stack::
! 495: * Stack pointer manipulation::
! 496: @end menu
! 497:
! 498: @node Data stack, Floating point stack, Stack Manipulation, Stack Manipulation
1.1 anton 499: @subsection Data stack
500: doc-drop
501: doc-nip
502: doc-dup
503: doc-over
504: doc-tuck
505: doc-swap
506: doc-rot
507: doc--rot
508: doc-?dup
509: doc-pick
510: doc-roll
511: doc-2drop
512: doc-2nip
513: doc-2dup
514: doc-2over
515: doc-2tuck
516: doc-2swap
517: doc-2rot
518:
1.4 ! anton 519: @node Floating point stack, Return stack, Data stack, Stack Manipulation
1.1 anton 520: @subsection Floating point stack
521: doc-fdrop
522: doc-fnip
523: doc-fdup
524: doc-fover
525: doc-ftuck
526: doc-fswap
527: doc-frot
528:
1.4 ! anton 529: @node Return stack, Locals stack, Floating point stack, Stack Manipulation
1.1 anton 530: @subsection Return stack
531: doc->r
532: doc-r>
533: doc-r@
534: doc-rdrop
535: doc-2>r
536: doc-2r>
537: doc-2r@
538: doc-2rdrop
539:
1.4 ! anton 540: @node Locals stack, Stack pointer manipulation, Return stack, Stack Manipulation
1.1 anton 541: @subsection Locals stack
542:
1.4 ! anton 543: @node Stack pointer manipulation, , Locals stack, Stack Manipulation
1.1 anton 544: @subsection Stack pointer manipulation
545: doc-sp@
546: doc-sp!
547: doc-fp@
548: doc-fp!
549: doc-rp@
550: doc-rp!
551: doc-lp@
552: doc-lp!
553:
1.4 ! anton 554: @node Memory access, Control Structures, Stack Manipulation, Words
1.1 anton 555: @section Memory access
556:
1.4 ! anton 557: @menu
! 558: * Stack-Memory transfers::
! 559: * Address arithmetic::
! 560: * Memory block access::
! 561: @end menu
! 562:
! 563: @node Stack-Memory transfers, Address arithmetic, Memory access, Memory access
1.1 anton 564: @subsection Stack-Memory transfers
565:
566: doc-@
567: doc-!
568: doc-+!
569: doc-c@
570: doc-c!
571: doc-2@
572: doc-2!
573: doc-f@
574: doc-f!
575: doc-sf@
576: doc-sf!
577: doc-df@
578: doc-df!
579:
1.4 ! anton 580: @node Address arithmetic, Memory block access, Stack-Memory transfers, Memory access
1.1 anton 581: @subsection Address arithmetic
582:
583: ANS Forth does not specify the sizes of the data types. Instead, it
584: offers a number of words for computing sizes and doing address
585: arithmetic. Basically, address arithmetic is performed in terms of
586: address units (aus); on most systems the address unit is one byte. Note
587: that a character may have more than one au, so @code{chars} is no noop
588: (on systems where it is a noop, it compiles to nothing).
589:
590: ANS Forth also defines words for aligning addresses for specific
591: addresses. Many computers require that accesses to specific data types
592: must only occur at specific addresses; e.g., that cells may only be
593: accessed at addresses divisible by 4. Even if a machine allows unaligned
594: accesses, it can usually perform aligned accesses faster.
595:
596: For the performance-concious: alignment operations are usually only
597: necessary during the definition of a data structure, not during the
598: (more frequent) accesses to it.
599:
600: ANS Forth defines no words for character-aligning addresses. This is not
601: an oversight, but reflects the fact that addresses that are not
602: char-aligned have no use in the standard and therefore will not be
603: created.
604:
605: The standard guarantees that addresses returned by @code{CREATE}d words
606: are cell-aligned; in addition, gforth guarantees that these addresses
607: are aligned for all purposes.
608:
609: doc-chars
610: doc-char+
611: doc-cells
612: doc-cell+
613: doc-align
614: doc-aligned
615: doc-floats
616: doc-float+
617: doc-falign
618: doc-faligned
619: doc-sfloats
620: doc-sfloat+
621: doc-sfalign
622: doc-sfaligned
623: doc-dfloats
624: doc-dfloat+
625: doc-dfalign
626: doc-dfaligned
627: doc-address-unit-bits
628:
1.4 ! anton 629: @node Memory block access, , Address arithmetic, Memory access
1.1 anton 630: @subsection Memory block access
631:
632: doc-move
633: doc-erase
634:
635: While the previous words work on address units, the rest works on
636: characters.
637:
638: doc-cmove
639: doc-cmove>
640: doc-fill
641: doc-blank
642:
1.4 ! anton 643: @node Control Structures, Locals, Memory access, Words
1.1 anton 644: @section Control Structures
645:
646: Control structures in Forth cannot be used in interpret state, only in
647: compile state, i.e., in a colon definition. We do not like this
648: limitation, but have not seen a satisfying way around it yet, although
649: many schemes have been proposed.
650:
1.4 ! anton 651: @menu
! 652: * Selection::
! 653: * Simple Loops::
! 654: * Counted Loops::
! 655: * Arbitrary control structures::
! 656: * Calls and returns::
! 657: * Exception Handling::
! 658: @end menu
! 659:
! 660: @node Selection, Simple Loops, Control Structures, Control Structures
1.1 anton 661: @subsection Selection
662:
663: @example
664: @var{flag}
665: IF
666: @var{code}
667: ENDIF
668: @end example
669: or
670: @example
671: @var{flag}
672: IF
673: @var{code1}
674: ELSE
675: @var{code2}
676: ENDIF
677: @end example
678:
1.4 ! anton 679: You can use @code{THEN} instead of @code{ENDIF}. Indeed, @code{THEN} is
1.1 anton 680: standard, and @code{ENDIF} is not, although it is quite popular. We
681: recommend using @code{ENDIF}, because it is less confusing for people
682: who also know other languages (and is not prone to reinforcing negative
683: prejudices against Forth in these people). Adding @code{ENDIF} to a
684: system that only supplies @code{THEN} is simple:
685: @example
686: : endif POSTPONE then ; immediate
687: @end example
688:
689: [According to @cite{Webster's New Encyclopedic Dictionary}, @dfn{then
690: (adv.)} has the following meanings:
691: @quotation
692: ... 2b: following next after in order ... 3d: as a necessary consequence
693: (if you were there, then you saw them).
694: @end quotation
695: Forth's @code{THEN} has the meaning 2b, whereas @code{THEN} in Pascal
696: and many other programming languages has the meaning 3d.]
697:
698: We also provide the words @code{?dup-if} and @code{?dup-0=-if}, so you
699: can avoid using @code{?dup}.
700:
701: @example
702: @var{n}
703: CASE
704: @var{n1} OF @var{code1} ENDOF
705: @var{n2} OF @var{code2} ENDOF
1.4 ! anton 706: @dots{}
1.1 anton 707: ENDCASE
708: @end example
709:
710: Executes the first @var{codei}, where the @var{ni} is equal to
711: @var{n}. A default case can be added by simply writing the code after
712: the last @code{ENDOF}. It may use @var{n}, which is on top of the stack,
713: but must not consume it.
714:
1.4 ! anton 715: @node Simple Loops, Counted Loops, Selection, Control Structures
1.1 anton 716: @subsection Simple Loops
717:
718: @example
719: BEGIN
720: @var{code1}
721: @var{flag}
722: WHILE
723: @var{code2}
724: REPEAT
725: @end example
726:
727: @var{code1} is executed and @var{flag} is computed. If it is true,
728: @var{code2} is executed and the loop is restarted; If @var{flag} is false, execution continues after the @code{REPEAT}.
729:
730: @example
731: BEGIN
732: @var{code}
733: @var{flag}
734: UNTIL
735: @end example
736:
737: @var{code} is executed. The loop is restarted if @code{flag} is false.
738:
739: @example
740: BEGIN
741: @var{code}
742: AGAIN
743: @end example
744:
745: This is an endless loop.
746:
1.4 ! anton 747: @node Counted Loops, Arbitrary control structures, Simple Loops, Control Structures
1.1 anton 748: @subsection Counted Loops
749:
750: The basic counted loop is:
751: @example
752: @var{limit} @var{start}
753: ?DO
754: @var{body}
755: LOOP
756: @end example
757:
758: This performs one iteration for every integer, starting from @var{start}
759: and up to, but excluding @var{limit}. The counter, aka index, can be
760: accessed with @code{i}. E.g., the loop
761: @example
762: 10 0 ?DO
763: i .
764: LOOP
765: @end example
766: prints
767: @example
768: 0 1 2 3 4 5 6 7 8 9
769: @end example
770: The index of the innermost loop can be accessed with @code{i}, the index
771: of the next loop with @code{j}, and the index of the third loop with
772: @code{k}.
773:
774: The loop control data are kept on the return stack, so there are some
775: restrictions on mixing return stack accesses and counted loop
776: words. E.g., if you put values on the return stack outside the loop, you
777: cannot read them inside the loop. If you put values on the return stack
778: within a loop, you have to remove them before the end of the loop and
779: before accessing the index of the loop.
780:
781: There are several variations on the counted loop:
782:
783: @code{LEAVE} leaves the innermost counted loop immediately.
784:
785: @code{LOOP} can be replaced with @code{@var{n} +LOOP}; this updates the
786: index by @var{n} instead of by 1. The loop is terminated when the border
787: between @var{limit-1} and @var{limit} is crossed. E.g.:
788:
1.2 anton 789: @code{4 0 ?DO i . 2 +LOOP} prints @code{0 2}
1.1 anton 790:
1.2 anton 791: @code{4 1 ?DO i . 2 +LOOP} prints @code{1 3}
1.1 anton 792:
793: The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative:
794:
1.2 anton 795: @code{-1 0 ?DO i . -1 +LOOP} prints @code{0 -1}
1.1 anton 796:
1.2 anton 797: @code{ 0 0 ?DO i . -1 +LOOP} prints nothing
1.1 anton 798:
799: Therefore we recommend avoiding using @code{@var{n} +LOOP} with negative
800: @var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative
801: case behaves symmetrical to the positive case:
802:
1.2 anton 803: @code{-2 0 ?DO i . -1 +LOOP} prints @code{0 -1}
1.1 anton 804:
1.2 anton 805: @code{-1 0 ?DO i . -1 +LOOP} prints @code{0}
1.1 anton 806:
1.2 anton 807: @code{ 0 0 ?DO i . -1 +LOOP} prints nothing
1.1 anton 808:
1.2 anton 809: The loop is terminated when the border between @var{limit@minus{}sgn(n)} and
1.1 anton 810: @var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS
811: Forth standard.
812:
813: @code{?DO} can be replaced by @code{DO}. @code{DO} enters the loop even
814: when the start and the limit value are equal. We do not recommend using
815: @code{DO}. It will just give you maintenance troubles.
816:
817: @code{UNLOOP} is used to prepare for an abnormal loop exit, e.g., via
818: @code{EXIT}. @code{UNLOOP} removes the loop control parameters from the
819: return stack so @code{EXIT} can get to its return address.
820:
821: Another counted loop is
822: @example
823: @var{n}
824: FOR
825: @var{body}
826: NEXT
827: @end example
828: This is the preferred loop of native code compiler writers who are too
829: lazy to optimize @code{?DO} loops properly. In GNU Forth, this loop
830: iterates @var{n+1} times; @code{i} produces values starting with @var{n}
831: and ending with 0. Other Forth systems may behave differently, even if
832: they support @code{FOR} loops.
833:
1.4 ! anton 834: @node Arbitrary control structures, Calls and returns, Counted Loops, Control Structures
1.2 anton 835: @subsection Arbitrary control structures
836:
837: ANS Forth permits and supports using control structures in a non-nested
838: way. Information about incomplete control structures is stored on the
839: control-flow stack. This stack may be implemented on the Forth data
840: stack, and this is what we have done in gforth.
841:
842: An @i{orig} entry represents an unresolved forward branch, a @i{dest}
843: entry represents a backward branch target. A few words are the basis for
844: building any control structure possible (except control structures that
845: need storage, like calls, coroutines, and backtracking).
846:
1.3 anton 847: doc-if
848: doc-ahead
849: doc-then
850: doc-begin
851: doc-until
852: doc-again
853: doc-cs-pick
854: doc-cs-roll
1.2 anton 855:
856: On many systems control-flow stack items take one word, in gforth they
857: currently take three (this may change in the future). Therefore it is a
858: really good idea to manipulate the control flow stack with
859: @code{cs-pick} and @code{cs-roll}, not with data stack manipulation
860: words.
861:
862: Some standard control structure words are built from these words:
863:
1.3 anton 864: doc-else
865: doc-while
866: doc-repeat
1.2 anton 867:
868: Counted loop words constitute a separate group of words:
869:
1.3 anton 870: doc-?do
871: doc-do
872: doc-for
873: doc-loop
874: doc-s+loop
875: doc-+loop
876: doc-next
877: doc-leave
878: doc-?leave
879: doc-unloop
880: doc-undo
1.2 anton 881:
882: The standard does not allow using @code{cs-pick} and @code{cs-roll} on
883: @i{do-sys}. Our system allows it, but it's your job to ensure that for
884: every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path
1.3 anton 885: through the definition (@code{LOOP} etc. compile an @code{UNLOOP} on the
886: fall-through path). Also, you have to ensure that all @code{LEAVE}s are
887: resolved (by using one of the loop-ending words or @code{UNDO}).
1.2 anton 888:
889: Another group of control structure words are
890:
1.3 anton 891: doc-case
892: doc-endcase
893: doc-of
894: doc-endof
1.2 anton 895:
896: @i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and
897: @code{cs-roll}.
898:
1.3 anton 899: @subsubsection Programming Style
900:
901: In order to ensure readability we recommend that you do not create
902: arbitrary control structures directly, but define new control structure
903: words for the control structure you want and use these words in your
904: program.
905:
906: E.g., instead of writing
907:
908: @example
909: begin
910: ...
911: if [ 1 cs-roll ]
912: ...
913: again then
914: @end example
915:
916: we recommend defining control structure words, e.g.,
917:
918: @example
919: : while ( dest -- orig dest )
920: POSTPONE if
921: 1 cs-roll ; immediate
922:
923: : repeat ( orig dest -- )
924: POSTPONE again
925: POSTPONE then ; immediate
926: @end example
927:
928: and then using these to create the control structure:
929:
930: @example
931: begin
932: ...
933: while
934: ...
935: repeat
936: @end example
937:
938: That's much easier to read, isn't it? Of course, @code{BEGIN} and
939: @code{WHILE} are predefined, so in this example it would not be
940: necessary to define them.
941:
1.4 ! anton 942: @node Calls and returns, Exception Handling, Arbitrary control structures, Control Structures
1.3 anton 943: @subsection Calls and returns
944:
945: A definition can be called simply be writing the name of the
946: definition. When the end of the definition is reached, it returns. An earlier return can be forced using
947:
948: doc-exit
949:
950: Don't forget to clean up the return stack and @code{UNLOOP} any
951: outstanding @code{?DO}...@code{LOOP}s before @code{EXIT}ing. The
952: primitive compiled by @code{EXIT} is
953:
954: doc-;s
955:
1.4 ! anton 956: @node Exception Handling, , Calls and returns, Control Structures
1.3 anton 957: @subsection Exception Handling
958:
959: doc-catch
960: doc-throw
961:
1.4 ! anton 962: @node Locals, Defining Words, Control Structures, Words
1.1 anton 963: @section Locals
964:
1.2 anton 965: Local variables can make Forth programming more enjoyable and Forth
966: programs easier to read. Unfortunately, the locals of ANS Forth are
967: laden with restrictions. Therefore, we provide not only the ANS Forth
968: locals wordset, but also our own, more powerful locals wordset (we
969: implemented the ANS Forth locals wordset through our locals wordset).
970:
971: @menu
1.4 ! anton 972: * gforth locals::
! 973: * ANS Forth locals::
1.2 anton 974: @end menu
975:
1.4 ! anton 976: @node gforth locals, ANS Forth locals, Locals, Locals
1.2 anton 977: @subsection gforth locals
978:
979: Locals can be defined with
980:
981: @example
982: @{ local1 local2 ... -- comment @}
983: @end example
984: or
985: @example
986: @{ local1 local2 ... @}
987: @end example
988:
989: E.g.,
990: @example
991: : max @{ n1 n2 -- n3 @}
992: n1 n2 > if
993: n1
994: else
995: n2
996: endif ;
997: @end example
998:
999: The similarity of locals definitions with stack comments is intended. A
1000: locals definition often replaces the stack comment of a word. The order
1001: of the locals corresponds to the order in a stack comment and everything
1002: after the @code{--} is really a comment.
1003:
1004: This similarity has one disadvantage: It is too easy to confuse locals
1005: declarations with stack comments, causing bugs and making them hard to
1006: find. However, this problem can be avoided by appropriate coding
1007: conventions: Do not use both notations in the same program. If you do,
1008: they should be distinguished using additional means, e.g. by position.
1009:
1010: The name of the local may be preceded by a type specifier, e.g.,
1011: @code{F:} for a floating point value:
1012:
1013: @example
1014: : CX* @{ F: Ar F: Ai F: Br F: Bi -- Cr Ci @}
1015: \ complex multiplication
1016: Ar Br f* Ai Bi f* f-
1017: Ar Bi f* Ai Br f* f+ ;
1018: @end example
1019:
1020: GNU Forth currently supports cells (@code{W:}, @code{W^}), doubles
1021: (@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters
1022: (@code{C:}, @code{C^}) in two flavours: a value-flavoured local (defined
1023: with @code{W:}, @code{D:} etc.) produces its value and can be changed
1024: with @code{TO}. A variable-flavoured local (defined with @code{W^} etc.)
1025: produces its address (which becomes invalid when the variable's scope is
1026: left). E.g., the standard word @code{emit} can be defined in therms of
1027: @code{type} like this:
1028:
1029: @example
1030: : emit @{ C^ char* -- @}
1031: char* 1 type ;
1032: @end example
1033:
1034: A local without type specifier is a @code{W:} local. Both flavours of
1035: locals are initialized with values from the data or FP stack.
1036:
1037: Currently there is no way to define locals with user-defined data
1038: structures, but we are working on it.
1039:
1040: GNU Forth allows defining locals everywhere in a colon definition. This poses the following questions:
1041:
1.4 ! anton 1042: @menu
! 1043: * Where are locals visible by name?::
! 1044: * How long do locals live? ::
! 1045: * Programming Style::
! 1046: * Implementation::
! 1047: @end menu
! 1048:
! 1049: @node Where are locals visible by name?, How long do locals live?, gforth locals, gforth locals
1.2 anton 1050: @subsubsection Where are locals visible by name?
1051:
1052: Basically, the answer is that locals are visible where you would expect
1053: it in block-structured languages, and sometimes a little longer. If you
1054: want to restrict the scope of a local, enclose its definition in
1055: @code{SCOPE}...@code{ENDSCOPE}.
1056:
1057: doc-scope
1058: doc-endscope
1059:
1060: These words behave like control structure words, so you can use them
1061: with @code{CS-PICK} and @code{CS-ROLL} to restrict the scope in
1062: arbitrary ways.
1063:
1064: If you want a more exact answer to the visibility question, here's the
1065: basic principle: A local is visible in all places that can only be
1066: reached through the definition of the local@footnote{In compiler
1067: construction terminology, all places dominated by the definition of the
1068: local.}. In other words, it is not visible in places that can be reached
1069: without going through the definition of the local. E.g., locals defined
1070: in @code{IF}...@code{ENDIF} are visible until the @code{ENDIF}, locals
1071: defined in @code{BEGIN}...@code{UNTIL} are visible after the
1072: @code{UNTIL} (until, e.g., a subsequent @code{ENDSCOPE}).
1073:
1074: The reasoning behind this solution is: We want to have the locals
1075: visible as long as it is meaningful. The user can always make the
1076: visibility shorter by using explicit scoping. In a place that can
1077: only be reached through the definition of a local, the meaning of a
1078: local name is clear. In other places it is not: How is the local
1079: initialized at the control flow path that does not contain the
1080: definition? Which local is meant, if the same name is defined twice in
1081: two independent control flow paths?
1082:
1083: This should be enough detail for nearly all users, so you can skip the
1084: rest of this section. If you relly must know all the gory details and
1085: options, read on.
1086:
1087: In order to implement this rule, the compiler has to know which places
1088: are unreachable. It knows this automatically after @code{AHEAD},
1089: @code{AGAIN}, @code{EXIT} and @code{LEAVE}; in other cases (e.g., after
1090: most @code{THROW}s), you can use the word @code{UNREACHABLE} to tell the
1091: compiler that the control flow never reaches that place. If
1092: @code{UNREACHABLE} is not used where it could, the only consequence is
1093: that the visibility of some locals is more limited than the rule above
1094: says. If @code{UNREACHABLE} is used where it should not (i.e., if you
1095: lie to the compiler), buggy code will be produced.
1096:
1097: Another problem with this rule is that at @code{BEGIN}, the compiler
1.3 anton 1098: does not know which locals will be visible on the incoming
1099: back-edge. All problems discussed in the following are due to this
1100: ignorance of the compiler (we discuss the problems using @code{BEGIN}
1101: loops as examples; the discussion also applies to @code{?DO} and other
1.2 anton 1102: loops). Perhaps the most insidious example is:
1103: @example
1104: AHEAD
1105: BEGIN
1106: x
1107: [ 1 CS-ROLL ] THEN
1.4 ! anton 1108: @{ x @}
1.2 anton 1109: ...
1110: UNTIL
1111: @end example
1112:
1113: This should be legal according to the visibility rule. The use of
1114: @code{x} can only be reached through the definition; but that appears
1115: textually below the use.
1116:
1117: From this example it is clear that the visibility rules cannot be fully
1118: implemented without major headaches. Our implementation treats common
1119: cases as advertised and the exceptions are treated in a safe way: The
1120: compiler makes a reasonable guess about the locals visible after a
1121: @code{BEGIN}; if it is too pessimistic, the
1122: user will get a spurious error about the local not being defined; if the
1123: compiler is too optimistic, it will notice this later and issue a
1124: warning. In the case above the compiler would complain about @code{x}
1125: being undefined at its use. You can see from the obscure examples in
1126: this section that it takes quite unusual control structures to get the
1127: compiler into trouble, and even then it will often do fine.
1128:
1129: If the @code{BEGIN} is reachable from above, the most optimistic guess
1130: is that all locals visible before the @code{BEGIN} will also be
1131: visible after the @code{BEGIN}. This guess is valid for all loops that
1132: are entered only through the @code{BEGIN}, in particular, for normal
1133: @code{BEGIN}...@code{WHILE}...@code{REPEAT} and
1134: @code{BEGIN}...@code{UNTIL} loops and it is implemented in our
1135: compiler. When the branch to the @code{BEGIN} is finally generated by
1136: @code{AGAIN} or @code{UNTIL}, the compiler checks the guess and
1137: warns the user if it was too optimisitic:
1138: @example
1139: IF
1.4 ! anton 1140: @{ x @}
1.2 anton 1141: BEGIN
1142: \ x ?
1143: [ 1 cs-roll ] THEN
1144: ...
1145: UNTIL
1146: @end example
1147:
1148: Here, @code{x} lives only until the @code{BEGIN}, but the compiler
1149: optimistically assumes that it lives until the @code{THEN}. It notices
1150: this difference when it compiles the @code{UNTIL} and issues a
1151: warning. The user can avoid the warning, and make sure that @code{x}
1152: is not used in the wrong area by using explicit scoping:
1153: @example
1154: IF
1155: SCOPE
1.4 ! anton 1156: @{ x @}
1.2 anton 1157: ENDSCOPE
1158: BEGIN
1159: [ 1 cs-roll ] THEN
1160: ...
1161: UNTIL
1162: @end example
1163:
1164: Since the guess is optimistic, there will be no spurious error messages
1165: about undefined locals.
1166:
1167: If the @code{BEGIN} is not reachable from above (e.g., after
1168: @code{AHEAD} or @code{EXIT}), the compiler cannot even make an
1169: optimistic guess, as the locals visible after the @code{BEGIN} may be
1170: defined later. Therefore, the compiler assumes that no locals are
1171: visible after the @code{BEGIN}. However, the useer can use
1172: @code{ASSUME-LIVE} to make the compiler assume that the same locals are
1173: visible at the BEGIN as at the point where the item was created.
1174:
1175: doc-assume-live
1176:
1177: E.g.,
1178: @example
1.4 ! anton 1179: @{ x @}
1.2 anton 1180: AHEAD
1181: ASSUME-LIVE
1182: BEGIN
1183: x
1184: [ 1 CS-ROLL ] THEN
1185: ...
1186: UNTIL
1187: @end example
1188:
1189: Other cases where the locals are defined before the @code{BEGIN} can be
1190: handled by inserting an appropriate @code{CS-ROLL} before the
1191: @code{ASSUME-LIVE} (and changing the control-flow stack manipulation
1192: behind the @code{ASSUME-LIVE}).
1193:
1194: Cases where locals are defined after the @code{BEGIN} (but should be
1195: visible immediately after the @code{BEGIN}) can only be handled by
1196: rearranging the loop. E.g., the ``most insidious'' example above can be
1197: arranged into:
1198: @example
1199: BEGIN
1.4 ! anton 1200: @{ x @}
1.2 anton 1201: ... 0=
1202: WHILE
1203: x
1204: REPEAT
1205: @end example
1206:
1.4 ! anton 1207: @node How long do locals live?, Programming Style, Where are locals visible by name?, gforth locals
1.2 anton 1208: @subsubsection How long do locals live?
1209:
1210: The right answer for the lifetime question would be: A local lives at
1211: least as long as it can be accessed. For a value-flavoured local this
1212: means: until the end of its visibility. However, a variable-flavoured
1213: local could be accessed through its address far beyond its visibility
1214: scope. Ultimately, this would mean that such locals would have to be
1215: garbage collected. Since this entails un-Forth-like implementation
1216: complexities, I adopted the same cowardly solution as some other
1217: languages (e.g., C): The local lives only as long as it is visible;
1218: afterwards its address is invalid (and programs that access it
1219: afterwards are erroneous).
1220:
1.4 ! anton 1221: @node Programming Style, Implementation, How long do locals live?, gforth locals
1.2 anton 1222: @subsubsection Programming Style
1223:
1224: The freedom to define locals anywhere has the potential to change
1225: programming styles dramatically. In particular, the need to use the
1226: return stack for intermediate storage vanishes. Moreover, all stack
1227: manipulations (except @code{PICK}s and @code{ROLL}s with run-time
1228: determined arguments) can be eliminated: If the stack items are in the
1229: wrong order, just write a locals definition for all of them; then
1230: write the items in the order you want.
1231:
1232: This seems a little far-fetched and eliminating stack manipulations is
1.4 ! anton 1233: unlikely to become a conscious programming objective. Still, the number
! 1234: of stack manipulations will be reduced dramatically if local variables
! 1235: are used liberally (e.g., compare @code{max} in @ref{gforth locals} with
! 1236: a traditional implementation of @code{max}).
1.2 anton 1237:
1238: This shows one potential benefit of locals: making Forth programs more
1239: readable. Of course, this benefit will only be realized if the
1240: programmers continue to honour the principle of factoring instead of
1241: using the added latitude to make the words longer.
1242:
1243: Using @code{TO} can and should be avoided. Without @code{TO},
1244: every value-flavoured local has only a single assignment and many
1245: advantages of functional languages apply to Forth. I.e., programs are
1246: easier to analyse, to optimize and to read: It is clear from the
1247: definition what the local stands for, it does not turn into something
1248: different later.
1249:
1250: E.g., a definition using @code{TO} might look like this:
1251: @example
1252: : strcmp @{ addr1 u1 addr2 u2 -- n @}
1253: u1 u2 min 0
1254: ?do
1255: addr1 c@ addr2 c@ - ?dup
1256: if
1257: unloop exit
1258: then
1259: addr1 char+ TO addr1
1260: addr2 char+ TO addr2
1261: loop
1262: u1 u2 - ;
1263: @end example
1264: Here, @code{TO} is used to update @code{addr1} and @code{addr2} at
1265: every loop iteration. @code{strcmp} is a typical example of the
1266: readability problems of using @code{TO}. When you start reading
1267: @code{strcmp}, you think that @code{addr1} refers to the start of the
1268: string. Only near the end of the loop you realize that it is something
1269: else.
1270:
1271: This can be avoided by defining two locals at the start of the loop that
1272: are initialized with the right value for the current iteration.
1273: @example
1274: : strcmp @{ addr1 u1 addr2 u2 -- n @}
1275: addr1 addr2
1276: u1 u2 min 0
1277: ?do @{ s1 s2 @}
1278: s1 c@ s2 c@ - ?dup
1279: if
1280: unloop exit
1281: then
1282: s1 char+ s2 char+
1283: loop
1284: 2drop
1285: u1 u2 - ;
1286: @end example
1287: Here it is clear from the start that @code{s1} has a different value
1288: in every loop iteration.
1289:
1.4 ! anton 1290: @node Implementation, , Programming Style, gforth locals
1.2 anton 1291: @subsubsection Implementation
1292:
1293: GNU Forth uses an extra locals stack. The most compelling reason for
1294: this is that the return stack is not float-aligned; using an extra stack
1295: also eliminates the problems and restrictions of using the return stack
1296: as locals stack. Like the other stacks, the locals stack grows toward
1297: lower addresses. A few primitives allow an efficient implementation:
1298:
1299: doc-@local#
1300: doc-f@local#
1301: doc-laddr#
1302: doc-lp+!#
1303: doc-lp!
1304: doc->l
1305: doc-f>l
1306:
1307: In addition to these primitives, some specializations of these
1308: primitives for commonly occurring inline arguments are provided for
1309: efficiency reasons, e.g., @code{@@local0} as specialization of
1310: @code{@@local#} for the inline argument 0. The following compiling words
1311: compile the right specialized version, or the general version, as
1312: appropriate:
1313:
1314: doc-compile-@@local
1315: doc-compile-f@@local
1316: doc-compile-lp+!
1317:
1318: Combinations of conditional branches and @code{lp+!#} like
1319: @code{?branch-lp+!#} (the locals pointer is only changed if the branch
1320: is taken) are provided for efficiency and correctness in loops.
1321:
1322: A special area in the dictionary space is reserved for keeping the
1323: local variable names. @code{@{} switches the dictionary pointer to this
1324: area and @code{@}} switches it back and generates the locals
1325: initializing code. @code{W:} etc.@ are normal defining words. This
1326: special area is cleared at the start of every colon definition.
1327:
1328: A special feature of GNU Forths dictionary is used to implement the
1329: definition of locals without type specifiers: every wordlist (aka
1330: vocabulary) has its own methods for searching
1.4 ! anton 1331: etc. (@pxref{Wordlists}). For the present purpose we defined a wordlist
1.2 anton 1332: with a special search method: When it is searched for a word, it
1333: actually creates that word using @code{W:}. @code{@{} changes the search
1334: order to first search the wordlist containing @code{@}}, @code{W:} etc.,
1335: and then the wordlist for defining locals without type specifiers.
1336:
1337: The lifetime rules support a stack discipline within a colon
1338: definition: The lifetime of a local is either nested with other locals
1339: lifetimes or it does not overlap them.
1340:
1341: At @code{BEGIN}, @code{IF}, and @code{AHEAD} no code for locals stack
1342: pointer manipulation is generated. Between control structure words
1343: locals definitions can push locals onto the locals stack. @code{AGAIN}
1344: is the simplest of the other three control flow words. It has to
1345: restore the locals stack depth of the corresponding @code{BEGIN}
1346: before branching. The code looks like this:
1347: @format
1348: @code{lp+!#} current-locals-size @minus{} dest-locals-size
1349: @code{branch} <begin>
1350: @end format
1351:
1352: @code{UNTIL} is a little more complicated: If it branches back, it
1353: must adjust the stack just like @code{AGAIN}. But if it falls through,
1354: the locals stack must not be changed. The compiler generates the
1355: following code:
1356: @format
1357: @code{?branch-lp+!#} <begin> current-locals-size @minus{} dest-locals-size
1358: @end format
1359: The locals stack pointer is only adjusted if the branch is taken.
1360:
1361: @code{THEN} can produce somewhat inefficient code:
1362: @format
1363: @code{lp+!#} current-locals-size @minus{} orig-locals-size
1364: <orig target>:
1365: @code{lp+!#} orig-locals-size @minus{} new-locals-size
1366: @end format
1367: The second @code{lp+!#} adjusts the locals stack pointer from the
1.4 ! anton 1368: level at the @var{orig} point to the level after the @code{THEN}. The
1.2 anton 1369: first @code{lp+!#} adjusts the locals stack pointer from the current
1370: level to the level at the orig point, so the complete effect is an
1371: adjustment from the current level to the right level after the
1372: @code{THEN}.
1373:
1374: In a conventional Forth implementation a dest control-flow stack entry
1375: is just the target address and an orig entry is just the address to be
1376: patched. Our locals implementation adds a wordlist to every orig or dest
1377: item. It is the list of locals visible (or assumed visible) at the point
1378: described by the entry. Our implementation also adds a tag to identify
1379: the kind of entry, in particular to differentiate between live and dead
1380: (reachable and unreachable) orig entries.
1381:
1382: A few unusual operations have to be performed on locals wordlists:
1383:
1384: doc-common-list
1385: doc-sub-list?
1386: doc-list-size
1387:
1388: Several features of our locals wordlist implementation make these
1389: operations easy to implement: The locals wordlists are organised as
1390: linked lists; the tails of these lists are shared, if the lists
1391: contain some of the same locals; and the address of a name is greater
1392: than the address of the names behind it in the list.
1393:
1394: Another important implementation detail is the variable
1395: @code{dead-code}. It is used by @code{BEGIN} and @code{THEN} to
1396: determine if they can be reached directly or only through the branch
1397: that they resolve. @code{dead-code} is set by @code{UNREACHABLE},
1398: @code{AHEAD}, @code{EXIT} etc., and cleared at the start of a colon
1399: definition, by @code{BEGIN} and usually by @code{THEN}.
1400:
1401: Counted loops are similar to other loops in most respects, but
1402: @code{LEAVE} requires special attention: It performs basically the same
1403: service as @code{AHEAD}, but it does not create a control-flow stack
1404: entry. Therefore the information has to be stored elsewhere;
1405: traditionally, the information was stored in the target fields of the
1406: branches created by the @code{LEAVE}s, by organizing these fields into a
1407: linked list. Unfortunately, this clever trick does not provide enough
1408: space for storing our extended control flow information. Therefore, we
1409: introduce another stack, the leave stack. It contains the control-flow
1410: stack entries for all unresolved @code{LEAVE}s.
1411:
1412: Local names are kept until the end of the colon definition, even if
1413: they are no longer visible in any control-flow path. In a few cases
1414: this may lead to increased space needs for the locals name area, but
1415: usually less than reclaiming this space would cost in code size.
1416:
1417:
1.4 ! anton 1418: @node ANS Forth locals, , gforth locals, Locals
1.2 anton 1419: @subsection ANS Forth locals
1420:
1421: The ANS Forth locals wordset does not define a syntax for locals, but
1422: words that make it possible to define various syntaxes. One of the
1423: possible syntaxes is a subset of the syntax we used in the gforth locals
1424: wordset, i.e.:
1425:
1426: @example
1427: @{ local1 local2 ... -- comment @}
1428: @end example
1429: or
1430: @example
1431: @{ local1 local2 ... @}
1432: @end example
1433:
1434: The order of the locals corresponds to the order in a stack comment. The
1435: restrictions are:
1.1 anton 1436:
1.2 anton 1437: @itemize @bullet
1438: @item
1439: Locals can only be cell-sized values (no type specifers are allowed).
1440: @item
1441: Locals can be defined only outside control structures.
1442: @item
1443: Locals can interfere with explicit usage of the return stack. For the
1444: exact (and long) rules, see the standard. If you don't use return stack
1445: accessing words in a definition using locals, you will we all right. The
1446: purpose of this rule is to make locals implementation on the return
1447: stack easier.
1448: @item
1449: The whole definition must be in one line.
1450: @end itemize
1451:
1452: Locals defined in this way behave like @code{VALUE}s
1.4 ! anton 1453: (@xref{Values}). I.e., they are initialized from the stack. Using their
1.2 anton 1454: name produces their value. Their value can be changed using @code{TO}.
1455:
1456: Since this syntax is supported by gforth directly, you need not do
1457: anything to use it. If you want to port a program using this syntax to
1458: another ANS Forth system, use @file{anslocal.fs} to implement the syntax
1459: on the other system.
1460:
1461: Note that a syntax shown in the standard, section A.13 looks
1462: similar, but is quite different in having the order of locals
1463: reversed. Beware!
1464:
1465: The ANS Forth locals wordset itself consists of the following word
1466:
1467: doc-(local)
1468:
1469: The ANS Forth locals extension wordset defines a syntax, but it is so
1470: awful that we strongly recommend not to use it. We have implemented this
1471: syntax to make porting to gforth easy, but do not document it here. The
1472: problem with this syntax is that the locals are defined in an order
1473: reversed with respect to the standard stack comment notation, making
1474: programs harder to read, and easier to misread and miswrite. The only
1475: merit of this syntax is that it is easy to implement using the ANS Forth
1476: locals wordset.
1.3 anton 1477:
1.4 ! anton 1478: @node Defining Words, Wordlists, Locals, Words
! 1479: @section Defining Words
! 1480:
! 1481: @node Values, , Defining Words, Defining Words
! 1482: @subsection Values
! 1483:
! 1484: @node Wordlists, Files, Defining Words, Words
! 1485: @section Wordlists
! 1486:
! 1487: @node Files, Blocks, Wordlists, Words
! 1488: @section Files
! 1489:
! 1490: @node Blocks, Other I/O, Files, Words
! 1491: @section Blocks
! 1492:
! 1493: @node Other I/O, Programming Tools, Blocks, Words
! 1494: @section Other I/O
! 1495:
! 1496: @node Programming Tools, Threading Words, Other I/O, Words
! 1497: @section Programming Tools
! 1498:
! 1499: @node
! 1500: @subsection Debugging
! 1501:
! 1502: The simple debugging aids provided in @file{debugging.fs}
! 1503: are meant to support a different style of debugging than the
! 1504: tracing/stepping debuggers used in languages with long turn-around
! 1505: times.
! 1506:
! 1507: A much better (faster) way in fast-compilig languages is to add
! 1508: printing code at well-selected places, let the program run, look at
! 1509: the output, see where things went wrong, add more printing code, etc.,
! 1510: until the bug is found.
! 1511:
! 1512: The word @code{~~} is easy to insert. It just prints debugging
! 1513: information (by default the source location and the stack contents). It
! 1514: is also easy to remove (@kbd{C-x ~} in the Emacs Forth mode to
! 1515: query-replace them with nothing). The deferred words
! 1516: @code{printdebugdata} and @code{printdebugline} control the output of
! 1517: @code{~~}. The default source location output format works well with
! 1518: Emacs' compilation mode, so you can step through the program at the
! 1519: source level using @kbd{C-x `}.
! 1520:
! 1521: Note that the default actions clobber the contents of the pictured
! 1522: numeric output string, so you should not use @code{~~}, e.g., between
! 1523: @code{<#} and @code{#>}.
! 1524:
! 1525: doc-~~
! 1526: doc-printdebugdata
! 1527: doc-printdebugline
! 1528:
! 1529: @node
! 1530: @subsection Assertions
! 1531:
! 1532: @node Threading Words, , Programming Tools, Words
! 1533: @section Threading Words
! 1534:
! 1535: These words provide access to code addresses and other threading stuff
! 1536: in gforth (and, possibly, other interpretive Forths). It more or less
! 1537: abstracts away the differences between direct and indirect threading
! 1538: (and, for direct threading, the machine dependences). However, at
! 1539: present this wordset is still inclomplete. It is also pretty low-level;
! 1540: some day it will hopefully be made unnecessary by an internals words set
! 1541: that abstracts implementation details away completely.
! 1542:
! 1543: doc->code-address
! 1544: doc->does-code
! 1545: doc-code-address!
! 1546: doc-does-code!
! 1547: doc-does-handler!
! 1548: doc-/does-handler
! 1549:
! 1550: @node ANS conformance, Model, Words, Top
! 1551: @chapter ANS conformance
! 1552:
! 1553: @node Model, Emacs and GForth, ANS conformance, Top
! 1554: @chapter Model
! 1555:
! 1556: @node Emacs and GForth, Internals, Model, Top
! 1557: @chapter Emacs and GForth
! 1558:
! 1559: GForth comes with @file{gforth.el}, an improved version of
! 1560: @file{forth.el} by Goran Rydqvist (icluded in the TILE package). The
! 1561: improvements are a better (but still not perfect) handling of
! 1562: indentation. I have also added comment paragraph filling (@kbd{M-q}),
! 1563: commenting (@kbd{C-x \}) and uncommenting (@kbd{C-x |}) regions and
! 1564: removing debugging tracers (@kbd{C-x ~}). I left the stuff I do not use
! 1565: alone, even though some of it only makes sense for TILE. To get a
! 1566: description of these features, enter Forth mode and type @kbd{C-h m}.
! 1567:
! 1568: In addition, GForth supports Emacs quite well: The source code locations
! 1569: given in error messages, debugging output (from @code{~~}) and failed
! 1570: assertion messages are in the right format for Emacs' compilation mode
! 1571: (@pxref{Compilation, , Running Compilations under Emacs, emacs, Emacs
! 1572: Manual}) so the source location corresponding to an error or other
! 1573: message is only a few keystrokes away (@kbd{C-x `} for the next error,
! 1574: @kbd{C-c C-c} for the error under the cursor).
! 1575:
! 1576: Also, if you @code{include} @file{etags.fs}, a new @file{TAGS} file
! 1577: (@pxref{Tags, , Tags Tables, emacs, Emacs Manual}) will be produced that
! 1578: contains the definitions of all words defined afterwards. You can then
! 1579: find the source for a word using @kbd{M-.}. Note that emacs can use
! 1580: several tags files at the same time (e.g., one for the gforth sources
! 1581: and one for your program).
! 1582:
! 1583: To get all these benefits, add the following lines to your @file{.emacs}
! 1584: file:
! 1585:
! 1586: @example
! 1587: (autoload 'forth-mode "gforth.el")
! 1588: (setq auto-mode-alist (cons '("\\.fs\\'" . forth-mode) auto-mode-alist))
! 1589: @end example
! 1590:
! 1591: @node Internals, Bugs, Emacs and GForth, Top
1.3 anton 1592: @chapter Internals
1593:
1594: Reading this section is not necessary for programming with gforth. It
1595: should be helpful for finding your way in the gforth sources.
1596:
1.4 ! anton 1597: @menu
! 1598: * Portability::
! 1599: * Threading::
! 1600: * Primitives::
! 1601: * System Architecture::
! 1602: @end menu
! 1603:
! 1604: @node Portability, Threading, Internals, Internals
1.3 anton 1605: @section Portability
1606:
1607: One of the main goals of the effort is availability across a wide range
1608: of personal machines. fig-Forth, and, to a lesser extent, F83, achieved
1609: this goal by manually coding the engine in assembly language for several
1610: then-popular processors. This approach is very labor-intensive and the
1611: results are short-lived due to progress in computer architecture.
1612:
1613: Others have avoided this problem by coding in C, e.g., Mitch Bradley
1614: (cforth), Mikael Patel (TILE) and Dirk Zoller (pfe). This approach is
1615: particularly popular for UNIX-based Forths due to the large variety of
1616: architectures of UNIX machines. Unfortunately an implementation in C
1617: does not mix well with the goals of efficiency and with using
1618: traditional techniques: Indirect or direct threading cannot be expressed
1619: in C, and switch threading, the fastest technique available in C, is
1620: significantly slower. Another problem with C is that it's very
1621: cumbersome to express double integer arithmetic.
1622:
1623: Fortunately, there is a portable language that does not have these
1624: limitations: GNU C, the version of C processed by the GNU C compiler
1625: (@pxref{C Extensions, , Extensions to the C Language Family, gcc.info,
1626: GNU C Manual}). Its labels as values feature (@pxref{Labels as Values, ,
1627: Labels as Values, gcc.info, GNU C Manual}) makes direct and indirect
1628: threading possible, its @code{long long} type (@pxref{Long Long, ,
1629: Double-Word Integers, gcc.info, GNU C Manual}) corresponds to Forths
1630: double numbers. GNU C is available for free on all important (and many
1631: unimportant) UNIX machines, VMS, 80386s running MS-DOS, the Amiga, and
1632: the Atari ST, so a Forth written in GNU C can run on all these
1633: machines@footnote{Due to Apple's look-and-feel lawsuit it is not
1634: available on the Mac (@pxref{Boycott, , Protect Your Freedom--Fight
1635: ``Look And Feel'', gcc.info, GNU C Manual}).}.
1636:
1637: Writing in a portable language has the reputation of producing code that
1638: is slower than assembly. For our Forth engine we repeatedly looked at
1639: the code produced by the compiler and eliminated most compiler-induced
1640: inefficiencies by appropriate changes in the source-code.
1641:
1642: However, register allocation cannot be portably influenced by the
1643: programmer, leading to some inefficiencies on register-starved
1644: machines. We use explicit register declarations (@pxref{Explicit Reg
1645: Vars, , Variables in Specified Registers, gcc.info, GNU C Manual}) to
1646: improve the speed on some machines. They are turned on by using the
1647: @code{gcc} switch @code{-DFORCE_REG}. Unfortunately, this feature not
1648: only depends on the machine, but also on the compiler version: On some
1649: machines some compiler versions produce incorrect code when certain
1650: explicit register declarations are used. So by default
1651: @code{-DFORCE_REG} is not used.
1652:
1.4 ! anton 1653: @node Threading, Primitives, Portability, Internals
1.3 anton 1654: @section Threading
1655:
1656: GNU C's labels as values extension (available since @code{gcc-2.0},
1657: @pxref{Labels as Values, , Labels as Values, gcc.info, GNU C Manual})
1658: makes it possible to take the address of @var{label} by writing
1659: @code{&&@var{label}}. This address can then be used in a statement like
1660: @code{goto *@var{address}}. I.e., @code{goto *&&x} is the same as
1661: @code{goto x}.
1662:
1663: With this feature an indirect threaded NEXT looks like:
1664: @example
1665: cfa = *ip++;
1666: ca = *cfa;
1667: goto *ca;
1668: @end example
1669: For those unfamiliar with the names: @code{ip} is the Forth instruction
1670: pointer; the @code{cfa} (code-field address) corresponds to ANS Forths
1671: execution token and points to the code field of the next word to be
1672: executed; The @code{ca} (code address) fetched from there points to some
1673: executable code, e.g., a primitive or the colon definition handler
1674: @code{docol}.
1675:
1676: Direct threading is even simpler:
1677: @example
1678: ca = *ip++;
1679: goto *ca;
1680: @end example
1681:
1682: Of course we have packaged the whole thing neatly in macros called
1683: @code{NEXT} and @code{NEXT1} (the part of NEXT after fetching the cfa).
1684:
1.4 ! anton 1685: @menu
! 1686: * Scheduling::
! 1687: * Direct or Indirect Threaded?::
! 1688: * DOES>::
! 1689: @end menu
! 1690:
! 1691: @node Scheduling, Direct or Indirect Threaded?, Threading, Threading
1.3 anton 1692: @subsection Scheduling
1693:
1694: There is a little complication: Pipelined and superscalar processors,
1695: i.e., RISC and some modern CISC machines can process independent
1696: instructions while waiting for the results of an instruction. The
1697: compiler usually reorders (schedules) the instructions in a way that
1698: achieves good usage of these delay slots. However, on our first tries
1699: the compiler did not do well on scheduling primitives. E.g., for
1700: @code{+} implemented as
1701: @example
1702: n=sp[0]+sp[1];
1703: sp++;
1704: sp[0]=n;
1705: NEXT;
1706: @end example
1707: the NEXT comes strictly after the other code, i.e., there is nearly no
1708: scheduling. After a little thought the problem becomes clear: The
1709: compiler cannot know that sp and ip point to different addresses (and
1.4 ! anton 1710: the version of @code{gcc} we used would not know it even if it was
! 1711: possible), so it could not move the load of the cfa above the store to
! 1712: the TOS. Indeed the pointers could be the same, if code on or very near
! 1713: the top of stack were executed. In the interest of speed we chose to
! 1714: forbid this probably unused ``feature'' and helped the compiler in
! 1715: scheduling: NEXT is divided into the loading part (@code{NEXT_P1}) and
! 1716: the goto part (@code{NEXT_P2}). @code{+} now looks like:
1.3 anton 1717: @example
1718: n=sp[0]+sp[1];
1719: sp++;
1720: NEXT_P1;
1721: sp[0]=n;
1722: NEXT_P2;
1723: @end example
1.4 ! anton 1724: This can be scheduled optimally by the compiler.
1.3 anton 1725:
1726: This division can be turned off with the switch @code{-DCISC_NEXT}. This
1727: switch is on by default on machines that do not profit from scheduling
1728: (e.g., the 80386), in order to preserve registers.
1729:
1.4 ! anton 1730: @node Direct or Indirect Threaded?, DOES>, Scheduling, Threading
1.3 anton 1731: @subsection Direct or Indirect Threaded?
1732:
1733: Both! After packaging the nasty details in macro definitions we
1734: realized that we could switch between direct and indirect threading by
1735: simply setting a compilation flag (@code{-DDIRECT_THREADED}) and
1736: defining a few machine-specific macros for the direct-threading case.
1737: On the Forth level we also offer access words that hide the
1738: differences between the threading methods (@pxref{Threading Words}).
1739:
1740: Indirect threading is implemented completely
1741: machine-independently. Direct threading needs routines for creating
1742: jumps to the executable code (e.g. to docol or dodoes). These routines
1743: are inherently machine-dependent, but they do not amount to many source
1744: lines. I.e., even porting direct threading to a new machine is a small
1745: effort.
1746:
1.4 ! anton 1747: @node DOES>, , Direct or Indirect Threaded?, Threading
1.3 anton 1748: @subsection DOES>
1749: One of the most complex parts of a Forth engine is @code{dodoes}, i.e.,
1750: the chunk of code executed by every word defined by a
1751: @code{CREATE}...@code{DOES>} pair. The main problem here is: How to find
1752: the Forth code to be executed, i.e. the code after the @code{DOES>} (the
1753: DOES-code)? There are two solutions:
1754:
1755: In fig-Forth the code field points directly to the dodoes and the
1756: DOES-code address is stored in the cell after the code address
1757: (i.e. at cfa cell+). It may seem that this solution is illegal in the
1758: Forth-79 and all later standards, because in fig-Forth this address
1759: lies in the body (which is illegal in these standards). However, by
1760: making the code field larger for all words this solution becomes legal
1761: again. We use this approach for the indirect threaded version. Leaving
1762: a cell unused in most words is a bit wasteful, but on the machines we
1763: are targetting this is hardly a problem. The other reason for having a
1764: code field size of two cells is to avoid having different image files
1.4 ! anton 1765: for direct and indirect threaded systems (@pxref{System Architecture}).
1.3 anton 1766:
1767: The other approach is that the code field points or jumps to the cell
1768: after @code{DOES}. In this variant there is a jump to @code{dodoes} at
1769: this address. @code{dodoes} can then get the DOES-code address by
1770: computing the code address, i.e., the address of the jump to dodoes,
1771: and add the length of that jump field. A variant of this is to have a
1772: call to @code{dodoes} after the @code{DOES>}; then the return address
1773: (which can be found in the return register on RISCs) is the DOES-code
1774: address. Since the two cells available in the code field are usually
1775: used up by the jump to the code address in direct threading, we use
1776: this approach for direct threading. We did not want to add another
1777: cell to the code field.
1778:
1.4 ! anton 1779: @node Primitives, System Architecture, Threading, Internals
1.3 anton 1780: @section Primitives
1781:
1.4 ! anton 1782: @menu
! 1783: * Automatic Generation::
! 1784: * TOS Optimization::
! 1785: * Produced code::
! 1786: @end menu
! 1787:
! 1788: @node Automatic Generation, TOS Optimization, Primitives, Primitives
1.3 anton 1789: @subsection Automatic Generation
1790:
1791: Since the primitives are implemented in a portable language, there is no
1792: longer any need to minimize the number of primitives. On the contrary,
1793: having many primitives is an advantage: speed. In order to reduce the
1794: number of errors in primitives and to make programming them easier, we
1795: provide a tool, the primitive generator (@file{prims2x.fs}), that
1796: automatically generates most (and sometimes all) of the C code for a
1797: primitive from the stack effect notation. The source for a primitive
1798: has the following form:
1799:
1800: @format
1801: @var{Forth-name} @var{stack-effect} @var{category} [@var{pronounc.}]
1802: [@code{""}@var{glossary entry}@code{""}]
1803: @var{C code}
1804: [@code{:}
1805: @var{Forth code}]
1806: @end format
1807:
1808: The items in brackets are optional. The category and glossary fields
1809: are there for generating the documentation, the Forth code is there
1810: for manual implementations on machines without GNU C. E.g., the source
1811: for the primitive @code{+} is:
1812: @example
1813: + n1 n2 -- n core plus
1814: n = n1+n2;
1815: @end example
1816:
1817: This looks like a specification, but in fact @code{n = n1+n2} is C
1818: code. Our primitive generation tool extracts a lot of information from
1819: the stack effect notations@footnote{We use a one-stack notation, even
1820: though we have separate data and floating-point stacks; The separate
1821: notation can be generated easily from the unified notation.}: The number
1822: of items popped from and pushed on the stack, their type, and by what
1823: name they are referred to in the C code. It then generates a C code
1824: prelude and postlude for each primitive. The final C code for @code{+}
1825: looks like this:
1826:
1827: @example
1828: I_plus: /* + ( n1 n2 -- n ) */ /* label, stack effect */
1829: /* */ /* documentation */
1.4 ! anton 1830: @{
1.3 anton 1831: DEF_CA /* definition of variable ca (indirect threading) */
1832: Cell n1; /* definitions of variables */
1833: Cell n2;
1834: Cell n;
1835: n1 = (Cell) sp[1]; /* input */
1836: n2 = (Cell) TOS;
1837: sp += 1; /* stack adjustment */
1838: NAME("+") /* debugging output (with -DDEBUG) */
1.4 ! anton 1839: @{
1.3 anton 1840: n = n1+n2; /* C code taken from the source */
1.4 ! anton 1841: @}
1.3 anton 1842: NEXT_P1; /* NEXT part 1 */
1843: TOS = (Cell)n; /* output */
1844: NEXT_P2; /* NEXT part 2 */
1.4 ! anton 1845: @}
1.3 anton 1846: @end example
1847:
1848: This looks long and inefficient, but the GNU C compiler optimizes quite
1849: well and produces optimal code for @code{+} on, e.g., the R3000 and the
1850: HP RISC machines: Defining the @code{n}s does not produce any code, and
1851: using them as intermediate storage also adds no cost.
1852:
1853: There are also other optimizations, that are not illustrated by this
1854: example: Assignments between simple variables are usually for free (copy
1855: propagation). If one of the stack items is not used by the primitive
1856: (e.g. in @code{drop}), the compiler eliminates the load from the stack
1857: (dead code elimination). On the other hand, there are some things that
1858: the compiler does not do, therefore they are performed by
1859: @file{prims2x.fs}: The compiler does not optimize code away that stores
1860: a stack item to the place where it just came from (e.g., @code{over}).
1861:
1862: While programming a primitive is usually easy, there are a few cases
1863: where the programmer has to take the actions of the generator into
1864: account, most notably @code{?dup}, but also words that do not (always)
1865: fall through to NEXT.
1866:
1.4 ! anton 1867: @node TOS Optimization, Produced code, Automatic Generation, Primitives
1.3 anton 1868: @subsection TOS Optimization
1869:
1870: An important optimization for stack machine emulators, e.g., Forth
1871: engines, is keeping one or more of the top stack items in
1.4 ! anton 1872: registers. If a word has the stack effect @var{in1}...@var{inx} @code{--}
! 1873: @var{out1}...@var{outy}, keeping the top @var{n} items in registers
1.3 anton 1874: @itemize
1875: @item
1876: is better than keeping @var{n-1} items, if @var{x>=n} and @var{y>=n},
1877: due to fewer loads from and stores to the stack.
1878: @item is slower than keeping @var{n-1} items, if @var{x<>y} and @var{x<n} and
1879: @var{y<n}, due to additional moves between registers.
1880: @end itemize
1881:
1882: In particular, keeping one item in a register is never a disadvantage,
1883: if there are enough registers. Keeping two items in registers is a
1884: disadvantage for frequent words like @code{?branch}, constants,
1885: variables, literals and @code{i}. Therefore our generator only produces
1886: code that keeps zero or one items in registers. The generated C code
1887: covers both cases; the selection between these alternatives is made at
1888: C-compile time using the switch @code{-DUSE_TOS}. @code{TOS} in the C
1889: code for @code{+} is just a simple variable name in the one-item case,
1890: otherwise it is a macro that expands into @code{sp[0]}. Note that the
1891: GNU C compiler tries to keep simple variables like @code{TOS} in
1892: registers, and it usually succeeds, if there are enough registers.
1893:
1894: The primitive generator performs the TOS optimization for the
1895: floating-point stack, too (@code{-DUSE_FTOS}). For floating-point
1896: operations the benefit of this optimization is even larger:
1897: floating-point operations take quite long on most processors, but can be
1898: performed in parallel with other operations as long as their results are
1899: not used. If the FP-TOS is kept in a register, this works. If
1900: it is kept on the stack, i.e., in memory, the store into memory has to
1901: wait for the result of the floating-point operation, lengthening the
1902: execution time of the primitive considerably.
1903:
1904: The TOS optimization makes the automatic generation of primitives a
1905: bit more complicated. Just replacing all occurrences of @code{sp[0]} by
1906: @code{TOS} is not sufficient. There are some special cases to
1907: consider:
1908: @itemize
1909: @item In the case of @code{dup ( w -- w w )} the generator must not
1910: eliminate the store to the original location of the item on the stack,
1911: if the TOS optimization is turned on.
1.4 ! anton 1912: @item Primitives with stack effects of the form @code{--}
! 1913: @var{out1}...@var{outy} must store the TOS to the stack at the start.
! 1914: Likewise, primitives with the stack effect @var{in1}...@var{inx} @code{--}
1.3 anton 1915: must load the TOS from the stack at the end. But for the null stack
1916: effect @code{--} no stores or loads should be generated.
1917: @end itemize
1918:
1.4 ! anton 1919: @node Produced code, , TOS Optimization, Primitives
1.3 anton 1920: @subsection Produced code
1921:
1922: To see what assembly code is produced for the primitives on your machine
1923: with your compiler and your flag settings, type @code{make engine.s} and
1.4 ! anton 1924: look at the resulting file @file{engine.s}.
1.3 anton 1925:
1.4 ! anton 1926: @node System Architecture, , Primitives, Internals
1.3 anton 1927: @section System Architecture
1928:
1929: Our Forth system consists not only of primitives, but also of
1930: definitions written in Forth. Since the Forth compiler itself belongs
1931: to those definitions, it is not possible to start the system with the
1932: primitives and the Forth source alone. Therefore we provide the Forth
1933: code as an image file in nearly executable form. At the start of the
1934: system a C routine loads the image file into memory, sets up the
1935: memory (stacks etc.) according to information in the image file, and
1936: starts executing Forth code.
1937:
1938: The image file format is a compromise between the goals of making it
1939: easy to generate image files and making them portable. The easiest way
1940: to generate an image file is to just generate a memory dump. However,
1941: this kind of image file cannot be used on a different machine, or on
1942: the next version of the engine on the same machine, it even might not
1943: work with the same engine compiled by a different version of the C
1944: compiler. We would like to have as few versions of the image file as
1945: possible, because we do not want to distribute many versions of the
1946: same image file, and to make it easy for the users to use their image
1947: files on many machines. We currently need to create a different image
1948: file for machines with different cell sizes and different byte order
1949: (little- or big-endian)@footnote{We consider adding information to the
1950: image file that enables the loader to change the byte order.}.
1951:
1952: Forth code that is going to end up in a portable image file has to
1.4 ! anton 1953: comply to some restrictions: addresses have to be stored in memory with
! 1954: special words (@code{A!}, @code{A,}, etc.) in order to make the code
! 1955: relocatable. Cells, floats, etc., have to be stored at the natural
! 1956: alignment boundaries@footnote{E.g., store floats (8 bytes) at an address
! 1957: dividable by~8. This happens automatically in our system when you use
! 1958: the ANS Forth alignment words.}, in order to avoid alignment faults on
! 1959: machines with stricter alignment. The image file is produced by a
! 1960: metacompiler (@file{cross.fs}).
1.3 anton 1961:
1962: So, unlike the image file of Mitch Bradleys @code{cforth}, our image
1963: file is not directly executable, but has to undergo some manipulations
1964: during loading. Address relocation is performed at image load-time, not
1965: at run-time. The loader also has to replace tokens standing for
1966: primitive calls with the appropriate code-field addresses (or code
1967: addresses in the case of direct threading).
1.4 ! anton 1968:
! 1969: @node Bugs, Pedigree, Internals, Top
! 1970: @chapter Bugs
! 1971:
! 1972: @node Pedigree, Word Index, Bugs, Top
! 1973: @chapter Pedigree
! 1974:
! 1975: @node Word Index, Node Index, Pedigree, Top
! 1976: @chapter Word Index
! 1977:
! 1978: @node Node Index, , Word Index, Top
! 1979: @chapter Node Index
1.1 anton 1980:
1981: @contents
1982: @bye
1983:
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