Annotation of gforth/gforth.ds, revision 1.17
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
1.17 ! anton 5: @settitle Gforth 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
1.17 ! anton 10: This file documents Gforth 0.1
1.1 anton 11:
1.17 ! anton 12: Copyright @copyright{} 1994 Gforth Development Group
1.1 anton 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
1.17 ! anton 41: @center @titlefont{Gforth Manual}
1.1 anton 42: @sp 2
1.17 ! anton 43: @center for version 0.1
1.1 anton 44: @sp 2
45: @center Anton Ertl
1.17 ! anton 46: @sp 3
! 47: @center This manual is under construction
1.1 anton 48:
49: @comment The following two commands start the copyright page.
50: @page
51: @vskip 0pt plus 1filll
1.17 ! anton 52: Copyright @copyright{} 1994 Gforth Development Group
1.1 anton 53:
54: @comment !! Published by ... or You can get a copy of this manual ...
55:
56: Permission is granted to make and distribute verbatim copies of
57: this manual provided the copyright notice and this permission notice
58: are preserved on all copies.
59:
60: Permission is granted to copy and distribute modified versions of this
61: manual under the conditions for verbatim copying, provided also that the
62: sections entitled "Distribution" and "General Public License" are
63: included exactly as in the original, and provided that the entire
64: resulting derived work is distributed under the terms of a permission
65: notice identical to this one.
66:
67: Permission is granted to copy and distribute translations of this manual
68: into another language, under the above conditions for modified versions,
69: except that the sections entitled "Distribution" and "General Public
70: License" may be included in a translation approved by the author instead
71: of in the original English.
72: @end titlepage
73:
74:
75: @node Top, License, (dir), (dir)
76: @ifinfo
1.17 ! anton 77: Gforth is a free implementation of ANS Forth available on many
1.1 anton 78: personal machines. This manual corresponds to version 0.0.
79: @end ifinfo
80:
81: @menu
1.4 anton 82: * License::
1.17 ! anton 83: * Goals:: About the Gforth Project
1.4 anton 84: * Other Books:: Things you might want to read
1.17 ! anton 85: * Invocation:: Starting Gforth
! 86: * Words:: Forth words available in Gforth
1.4 anton 87: * ANS conformance:: Implementation-defined options etc.
1.17 ! anton 88: * Model:: The abstract machine of Gforth
! 89: * Emacs and Gforth:: The Gforth Mode
1.4 anton 90: * Internals:: Implementation details
91: * Bugs:: How to report them
1.17 ! anton 92: * Pedigree:: Ancestors of Gforth
1.4 anton 93: * Word Index:: An item for each Forth word
94: * Node Index:: An item for each node
1.1 anton 95: @end menu
96:
97: @node License, Goals, Top, Top
98: @unnumbered License
99: !! Insert GPL here
100:
101: @iftex
102: @unnumbered Preface
1.17 ! anton 103: This manual documents Gforth. The reader is expected to know
1.1 anton 104: Forth. This manual is primarily a reference manual. @xref{Other Books}
105: for introductory material.
106: @end iftex
107:
108: @node Goals, Other Books, License, Top
109: @comment node-name, next, previous, up
1.17 ! anton 110: @chapter Goals of Gforth
1.1 anton 111: @cindex Goals
1.17 ! anton 112: The goal of the Gforth Project is to develop a standard model for
1.1 anton 113: ANSI Forth. This can be split into several subgoals:
114:
115: @itemize @bullet
116: @item
1.17 ! anton 117: Gforth should conform to the ANSI Forth standard.
1.1 anton 118: @item
119: It should be a model, i.e. it should define all the
120: implementation-dependent things.
121: @item
122: It should become standard, i.e. widely accepted and used. This goal
123: is the most difficult one.
124: @end itemize
125:
1.17 ! anton 126: To achieve these goals Gforth should be
1.1 anton 127: @itemize @bullet
128: @item
129: Similar to previous models (fig-Forth, F83)
130: @item
131: Powerful. It should provide for all the things that are considered
132: necessary today and even some that are not yet considered necessary.
133: @item
134: Efficient. It should not get the reputation of being exceptionally
135: slow.
136: @item
137: Free.
138: @item
139: Available on many machines/easy to port.
140: @end itemize
141:
1.17 ! anton 142: Have we achieved these goals? Gforth conforms to the ANS Forth
! 143: standard. It may be considered a model, but we have not yet documented
1.1 anton 144: which parts of the model are stable and which parts we are likely to
1.17 ! anton 145: change. It certainly has not yet become a de facto standard. It has some
! 146: similarities and some differences to previous models. It has some
! 147: powerful features, but not yet everything that we envisioned. We
! 148: certainly have achieved our execution speed goals (@pxref{Performance}).
! 149: It is free and available on many machines.
1.1 anton 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
1.17 ! anton 155: is not recommended to learn Forth by using Gforth and a book that is
1.1 anton 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
1.17 ! anton 176: Gforth image will be invoked like this:
1.1 anton 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
1.17 ! anton 288: The name of the word. BTW, Gforth is case insensitive, so you can
1.14 anton 289: type the words in in lower case (However, @pxref{core-idef}).
1.1 anton 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,
1.17 ! anton 296: i.e., a stack sequence is written as it is typed in. Note that Gforth
1.1 anton 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
1.16 anton 412:
413: The outer (aka text) interpreter converts numbers containing a dot into
414: a double precision number. Note that only numbers with the dot as last
415: character are standard-conforming.
416:
1.1 anton 417: doc-d+
418: doc-d-
419: doc-dnegate
420: doc-dabs
421: doc-dmin
422: doc-dmax
423:
1.4 anton 424: @node Floating Point, , Double precision, Arithmetic
425: @subsection Floating Point
1.16 anton 426:
427: The format of floating point numbers recognized by the outer (aka text)
428: interpreter is: a signed decimal number, possibly containing a decimal
429: point (@code{.}), followed by @code{E} or @code{e}, optionally followed
430: by a signed integer (the exponent). E.g., @code{1e} ist the same as
431: @code{+1.0e+1}. Note that a number without @code{e}
432: is not interpreted as floating-point number, but as double (if the
433: number contains a @code{.}) or single precision integer. Also,
434: conversions between string and floating point numbers always use base
435: 10, irrespective of the value of @code{BASE}. If @code{BASE} contains a
436: value greater then 14, the @code{E} may be interpreted as digit and the
437: number will be interpreted as integer, unless it has a signed exponent
438: (both @code{+} and @code{-} are allowed as signs).
1.4 anton 439:
440: Angles in floating point operations are given in radians (a full circle
1.17 ! anton 441: has 2 pi radians). Note, that Gforth has a separate floating point
1.4 anton 442: stack, but we use the unified notation.
443:
444: Floating point numbers have a number of unpleasant surprises for the
445: unwary (e.g., floating point addition is not associative) and even a few
446: for the wary. You should not use them unless you know what you are doing
447: or you don't care that the results you get are totally bogus. If you
448: want to learn about the problems of floating point numbers (and how to
1.11 anton 449: avoid them), you might start with @cite{David Goldberg, What Every
1.6 anton 450: Computer Scientist Should Know About Floating-Point Arithmetic, ACM
451: Computing Surveys 23(1):5@minus{}48, March 1991}.
1.4 anton 452:
453: doc-f+
454: doc-f-
455: doc-f*
456: doc-f/
457: doc-fnegate
458: doc-fabs
459: doc-fmax
460: doc-fmin
461: doc-floor
462: doc-fround
463: doc-f**
464: doc-fsqrt
465: doc-fexp
466: doc-fexpm1
467: doc-fln
468: doc-flnp1
469: doc-flog
1.6 anton 470: doc-falog
1.4 anton 471: doc-fsin
472: doc-fcos
473: doc-fsincos
474: doc-ftan
475: doc-fasin
476: doc-facos
477: doc-fatan
478: doc-fatan2
479: doc-fsinh
480: doc-fcosh
481: doc-ftanh
482: doc-fasinh
483: doc-facosh
484: doc-fatanh
485:
486: @node Stack Manipulation, Memory access, Arithmetic, Words
1.1 anton 487: @section Stack Manipulation
488:
1.17 ! anton 489: Gforth has a data stack (aka parameter stack) for characters, cells,
1.1 anton 490: addresses, and double cells, a floating point stack for floating point
491: numbers, a return stack for storing the return addresses of colon
492: definitions and other data, and a locals stack for storing local
493: variables. Note that while every sane Forth has a separate floating
494: point stack, this is not strictly required; an ANS Forth system could
495: theoretically keep floating point numbers on the data stack. As an
496: additional difficulty, you don't know how many cells a floating point
497: number takes. It is reportedly possible to write words in a way that
498: they work also for a unified stack model, but we do not recommend trying
1.4 anton 499: it. Instead, just say that your program has an environmental dependency
500: on a separate FP stack.
501:
502: Also, a Forth system is allowed to keep the local variables on the
1.1 anton 503: return stack. This is reasonable, as local variables usually eliminate
504: the need to use the return stack explicitly. So, if you want to produce
505: a standard complying program and if you are using local variables in a
506: word, forget about return stack manipulations in that word (see the
507: standard document for the exact rules).
508:
1.4 anton 509: @menu
510: * Data stack::
511: * Floating point stack::
512: * Return stack::
513: * Locals stack::
514: * Stack pointer manipulation::
515: @end menu
516:
517: @node Data stack, Floating point stack, Stack Manipulation, Stack Manipulation
1.1 anton 518: @subsection Data stack
519: doc-drop
520: doc-nip
521: doc-dup
522: doc-over
523: doc-tuck
524: doc-swap
525: doc-rot
526: doc--rot
527: doc-?dup
528: doc-pick
529: doc-roll
530: doc-2drop
531: doc-2nip
532: doc-2dup
533: doc-2over
534: doc-2tuck
535: doc-2swap
536: doc-2rot
537:
1.4 anton 538: @node Floating point stack, Return stack, Data stack, Stack Manipulation
1.1 anton 539: @subsection Floating point stack
540: doc-fdrop
541: doc-fnip
542: doc-fdup
543: doc-fover
544: doc-ftuck
545: doc-fswap
546: doc-frot
547:
1.4 anton 548: @node Return stack, Locals stack, Floating point stack, Stack Manipulation
1.1 anton 549: @subsection Return stack
550: doc->r
551: doc-r>
552: doc-r@
553: doc-rdrop
554: doc-2>r
555: doc-2r>
556: doc-2r@
557: doc-2rdrop
558:
1.4 anton 559: @node Locals stack, Stack pointer manipulation, Return stack, Stack Manipulation
1.1 anton 560: @subsection Locals stack
561:
1.4 anton 562: @node Stack pointer manipulation, , Locals stack, Stack Manipulation
1.1 anton 563: @subsection Stack pointer manipulation
564: doc-sp@
565: doc-sp!
566: doc-fp@
567: doc-fp!
568: doc-rp@
569: doc-rp!
570: doc-lp@
571: doc-lp!
572:
1.4 anton 573: @node Memory access, Control Structures, Stack Manipulation, Words
1.1 anton 574: @section Memory access
575:
1.4 anton 576: @menu
577: * Stack-Memory transfers::
578: * Address arithmetic::
579: * Memory block access::
580: @end menu
581:
582: @node Stack-Memory transfers, Address arithmetic, Memory access, Memory access
1.1 anton 583: @subsection Stack-Memory transfers
584:
585: doc-@
586: doc-!
587: doc-+!
588: doc-c@
589: doc-c!
590: doc-2@
591: doc-2!
592: doc-f@
593: doc-f!
594: doc-sf@
595: doc-sf!
596: doc-df@
597: doc-df!
598:
1.4 anton 599: @node Address arithmetic, Memory block access, Stack-Memory transfers, Memory access
1.1 anton 600: @subsection Address arithmetic
601:
602: ANS Forth does not specify the sizes of the data types. Instead, it
603: offers a number of words for computing sizes and doing address
604: arithmetic. Basically, address arithmetic is performed in terms of
605: address units (aus); on most systems the address unit is one byte. Note
606: that a character may have more than one au, so @code{chars} is no noop
607: (on systems where it is a noop, it compiles to nothing).
608:
609: ANS Forth also defines words for aligning addresses for specific
610: addresses. Many computers require that accesses to specific data types
611: must only occur at specific addresses; e.g., that cells may only be
612: accessed at addresses divisible by 4. Even if a machine allows unaligned
613: accesses, it can usually perform aligned accesses faster.
614:
1.17 ! anton 615: For the performance-conscious: alignment operations are usually only
1.1 anton 616: necessary during the definition of a data structure, not during the
617: (more frequent) accesses to it.
618:
619: ANS Forth defines no words for character-aligning addresses. This is not
620: an oversight, but reflects the fact that addresses that are not
621: char-aligned have no use in the standard and therefore will not be
622: created.
623:
624: The standard guarantees that addresses returned by @code{CREATE}d words
1.17 ! anton 625: are cell-aligned; in addition, Gforth guarantees that these addresses
1.1 anton 626: are aligned for all purposes.
627:
1.9 anton 628: Note that the standard defines a word @code{char}, which has nothing to
629: do with address arithmetic.
630:
1.1 anton 631: doc-chars
632: doc-char+
633: doc-cells
634: doc-cell+
635: doc-align
636: doc-aligned
637: doc-floats
638: doc-float+
639: doc-falign
640: doc-faligned
641: doc-sfloats
642: doc-sfloat+
643: doc-sfalign
644: doc-sfaligned
645: doc-dfloats
646: doc-dfloat+
647: doc-dfalign
648: doc-dfaligned
1.10 anton 649: doc-maxalign
650: doc-maxaligned
651: doc-cfalign
652: doc-cfaligned
1.1 anton 653: doc-address-unit-bits
654:
1.4 anton 655: @node Memory block access, , Address arithmetic, Memory access
1.1 anton 656: @subsection Memory block access
657:
658: doc-move
659: doc-erase
660:
661: While the previous words work on address units, the rest works on
662: characters.
663:
664: doc-cmove
665: doc-cmove>
666: doc-fill
667: doc-blank
668:
1.4 anton 669: @node Control Structures, Locals, Memory access, Words
1.1 anton 670: @section Control Structures
671:
672: Control structures in Forth cannot be used in interpret state, only in
673: compile state, i.e., in a colon definition. We do not like this
674: limitation, but have not seen a satisfying way around it yet, although
675: many schemes have been proposed.
676:
1.4 anton 677: @menu
678: * Selection::
679: * Simple Loops::
680: * Counted Loops::
681: * Arbitrary control structures::
682: * Calls and returns::
683: * Exception Handling::
684: @end menu
685:
686: @node Selection, Simple Loops, Control Structures, Control Structures
1.1 anton 687: @subsection Selection
688:
689: @example
690: @var{flag}
691: IF
692: @var{code}
693: ENDIF
694: @end example
695: or
696: @example
697: @var{flag}
698: IF
699: @var{code1}
700: ELSE
701: @var{code2}
702: ENDIF
703: @end example
704:
1.4 anton 705: You can use @code{THEN} instead of @code{ENDIF}. Indeed, @code{THEN} is
1.1 anton 706: standard, and @code{ENDIF} is not, although it is quite popular. We
707: recommend using @code{ENDIF}, because it is less confusing for people
708: who also know other languages (and is not prone to reinforcing negative
709: prejudices against Forth in these people). Adding @code{ENDIF} to a
710: system that only supplies @code{THEN} is simple:
711: @example
712: : endif POSTPONE then ; immediate
713: @end example
714:
715: [According to @cite{Webster's New Encyclopedic Dictionary}, @dfn{then
716: (adv.)} has the following meanings:
717: @quotation
718: ... 2b: following next after in order ... 3d: as a necessary consequence
719: (if you were there, then you saw them).
720: @end quotation
721: Forth's @code{THEN} has the meaning 2b, whereas @code{THEN} in Pascal
722: and many other programming languages has the meaning 3d.]
723:
724: We also provide the words @code{?dup-if} and @code{?dup-0=-if}, so you
725: can avoid using @code{?dup}.
726:
727: @example
728: @var{n}
729: CASE
730: @var{n1} OF @var{code1} ENDOF
731: @var{n2} OF @var{code2} ENDOF
1.4 anton 732: @dots{}
1.1 anton 733: ENDCASE
734: @end example
735:
736: Executes the first @var{codei}, where the @var{ni} is equal to
737: @var{n}. A default case can be added by simply writing the code after
738: the last @code{ENDOF}. It may use @var{n}, which is on top of the stack,
739: but must not consume it.
740:
1.4 anton 741: @node Simple Loops, Counted Loops, Selection, Control Structures
1.1 anton 742: @subsection Simple Loops
743:
744: @example
745: BEGIN
746: @var{code1}
747: @var{flag}
748: WHILE
749: @var{code2}
750: REPEAT
751: @end example
752:
753: @var{code1} is executed and @var{flag} is computed. If it is true,
754: @var{code2} is executed and the loop is restarted; If @var{flag} is false, execution continues after the @code{REPEAT}.
755:
756: @example
757: BEGIN
758: @var{code}
759: @var{flag}
760: UNTIL
761: @end example
762:
763: @var{code} is executed. The loop is restarted if @code{flag} is false.
764:
765: @example
766: BEGIN
767: @var{code}
768: AGAIN
769: @end example
770:
771: This is an endless loop.
772:
1.4 anton 773: @node Counted Loops, Arbitrary control structures, Simple Loops, Control Structures
1.1 anton 774: @subsection Counted Loops
775:
776: The basic counted loop is:
777: @example
778: @var{limit} @var{start}
779: ?DO
780: @var{body}
781: LOOP
782: @end example
783:
784: This performs one iteration for every integer, starting from @var{start}
785: and up to, but excluding @var{limit}. The counter, aka index, can be
786: accessed with @code{i}. E.g., the loop
787: @example
788: 10 0 ?DO
789: i .
790: LOOP
791: @end example
792: prints
793: @example
794: 0 1 2 3 4 5 6 7 8 9
795: @end example
796: The index of the innermost loop can be accessed with @code{i}, the index
797: of the next loop with @code{j}, and the index of the third loop with
798: @code{k}.
799:
800: The loop control data are kept on the return stack, so there are some
801: restrictions on mixing return stack accesses and counted loop
802: words. E.g., if you put values on the return stack outside the loop, you
803: cannot read them inside the loop. If you put values on the return stack
804: within a loop, you have to remove them before the end of the loop and
805: before accessing the index of the loop.
806:
807: There are several variations on the counted loop:
808:
809: @code{LEAVE} leaves the innermost counted loop immediately.
810:
811: @code{LOOP} can be replaced with @code{@var{n} +LOOP}; this updates the
812: index by @var{n} instead of by 1. The loop is terminated when the border
813: between @var{limit-1} and @var{limit} is crossed. E.g.:
814:
1.2 anton 815: @code{4 0 ?DO i . 2 +LOOP} prints @code{0 2}
1.1 anton 816:
1.2 anton 817: @code{4 1 ?DO i . 2 +LOOP} prints @code{1 3}
1.1 anton 818:
819: The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative:
820:
1.2 anton 821: @code{-1 0 ?DO i . -1 +LOOP} prints @code{0 -1}
1.1 anton 822:
1.2 anton 823: @code{ 0 0 ?DO i . -1 +LOOP} prints nothing
1.1 anton 824:
825: Therefore we recommend avoiding using @code{@var{n} +LOOP} with negative
826: @var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative
827: case behaves symmetrical to the positive case:
828:
1.7 pazsan 829: @code{-2 0 ?DO i . -1 S+LOOP} prints @code{0 -1}
1.1 anton 830:
1.7 pazsan 831: @code{-1 0 ?DO i . -1 S+LOOP} prints @code{0}
1.1 anton 832:
1.7 pazsan 833: @code{ 0 0 ?DO i . -1 S+LOOP} prints nothing
1.1 anton 834:
1.2 anton 835: The loop is terminated when the border between @var{limit@minus{}sgn(n)} and
1.1 anton 836: @var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS
837: Forth standard.
838:
839: @code{?DO} can be replaced by @code{DO}. @code{DO} enters the loop even
840: when the start and the limit value are equal. We do not recommend using
841: @code{DO}. It will just give you maintenance troubles.
842:
843: @code{UNLOOP} is used to prepare for an abnormal loop exit, e.g., via
844: @code{EXIT}. @code{UNLOOP} removes the loop control parameters from the
845: return stack so @code{EXIT} can get to its return address.
846:
847: Another counted loop is
848: @example
849: @var{n}
850: FOR
851: @var{body}
852: NEXT
853: @end example
854: This is the preferred loop of native code compiler writers who are too
1.17 ! anton 855: lazy to optimize @code{?DO} loops properly. In Gforth, this loop
1.1 anton 856: iterates @var{n+1} times; @code{i} produces values starting with @var{n}
857: and ending with 0. Other Forth systems may behave differently, even if
858: they support @code{FOR} loops.
859:
1.4 anton 860: @node Arbitrary control structures, Calls and returns, Counted Loops, Control Structures
1.2 anton 861: @subsection Arbitrary control structures
862:
863: ANS Forth permits and supports using control structures in a non-nested
864: way. Information about incomplete control structures is stored on the
865: control-flow stack. This stack may be implemented on the Forth data
1.17 ! anton 866: stack, and this is what we have done in Gforth.
1.2 anton 867:
868: An @i{orig} entry represents an unresolved forward branch, a @i{dest}
869: entry represents a backward branch target. A few words are the basis for
870: building any control structure possible (except control structures that
871: need storage, like calls, coroutines, and backtracking).
872:
1.3 anton 873: doc-if
874: doc-ahead
875: doc-then
876: doc-begin
877: doc-until
878: doc-again
879: doc-cs-pick
880: doc-cs-roll
1.2 anton 881:
1.17 ! anton 882: On many systems control-flow stack items take one word, in Gforth they
1.2 anton 883: currently take three (this may change in the future). Therefore it is a
884: really good idea to manipulate the control flow stack with
885: @code{cs-pick} and @code{cs-roll}, not with data stack manipulation
886: words.
887:
888: Some standard control structure words are built from these words:
889:
1.3 anton 890: doc-else
891: doc-while
892: doc-repeat
1.2 anton 893:
894: Counted loop words constitute a separate group of words:
895:
1.3 anton 896: doc-?do
897: doc-do
898: doc-for
899: doc-loop
900: doc-s+loop
901: doc-+loop
902: doc-next
903: doc-leave
904: doc-?leave
905: doc-unloop
1.10 anton 906: doc-done
1.2 anton 907:
908: The standard does not allow using @code{cs-pick} and @code{cs-roll} on
909: @i{do-sys}. Our system allows it, but it's your job to ensure that for
910: every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path
1.3 anton 911: through the definition (@code{LOOP} etc. compile an @code{UNLOOP} on the
912: fall-through path). Also, you have to ensure that all @code{LEAVE}s are
1.7 pazsan 913: resolved (by using one of the loop-ending words or @code{DONE}).
1.2 anton 914:
915: Another group of control structure words are
916:
1.3 anton 917: doc-case
918: doc-endcase
919: doc-of
920: doc-endof
1.2 anton 921:
922: @i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and
923: @code{cs-roll}.
924:
1.3 anton 925: @subsubsection Programming Style
926:
927: In order to ensure readability we recommend that you do not create
928: arbitrary control structures directly, but define new control structure
929: words for the control structure you want and use these words in your
930: program.
931:
932: E.g., instead of writing
933:
934: @example
935: begin
936: ...
937: if [ 1 cs-roll ]
938: ...
939: again then
940: @end example
941:
942: we recommend defining control structure words, e.g.,
943:
944: @example
945: : while ( dest -- orig dest )
946: POSTPONE if
947: 1 cs-roll ; immediate
948:
949: : repeat ( orig dest -- )
950: POSTPONE again
951: POSTPONE then ; immediate
952: @end example
953:
954: and then using these to create the control structure:
955:
956: @example
957: begin
958: ...
959: while
960: ...
961: repeat
962: @end example
963:
964: That's much easier to read, isn't it? Of course, @code{BEGIN} and
965: @code{WHILE} are predefined, so in this example it would not be
966: necessary to define them.
967:
1.4 anton 968: @node Calls and returns, Exception Handling, Arbitrary control structures, Control Structures
1.3 anton 969: @subsection Calls and returns
970:
971: A definition can be called simply be writing the name of the
1.17 ! anton 972: definition. When the end of the definition is reached, it returns. An
! 973: earlier return can be forced using
1.3 anton 974:
975: doc-exit
976:
977: Don't forget to clean up the return stack and @code{UNLOOP} any
978: outstanding @code{?DO}...@code{LOOP}s before @code{EXIT}ing. The
979: primitive compiled by @code{EXIT} is
980:
981: doc-;s
982:
1.4 anton 983: @node Exception Handling, , Calls and returns, Control Structures
1.3 anton 984: @subsection Exception Handling
985:
986: doc-catch
987: doc-throw
988:
1.4 anton 989: @node Locals, Defining Words, Control Structures, Words
1.1 anton 990: @section Locals
991:
1.2 anton 992: Local variables can make Forth programming more enjoyable and Forth
993: programs easier to read. Unfortunately, the locals of ANS Forth are
994: laden with restrictions. Therefore, we provide not only the ANS Forth
995: locals wordset, but also our own, more powerful locals wordset (we
996: implemented the ANS Forth locals wordset through our locals wordset).
997:
998: @menu
1.17 ! anton 999: * Gforth locals::
1.4 anton 1000: * ANS Forth locals::
1.2 anton 1001: @end menu
1002:
1.17 ! anton 1003: @node Gforth locals, ANS Forth locals, Locals, Locals
! 1004: @subsection Gforth locals
1.2 anton 1005:
1006: Locals can be defined with
1007:
1008: @example
1009: @{ local1 local2 ... -- comment @}
1010: @end example
1011: or
1012: @example
1013: @{ local1 local2 ... @}
1014: @end example
1015:
1016: E.g.,
1017: @example
1018: : max @{ n1 n2 -- n3 @}
1019: n1 n2 > if
1020: n1
1021: else
1022: n2
1023: endif ;
1024: @end example
1025:
1026: The similarity of locals definitions with stack comments is intended. A
1027: locals definition often replaces the stack comment of a word. The order
1028: of the locals corresponds to the order in a stack comment and everything
1029: after the @code{--} is really a comment.
1030:
1031: This similarity has one disadvantage: It is too easy to confuse locals
1032: declarations with stack comments, causing bugs and making them hard to
1033: find. However, this problem can be avoided by appropriate coding
1034: conventions: Do not use both notations in the same program. If you do,
1035: they should be distinguished using additional means, e.g. by position.
1036:
1037: The name of the local may be preceded by a type specifier, e.g.,
1038: @code{F:} for a floating point value:
1039:
1040: @example
1041: : CX* @{ F: Ar F: Ai F: Br F: Bi -- Cr Ci @}
1042: \ complex multiplication
1043: Ar Br f* Ai Bi f* f-
1044: Ar Bi f* Ai Br f* f+ ;
1045: @end example
1046:
1.17 ! anton 1047: Gforth currently supports cells (@code{W:}, @code{W^}), doubles
1.2 anton 1048: (@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters
1049: (@code{C:}, @code{C^}) in two flavours: a value-flavoured local (defined
1050: with @code{W:}, @code{D:} etc.) produces its value and can be changed
1051: with @code{TO}. A variable-flavoured local (defined with @code{W^} etc.)
1052: produces its address (which becomes invalid when the variable's scope is
1053: left). E.g., the standard word @code{emit} can be defined in therms of
1054: @code{type} like this:
1055:
1056: @example
1057: : emit @{ C^ char* -- @}
1058: char* 1 type ;
1059: @end example
1060:
1061: A local without type specifier is a @code{W:} local. Both flavours of
1062: locals are initialized with values from the data or FP stack.
1063:
1064: Currently there is no way to define locals with user-defined data
1065: structures, but we are working on it.
1066:
1.17 ! anton 1067: Gforth allows defining locals everywhere in a colon definition. This
1.7 pazsan 1068: poses the following questions:
1.2 anton 1069:
1.4 anton 1070: @menu
1071: * Where are locals visible by name?::
1.14 anton 1072: * How long do locals live?::
1.4 anton 1073: * Programming Style::
1074: * Implementation::
1075: @end menu
1076:
1.17 ! anton 1077: @node Where are locals visible by name?, How long do locals live?, Gforth locals, Gforth locals
1.2 anton 1078: @subsubsection Where are locals visible by name?
1079:
1080: Basically, the answer is that locals are visible where you would expect
1081: it in block-structured languages, and sometimes a little longer. If you
1082: want to restrict the scope of a local, enclose its definition in
1083: @code{SCOPE}...@code{ENDSCOPE}.
1084:
1085: doc-scope
1086: doc-endscope
1087:
1088: These words behave like control structure words, so you can use them
1089: with @code{CS-PICK} and @code{CS-ROLL} to restrict the scope in
1090: arbitrary ways.
1091:
1092: If you want a more exact answer to the visibility question, here's the
1093: basic principle: A local is visible in all places that can only be
1094: reached through the definition of the local@footnote{In compiler
1095: construction terminology, all places dominated by the definition of the
1096: local.}. In other words, it is not visible in places that can be reached
1097: without going through the definition of the local. E.g., locals defined
1098: in @code{IF}...@code{ENDIF} are visible until the @code{ENDIF}, locals
1099: defined in @code{BEGIN}...@code{UNTIL} are visible after the
1100: @code{UNTIL} (until, e.g., a subsequent @code{ENDSCOPE}).
1101:
1102: The reasoning behind this solution is: We want to have the locals
1103: visible as long as it is meaningful. The user can always make the
1104: visibility shorter by using explicit scoping. In a place that can
1105: only be reached through the definition of a local, the meaning of a
1106: local name is clear. In other places it is not: How is the local
1107: initialized at the control flow path that does not contain the
1108: definition? Which local is meant, if the same name is defined twice in
1109: two independent control flow paths?
1110:
1111: This should be enough detail for nearly all users, so you can skip the
1112: rest of this section. If you relly must know all the gory details and
1113: options, read on.
1114:
1115: In order to implement this rule, the compiler has to know which places
1116: are unreachable. It knows this automatically after @code{AHEAD},
1117: @code{AGAIN}, @code{EXIT} and @code{LEAVE}; in other cases (e.g., after
1118: most @code{THROW}s), you can use the word @code{UNREACHABLE} to tell the
1119: compiler that the control flow never reaches that place. If
1120: @code{UNREACHABLE} is not used where it could, the only consequence is
1121: that the visibility of some locals is more limited than the rule above
1122: says. If @code{UNREACHABLE} is used where it should not (i.e., if you
1123: lie to the compiler), buggy code will be produced.
1124:
1125: Another problem with this rule is that at @code{BEGIN}, the compiler
1.3 anton 1126: does not know which locals will be visible on the incoming
1127: back-edge. All problems discussed in the following are due to this
1128: ignorance of the compiler (we discuss the problems using @code{BEGIN}
1129: loops as examples; the discussion also applies to @code{?DO} and other
1.2 anton 1130: loops). Perhaps the most insidious example is:
1131: @example
1132: AHEAD
1133: BEGIN
1134: x
1135: [ 1 CS-ROLL ] THEN
1.4 anton 1136: @{ x @}
1.2 anton 1137: ...
1138: UNTIL
1139: @end example
1140:
1141: This should be legal according to the visibility rule. The use of
1142: @code{x} can only be reached through the definition; but that appears
1143: textually below the use.
1144:
1145: From this example it is clear that the visibility rules cannot be fully
1146: implemented without major headaches. Our implementation treats common
1147: cases as advertised and the exceptions are treated in a safe way: The
1148: compiler makes a reasonable guess about the locals visible after a
1149: @code{BEGIN}; if it is too pessimistic, the
1150: user will get a spurious error about the local not being defined; if the
1151: compiler is too optimistic, it will notice this later and issue a
1152: warning. In the case above the compiler would complain about @code{x}
1153: being undefined at its use. You can see from the obscure examples in
1154: this section that it takes quite unusual control structures to get the
1155: compiler into trouble, and even then it will often do fine.
1156:
1157: If the @code{BEGIN} is reachable from above, the most optimistic guess
1158: is that all locals visible before the @code{BEGIN} will also be
1159: visible after the @code{BEGIN}. This guess is valid for all loops that
1160: are entered only through the @code{BEGIN}, in particular, for normal
1161: @code{BEGIN}...@code{WHILE}...@code{REPEAT} and
1162: @code{BEGIN}...@code{UNTIL} loops and it is implemented in our
1163: compiler. When the branch to the @code{BEGIN} is finally generated by
1164: @code{AGAIN} or @code{UNTIL}, the compiler checks the guess and
1165: warns the user if it was too optimisitic:
1166: @example
1167: IF
1.4 anton 1168: @{ x @}
1.2 anton 1169: BEGIN
1170: \ x ?
1171: [ 1 cs-roll ] THEN
1172: ...
1173: UNTIL
1174: @end example
1175:
1176: Here, @code{x} lives only until the @code{BEGIN}, but the compiler
1177: optimistically assumes that it lives until the @code{THEN}. It notices
1178: this difference when it compiles the @code{UNTIL} and issues a
1179: warning. The user can avoid the warning, and make sure that @code{x}
1180: is not used in the wrong area by using explicit scoping:
1181: @example
1182: IF
1183: SCOPE
1.4 anton 1184: @{ x @}
1.2 anton 1185: ENDSCOPE
1186: BEGIN
1187: [ 1 cs-roll ] THEN
1188: ...
1189: UNTIL
1190: @end example
1191:
1192: Since the guess is optimistic, there will be no spurious error messages
1193: about undefined locals.
1194:
1195: If the @code{BEGIN} is not reachable from above (e.g., after
1196: @code{AHEAD} or @code{EXIT}), the compiler cannot even make an
1197: optimistic guess, as the locals visible after the @code{BEGIN} may be
1198: defined later. Therefore, the compiler assumes that no locals are
1.17 ! anton 1199: visible after the @code{BEGIN}. However, the user can use
1.2 anton 1200: @code{ASSUME-LIVE} to make the compiler assume that the same locals are
1.17 ! anton 1201: visible at the BEGIN as at the point where the top control-flow stack
! 1202: item was created.
1.2 anton 1203:
1204: doc-assume-live
1205:
1206: E.g.,
1207: @example
1.4 anton 1208: @{ x @}
1.2 anton 1209: AHEAD
1210: ASSUME-LIVE
1211: BEGIN
1212: x
1213: [ 1 CS-ROLL ] THEN
1214: ...
1215: UNTIL
1216: @end example
1217:
1218: Other cases where the locals are defined before the @code{BEGIN} can be
1219: handled by inserting an appropriate @code{CS-ROLL} before the
1220: @code{ASSUME-LIVE} (and changing the control-flow stack manipulation
1221: behind the @code{ASSUME-LIVE}).
1222:
1223: Cases where locals are defined after the @code{BEGIN} (but should be
1224: visible immediately after the @code{BEGIN}) can only be handled by
1225: rearranging the loop. E.g., the ``most insidious'' example above can be
1226: arranged into:
1227: @example
1228: BEGIN
1.4 anton 1229: @{ x @}
1.2 anton 1230: ... 0=
1231: WHILE
1232: x
1233: REPEAT
1234: @end example
1235:
1.17 ! anton 1236: @node How long do locals live?, Programming Style, Where are locals visible by name?, Gforth locals
1.2 anton 1237: @subsubsection How long do locals live?
1238:
1239: The right answer for the lifetime question would be: A local lives at
1240: least as long as it can be accessed. For a value-flavoured local this
1241: means: until the end of its visibility. However, a variable-flavoured
1242: local could be accessed through its address far beyond its visibility
1243: scope. Ultimately, this would mean that such locals would have to be
1244: garbage collected. Since this entails un-Forth-like implementation
1245: complexities, I adopted the same cowardly solution as some other
1246: languages (e.g., C): The local lives only as long as it is visible;
1247: afterwards its address is invalid (and programs that access it
1248: afterwards are erroneous).
1249:
1.17 ! anton 1250: @node Programming Style, Implementation, How long do locals live?, Gforth locals
1.2 anton 1251: @subsubsection Programming Style
1252:
1253: The freedom to define locals anywhere has the potential to change
1254: programming styles dramatically. In particular, the need to use the
1255: return stack for intermediate storage vanishes. Moreover, all stack
1256: manipulations (except @code{PICK}s and @code{ROLL}s with run-time
1257: determined arguments) can be eliminated: If the stack items are in the
1258: wrong order, just write a locals definition for all of them; then
1259: write the items in the order you want.
1260:
1261: This seems a little far-fetched and eliminating stack manipulations is
1.4 anton 1262: unlikely to become a conscious programming objective. Still, the number
1263: of stack manipulations will be reduced dramatically if local variables
1.17 ! anton 1264: are used liberally (e.g., compare @code{max} in @ref{Gforth locals} with
1.4 anton 1265: a traditional implementation of @code{max}).
1.2 anton 1266:
1267: This shows one potential benefit of locals: making Forth programs more
1268: readable. Of course, this benefit will only be realized if the
1269: programmers continue to honour the principle of factoring instead of
1270: using the added latitude to make the words longer.
1271:
1272: Using @code{TO} can and should be avoided. Without @code{TO},
1273: every value-flavoured local has only a single assignment and many
1274: advantages of functional languages apply to Forth. I.e., programs are
1275: easier to analyse, to optimize and to read: It is clear from the
1276: definition what the local stands for, it does not turn into something
1277: different later.
1278:
1279: E.g., a definition using @code{TO} might look like this:
1280: @example
1281: : strcmp @{ addr1 u1 addr2 u2 -- n @}
1282: u1 u2 min 0
1283: ?do
1284: addr1 c@ addr2 c@ - ?dup
1285: if
1286: unloop exit
1287: then
1288: addr1 char+ TO addr1
1289: addr2 char+ TO addr2
1290: loop
1291: u1 u2 - ;
1292: @end example
1293: Here, @code{TO} is used to update @code{addr1} and @code{addr2} at
1294: every loop iteration. @code{strcmp} is a typical example of the
1295: readability problems of using @code{TO}. When you start reading
1296: @code{strcmp}, you think that @code{addr1} refers to the start of the
1297: string. Only near the end of the loop you realize that it is something
1298: else.
1299:
1300: This can be avoided by defining two locals at the start of the loop that
1301: are initialized with the right value for the current iteration.
1302: @example
1303: : strcmp @{ addr1 u1 addr2 u2 -- n @}
1304: addr1 addr2
1305: u1 u2 min 0
1306: ?do @{ s1 s2 @}
1307: s1 c@ s2 c@ - ?dup
1308: if
1309: unloop exit
1310: then
1311: s1 char+ s2 char+
1312: loop
1313: 2drop
1314: u1 u2 - ;
1315: @end example
1316: Here it is clear from the start that @code{s1} has a different value
1317: in every loop iteration.
1318:
1.17 ! anton 1319: @node Implementation, , Programming Style, Gforth locals
1.2 anton 1320: @subsubsection Implementation
1321:
1.17 ! anton 1322: Gforth uses an extra locals stack. The most compelling reason for
1.2 anton 1323: this is that the return stack is not float-aligned; using an extra stack
1324: also eliminates the problems and restrictions of using the return stack
1325: as locals stack. Like the other stacks, the locals stack grows toward
1326: lower addresses. A few primitives allow an efficient implementation:
1327:
1328: doc-@local#
1329: doc-f@local#
1330: doc-laddr#
1331: doc-lp+!#
1332: doc-lp!
1333: doc->l
1334: doc-f>l
1335:
1336: In addition to these primitives, some specializations of these
1337: primitives for commonly occurring inline arguments are provided for
1338: efficiency reasons, e.g., @code{@@local0} as specialization of
1339: @code{@@local#} for the inline argument 0. The following compiling words
1340: compile the right specialized version, or the general version, as
1341: appropriate:
1342:
1.12 anton 1343: doc-compile-@local
1344: doc-compile-f@local
1.2 anton 1345: doc-compile-lp+!
1346:
1347: Combinations of conditional branches and @code{lp+!#} like
1348: @code{?branch-lp+!#} (the locals pointer is only changed if the branch
1349: is taken) are provided for efficiency and correctness in loops.
1350:
1351: A special area in the dictionary space is reserved for keeping the
1352: local variable names. @code{@{} switches the dictionary pointer to this
1353: area and @code{@}} switches it back and generates the locals
1354: initializing code. @code{W:} etc.@ are normal defining words. This
1355: special area is cleared at the start of every colon definition.
1356:
1.17 ! anton 1357: A special feature of Gforth's dictionary is used to implement the
1.2 anton 1358: definition of locals without type specifiers: every wordlist (aka
1359: vocabulary) has its own methods for searching
1.4 anton 1360: etc. (@pxref{Wordlists}). For the present purpose we defined a wordlist
1.2 anton 1361: with a special search method: When it is searched for a word, it
1362: actually creates that word using @code{W:}. @code{@{} changes the search
1363: order to first search the wordlist containing @code{@}}, @code{W:} etc.,
1364: and then the wordlist for defining locals without type specifiers.
1365:
1366: The lifetime rules support a stack discipline within a colon
1367: definition: The lifetime of a local is either nested with other locals
1368: lifetimes or it does not overlap them.
1369:
1370: At @code{BEGIN}, @code{IF}, and @code{AHEAD} no code for locals stack
1371: pointer manipulation is generated. Between control structure words
1372: locals definitions can push locals onto the locals stack. @code{AGAIN}
1373: is the simplest of the other three control flow words. It has to
1374: restore the locals stack depth of the corresponding @code{BEGIN}
1375: before branching. The code looks like this:
1376: @format
1377: @code{lp+!#} current-locals-size @minus{} dest-locals-size
1378: @code{branch} <begin>
1379: @end format
1380:
1381: @code{UNTIL} is a little more complicated: If it branches back, it
1382: must adjust the stack just like @code{AGAIN}. But if it falls through,
1383: the locals stack must not be changed. The compiler generates the
1384: following code:
1385: @format
1386: @code{?branch-lp+!#} <begin> current-locals-size @minus{} dest-locals-size
1387: @end format
1388: The locals stack pointer is only adjusted if the branch is taken.
1389:
1390: @code{THEN} can produce somewhat inefficient code:
1391: @format
1392: @code{lp+!#} current-locals-size @minus{} orig-locals-size
1393: <orig target>:
1394: @code{lp+!#} orig-locals-size @minus{} new-locals-size
1395: @end format
1396: The second @code{lp+!#} adjusts the locals stack pointer from the
1.4 anton 1397: level at the @var{orig} point to the level after the @code{THEN}. The
1.2 anton 1398: first @code{lp+!#} adjusts the locals stack pointer from the current
1399: level to the level at the orig point, so the complete effect is an
1400: adjustment from the current level to the right level after the
1401: @code{THEN}.
1402:
1403: In a conventional Forth implementation a dest control-flow stack entry
1404: is just the target address and an orig entry is just the address to be
1405: patched. Our locals implementation adds a wordlist to every orig or dest
1406: item. It is the list of locals visible (or assumed visible) at the point
1407: described by the entry. Our implementation also adds a tag to identify
1408: the kind of entry, in particular to differentiate between live and dead
1409: (reachable and unreachable) orig entries.
1410:
1411: A few unusual operations have to be performed on locals wordlists:
1412:
1413: doc-common-list
1414: doc-sub-list?
1415: doc-list-size
1416:
1417: Several features of our locals wordlist implementation make these
1418: operations easy to implement: The locals wordlists are organised as
1419: linked lists; the tails of these lists are shared, if the lists
1420: contain some of the same locals; and the address of a name is greater
1421: than the address of the names behind it in the list.
1422:
1423: Another important implementation detail is the variable
1424: @code{dead-code}. It is used by @code{BEGIN} and @code{THEN} to
1425: determine if they can be reached directly or only through the branch
1426: that they resolve. @code{dead-code} is set by @code{UNREACHABLE},
1427: @code{AHEAD}, @code{EXIT} etc., and cleared at the start of a colon
1428: definition, by @code{BEGIN} and usually by @code{THEN}.
1429:
1430: Counted loops are similar to other loops in most respects, but
1431: @code{LEAVE} requires special attention: It performs basically the same
1432: service as @code{AHEAD}, but it does not create a control-flow stack
1433: entry. Therefore the information has to be stored elsewhere;
1434: traditionally, the information was stored in the target fields of the
1435: branches created by the @code{LEAVE}s, by organizing these fields into a
1436: linked list. Unfortunately, this clever trick does not provide enough
1437: space for storing our extended control flow information. Therefore, we
1438: introduce another stack, the leave stack. It contains the control-flow
1439: stack entries for all unresolved @code{LEAVE}s.
1440:
1441: Local names are kept until the end of the colon definition, even if
1442: they are no longer visible in any control-flow path. In a few cases
1443: this may lead to increased space needs for the locals name area, but
1444: usually less than reclaiming this space would cost in code size.
1445:
1446:
1.17 ! anton 1447: @node ANS Forth locals, , Gforth locals, Locals
1.2 anton 1448: @subsection ANS Forth locals
1449:
1450: The ANS Forth locals wordset does not define a syntax for locals, but
1451: words that make it possible to define various syntaxes. One of the
1.17 ! anton 1452: possible syntaxes is a subset of the syntax we used in the Gforth locals
1.2 anton 1453: wordset, i.e.:
1454:
1455: @example
1456: @{ local1 local2 ... -- comment @}
1457: @end example
1458: or
1459: @example
1460: @{ local1 local2 ... @}
1461: @end example
1462:
1463: The order of the locals corresponds to the order in a stack comment. The
1464: restrictions are:
1.1 anton 1465:
1.2 anton 1466: @itemize @bullet
1467: @item
1.17 ! anton 1468: Locals can only be cell-sized values (no type specifiers are allowed).
1.2 anton 1469: @item
1470: Locals can be defined only outside control structures.
1471: @item
1472: Locals can interfere with explicit usage of the return stack. For the
1473: exact (and long) rules, see the standard. If you don't use return stack
1.17 ! anton 1474: accessing words in a definition using locals, you will be all right. The
1.2 anton 1475: purpose of this rule is to make locals implementation on the return
1476: stack easier.
1477: @item
1478: The whole definition must be in one line.
1479: @end itemize
1480:
1481: Locals defined in this way behave like @code{VALUE}s
1.4 anton 1482: (@xref{Values}). I.e., they are initialized from the stack. Using their
1.2 anton 1483: name produces their value. Their value can be changed using @code{TO}.
1484:
1.17 ! anton 1485: Since this syntax is supported by Gforth directly, you need not do
1.2 anton 1486: anything to use it. If you want to port a program using this syntax to
1487: another ANS Forth system, use @file{anslocal.fs} to implement the syntax
1488: on the other system.
1489:
1490: Note that a syntax shown in the standard, section A.13 looks
1491: similar, but is quite different in having the order of locals
1492: reversed. Beware!
1493:
1494: The ANS Forth locals wordset itself consists of the following word
1495:
1496: doc-(local)
1497:
1498: The ANS Forth locals extension wordset defines a syntax, but it is so
1499: awful that we strongly recommend not to use it. We have implemented this
1.17 ! anton 1500: syntax to make porting to Gforth easy, but do not document it here. The
1.2 anton 1501: problem with this syntax is that the locals are defined in an order
1502: reversed with respect to the standard stack comment notation, making
1503: programs harder to read, and easier to misread and miswrite. The only
1504: merit of this syntax is that it is easy to implement using the ANS Forth
1505: locals wordset.
1.3 anton 1506:
1.4 anton 1507: @node Defining Words, Wordlists, Locals, Words
1508: @section Defining Words
1509:
1.14 anton 1510: @menu
1511: * Values::
1512: @end menu
1513:
1.4 anton 1514: @node Values, , Defining Words, Defining Words
1515: @subsection Values
1516:
1517: @node Wordlists, Files, Defining Words, Words
1518: @section Wordlists
1519:
1520: @node Files, Blocks, Wordlists, Words
1521: @section Files
1522:
1523: @node Blocks, Other I/O, Files, Words
1524: @section Blocks
1525:
1526: @node Other I/O, Programming Tools, Blocks, Words
1527: @section Other I/O
1528:
1529: @node Programming Tools, Threading Words, Other I/O, Words
1530: @section Programming Tools
1531:
1.5 anton 1532: @menu
1533: * Debugging:: Simple and quick.
1534: * Assertions:: Making your programs self-checking.
1535: @end menu
1536:
1537: @node Debugging, Assertions, Programming Tools, Programming Tools
1.4 anton 1538: @subsection Debugging
1539:
1540: The simple debugging aids provided in @file{debugging.fs}
1541: are meant to support a different style of debugging than the
1542: tracing/stepping debuggers used in languages with long turn-around
1543: times.
1544:
1545: A much better (faster) way in fast-compilig languages is to add
1546: printing code at well-selected places, let the program run, look at
1547: the output, see where things went wrong, add more printing code, etc.,
1548: until the bug is found.
1549:
1550: The word @code{~~} is easy to insert. It just prints debugging
1551: information (by default the source location and the stack contents). It
1552: is also easy to remove (@kbd{C-x ~} in the Emacs Forth mode to
1553: query-replace them with nothing). The deferred words
1554: @code{printdebugdata} and @code{printdebugline} control the output of
1555: @code{~~}. The default source location output format works well with
1556: Emacs' compilation mode, so you can step through the program at the
1.5 anton 1557: source level using @kbd{C-x `} (the advantage over a stepping debugger
1558: is that you can step in any direction and you know where the crash has
1559: happened or where the strange data has occurred).
1.4 anton 1560:
1561: Note that the default actions clobber the contents of the pictured
1562: numeric output string, so you should not use @code{~~}, e.g., between
1563: @code{<#} and @code{#>}.
1564:
1565: doc-~~
1566: doc-printdebugdata
1567: doc-printdebugline
1568:
1.5 anton 1569: @node Assertions, , Debugging, Programming Tools
1.4 anton 1570: @subsection Assertions
1571:
1.5 anton 1572: It is a good idea to make your programs self-checking, in particular, if
1573: you use an assumption (e.g., that a certain field of a data structure is
1.17 ! anton 1574: never zero) that may become wrong during maintenance. Gforth supports
1.5 anton 1575: assertions for this purpose. They are used like this:
1576:
1577: @example
1578: assert( @var{flag} )
1579: @end example
1580:
1581: The code between @code{assert(} and @code{)} should compute a flag, that
1582: should be true if everything is alright and false otherwise. It should
1583: not change anything else on the stack. The overall stack effect of the
1584: assertion is @code{( -- )}. E.g.
1585:
1586: @example
1587: assert( 1 1 + 2 = ) \ what we learn in school
1588: assert( dup 0<> ) \ assert that the top of stack is not zero
1589: assert( false ) \ this code should not be reached
1590: @end example
1591:
1592: The need for assertions is different at different times. During
1593: debugging, we want more checking, in production we sometimes care more
1594: for speed. Therefore, assertions can be turned off, i.e., the assertion
1595: becomes a comment. Depending on the importance of an assertion and the
1596: time it takes to check it, you may want to turn off some assertions and
1.17 ! anton 1597: keep others turned on. Gforth provides several levels of assertions for
1.5 anton 1598: this purpose:
1599:
1600: doc-assert0(
1601: doc-assert1(
1602: doc-assert2(
1603: doc-assert3(
1604: doc-assert(
1605: doc-)
1606:
1607: @code{Assert(} is the same as @code{assert1(}. The variable
1608: @code{assert-level} specifies the highest assertions that are turned
1609: on. I.e., at the default @code{assert-level} of one, @code{assert0(} and
1610: @code{assert1(} assertions perform checking, while @code{assert2(} and
1611: @code{assert3(} assertions are treated as comments.
1612:
1613: Note that the @code{assert-level} is evaluated at compile-time, not at
1614: run-time. I.e., you cannot turn assertions on or off at run-time, you
1615: have to set the @code{assert-level} appropriately before compiling a
1616: piece of code. You can compile several pieces of code at several
1617: @code{assert-level}s (e.g., a trusted library at level 1 and newly
1618: written code at level 3).
1619:
1620: doc-assert-level
1621:
1622: If an assertion fails, a message compatible with Emacs' compilation mode
1623: is produced and the execution is aborted (currently with @code{ABORT"}.
1624: If there is interest, we will introduce a special throw code. But if you
1625: intend to @code{catch} a specific condition, using @code{throw} is
1626: probably more appropriate than an assertion).
1627:
1.4 anton 1628: @node Threading Words, , Programming Tools, Words
1629: @section Threading Words
1630:
1631: These words provide access to code addresses and other threading stuff
1.17 ! anton 1632: in Gforth (and, possibly, other interpretive Forths). It more or less
1.4 anton 1633: abstracts away the differences between direct and indirect threading
1634: (and, for direct threading, the machine dependences). However, at
1635: present this wordset is still inclomplete. It is also pretty low-level;
1636: some day it will hopefully be made unnecessary by an internals words set
1637: that abstracts implementation details away completely.
1638:
1639: doc->code-address
1640: doc->does-code
1641: doc-code-address!
1642: doc-does-code!
1643: doc-does-handler!
1644: doc-/does-handler
1645:
1.14 anton 1646:
1647:
1.4 anton 1648: @node ANS conformance, Model, Words, Top
1649: @chapter ANS conformance
1650:
1.17 ! anton 1651: To the best of our knowledge, Gforth is an
1.14 anton 1652:
1.15 anton 1653: ANS Forth System
1654: @itemize
1655: @item providing the Core Extensions word set
1656: @item providing the Block word set
1657: @item providing the Block Extensions word set
1658: @item providing the Double-Number word set
1659: @item providing the Double-Number Extensions word set
1660: @item providing the Exception word set
1661: @item providing the Exception Extensions word set
1662: @item providing the Facility word set
1663: @item providing @code{MS} and @code{TIME&DATE} from the Facility Extensions word set
1664: @item providing the File Access word set
1665: @item providing the File Access Extensions word set
1666: @item providing the Floating-Point word set
1667: @item providing the Floating-Point Extensions word set
1668: @item providing the Locals word set
1669: @item providing the Locals Extensions word set
1670: @item providing the Memory-Allocation word set
1671: @item providing the Memory-Allocation Extensions word set (that one's easy)
1672: @item providing the Programming-Tools word set
1673: @item providing @code{AHEAD}, @code{BYE}, @code{CS-PICK}, @code{CS-ROLL}, @code{STATE}, @code{[ELSE]}, @code{[IF]}, @code{[THEN]} from the Programming-Tools Extensions word set
1674: @item providing the Search-Order word set
1675: @item providing the Search-Order Extensions word set
1676: @item providing the String word set
1677: @item providing the String Extensions word set (another easy one)
1678: @end itemize
1679:
1680: In addition, ANS Forth systems are required to document certain
1681: implementation choices. This chapter tries to meet these
1682: requirements. In many cases it gives a way to ask the system for the
1683: information instead of providing the information directly, in
1684: particular, if the information depends on the processor, the operating
1685: system or the installation options chosen, or if they are likely to
1.17 ! anton 1686: change during the maintenance of Gforth.
1.15 anton 1687:
1.14 anton 1688: @comment The framework for the rest has been taken from pfe.
1689:
1690: @menu
1691: * The Core Words::
1692: * The optional Block word set::
1693: * The optional Double Number word set::
1694: * The optional Exception word set::
1695: * The optional Facility word set::
1696: * The optional File-Access word set::
1697: * The optional Floating-Point word set::
1698: * The optional Locals word set::
1699: * The optional Memory-Allocation word set::
1700: * The optional Programming-Tools word set::
1701: * The optional Search-Order word set::
1702: @end menu
1703:
1704:
1705: @c =====================================================================
1706: @node The Core Words, The optional Block word set, ANS conformance, ANS conformance
1707: @comment node-name, next, previous, up
1708: @section The Core Words
1709: @c =====================================================================
1710:
1711: @menu
1.15 anton 1712: * core-idef:: Implementation Defined Options
1713: * core-ambcond:: Ambiguous Conditions
1714: * core-other:: Other System Documentation
1.14 anton 1715: @end menu
1716:
1717: @c ---------------------------------------------------------------------
1718: @node core-idef, core-ambcond, The Core Words, The Core Words
1719: @subsection Implementation Defined Options
1720: @c ---------------------------------------------------------------------
1721:
1722: @table @i
1723:
1724: @item (Cell) aligned addresses:
1.17 ! anton 1725: processor-dependent. Gforth's alignment words perform natural alignment
1.14 anton 1726: (e.g., an address aligned for a datum of size 8 is divisible by
1727: 8). Unaligned accesses usually result in a @code{-23 THROW}.
1728:
1729: @item @code{EMIT} and non-graphic characters:
1730: The character is output using the C library function (actually, macro)
1731: @code{putchar}.
1732:
1733: @item character editing of @code{ACCEPT} and @code{EXPECT}:
1734: This is modeled on the GNU readline library (@pxref{Readline
1735: Interaction, , Command Line Editing, readline, The GNU Readline
1736: Library}) with Emacs-like key bindings. @kbd{Tab} deviates a little by
1737: producing a full word completion every time you type it (instead of
1738: producing the common prefix of all completions).
1739:
1740: @item character set:
1741: The character set of your computer and display device. Gforth is
1742: 8-bit-clean (but some other component in your system may make trouble).
1743:
1744: @item Character-aligned address requirements:
1745: installation-dependent. Currently a character is represented by a C
1746: @code{unsigned char}; in the future we might switch to @code{wchar_t}
1747: (Comments on that requested).
1748:
1749: @item character-set extensions and matching of names:
1.17 ! anton 1750: Any character except the ASCII NUL charcter can be used in a
! 1751: name. Matching is case-insensitive. The matching is performed using the
! 1752: C function @code{strncasecmp}, whose function is probably influenced by
! 1753: the locale. E.g., the @code{C} locale does not know about accents and
1.14 anton 1754: umlauts, so they are matched case-sensitively in that locale. For
1755: portability reasons it is best to write programs such that they work in
1756: the @code{C} locale. Then one can use libraries written by a Polish
1757: programmer (who might use words containing ISO Latin-2 encoded
1758: characters) and by a French programmer (ISO Latin-1) in the same program
1759: (of course, @code{WORDS} will produce funny results for some of the
1760: words (which ones, depends on the font you are using)). Also, the locale
1761: you prefer may not be available in other operating systems. Hopefully,
1762: Unicode will solve these problems one day.
1763:
1764: @item conditions under which control characters match a space delimiter:
1765: If @code{WORD} is called with the space character as a delimiter, all
1766: white-space characters (as identified by the C macro @code{isspace()})
1767: are delimiters. @code{PARSE}, on the other hand, treats space like other
1768: delimiters. @code{PARSE-WORD} treats space like @code{WORD}, but behaves
1769: like @code{PARSE} otherwise. @code{(NAME)}, which is used by the outer
1770: interpreter (aka text interpreter) by default, treats all white-space
1771: characters as delimiters.
1772:
1773: @item format of the control flow stack:
1774: The data stack is used as control flow stack. The size of a control flow
1775: stack item in cells is given by the constant @code{cs-item-size}. At the
1776: time of this writing, an item consists of a (pointer to a) locals list
1777: (third), an address in the code (second), and a tag for identifying the
1778: item (TOS). The following tags are used: @code{defstart},
1779: @code{live-orig}, @code{dead-orig}, @code{dest}, @code{do-dest},
1780: @code{scopestart}.
1781:
1782: @item conversion of digits > 35
1783: The characters @code{[\]^_'} are the digits with the decimal value
1784: 36@minus{}41. There is no way to input many of the larger digits.
1785:
1786: @item display after input terminates in @code{ACCEPT} and @code{EXPECT}:
1787: The cursor is moved to the end of the entered string. If the input is
1788: terminated using the @kbd{Return} key, a space is typed.
1789:
1790: @item exception abort sequence of @code{ABORT"}:
1791: The error string is stored into the variable @code{"error} and a
1792: @code{-2 throw} is performed.
1793:
1794: @item input line terminator:
1795: For interactive input, @kbd{C-m} and @kbd{C-j} terminate lines. One of
1796: these characters is typically produced when you type the @kbd{Enter} or
1797: @kbd{Return} key.
1798:
1799: @item maximum size of a counted string:
1800: @code{s" /counted-string" environment? drop .}. Currently 255 characters
1801: on all ports, but this may change.
1802:
1803: @item maximum size of a parsed string:
1804: Given by the constant @code{/line}. Currently 255 characters.
1805:
1806: @item maximum size of a definition name, in characters:
1807: 31
1808:
1809: @item maximum string length for @code{ENVIRONMENT?}, in characters:
1810: 31
1811:
1812: @item method of selecting the user input device:
1.17 ! anton 1813: The user input device is the standard input. There is currently no way to
! 1814: change it from within Gforth. However, the input can typically be
! 1815: redirected in the command line that starts Gforth.
1.14 anton 1816:
1817: @item method of selecting the user output device:
1818: The user output device is the standard output. It cannot be redirected
1.17 ! anton 1819: from within Gforth, but typically from the command line that starts
! 1820: Gforth. Gforth uses buffered output, so output on a terminal does not
1.14 anton 1821: become visible before the next newline or buffer overflow. Output on
1822: non-terminals is invisible until the buffer overflows.
1823:
1824: @item methods of dictionary compilation:
1.17 ! anton 1825: What are we expected to document here?
1.14 anton 1826:
1827: @item number of bits in one address unit:
1828: @code{s" address-units-bits" environment? drop .}. 8 in all current
1829: ports.
1830:
1831: @item number representation and arithmetic:
1832: Processor-dependent. Binary two's complement on all current ports.
1833:
1834: @item ranges for integer types:
1835: Installation-dependent. Make environmental queries for @code{MAX-N},
1836: @code{MAX-U}, @code{MAX-D} and @code{MAX-UD}. The lower bounds for
1837: unsigned (and positive) types is 0. The lower bound for signed types on
1838: two's complement and one's complement machines machines can be computed
1839: by adding 1 to the upper bound.
1840:
1841: @item read-only data space regions:
1842: The whole Forth data space is writable.
1843:
1844: @item size of buffer at @code{WORD}:
1845: @code{PAD HERE - .}. 104 characters on 32-bit machines. The buffer is
1846: shared with the pictured numeric output string. If overwriting
1847: @code{PAD} is acceptable, it is as large as the remaining dictionary
1848: space, although only as much can be sensibly used as fits in a counted
1849: string.
1850:
1851: @item size of one cell in address units:
1852: @code{1 cells .}.
1853:
1854: @item size of one character in address units:
1855: @code{1 chars .}. 1 on all current ports.
1856:
1857: @item size of the keyboard terminal buffer:
1858: Varies. You can determine the size at a specific time using @code{lp@
1859: tib - .}. It is shared with the locals stack and TIBs of files that
1860: include the current file. You can change the amount of space for TIBs
1.17 ! anton 1861: and locals stack at Gforth startup with the command line option
1.14 anton 1862: @code{-l}.
1863:
1864: @item size of the pictured numeric output buffer:
1865: @code{PAD HERE - .}. 104 characters on 32-bit machines. The buffer is
1866: shared with @code{WORD}.
1867:
1868: @item size of the scratch area returned by @code{PAD}:
1869: The remainder of dictionary space. You can even use the unused part of
1870: the data stack space. The current size can be computed with @code{sp@
1871: pad - .}.
1872:
1873: @item system case-sensitivity characteristics:
1874: Dictionary searches are case insensitive. However, as explained above
1875: under @i{character-set extensions}, the matching for non-ASCII
1876: characters is determined by the locale you are using. In the default
1877: @code{C} locale all non-ASCII characters are matched case-sensitively.
1878:
1879: @item system prompt:
1880: @code{ ok} in interpret state, @code{ compiled} in compile state.
1881:
1882: @item division rounding:
1883: installation dependent. @code{s" floored" environment? drop .}. We leave
1884: the choice to gcc (what to use for @code{/}) and to you (whether to use
1885: @code{fm/mod}, @code{sm/rem} or simply @code{/}).
1886:
1887: @item values of @code{STATE} when true:
1888: -1.
1889:
1890: @item values returned after arithmetic overflow:
1891: On two's complement machines, arithmetic is performed modulo
1892: 2**bits-per-cell for single arithmetic and 4**bits-per-cell for double
1893: arithmetic (with appropriate mapping for signed types). Division by zero
1894: typically results in a @code{-55 throw} (floatingpoint unidentified
1895: fault), although a @code{-10 throw} (divide by zero) would be more
1896: appropriate.
1897:
1898: @item whether the current definition can be found after @t{DOES>}:
1899: No.
1900:
1901: @end table
1902:
1903: @c ---------------------------------------------------------------------
1904: @node core-ambcond, core-other, core-idef, The Core Words
1905: @subsection Ambiguous conditions
1906: @c ---------------------------------------------------------------------
1907:
1908: @table @i
1909:
1910: @item a name is neither a word nor a number:
1911: @code{-13 throw} (Undefined word)
1912:
1913: @item a definition name exceeds the maximum length allowed:
1914: @code{-19 throw} (Word name too long)
1915:
1916: @item addressing a region not inside the various data spaces of the forth system:
1917: The stacks, code space and name space are accessible. Machine code space is
1918: typically readable. Accessing other addresses gives results dependent on
1919: the operating system. On decent systems: @code{-9 throw} (Invalid memory
1920: address).
1921:
1922: @item argument type incompatible with parameter:
1923: This is usually not caught. Some words perform checks, e.g., the control
1924: flow words, and issue a @code{ABORT"} or @code{-12 THROW} (Argument type
1925: mismatch).
1926:
1927: @item attempting to obtain the execution token of a word with undefined execution semantics:
1928: You get an execution token representing the compilation semantics
1929: instead.
1930:
1931: @item dividing by zero:
1932: typically results in a @code{-55 throw} (floating point unidentified
1933: fault), although a @code{-10 throw} (divide by zero) would be more
1934: appropriate.
1935:
1936: @item insufficient data stack or return stack space:
1937: Not checked. This typically results in mysterious illegal memory
1938: accesses, producing @code{-9 throw} (Invalid memory address) or
1939: @code{-23 throw} (Address alignment exception).
1940:
1941: @item insufficient space for loop control parameters:
1942: like other return stack overflows.
1943:
1944: @item insufficient space in the dictionary:
1945: Not checked. Similar results as stack overflows. However, typically the
1946: error appears at a different place when one inserts or removes code.
1947:
1948: @item interpreting a word with undefined interpretation semantics:
1949: For some words, we defined interpretation semantics. For the others:
1950: @code{-14 throw} (Interpreting a compile-only word). Note that this is
1951: checked only by the outer (aka text) interpreter; if the word is
1952: @code{execute}d in some other way, it will typically perform it's
1953: compilation semantics even in interpret state. (We could change @code{'}
1954: and relatives not to give the xt of such words, but we think that would
1955: be too restrictive).
1956:
1957: @item modifying the contents of the input buffer or a string literal:
1958: These are located in writable memory and can be modified.
1959:
1960: @item overflow of the pictured numeric output string:
1961: Not checked.
1962:
1963: @item parsed string overflow:
1964: @code{PARSE} cannot overflow. @code{WORD} does not check for overflow.
1965:
1966: @item producing a result out of range:
1967: On two's complement machines, arithmetic is performed modulo
1968: 2**bits-per-cell for single arithmetic and 4**bits-per-cell for double
1969: arithmetic (with appropriate mapping for signed types). Division by zero
1970: typically results in a @code{-55 throw} (floatingpoint unidentified
1971: fault), although a @code{-10 throw} (divide by zero) would be more
1972: appropriate. @code{convert} and @code{>number} currently overflow
1973: silently.
1974:
1975: @item reading from an empty data or return stack:
1976: The data stack is checked by the outer (aka text) interpreter after
1977: every word executed. If it has underflowed, a @code{-4 throw} (Stack
1978: underflow) is performed. Apart from that, the stacks are not checked and
1979: underflows can result in similar behaviour as overflows (of adjacent
1980: stacks).
1981:
1982: @item unexepected end of the input buffer, resulting in an attempt to use a zero-length string as a name:
1983: @code{Create} and its descendants perform a @code{-16 throw} (Attempt to
1984: use zero-length string as a name). Words like @code{'} probably will not
1985: find what they search. Note that it is possible to create zero-length
1986: names with @code{nextname} (should it not?).
1987:
1988: @item @code{>IN} greater than input buffer:
1989: The next invocation of a parsing word returns a string wih length 0.
1990:
1991: @item @code{RECURSE} appears after @code{DOES>}:
1992: Compiles a recursive call to the defining word not to the defined word.
1993:
1994: @item argument input source different than current input source for @code{RESTORE-INPUT}:
1995: !!???If the argument input source is a valid input source then it gets
1996: restored. Otherwise causes @code{-12 THROW} which unless caught issues
1997: the message "argument type mismatch" and aborts.
1998:
1999: @item data space containing definitions gets de-allocated:
2000: Deallocation with @code{allot} is not checked. This typically resuls in
2001: memory access faults or execution of illegal instructions.
2002:
2003: @item data space read/write with incorrect alignment:
2004: Processor-dependent. Typically results in a @code{-23 throw} (Address
2005: alignment exception). Under Linux on a 486 or later processor with
2006: alignment turned on, incorrect alignment results in a @code{-9 throw}
2007: (Invalid memory address). There are reportedly some processors with
2008: alignment restrictions that do not report them.
2009:
2010: @item data space pointer not properly aligned, @code{,}, @code{C,}:
2011: Like other alignment errors.
2012:
2013: @item less than u+2 stack items (@code{PICK} and @code{ROLL}):
2014: Not checked. May cause an illegal memory access.
2015:
2016: @item loop control parameters not available:
2017: Not checked. The counted loop words simply assume that the top of return
2018: stack items are loop control parameters and behave accordingly.
2019:
2020: @item most recent definition does not have a name (@code{IMMEDIATE}):
2021: @code{abort" last word was headerless"}.
2022:
2023: @item name not defined by @code{VALUE} used by @code{TO}:
2024: @code{-32 throw} (Invalid name argument)
2025:
1.15 anton 2026: @item name not found (@code{'}, @code{POSTPONE}, @code{[']}, @code{[COMPILE]}):
1.14 anton 2027: @code{-13 throw} (Undefined word)
2028:
2029: @item parameters are not of the same type (@code{DO}, @code{?DO}, @code{WITHIN}):
2030: Gforth behaves as if they were of the same type. I.e., you can predict
2031: the behaviour by interpreting all parameters as, e.g., signed.
2032:
2033: @item @code{POSTPONE} or @code{[COMPILE]} applied to @code{TO}:
2034: Assume @code{: X POSTPONE TO ; IMMEDIATE}. @code{X} is equivalent to
2035: @code{TO}.
2036:
2037: @item String longer than a counted string returned by @code{WORD}:
2038: Not checked. The string will be ok, but the count will, of course,
2039: contain only the least significant bits of the length.
2040:
1.15 anton 2041: @item u greater than or equal to the number of bits in a cell (@code{LSHIFT}, @code{RSHIFT}):
1.14 anton 2042: Processor-dependent. Typical behaviours are returning 0 and using only
2043: the low bits of the shift count.
2044:
2045: @item word not defined via @code{CREATE}:
2046: @code{>BODY} produces the PFA of the word no matter how it was defined.
2047:
2048: @code{DOES>} changes the execution semantics of the last defined word no
2049: matter how it was defined. E.g., @code{CONSTANT DOES>} is equivalent to
2050: @code{CREATE , DOES>}.
2051:
2052: @item words improperly used outside @code{<#} and @code{#>}:
2053: Not checked. As usual, you can expect memory faults.
2054:
2055: @end table
2056:
2057:
2058: @c ---------------------------------------------------------------------
2059: @node core-other, , core-ambcond, The Core Words
2060: @subsection Other system documentation
2061: @c ---------------------------------------------------------------------
2062:
2063: @table @i
2064:
2065: @item nonstandard words using @code{PAD}:
2066: None.
2067:
2068: @item operator's terminal facilities available:
2069: !!??
2070:
2071: @item program data space available:
2072: @code{sp@ here - .} gives the space remaining for dictionary and data
2073: stack together.
2074:
2075: @item return stack space available:
2076: !!??
2077:
2078: @item stack space available:
2079: @code{sp@ here - .} gives the space remaining for dictionary and data
2080: stack together.
2081:
2082: @item system dictionary space required, in address units:
2083: Type @code{here forthstart - .} after startup. At the time of this
2084: writing, this gives 70108 (bytes) on a 32-bit system.
2085: @end table
2086:
2087:
2088: @c =====================================================================
2089: @node The optional Block word set, The optional Double Number word set, The Core Words, ANS conformance
2090: @section The optional Block word set
2091: @c =====================================================================
2092:
2093: @menu
1.15 anton 2094: * block-idef:: Implementation Defined Options
2095: * block-ambcond:: Ambiguous Conditions
2096: * block-other:: Other System Documentation
1.14 anton 2097: @end menu
2098:
2099:
2100: @c ---------------------------------------------------------------------
2101: @node block-idef, block-ambcond, The optional Block word set, The optional Block word set
2102: @subsection Implementation Defined Options
2103: @c ---------------------------------------------------------------------
2104:
2105: @table @i
2106:
2107: @item the format for display by @code{LIST}:
2108: First the screen number is displayed, then 16 lines of 64 characters,
2109: each line preceded by the line number.
2110:
2111: @item the length of a line affected by @code{\}:
2112: 64 characters.
2113: @end table
2114:
2115:
2116: @c ---------------------------------------------------------------------
2117: @node block-ambcond, block-other, block-idef, The optional Block word set
2118: @subsection Ambiguous conditions
2119: @c ---------------------------------------------------------------------
2120:
2121: @table @i
2122:
2123: @item correct block read was not possible:
2124: Typically results in a @code{throw} of some OS-derived value (between
2125: -512 and -2048). If the blocks file was just not long enough, blanks are
2126: supplied for the missing portion.
2127:
2128: @item I/O exception in block transfer:
2129: Typically results in a @code{throw} of some OS-derived value (between
2130: -512 and -2048).
2131:
2132: @item invalid block number:
2133: @code{-35 throw} (Invalid block number)
2134:
2135: @item a program directly alters the contents of @code{BLK}:
2136: The input stream is switched to that other block, at the same
2137: position. If the storing to @code{BLK} happens when interpreting
2138: non-block input, the system will get quite confused when the block ends.
2139:
2140: @item no current block buffer for @code{UPDATE}:
2141: @code{UPDATE} has no effect.
2142:
2143: @end table
2144:
2145:
2146: @c ---------------------------------------------------------------------
2147: @node block-other, , block-ambcond, The optional Block word set
2148: @subsection Other system documentation
2149: @c ---------------------------------------------------------------------
2150:
2151: @table @i
2152:
2153: @item any restrictions a multiprogramming system places on the use of buffer addresses:
2154: No restrictions (yet).
2155:
2156: @item the number of blocks available for source and data:
2157: depends on your disk space.
2158:
2159: @end table
2160:
2161:
2162: @c =====================================================================
2163: @node The optional Double Number word set, The optional Exception word set, The optional Block word set, ANS conformance
2164: @section The optional Double Number word set
2165: @c =====================================================================
2166:
2167: @menu
1.15 anton 2168: * double-ambcond:: Ambiguous Conditions
1.14 anton 2169: @end menu
2170:
2171:
2172: @c ---------------------------------------------------------------------
1.15 anton 2173: @node double-ambcond, , The optional Double Number word set, The optional Double Number word set
1.14 anton 2174: @subsection Ambiguous conditions
2175: @c ---------------------------------------------------------------------
2176:
2177: @table @i
2178:
1.15 anton 2179: @item @var{d} outside of range of @var{n} in @code{D>S}:
1.14 anton 2180: The least significant cell of @var{d} is produced.
2181:
2182: @end table
2183:
2184:
2185: @c =====================================================================
2186: @node The optional Exception word set, The optional Facility word set, The optional Double Number word set, ANS conformance
2187: @section The optional Exception word set
2188: @c =====================================================================
2189:
2190: @menu
1.15 anton 2191: * exception-idef:: Implementation Defined Options
1.14 anton 2192: @end menu
2193:
2194:
2195: @c ---------------------------------------------------------------------
1.15 anton 2196: @node exception-idef, , The optional Exception word set, The optional Exception word set
1.14 anton 2197: @subsection Implementation Defined Options
2198: @c ---------------------------------------------------------------------
2199:
2200: @table @i
2201: @item @code{THROW}-codes used in the system:
2202: The codes -256@minus{}-511 are used for reporting signals (see
2203: @file{errore.fs}). The codes -512@minus{}-2047 are used for OS errors
2204: (for file and memory allocation operations). The mapping from OS error
2205: numbers to throw code is -512@minus{}@var{errno}. One side effect of
2206: this mapping is that undefined OS errors produce a message with a
2207: strange number; e.g., @code{-1000 THROW} results in @code{Unknown error
2208: 488} on my system.
2209: @end table
2210:
2211: @c =====================================================================
2212: @node The optional Facility word set, The optional File-Access word set, The optional Exception word set, ANS conformance
2213: @section The optional Facility word set
2214: @c =====================================================================
2215:
2216: @menu
1.15 anton 2217: * facility-idef:: Implementation Defined Options
2218: * facility-ambcond:: Ambiguous Conditions
1.14 anton 2219: @end menu
2220:
2221:
2222: @c ---------------------------------------------------------------------
2223: @node facility-idef, facility-ambcond, The optional Facility word set, The optional Facility word set
2224: @subsection Implementation Defined Options
2225: @c ---------------------------------------------------------------------
2226:
2227: @table @i
2228:
2229: @item encoding of keyboard events (@code{EKEY}):
2230: Not yet implemeted.
2231:
2232: @item duration of a system clock tick
2233: System dependent. With respect to @code{MS}, the time is specified in
2234: microseconds. How well the OS and the hardware implement this, is
2235: another question.
2236:
2237: @item repeatability to be expected from the execution of @code{MS}:
2238: System dependent. On Unix, a lot depends on load. If the system is
1.17 ! anton 2239: lightly loaded, and the delay is short enough that Gforth does not get
1.14 anton 2240: swapped out, the performance should be acceptable. Under MS-DOS and
2241: other single-tasking systems, it should be good.
2242:
2243: @end table
2244:
2245:
2246: @c ---------------------------------------------------------------------
1.15 anton 2247: @node facility-ambcond, , facility-idef, The optional Facility word set
1.14 anton 2248: @subsection Ambiguous conditions
2249: @c ---------------------------------------------------------------------
2250:
2251: @table @i
2252:
2253: @item @code{AT-XY} can't be performed on user output device:
2254: Largely terminal dependant. No range checks are done on the arguments.
2255: No errors are reported. You may see some garbage appearing, you may see
2256: simply nothing happen.
2257:
2258: @end table
2259:
2260:
2261: @c =====================================================================
2262: @node The optional File-Access word set, The optional Floating-Point word set, The optional Facility word set, ANS conformance
2263: @section The optional File-Access word set
2264: @c =====================================================================
2265:
2266: @menu
1.15 anton 2267: * file-idef:: Implementation Defined Options
2268: * file-ambcond:: Ambiguous Conditions
1.14 anton 2269: @end menu
2270:
2271:
2272: @c ---------------------------------------------------------------------
2273: @node file-idef, file-ambcond, The optional File-Access word set, The optional File-Access word set
2274: @subsection Implementation Defined Options
2275: @c ---------------------------------------------------------------------
2276:
2277: @table @i
2278:
2279: @item File access methods used:
2280: @code{R/O}, @code{R/W} and @code{BIN} work as you would
2281: expect. @code{W/O} translates into the C file opening mode @code{w} (or
2282: @code{wb}): The file is cleared, if it exists, and created, if it does
1.15 anton 2283: not (both with @code{open-file} and @code{create-file}). Under Unix
1.14 anton 2284: @code{create-file} creates a file with 666 permissions modified by your
2285: umask.
2286:
2287: @item file exceptions:
2288: The file words do not raise exceptions (except, perhaps, memory access
2289: faults when you pass illegal addresses or file-ids).
2290:
2291: @item file line terminator:
2292: System-dependent. Gforth uses C's newline character as line
2293: terminator. What the actual character code(s) of this are is
2294: system-dependent.
2295:
2296: @item file name format
2297: System dependent. Gforth just uses the file name format of your OS.
2298:
2299: @item information returned by @code{FILE-STATUS}:
2300: @code{FILE-STATUS} returns the most powerful file access mode allowed
2301: for the file: Either @code{R/O}, @code{W/O} or @code{R/W}. If the file
2302: cannot be accessed, @code{R/O BIN} is returned. @code{BIN} is applicable
2303: along with the retured mode.
2304:
2305: @item input file state after an exception when including source:
2306: All files that are left via the exception are closed.
2307:
2308: @item @var{ior} values and meaning:
1.15 anton 2309: The @var{ior}s returned by the file and memory allocation words are
2310: intended as throw codes. They typically are in the range
2311: -512@minus{}-2047 of OS errors. The mapping from OS error numbers to
2312: @var{ior}s is -512@minus{}@var{errno}.
1.14 anton 2313:
2314: @item maximum depth of file input nesting:
2315: limited by the amount of return stack, locals/TIB stack, and the number
2316: of open files available. This should not give you troubles.
2317:
2318: @item maximum size of input line:
2319: @code{/line}. Currently 255.
2320:
2321: @item methods of mapping block ranges to files:
2322: Currently, the block words automatically access the file
2323: @file{blocks.fb} in the currend working directory. More sophisticated
2324: methods could be implemented if there is demand (and a volunteer).
2325:
2326: @item number of string buffers provided by @code{S"}:
2327: 1
2328:
2329: @item size of string buffer used by @code{S"}:
2330: @code{/line}. currently 255.
2331:
2332: @end table
2333:
2334: @c ---------------------------------------------------------------------
1.15 anton 2335: @node file-ambcond, , file-idef, The optional File-Access word set
1.14 anton 2336: @subsection Ambiguous conditions
2337: @c ---------------------------------------------------------------------
2338:
2339: @table @i
2340:
2341: @item attempting to position a file outside it's boundaries:
2342: @code{REPOSITION-FILE} is performed as usual: Afterwards,
2343: @code{FILE-POSITION} returns the value given to @code{REPOSITION-FILE}.
2344:
2345: @item attempting to read from file positions not yet written:
2346: End-of-file, i.e., zero characters are read and no error is reported.
2347:
2348: @item @var{file-id} is invalid (@code{INCLUDE-FILE}):
2349: An appropriate exception may be thrown, but a memory fault or other
2350: problem is more probable.
2351:
2352: @item I/O exception reading or closing @var{file-id} (@code{include-file}, @code{included}):
2353: The @var{ior} produced by the operation, that discovered the problem, is
2354: thrown.
2355:
2356: @item named file cannot be opened (@code{included}):
2357: The @var{ior} produced by @code{open-file} is thrown.
2358:
2359: @item requesting an unmapped block number:
2360: There are no unmapped legal block numbers. On some operating systems,
2361: writing a block with a large number may overflow the file system and
2362: have an error message as consequence.
2363:
2364: @item using @code{source-id} when @code{blk} is non-zero:
2365: @code{source-id} performs its function. Typically it will give the id of
2366: the source which loaded the block. (Better ideas?)
2367:
2368: @end table
2369:
2370:
2371: @c =====================================================================
2372: @node The optional Floating-Point word set, The optional Locals word set, The optional File-Access word set, ANS conformance
1.15 anton 2373: @section The optional Floating-Point word set
1.14 anton 2374: @c =====================================================================
2375:
2376: @menu
1.15 anton 2377: * floating-idef:: Implementation Defined Options
2378: * floating-ambcond:: Ambiguous Conditions
1.14 anton 2379: @end menu
2380:
2381:
2382: @c ---------------------------------------------------------------------
2383: @node floating-idef, floating-ambcond, The optional Floating-Point word set, The optional Floating-Point word set
2384: @subsection Implementation Defined Options
2385: @c ---------------------------------------------------------------------
2386:
2387: @table @i
2388:
1.15 anton 2389: @item format and range of floating point numbers:
2390: System-dependent; the @code{double} type of C.
1.14 anton 2391:
1.15 anton 2392: @item results of @code{REPRESENT} when @var{float} is out of range:
2393: System dependent; @code{REPRESENT} is implemented using the C library
2394: function @code{ecvt()} and inherits its behaviour in this respect.
1.14 anton 2395:
1.15 anton 2396: @item rounding or truncation of floating-point numbers:
2397: What's the question?!!
1.14 anton 2398:
1.15 anton 2399: @item size of floating-point stack:
2400: @code{s" FLOATING-STACK" environment? drop .}. Can be changed at startup
2401: with the command-line option @code{-f}.
1.14 anton 2402:
1.15 anton 2403: @item width of floating-point stack:
2404: @code{1 floats}.
1.14 anton 2405:
2406: @end table
2407:
2408:
2409: @c ---------------------------------------------------------------------
1.15 anton 2410: @node floating-ambcond, , floating-idef, The optional Floating-Point word set
2411: @subsection Ambiguous conditions
1.14 anton 2412: @c ---------------------------------------------------------------------
2413:
2414: @table @i
2415:
1.15 anton 2416: @item @code{df@@} or @code{df!} used with an address that is not double-float aligned:
2417: System-dependent. Typically results in an alignment fault like other
2418: alignment violations.
1.14 anton 2419:
1.15 anton 2420: @item @code{f@@} or @code{f!} used with an address that is not float aligned:
2421: System-dependent. Typically results in an alignment fault like other
2422: alignment violations.
1.14 anton 2423:
1.15 anton 2424: @item Floating-point result out of range:
2425: System-dependent. Can result in a @code{-55 THROW} (Floating-point
2426: unidentified fault), or can produce a special value representing, e.g.,
2427: Infinity.
1.14 anton 2428:
1.15 anton 2429: @item @code{sf@@} or @code{sf!} used with an address that is not single-float aligned:
2430: System-dependent. Typically results in an alignment fault like other
2431: alignment violations.
1.14 anton 2432:
1.15 anton 2433: @item BASE is not decimal (@code{REPRESENT}, @code{F.}, @code{FE.}, @code{FS.}):
2434: The floating-point number is converted into decimal nonetheless.
1.14 anton 2435:
1.15 anton 2436: @item Both arguments are equal to zero (@code{FATAN2}):
2437: System-dependent. @code{FATAN2} is implemented using the C library
2438: function @code{atan2()}.
1.14 anton 2439:
1.15 anton 2440: @item Using ftan on an argument @var{r1} where cos(@var{r1}) is zero:
2441: System-dependent. Anyway, typically the cos of @var{r1} will not be zero
2442: because of small errors and the tan will be a very large (or very small)
2443: but finite number.
1.14 anton 2444:
1.15 anton 2445: @item @var{d} cannot be presented precisely as a float in @code{D>F}:
2446: The result is rounded to the nearest float.
1.14 anton 2447:
1.15 anton 2448: @item dividing by zero:
2449: @code{-55 throw} (Floating-point unidentified fault)
1.14 anton 2450:
1.15 anton 2451: @item exponent too big for conversion (@code{DF!}, @code{DF@@}, @code{SF!}, @code{SF@@}):
2452: System dependent. On IEEE-FP based systems the number is converted into
2453: an infinity.
1.14 anton 2454:
1.15 anton 2455: @item @var{float}<1 (@code{facosh}):
2456: @code{-55 throw} (Floating-point unidentified fault)
1.14 anton 2457:
1.15 anton 2458: @item @var{float}=<-1 (@code{flnp1}):
2459: @code{-55 throw} (Floating-point unidentified fault). On IEEE-FP systems
2460: negative infinity is typically produced for @var{float}=-1.
1.14 anton 2461:
1.15 anton 2462: @item @var{float}=<0 (@code{fln}, @code{flog}):
2463: @code{-55 throw} (Floating-point unidentified fault). On IEEE-FP systems
2464: negative infinity is typically produced for @var{float}=0.
1.14 anton 2465:
1.15 anton 2466: @item @var{float}<0 (@code{fasinh}, @code{fsqrt}):
2467: @code{-55 throw} (Floating-point unidentified fault). @code{fasinh}
2468: produces values for these inputs on my Linux box (Bug in the C library?)
1.14 anton 2469:
1.15 anton 2470: @item |@var{float}|>1 (@code{facos}, @code{fasin}, @code{fatanh}):
2471: @code{-55 throw} (Floating-point unidentified fault).
1.14 anton 2472:
1.15 anton 2473: @item integer part of float cannot be represented by @var{d} in @code{f>d}:
2474: @code{-55 throw} (Floating-point unidentified fault).
1.14 anton 2475:
1.15 anton 2476: @item string larger than pictured numeric output area (@code{f.}, @code{fe.}, @code{fs.}):
2477: This does not happen.
2478: @end table
1.14 anton 2479:
2480:
2481:
2482: @c =====================================================================
1.15 anton 2483: @node The optional Locals word set, The optional Memory-Allocation word set, The optional Floating-Point word set, ANS conformance
2484: @section The optional Locals word set
1.14 anton 2485: @c =====================================================================
2486:
2487: @menu
1.15 anton 2488: * locals-idef:: Implementation Defined Options
2489: * locals-ambcond:: Ambiguous Conditions
1.14 anton 2490: @end menu
2491:
2492:
2493: @c ---------------------------------------------------------------------
1.15 anton 2494: @node locals-idef, locals-ambcond, The optional Locals word set, The optional Locals word set
1.14 anton 2495: @subsection Implementation Defined Options
2496: @c ---------------------------------------------------------------------
2497:
2498: @table @i
2499:
1.15 anton 2500: @item maximum number of locals in a definition:
2501: @code{s" #locals" environment? drop .}. Currently 15. This is a lower
2502: bound, e.g., on a 32-bit machine there can be 41 locals of up to 8
2503: characters. The number of locals in a definition is bounded by the size
2504: of locals-buffer, which contains the names of the locals.
1.14 anton 2505:
2506: @end table
2507:
2508:
2509: @c ---------------------------------------------------------------------
1.15 anton 2510: @node locals-ambcond, , locals-idef, The optional Locals word set
1.14 anton 2511: @subsection Ambiguous conditions
2512: @c ---------------------------------------------------------------------
2513:
2514: @table @i
2515:
1.15 anton 2516: @item executing a named local in interpretation state:
2517: @code{-14 throw} (Interpreting a compile-only word).
1.14 anton 2518:
1.15 anton 2519: @item @var{name} not defined by @code{VALUE} or @code{(LOCAL)} (@code{TO}):
2520: @code{-32 throw} (Invalid name argument)
1.14 anton 2521:
2522: @end table
2523:
2524:
2525: @c =====================================================================
1.15 anton 2526: @node The optional Memory-Allocation word set, The optional Programming-Tools word set, The optional Locals word set, ANS conformance
2527: @section The optional Memory-Allocation word set
1.14 anton 2528: @c =====================================================================
2529:
2530: @menu
1.15 anton 2531: * memory-idef:: Implementation Defined Options
1.14 anton 2532: @end menu
2533:
2534:
2535: @c ---------------------------------------------------------------------
1.15 anton 2536: @node memory-idef, , The optional Memory-Allocation word set, The optional Memory-Allocation word set
1.14 anton 2537: @subsection Implementation Defined Options
2538: @c ---------------------------------------------------------------------
2539:
2540: @table @i
2541:
1.15 anton 2542: @item values and meaning of @var{ior}:
2543: The @var{ior}s returned by the file and memory allocation words are
2544: intended as throw codes. They typically are in the range
2545: -512@minus{}-2047 of OS errors. The mapping from OS error numbers to
2546: @var{ior}s is -512@minus{}@var{errno}.
1.14 anton 2547:
2548: @end table
2549:
2550: @c =====================================================================
1.15 anton 2551: @node The optional Programming-Tools word set, The optional Search-Order word set, The optional Memory-Allocation word set, ANS conformance
2552: @section The optional Programming-Tools word set
1.14 anton 2553: @c =====================================================================
2554:
2555: @menu
1.15 anton 2556: * programming-idef:: Implementation Defined Options
2557: * programming-ambcond:: Ambiguous Conditions
1.14 anton 2558: @end menu
2559:
2560:
2561: @c ---------------------------------------------------------------------
1.15 anton 2562: @node programming-idef, programming-ambcond, The optional Programming-Tools word set, The optional Programming-Tools word set
1.14 anton 2563: @subsection Implementation Defined Options
2564: @c ---------------------------------------------------------------------
2565:
2566: @table @i
2567:
1.15 anton 2568: @item ending sequence for input following @code{;code} and @code{code}:
2569: Not implemented (yet).
1.14 anton 2570:
1.15 anton 2571: @item manner of processing input following @code{;code} and @code{code}:
2572: Not implemented (yet).
2573:
2574: @item search order capability for @code{EDITOR} and @code{ASSEMBLER}:
2575: Not implemented (yet). If they were implemented, they would use the
2576: search order wordset.
2577:
2578: @item source and format of display by @code{SEE}:
2579: The source for @code{see} is the intermediate code used by the inner
2580: interpreter. The current @code{see} tries to output Forth source code
2581: as well as possible.
2582:
1.14 anton 2583: @end table
2584:
2585: @c ---------------------------------------------------------------------
1.15 anton 2586: @node programming-ambcond, , programming-idef, The optional Programming-Tools word set
1.14 anton 2587: @subsection Ambiguous conditions
2588: @c ---------------------------------------------------------------------
2589:
2590: @table @i
2591:
1.15 anton 2592: @item deleting the compilation wordlist (@code{FORGET}):
2593: Not implemented (yet).
1.14 anton 2594:
1.15 anton 2595: @item fewer than @var{u}+1 items on the control flow stack (@code{CS-PICK}, @code{CS-ROLL}):
2596: This typically results in an @code{abort"} with a descriptive error
2597: message (may change into a @code{-22 throw} (Control structure mismatch)
2598: in the future). You may also get a memory access error. If you are
2599: unlucky, this ambiguous condition is not caught.
2600:
2601: @item @var{name} can't be found (@code{forget}):
2602: Not implemented (yet).
1.14 anton 2603:
1.15 anton 2604: @item @var{name} not defined via @code{CREATE}:
2605: @code{;code} is not implemented (yet). If it were, it would behave like
2606: @code{DOES>} in this respect, i.e., change the execution semantics of
2607: the last defined word no matter how it was defined.
1.14 anton 2608:
1.15 anton 2609: @item @code{POSTPONE} applied to @code{[IF]}:
2610: After defining @code{: X POSTPONE [IF] ; IMMEDIATE}. @code{X} is
2611: equivalent to @code{[IF]}.
1.14 anton 2612:
1.15 anton 2613: @item reaching the end of the input source before matching @code{[ELSE]} or @code{[THEN]}:
2614: Continue in the same state of conditional compilation in the next outer
2615: input source. Currently there is no warning to the user about this.
1.14 anton 2616:
1.15 anton 2617: @item removing a needed definition (@code{FORGET}):
2618: Not implemented (yet).
1.14 anton 2619:
2620: @end table
2621:
2622:
2623: @c =====================================================================
1.15 anton 2624: @node The optional Search-Order word set, , The optional Programming-Tools word set, ANS conformance
2625: @section The optional Search-Order word set
1.14 anton 2626: @c =====================================================================
2627:
2628: @menu
1.15 anton 2629: * search-idef:: Implementation Defined Options
2630: * search-ambcond:: Ambiguous Conditions
1.14 anton 2631: @end menu
2632:
2633:
2634: @c ---------------------------------------------------------------------
1.15 anton 2635: @node search-idef, search-ambcond, The optional Search-Order word set, The optional Search-Order word set
1.14 anton 2636: @subsection Implementation Defined Options
2637: @c ---------------------------------------------------------------------
2638:
2639: @table @i
2640:
1.15 anton 2641: @item maximum number of word lists in search order:
2642: @code{s" wordlists" environment? drop .}. Currently 16.
2643:
2644: @item minimum search order:
2645: @code{root root}.
1.14 anton 2646:
2647: @end table
2648:
2649: @c ---------------------------------------------------------------------
1.15 anton 2650: @node search-ambcond, , search-idef, The optional Search-Order word set
1.14 anton 2651: @subsection Ambiguous conditions
2652: @c ---------------------------------------------------------------------
2653:
2654: @table @i
2655:
1.15 anton 2656: @item changing the compilation wordlist (during compilation):
2657: The definition is put into the wordlist that is the compilation wordlist
2658: when @code{REVEAL} is executed (by @code{;}, @code{DOES>},
2659: @code{RECURSIVE}, etc.).
1.14 anton 2660:
1.15 anton 2661: @item search order empty (@code{previous}):
2662: @code{abort" Vocstack empty"}.
1.14 anton 2663:
1.15 anton 2664: @item too many word lists in search order (@code{also}):
2665: @code{abort" Vocstack full"}.
1.14 anton 2666:
2667: @end table
1.13 anton 2668:
2669:
1.17 ! anton 2670: @node Model, Emacs and Gforth, ANS conformance, Top
1.4 anton 2671: @chapter Model
2672:
1.17 ! anton 2673: @node Emacs and Gforth, Internals, Model, Top
! 2674: @chapter Emacs and Gforth
1.4 anton 2675:
1.17 ! anton 2676: Gforth comes with @file{gforth.el}, an improved version of
1.4 anton 2677: @file{forth.el} by Goran Rydqvist (icluded in the TILE package). The
2678: improvements are a better (but still not perfect) handling of
2679: indentation. I have also added comment paragraph filling (@kbd{M-q}),
1.8 anton 2680: commenting (@kbd{C-x \}) and uncommenting (@kbd{C-u C-x \}) regions and
2681: removing debugging tracers (@kbd{C-x ~}, @pxref{Debugging}). I left the
2682: stuff I do not use alone, even though some of it only makes sense for
2683: TILE. To get a description of these features, enter Forth mode and type
2684: @kbd{C-h m}.
1.4 anton 2685:
1.17 ! anton 2686: In addition, Gforth supports Emacs quite well: The source code locations
1.4 anton 2687: given in error messages, debugging output (from @code{~~}) and failed
2688: assertion messages are in the right format for Emacs' compilation mode
2689: (@pxref{Compilation, , Running Compilations under Emacs, emacs, Emacs
2690: Manual}) so the source location corresponding to an error or other
2691: message is only a few keystrokes away (@kbd{C-x `} for the next error,
2692: @kbd{C-c C-c} for the error under the cursor).
2693:
2694: Also, if you @code{include} @file{etags.fs}, a new @file{TAGS} file
2695: (@pxref{Tags, , Tags Tables, emacs, Emacs Manual}) will be produced that
2696: contains the definitions of all words defined afterwards. You can then
2697: find the source for a word using @kbd{M-.}. Note that emacs can use
1.17 ! anton 2698: several tags files at the same time (e.g., one for the Gforth sources
1.4 anton 2699: and one for your program).
2700:
2701: To get all these benefits, add the following lines to your @file{.emacs}
2702: file:
2703:
2704: @example
2705: (autoload 'forth-mode "gforth.el")
2706: (setq auto-mode-alist (cons '("\\.fs\\'" . forth-mode) auto-mode-alist))
2707: @end example
2708:
1.17 ! anton 2709: @node Internals, Bugs, Emacs and Gforth, Top
1.3 anton 2710: @chapter Internals
2711:
1.17 ! anton 2712: Reading this section is not necessary for programming with Gforth. It
! 2713: should be helpful for finding your way in the Gforth sources.
1.3 anton 2714:
1.4 anton 2715: @menu
2716: * Portability::
2717: * Threading::
2718: * Primitives::
2719: * System Architecture::
1.17 ! anton 2720: * Performance::
1.4 anton 2721: @end menu
2722:
2723: @node Portability, Threading, Internals, Internals
1.3 anton 2724: @section Portability
2725:
2726: One of the main goals of the effort is availability across a wide range
2727: of personal machines. fig-Forth, and, to a lesser extent, F83, achieved
2728: this goal by manually coding the engine in assembly language for several
2729: then-popular processors. This approach is very labor-intensive and the
2730: results are short-lived due to progress in computer architecture.
2731:
2732: Others have avoided this problem by coding in C, e.g., Mitch Bradley
2733: (cforth), Mikael Patel (TILE) and Dirk Zoller (pfe). This approach is
2734: particularly popular for UNIX-based Forths due to the large variety of
2735: architectures of UNIX machines. Unfortunately an implementation in C
2736: does not mix well with the goals of efficiency and with using
2737: traditional techniques: Indirect or direct threading cannot be expressed
2738: in C, and switch threading, the fastest technique available in C, is
2739: significantly slower. Another problem with C is that it's very
2740: cumbersome to express double integer arithmetic.
2741:
2742: Fortunately, there is a portable language that does not have these
2743: limitations: GNU C, the version of C processed by the GNU C compiler
2744: (@pxref{C Extensions, , Extensions to the C Language Family, gcc.info,
2745: GNU C Manual}). Its labels as values feature (@pxref{Labels as Values, ,
2746: Labels as Values, gcc.info, GNU C Manual}) makes direct and indirect
2747: threading possible, its @code{long long} type (@pxref{Long Long, ,
2748: Double-Word Integers, gcc.info, GNU C Manual}) corresponds to Forths
2749: double numbers. GNU C is available for free on all important (and many
2750: unimportant) UNIX machines, VMS, 80386s running MS-DOS, the Amiga, and
2751: the Atari ST, so a Forth written in GNU C can run on all these
1.17 ! anton 2752: machines.
1.3 anton 2753:
2754: Writing in a portable language has the reputation of producing code that
2755: is slower than assembly. For our Forth engine we repeatedly looked at
2756: the code produced by the compiler and eliminated most compiler-induced
2757: inefficiencies by appropriate changes in the source-code.
2758:
2759: However, register allocation cannot be portably influenced by the
2760: programmer, leading to some inefficiencies on register-starved
2761: machines. We use explicit register declarations (@pxref{Explicit Reg
2762: Vars, , Variables in Specified Registers, gcc.info, GNU C Manual}) to
2763: improve the speed on some machines. They are turned on by using the
2764: @code{gcc} switch @code{-DFORCE_REG}. Unfortunately, this feature not
2765: only depends on the machine, but also on the compiler version: On some
2766: machines some compiler versions produce incorrect code when certain
2767: explicit register declarations are used. So by default
2768: @code{-DFORCE_REG} is not used.
2769:
1.4 anton 2770: @node Threading, Primitives, Portability, Internals
1.3 anton 2771: @section Threading
2772:
2773: GNU C's labels as values extension (available since @code{gcc-2.0},
2774: @pxref{Labels as Values, , Labels as Values, gcc.info, GNU C Manual})
2775: makes it possible to take the address of @var{label} by writing
2776: @code{&&@var{label}}. This address can then be used in a statement like
2777: @code{goto *@var{address}}. I.e., @code{goto *&&x} is the same as
2778: @code{goto x}.
2779:
2780: With this feature an indirect threaded NEXT looks like:
2781: @example
2782: cfa = *ip++;
2783: ca = *cfa;
2784: goto *ca;
2785: @end example
2786: For those unfamiliar with the names: @code{ip} is the Forth instruction
2787: pointer; the @code{cfa} (code-field address) corresponds to ANS Forths
2788: execution token and points to the code field of the next word to be
2789: executed; The @code{ca} (code address) fetched from there points to some
2790: executable code, e.g., a primitive or the colon definition handler
2791: @code{docol}.
2792:
2793: Direct threading is even simpler:
2794: @example
2795: ca = *ip++;
2796: goto *ca;
2797: @end example
2798:
2799: Of course we have packaged the whole thing neatly in macros called
2800: @code{NEXT} and @code{NEXT1} (the part of NEXT after fetching the cfa).
2801:
1.4 anton 2802: @menu
2803: * Scheduling::
2804: * Direct or Indirect Threaded?::
2805: * DOES>::
2806: @end menu
2807:
2808: @node Scheduling, Direct or Indirect Threaded?, Threading, Threading
1.3 anton 2809: @subsection Scheduling
2810:
2811: There is a little complication: Pipelined and superscalar processors,
2812: i.e., RISC and some modern CISC machines can process independent
2813: instructions while waiting for the results of an instruction. The
2814: compiler usually reorders (schedules) the instructions in a way that
2815: achieves good usage of these delay slots. However, on our first tries
2816: the compiler did not do well on scheduling primitives. E.g., for
2817: @code{+} implemented as
2818: @example
2819: n=sp[0]+sp[1];
2820: sp++;
2821: sp[0]=n;
2822: NEXT;
2823: @end example
2824: the NEXT comes strictly after the other code, i.e., there is nearly no
2825: scheduling. After a little thought the problem becomes clear: The
2826: compiler cannot know that sp and ip point to different addresses (and
1.4 anton 2827: the version of @code{gcc} we used would not know it even if it was
2828: possible), so it could not move the load of the cfa above the store to
2829: the TOS. Indeed the pointers could be the same, if code on or very near
2830: the top of stack were executed. In the interest of speed we chose to
2831: forbid this probably unused ``feature'' and helped the compiler in
2832: scheduling: NEXT is divided into the loading part (@code{NEXT_P1}) and
2833: the goto part (@code{NEXT_P2}). @code{+} now looks like:
1.3 anton 2834: @example
2835: n=sp[0]+sp[1];
2836: sp++;
2837: NEXT_P1;
2838: sp[0]=n;
2839: NEXT_P2;
2840: @end example
1.4 anton 2841: This can be scheduled optimally by the compiler.
1.3 anton 2842:
2843: This division can be turned off with the switch @code{-DCISC_NEXT}. This
2844: switch is on by default on machines that do not profit from scheduling
2845: (e.g., the 80386), in order to preserve registers.
2846:
1.4 anton 2847: @node Direct or Indirect Threaded?, DOES>, Scheduling, Threading
1.3 anton 2848: @subsection Direct or Indirect Threaded?
2849:
2850: Both! After packaging the nasty details in macro definitions we
2851: realized that we could switch between direct and indirect threading by
2852: simply setting a compilation flag (@code{-DDIRECT_THREADED}) and
2853: defining a few machine-specific macros for the direct-threading case.
2854: On the Forth level we also offer access words that hide the
2855: differences between the threading methods (@pxref{Threading Words}).
2856:
2857: Indirect threading is implemented completely
2858: machine-independently. Direct threading needs routines for creating
2859: jumps to the executable code (e.g. to docol or dodoes). These routines
2860: are inherently machine-dependent, but they do not amount to many source
2861: lines. I.e., even porting direct threading to a new machine is a small
2862: effort.
2863:
1.4 anton 2864: @node DOES>, , Direct or Indirect Threaded?, Threading
1.3 anton 2865: @subsection DOES>
2866: One of the most complex parts of a Forth engine is @code{dodoes}, i.e.,
2867: the chunk of code executed by every word defined by a
2868: @code{CREATE}...@code{DOES>} pair. The main problem here is: How to find
2869: the Forth code to be executed, i.e. the code after the @code{DOES>} (the
2870: DOES-code)? There are two solutions:
2871:
2872: In fig-Forth the code field points directly to the dodoes and the
2873: DOES-code address is stored in the cell after the code address
2874: (i.e. at cfa cell+). It may seem that this solution is illegal in the
2875: Forth-79 and all later standards, because in fig-Forth this address
2876: lies in the body (which is illegal in these standards). However, by
2877: making the code field larger for all words this solution becomes legal
2878: again. We use this approach for the indirect threaded version. Leaving
2879: a cell unused in most words is a bit wasteful, but on the machines we
2880: are targetting this is hardly a problem. The other reason for having a
2881: code field size of two cells is to avoid having different image files
1.4 anton 2882: for direct and indirect threaded systems (@pxref{System Architecture}).
1.3 anton 2883:
2884: The other approach is that the code field points or jumps to the cell
2885: after @code{DOES}. In this variant there is a jump to @code{dodoes} at
2886: this address. @code{dodoes} can then get the DOES-code address by
2887: computing the code address, i.e., the address of the jump to dodoes,
2888: and add the length of that jump field. A variant of this is to have a
2889: call to @code{dodoes} after the @code{DOES>}; then the return address
2890: (which can be found in the return register on RISCs) is the DOES-code
2891: address. Since the two cells available in the code field are usually
2892: used up by the jump to the code address in direct threading, we use
2893: this approach for direct threading. We did not want to add another
2894: cell to the code field.
2895:
1.4 anton 2896: @node Primitives, System Architecture, Threading, Internals
1.3 anton 2897: @section Primitives
2898:
1.4 anton 2899: @menu
2900: * Automatic Generation::
2901: * TOS Optimization::
2902: * Produced code::
2903: @end menu
2904:
2905: @node Automatic Generation, TOS Optimization, Primitives, Primitives
1.3 anton 2906: @subsection Automatic Generation
2907:
2908: Since the primitives are implemented in a portable language, there is no
2909: longer any need to minimize the number of primitives. On the contrary,
2910: having many primitives is an advantage: speed. In order to reduce the
2911: number of errors in primitives and to make programming them easier, we
2912: provide a tool, the primitive generator (@file{prims2x.fs}), that
2913: automatically generates most (and sometimes all) of the C code for a
2914: primitive from the stack effect notation. The source for a primitive
2915: has the following form:
2916:
2917: @format
2918: @var{Forth-name} @var{stack-effect} @var{category} [@var{pronounc.}]
2919: [@code{""}@var{glossary entry}@code{""}]
2920: @var{C code}
2921: [@code{:}
2922: @var{Forth code}]
2923: @end format
2924:
2925: The items in brackets are optional. The category and glossary fields
2926: are there for generating the documentation, the Forth code is there
2927: for manual implementations on machines without GNU C. E.g., the source
2928: for the primitive @code{+} is:
2929: @example
2930: + n1 n2 -- n core plus
2931: n = n1+n2;
2932: @end example
2933:
2934: This looks like a specification, but in fact @code{n = n1+n2} is C
2935: code. Our primitive generation tool extracts a lot of information from
2936: the stack effect notations@footnote{We use a one-stack notation, even
2937: though we have separate data and floating-point stacks; The separate
2938: notation can be generated easily from the unified notation.}: The number
2939: of items popped from and pushed on the stack, their type, and by what
2940: name they are referred to in the C code. It then generates a C code
2941: prelude and postlude for each primitive. The final C code for @code{+}
2942: looks like this:
2943:
2944: @example
2945: I_plus: /* + ( n1 n2 -- n ) */ /* label, stack effect */
2946: /* */ /* documentation */
1.4 anton 2947: @{
1.3 anton 2948: DEF_CA /* definition of variable ca (indirect threading) */
2949: Cell n1; /* definitions of variables */
2950: Cell n2;
2951: Cell n;
2952: n1 = (Cell) sp[1]; /* input */
2953: n2 = (Cell) TOS;
2954: sp += 1; /* stack adjustment */
2955: NAME("+") /* debugging output (with -DDEBUG) */
1.4 anton 2956: @{
1.3 anton 2957: n = n1+n2; /* C code taken from the source */
1.4 anton 2958: @}
1.3 anton 2959: NEXT_P1; /* NEXT part 1 */
2960: TOS = (Cell)n; /* output */
2961: NEXT_P2; /* NEXT part 2 */
1.4 anton 2962: @}
1.3 anton 2963: @end example
2964:
2965: This looks long and inefficient, but the GNU C compiler optimizes quite
2966: well and produces optimal code for @code{+} on, e.g., the R3000 and the
2967: HP RISC machines: Defining the @code{n}s does not produce any code, and
2968: using them as intermediate storage also adds no cost.
2969:
2970: There are also other optimizations, that are not illustrated by this
2971: example: Assignments between simple variables are usually for free (copy
2972: propagation). If one of the stack items is not used by the primitive
2973: (e.g. in @code{drop}), the compiler eliminates the load from the stack
2974: (dead code elimination). On the other hand, there are some things that
2975: the compiler does not do, therefore they are performed by
2976: @file{prims2x.fs}: The compiler does not optimize code away that stores
2977: a stack item to the place where it just came from (e.g., @code{over}).
2978:
2979: While programming a primitive is usually easy, there are a few cases
2980: where the programmer has to take the actions of the generator into
2981: account, most notably @code{?dup}, but also words that do not (always)
2982: fall through to NEXT.
2983:
1.4 anton 2984: @node TOS Optimization, Produced code, Automatic Generation, Primitives
1.3 anton 2985: @subsection TOS Optimization
2986:
2987: An important optimization for stack machine emulators, e.g., Forth
2988: engines, is keeping one or more of the top stack items in
1.4 anton 2989: registers. If a word has the stack effect @var{in1}...@var{inx} @code{--}
2990: @var{out1}...@var{outy}, keeping the top @var{n} items in registers
1.3 anton 2991: @itemize
2992: @item
2993: is better than keeping @var{n-1} items, if @var{x>=n} and @var{y>=n},
2994: due to fewer loads from and stores to the stack.
2995: @item is slower than keeping @var{n-1} items, if @var{x<>y} and @var{x<n} and
2996: @var{y<n}, due to additional moves between registers.
2997: @end itemize
2998:
2999: In particular, keeping one item in a register is never a disadvantage,
3000: if there are enough registers. Keeping two items in registers is a
3001: disadvantage for frequent words like @code{?branch}, constants,
3002: variables, literals and @code{i}. Therefore our generator only produces
3003: code that keeps zero or one items in registers. The generated C code
3004: covers both cases; the selection between these alternatives is made at
3005: C-compile time using the switch @code{-DUSE_TOS}. @code{TOS} in the C
3006: code for @code{+} is just a simple variable name in the one-item case,
3007: otherwise it is a macro that expands into @code{sp[0]}. Note that the
3008: GNU C compiler tries to keep simple variables like @code{TOS} in
3009: registers, and it usually succeeds, if there are enough registers.
3010:
3011: The primitive generator performs the TOS optimization for the
3012: floating-point stack, too (@code{-DUSE_FTOS}). For floating-point
3013: operations the benefit of this optimization is even larger:
3014: floating-point operations take quite long on most processors, but can be
3015: performed in parallel with other operations as long as their results are
3016: not used. If the FP-TOS is kept in a register, this works. If
3017: it is kept on the stack, i.e., in memory, the store into memory has to
3018: wait for the result of the floating-point operation, lengthening the
3019: execution time of the primitive considerably.
3020:
3021: The TOS optimization makes the automatic generation of primitives a
3022: bit more complicated. Just replacing all occurrences of @code{sp[0]} by
3023: @code{TOS} is not sufficient. There are some special cases to
3024: consider:
3025: @itemize
3026: @item In the case of @code{dup ( w -- w w )} the generator must not
3027: eliminate the store to the original location of the item on the stack,
3028: if the TOS optimization is turned on.
1.4 anton 3029: @item Primitives with stack effects of the form @code{--}
3030: @var{out1}...@var{outy} must store the TOS to the stack at the start.
3031: Likewise, primitives with the stack effect @var{in1}...@var{inx} @code{--}
1.3 anton 3032: must load the TOS from the stack at the end. But for the null stack
3033: effect @code{--} no stores or loads should be generated.
3034: @end itemize
3035:
1.4 anton 3036: @node Produced code, , TOS Optimization, Primitives
1.3 anton 3037: @subsection Produced code
3038:
3039: To see what assembly code is produced for the primitives on your machine
3040: with your compiler and your flag settings, type @code{make engine.s} and
1.4 anton 3041: look at the resulting file @file{engine.s}.
1.3 anton 3042:
1.17 ! anton 3043: @node System Architecture, Performance, Primitives, Internals
1.3 anton 3044: @section System Architecture
3045:
3046: Our Forth system consists not only of primitives, but also of
3047: definitions written in Forth. Since the Forth compiler itself belongs
3048: to those definitions, it is not possible to start the system with the
3049: primitives and the Forth source alone. Therefore we provide the Forth
3050: code as an image file in nearly executable form. At the start of the
3051: system a C routine loads the image file into memory, sets up the
3052: memory (stacks etc.) according to information in the image file, and
3053: starts executing Forth code.
3054:
3055: The image file format is a compromise between the goals of making it
3056: easy to generate image files and making them portable. The easiest way
3057: to generate an image file is to just generate a memory dump. However,
3058: this kind of image file cannot be used on a different machine, or on
3059: the next version of the engine on the same machine, it even might not
3060: work with the same engine compiled by a different version of the C
3061: compiler. We would like to have as few versions of the image file as
3062: possible, because we do not want to distribute many versions of the
3063: same image file, and to make it easy for the users to use their image
3064: files on many machines. We currently need to create a different image
3065: file for machines with different cell sizes and different byte order
1.17 ! anton 3066: (little- or big-endian)@footnote{We are considering adding information to the
1.3 anton 3067: image file that enables the loader to change the byte order.}.
3068:
3069: Forth code that is going to end up in a portable image file has to
1.4 anton 3070: comply to some restrictions: addresses have to be stored in memory with
3071: special words (@code{A!}, @code{A,}, etc.) in order to make the code
3072: relocatable. Cells, floats, etc., have to be stored at the natural
3073: alignment boundaries@footnote{E.g., store floats (8 bytes) at an address
3074: dividable by~8. This happens automatically in our system when you use
3075: the ANS Forth alignment words.}, in order to avoid alignment faults on
3076: machines with stricter alignment. The image file is produced by a
3077: metacompiler (@file{cross.fs}).
1.3 anton 3078:
3079: So, unlike the image file of Mitch Bradleys @code{cforth}, our image
3080: file is not directly executable, but has to undergo some manipulations
3081: during loading. Address relocation is performed at image load-time, not
3082: at run-time. The loader also has to replace tokens standing for
3083: primitive calls with the appropriate code-field addresses (or code
3084: addresses in the case of direct threading).
1.4 anton 3085:
1.17 ! anton 3086: @node Performance, , System Architecture, Internals
! 3087: @section Performance
! 3088:
! 3089: On RISCs the Gforth engine is very close to optimal; i.e., it is usually
! 3090: impossible to write a significantly faster engine.
! 3091:
! 3092: On register-starved machines like the 386 architecture processors
! 3093: improvements are possible, because @code{gcc} does not utilize the
! 3094: registers as well as a human, even with explicit register declarations;
! 3095: e.g., Bernd Beuster wrote a Forth system fragment in assembly language
! 3096: and hand-tuned it for the 486; this system is 1.19 times faster on the
! 3097: Sieve benchmark on a 486DX2/66 than Gforth compiled with
! 3098: @code{gcc-2.6.3} with @code{-DFORCE_REG}.
! 3099:
! 3100: However, this potential advantage of assembly language implementations
! 3101: is not necessarily realized in complete Forth systems: We compared
! 3102: Gforth (compiled with @code{gcc-2.6.3} and @code{-DFORCE_REG}) with
! 3103: Win32Forth and LMI's NT Forth, two systems written in assembly, and with
! 3104: two systems written in C: PFE-0.9.11 (compiled with @code{gcc-2.6.3}
! 3105: with the default configuration for Linux: @code{-O2 -fomit-frame-pointer
! 3106: -DUSE_REGS}) and ThisForth Beta (compiled with gcc-2.6.3 -O3
! 3107: -fomit-frame-pointer). We benchmarked Gforth, PFE and ThisForth on a
! 3108: 486DX2/66 under Linux. Kenneth O'Heskin kindly provided the results for
! 3109: Win32Forth and NT Forth on a 486DX2/66 with similar memory performance
! 3110: under Windows NT.
! 3111:
! 3112: We used four small benchmarks: the ubiquitous Sieve; bubble-sorting and
! 3113: matrix multiplication come from the Stanford integer benchmarks and have
! 3114: been translated into Forth by Martin Fraeman; we used the versions
! 3115: included in the TILE Forth package; and a recursive Fibonacci number
! 3116: computation for benchmark calling performance. The following table shows
! 3117: the time taken for the benchmarks scaled by the time taken by Gforth (in
! 3118: other words, it shows the speedup factor that Gforth achieved over the
! 3119: other systems).
! 3120:
! 3121: @example
! 3122: relative Win32- NT This-
! 3123: time Gforth Forth Forth PFE Forth
! 3124: sieve 1.00 1.30 1.07 1.67 2.98
! 3125: bubble 1.00 1.30 1.40 1.66
! 3126: matmul 1.00 1.40 1.29 2.24
! 3127: fib 1.00 1.44 1.26 1.82 2.82
! 3128: @end example
! 3129:
! 3130: You may find the good performance of Gforth compared with the systems
! 3131: written in assembly language quite surprising. One important reason for
! 3132: the disappointing performance of these systems is probably that they are
! 3133: not written optimally for the 486 (e.g., they use the @code{lods}
! 3134: instruction). In addition, Win32Forth uses a comfortable, but costly
! 3135: method for relocating the Forth image: like @code{cforth}, it computes
! 3136: the actual addresses at run time, resulting in two address computations
! 3137: per NEXT (@pxref{System Architecture}).
! 3138:
! 3139: The speedup of Gforth over PFE and ThisForth can be easily explained
! 3140: with the self-imposed restriction to standard C (although the measured
! 3141: implementation of PFE uses a GNU C extension: global register
! 3142: variables), which makes efficient threading impossible. Moreover,
! 3143: current C compilers have a hard time optimizing other aspects of the
! 3144: ThisForth source.
! 3145:
! 3146: Note that the performance of Gforth on 386 architecture processors
! 3147: varies widely with the version of @code{gcc} used. E.g., @code{gcc-2.5.8}
! 3148: failed to allocate any of the virtual machine registers into real
! 3149: machine registers by itself and would not work correctly with explicit
! 3150: register declarations, giving a 1.3 times slower engine (on a 486DX2/66
! 3151: running the Sieve) than the one measured above.
! 3152:
1.4 anton 3153: @node Bugs, Pedigree, Internals, Top
3154: @chapter Bugs
3155:
1.17 ! anton 3156: Known bugs are described in the file BUGS in the Gforth distribution.
! 3157:
! 3158: If you find a bug, please send a bug report to !!. A bug report should
! 3159: describe the Gforth version used (it is announced at the start of an
! 3160: interactive Gforth session), the machine and operating system (on Unix
! 3161: systems you can use @code{uname -a} to produce this information), the
! 3162: installation options (!! a way to find them out), and a complete list of
! 3163: changes you (or your installer) have made to the Gforth sources (if
! 3164: any); it should contain a program (or a sequence of keyboard commands)
! 3165: that reproduces the bug and a description of what you think constitutes
! 3166: the buggy behaviour.
! 3167:
! 3168: For a thorough guide on reporting bugs read @ref{Bug Reporting, , How
! 3169: to Report Bugs, gcc.info, GNU C Manual}.
! 3170:
! 3171:
1.4 anton 3172: @node Pedigree, Word Index, Bugs, Top
3173: @chapter Pedigree
3174:
1.17 ! anton 3175: Gforth descends from BigForth (1993) and fig-Forth. Gforth and PFE (by
! 3176: Dirk Zoller) will cross-fertilize each other. Of course, a significant part of the design of Gforth was prescribed by ANS Forth.
! 3177:
! 3178: Bernd Paysan wrote BigForth, a child of VolksForth.
! 3179:
! 3180: VolksForth descends from F83. !! Authors? When?
! 3181:
! 3182: Laxen and Perry wrote F83 as a model implementation of the
! 3183: Forth-83 standard. !! Pedigree? When?
! 3184:
! 3185: A team led by Bill Ragsdale implemented fig-Forth on many processors in
! 3186: 1979. Dean Sanderson and Bill Ragsdale developed the original
! 3187: implementation of fig-Forth based on microForth.
! 3188:
! 3189: !! microForth pedigree
! 3190:
! 3191: A part of the information in this section comes from @cite{The Evolution
! 3192: of Forth} by Elizabeth D. Rather, Donald R. Colburn and Charles
! 3193: H. Moore, presented at the HOPL-II conference and preprinted in SIGPLAN
! 3194: Notices 28(3), 1993. You can find more historical and genealogical
! 3195: information about Forth there.
! 3196:
1.4 anton 3197: @node Word Index, Node Index, Pedigree, Top
3198: @chapter Word Index
3199:
1.17 ! anton 3200: This index is as incomplete as the manual.
! 3201:
! 3202: @printindex fn
! 3203:
1.4 anton 3204: @node Node Index, , Word Index, Top
3205: @chapter Node Index
1.17 ! anton 3206:
! 3207: This index is even less complete than the manual.
1.1 anton 3208:
3209: @contents
3210: @bye
3211:
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