[gforth] / gforth / Attic / gforth.ds

gforth: gforth/Attic/gforth.ds

1 :	anton	1.1	\input texinfo @c --texinfo--
2 :			@comment The source is gforth.ds, from which gforth.texi is generated
3 :			@comment %**start of header (This is for running Texinfo on a region.)
4 :	anton	1.4	@setfilename gforth.info
5 :	anton	1.17	@settitle Gforth Manual
6 :	anton	1.4	@comment @setchapternewpage odd
7 :	anton	1.1	@comment %**end of header (This is for running Texinfo on a region.)
8 :
9 :			@ifinfo
10 :	anton	1.17	This file documents Gforth 0.1
11 :	anton	1.1
12 :	anton	1.17	Copyright @copyright{} 1994 Gforth Development Group
13 :	anton	1.1
14 :			Permission is granted to make and distribute verbatim copies of
15 :			this manual provided the copyright notice and this permission notice
16 :			are preserved on all copies.
17 :
18 :	anton	1.4	@ignore
19 :	anton	1.1	Permission is granted to process this file through TeX and print the
20 :			results, provided the printed document carries a copying permission
21 :			notice identical to this one except for the removal of this paragraph
22 :			(this paragraph not being relevant to the printed manual).
23 :
24 :	anton	1.4	@end ignore
25 :	anton	1.1	Permission is granted to copy and distribute modified versions of this
26 :			manual under the conditions for verbatim copying, provided also that the
27 :			sections entitled "Distribution" and "General Public License" are
28 :			included exactly as in the original, and provided that the entire
29 :			resulting derived work is distributed under the terms of a permission
30 :			notice identical to this one.
31 :
32 :			Permission is granted to copy and distribute translations of this manual
33 :			into another language, under the above conditions for modified versions,
34 :			except that the sections entitled "Distribution" and "General Public
35 :			License" may be included in a translation approved by the author instead
36 :			of in the original English.
37 :			@end ifinfo
38 :
39 :			@titlepage
40 :			@sp 10
41 :	anton	1.17	@center @titlefont{Gforth Manual}
42 :	anton	1.1	@sp 2
43 :	anton	1.17	@center for version 0.1
44 :	anton	1.1	@sp 2
45 :			@center Anton Ertl
46 :	anton	1.17	@sp 3
47 :			@center This manual is under construction
48 :	anton	1.1
49 :			@comment The following two commands start the copyright page.
50 :			@page
51 :			@vskip 0pt plus 1filll
52 :	anton	1.17	Copyright @copyright{} 1994 Gforth Development Group
53 :	anton	1.1
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
77 :	anton	1.17	Gforth is a free implementation of ANS Forth available on many
78 :	anton	1.1	personal machines. This manual corresponds to version 0.0.
79 :			@end ifinfo
80 :
81 :			@menu
82 :	anton	1.4	* License::
83 :	anton	1.17	* Goals:: About the Gforth Project
84 :	anton	1.4	* Other Books:: Things you might want to read
85 :	anton	1.17	* Invocation:: Starting Gforth
86 :			* Words:: Forth words available in Gforth
87 :	anton	1.4	* ANS conformance:: Implementation-defined options etc.
88 :	anton	1.17	* Model:: The abstract machine of Gforth
89 :			* Emacs and Gforth:: The Gforth Mode
90 :	anton	1.4	* Internals:: Implementation details
91 :			* Bugs:: How to report them
92 :	anton	1.17	* Pedigree:: Ancestors of Gforth
93 :	anton	1.4	* Word Index:: An item for each Forth word
94 :			* Node Index:: An item for each node
95 :	anton	1.1	@end menu
96 :
97 :			@node License, Goals, Top, Top
98 :			@unnumbered License
99 :			!! Insert GPL here
100 :
101 :			@iftex
102 :			@unnumbered Preface
103 :	anton	1.17	This manual documents Gforth. The reader is expected to know
104 :	anton	1.1	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
110 :	anton	1.17	@chapter Goals of Gforth
111 :	anton	1.1	@cindex Goals
112 :	anton	1.17	The goal of the Gforth Project is to develop a standard model for
113 :	anton	1.1	ANSI Forth. This can be split into several subgoals:
114 :
115 :			@itemize @bullet
116 :			@item
117 :	anton	1.17	Gforth should conform to the ANSI Forth standard.
118 :	anton	1.1	@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 :
126 :	anton	1.17	To achieve these goals Gforth should be
127 :	anton	1.1	@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 :
142 :	anton	1.17	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
144 :	anton	1.1	which parts of the model are stable and which parts we are likely to
145 :	anton	1.17	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.
150 :	anton	1.1
151 :			@node Other Books, Invocation, Goals, Top
152 :			@chapter Other books on ANS Forth
153 :
154 :			As the standard is relatively new, there are not many books out yet. It
155 :	anton	1.17	is not recommended to learn Forth by using Gforth and a book that is
156 :	anton	1.1	not written for ANS Forth, as you will not know your mistakes from the
157 :			deviations of the book.
158 :
159 :			There is, of course, the standard, the definite reference if you want to
160 :			write ANS Forth programs. It will be available in printed form from
161 :			Global Engineering Documents !! somtime in spring or summer 1994. If you
162 :			are lucky, you can still get dpANS6 (the draft that was approved as
163 :			standard) by aftp from ftp.uu.net:/vendor/minerva/x3j14.
164 :
165 :			@cite{Forth: The new model} by Jack Woehr (!! Publisher) is an
166 :			introductory book based on a draft version of the standard. It does not
167 :			cover the whole standard. It also contains interesting background
168 :			information (Jack Woehr was in the ANS Forth Technical Committe). It is
169 :			not appropriate for complete newbies, but programmers experienced in
170 :			other languages should find it ok.
171 :
172 :			@node Invocation, Words, Other Books, Top
173 :			@chapter Invocation
174 :
175 :			You will usually just say @code{gforth}. In many other cases the default
176 :	anton	1.17	Gforth image will be invoked like this:
177 :	anton	1.1
178 :			@example
179 :			gforth [files] [-e forth-code]
180 :			@end example
181 :
182 :			executing the contents of the files and the Forth code in the order they
183 :			are given.
184 :
185 :			In general, the command line looks like this:
186 :
187 :			@example
188 :			gforth [initialization options] [image-specific options]
189 :			@end example
190 :
191 :			The initialization options must come before the rest of the command
192 :			line. They are:
193 :
194 :			@table @code
195 :			@item --image-file @var{file}
196 :			Loads the Forth image @var{file} instead of the default
197 :			@file{gforth.fi}.
198 :
199 :			@item --path @var{path}
200 :			Uses @var{path} for searching the image file and Forth source code
201 :			files instead of the default in the environment variable
202 :			@code{GFORTHPATH} or the path specified at installation time (typically
203 :			@file{/usr/local/lib/gforth:.}). A path is given as a @code{:}-separated
204 :			list.
205 :
206 :			@item --dictionary-size @var{size}
207 :			@item -m @var{size}
208 :			Allocate @var{size} space for the Forth dictionary space instead of
209 :			using the default specified in the image (typically 256K). The
210 :			@var{size} specification consists of an integer and a unit (e.g.,
211 :			@code{4M}). The unit can be one of @code{b} (bytes), @code{e} (element
212 :			size, in this case Cells), @code{k} (kilobytes), and @code{M}
213 :			(Megabytes). If no unit is specified, @code{e} is used.
214 :
215 :			@item --data-stack-size @var{size}
216 :			@item -d @var{size}
217 :			Allocate @var{size} space for the data stack instead of using the
218 :			default specified in the image (typically 16K).
219 :
220 :			@item --return-stack-size @var{size}
221 :			@item -r @var{size}
222 :			Allocate @var{size} space for the return stack instead of using the
223 :			default specified in the image (typically 16K).
224 :
225 :			@item --fp-stack-size @var{size}
226 :			@item -f @var{size}
227 :			Allocate @var{size} space for the floating point stack instead of
228 :			using the default specified in the image (typically 16K). In this case
229 :			the unit specifier @code{e} refers to floating point numbers.
230 :
231 :			@item --locals-stack-size @var{size}
232 :			@item -l @var{size}
233 :			Allocate @var{size} space for the locals stack instead of using the
234 :			default specified in the image (typically 16K).
235 :
236 :			@end table
237 :
238 :			As explained above, the image-specific command-line arguments for the
239 :			default image @file{gforth.fi} consist of a sequence of filenames and
240 :			@code{-e @var{forth-code}} options that are interpreted in the seqence
241 :			in which they are given. The @code{-e @var{forth-code}} or
242 :			@code{--evaluate @var{forth-code}} option evaluates the forth
243 :			code. This option takes only one argument; if you want to evaluate more
244 :			Forth words, you have to quote them or use several @code{-e}s. To exit
245 :			after processing the command line (instead of entering interactive mode)
246 :			append @code{-e bye} to the command line.
247 :
248 :			Not yet implemented:
249 :			On startup the system first executes the system initialization file
250 :			(unless the option @code{--no-init-file} is given; note that the system
251 :			resulting from using this option may not be ANS Forth conformant). Then
252 :			the user initialization file @file{.gforth.fs} is executed, unless the
253 :			option @code{--no-rc} is given; this file is first searched in @file{.},
254 :			then in @file{~}, then in the normal path (see above).
255 :
256 :	anton	1.4	@node Words, ANS conformance, Invocation, Top
257 :	anton	1.1	@chapter Forth Words
258 :
259 :			@menu
260 :	anton	1.4	* Notation::
261 :			* Arithmetic::
262 :			* Stack Manipulation::
263 :			* Memory access::
264 :			* Control Structures::
265 :			* Locals::
266 :			* Defining Words::
267 :			* Wordlists::
268 :			* Files::
269 :			* Blocks::
270 :			* Other I/O::
271 :			* Programming Tools::
272 :			* Threading Words::
273 :	anton	1.1	@end menu
274 :
275 :			@node Notation, Arithmetic, Words, Words
276 :			@section Notation
277 :
278 :			The Forth words are described in this section in the glossary notation
279 :			that has become a de-facto standard for Forth texts, i.e.
280 :
281 :	anton	1.4	@format
282 :	anton	1.1	@var{word} @var{Stack effect} @var{wordset} @var{pronunciation}
283 :	anton	1.4	@end format
284 :	anton	1.1	@var{Description}
285 :
286 :			@table @var
287 :			@item word
288 :	anton	1.17	The name of the word. BTW, Gforth is case insensitive, so you can
289 :	anton	1.14	type the words in in lower case (However, @pxref{core-idef}).
290 :	anton	1.1
291 :			@item Stack effect
292 :			The stack effect is written in the notation @code{@var{before} --
293 :			@var{after}}, where @var{before} and @var{after} describe the top of
294 :			stack entries before and after the execution of the word. The rest of
295 :			the stack is not touched by the word. The top of stack is rightmost,
296 :	anton	1.17	i.e., a stack sequence is written as it is typed in. Note that Gforth
297 :	anton	1.1	uses a separate floating point stack, but a unified stack
298 :			notation. Also, return stack effects are not shown in @var{stack
299 :			effect}, but in @var{Description}. The name of a stack item describes
300 :			the type and/or the function of the item. See below for a discussion of
301 :			the types.
302 :
303 :			@item pronunciation
304 :			How the word is pronounced
305 :
306 :			@item wordset
307 :			The ANS Forth standard is divided into several wordsets. A standard
308 :			system need not support all of them. So, the fewer wordsets your program
309 :			uses the more portable it will be in theory. However, we suspect that
310 :			most ANS Forth systems on personal machines will feature all
311 :			wordsets. Words that are not defined in the ANS standard have
312 :			@code{gforth} as wordset.
313 :
314 :			@item Description
315 :			A description of the behaviour of the word.
316 :			@end table
317 :
318 :	anton	1.4	The type of a stack item is specified by the character(s) the name
319 :			starts with:
320 :	anton	1.1
321 :			@table @code
322 :			@item f
323 :			Bool, i.e. @code{false} or @code{true}.
324 :			@item c
325 :			Char
326 :			@item w
327 :			Cell, can contain an integer or an address
328 :			@item n
329 :			signed integer
330 :			@item u
331 :			unsigned integer
332 :			@item d
333 :			double sized signed integer
334 :			@item ud
335 :			double sized unsigned integer
336 :			@item r
337 :			Float
338 :			@item a_
339 :			Cell-aligned address
340 :			@item c_
341 :			Char-aligned address (note that a Char is two bytes in Windows NT)
342 :			@item f_
343 :			Float-aligned address
344 :			@item df_
345 :			Address aligned for IEEE double precision float
346 :			@item sf_
347 :			Address aligned for IEEE single precision float
348 :			@item xt
349 :			Execution token, same size as Cell
350 :			@item wid
351 :			Wordlist ID, same size as Cell
352 :			@item f83name
353 :			Pointer to a name structure
354 :			@end table
355 :
356 :	anton	1.4	@node Arithmetic, Stack Manipulation, Notation, Words
357 :	anton	1.1	@section Arithmetic
358 :			Forth arithmetic is not checked, i.e., you will not hear about integer
359 :			overflow on addition or multiplication, you may hear about division by
360 :			zero if you are lucky. The operator is written after the operands, but
361 :			the operands are still in the original order. I.e., the infix @code{2-1}
362 :			corresponds to @code{2 1 -}. Forth offers a variety of division
363 :			operators. If you perform division with potentially negative operands,
364 :			you do not want to use @code{/} or @code{/mod} with its undefined
365 :			behaviour, but rather @code{fm/mod} or @code{sm/mod} (probably the
366 :	anton	1.4	former, @pxref{Mixed precision}).
367 :
368 :			@menu
369 :			* Single precision::
370 :			* Bitwise operations::
371 :			* Mixed precision:: operations with single and double-cell integers
372 :			* Double precision:: Double-cell integer arithmetic
373 :			* Floating Point::
374 :			@end menu
375 :	anton	1.1
376 :	anton	1.4	@node Single precision, Bitwise operations, Arithmetic, Arithmetic
377 :	anton	1.1	@subsection Single precision
378 :			doc-+
379 :			doc--
380 :			doc-*
381 :			doc-/
382 :			doc-mod
383 :			doc-/mod
384 :			doc-negate
385 :			doc-abs
386 :			doc-min
387 :			doc-max
388 :
389 :	anton	1.4	@node Bitwise operations, Mixed precision, Single precision, Arithmetic
390 :	anton	1.1	@subsection Bitwise operations
391 :			doc-and
392 :			doc-or
393 :			doc-xor
394 :			doc-invert
395 :			doc-2*
396 :			doc-2/
397 :
398 :	anton	1.4	@node Mixed precision, Double precision, Bitwise operations, Arithmetic
399 :	anton	1.1	@subsection Mixed precision
400 :			doc-m+
401 :			doc-*/
402 :			doc-*/mod
403 :			doc-m*
404 :			doc-um*
405 :			doc-m*/
406 :			doc-um/mod
407 :			doc-fm/mod
408 :			doc-sm/rem
409 :
410 :	anton	1.4	@node Double precision, Floating Point, Mixed precision, Arithmetic
411 :	anton	1.1	@subsection Double precision
412 :	anton	1.16
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 :
417 :	anton	1.1	doc-d+
418 :			doc-d-
419 :			doc-dnegate
420 :			doc-dabs
421 :			doc-dmin
422 :			doc-dmax
423 :
424 :	anton	1.4	@node Floating Point, , Double precision, Arithmetic
425 :			@subsection Floating Point
426 :	anton	1.16
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).
439 :	anton	1.4
440 :			Angles in floating point operations are given in radians (a full circle
441 :	anton	1.17	has 2 pi radians). Note, that Gforth has a separate floating point
442 :	anton	1.4	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
449 :	anton	1.11	avoid them), you might start with @cite{David Goldberg, What Every
450 :	anton	1.6	Computer Scientist Should Know About Floating-Point Arithmetic, ACM
451 :			Computing Surveys 23(1):5@minus{}48, March 1991}.
452 :	anton	1.4
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
470 :	anton	1.6	doc-falog
471 :	anton	1.4	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
487 :	anton	1.1	@section Stack Manipulation
488 :
489 :	anton	1.17	Gforth has a data stack (aka parameter stack) for characters, cells,
490 :	anton	1.1	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
499 :	anton	1.4	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
503 :	anton	1.1	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 :
509 :	anton	1.4	@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
518 :	anton	1.1	@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 :
538 :	anton	1.4	@node Floating point stack, Return stack, Data stack, Stack Manipulation
539 :	anton	1.1	@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 :
548 :	anton	1.4	@node Return stack, Locals stack, Floating point stack, Stack Manipulation
549 :	anton	1.1	@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 :
559 :	anton	1.4	@node Locals stack, Stack pointer manipulation, Return stack, Stack Manipulation
560 :	anton	1.1	@subsection Locals stack
561 :
562 :	anton	1.4	@node Stack pointer manipulation, , Locals stack, Stack Manipulation
563 :	anton	1.1	@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 :
573 :	anton	1.4	@node Memory access, Control Structures, Stack Manipulation, Words
574 :	anton	1.1	@section Memory access
575 :
576 :	anton	1.4	@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
583 :	anton	1.1	@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 :
599 :	anton	1.4	@node Address arithmetic, Memory block access, Stack-Memory transfers, Memory access
600 :	anton	1.1	@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 :
615 :	anton	1.17	For the performance-conscious: alignment operations are usually only
616 :	anton	1.1	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
625 :	anton	1.17	are cell-aligned; in addition, Gforth guarantees that these addresses
626 :	anton	1.1	are aligned for all purposes.
627 :
628 :	anton	1.9	Note that the standard defines a word @code{char}, which has nothing to
629 :			do with address arithmetic.
630 :
631 :	anton	1.1	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
649 :	anton	1.10	doc-maxalign
650 :			doc-maxaligned
651 :			doc-cfalign
652 :			doc-cfaligned
653 :	anton	1.1	doc-address-unit-bits
654 :
655 :	anton	1.4	@node Memory block access, , Address arithmetic, Memory access
656 :	anton	1.1	@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 :
669 :	anton	1.4	@node Control Structures, Locals, Memory access, Words
670 :	anton	1.1	@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 :
677 :	anton	1.4	@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
687 :	anton	1.1	@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 :
705 :	anton	1.4	You can use @code{THEN} instead of @code{ENDIF}. Indeed, @code{THEN} is
706 :	anton	1.1	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
732 :	anton	1.4	@dots{}
733 :	anton	1.1	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 :
741 :	anton	1.4	@node Simple Loops, Counted Loops, Selection, Control Structures
742 :	anton	1.1	@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 :
773 :	anton	1.4	@node Counted Loops, Arbitrary control structures, Simple Loops, Control Structures
774 :	anton	1.1	@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 :
815 :	anton	1.2	@code{4 0 ?DO i . 2 +LOOP} prints @code{0 2}
816 :	anton	1.1
817 :	anton	1.2	@code{4 1 ?DO i . 2 +LOOP} prints @code{1 3}
818 :	anton	1.1
819 :			The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative:
820 :
821 :	anton	1.2	@code{-1 0 ?DO i . -1 +LOOP} prints @code{0 -1}
822 :	anton	1.1
823 :	anton	1.2	@code{ 0 0 ?DO i . -1 +LOOP} prints nothing
824 :	anton	1.1
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 :
829 :	pazsan	1.7	@code{-2 0 ?DO i . -1 S+LOOP} prints @code{0 -1}
830 :	anton	1.1
831 :	pazsan	1.7	@code{-1 0 ?DO i . -1 S+LOOP} prints @code{0}
832 :	anton	1.1
833 :	pazsan	1.7	@code{ 0 0 ?DO i . -1 S+LOOP} prints nothing
834 :	anton	1.1
835 :	anton	1.2	The loop is terminated when the border between @var{limit@minus{}sgn(n)} and
836 :	anton	1.1	@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
855 :	anton	1.17	lazy to optimize @code{?DO} loops properly. In Gforth, this loop
856 :	anton	1.1	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 :
860 :	anton	1.4	@node Arbitrary control structures, Calls and returns, Counted Loops, Control Structures
861 :	anton	1.2	@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
866 :	anton	1.17	stack, and this is what we have done in Gforth.
867 :	anton	1.2
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 :
873 :	anton	1.3	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
881 :	anton	1.2
882 :	anton	1.17	On many systems control-flow stack items take one word, in Gforth they
883 :	anton	1.2	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 :
890 :	anton	1.3	doc-else
891 :			doc-while
892 :			doc-repeat
893 :	anton	1.2
894 :			Counted loop words constitute a separate group of words:
895 :
896 :	anton	1.3	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
906 :	anton	1.10	doc-done
907 :	anton	1.2
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
911 :	anton	1.3	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
913 :	pazsan	1.7	resolved (by using one of the loop-ending words or @code{DONE}).
914 :	anton	1.2
915 :			Another group of control structure words are
916 :
917 :	anton	1.3	doc-case
918 :			doc-endcase
919 :			doc-of
920 :			doc-endof
921 :	anton	1.2
922 :			@i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and
923 :			@code{cs-roll}.
924 :
925 :	anton	1.3	@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 :
968 :	anton	1.4	@node Calls and returns, Exception Handling, Arbitrary control structures, Control Structures
969 :	anton	1.3	@subsection Calls and returns
970 :
971 :			A definition can be called simply be writing the name of the
972 :	anton	1.17	definition. When the end of the definition is reached, it returns. An
973 :			earlier return can be forced using
974 :	anton	1.3
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 :
983 :	anton	1.4	@node Exception Handling, , Calls and returns, Control Structures
984 :	anton	1.3	@subsection Exception Handling
985 :
986 :			doc-catch
987 :			doc-throw
988 :
989 :	anton	1.4	@node Locals, Defining Words, Control Structures, Words
990 :	anton	1.1	@section Locals
991 :
992 :	anton	1.2	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
999 :	anton	1.17	* Gforth locals::
1000 :	anton	1.4	* ANS Forth locals::
1001 :	anton	1.2	@end menu
1002 :
1003 :	anton	1.17	@node Gforth locals, ANS Forth locals, Locals, Locals
1004 :			@subsection Gforth locals
1005 :	anton	1.2
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 :
1047 :	anton	1.17	Gforth currently supports cells (@code{W:}, @code{W^}), doubles
1048 :	anton	1.2	(@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 :
1067 :	anton	1.17	Gforth allows defining locals everywhere in a colon definition. This
1068 :	pazsan	1.7	poses the following questions:
1069 :	anton	1.2
1070 :	anton	1.4	@menu
1071 :			* Where are locals visible by name?::
1072 :	anton	1.14	* How long do locals live?::
1073 :	anton	1.4	* Programming Style::
1074 :			* Implementation::
1075 :			@end menu
1076 :
1077 :	anton	1.17	@node Where are locals visible by name?, How long do locals live?, Gforth locals, Gforth locals
1078 :	anton	1.2	@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
1126 :	anton	1.3	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
1130 :	anton	1.2	loops). Perhaps the most insidious example is:
1131 :			@example
1132 :			AHEAD
1133 :			BEGIN
1134 :			x
1135 :			[ 1 CS-ROLL ] THEN
1136 :	anton	1.4	@{ x @}
1137 :	anton	1.2	...
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
1168 :	anton	1.4	@{ x @}
1169 :	anton	1.2	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
1184 :	anton	1.4	@{ x @}
1185 :	anton	1.2	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
1199 :	anton	1.17	visible after the @code{BEGIN}. However, the user can use
1200 :	anton	1.2	@code{ASSUME-LIVE} to make the compiler assume that the same locals are
1201 :	anton	1.17	visible at the BEGIN as at the point where the top control-flow stack
1202 :			item was created.
1203 :	anton	1.2
1204 :			doc-assume-live
1205 :
1206 :			E.g.,
1207 :			@example
1208 :	anton	1.4	@{ x @}
1209 :	anton	1.2	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
1229 :	anton	1.4	@{ x @}
1230 :	anton	1.2	... 0=
1231 :			WHILE
1232 :			x
1233 :			REPEAT
1234 :			@end example
1235 :
1236 :	anton	1.17	@node How long do locals live?, Programming Style, Where are locals visible by name?, Gforth locals
1237 :	anton	1.2	@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 :
1250 :	anton	1.17	@node Programming Style, Implementation, How long do locals live?, Gforth locals
1251 :	anton	1.2	@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
1262 :	anton	1.4	unlikely to become a conscious programming objective. Still, the number
1263 :			of stack manipulations will be reduced dramatically if local variables
1264 :	anton	1.17	are used liberally (e.g., compare @code{max} in @ref{Gforth locals} with
1265 :	anton	1.4	a traditional implementation of @code{max}).
1266 :	anton	1.2
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 :
1319 :	anton	1.17	@node Implementation, , Programming Style, Gforth locals
1320 :	anton	1.2	@subsubsection Implementation
1321 :
1322 :	anton	1.17	Gforth uses an extra locals stack. The most compelling reason for
1323 :	anton	1.2	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 :
1343 :	anton	1.12	doc-compile-@local
1344 :			doc-compile-f@local
1345 :	anton	1.2	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 :
1357 :	anton	1.17	A special feature of Gforth's dictionary is used to implement the
1358 :	anton	1.2	definition of locals without type specifiers: every wordlist (aka
1359 :			vocabulary) has its own methods for searching
1360 :	anton	1.4	etc. (@pxref{Wordlists}). For the present purpose we defined a wordlist
1361 :	anton	1.2	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
1397 :	anton	1.4	level at the @var{orig} point to the level after the @code{THEN}. The
1398 :	anton	1.2	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 :
1447 :	anton	1.17	@node ANS Forth locals, , Gforth locals, Locals
1448 :	anton	1.2	@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
1452 :	anton	1.17	possible syntaxes is a subset of the syntax we used in the Gforth locals
1453 :	anton	1.2	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:
1465 :	anton	1.1
1466 :	anton	1.2	@itemize @bullet
1467 :			@item
1468 :	anton	1.17	Locals can only be cell-sized values (no type specifiers are allowed).
1469 :	anton	1.2	@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
1474 :	anton	1.17	accessing words in a definition using locals, you will be all right. The
1475 :	anton	1.2	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
1482 :	anton	1.4	(@xref{Values}). I.e., they are initialized from the stack. Using their
1483 :	anton	1.2	name produces their value. Their value can be changed using @code{TO}.
1484 :
1485 :	anton	1.17	Since this syntax is supported by Gforth directly, you need not do
1486 :	anton	1.2	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
1500 :	anton	1.17	syntax to make porting to Gforth easy, but do not document it here. The
1501 :	anton	1.2	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.
1506 :	anton	1.3
1507 :	anton	1.4	@node Defining Words, Wordlists, Locals, Words
1508 :			@section Defining Words
1509 :
1510 :	anton	1.14	@menu
1511 :			* Values::
1512 :			@end menu
1513 :
1514 :	anton	1.4	@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 :
1532 :	anton	1.5	@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
1538 :	anton	1.4	@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
1557 :	anton	1.5	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).
1560 :	anton	1.4
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 :
1569 :	anton	1.5	@node Assertions, , Debugging, Programming Tools
1570 :	anton	1.4	@subsection Assertions
1571 :
1572 :	anton	1.5	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
1574 :	anton	1.17	never zero) that may become wrong during maintenance. Gforth supports
1575 :	anton	1.5	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
1597 :	anton	1.17	keep others turned on. Gforth provides several levels of assertions for
1598 :	anton	1.5	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 :
1628 :	anton	1.4	@node Threading Words, , Programming Tools, Words
1629 :			@section Threading Words
1630 :
1631 :			These words provide access to code addresses and other threading stuff
1632 :	anton	1.17	in Gforth (and, possibly, other interpretive Forths). It more or less
1633 :	anton	1.4	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 :
1646 :	anton	1.14
1647 :
1648 :	anton	1.4	@node ANS conformance, Model, Words, Top
1649 :			@chapter ANS conformance
1650 :
1651 :	anton	1.17	To the best of our knowledge, Gforth is an
1652 :	anton	1.14
1653 :	anton	1.15	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
1686 :	anton	1.17	change during the maintenance of Gforth.
1687 :	anton	1.15
1688 :	anton	1.14	@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
1712 :	anton	1.15	* core-idef:: Implementation Defined Options
1713 :			* core-ambcond:: Ambiguous Conditions
1714 :			* core-other:: Other System Documentation
1715 :	anton	1.14	@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:
1725 :	anton	1.17	processor-dependent. Gforth's alignment words perform natural alignment
1726 :	anton	1.14	(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:
1750 :	anton	1.17	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
1754 :	anton	1.14	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:
1813 :	anton	1.17	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.
1816 :	anton	1.14
1817 :			@item method of selecting the user output device:
1818 :			The user output device is the standard output. It cannot be redirected
1819 :	anton	1.17	from within Gforth, but typically from the command line that starts
1820 :			Gforth. Gforth uses buffered output, so output on a terminal does not
1821 :	anton	1.14	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:
1825 :	anton	1.17	What are we expected to document here?
1826 :	anton	1.14
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
1861 :	anton	1.17	and locals stack at Gforth startup with the command line option
1862 :	anton	1.14	@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 :			2bits-per-cell for single arithmetic and 4bits-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 :			2bits-per-cell for single arithmetic and 4bits-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 :
2026 :	anton	1.15	@item name not found (@code{'}, @code{POSTPONE}, @code{[']}, @code{[COMPILE]}):
2027 :	anton	1.14	@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 :
2041 :	anton	1.15	@item u greater than or equal to the number of bits in a cell (@code{LSHIFT}, @code{RSHIFT}):
2042 :	anton	1.14	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
2094 :	anton	1.15	* block-idef:: Implementation Defined Options
2095 :			* block-ambcond:: Ambiguous Conditions
2096 :			* block-other:: Other System Documentation
2097 :	anton	1.14	@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
2168 :	anton	1.15	* double-ambcond:: Ambiguous Conditions
2169 :	anton	1.14	@end menu
2170 :
2171 :
2172 :			@c ---------------------------------------------------------------------
2173 :	anton	1.15	@node double-ambcond, , The optional Double Number word set, The optional Double Number word set
2174 :	anton	1.14	@subsection Ambiguous conditions
2175 :			@c ---------------------------------------------------------------------
2176 :
2177 :			@table @i
2178 :
2179 :	anton	1.15	@item @var{d} outside of range of @var{n} in @code{D>S}:
2180 :	anton	1.14	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
2191 :	anton	1.15	* exception-idef:: Implementation Defined Options
2192 :	anton	1.14	@end menu
2193 :
2194 :
2195 :			@c ---------------------------------------------------------------------
2196 :	anton	1.15	@node exception-idef, , The optional Exception word set, The optional Exception word set
2197 :	anton	1.14	@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
2217 :	anton	1.15	* facility-idef:: Implementation Defined Options
2218 :			* facility-ambcond:: Ambiguous Conditions
2219 :	anton	1.14	@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
2239 :	anton	1.17	lightly loaded, and the delay is short enough that Gforth does not get
2240 :	anton	1.14	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 ---------------------------------------------------------------------
2247 :	anton	1.15	@node facility-ambcond, , facility-idef, The optional Facility word set
2248 :	anton	1.14	@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
2267 :	anton	1.15	* file-idef:: Implementation Defined Options
2268 :			* file-ambcond:: Ambiguous Conditions
2269 :	anton	1.14	@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
2283 :	anton	1.15	not (both with @code{open-file} and @code{create-file}). Under Unix
2284 :	anton	1.14	@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:
2309 :	anton	1.15	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}.
2313 :	anton	1.14
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 ---------------------------------------------------------------------
2335 :	anton	1.15	@node file-ambcond, , file-idef, The optional File-Access word set
2336 :	anton	1.14	@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
2373 :	anton	1.15	@section The optional Floating-Point word set
2374 :	anton	1.14	@c =====================================================================
2375 :
2376 :			@menu
2377 :	anton	1.15	* floating-idef:: Implementation Defined Options
2378 :			* floating-ambcond:: Ambiguous Conditions
2379 :	anton	1.14	@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 :
2389 :	anton	1.15	@item format and range of floating point numbers:
2390 :			System-dependent; the @code{double} type of C.
2391 :	anton	1.14
2392 :	anton	1.15	@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.
2395 :	anton	1.14
2396 :	anton	1.15	@item rounding or truncation of floating-point numbers:
2397 :			What's the question?!!
2398 :	anton	1.14
2399 :	anton	1.15	@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}.
2402 :	anton	1.14
2403 :	anton	1.15	@item width of floating-point stack:
2404 :			@code{1 floats}.
2405 :	anton	1.14
2406 :			@end table
2407 :
2408 :
2409 :			@c ---------------------------------------------------------------------
2410 :	anton	1.15	@node floating-ambcond, , floating-idef, The optional Floating-Point word set
2411 :			@subsection Ambiguous conditions
2412 :	anton	1.14	@c ---------------------------------------------------------------------
2413 :
2414 :			@table @i
2415 :
2416 :	anton	1.15	@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.
2419 :	anton	1.14
2420 :	anton	1.15	@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.
2423 :	anton	1.14
2424 :	anton	1.15	@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.
2428 :	anton	1.14
2429 :	anton	1.15	@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.
2432 :	anton	1.14
2433 :	anton	1.15	@item BASE is not decimal (@code{REPRESENT}, @code{F.}, @code{FE.}, @code{FS.}):
2434 :			The floating-point number is converted into decimal nonetheless.
2435 :	anton	1.14
2436 :	anton	1.15	@item Both arguments are equal to zero (@code{FATAN2}):
2437 :			System-dependent. @code{FATAN2} is implemented using the C library
2438 :			function @code{atan2()}.
2439 :	anton	1.14
2440 :	anton	1.15	@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.
2444 :	anton	1.14
2445 :	anton	1.15	@item @var{d} cannot be presented precisely as a float in @code{D>F}:
2446 :			The result is rounded to the nearest float.
2447 :	anton	1.14
2448 :	anton	1.15	@item dividing by zero:
2449 :			@code{-55 throw} (Floating-point unidentified fault)
2450 :	anton	1.14
2451 :	anton	1.15	@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.
2454 :	anton	1.14
2455 :	anton	1.15	@item @var{float}<1 (@code{facosh}):
2456 :			@code{-55 throw} (Floating-point unidentified fault)
2457 :	anton	1.14
2458 :	anton	1.15	@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.
2461 :	anton	1.14
2462 :	anton	1.15	@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.
2465 :	anton	1.14
2466 :	anton	1.15	@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?)
2469 :	anton	1.14
2470 :	anton	1.15	@item \|@var{float}\|>1 (@code{facos}, @code{fasin}, @code{fatanh}):
2471 :			@code{-55 throw} (Floating-point unidentified fault).
2472 :	anton	1.14
2473 :	anton	1.15	@item integer part of float cannot be represented by @var{d} in @code{f>d}:
2474 :			@code{-55 throw} (Floating-point unidentified fault).
2475 :	anton	1.14
2476 :	anton	1.15	@item string larger than pictured numeric output area (@code{f.}, @code{fe.}, @code{fs.}):
2477 :			This does not happen.
2478 :			@end table
2479 :	anton	1.14
2480 :
2481 :
2482 :			@c =====================================================================
2483 :	anton	1.15	@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
2485 :	anton	1.14	@c =====================================================================
2486 :
2487 :			@menu
2488 :	anton	1.15	* locals-idef:: Implementation Defined Options
2489 :			* locals-ambcond:: Ambiguous Conditions
2490 :	anton	1.14	@end menu
2491 :
2492 :
2493 :			@c ---------------------------------------------------------------------
2494 :	anton	1.15	@node locals-idef, locals-ambcond, The optional Locals word set, The optional Locals word set
2495 :	anton	1.14	@subsection Implementation Defined Options
2496 :			@c ---------------------------------------------------------------------
2497 :
2498 :			@table @i
2499 :
2500 :	anton	1.15	@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.
2505 :	anton	1.14
2506 :			@end table
2507 :
2508 :
2509 :			@c ---------------------------------------------------------------------
2510 :	anton	1.15	@node locals-ambcond, , locals-idef, The optional Locals word set
2511 :	anton	1.14	@subsection Ambiguous conditions
2512 :			@c ---------------------------------------------------------------------
2513 :
2514 :			@table @i
2515 :
2516 :	anton	1.15	@item executing a named local in interpretation state:
2517 :			@code{-14 throw} (Interpreting a compile-only word).
2518 :	anton	1.14
2519 :	anton	1.15	@item @var{name} not defined by @code{VALUE} or @code{(LOCAL)} (@code{TO}):
2520 :			@code{-32 throw} (Invalid name argument)
2521 :	anton	1.14
2522 :			@end table
2523 :
2524 :
2525 :			@c =====================================================================
2526 :	anton	1.15	@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
2528 :	anton	1.14	@c =====================================================================
2529 :
2530 :			@menu
2531 :	anton	1.15	* memory-idef:: Implementation Defined Options
2532 :	anton	1.14	@end menu
2533 :
2534 :
2535 :			@c ---------------------------------------------------------------------
2536 :	anton	1.15	@node memory-idef, , The optional Memory-Allocation word set, The optional Memory-Allocation word set
2537 :	anton	1.14	@subsection Implementation Defined Options
2538 :			@c ---------------------------------------------------------------------
2539 :
2540 :			@table @i
2541 :
2542 :	anton	1.15	@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}.
2547 :	anton	1.14
2548 :			@end table
2549 :
2550 :			@c =====================================================================
2551 :	anton	1.15	@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
2553 :	anton	1.14	@c =====================================================================
2554 :
2555 :			@menu
2556 :	anton	1.15	* programming-idef:: Implementation Defined Options
2557 :			* programming-ambcond:: Ambiguous Conditions
2558 :	anton	1.14	@end menu
2559 :
2560 :
2561 :			@c ---------------------------------------------------------------------
2562 :	anton	1.15	@node programming-idef, programming-ambcond, The optional Programming-Tools word set, The optional Programming-Tools word set
2563 :	anton	1.14	@subsection Implementation Defined Options
2564 :			@c ---------------------------------------------------------------------
2565 :
2566 :			@table @i
2567 :
2568 :	anton	1.15	@item ending sequence for input following @code{;code} and @code{code}:
2569 :			Not implemented (yet).
2570 :	anton	1.14
2571 :	anton	1.15	@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 :
2583 :	anton	1.14	@end table
2584 :
2585 :			@c ---------------------------------------------------------------------
2586 :	anton	1.15	@node programming-ambcond, , programming-idef, The optional Programming-Tools word set
2587 :	anton	1.14	@subsection Ambiguous conditions
2588 :			@c ---------------------------------------------------------------------
2589 :
2590 :			@table @i
2591 :
2592 :	anton	1.15	@item deleting the compilation wordlist (@code{FORGET}):
2593 :			Not implemented (yet).
2594 :	anton	1.14
2595 :	anton	1.15	@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).
2603 :	anton	1.14
2604 :	anton	1.15	@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.
2608 :	anton	1.14
2609 :	anton	1.15	@item @code{POSTPONE} applied to @code{[IF]}:
2610 :			After defining @code{: X POSTPONE [IF] ; IMMEDIATE}. @code{X} is
2611 :			equivalent to @code{[IF]}.
2612 :	anton	1.14
2613 :	anton	1.15	@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.
2616 :	anton	1.14
2617 :	anton	1.15	@item removing a needed definition (@code{FORGET}):
2618 :			Not implemented (yet).
2619 :	anton	1.14
2620 :			@end table
2621 :
2622 :
2623 :			@c =====================================================================
2624 :	anton	1.15	@node The optional Search-Order word set, , The optional Programming-Tools word set, ANS conformance
2625 :			@section The optional Search-Order word set
2626 :	anton	1.14	@c =====================================================================
2627 :
2628 :			@menu
2629 :	anton	1.15	* search-idef:: Implementation Defined Options
2630 :			* search-ambcond:: Ambiguous Conditions
2631 :	anton	1.14	@end menu
2632 :
2633 :
2634 :			@c ---------------------------------------------------------------------
2635 :	anton	1.15	@node search-idef, search-ambcond, The optional Search-Order word set, The optional Search-Order word set
2636 :	anton	1.14	@subsection Implementation Defined Options
2637 :			@c ---------------------------------------------------------------------
2638 :
2639 :			@table @i
2640 :
2641 :	anton	1.15	@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}.
2646 :	anton	1.14
2647 :			@end table
2648 :
2649 :			@c ---------------------------------------------------------------------
2650 :	anton	1.15	@node search-ambcond, , search-idef, The optional Search-Order word set
2651 :	anton	1.14	@subsection Ambiguous conditions
2652 :			@c ---------------------------------------------------------------------
2653 :
2654 :			@table @i
2655 :
2656 :	anton	1.15	@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.).
2660 :	anton	1.14
2661 :	anton	1.15	@item search order empty (@code{previous}):
2662 :			@code{abort" Vocstack empty"}.
2663 :	anton	1.14
2664 :	anton	1.15	@item too many word lists in search order (@code{also}):
2665 :			@code{abort" Vocstack full"}.
2666 :	anton	1.14
2667 :			@end table
2668 :	anton	1.13
2669 :
2670 :	anton	1.17	@node Model, Emacs and Gforth, ANS conformance, Top
2671 :	anton	1.4	@chapter Model
2672 :
2673 :	anton	1.17	@node Emacs and Gforth, Internals, Model, Top
2674 :			@chapter Emacs and Gforth
2675 :	anton	1.4
2676 :	anton	1.17	Gforth comes with @file{gforth.el}, an improved version of
2677 :	anton	1.4	@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}),
2680 :	anton	1.8	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}.
2685 :	anton	1.4
2686 :	anton	1.17	In addition, Gforth supports Emacs quite well: The source code locations
2687 :	anton	1.4	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
2698 :	anton	1.17	several tags files at the same time (e.g., one for the Gforth sources
2699 :	anton	1.4	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 :
2709 :	anton	1.17	@node Internals, Bugs, Emacs and Gforth, Top
2710 :	anton	1.3	@chapter Internals
2711 :
2712 :	anton	1.17	Reading this section is not necessary for programming with Gforth. It
2713 :			should be helpful for finding your way in the Gforth sources.
2714 :	anton	1.3
2715 :	anton	1.4	@menu
2716 :			* Portability::
2717 :			* Threading::
2718 :			* Primitives::
2719 :			* System Architecture::
2720 :	anton	1.17	* Performance::
2721 :	anton	1.4	@end menu
2722 :
2723 :			@node Portability, Threading, Internals, Internals
2724 :	anton	1.3	@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
2752 :	anton	1.17	machines.
2753 :	anton	1.3
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 :
2770 :	anton	1.4	@node Threading, Primitives, Portability, Internals
2771 :	anton	1.3	@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 :
2802 :	anton	1.4	@menu
2803 :			* Scheduling::
2804 :			* Direct or Indirect Threaded?::
2805 :			* DOES>::
2806 :			@end menu
2807 :
2808 :			@node Scheduling, Direct or Indirect Threaded?, Threading, Threading
2809 :	anton	1.3	@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
2827 :	anton	1.4	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:
2834 :	anton	1.3	@example
2835 :			n=sp[0]+sp[1];
2836 :			sp++;
2837 :			NEXT_P1;
2838 :			sp[0]=n;
2839 :			NEXT_P2;
2840 :			@end example
2841 :	anton	1.4	This can be scheduled optimally by the compiler.
2842 :	anton	1.3
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 :
2847 :	anton	1.4	@node Direct or Indirect Threaded?, DOES>, Scheduling, Threading
2848 :	anton	1.3	@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 :
2864 :	anton	1.4	@node DOES>, , Direct or Indirect Threaded?, Threading
2865 :	anton	1.3	@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
2882 :	anton	1.4	for direct and indirect threaded systems (@pxref{System Architecture}).
2883 :	anton	1.3
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 :
2896 :	anton	1.4	@node Primitives, System Architecture, Threading, Internals
2897 :	anton	1.3	@section Primitives
2898 :
2899 :	anton	1.4	@menu
2900 :			* Automatic Generation::
2901 :			* TOS Optimization::
2902 :			* Produced code::
2903 :			@end menu
2904 :
2905 :			@node Automatic Generation, TOS Optimization, Primitives, Primitives
2906 :	anton	1.3	@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 */
2947 :	anton	1.4	@{
2948 :	anton	1.3	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) */
2956 :	anton	1.4	@{
2957 :	anton	1.3	n = n1+n2; /* C code taken from the source */
2958 :	anton	1.4	@}
2959 :	anton	1.3	NEXT_P1; /* NEXT part 1 */
2960 :			TOS = (Cell)n; /* output */
2961 :			NEXT_P2; /* NEXT part 2 */
2962 :	anton	1.4	@}
2963 :	anton	1.3	@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 :
2984 :	anton	1.4	@node TOS Optimization, Produced code, Automatic Generation, Primitives
2985 :	anton	1.3	@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
2989 :	anton	1.4	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
2991 :	anton	1.3	@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.
3029 :	anton	1.4	@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{--}
3032 :	anton	1.3	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 :
3036 :	anton	1.4	@node Produced code, , TOS Optimization, Primitives
3037 :	anton	1.3	@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
3041 :	anton	1.4	look at the resulting file @file{engine.s}.
3042 :	anton	1.3
3043 :	anton	1.17	@node System Architecture, Performance, Primitives, Internals
3044 :	anton	1.3	@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
3066 :	anton	1.17	(little- or big-endian)@footnote{We are considering adding information to the
3067 :	anton	1.3	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
3070 :	anton	1.4	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}).
3078 :	anton	1.3
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).
3085 :	anton	1.4
3086 :	anton	1.17	@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 :
3153 :	anton	1.4	@node Bugs, Pedigree, Internals, Top
3154 :			@chapter Bugs
3155 :
3156 :	anton	1.17	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 :
3172 :	anton	1.4	@node Pedigree, Word Index, Bugs, Top
3173 :			@chapter Pedigree
3174 :
3175 :	anton	1.17	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 :
3197 :	anton	1.4	@node Word Index, Node Index, Pedigree, Top
3198 :			@chapter Word Index
3199 :
3200 :	anton	1.17	This index is as incomplete as the manual.
3201 :
3202 :			@printindex fn
3203 :
3204 :	anton	1.4	@node Node Index, , Word Index, Top
3205 :			@chapter Node Index
3206 :	anton	1.17
3207 :			This index is even less complete than the manual.
3208 :	anton	1.1
3209 :			@contents
3210 :			@bye
3211 :

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