version 1.1, 1994/10/24 19:15:57
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version 1.6, 1995/01/18 18:41:37
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\input texinfo @c -*-texinfo-*- |
\input texinfo @c -*-texinfo-*- |
@comment The source is gforth.ds, from which gforth.texi is generated |
@comment The source is gforth.ds, from which gforth.texi is generated |
@comment %**start of header (This is for running Texinfo on a region.) |
@comment %**start of header (This is for running Texinfo on a region.) |
@setfilename gforth-info |
@setfilename gforth.info |
@settitle GNU Forth Manual |
@settitle GNU Forth Manual |
@setchapternewpage odd |
@comment @setchapternewpage odd |
@comment %**end of header (This is for running Texinfo on a region.) |
@comment %**end of header (This is for running Texinfo on a region.) |
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@ifinfo |
@ifinfo |
Line 15 Copyright @copyright{} 1994 GNU Forth De
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Line 15 Copyright @copyright{} 1994 GNU Forth De
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this manual provided the copyright notice and this permission notice |
this manual provided the copyright notice and this permission notice |
are preserved on all copies. |
are preserved on all copies. |
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@ignore |
@ignore |
Permission is granted to process this file through TeX and print the |
Permission is granted to process this file through TeX and print the |
results, provided the printed document carries a copying permission |
results, provided the printed document carries a copying permission |
notice identical to this one except for the removal of this paragraph |
notice identical to this one except for the removal of this paragraph |
(this paragraph not being relevant to the printed manual). |
(this paragraph not being relevant to the printed manual). |
|
|
@end ignore |
@end ignore |
Permission is granted to copy and distribute modified versions of this |
Permission is granted to copy and distribute modified versions of this |
manual under the conditions for verbatim copying, provided also that the |
manual under the conditions for verbatim copying, provided also that the |
sections entitled "Distribution" and "General Public License" are |
sections entitled "Distribution" and "General Public License" are |
Line 77 personal machines. This manual correspon
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Line 77 personal machines. This manual correspon
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@end ifinfo |
@end ifinfo |
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|
@menu |
@menu |
* License:: |
* License:: |
* Goals:: About the GNU Forth Project |
* Goals:: About the GNU Forth Project |
* Other Books:: Things you might want to read |
* Other Books:: Things you might want to read |
* Invocation:: Starting GNU Forth |
* Invocation:: Starting GNU Forth |
* Words:: Forth words available in GNU Forth |
* Words:: Forth words available in GNU Forth |
* ANS conformance:: Implementation-defined options etc. |
* ANS conformance:: Implementation-defined options etc. |
* Model:: The abstract machine of GNU Forth |
* Model:: The abstract machine of GNU Forth |
* Emacs and GForth:: The GForth Mode |
* Emacs and GForth:: The GForth Mode |
* Internals:: Implementation details |
* Internals:: Implementation details |
* Bugs:: How to report them |
* Bugs:: How to report them |
* Pedigree:: Ancestors of GNU Forth |
* Pedigree:: Ancestors of GNU Forth |
* Word Index:: An item for each Forth word |
* Word Index:: An item for each Forth word |
* Node Index:: An item for each node |
* Node Index:: An item for each node |
@end menu |
@end menu |
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@node License, Goals, Top, Top |
@node License, Goals, Top, Top |
Line 253 the user initialization file @file{.gfor
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Line 253 the user initialization file @file{.gfor
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option @code{--no-rc} is given; this file is first searched in @file{.}, |
option @code{--no-rc} is given; this file is first searched in @file{.}, |
then in @file{~}, then in the normal path (see above). |
then in @file{~}, then in the normal path (see above). |
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@node Words, , Invocation, Top |
@node Words, ANS conformance, Invocation, Top |
@chapter Forth Words |
@chapter Forth Words |
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@menu |
@menu |
* Notation:: |
* Notation:: |
* Arithmetic:: |
* Arithmetic:: |
* Stack Manipulation:: |
* Stack Manipulation:: |
* Memory access:: |
* Memory access:: |
* Control Structures:: |
* Control Structures:: |
* Local Variables:: |
* Locals:: |
* Defining Words:: |
* Defining Words:: |
* Vocabularies:: |
* Wordlists:: |
* Files:: |
* Files:: |
* Blocks:: |
* Blocks:: |
* Other I/O:: |
* Other I/O:: |
* Programming Tools:: |
* Programming Tools:: |
|
* Threading Words:: |
@end menu |
@end menu |
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@node Notation, Arithmetic, Words, Words |
@node Notation, Arithmetic, Words, Words |
Line 277 then in @file{~}, then in the normal pat
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Line 278 then in @file{~}, then in the normal pat
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The Forth words are described in this section in the glossary notation |
The Forth words are described in this section in the glossary notation |
that has become a de-facto standard for Forth texts, i.e. |
that has become a de-facto standard for Forth texts, i.e. |
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@quotation |
@format |
@var{word} @var{Stack effect} @var{wordset} @var{pronunciation} |
@var{word} @var{Stack effect} @var{wordset} @var{pronunciation} |
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@end format |
@var{Description} |
@var{Description} |
@end quotation |
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@table @var |
@table @var |
@item word |
@item word |
Line 314 wordsets. Words that are not defined in
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Line 315 wordsets. Words that are not defined in
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A description of the behaviour of the word. |
A description of the behaviour of the word. |
@end table |
@end table |
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The name of a stack item corresponds in the following way with its type: |
The type of a stack item is specified by the character(s) the name |
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starts with: |
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@table @code |
@table @code |
@item name starts with |
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Type |
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@item f |
@item f |
Bool, i.e. @code{false} or @code{true}. |
Bool, i.e. @code{false} or @code{true}. |
@item c |
@item c |
Line 353 Wordlist ID, same size as Cell
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Line 353 Wordlist ID, same size as Cell
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Pointer to a name structure |
Pointer to a name structure |
@end table |
@end table |
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@node Arithmetic, , Notation, Words |
@node Arithmetic, Stack Manipulation, Notation, Words |
@section Arithmetic |
@section Arithmetic |
Forth arithmetic is not checked, i.e., you will not hear about integer |
Forth arithmetic is not checked, i.e., you will not hear about integer |
overflow on addition or multiplication, you may hear about division by |
overflow on addition or multiplication, you may hear about division by |
Line 363 corresponds to @code{2 1 -}. Forth offer
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Line 363 corresponds to @code{2 1 -}. Forth offer
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operators. If you perform division with potentially negative operands, |
operators. If you perform division with potentially negative operands, |
you do not want to use @code{/} or @code{/mod} with its undefined |
you do not want to use @code{/} or @code{/mod} with its undefined |
behaviour, but rather @code{fm/mod} or @code{sm/mod} (probably the |
behaviour, but rather @code{fm/mod} or @code{sm/mod} (probably the |
former). |
former, @pxref{Mixed precision}). |
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@menu |
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* Single precision:: |
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* Bitwise operations:: |
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* Mixed precision:: operations with single and double-cell integers |
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* Double precision:: Double-cell integer arithmetic |
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* Floating Point:: |
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@end menu |
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@node Single precision, Bitwise operations, Arithmetic, Arithmetic |
@subsection Single precision |
@subsection Single precision |
doc-+ |
doc-+ |
doc-- |
doc-- |
Line 377 doc-abs
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Line 386 doc-abs
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doc-min |
doc-min |
doc-max |
doc-max |
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@node Bitwise operations, Mixed precision, Single precision, Arithmetic |
@subsection Bitwise operations |
@subsection Bitwise operations |
doc-and |
doc-and |
doc-or |
doc-or |
Line 385 doc-invert
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Line 395 doc-invert
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doc-2* |
doc-2* |
doc-2/ |
doc-2/ |
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@node Mixed precision, Double precision, Bitwise operations, Arithmetic |
@subsection Mixed precision |
@subsection Mixed precision |
doc-m+ |
doc-m+ |
doc-*/ |
doc-*/ |
Line 396 doc-um/mod
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Line 407 doc-um/mod
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doc-fm/mod |
doc-fm/mod |
doc-sm/rem |
doc-sm/rem |
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@node Double precision, Floating Point, Mixed precision, Arithmetic |
@subsection Double precision |
@subsection Double precision |
doc-d+ |
doc-d+ |
doc-d- |
doc-d- |
Line 404 doc-dabs
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Line 416 doc-dabs
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doc-dmin |
doc-dmin |
doc-dmax |
doc-dmax |
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@node Stack Manipulation,,, |
@node Floating Point, , Double precision, Arithmetic |
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@subsection Floating Point |
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Angles in floating point operations are given in radians (a full circle |
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has 2 pi radians). Note, that gforth has a separate floating point |
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stack, but we use the unified notation. |
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Floating point numbers have a number of unpleasant surprises for the |
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unwary (e.g., floating point addition is not associative) and even a few |
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for the wary. You should not use them unless you know what you are doing |
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or you don't care that the results you get are totally bogus. If you |
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want to learn about the problems of floating point numbers (and how to |
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avoid them), you might start with @cite{David (?) Goldberg, What Every |
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Computer Scientist Should Know About Floating-Point Arithmetic, ACM |
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Computing Surveys 23(1):5@minus{}48, March 1991}. |
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doc-f+ |
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doc-f- |
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doc-f* |
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doc-f/ |
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doc-fnegate |
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doc-fabs |
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doc-fmax |
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doc-fmin |
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doc-floor |
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doc-fround |
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doc-f** |
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doc-fsqrt |
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doc-fexp |
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doc-fexpm1 |
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doc-fln |
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doc-flnp1 |
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doc-flog |
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doc-falog |
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doc-fsin |
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doc-fcos |
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doc-fsincos |
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doc-ftan |
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doc-fasin |
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doc-facos |
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doc-fatan |
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doc-fatan2 |
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doc-fsinh |
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doc-fcosh |
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doc-ftanh |
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doc-fasinh |
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doc-facosh |
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doc-fatanh |
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@node Stack Manipulation, Memory access, Arithmetic, Words |
@section Stack Manipulation |
@section Stack Manipulation |
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gforth has a data stack (aka parameter stack) for characters, cells, |
gforth has a data stack (aka parameter stack) for characters, cells, |
Line 417 theoretically keep floating point number
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Line 478 theoretically keep floating point number
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additional difficulty, you don't know how many cells a floating point |
additional difficulty, you don't know how many cells a floating point |
number takes. It is reportedly possible to write words in a way that |
number takes. It is reportedly possible to write words in a way that |
they work also for a unified stack model, but we do not recommend trying |
they work also for a unified stack model, but we do not recommend trying |
it. Also, a Forth system is allowed to keep the local variables on the |
it. Instead, just say that your program has an environmental dependency |
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on a separate FP stack. |
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Also, a Forth system is allowed to keep the local variables on the |
return stack. This is reasonable, as local variables usually eliminate |
return stack. This is reasonable, as local variables usually eliminate |
the need to use the return stack explicitly. So, if you want to produce |
the need to use the return stack explicitly. So, if you want to produce |
a standard complying program and if you are using local variables in a |
a standard complying program and if you are using local variables in a |
word, forget about return stack manipulations in that word (see the |
word, forget about return stack manipulations in that word (see the |
standard document for the exact rules). |
standard document for the exact rules). |
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@menu |
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* Data stack:: |
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* Floating point stack:: |
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* Return stack:: |
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* Locals stack:: |
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* Stack pointer manipulation:: |
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@end menu |
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@node Data stack, Floating point stack, Stack Manipulation, Stack Manipulation |
@subsection Data stack |
@subsection Data stack |
doc-drop |
doc-drop |
doc-nip |
doc-nip |
Line 444 doc-2tuck
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Line 517 doc-2tuck
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doc-2swap |
doc-2swap |
doc-2rot |
doc-2rot |
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@node Floating point stack, Return stack, Data stack, Stack Manipulation |
@subsection Floating point stack |
@subsection Floating point stack |
doc-fdrop |
doc-fdrop |
doc-fnip |
doc-fnip |
Line 453 doc-ftuck
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Line 527 doc-ftuck
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doc-fswap |
doc-fswap |
doc-frot |
doc-frot |
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@node Return stack, Locals stack, Floating point stack, Stack Manipulation |
@subsection Return stack |
@subsection Return stack |
doc->r |
doc->r |
doc-r> |
doc-r> |
Line 463 doc-2r>
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Line 538 doc-2r>
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doc-2r@ |
doc-2r@ |
doc-2rdrop |
doc-2rdrop |
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@node Locals stack, Stack pointer manipulation, Return stack, Stack Manipulation |
@subsection Locals stack |
@subsection Locals stack |
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@node Stack pointer manipulation, , Locals stack, Stack Manipulation |
@subsection Stack pointer manipulation |
@subsection Stack pointer manipulation |
doc-sp@ |
doc-sp@ |
doc-sp! |
doc-sp! |
Line 475 doc-rp!
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Line 552 doc-rp!
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doc-lp@ |
doc-lp@ |
doc-lp! |
doc-lp! |
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@node Memory access |
@node Memory access, Control Structures, Stack Manipulation, Words |
@section Memory access |
@section Memory access |
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@menu |
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* Stack-Memory transfers:: |
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* Address arithmetic:: |
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* Memory block access:: |
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@end menu |
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@node Stack-Memory transfers, Address arithmetic, Memory access, Memory access |
@subsection Stack-Memory transfers |
@subsection Stack-Memory transfers |
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doc-@ |
doc-@ |
Line 494 doc-sf!
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Line 578 doc-sf!
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doc-df@ |
doc-df@ |
doc-df! |
doc-df! |
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@node Address arithmetic, Memory block access, Stack-Memory transfers, Memory access |
@subsection Address arithmetic |
@subsection Address arithmetic |
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ANS Forth does not specify the sizes of the data types. Instead, it |
ANS Forth does not specify the sizes of the data types. Instead, it |
Line 542 doc-dfalign
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Line 627 doc-dfalign
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doc-dfaligned |
doc-dfaligned |
doc-address-unit-bits |
doc-address-unit-bits |
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@node Memory block access, , Address arithmetic, Memory access |
@subsection Memory block access |
@subsection Memory block access |
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doc-move |
doc-move |
Line 555 doc-cmove>
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Line 641 doc-cmove>
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doc-fill |
doc-fill |
doc-blank |
doc-blank |
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@node Control Structures |
@node Control Structures, Locals, Memory access, Words |
@section Control Structures |
@section Control Structures |
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Control structures in Forth cannot be used in interpret state, only in |
Control structures in Forth cannot be used in interpret state, only in |
Line 563 compile state, i.e., in a colon definiti
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Line 649 compile state, i.e., in a colon definiti
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limitation, but have not seen a satisfying way around it yet, although |
limitation, but have not seen a satisfying way around it yet, although |
many schemes have been proposed. |
many schemes have been proposed. |
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@menu |
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* Selection:: |
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* Simple Loops:: |
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* Counted Loops:: |
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* Arbitrary control structures:: |
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* Calls and returns:: |
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* Exception Handling:: |
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@end menu |
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@node Selection, Simple Loops, Control Structures, Control Structures |
@subsection Selection |
@subsection Selection |
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@example |
@example |
Line 581 ELSE
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Line 677 ELSE
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ENDIF |
ENDIF |
@end example |
@end example |
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You can use @code{THEN} instead of {ENDIF}. Indeed, @code{THEN} is |
You can use @code{THEN} instead of @code{ENDIF}. Indeed, @code{THEN} is |
standard, and @code{ENDIF} is not, although it is quite popular. We |
standard, and @code{ENDIF} is not, although it is quite popular. We |
recommend using @code{ENDIF}, because it is less confusing for people |
recommend using @code{ENDIF}, because it is less confusing for people |
who also know other languages (and is not prone to reinforcing negative |
who also know other languages (and is not prone to reinforcing negative |
Line 608 can avoid using @code{?dup}.
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Line 704 can avoid using @code{?dup}.
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CASE |
CASE |
@var{n1} OF @var{code1} ENDOF |
@var{n1} OF @var{code1} ENDOF |
@var{n2} OF @var{code2} ENDOF |
@var{n2} OF @var{code2} ENDOF |
@dots |
@dots{} |
ENDCASE |
ENDCASE |
@end example |
@end example |
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Line 617 Executes the first @var{codei}, where th
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Line 713 Executes the first @var{codei}, where th
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the last @code{ENDOF}. It may use @var{n}, which is on top of the stack, |
the last @code{ENDOF}. It may use @var{n}, which is on top of the stack, |
but must not consume it. |
but must not consume it. |
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@node Simple Loops, Counted Loops, Selection, Control Structures |
@subsection Simple Loops |
@subsection Simple Loops |
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@example |
@example |
Line 648 AGAIN
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Line 745 AGAIN
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This is an endless loop. |
This is an endless loop. |
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@node Counted Loops, Arbitrary control structures, Simple Loops, Control Structures |
@subsection Counted Loops |
@subsection Counted Loops |
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The basic counted loop is: |
The basic counted loop is: |
Line 689 There are several variations on the coun
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Line 787 There are several variations on the coun
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index by @var{n} instead of by 1. The loop is terminated when the border |
index by @var{n} instead of by 1. The loop is terminated when the border |
between @var{limit-1} and @var{limit} is crossed. E.g.: |
between @var{limit-1} and @var{limit} is crossed. E.g.: |
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4 0 ?DO i . 2 +LOOP prints 0 2 |
@code{4 0 ?DO i . 2 +LOOP} prints @code{0 2} |
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4 1 ?DO i . 2 +LOOP prints 1 3 |
@code{4 1 ?DO i . 2 +LOOP} prints @code{1 3} |
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The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative: |
The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative: |
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-1 0 ?DO i . -1 +LOOP prints 0 -1 |
@code{-1 0 ?DO i . -1 +LOOP} prints @code{0 -1} |
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0 0 ?DO i . -1 +LOOP prints nothing |
@code{ 0 0 ?DO i . -1 +LOOP} prints nothing |
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Therefore we recommend avoiding using @code{@var{n} +LOOP} with negative |
Therefore we recommend avoiding using @code{@var{n} +LOOP} with negative |
@var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative |
@var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative |
case behaves symmetrical to the positive case: |
case behaves symmetrical to the positive case: |
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-2 0 ?DO i . -1 +LOOP prints 0 -1 |
@code{-2 0 ?DO i . -1 +LOOP} prints @code{0 -1} |
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-1 0 ?DO i . -1 +LOOP prints 0 |
@code{-1 0 ?DO i . -1 +LOOP} prints @code{0} |
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0 0 ?DO i . -1 +LOOP prints nothing |
@code{ 0 0 ?DO i . -1 +LOOP} prints nothing |
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The loop is terminated when the border between @var{limit-sgn(n)} and |
The loop is terminated when the border between @var{limit@minus{}sgn(n)} and |
@var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS |
@var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS |
Forth standard. |
Forth standard. |
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Line 734 iterates @var{n+1} times; @code{i} produ
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Line 832 iterates @var{n+1} times; @code{i} produ
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and ending with 0. Other Forth systems may behave differently, even if |
and ending with 0. Other Forth systems may behave differently, even if |
they support @code{FOR} loops. |
they support @code{FOR} loops. |
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@node Locals |
@node Arbitrary control structures, Calls and returns, Counted Loops, Control Structures |
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@subsection Arbitrary control structures |
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ANS Forth permits and supports using control structures in a non-nested |
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way. Information about incomplete control structures is stored on the |
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control-flow stack. This stack may be implemented on the Forth data |
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stack, and this is what we have done in gforth. |
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An @i{orig} entry represents an unresolved forward branch, a @i{dest} |
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entry represents a backward branch target. A few words are the basis for |
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building any control structure possible (except control structures that |
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need storage, like calls, coroutines, and backtracking). |
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doc-if |
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doc-ahead |
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doc-then |
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doc-begin |
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doc-until |
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doc-again |
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doc-cs-pick |
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doc-cs-roll |
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On many systems control-flow stack items take one word, in gforth they |
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currently take three (this may change in the future). Therefore it is a |
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really good idea to manipulate the control flow stack with |
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@code{cs-pick} and @code{cs-roll}, not with data stack manipulation |
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words. |
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Some standard control structure words are built from these words: |
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doc-else |
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doc-while |
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doc-repeat |
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Counted loop words constitute a separate group of words: |
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doc-?do |
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doc-do |
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doc-for |
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doc-loop |
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doc-s+loop |
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doc-+loop |
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doc-next |
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doc-leave |
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doc-?leave |
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doc-unloop |
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doc-undo |
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The standard does not allow using @code{cs-pick} and @code{cs-roll} on |
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@i{do-sys}. Our system allows it, but it's your job to ensure that for |
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every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path |
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through the definition (@code{LOOP} etc. compile an @code{UNLOOP} on the |
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fall-through path). Also, you have to ensure that all @code{LEAVE}s are |
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resolved (by using one of the loop-ending words or @code{UNDO}). |
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Another group of control structure words are |
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doc-case |
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doc-endcase |
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doc-of |
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doc-endof |
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@i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and |
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@code{cs-roll}. |
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@subsubsection Programming Style |
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In order to ensure readability we recommend that you do not create |
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arbitrary control structures directly, but define new control structure |
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words for the control structure you want and use these words in your |
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program. |
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E.g., instead of writing |
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@example |
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begin |
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... |
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if [ 1 cs-roll ] |
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... |
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again then |
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@end example |
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we recommend defining control structure words, e.g., |
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@example |
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: while ( dest -- orig dest ) |
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POSTPONE if |
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1 cs-roll ; immediate |
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: repeat ( orig dest -- ) |
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POSTPONE again |
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POSTPONE then ; immediate |
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@end example |
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and then using these to create the control structure: |
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@example |
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begin |
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... |
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while |
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... |
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repeat |
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@end example |
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That's much easier to read, isn't it? Of course, @code{BEGIN} and |
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@code{WHILE} are predefined, so in this example it would not be |
|
necessary to define them. |
|
|
|
@node Calls and returns, Exception Handling, Arbitrary control structures, Control Structures |
|
@subsection Calls and returns |
|
|
|
A definition can be called simply be writing the name of the |
|
definition. When the end of the definition is reached, it returns. An earlier return can be forced using |
|
|
|
doc-exit |
|
|
|
Don't forget to clean up the return stack and @code{UNLOOP} any |
|
outstanding @code{?DO}...@code{LOOP}s before @code{EXIT}ing. The |
|
primitive compiled by @code{EXIT} is |
|
|
|
doc-;s |
|
|
|
@node Exception Handling, , Calls and returns, Control Structures |
|
@subsection Exception Handling |
|
|
|
doc-catch |
|
doc-throw |
|
|
|
@node Locals, Defining Words, Control Structures, Words |
@section Locals |
@section Locals |
|
|
|
Local variables can make Forth programming more enjoyable and Forth |
|
programs easier to read. Unfortunately, the locals of ANS Forth are |
|
laden with restrictions. Therefore, we provide not only the ANS Forth |
|
locals wordset, but also our own, more powerful locals wordset (we |
|
implemented the ANS Forth locals wordset through our locals wordset). |
|
|
|
@menu |
|
* gforth locals:: |
|
* ANS Forth locals:: |
|
@end menu |
|
|
|
@node gforth locals, ANS Forth locals, Locals, Locals |
|
@subsection gforth locals |
|
|
|
Locals can be defined with |
|
|
|
@example |
|
@{ local1 local2 ... -- comment @} |
|
@end example |
|
or |
|
@example |
|
@{ local1 local2 ... @} |
|
@end example |
|
|
|
E.g., |
|
@example |
|
: max @{ n1 n2 -- n3 @} |
|
n1 n2 > if |
|
n1 |
|
else |
|
n2 |
|
endif ; |
|
@end example |
|
|
|
The similarity of locals definitions with stack comments is intended. A |
|
locals definition often replaces the stack comment of a word. The order |
|
of the locals corresponds to the order in a stack comment and everything |
|
after the @code{--} is really a comment. |
|
|
|
This similarity has one disadvantage: It is too easy to confuse locals |
|
declarations with stack comments, causing bugs and making them hard to |
|
find. However, this problem can be avoided by appropriate coding |
|
conventions: Do not use both notations in the same program. If you do, |
|
they should be distinguished using additional means, e.g. by position. |
|
|
|
The name of the local may be preceded by a type specifier, e.g., |
|
@code{F:} for a floating point value: |
|
|
|
@example |
|
: CX* @{ F: Ar F: Ai F: Br F: Bi -- Cr Ci @} |
|
\ complex multiplication |
|
Ar Br f* Ai Bi f* f- |
|
Ar Bi f* Ai Br f* f+ ; |
|
@end example |
|
|
|
GNU Forth currently supports cells (@code{W:}, @code{W^}), doubles |
|
(@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters |
|
(@code{C:}, @code{C^}) in two flavours: a value-flavoured local (defined |
|
with @code{W:}, @code{D:} etc.) produces its value and can be changed |
|
with @code{TO}. A variable-flavoured local (defined with @code{W^} etc.) |
|
produces its address (which becomes invalid when the variable's scope is |
|
left). E.g., the standard word @code{emit} can be defined in therms of |
|
@code{type} like this: |
|
|
|
@example |
|
: emit @{ C^ char* -- @} |
|
char* 1 type ; |
|
@end example |
|
|
|
A local without type specifier is a @code{W:} local. Both flavours of |
|
locals are initialized with values from the data or FP stack. |
|
|
|
Currently there is no way to define locals with user-defined data |
|
structures, but we are working on it. |
|
|
|
GNU Forth allows defining locals everywhere in a colon definition. This poses the following questions: |
|
|
|
@menu |
|
* Where are locals visible by name?:: |
|
* How long do locals live? :: |
|
* Programming Style:: |
|
* Implementation:: |
|
@end menu |
|
|
|
@node Where are locals visible by name?, How long do locals live?, gforth locals, gforth locals |
|
@subsubsection Where are locals visible by name? |
|
|
|
Basically, the answer is that locals are visible where you would expect |
|
it in block-structured languages, and sometimes a little longer. If you |
|
want to restrict the scope of a local, enclose its definition in |
|
@code{SCOPE}...@code{ENDSCOPE}. |
|
|
|
doc-scope |
|
doc-endscope |
|
|
|
These words behave like control structure words, so you can use them |
|
with @code{CS-PICK} and @code{CS-ROLL} to restrict the scope in |
|
arbitrary ways. |
|
|
|
If you want a more exact answer to the visibility question, here's the |
|
basic principle: A local is visible in all places that can only be |
|
reached through the definition of the local@footnote{In compiler |
|
construction terminology, all places dominated by the definition of the |
|
local.}. In other words, it is not visible in places that can be reached |
|
without going through the definition of the local. E.g., locals defined |
|
in @code{IF}...@code{ENDIF} are visible until the @code{ENDIF}, locals |
|
defined in @code{BEGIN}...@code{UNTIL} are visible after the |
|
@code{UNTIL} (until, e.g., a subsequent @code{ENDSCOPE}). |
|
|
|
The reasoning behind this solution is: We want to have the locals |
|
visible as long as it is meaningful. The user can always make the |
|
visibility shorter by using explicit scoping. In a place that can |
|
only be reached through the definition of a local, the meaning of a |
|
local name is clear. In other places it is not: How is the local |
|
initialized at the control flow path that does not contain the |
|
definition? Which local is meant, if the same name is defined twice in |
|
two independent control flow paths? |
|
|
|
This should be enough detail for nearly all users, so you can skip the |
|
rest of this section. If you relly must know all the gory details and |
|
options, read on. |
|
|
|
In order to implement this rule, the compiler has to know which places |
|
are unreachable. It knows this automatically after @code{AHEAD}, |
|
@code{AGAIN}, @code{EXIT} and @code{LEAVE}; in other cases (e.g., after |
|
most @code{THROW}s), you can use the word @code{UNREACHABLE} to tell the |
|
compiler that the control flow never reaches that place. If |
|
@code{UNREACHABLE} is not used where it could, the only consequence is |
|
that the visibility of some locals is more limited than the rule above |
|
says. If @code{UNREACHABLE} is used where it should not (i.e., if you |
|
lie to the compiler), buggy code will be produced. |
|
|
|
Another problem with this rule is that at @code{BEGIN}, the compiler |
|
does not know which locals will be visible on the incoming |
|
back-edge. All problems discussed in the following are due to this |
|
ignorance of the compiler (we discuss the problems using @code{BEGIN} |
|
loops as examples; the discussion also applies to @code{?DO} and other |
|
loops). Perhaps the most insidious example is: |
|
@example |
|
AHEAD |
|
BEGIN |
|
x |
|
[ 1 CS-ROLL ] THEN |
|
@{ x @} |
|
... |
|
UNTIL |
|
@end example |
|
|
|
This should be legal according to the visibility rule. The use of |
|
@code{x} can only be reached through the definition; but that appears |
|
textually below the use. |
|
|
|
From this example it is clear that the visibility rules cannot be fully |
|
implemented without major headaches. Our implementation treats common |
|
cases as advertised and the exceptions are treated in a safe way: The |
|
compiler makes a reasonable guess about the locals visible after a |
|
@code{BEGIN}; if it is too pessimistic, the |
|
user will get a spurious error about the local not being defined; if the |
|
compiler is too optimistic, it will notice this later and issue a |
|
warning. In the case above the compiler would complain about @code{x} |
|
being undefined at its use. You can see from the obscure examples in |
|
this section that it takes quite unusual control structures to get the |
|
compiler into trouble, and even then it will often do fine. |
|
|
|
If the @code{BEGIN} is reachable from above, the most optimistic guess |
|
is that all locals visible before the @code{BEGIN} will also be |
|
visible after the @code{BEGIN}. This guess is valid for all loops that |
|
are entered only through the @code{BEGIN}, in particular, for normal |
|
@code{BEGIN}...@code{WHILE}...@code{REPEAT} and |
|
@code{BEGIN}...@code{UNTIL} loops and it is implemented in our |
|
compiler. When the branch to the @code{BEGIN} is finally generated by |
|
@code{AGAIN} or @code{UNTIL}, the compiler checks the guess and |
|
warns the user if it was too optimisitic: |
|
@example |
|
IF |
|
@{ x @} |
|
BEGIN |
|
\ x ? |
|
[ 1 cs-roll ] THEN |
|
... |
|
UNTIL |
|
@end example |
|
|
|
Here, @code{x} lives only until the @code{BEGIN}, but the compiler |
|
optimistically assumes that it lives until the @code{THEN}. It notices |
|
this difference when it compiles the @code{UNTIL} and issues a |
|
warning. The user can avoid the warning, and make sure that @code{x} |
|
is not used in the wrong area by using explicit scoping: |
|
@example |
|
IF |
|
SCOPE |
|
@{ x @} |
|
ENDSCOPE |
|
BEGIN |
|
[ 1 cs-roll ] THEN |
|
... |
|
UNTIL |
|
@end example |
|
|
|
Since the guess is optimistic, there will be no spurious error messages |
|
about undefined locals. |
|
|
|
If the @code{BEGIN} is not reachable from above (e.g., after |
|
@code{AHEAD} or @code{EXIT}), the compiler cannot even make an |
|
optimistic guess, as the locals visible after the @code{BEGIN} may be |
|
defined later. Therefore, the compiler assumes that no locals are |
|
visible after the @code{BEGIN}. However, the useer can use |
|
@code{ASSUME-LIVE} to make the compiler assume that the same locals are |
|
visible at the BEGIN as at the point where the item was created. |
|
|
|
doc-assume-live |
|
|
|
E.g., |
|
@example |
|
@{ x @} |
|
AHEAD |
|
ASSUME-LIVE |
|
BEGIN |
|
x |
|
[ 1 CS-ROLL ] THEN |
|
... |
|
UNTIL |
|
@end example |
|
|
|
Other cases where the locals are defined before the @code{BEGIN} can be |
|
handled by inserting an appropriate @code{CS-ROLL} before the |
|
@code{ASSUME-LIVE} (and changing the control-flow stack manipulation |
|
behind the @code{ASSUME-LIVE}). |
|
|
|
Cases where locals are defined after the @code{BEGIN} (but should be |
|
visible immediately after the @code{BEGIN}) can only be handled by |
|
rearranging the loop. E.g., the ``most insidious'' example above can be |
|
arranged into: |
|
@example |
|
BEGIN |
|
@{ x @} |
|
... 0= |
|
WHILE |
|
x |
|
REPEAT |
|
@end example |
|
|
|
@node How long do locals live?, Programming Style, Where are locals visible by name?, gforth locals |
|
@subsubsection How long do locals live? |
|
|
|
The right answer for the lifetime question would be: A local lives at |
|
least as long as it can be accessed. For a value-flavoured local this |
|
means: until the end of its visibility. However, a variable-flavoured |
|
local could be accessed through its address far beyond its visibility |
|
scope. Ultimately, this would mean that such locals would have to be |
|
garbage collected. Since this entails un-Forth-like implementation |
|
complexities, I adopted the same cowardly solution as some other |
|
languages (e.g., C): The local lives only as long as it is visible; |
|
afterwards its address is invalid (and programs that access it |
|
afterwards are erroneous). |
|
|
|
@node Programming Style, Implementation, How long do locals live?, gforth locals |
|
@subsubsection Programming Style |
|
|
|
The freedom to define locals anywhere has the potential to change |
|
programming styles dramatically. In particular, the need to use the |
|
return stack for intermediate storage vanishes. Moreover, all stack |
|
manipulations (except @code{PICK}s and @code{ROLL}s with run-time |
|
determined arguments) can be eliminated: If the stack items are in the |
|
wrong order, just write a locals definition for all of them; then |
|
write the items in the order you want. |
|
|
|
This seems a little far-fetched and eliminating stack manipulations is |
|
unlikely to become a conscious programming objective. Still, the number |
|
of stack manipulations will be reduced dramatically if local variables |
|
are used liberally (e.g., compare @code{max} in @ref{gforth locals} with |
|
a traditional implementation of @code{max}). |
|
|
|
This shows one potential benefit of locals: making Forth programs more |
|
readable. Of course, this benefit will only be realized if the |
|
programmers continue to honour the principle of factoring instead of |
|
using the added latitude to make the words longer. |
|
|
|
Using @code{TO} can and should be avoided. Without @code{TO}, |
|
every value-flavoured local has only a single assignment and many |
|
advantages of functional languages apply to Forth. I.e., programs are |
|
easier to analyse, to optimize and to read: It is clear from the |
|
definition what the local stands for, it does not turn into something |
|
different later. |
|
|
|
E.g., a definition using @code{TO} might look like this: |
|
@example |
|
: strcmp @{ addr1 u1 addr2 u2 -- n @} |
|
u1 u2 min 0 |
|
?do |
|
addr1 c@ addr2 c@ - ?dup |
|
if |
|
unloop exit |
|
then |
|
addr1 char+ TO addr1 |
|
addr2 char+ TO addr2 |
|
loop |
|
u1 u2 - ; |
|
@end example |
|
Here, @code{TO} is used to update @code{addr1} and @code{addr2} at |
|
every loop iteration. @code{strcmp} is a typical example of the |
|
readability problems of using @code{TO}. When you start reading |
|
@code{strcmp}, you think that @code{addr1} refers to the start of the |
|
string. Only near the end of the loop you realize that it is something |
|
else. |
|
|
|
This can be avoided by defining two locals at the start of the loop that |
|
are initialized with the right value for the current iteration. |
|
@example |
|
: strcmp @{ addr1 u1 addr2 u2 -- n @} |
|
addr1 addr2 |
|
u1 u2 min 0 |
|
?do @{ s1 s2 @} |
|
s1 c@ s2 c@ - ?dup |
|
if |
|
unloop exit |
|
then |
|
s1 char+ s2 char+ |
|
loop |
|
2drop |
|
u1 u2 - ; |
|
@end example |
|
Here it is clear from the start that @code{s1} has a different value |
|
in every loop iteration. |
|
|
|
@node Implementation, , Programming Style, gforth locals |
|
@subsubsection Implementation |
|
|
|
GNU Forth uses an extra locals stack. The most compelling reason for |
|
this is that the return stack is not float-aligned; using an extra stack |
|
also eliminates the problems and restrictions of using the return stack |
|
as locals stack. Like the other stacks, the locals stack grows toward |
|
lower addresses. A few primitives allow an efficient implementation: |
|
|
|
doc-@local# |
|
doc-f@local# |
|
doc-laddr# |
|
doc-lp+!# |
|
doc-lp! |
|
doc->l |
|
doc-f>l |
|
|
|
In addition to these primitives, some specializations of these |
|
primitives for commonly occurring inline arguments are provided for |
|
efficiency reasons, e.g., @code{@@local0} as specialization of |
|
@code{@@local#} for the inline argument 0. The following compiling words |
|
compile the right specialized version, or the general version, as |
|
appropriate: |
|
|
|
doc-compile-@@local |
|
doc-compile-f@@local |
|
doc-compile-lp+! |
|
|
|
Combinations of conditional branches and @code{lp+!#} like |
|
@code{?branch-lp+!#} (the locals pointer is only changed if the branch |
|
is taken) are provided for efficiency and correctness in loops. |
|
|
|
A special area in the dictionary space is reserved for keeping the |
|
local variable names. @code{@{} switches the dictionary pointer to this |
|
area and @code{@}} switches it back and generates the locals |
|
initializing code. @code{W:} etc.@ are normal defining words. This |
|
special area is cleared at the start of every colon definition. |
|
|
|
A special feature of GNU Forths dictionary is used to implement the |
|
definition of locals without type specifiers: every wordlist (aka |
|
vocabulary) has its own methods for searching |
|
etc. (@pxref{Wordlists}). For the present purpose we defined a wordlist |
|
with a special search method: When it is searched for a word, it |
|
actually creates that word using @code{W:}. @code{@{} changes the search |
|
order to first search the wordlist containing @code{@}}, @code{W:} etc., |
|
and then the wordlist for defining locals without type specifiers. |
|
|
|
The lifetime rules support a stack discipline within a colon |
|
definition: The lifetime of a local is either nested with other locals |
|
lifetimes or it does not overlap them. |
|
|
|
At @code{BEGIN}, @code{IF}, and @code{AHEAD} no code for locals stack |
|
pointer manipulation is generated. Between control structure words |
|
locals definitions can push locals onto the locals stack. @code{AGAIN} |
|
is the simplest of the other three control flow words. It has to |
|
restore the locals stack depth of the corresponding @code{BEGIN} |
|
before branching. The code looks like this: |
|
@format |
|
@code{lp+!#} current-locals-size @minus{} dest-locals-size |
|
@code{branch} <begin> |
|
@end format |
|
|
|
@code{UNTIL} is a little more complicated: If it branches back, it |
|
must adjust the stack just like @code{AGAIN}. But if it falls through, |
|
the locals stack must not be changed. The compiler generates the |
|
following code: |
|
@format |
|
@code{?branch-lp+!#} <begin> current-locals-size @minus{} dest-locals-size |
|
@end format |
|
The locals stack pointer is only adjusted if the branch is taken. |
|
|
|
@code{THEN} can produce somewhat inefficient code: |
|
@format |
|
@code{lp+!#} current-locals-size @minus{} orig-locals-size |
|
<orig target>: |
|
@code{lp+!#} orig-locals-size @minus{} new-locals-size |
|
@end format |
|
The second @code{lp+!#} adjusts the locals stack pointer from the |
|
level at the @var{orig} point to the level after the @code{THEN}. The |
|
first @code{lp+!#} adjusts the locals stack pointer from the current |
|
level to the level at the orig point, so the complete effect is an |
|
adjustment from the current level to the right level after the |
|
@code{THEN}. |
|
|
|
In a conventional Forth implementation a dest control-flow stack entry |
|
is just the target address and an orig entry is just the address to be |
|
patched. Our locals implementation adds a wordlist to every orig or dest |
|
item. It is the list of locals visible (or assumed visible) at the point |
|
described by the entry. Our implementation also adds a tag to identify |
|
the kind of entry, in particular to differentiate between live and dead |
|
(reachable and unreachable) orig entries. |
|
|
|
A few unusual operations have to be performed on locals wordlists: |
|
|
|
doc-common-list |
|
doc-sub-list? |
|
doc-list-size |
|
|
|
Several features of our locals wordlist implementation make these |
|
operations easy to implement: The locals wordlists are organised as |
|
linked lists; the tails of these lists are shared, if the lists |
|
contain some of the same locals; and the address of a name is greater |
|
than the address of the names behind it in the list. |
|
|
|
Another important implementation detail is the variable |
|
@code{dead-code}. It is used by @code{BEGIN} and @code{THEN} to |
|
determine if they can be reached directly or only through the branch |
|
that they resolve. @code{dead-code} is set by @code{UNREACHABLE}, |
|
@code{AHEAD}, @code{EXIT} etc., and cleared at the start of a colon |
|
definition, by @code{BEGIN} and usually by @code{THEN}. |
|
|
|
Counted loops are similar to other loops in most respects, but |
|
@code{LEAVE} requires special attention: It performs basically the same |
|
service as @code{AHEAD}, but it does not create a control-flow stack |
|
entry. Therefore the information has to be stored elsewhere; |
|
traditionally, the information was stored in the target fields of the |
|
branches created by the @code{LEAVE}s, by organizing these fields into a |
|
linked list. Unfortunately, this clever trick does not provide enough |
|
space for storing our extended control flow information. Therefore, we |
|
introduce another stack, the leave stack. It contains the control-flow |
|
stack entries for all unresolved @code{LEAVE}s. |
|
|
|
Local names are kept until the end of the colon definition, even if |
|
they are no longer visible in any control-flow path. In a few cases |
|
this may lead to increased space needs for the locals name area, but |
|
usually less than reclaiming this space would cost in code size. |
|
|
|
|
|
@node ANS Forth locals, , gforth locals, Locals |
|
@subsection ANS Forth locals |
|
|
|
The ANS Forth locals wordset does not define a syntax for locals, but |
|
words that make it possible to define various syntaxes. One of the |
|
possible syntaxes is a subset of the syntax we used in the gforth locals |
|
wordset, i.e.: |
|
|
|
@example |
|
@{ local1 local2 ... -- comment @} |
|
@end example |
|
or |
|
@example |
|
@{ local1 local2 ... @} |
|
@end example |
|
|
|
The order of the locals corresponds to the order in a stack comment. The |
|
restrictions are: |
|
|
|
@itemize @bullet |
|
@item |
|
Locals can only be cell-sized values (no type specifers are allowed). |
|
@item |
|
Locals can be defined only outside control structures. |
|
@item |
|
Locals can interfere with explicit usage of the return stack. For the |
|
exact (and long) rules, see the standard. If you don't use return stack |
|
accessing words in a definition using locals, you will we all right. The |
|
purpose of this rule is to make locals implementation on the return |
|
stack easier. |
|
@item |
|
The whole definition must be in one line. |
|
@end itemize |
|
|
|
Locals defined in this way behave like @code{VALUE}s |
|
(@xref{Values}). I.e., they are initialized from the stack. Using their |
|
name produces their value. Their value can be changed using @code{TO}. |
|
|
|
Since this syntax is supported by gforth directly, you need not do |
|
anything to use it. If you want to port a program using this syntax to |
|
another ANS Forth system, use @file{anslocal.fs} to implement the syntax |
|
on the other system. |
|
|
|
Note that a syntax shown in the standard, section A.13 looks |
|
similar, but is quite different in having the order of locals |
|
reversed. Beware! |
|
|
|
The ANS Forth locals wordset itself consists of the following word |
|
|
|
doc-(local) |
|
|
|
The ANS Forth locals extension wordset defines a syntax, but it is so |
|
awful that we strongly recommend not to use it. We have implemented this |
|
syntax to make porting to gforth easy, but do not document it here. The |
|
problem with this syntax is that the locals are defined in an order |
|
reversed with respect to the standard stack comment notation, making |
|
programs harder to read, and easier to misread and miswrite. The only |
|
merit of this syntax is that it is easy to implement using the ANS Forth |
|
locals wordset. |
|
|
|
@node Defining Words, Wordlists, Locals, Words |
|
@section Defining Words |
|
|
|
@node Values, , Defining Words, Defining Words |
|
@subsection Values |
|
|
|
@node Wordlists, Files, Defining Words, Words |
|
@section Wordlists |
|
|
|
@node Files, Blocks, Wordlists, Words |
|
@section Files |
|
|
|
@node Blocks, Other I/O, Files, Words |
|
@section Blocks |
|
|
|
@node Other I/O, Programming Tools, Blocks, Words |
|
@section Other I/O |
|
|
|
@node Programming Tools, Threading Words, Other I/O, Words |
|
@section Programming Tools |
|
|
|
@menu |
|
* Debugging:: Simple and quick. |
|
* Assertions:: Making your programs self-checking. |
|
@end menu |
|
|
|
@node Debugging, Assertions, Programming Tools, Programming Tools |
|
@subsection Debugging |
|
|
|
The simple debugging aids provided in @file{debugging.fs} |
|
are meant to support a different style of debugging than the |
|
tracing/stepping debuggers used in languages with long turn-around |
|
times. |
|
|
|
A much better (faster) way in fast-compilig languages is to add |
|
printing code at well-selected places, let the program run, look at |
|
the output, see where things went wrong, add more printing code, etc., |
|
until the bug is found. |
|
|
|
The word @code{~~} is easy to insert. It just prints debugging |
|
information (by default the source location and the stack contents). It |
|
is also easy to remove (@kbd{C-x ~} in the Emacs Forth mode to |
|
query-replace them with nothing). The deferred words |
|
@code{printdebugdata} and @code{printdebugline} control the output of |
|
@code{~~}. The default source location output format works well with |
|
Emacs' compilation mode, so you can step through the program at the |
|
source level using @kbd{C-x `} (the advantage over a stepping debugger |
|
is that you can step in any direction and you know where the crash has |
|
happened or where the strange data has occurred). |
|
|
|
Note that the default actions clobber the contents of the pictured |
|
numeric output string, so you should not use @code{~~}, e.g., between |
|
@code{<#} and @code{#>}. |
|
|
|
doc-~~ |
|
doc-printdebugdata |
|
doc-printdebugline |
|
|
|
@node Assertions, , Debugging, Programming Tools |
|
@subsection Assertions |
|
|
|
It is a good idea to make your programs self-checking, in particular, if |
|
you use an assumption (e.g., that a certain field of a data structure is |
|
never zero) that may become wrong during maintenance. GForth supports |
|
assertions for this purpose. They are used like this: |
|
|
|
@example |
|
assert( @var{flag} ) |
|
@end example |
|
|
|
The code between @code{assert(} and @code{)} should compute a flag, that |
|
should be true if everything is alright and false otherwise. It should |
|
not change anything else on the stack. The overall stack effect of the |
|
assertion is @code{( -- )}. E.g. |
|
|
|
@example |
|
assert( 1 1 + 2 = ) \ what we learn in school |
|
assert( dup 0<> ) \ assert that the top of stack is not zero |
|
assert( false ) \ this code should not be reached |
|
@end example |
|
|
|
The need for assertions is different at different times. During |
|
debugging, we want more checking, in production we sometimes care more |
|
for speed. Therefore, assertions can be turned off, i.e., the assertion |
|
becomes a comment. Depending on the importance of an assertion and the |
|
time it takes to check it, you may want to turn off some assertions and |
|
keep others turned on. GForth provides several levels of assertions for |
|
this purpose: |
|
|
|
doc-assert0( |
|
doc-assert1( |
|
doc-assert2( |
|
doc-assert3( |
|
doc-assert( |
|
doc-) |
|
|
|
@code{Assert(} is the same as @code{assert1(}. The variable |
|
@code{assert-level} specifies the highest assertions that are turned |
|
on. I.e., at the default @code{assert-level} of one, @code{assert0(} and |
|
@code{assert1(} assertions perform checking, while @code{assert2(} and |
|
@code{assert3(} assertions are treated as comments. |
|
|
|
Note that the @code{assert-level} is evaluated at compile-time, not at |
|
run-time. I.e., you cannot turn assertions on or off at run-time, you |
|
have to set the @code{assert-level} appropriately before compiling a |
|
piece of code. You can compile several pieces of code at several |
|
@code{assert-level}s (e.g., a trusted library at level 1 and newly |
|
written code at level 3). |
|
|
|
doc-assert-level |
|
|
|
If an assertion fails, a message compatible with Emacs' compilation mode |
|
is produced and the execution is aborted (currently with @code{ABORT"}. |
|
If there is interest, we will introduce a special throw code. But if you |
|
intend to @code{catch} a specific condition, using @code{throw} is |
|
probably more appropriate than an assertion). |
|
|
|
@node Threading Words, , Programming Tools, Words |
|
@section Threading Words |
|
|
|
These words provide access to code addresses and other threading stuff |
|
in gforth (and, possibly, other interpretive Forths). It more or less |
|
abstracts away the differences between direct and indirect threading |
|
(and, for direct threading, the machine dependences). However, at |
|
present this wordset is still inclomplete. It is also pretty low-level; |
|
some day it will hopefully be made unnecessary by an internals words set |
|
that abstracts implementation details away completely. |
|
|
|
doc->code-address |
|
doc->does-code |
|
doc-code-address! |
|
doc-does-code! |
|
doc-does-handler! |
|
doc-/does-handler |
|
|
|
@node ANS conformance, Model, Words, Top |
|
@chapter ANS conformance |
|
|
|
@node Model, Emacs and GForth, ANS conformance, Top |
|
@chapter Model |
|
|
|
@node Emacs and GForth, Internals, Model, Top |
|
@chapter Emacs and GForth |
|
|
|
GForth comes with @file{gforth.el}, an improved version of |
|
@file{forth.el} by Goran Rydqvist (icluded in the TILE package). The |
|
improvements are a better (but still not perfect) handling of |
|
indentation. I have also added comment paragraph filling (@kbd{M-q}), |
|
commenting (@kbd{C-x \}) and uncommenting (@kbd{C-x |}) regions and |
|
removing debugging tracers (@kbd{C-x ~}). I left the stuff I do not use |
|
alone, even though some of it only makes sense for TILE. To get a |
|
description of these features, enter Forth mode and type @kbd{C-h m}. |
|
|
|
In addition, GForth supports Emacs quite well: The source code locations |
|
given in error messages, debugging output (from @code{~~}) and failed |
|
assertion messages are in the right format for Emacs' compilation mode |
|
(@pxref{Compilation, , Running Compilations under Emacs, emacs, Emacs |
|
Manual}) so the source location corresponding to an error or other |
|
message is only a few keystrokes away (@kbd{C-x `} for the next error, |
|
@kbd{C-c C-c} for the error under the cursor). |
|
|
|
Also, if you @code{include} @file{etags.fs}, a new @file{TAGS} file |
|
(@pxref{Tags, , Tags Tables, emacs, Emacs Manual}) will be produced that |
|
contains the definitions of all words defined afterwards. You can then |
|
find the source for a word using @kbd{M-.}. Note that emacs can use |
|
several tags files at the same time (e.g., one for the gforth sources |
|
and one for your program). |
|
|
|
To get all these benefits, add the following lines to your @file{.emacs} |
|
file: |
|
|
|
@example |
|
(autoload 'forth-mode "gforth.el") |
|
(setq auto-mode-alist (cons '("\\.fs\\'" . forth-mode) auto-mode-alist)) |
|
@end example |
|
|
|
@node Internals, Bugs, Emacs and GForth, Top |
|
@chapter Internals |
|
|
|
Reading this section is not necessary for programming with gforth. It |
|
should be helpful for finding your way in the gforth sources. |
|
|
|
@menu |
|
* Portability:: |
|
* Threading:: |
|
* Primitives:: |
|
* System Architecture:: |
|
@end menu |
|
|
|
@node Portability, Threading, Internals, Internals |
|
@section Portability |
|
|
|
One of the main goals of the effort is availability across a wide range |
|
of personal machines. fig-Forth, and, to a lesser extent, F83, achieved |
|
this goal by manually coding the engine in assembly language for several |
|
then-popular processors. This approach is very labor-intensive and the |
|
results are short-lived due to progress in computer architecture. |
|
|
|
Others have avoided this problem by coding in C, e.g., Mitch Bradley |
|
(cforth), Mikael Patel (TILE) and Dirk Zoller (pfe). This approach is |
|
particularly popular for UNIX-based Forths due to the large variety of |
|
architectures of UNIX machines. Unfortunately an implementation in C |
|
does not mix well with the goals of efficiency and with using |
|
traditional techniques: Indirect or direct threading cannot be expressed |
|
in C, and switch threading, the fastest technique available in C, is |
|
significantly slower. Another problem with C is that it's very |
|
cumbersome to express double integer arithmetic. |
|
|
|
Fortunately, there is a portable language that does not have these |
|
limitations: GNU C, the version of C processed by the GNU C compiler |
|
(@pxref{C Extensions, , Extensions to the C Language Family, gcc.info, |
|
GNU C Manual}). Its labels as values feature (@pxref{Labels as Values, , |
|
Labels as Values, gcc.info, GNU C Manual}) makes direct and indirect |
|
threading possible, its @code{long long} type (@pxref{Long Long, , |
|
Double-Word Integers, gcc.info, GNU C Manual}) corresponds to Forths |
|
double numbers. GNU C is available for free on all important (and many |
|
unimportant) UNIX machines, VMS, 80386s running MS-DOS, the Amiga, and |
|
the Atari ST, so a Forth written in GNU C can run on all these |
|
machines@footnote{Due to Apple's look-and-feel lawsuit it is not |
|
available on the Mac (@pxref{Boycott, , Protect Your Freedom---Fight |
|
``Look And Feel'', gcc.info, GNU C Manual}).}. |
|
|
|
Writing in a portable language has the reputation of producing code that |
|
is slower than assembly. For our Forth engine we repeatedly looked at |
|
the code produced by the compiler and eliminated most compiler-induced |
|
inefficiencies by appropriate changes in the source-code. |
|
|
|
However, register allocation cannot be portably influenced by the |
|
programmer, leading to some inefficiencies on register-starved |
|
machines. We use explicit register declarations (@pxref{Explicit Reg |
|
Vars, , Variables in Specified Registers, gcc.info, GNU C Manual}) to |
|
improve the speed on some machines. They are turned on by using the |
|
@code{gcc} switch @code{-DFORCE_REG}. Unfortunately, this feature not |
|
only depends on the machine, but also on the compiler version: On some |
|
machines some compiler versions produce incorrect code when certain |
|
explicit register declarations are used. So by default |
|
@code{-DFORCE_REG} is not used. |
|
|
|
@node Threading, Primitives, Portability, Internals |
|
@section Threading |
|
|
|
GNU C's labels as values extension (available since @code{gcc-2.0}, |
|
@pxref{Labels as Values, , Labels as Values, gcc.info, GNU C Manual}) |
|
makes it possible to take the address of @var{label} by writing |
|
@code{&&@var{label}}. This address can then be used in a statement like |
|
@code{goto *@var{address}}. I.e., @code{goto *&&x} is the same as |
|
@code{goto x}. |
|
|
|
With this feature an indirect threaded NEXT looks like: |
|
@example |
|
cfa = *ip++; |
|
ca = *cfa; |
|
goto *ca; |
|
@end example |
|
For those unfamiliar with the names: @code{ip} is the Forth instruction |
|
pointer; the @code{cfa} (code-field address) corresponds to ANS Forths |
|
execution token and points to the code field of the next word to be |
|
executed; The @code{ca} (code address) fetched from there points to some |
|
executable code, e.g., a primitive or the colon definition handler |
|
@code{docol}. |
|
|
|
Direct threading is even simpler: |
|
@example |
|
ca = *ip++; |
|
goto *ca; |
|
@end example |
|
|
|
Of course we have packaged the whole thing neatly in macros called |
|
@code{NEXT} and @code{NEXT1} (the part of NEXT after fetching the cfa). |
|
|
|
@menu |
|
* Scheduling:: |
|
* Direct or Indirect Threaded?:: |
|
* DOES>:: |
|
@end menu |
|
|
|
@node Scheduling, Direct or Indirect Threaded?, Threading, Threading |
|
@subsection Scheduling |
|
|
|
There is a little complication: Pipelined and superscalar processors, |
|
i.e., RISC and some modern CISC machines can process independent |
|
instructions while waiting for the results of an instruction. The |
|
compiler usually reorders (schedules) the instructions in a way that |
|
achieves good usage of these delay slots. However, on our first tries |
|
the compiler did not do well on scheduling primitives. E.g., for |
|
@code{+} implemented as |
|
@example |
|
n=sp[0]+sp[1]; |
|
sp++; |
|
sp[0]=n; |
|
NEXT; |
|
@end example |
|
the NEXT comes strictly after the other code, i.e., there is nearly no |
|
scheduling. After a little thought the problem becomes clear: The |
|
compiler cannot know that sp and ip point to different addresses (and |
|
the version of @code{gcc} we used would not know it even if it was |
|
possible), so it could not move the load of the cfa above the store to |
|
the TOS. Indeed the pointers could be the same, if code on or very near |
|
the top of stack were executed. In the interest of speed we chose to |
|
forbid this probably unused ``feature'' and helped the compiler in |
|
scheduling: NEXT is divided into the loading part (@code{NEXT_P1}) and |
|
the goto part (@code{NEXT_P2}). @code{+} now looks like: |
|
@example |
|
n=sp[0]+sp[1]; |
|
sp++; |
|
NEXT_P1; |
|
sp[0]=n; |
|
NEXT_P2; |
|
@end example |
|
This can be scheduled optimally by the compiler. |
|
|
|
This division can be turned off with the switch @code{-DCISC_NEXT}. This |
|
switch is on by default on machines that do not profit from scheduling |
|
(e.g., the 80386), in order to preserve registers. |
|
|
|
@node Direct or Indirect Threaded?, DOES>, Scheduling, Threading |
|
@subsection Direct or Indirect Threaded? |
|
|
|
Both! After packaging the nasty details in macro definitions we |
|
realized that we could switch between direct and indirect threading by |
|
simply setting a compilation flag (@code{-DDIRECT_THREADED}) and |
|
defining a few machine-specific macros for the direct-threading case. |
|
On the Forth level we also offer access words that hide the |
|
differences between the threading methods (@pxref{Threading Words}). |
|
|
|
Indirect threading is implemented completely |
|
machine-independently. Direct threading needs routines for creating |
|
jumps to the executable code (e.g. to docol or dodoes). These routines |
|
are inherently machine-dependent, but they do not amount to many source |
|
lines. I.e., even porting direct threading to a new machine is a small |
|
effort. |
|
|
|
@node DOES>, , Direct or Indirect Threaded?, Threading |
|
@subsection DOES> |
|
One of the most complex parts of a Forth engine is @code{dodoes}, i.e., |
|
the chunk of code executed by every word defined by a |
|
@code{CREATE}...@code{DOES>} pair. The main problem here is: How to find |
|
the Forth code to be executed, i.e. the code after the @code{DOES>} (the |
|
DOES-code)? There are two solutions: |
|
|
|
In fig-Forth the code field points directly to the dodoes and the |
|
DOES-code address is stored in the cell after the code address |
|
(i.e. at cfa cell+). It may seem that this solution is illegal in the |
|
Forth-79 and all later standards, because in fig-Forth this address |
|
lies in the body (which is illegal in these standards). However, by |
|
making the code field larger for all words this solution becomes legal |
|
again. We use this approach for the indirect threaded version. Leaving |
|
a cell unused in most words is a bit wasteful, but on the machines we |
|
are targetting this is hardly a problem. The other reason for having a |
|
code field size of two cells is to avoid having different image files |
|
for direct and indirect threaded systems (@pxref{System Architecture}). |
|
|
|
The other approach is that the code field points or jumps to the cell |
|
after @code{DOES}. In this variant there is a jump to @code{dodoes} at |
|
this address. @code{dodoes} can then get the DOES-code address by |
|
computing the code address, i.e., the address of the jump to dodoes, |
|
and add the length of that jump field. A variant of this is to have a |
|
call to @code{dodoes} after the @code{DOES>}; then the return address |
|
(which can be found in the return register on RISCs) is the DOES-code |
|
address. Since the two cells available in the code field are usually |
|
used up by the jump to the code address in direct threading, we use |
|
this approach for direct threading. We did not want to add another |
|
cell to the code field. |
|
|
|
@node Primitives, System Architecture, Threading, Internals |
|
@section Primitives |
|
|
|
@menu |
|
* Automatic Generation:: |
|
* TOS Optimization:: |
|
* Produced code:: |
|
@end menu |
|
|
|
@node Automatic Generation, TOS Optimization, Primitives, Primitives |
|
@subsection Automatic Generation |
|
|
|
Since the primitives are implemented in a portable language, there is no |
|
longer any need to minimize the number of primitives. On the contrary, |
|
having many primitives is an advantage: speed. In order to reduce the |
|
number of errors in primitives and to make programming them easier, we |
|
provide a tool, the primitive generator (@file{prims2x.fs}), that |
|
automatically generates most (and sometimes all) of the C code for a |
|
primitive from the stack effect notation. The source for a primitive |
|
has the following form: |
|
|
|
@format |
|
@var{Forth-name} @var{stack-effect} @var{category} [@var{pronounc.}] |
|
[@code{""}@var{glossary entry}@code{""}] |
|
@var{C code} |
|
[@code{:} |
|
@var{Forth code}] |
|
@end format |
|
|
|
The items in brackets are optional. The category and glossary fields |
|
are there for generating the documentation, the Forth code is there |
|
for manual implementations on machines without GNU C. E.g., the source |
|
for the primitive @code{+} is: |
|
@example |
|
+ n1 n2 -- n core plus |
|
n = n1+n2; |
|
@end example |
|
|
|
This looks like a specification, but in fact @code{n = n1+n2} is C |
|
code. Our primitive generation tool extracts a lot of information from |
|
the stack effect notations@footnote{We use a one-stack notation, even |
|
though we have separate data and floating-point stacks; The separate |
|
notation can be generated easily from the unified notation.}: The number |
|
of items popped from and pushed on the stack, their type, and by what |
|
name they are referred to in the C code. It then generates a C code |
|
prelude and postlude for each primitive. The final C code for @code{+} |
|
looks like this: |
|
|
|
@example |
|
I_plus: /* + ( n1 n2 -- n ) */ /* label, stack effect */ |
|
/* */ /* documentation */ |
|
@{ |
|
DEF_CA /* definition of variable ca (indirect threading) */ |
|
Cell n1; /* definitions of variables */ |
|
Cell n2; |
|
Cell n; |
|
n1 = (Cell) sp[1]; /* input */ |
|
n2 = (Cell) TOS; |
|
sp += 1; /* stack adjustment */ |
|
NAME("+") /* debugging output (with -DDEBUG) */ |
|
@{ |
|
n = n1+n2; /* C code taken from the source */ |
|
@} |
|
NEXT_P1; /* NEXT part 1 */ |
|
TOS = (Cell)n; /* output */ |
|
NEXT_P2; /* NEXT part 2 */ |
|
@} |
|
@end example |
|
|
|
This looks long and inefficient, but the GNU C compiler optimizes quite |
|
well and produces optimal code for @code{+} on, e.g., the R3000 and the |
|
HP RISC machines: Defining the @code{n}s does not produce any code, and |
|
using them as intermediate storage also adds no cost. |
|
|
|
There are also other optimizations, that are not illustrated by this |
|
example: Assignments between simple variables are usually for free (copy |
|
propagation). If one of the stack items is not used by the primitive |
|
(e.g. in @code{drop}), the compiler eliminates the load from the stack |
|
(dead code elimination). On the other hand, there are some things that |
|
the compiler does not do, therefore they are performed by |
|
@file{prims2x.fs}: The compiler does not optimize code away that stores |
|
a stack item to the place where it just came from (e.g., @code{over}). |
|
|
|
While programming a primitive is usually easy, there are a few cases |
|
where the programmer has to take the actions of the generator into |
|
account, most notably @code{?dup}, but also words that do not (always) |
|
fall through to NEXT. |
|
|
|
@node TOS Optimization, Produced code, Automatic Generation, Primitives |
|
@subsection TOS Optimization |
|
|
|
An important optimization for stack machine emulators, e.g., Forth |
|
engines, is keeping one or more of the top stack items in |
|
registers. If a word has the stack effect @var{in1}...@var{inx} @code{--} |
|
@var{out1}...@var{outy}, keeping the top @var{n} items in registers |
|
@itemize |
|
@item |
|
is better than keeping @var{n-1} items, if @var{x>=n} and @var{y>=n}, |
|
due to fewer loads from and stores to the stack. |
|
@item is slower than keeping @var{n-1} items, if @var{x<>y} and @var{x<n} and |
|
@var{y<n}, due to additional moves between registers. |
|
@end itemize |
|
|
|
In particular, keeping one item in a register is never a disadvantage, |
|
if there are enough registers. Keeping two items in registers is a |
|
disadvantage for frequent words like @code{?branch}, constants, |
|
variables, literals and @code{i}. Therefore our generator only produces |
|
code that keeps zero or one items in registers. The generated C code |
|
covers both cases; the selection between these alternatives is made at |
|
C-compile time using the switch @code{-DUSE_TOS}. @code{TOS} in the C |
|
code for @code{+} is just a simple variable name in the one-item case, |
|
otherwise it is a macro that expands into @code{sp[0]}. Note that the |
|
GNU C compiler tries to keep simple variables like @code{TOS} in |
|
registers, and it usually succeeds, if there are enough registers. |
|
|
|
The primitive generator performs the TOS optimization for the |
|
floating-point stack, too (@code{-DUSE_FTOS}). For floating-point |
|
operations the benefit of this optimization is even larger: |
|
floating-point operations take quite long on most processors, but can be |
|
performed in parallel with other operations as long as their results are |
|
not used. If the FP-TOS is kept in a register, this works. If |
|
it is kept on the stack, i.e., in memory, the store into memory has to |
|
wait for the result of the floating-point operation, lengthening the |
|
execution time of the primitive considerably. |
|
|
|
The TOS optimization makes the automatic generation of primitives a |
|
bit more complicated. Just replacing all occurrences of @code{sp[0]} by |
|
@code{TOS} is not sufficient. There are some special cases to |
|
consider: |
|
@itemize |
|
@item In the case of @code{dup ( w -- w w )} the generator must not |
|
eliminate the store to the original location of the item on the stack, |
|
if the TOS optimization is turned on. |
|
@item Primitives with stack effects of the form @code{--} |
|
@var{out1}...@var{outy} must store the TOS to the stack at the start. |
|
Likewise, primitives with the stack effect @var{in1}...@var{inx} @code{--} |
|
must load the TOS from the stack at the end. But for the null stack |
|
effect @code{--} no stores or loads should be generated. |
|
@end itemize |
|
|
|
@node Produced code, , TOS Optimization, Primitives |
|
@subsection Produced code |
|
|
|
To see what assembly code is produced for the primitives on your machine |
|
with your compiler and your flag settings, type @code{make engine.s} and |
|
look at the resulting file @file{engine.s}. |
|
|
|
@node System Architecture, , Primitives, Internals |
|
@section System Architecture |
|
|
|
Our Forth system consists not only of primitives, but also of |
|
definitions written in Forth. Since the Forth compiler itself belongs |
|
to those definitions, it is not possible to start the system with the |
|
primitives and the Forth source alone. Therefore we provide the Forth |
|
code as an image file in nearly executable form. At the start of the |
|
system a C routine loads the image file into memory, sets up the |
|
memory (stacks etc.) according to information in the image file, and |
|
starts executing Forth code. |
|
|
|
The image file format is a compromise between the goals of making it |
|
easy to generate image files and making them portable. The easiest way |
|
to generate an image file is to just generate a memory dump. However, |
|
this kind of image file cannot be used on a different machine, or on |
|
the next version of the engine on the same machine, it even might not |
|
work with the same engine compiled by a different version of the C |
|
compiler. We would like to have as few versions of the image file as |
|
possible, because we do not want to distribute many versions of the |
|
same image file, and to make it easy for the users to use their image |
|
files on many machines. We currently need to create a different image |
|
file for machines with different cell sizes and different byte order |
|
(little- or big-endian)@footnote{We consider adding information to the |
|
image file that enables the loader to change the byte order.}. |
|
|
|
Forth code that is going to end up in a portable image file has to |
|
comply to some restrictions: addresses have to be stored in memory with |
|
special words (@code{A!}, @code{A,}, etc.) in order to make the code |
|
relocatable. Cells, floats, etc., have to be stored at the natural |
|
alignment boundaries@footnote{E.g., store floats (8 bytes) at an address |
|
dividable by~8. This happens automatically in our system when you use |
|
the ANS Forth alignment words.}, in order to avoid alignment faults on |
|
machines with stricter alignment. The image file is produced by a |
|
metacompiler (@file{cross.fs}). |
|
|
|
So, unlike the image file of Mitch Bradleys @code{cforth}, our image |
|
file is not directly executable, but has to undergo some manipulations |
|
during loading. Address relocation is performed at image load-time, not |
|
at run-time. The loader also has to replace tokens standing for |
|
primitive calls with the appropriate code-field addresses (or code |
|
addresses in the case of direct threading). |
|
|
|
@node Bugs, Pedigree, Internals, Top |
|
@chapter Bugs |
|
|
|
@node Pedigree, Word Index, Bugs, Top |
|
@chapter Pedigree |
|
|
|
@node Word Index, Node Index, Pedigree, Top |
|
@chapter Word Index |
|
|
|
@node Node Index, , Word Index, Top |
|
@chapter Node Index |
|
|
@contents |
@contents |
@bye |
@bye |