### Diff for /gforth/Attic/gforth.ds between versions 1.2 and 1.40

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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 Gforth 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.)

@ifinfo  @ifinfo
This file documents GNU Forth 0.0  This file documents Gforth 0.2

Permission is granted to make and distribute verbatim copies of       Permission is granted to make and distribute verbatim copies of
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.

@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
of in the original English.       of in the original English.
@end ifinfo  @end ifinfo

@finalout
@titlepage  @titlepage
@sp 10  @sp 10
@center @titlefont{GNU Forth Manual}  @center @titlefont{Gforth Manual}
@sp 2  @sp 2
@center for version 0.0  @center for version 0.2
@sp 2  @sp 2
@center Anton Ertl  @center Anton Ertl
@center Bernd Paysan
@sp 3
@center This manual is under construction

@comment  The following two commands start the copyright page.  @comment  The following two commands start the copyright page.
@page  @page
@vskip 0pt plus 1filll  @vskip 0pt plus 1filll

@comment !! Published by ... or You can get a copy of this manual ...  @comment !! Published by ... or You can get a copy of this manual ...

@ifinfo  @ifinfo
GNU Forth is a free implementation of ANS Forth available on many  Gforth is a free implementation of ANS Forth available on many
personal machines. This manual corresponds to version 0.0.  personal machines. This manual corresponds to version 0.2.
@end ifinfo  @end ifinfo

* Goals::               About the GNU Forth Project  * Goals::                       About the Gforth Project
* Other Books::         Things you might want to read  * Other Books::                 Things you might want to read
* Invocation::          Starting GNU Forth  * Invocation::                  Starting Gforth
* Words::               Forth words available in GNU Forth  * Words::                       Forth words available in Gforth
* ANS conformance::     Implementation-defined options etc.  * Tools::                       Programming tools
* Model::               The abstract machine of GNU Forth  * ANS conformance::             Implementation-defined options etc.
* Emacs and GForth::    The GForth Mode  * Model::                       The abstract machine of Gforth
* Internals::           Implementation details  * Integrating Gforth::          Forth as scripting language for applications.
* Bugs::                How to report them  * Emacs and Gforth::            The Gforth Mode
* Pedigree::            Ancestors of GNU Forth  * Internals::                   Implementation details
* Word Index::          An item for each Forth word  * Bugs::                        How to report them
* Node Index::          An item for each node  * Origin::                      Authors and ancestors of Gforth
* Word Index::                  An item for each Forth word
* Node Index::                  An item for each node

!! Insert GPL here  @center Version 2, June 1991

@display
675 Mass Ave, Cambridge, MA 02139, USA

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
@end display

@unnumberedsec Preamble

The licenses for most software are designed to take away your
freedom to share and change it.  By contrast, the GNU General Public
License is intended to guarantee your freedom to share and change free
software---to make sure the software is free for all its users.  This
General Public License applies to most of the Free Software
Foundation's software and to any other program whose authors commit to
using it.  (Some other Free Software Foundation software is covered by
the GNU Library General Public License instead.)  You can apply it to

When we speak of free software, we are referring to freedom, not
price.  Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
this service if you wish), that you receive source code or can get it
if you want it, that you can change the software or use pieces of it
in new free programs; and that you know you can do these things.

To protect your rights, we need to make restrictions that forbid
anyone to deny you these rights or to ask you to surrender the rights.
These restrictions translate to certain responsibilities for you if you
distribute copies of the software, or if you modify it.

For example, if you distribute copies of such a program, whether
gratis or for a fee, you must give the recipients all the rights that
you have.  You must make sure that they, too, receive or can get the
source code.  And you must show them these terms so they know their
rights.

We protect your rights with two steps: (1) copyright the software, and
(2) offer you this license which gives you legal permission to copy,
distribute and/or modify the software.

Also, for each author's protection and ours, we want to make certain
that everyone understands that there is no warranty for this free
software.  If the software is modified by someone else and passed on, we
want its recipients to know that what they have is not the original, so
that any problems introduced by others will not reflect on the original
authors' reputations.

Finally, any free program is threatened constantly by software
patents.  We wish to avoid the danger that redistributors of a free
program will individually obtain patent licenses, in effect making the
program proprietary.  To prevent this, we have made it clear that any
patent must be licensed for everyone's free use or not licensed at all.

The precise terms and conditions for copying, distribution and
modification follow.

@iftex  @iftex
@unnumberedsec TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
@end iftex
@ifinfo
@center TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
@end ifinfo

@enumerate 0
@item
This License applies to any program or other work which contains
a notice placed by the copyright holder saying it may be distributed
under the terms of this General Public License.  The Program'', below,
refers to any such program or work, and a work based on the Program''
means either the Program or any derivative work under copyright law:
that is to say, a work containing the Program or a portion of it,
either verbatim or with modifications and/or translated into another
language.  (Hereinafter, translation is included without limitation in
the term modification''.)  Each licensee is addressed as you''.

Activities other than copying, distribution and modification are not
covered by this License; they are outside its scope.  The act of
running the Program is not restricted, and the output from the Program
is covered only if its contents constitute a work based on the
Program (independent of having been made by running the Program).
Whether that is true depends on what the Program does.

@item
You may copy and distribute verbatim copies of the Program's
source code as you receive it, in any medium, provided that you
conspicuously and appropriately publish on each copy an appropriate
copyright notice and disclaimer of warranty; keep intact all the
notices that refer to this License and to the absence of any warranty;
and give any other recipients of the Program a copy of this License
along with the Program.

You may charge a fee for the physical act of transferring a copy, and
you may at your option offer warranty protection in exchange for a fee.

@item
You may modify your copy or copies of the Program or any portion
of it, thus forming a work based on the Program, and copy and
distribute such modifications or work under the terms of Section 1
above, provided that you also meet all of these conditions:

@enumerate a
@item
You must cause the modified files to carry prominent notices
stating that you changed the files and the date of any change.

@item
You must cause any work that you distribute or publish, that in
whole or in part contains or is derived from the Program or any
part thereof, to be licensed as a whole at no charge to all third
parties under the terms of this License.

@item
If the modified program normally reads commands interactively
when run, you must cause it, when started running for such
interactive use in the most ordinary way, to print or display an
announcement including an appropriate copyright notice and a
notice that there is no warranty (or else, saying that you provide
a warranty) and that users may redistribute the program under
these conditions, and telling the user how to view a copy of this
License.  (Exception: if the Program itself is interactive but
does not normally print such an announcement, your work based on
the Program is not required to print an announcement.)
@end enumerate

These requirements apply to the modified work as a whole.  If
identifiable sections of that work are not derived from the Program,
and can be reasonably considered independent and separate works in
themselves, then this License, and its terms, do not apply to those
sections when you distribute them as separate works.  But when you
distribute the same sections as part of a whole which is a work based
on the Program, the distribution of the whole must be on the terms of
entire whole, and thus to each and every part regardless of who wrote it.

Thus, it is not the intent of this section to claim rights or contest
your rights to work written entirely by you; rather, the intent is to
exercise the right to control the distribution of derivative or
collective works based on the Program.

In addition, mere aggregation of another work not based on the Program
with the Program (or with a work based on the Program) on a volume of
a storage or distribution medium does not bring the other work under

@item
You may copy and distribute the Program (or a work based on it,
under Section 2) in object code or executable form under the terms of
Sections 1 and 2 above provided that you also do one of the following:

@enumerate a
@item
Accompany it with the complete corresponding machine-readable
1 and 2 above on a medium customarily used for software interchange; or,

@item
Accompany it with a written offer, valid for at least three
years, to give any third party, for a charge no more than your
cost of physically performing source distribution, a complete
machine-readable copy of the corresponding source code, to be
distributed under the terms of Sections 1 and 2 above on a medium
customarily used for software interchange; or,

@item
Accompany it with the information you received as to the offer
to distribute corresponding source code.  (This alternative is
allowed only for noncommercial distribution and only if you
received the program in object code or executable form with such
an offer, in accord with Subsection b above.)
@end enumerate

The source code for a work means the preferred form of the work for
making modifications to it.  For an executable work, complete source
code means all the source code for all modules it contains, plus any
associated interface definition files, plus the scripts used to
control compilation and installation of the executable.  However, as a
special exception, the source code distributed need not include
anything that is normally distributed (in either source or binary
form) with the major components (compiler, kernel, and so on) of the
operating system on which the executable runs, unless that component
itself accompanies the executable.

If distribution of executable or object code is made by offering
access to copy the source code from the same place counts as
distribution of the source code, even though third parties are not
compelled to copy the source along with the object code.

@item
You may not copy, modify, sublicense, or distribute the Program
except as expressly provided under this License.  Any attempt
otherwise to copy, modify, sublicense or distribute the Program is
However, parties who have received copies, or rights, from you under
this License will not have their licenses terminated so long as such
parties remain in full compliance.

@item
You are not required to accept this License, since you have not
signed it.  However, nothing else grants you permission to modify or
distribute the Program or its derivative works.  These actions are
prohibited by law if you do not accept this License.  Therefore, by
modifying or distributing the Program (or any work based on the
Program), you indicate your acceptance of this License to do so, and
all its terms and conditions for copying, distributing or modifying
the Program or works based on it.

@item
Each time you redistribute the Program (or any work based on the
original licensor to copy, distribute or modify the Program subject to
these terms and conditions.  You may not impose any further
restrictions on the recipients' exercise of the rights granted herein.
You are not responsible for enforcing compliance by third parties to

@item
If, as a consequence of a court judgment or allegation of patent
infringement or for any other reason (not limited to patent issues),
conditions are imposed on you (whether by court order, agreement or
excuse you from the conditions of this License.  If you cannot
distribute so as to satisfy simultaneously your obligations under this
License and any other pertinent obligations, then as a consequence you
may not distribute the Program at all.  For example, if a patent
license would not permit royalty-free redistribution of the Program by
all those who receive copies directly or indirectly through you, then
the only way you could satisfy both it and this License would be to
refrain entirely from distribution of the Program.

If any portion of this section is held invalid or unenforceable under
any particular circumstance, the balance of the section is intended to
apply and the section as a whole is intended to apply in other
circumstances.

It is not the purpose of this section to induce you to infringe any
patents or other property right claims or to contest validity of any
such claims; this section has the sole purpose of protecting the
integrity of the free software distribution system, which is
generous contributions to the wide range of software distributed
through that system in reliance on consistent application of that
system; it is up to the author/donor to decide if he or she is willing
to distribute software through any other system and a licensee cannot
impose that choice.

This section is intended to make thoroughly clear what is believed to
be a consequence of the rest of this License.

@item
If the distribution and/or use of the Program is restricted in
certain countries either by patents or by copyrighted interfaces, the
may add an explicit geographical distribution limitation excluding
those countries, so that distribution is permitted only in or among
countries not thus excluded.  In such case, this License incorporates
the limitation as if written in the body of this License.

@item
The Free Software Foundation may publish revised and/or new versions
of the General Public License from time to time.  Such new versions will
be similar in spirit to the present version, but may differ in detail to

Each version is given a distinguishing version number.  If the Program
specifies a version number of this License which applies to it and any
later version'', you have the option of following the terms and conditions
either of that version or of any later version published by the Free
Software Foundation.  If the Program does not specify a version number of
Foundation.

@item
If you wish to incorporate parts of the Program into other free
programs whose distribution conditions are different, write to the author
to ask for permission.  For software which is copyrighted by the Free
Software Foundation, write to the Free Software Foundation; we sometimes
make exceptions for this.  Our decision will be guided by the two goals
of preserving the free status of all derivatives of our free software and
of promoting the sharing and reuse of software generally.

@iftex
@end iftex
@ifinfo
@center NO WARRANTY
@end ifinfo

@item
BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW.  EXCEPT WHEN
OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
PROVIDE THE PROGRAM AS IS'' WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  THE ENTIRE RISK AS
TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.  SHOULD THE
PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
REPAIR OR CORRECTION.

@item
IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING
OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED
TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY
YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.
@end enumerate

@iftex
@heading END OF TERMS AND CONDITIONS
@end iftex
@ifinfo
@center END OF TERMS AND CONDITIONS
@end ifinfo

@page
@unnumberedsec How to Apply These Terms to Your New Programs

If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these terms.

To do so, attach the following notices to the program.  It is safest
to attach them to the start of each source file to most effectively
convey the exclusion of warranty; and each file should have at least
the copyright'' line and a pointer to where the full notice is found.

@smallexample
@var{one line to give the program's name and a brief idea of what it does.}
Copyright (C) 19@var{yy}  @var{name of author}

This program is free software; you can redistribute it and/or modify
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
@end smallexample

Also add information on how to contact you by electronic and paper mail.

If the program is interactive, make it output a short notice like this
when it starts in an interactive mode:

@smallexample
Gnomovision version 69, Copyright (C) 19@var{yy} @var{name of author}
Gnomovision comes with ABSOLUTELY NO WARRANTY; for details
type show w'.
This is free software, and you are welcome to redistribute it
under certain conditions; type show c' for details.
@end smallexample

The hypothetical commands @samp{show w} and @samp{show c} should show
the appropriate parts of the General Public License.  Of course, the
commands you use may be called something other than @samp{show w} and
@samp{show c}; they could even be mouse-clicks or menu items---whatever

You should also get your employer (if you work as a programmer) or your
school, if any, to sign a copyright disclaimer'' for the program, if
necessary.  Here is a sample; alter the names:

@smallexample
Yoyodyne, Inc., hereby disclaims all copyright interest in the program
Gnomovision' (which makes passes at compilers) written by James Hacker.

@var{signature of Ty Coon}, 1 April 1989
Ty Coon, President of Vice
@end smallexample

proprietary programs.  If your program is a subroutine library, you may
consider it more useful to permit linking proprietary applications with the
library.  If this is what you want to do, use the GNU Library General

@iftex
@node    Preface
@comment node-name,     next,           previous, up
@unnumbered Preface  @unnumbered Preface
This manual documents GNU Forth. The reader is expected to know  @cindex Preface
This manual documents Gforth. The reader is expected to know
Forth. This manual is primarily a reference manual. @xref{Other Books}  Forth. This manual is primarily a reference manual. @xref{Other Books}
for introductory material.  for introductory material.
@end iftex  @end iftex

@node    Goals, Other Books, License, Top  @node    Goals, Other Books, License, Top
@comment node-name,     next,           previous, up  @comment node-name,     next,           previous, up
@chapter Goals of GNU Forth  @chapter Goals of Gforth
@cindex Goals  @cindex Goals
The goal of the GNU Forth Project is to develop a standard model for  The goal of the Gforth Project is to develop a standard model for
ANSI Forth. This can be split into several subgoals:  ANSI Forth. This can be split into several subgoals:

@itemize @bullet  @itemize @bullet
@item  @item
GNU Forth should conform to the ANSI Forth standard.  Gforth should conform to the ANSI Forth standard.
@item  @item
It should be a model, i.e. it should define all the  It should be a model, i.e. it should define all the
implementation-dependent things.  implementation-dependent things.
Line 121  It should become standard, i.e. widely a Line 518  It should become standard, i.e. widely a
is the most difficult one.  is the most difficult one.
@end itemize  @end itemize

To achieve these goals GNU Forth should be  To achieve these goals Gforth should be
@itemize @bullet  @itemize @bullet
@item  @item
Similar to previous models (fig-Forth, F83)  Similar to previous models (fig-Forth, F83)
Line 137  Free. Line 534  Free.
Available on many machines/easy to port.  Available on many machines/easy to port.
@end itemize  @end itemize

Have we achieved these goals? GNU Forth conforms to the ANS Forth  Have we achieved these goals? Gforth conforms to the ANS Forth
standard; it may be considered a model, but we have not yet documented  standard. It may be considered a model, but we have not yet documented
which parts of the model are stable and which parts we are likely to  which parts of the model are stable and which parts we are likely to
change; it certainly has not yet become a de facto standard. It has some  change. It certainly has not yet become a de facto standard. It has some
similarities and some differences to previous models; It has some  similarities and some differences to previous models. It has some
powerful features, but not yet everything that we envisioned; on RISCs  powerful features, but not yet everything that we envisioned. We
it is as fast as interpreters programmed in assembly, on  certainly have achieved our execution speed goals (@pxref{Performance}).
register-starved machines it is not so fast, but still faster than any  It is free and available on many machines.
other C-based interpretive implementation; it is free and available on
many machines.

@node Other Books, Invocation, Goals, Top  @node Other Books, Invocation, Goals, Top
@chapter Other books on ANS Forth  @chapter Other books on ANS Forth

As the standard is relatively new, there are not many books out yet. It  As the standard is relatively new, there are not many books out yet. It
is not recommended to learn Forth by using GNU Forth and a book that is  is not recommended to learn Forth by using Gforth and a book that is
not written for ANS Forth, as you will not know your mistakes from the  not written for ANS Forth, as you will not know your mistakes from the
deviations of the book.  deviations of the book.

There is, of course, the standard, the definite reference if you want to  There is, of course, the standard, the definite reference if you want to
write ANS Forth programs. It will be available in printed form from  write ANS Forth programs. It is available in printed form from the
Global Engineering Documents !! somtime in spring or summer 1994. If you  National Standards Institute Sales Department (Tel.: USA (212) 642-4900;
are lucky, you can still get dpANS6 (the draft that was approved as  Fax.: USA (212) 302-1286) as document @cite{X3.215-1994} for about $200. You standard) by aftp from ftp.uu.net:/vendor/minerva/x3j14. can also get it from Global Engineering Documents (Tel.: USA (800) 854-7179; Fax.: (303) 843-9880) for about$300.

@cite{dpANS6}, the last draft of the standard, which was then submitted to ANSI
for publication is available electronically and for free in some MS Word
format, and it has been converted to HTML. Some pointers to these
versions can be found through
@*@file{http://www.complang.tuwien.ac.at/projects/forth.html}.

@cite{Forth: The new model} by Jack Woehr (!! Publisher) is an  @cite{Forth: The new model} by Jack Woehr (Prentice-Hall, 1993) is an
introductory book based on a draft version of the standard. It does not  introductory book based on a draft version of the standard. It does not
cover the whole standard. It also contains interesting background  cover the whole standard. It also contains interesting background
information (Jack Woehr was in the ANS Forth Technical Committe). It is  information (Jack Woehr was in the ANS Forth Technical Committe). It is
Line 173  other languages should find it ok. Line 575  other languages should find it ok.
@chapter Invocation  @chapter Invocation

You will usually just say @code{gforth}. In many other cases the default  You will usually just say @code{gforth}. In many other cases the default
GNU Forth image will be invoked like this:  Gforth image will be invoked like this:

@example  @example
gforth [files] [-e forth-code]  gforth [files] [-e forth-code]
Line 193  line. They are: Line 595  line. They are:

@table @code  @table @code
@item --image-file @var{file}  @item --image-file @var{file}
@item -i @var{file}
@file{gforth.fi}.  @file{gforth.fi}.

@item --path @var{path}  @item --path @var{path}
Uses @var{path} for searching the image file and Forth source code  @item -p @var{path}
files instead of the default in the environment variable  Uses @var{path} for searching the image file and Forth source code files
@code{GFORTHPATH} or the path specified at installation time (typically  instead of the default in the environment variable @code{GFORTHPATH} or
@file{/usr/local/lib/gforth:.}). A path is given as a @code{:}-separated  the path specified at installation time (e.g.,
list.  @file{/usr/local/share/gforth/0.2.0:.}). A path is given as a list of
directories, separated by @samp{:} (on Unix) or @samp{;} (on other OSs).

@item --dictionary-size @var{size}  @item --dictionary-size @var{size}
@item -m @var{size}  @item -m @var{size}
Line 245  Forth words, you have to quote them or u Line 649  Forth words, you have to quote them or u
after processing the command line (instead of entering interactive mode)  after processing the command line (instead of entering interactive mode)
append @code{-e bye} to the command line.  append @code{-e bye} to the command line.

If you have several versions of Gforth installed, @code{gforth} will
invoke the version that was installed last. @code{gforth-@var{version}}
invokes a specific version. You may want to use the option
@code{--path}, if your environment contains the variable
@code{GFORTHPATH}.

Not yet implemented:  Not yet implemented:
On startup the system first executes the system initialization file  On startup the system first executes the system initialization file
(unless the option @code{--no-init-file} is given; note that the system  (unless the option @code{--no-init-file} is given; note that the system
Line 253  the user initialization file @file{.gfor Line 663  the user initialization file @file{.gfor
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).

@node Words,  , Invocation, Top  @node Words, Tools, Invocation, Top
@chapter Forth Words  @chapter Forth Words

* Notation::  * Notation::
* Arithmetic::  * Arithmetic::
* Stack Manipulation::  * Stack Manipulation::
* Memory access::  * Memory access::
* Control Structures::  * Control Structures::
* Local Variables::  * Locals::
* Defining Words::  * Defining Words::
* Vocabularies::  * Tokens for Words::
* Files::  * Wordlists::
* Blocks::  * Files::
* Other I/O::  * Blocks::
* Programming Tools::  * Other I/O::
* Programming Tools::
* Assembler and Code words::

@node Notation, Arithmetic, Words, Words  @node Notation, Arithmetic, Words, Words
Line 277  then in @file{~}, then in the normal pat Line 690  then in @file{~}, then in the normal pat
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.

@quotation  @format
@var{word}     @var{Stack effect}   @var{wordset}   @var{pronunciation}  @var{word}     @var{Stack effect}   @var{wordset}   @var{pronunciation}
@end format
@var{Description}  @var{Description}
@end quotation

@table @var  @table @var
@item word  @item word
The name of the word. BTW, GNU Forth is case insensitive, so you can  The name of the word. BTW, Gforth is case insensitive, so you can
type the words in in lower case.  type the words in in lower case (However, @pxref{core-idef}).

@item Stack effect  @item Stack effect
The stack effect is written in the notation @code{@var{before} --  The stack effect is written in the notation @code{@var{before} --
@var{after}}, where @var{before} and @var{after} describe the top of  @var{after}}, where @var{before} and @var{after} describe the top of
stack entries before and after the execution of the word. The rest of  stack entries before and after the execution of the word. The rest of
the stack is not touched by the word. The top of stack is rightmost,  the stack is not touched by the word. The top of stack is rightmost,
i.e., a stack sequence is written as it is typed in. Note that GNU Forth  i.e., a stack sequence is written as it is typed in. Note that Gforth
uses a separate floating point stack, but a unified stack  uses a separate floating point stack, but a unified stack
notation. Also, return stack effects are not shown in @var{stack  notation. Also, return stack effects are not shown in @var{stack
effect}, but in @var{Description}. The name of a stack item describes  effect}, but in @var{Description}. The name of a stack item describes
the type and/or the function of the item. See below for a discussion of  the type and/or the function of the item. See below for a discussion of
the types.  the types.

All words have two stack effects: A compile-time stack effect and a
run-time stack effect. The compile-time stack-effect of most words is
@var{ -- }. If the compile-time stack-effect of a word deviates from
this standard behaviour, or the word does other unusual things at
compile time, both stack effects are shown; otherwise only the run-time
stack effect is shown.

@item pronunciation  @item pronunciation
How the word is pronounced  How the word is pronounced

Line 308  system need not support all of them. So, Line 728  system need not support all of them. So,
uses the more portable it will be in theory. However, we suspect that  uses the more portable it will be in theory. However, we suspect that
most ANS Forth systems on personal machines will feature all  most ANS Forth systems on personal machines will feature all
wordsets. Words that are not defined in the ANS standard have  wordsets. Words that are not defined in the ANS standard have
@code{gforth} as wordset.  @code{gforth} or @code{gforth-internal} as wordset. @code{gforth}
describes words that will work in future releases of Gforth;
@code{gforth-internal} words are more volatile. Environmental query
strings are also displayed like words; you can recognize them by the
@code{environment} in the wordset field.

@item Description  @item Description
A description of the behaviour of the word.  A description of the behaviour of the word.
@end table  @end table

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
starts with:

@table @code  @table @code
@item name starts with
Type
@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 334  double sized signed integer Line 757  double sized signed integer
@item ud  @item ud
double sized unsigned integer  double sized unsigned integer
@item r  @item r
Float  Float (on the FP stack)
@item a_  @item a_
@item c_  @item c_
Char-aligned address (note that a Char is two bytes in Windows NT)  Char-aligned address (note that a Char may have two bytes in Windows NT)
@item f_  @item f_
@item df_  @item df_
Line 351  Execution token, same size as Cell Line 774  Execution token, same size as Cell
Wordlist ID, same size as Cell  Wordlist ID, same size as Cell
@item f83name  @item f83name
Pointer to a name structure  Pointer to a name structure
@item "
string in the input stream (not the stack). The terminating character is
a blank by default. If it is not a blank, it is shown in @code{<>}
quotes.

@end table  @end table

@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
Line 363  corresponds to @code{2 1 -}. Forth offer Line 791  corresponds to @code{2 1 -}. Forth offer
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}).

* Single precision::
* Bitwise operations::
* Mixed precision::             operations with single and double-cell integers
* Double precision::            Double-cell integer arithmetic
* Floating Point::

@node Single precision, Bitwise operations, Arithmetic, Arithmetic
@subsection Single precision  @subsection Single precision
doc-+  doc-+
doc--  doc--
Line 377  doc-abs Line 814  doc-abs
doc-min  doc-min
doc-max  doc-max

@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 Line 823  doc-invert
doc-2*  doc-2*
doc-2/  doc-2/

@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 Line 835  doc-um/mod
doc-fm/mod  doc-fm/mod
doc-sm/rem  doc-sm/rem

@node Double precision, Floating Point, Mixed precision, Arithmetic
@subsection Double precision  @subsection Double precision

The outer (aka text) interpreter converts numbers containing a dot into
a double precision number. Note that only numbers with the dot as last
character are standard-conforming.

doc-d+  doc-d+
doc-d-  doc-d-
doc-dnegate  doc-dnegate
Line 404  doc-dabs Line 849  doc-dabs
doc-dmin  doc-dmin
doc-dmax  doc-dmax

@node Stack Manipulation,,,  @node Floating Point,  , Double precision, Arithmetic
@subsection Floating Point

The format of floating point numbers recognized by the outer (aka text)
interpreter is: a signed decimal number, possibly containing a decimal
point (@code{.}), followed by @code{E} or @code{e}, optionally followed
by a signed integer (the exponent). E.g., @code{1e} ist the same as
@code{+1.0e+0}. Note that a number without @code{e}
is not interpreted as floating-point number, but as double (if the
number contains a @code{.}) or single precision integer. Also,
conversions between string and floating point numbers always use base
10, irrespective of the value of @code{BASE}. If @code{BASE} contains a
value greater then 14, the @code{E} may be interpreted as digit and the
number will be interpreted as integer, unless it has a signed exponent
(both @code{+} and @code{-} are allowed as signs).

Angles in floating point operations are given in radians (a full circle
has 2 pi radians). Note, that Gforth has a separate floating point
stack, but we use the unified notation.

Floating point numbers have a number of unpleasant surprises for the
unwary (e.g., floating point addition is not associative) and even a few
for the wary. You should not use them unless you know what you are doing
or you don't care that the results you get are totally bogus. If you
want to learn about the problems of floating point numbers (and how to
Computer Scientist Should Know About Floating-Point Arithmetic, ACM
Computing Surveys 23(1):5@minus{}48, March 1991}.

doc-f+
doc-f-
doc-f*
doc-f/
doc-fnegate
doc-fabs
doc-fmax
doc-fmin
doc-floor
doc-fround
doc-f**
doc-fsqrt
doc-fexp
doc-fexpm1
doc-fln
doc-flnp1
doc-flog
doc-falog
doc-fsin
doc-fcos
doc-fsincos
doc-ftan
doc-fasin
doc-facos
doc-fatan
doc-fatan2
doc-fsinh
doc-fcosh
doc-ftanh
doc-fasinh
doc-facosh
doc-fatanh

@node Stack Manipulation, Memory access, Arithmetic, Words
@section Stack Manipulation  @section Stack Manipulation

gforth has a data stack (aka parameter stack) for characters, cells,  Gforth has a data stack (aka parameter stack) for characters, cells,
addresses, and double cells, a floating point stack for floating point  addresses, and double cells, a floating point stack for floating point
numbers, a return stack for storing the return addresses of colon  numbers, a return stack for storing the return addresses of colon
definitions and other data, and a locals stack for storing local  definitions and other data, and a locals stack for storing local
Line 417  theoretically keep floating point number Line 924  theoretically keep floating point number
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
on a separate FP stack.

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).

* Data stack::
* Floating point stack::
* Return stack::
* Locals stack::
* Stack pointer manipulation::

@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 Line 963  doc-2tuck
doc-2swap  doc-2swap
doc-2rot  doc-2rot

@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 Line 973  doc-ftuck
doc-fswap  doc-fswap
doc-frot  doc-frot

@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> Line 984  doc-2r>
doc-2r@  doc-2r@
doc-2rdrop  doc-2rdrop

@node Locals stack, Stack pointer manipulation, Return stack, Stack Manipulation
@subsection Locals stack  @subsection Locals stack

@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! Line 998  doc-rp!
doc-lp@  doc-lp@
doc-lp!  doc-lp!

@node Memory access  @node Memory access, Control Structures, Stack Manipulation, Words
@section Memory access  @section Memory access

* Stack-Memory transfers::
* Memory block access::

@node Stack-Memory transfers, Address arithmetic, Memory access, Memory access
@subsection Stack-Memory transfers  @subsection Stack-Memory transfers

doc-@  doc-@
Line 494  doc-sf! Line 1024  doc-sf!
doc-df@  doc-df@
doc-df!  doc-df!

@node Address arithmetic, Memory block access, Stack-Memory transfers, Memory access

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 509  must only occur at specific addresses; e Line 1040  must only occur at specific addresses; e
accessed at addresses divisible by 4. Even if a machine allows unaligned  accessed at addresses divisible by 4. Even if a machine allows unaligned
accesses, it can usually perform aligned accesses faster.   accesses, it can usually perform aligned accesses faster.

For the performance-concious: alignment operations are usually only  For the performance-conscious: alignment operations are usually only
necessary during the definition of a data structure, not during the  necessary during the definition of a data structure, not during the
(more frequent) accesses to it.  (more frequent) accesses to it.

Line 519  char-aligned have no use in the standard Line 1050  char-aligned have no use in the standard
created.  created.

The standard guarantees that addresses returned by @code{CREATE}d words  The standard guarantees that addresses returned by @code{CREATE}d words
are aligned for all purposes.  are aligned for all purposes.

Note that the standard defines a word @code{char}, which has nothing to

doc-chars  doc-chars
doc-char+  doc-char+
doc-cells  doc-cells
Line 540  doc-dfloats Line 1074  doc-dfloats
doc-dfloat+  doc-dfloat+
doc-dfalign  doc-dfalign
doc-dfaligned  doc-dfaligned
doc-maxalign
doc-maxaligned
doc-cfalign
doc-cfaligned

@node Memory block access,  , Address arithmetic, Memory access
@subsection Memory block access  @subsection Memory block access

doc-move  doc-move
Line 555  doc-cmove> Line 1094  doc-cmove>
doc-fill  doc-fill
doc-blank  doc-blank

@node Control Structures  @node Control Structures, Locals, Memory access, Words
@section Control Structures  @section Control Structures

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 Line 1102  compile state, i.e., in a colon definiti
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.

* Selection::
* Simple Loops::
* Counted Loops::
* Arbitrary control structures::
* Calls and returns::
* Exception Handling::

@node Selection, Simple Loops, Control Structures, Control Structures
@subsection Selection  @subsection Selection

@example  @example
Line 581  ELSE Line 1130  ELSE
ENDIF  ENDIF
@end example  @end example

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 600  system that only supplies @code{THEN} is Line 1149  system that only supplies @code{THEN} is
Forth's @code{THEN} has the meaning 2b, whereas @code{THEN} in Pascal  Forth's @code{THEN} has the meaning 2b, whereas @code{THEN} in Pascal
and many other programming languages has the meaning 3d.]  and many other programming languages has the meaning 3d.]

We also provide the words @code{?dup-if} and @code{?dup-0=-if}, so you  Gforth also provides the words @code{?dup-if} and @code{?dup-0=-if}, so
can avoid using @code{?dup}.  you can avoid using @code{?dup}. Using these alternatives is also more
efficient than using @code{?dup}. Definitions in plain standard Forth
for @code{ENDIF}, @code{?DUP-IF} and @code{?DUP-0=-IF} are provided in
@file{compat/control.fs}.

@example  @example
@var{n}  @var{n}
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

Line 617  Executes the first @var{codei}, where th Line 1169  Executes the first @var{codei}, where th
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.

@node Simple Loops, Counted Loops, Selection, Control Structures
@subsection Simple Loops  @subsection Simple Loops

@example  @example
Line 648  AGAIN Line 1201  AGAIN

This is an endless loop.  This is an endless loop.

@node Counted Loops, Arbitrary control structures, Simple Loops, Control Structures
@subsection Counted Loops  @subsection Counted Loops

The basic counted loop is:  The basic counted loop is:
Line 685  There are several variations on the coun Line 1239  There are several variations on the coun

@code{LEAVE} leaves the innermost counted loop immediately.  @code{LEAVE} leaves the innermost counted loop immediately.

If @var{start} is greater than @var{limit}, a @code{?DO} loop is entered
(and @code{LOOP} iterates until they become equal by wrap-around
arithmetic). This behaviour is usually not what you want. Therefore,
Gforth offers @code{+DO} and @code{U+DO} (as replacements for
@code{?DO}), which do not enter the loop if @var{start} is greater than
@var{limit}; @code{+DO} is for signed loop parameters, @code{U+DO} for
unsigned loop parameters.

@code{LOOP} can be replaced with @code{@var{n} +LOOP}; this updates the  @code{LOOP} can be replaced with @code{@var{n} +LOOP}; this updates the
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.:

@code{4 0 ?DO  i .  2 +LOOP}   prints @code{0 2}  @code{4 0 +DO  i .  2 +LOOP}   prints @code{0 2}

@code{4 1 ?DO  i .  2 +LOOP}   prints @code{1 3}  @code{4 1 +DO  i .  2 +LOOP}   prints @code{1 3}

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:

Line 699  The behaviour of @code{@var{n} +LOOP} is Line 1261  The behaviour of @code{@var{n} +LOOP} is

@code{ 0 0 ?DO  i .  -1 +LOOP}  prints nothing  @code{ 0 0 ?DO  i .  -1 +LOOP}  prints nothing

Therefore we recommend avoiding using @code{@var{n} +LOOP} with negative  Therefore we recommend avoiding @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{u} -LOOP}, which reduces the
case behaves symmetrical to the positive case:  index by @var{u} each iteration. The loop is terminated when the border
between @var{limit+1} and @var{limit} is crossed. Gforth also provides
@code{-2 0 ?DO  i .  -1 +LOOP}  prints @code{0 -1}  @code{-DO} and @code{U-DO} for down-counting loops. E.g.:

@code{-1 0 ?DO  i .  -1 +LOOP}  prints @code{0}  @code{-2 0 -DO  i .  1 -LOOP}  prints @code{0 -1}

@code{ 0 0 ?DO  i .  -1 +LOOP}  prints nothing  @code{-1 0 -DO  i .  1 -LOOP}  prints @code{0}

The loop is terminated when the border between @var{limit@minus{}sgn(n)} and  @code{ 0 0 -DO  i .  1 -LOOP}  prints nothing
@var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS
Forth standard.  Unfortunately, @code{+DO}, @code{U+DO}, @code{-DO}, @code{U-DO} and
@code{-LOOP} are not in the ANS Forth standard. However, an
@code{?DO} can be replaced by @code{DO}. @code{DO} enters the loop even  implementation for these words that uses only standard words is provided
when the start and the limit value are equal. We do not recommend using  in @file{compat/loops.fs}.
@code{DO}. It will just give you maintenance troubles.
@code{?DO} can also be replaced by @code{DO}. @code{DO} always enters
the loop, independent of the loop parameters. Do not use @code{DO}, even
if you know that the loop is entered in any case. Such knowledge tends
to become invalid during maintenance of a program, and then the
@code{DO} will make trouble.

@code{UNLOOP} is used to prepare for an abnormal loop exit, e.g., via  @code{UNLOOP} is used to prepare for an abnormal loop exit, e.g., via
@code{EXIT}. @code{UNLOOP} removes the loop control parameters from the  @code{EXIT}. @code{UNLOOP} removes the loop control parameters from the
Line 729  FOR Line 1296  FOR
NEXT  NEXT
@end example  @end example
This is the preferred loop of native code compiler writers who are too  This is the preferred loop of native code compiler writers who are too
lazy to optimize @code{?DO} loops properly. In GNU Forth, this loop  lazy to optimize @code{?DO} loops properly. In Gforth, this loop
iterates @var{n+1} times; @code{i} produces values starting with @var{n}  iterates @var{n+1} times; @code{i} produces values starting with @var{n}
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. To avoid problems, don't use @code{FOR}
loops.

@node Arbitrary control structures, Calls and returns, Counted Loops, Control Structures
@subsection Arbitrary control structures  @subsection Arbitrary control structures

ANS Forth permits and supports using control structures in a non-nested  ANS Forth permits and supports using control structures in a non-nested
way. Information about incomplete control structures is stored on the  way. Information about incomplete control structures is stored on the
control-flow stack. This stack may be implemented on the Forth data  control-flow stack. This stack may be implemented on the Forth data
stack, and this is what we have done in gforth.  stack, and this is what we have done in Gforth.

An @i{orig} entry represents an unresolved forward branch, a @i{dest}  An @i{orig} entry represents an unresolved forward branch, a @i{dest}
entry represents a backward branch target. A few words are the basis for  entry represents a backward branch target. A few words are the basis for
building any control structure possible (except control structures that  building any control structure possible (except control structures that
need storage, like calls, coroutines, and backtracking).  need storage, like calls, coroutines, and backtracking).

if  doc-if
then  doc-then
begin  doc-begin
until  doc-until
again  doc-again
cs-pick  doc-cs-pick
cs-roll  doc-cs-roll

On many systems control-flow stack items take one word, in gforth they  On many systems control-flow stack items take one word, in Gforth they
currently take three (this may change in the future). Therefore it is a  currently take three (this may change in the future). Therefore it is a
really good idea to manipulate the control flow stack with  really good idea to manipulate the control flow stack with
@code{cs-pick} and @code{cs-roll}, not with data stack manipulation  @code{cs-pick} and @code{cs-roll}, not with data stack manipulation
Line 763  words. Line 1332  words.

Some standard control structure words are built from these words:  Some standard control structure words are built from these words:

else  doc-else
while  doc-while
repeat  doc-repeat

Gforth adds some more control-structure words:

doc-endif
doc-?dup-if
doc-?dup-0=-if

Counted loop words constitute a separate group of words:  Counted loop words constitute a separate group of words:

?do  doc-?do
do  doc-+do
for  doc-u+do
loop  doc--do
s+loop  doc-u-do
+loop  doc-do
next  doc-for
leave  doc-loop
?leave  doc-+loop
unloop  doc--loop
undo  doc-next
doc-leave
doc-?leave
doc-unloop
doc-done

The standard does not allow using @code{cs-pick} and @code{cs-roll} on  The standard does not allow using @code{cs-pick} and @code{cs-roll} on
@i{do-sys}. Our system allows it, but it's your job to ensure that for  @i{do-sys}. Our system allows it, but it's your job to ensure that for
every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path  every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path
through the program (@code{LOOP} etc. compile an @code{UNLOOP}). Also,  through the definition (@code{LOOP} etc. compile an @code{UNLOOP} on the
you have to ensure that all @code{LEAVE}s are resolved (by using one of  fall-through path). Also, you have to ensure that all @code{LEAVE}s are
the loop-ending words or @code{UNDO}).  resolved (by using one of the loop-ending words or @code{DONE}).

Another group of control structure words are  Another group of control structure words are

case  doc-case
endcase  doc-endcase
of  doc-of
endof  doc-endof

@i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and  @i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and
@code{cs-roll}.  @code{cs-roll}.

@node Locals  @subsubsection Programming Style

In order to ensure readability we recommend that you do not create
arbitrary control structures directly, but define new control structure
words for the control structure you want and use these words in your
program.

@example
begin
...
if [ 1 cs-roll ]
...
again then
@end example

we recommend defining control structure words, e.g.,

@example
: while ( dest -- orig dest )
POSTPONE if
1 cs-roll ; immediate

: repeat ( orig dest -- )
POSTPONE again
POSTPONE then ; immediate
@end example

and then using these to create the control structure:

@example
begin
...
while
...
repeat
@end example

That's much easier to read, isn't it? Of course, @code{REPEAT} and
@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  Local variables can make Forth programming more enjoyable and Forth
Line 807  laden with restrictions. Therefore, we p Line 1450  laden with restrictions. Therefore, we p
locals wordset, but also our own, more powerful locals wordset (we  locals wordset, but also our own, more powerful locals wordset (we
implemented the ANS Forth locals wordset through our locals wordset).  implemented the ANS Forth locals wordset through our locals wordset).

The ideas in this section have also been published in the paper
@cite{Automatic Scoping of Local Variables} by M. Anton Ertl, presented
at EuroForth '94; it is available at
@*@file{http://www.complang.tuwien.ac.at/papers/ertl94l.ps.gz}.

* Gforth locals::
* ANS Forth locals::

@subsection gforth locals  @node Gforth locals, ANS Forth locals, Locals, Locals
@subsection Gforth locals

Locals can be defined with  Locals can be defined with

Line 853  The name of the local may be preceded by Line 1504  The name of the local may be preceded by
Ar Bi f* Ai Br f* f+ ;   Ar Bi f* Ai Br f* f+ ;
@end example  @end example

GNU Forth currently supports cells (@code{W:}, @code{W^}), doubles  Gforth currently supports cells (@code{W:}, @code{W^}), doubles
(@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters  (@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters
(@code{C:}, @code{C^}) in two flavours: a value-flavoured local (defined  (@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{W:}, @code{D:} etc.) produces its value and can be changed
Line 873  locals are initialized with values from Line 1524  locals are initialized with values from
Currently there is no way to define locals with user-defined data  Currently there is no way to define locals with user-defined data
structures, but we are working on it.  structures, but we are working on it.

GNU Forth allows defining locals everywhere in a colon definition. This poses the following questions:  Gforth allows defining locals everywhere in a colon definition. This
poses the following questions:

* Where are locals visible by name?::
* How long do locals live?::
* Programming Style::
* Implementation::

@node Where are locals visible by name?, How long do locals live?, Gforth locals, Gforth locals
@subsubsection Where are locals visible by name?  @subsubsection Where are locals visible by name?

Basically, the answer is that locals are visible where you would expect  Basically, the answer is that locals are visible where you would expect
Line 923  says. If @code{UNREACHABLE} is used wher Line 1583  says. If @code{UNREACHABLE} is used wher
lie to the compiler), buggy code will be produced.  lie to the compiler), buggy code will be produced.

Another problem with this rule is that at @code{BEGIN}, the compiler  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  does not know which locals will be visible on the incoming
. All problems discussed in the following are due to this ignorance of  back-edge. All problems discussed in the following are due to this
the compiler (we discuss the problems using @code{BEGIN} loops as  ignorance of the compiler (we discuss the problems using @code{BEGIN}
examples; the discussion also applies to @code{?DO} and other  loops as examples; the discussion also applies to @code{?DO} and other
loops). Perhaps the most insidious example is:  loops). Perhaps the most insidious example is:
@example  @example
BEGIN  BEGIN
x    x
[ 1 CS-ROLL ] THEN  [ 1 CS-ROLL ] THEN
{ x }    @{ x @}
...    ...
UNTIL  UNTIL
@end example  @end example
Line 965  compiler. When the branch to the @code{B Line 1625  compiler. When the branch to the @code{B
warns the user if it was too optimisitic:  warns the user if it was too optimisitic:
@example  @example
IF  IF
{ x }    @{ x @}
BEGIN  BEGIN
\ x ?     \ x ?
[ 1 cs-roll ] THEN  [ 1 cs-roll ] THEN
Line 981  is not used in the wrong area by using e Line 1641  is not used in the wrong area by using e
@example  @example
IF  IF
SCOPE    SCOPE
{ x }    @{ x @}
ENDSCOPE    ENDSCOPE
BEGIN  BEGIN
[ 1 cs-roll ] THEN  [ 1 cs-roll ] THEN
Line 996  If the @code{BEGIN} is not reachable fro Line 1656  If the @code{BEGIN} is not reachable fro
@code{AHEAD} or @code{EXIT}), the compiler cannot even make an  @code{AHEAD} or @code{EXIT}), the compiler cannot even make an
optimistic guess, as the locals visible after the @code{BEGIN} may be  optimistic guess, as the locals visible after the @code{BEGIN} may be
defined later. Therefore, the compiler assumes that no locals are  defined later. Therefore, the compiler assumes that no locals are
visible after the @code{BEGIN}. However, the useer can use  visible after the @code{BEGIN}. However, the user can use
@code{ASSUME-LIVE} to make the compiler assume that the same locals are  @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.  visible at the BEGIN as at the point where the top control-flow stack
item was created.

doc-assume-live  doc-assume-live

E.g.,  E.g.,
@example  @example
{ x }  @{ x @}
ASSUME-LIVE  ASSUME-LIVE
BEGIN  BEGIN
Line 1025  rearranging the loop. E.g., the most i Line 1686  rearranging the loop. E.g., the most i
arranged into:  arranged into:
@example  @example
BEGIN  BEGIN
{ x }    @{ x @}
... 0=    ... 0=
WHILE  WHILE
x    x
REPEAT  REPEAT
@end example  @end example

@node How long do locals live?, Programming Style, Where are locals visible by name?, Gforth locals
@subsubsection How long do locals live?  @subsubsection How long do locals live?

The right answer for the lifetime question would be: A local lives at  The right answer for the lifetime question would be: A local lives at
Line 1045  languages (e.g., C): The local lives onl Line 1707  languages (e.g., C): The local lives onl
afterwards its address is invalid (and programs that access it  afterwards its address is invalid (and programs that access it
afterwards are erroneous).  afterwards are erroneous).

@node Programming Style, Implementation, How long do locals live?, Gforth locals
@subsubsection Programming Style  @subsubsection Programming Style

The freedom to define locals anywhere has the potential to change  The freedom to define locals anywhere has the potential to change
Line 1056  wrong order, just write a locals definit Line 1719  wrong order, just write a locals definit
write the items in the order you want.  write the items in the order you want.

This seems a little far-fetched and eliminating stack manipulations is  This seems a little far-fetched and eliminating stack manipulations is
unlikely to become a conscious programming objective. Still, the  unlikely to become a conscious programming objective. Still, the number
number of stack manipulations will be reduced dramatically if local  of stack manipulations will be reduced dramatically if local variables
variables are used liberally (e.g., compare @code{max} in \sect{misc}  are used liberally (e.g., compare @code{max} in @ref{Gforth locals} with

This shows one potential benefit of locals: making Forth programs more  This shows one potential benefit of locals: making Forth programs more
readable. Of course, this benefit will only be realized if the  readable. Of course, this benefit will only be realized if the
Line 1078  E.g., a definition using @code{TO} might Line 1741  E.g., a definition using @code{TO} might
u1 u2 min 0   u1 u2 min 0
?do   ?do
if     ?dup-if
unloop exit       unloop exit
then     then
Line 1101  are initialized with the right value for Line 1764  are initialized with the right value for
u1 u2 min 0    u1 u2 min 0
?do @{ s1 s2 @}   ?do @{ s1 s2 @}
s1 c@ s2 c@ - ?dup      s1 c@@ s2 c@@ -
if     ?dup-if
unloop exit       unloop exit
then     then
s1 char+ s2 char+     s1 char+ s2 char+
Line 1113  are initialized with the right value for Line 1776  are initialized with the right value for
Here it is clear from the start that @code{s1} has a different value  Here it is clear from the start that @code{s1} has a different value
in every loop iteration.  in every loop iteration.

@node Implementation,  , Programming Style, Gforth locals
@subsubsection Implementation  @subsubsection Implementation

GNU Forth uses an extra locals stack. The most compelling reason for  Gforth uses an extra locals stack. The most compelling reason for
this is that the return stack is not float-aligned; using an extra stack  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  also eliminates the problems and restrictions of using the return stack
as locals stack. Like the other stacks, the locals stack grows toward  as locals stack. Like the other stacks, the locals stack grows toward
Line 1136 local0 Line 1800 local0
compile the right specialized version, or the general version, as  compile the right specialized version, or the general version, as
appropriate:  appropriate:

doc-compile-@@local  doc-compile-@local
doc-compile-f@@local  doc-compile-f@local
doc-compile-lp+!  doc-compile-lp+!

Combinations of conditional branches and @code{lp+!#} like  Combinations of conditional branches and @code{lp+!#} like
Line 1150  area and @code{@}} switches it back and Line 1814  area and @code{@}} switches it back and
initializing code. @code{W:} etc.@ are normal defining words. This  initializing code. @code{W:} etc.@ are normal defining words. This
special area is cleared at the start of every colon definition.  special area is cleared at the start of every colon definition.

A special feature of GNU Forths dictionary is used to implement the  A special feature of Gforth's dictionary is used to implement the
definition of locals without type specifiers: every wordlist (aka  definition of locals without type specifiers: every wordlist (aka
vocabulary) has its own methods for searching  vocabulary) has its own methods for searching
etc. (@xref{dictionary}). For the present purpose we defined a wordlist  etc. (@pxref{Wordlists}). For the present purpose we defined a wordlist
with a special search method: When it is searched for a word, it  with a special search method: When it is searched for a word, it
actually creates that word using @code{W:}. @code{@{} changes the search  actually creates that word using @code{W:}. @code{@{} changes the search
order to first search the wordlist containing @code{@}}, @code{W:} etc.,  order to first search the wordlist containing @code{@}}, @code{W:} etc.,
Line 1190  The locals stack pointer is only adjuste Line 1854  The locals stack pointer is only adjuste
@code{lp+!#} orig-locals-size @minus{} new-locals-size  @code{lp+!#} orig-locals-size @minus{} new-locals-size
@end format  @end format
The second @code{lp+!#} adjusts the locals stack pointer from the  The second @code{lp+!#} adjusts the locals stack pointer from the
level at the {\em orig} point to the level after the @code{THEN}. 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  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  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  adjustment from the current level to the right level after the
Line 1240  this may lead to increased space needs f Line 1904  this may lead to increased space needs f
usually less than reclaiming this space would cost in code size.  usually less than reclaiming this space would cost in code size.

@node ANS Forth locals,  , Gforth locals, Locals
@subsection ANS Forth locals  @subsection ANS Forth locals

The ANS Forth locals wordset does not define a syntax for locals, but  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  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  possible syntaxes is a subset of the syntax we used in the Gforth locals
wordset, i.e.:  wordset, i.e.:

@example  @example
Line 1260  restrictions are: Line 1925  restrictions are:

@itemize @bullet  @itemize @bullet
@item  @item
Locals can only be cell-sized values (no type specifers are allowed).  Locals can only be cell-sized values (no type specifiers are allowed).
@item  @item
Locals can be defined only outside control structures.  Locals can be defined only outside control structures.
@item  @item
Locals can interfere with explicit usage of the return stack. For the  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  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  accessing words in a definition using locals, you will be all right. The
purpose of this rule is to make locals implementation on the return  purpose of this rule is to make locals implementation on the return
stack easier.  stack easier.
@item  @item
The whole definition must be in one line.  The whole definition must be in one line.
@end itemize  @end itemize

Locals defined in this way behave like @code{VALUE}s  Locals defined in this way behave like @code{VALUE}s (@xref{Simple
(@xref{values}). I.e., they are initialized from the stack. Using their  Defining Words}). I.e., they are initialized from the stack. Using their
name produces their value. Their value can be changed using @code{TO}.  name produces their value. Their value can be changed using @code{TO}.

Since this syntax is supported by gforth directly, you need not do  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  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  another ANS Forth system, use @file{compat/anslocal.fs} to implement the
on the other system.  syntax on the other system.

Note that a syntax shown in the standard, section A.13 looks  Note that a syntax shown in the standard, section A.13 looks
similar, but is quite different in having the order of locals  similar, but is quite different in having the order of locals
Line 1292  doc-(local) Line 1957  doc-(local)

The ANS Forth locals extension wordset defines a syntax, but it is so  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  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  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  problem with this syntax is that the locals are defined in an order
reversed with respect to the standard stack comment notation, making  reversed with respect to the standard stack comment notation, making
programs harder to read, and easier to misread and miswrite. The only  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  merit of this syntax is that it is easy to implement using the ANS Forth
locals wordset.  locals wordset.

@node Defining Words, Tokens for Words, Locals, Words
@section Defining Words

* Simple Defining Words::
* Colon Definitions::
* User-defined Defining Words::
* Supplying names::
* Interpretation and Compilation Semantics::

@node Simple Defining Words, Colon Definitions, Defining Words, Defining Words
@subsection Simple Defining Words

doc-constant
doc-2constant
doc-fconstant
doc-variable
doc-2variable
doc-fvariable
doc-create
doc-user
doc-value
doc-to
doc-defer
doc-is

@node Colon Definitions, User-defined Defining Words, Simple Defining Words, Defining Words
@subsection Colon Definitions

@example
: name ( ... -- ... )
word1 word2 word3 ;
@end example

creates a word called @code{name}, that, upon execution, executes
@code{word1 word2 word3}. @code{name} is a @dfn{(colon) definition}.

The explanation above is somewhat superficial. @xref{Interpretation and
Compilation Semantics} for an in-depth discussion of some of the issues
involved.

doc-:
doc-;

@node User-defined Defining Words, Supplying names, Colon Definitions, Defining Words
@subsection User-defined Defining Words

You can create new defining words simply by wrapping defining-time code
around existing defining words and putting the sequence in a colon
definition.

If you want the words defined with your defining words to behave
differently from words defined with standard defining words, you can
write your defining word like this:

@example
: def-word ( "name" -- )
Create @var{code1}
DOES> ( ... -- ... )
@var{code2} ;

def-word name
@end example

Technically, this fragment defines a defining word @code{def-word}, and
a word @code{name}; when you execute @code{name}, the address of the
body of @code{name} is put on the data stack and @var{code2} is executed
returns immediately after the @code{CREATE}).

In other words, if you make the following definitions:

@example
: def-word1 ( "name" -- )
Create @var{code1} ;

: action1 ( ... -- ... )
@var{code2} ;

def-word name1
@end example

Using @code{name1 action1} is equivalent to using @code{name}.

E.g., you can implement @code{Constant} in this way:

@example
: constant ( w "name" -- )
create ,
DOES> ( -- w )
@@ ;
@end example

When you create a constant with @code{5 constant five}, first a new word
@code{five} is created, then the value 5 is laid down in the body of
@code{five} with @code{,}. When @code{five} is invoked, the address of
the body is put on the stack, and @code{@@} retrieves the value 5.

In the example above the stack comment after the @code{DOES>} specifies
the stack effect of the defined words, not the stack effect of the
following code (the following code expects the address of the body on
the top of stack, which is not reflected in the stack comment). This is
the convention that I use and recommend (it clashes a bit with using
locals declarations for stack effect specification, though).

@subsubsection Applications of @code{CREATE..DOES>}

You may wonder how to use this feature. Here are some usage patterns:

When you see a sequence of code occurring several times, and you can
identify a meaning, you will factor it out as a colon definition. When
you see similar colon definitions, you can factor them using
@code{CREATE..DOES>}. E.g., an assembler usually defines several words
that look very similar:
@example
: ori, ( reg-taget reg-source n -- )
0 asm-reg-reg-imm ;
: andi, ( reg-taget reg-source n -- )
1 asm-reg-reg-imm ;
@end example

This could be factored with:
@example
: reg-reg-imm ( op-code -- )
create ,
DOES> ( reg-taget reg-source n -- )
@@ asm-reg-reg-imm ;

0 reg-reg-imm ori,
1 reg-reg-imm andi,
@end example

Another view of @code{CREATE..DOES>} is to consider it as a crude way to
supply a part of the parameters for a word (known as @dfn{currying} in
the functional language community). E.g., @code{+} needs two
parameters. Creating versions of @code{+} with one parameter fixed can
be done like this:
@example
: curry+ ( n1 -- )
create ,
DOES> ( n2 -- n1+n2 )
@@ + ;

3 curry+ 3+
-2 curry+ 2-
@end example

@subsubsection The gory details of @code{CREATE..DOES>}

doc-does>

This means that you need not use @code{CREATE} and @code{DOES>} in the
same definition; E.g., you can put the @code{DOES>}-part in a separate
definition. This allows us to, e.g., select among different DOES>-parts:
@example
: does1
DOES> ( ... -- ... )
... ;

: does2
DOES> ( ... -- ... )
... ;

: def-word ( ... -- ... )
create ...
IF
does1
ELSE
does2
ENDIF ;
@end example

In a standard program you can apply a @code{DOES>}-part only if the last
word was defined with @code{CREATE}. In Gforth, the @code{DOES>}-part
will override the behaviour of the last word defined in any case. In a
standard program, you can use @code{DOES>} only in a colon
definition. In Gforth, you can also use it in interpretation state, in a
kind of one-shot mode:
@example
CREATE name ( ... -- ... )
@var{initialization}
DOES>
@var{code} ;
@end example
This is equivalwent to the standard
@example
:noname
DOES>
@var{code} ;
CREATE name EXECUTE ( ... -- ... )
@var{initialization}
@end example

You can get the address of the body of a word with

doc->body

@node Supplying names, Interpretation and Compilation Semantics, User-defined Defining Words, Defining Words
@subsection Supplying names for the defined words

By default, defining words take the names for the defined words from the
input stream. Sometimes you want to supply the name from a string. You
can do this with

doc-nextname

E.g.,

@example
s" foo" nextname create
@end example
is equivalent to
@example
create foo
@end example

Sometimes you want to define a word without a name. You can do this with

doc-noname

To make any use of the newly defined word, you need its execution
token. You can get it with

doc-lastxt

E.g., you can initialize a deferred word with an anonymous colon
definition:
@example
Defer deferred
noname : ( ... -- ... )
... ;
lastxt IS deferred
@end example

@code{lastxt} also works when the last word was not defined as
@code{noname}.

The standard has also recognized the need for anonymous words and
provides

doc-:noname

This leaves the execution token for the word on the stack after the
closing @code{;}. You can rewrite the last example with @code{:noname}:
@example
Defer deferred
:noname ( ... -- ... )
... ;
IS deferred
@end example

@node Interpretation and Compilation Semantics,  , Supplying names, Defining Words
@subsection Interpretation and Compilation Semantics

The @dfn{interpretation semantics} of a word are what the text
interpreter does when it encounters the word in interpret state. It also
appears in some other contexts, e.g., the execution token returned by
@code{' @var{word}} identifies the interpretation semantics of
@var{word} (in other words, @code{' @var{word} execute} is equivalent to
interpret-state text interpretation of @code{@var{word}}).

The @dfn{compilation semantics} of a word are what the text interpreter
does when it encounters the word in compile state. It also appears in
other contexts, e.g, @code{POSTPONE @var{word}} compiles@footnote{In
standard terminology, appends to the current definition''.} the
compilation semantics of @var{word}.

The standard also talks about @dfn{execution semantics}. They are used
only for defining the interpretation and compilation semantics of many
words. By default, the interpretation semantics of a word are to
@code{execute} its execution semantics, and the compilation semantics of
a word are to @code{compile,} its execution semantics.@footnote{In
standard terminology: The default interpretation semantics are its
execution semantics; the default compilation semantics are to append its
execution semantics to the execution semantics of the current
definition.}

You can change the compilation semantics into @code{execute}ing the
execution semantics with

doc-immediate

You can remove the interpretation semantics of a word with

doc-compile-only
doc-restrict

Note that ticking (@code{'}) compile-only words gives an error
(Interpreting a compile-only word'').

Gforth also allows you to define words with arbitrary combinations of
interpretation and compilation semantics.

doc-interpret/compile:

This feature was introduced for implementing @code{TO} and @code{S"}. I
recommend that you do not define such words, as cute as they may be:
they make it hard to get at both parts of the word in some contexts.
E.g., assume you want to get an execution token for the compilation
part. Instead, define two words, one that embodies the interpretation
part, and one that embodies the compilation part.

There is, however, a potentially useful application of this feature:
Providing differing implementations for the default semantics. While
this introduces redundancy and is therefore usually a bad idea, a
performance improvement may be worth the trouble. E.g., consider the
word @code{foobar}:

@example
: foobar
foo bar ;
@end example

Let us assume that @code{foobar} is called so frequently that the
calling overhead would take a significant amount of the run-time. We can
optimize it with @code{interpret/compile:}:

@example
:noname
foo bar ;
:noname
POSTPONE foo POSTPONE bar ;
interpret/compile: foobar
@end example

This definition has the same interpretation semantics and essentially
the same compilation semantics as the simple definition of
@code{foobar}, but the implementation of the compilation semantics is
more efficient with respect to run-time.

Some people try to use state-smart words to emulate the feature provided
by @code{interpret/compile:} (words are state-smart if they check
@code{STATE} during execution). E.g., they would try to code
@code{foobar} like this:

@example
: foobar
STATE @@
IF ( compilation state )
POSTPONE foo POSTPONE bar
ELSE
foo bar
ENDIF ; immediate
@end example

While this works if @code{foobar} is processed only by the text
interpreter, it does not work in other contexts (like @code{'} or
@code{POSTPONE}). E.g., @code{' foobar} will produce an execution token
for a state-smart word, not for the interpretation semantics of the
original @code{foobar}; when you execute this execution token (directly
with @code{EXECUTE} or indirectly through @code{COMPILE,}) in compile
state, the result will not be what you expected (i.e., it will not
perform @code{foo bar}). State-smart words are a bad idea. Simply don't
write them!

It is also possible to write defining words that define words with
arbitrary combinations of interpretation and compilation semantics (or,
preferably, arbitrary combinations of implementations of the default
semantics). In general, this looks like:

@example
: def-word
create-interpret/compile
@var{code1}
interpretation>
@var{code2}
<interpretation
compilation>
@var{code3}
<compilation ;
@end example

For a @var{word} defined with @code{def-word}, the interpretation
semantics are to push the address of the body of @var{word} and perform
@var{code2}, and the compilation semantics are to push the address of
the body of @var{word} and perform @var{code3}. E.g., @code{constant}
can also be defined like this:

@example
: constant ( n "name" -- )
create-interpret/compile
,
interpretation> ( -- n )
@@
<interpretation
compilation> ( compilation. -- ; run-time. -- n )
@@ postpone literal
<compilation ;
@end example

doc-create-interpret/compile
doc-interpretation>
doc-<interpretation
doc-compilation>
doc-<compilation

Note that words defined with @code{interpret/compile:} and
@code{create-interpret/compile} have an extended header structure that
differs from other words; however, unless you try to access them with
plain address arithmetic, you should not notice this. Words for
accessing the header structure usually know how to deal with this; e.g.,
@code{' word >body} also gives you the body of a word created with
@code{create-interpret/compile}.

@node Tokens for Words, Wordlists, Defining Words, Words
@section Tokens for Words

This chapter describes the creation and use of tokens that represent
words on the stack (and in data space).

Named words have interpretation and compilation semantics. Unnamed words
just have execution semantics.

An @dfn{execution token} represents the execution semantics of an
unnamed word. An execution token occupies one cell. As explained in
section @ref{Supplying names}, the execution token of the last words
defined can be produced with

short-lastxt

You can perform the semantics represented by an execution token with
doc-execute
You can compile the word with
doc-compile,

In Gforth, the abstract data type @emph{execution token} is implemented

The interpretation semantics of a named word are also represented by an
execution token. You can get it with

doc-[']
doc-'

For literals, you use @code{'} in interpreted code and @code{[']} in
compiled code. Gforth's @code{'} and @code{[']} behave somewhat unusual
by complaining about compile-only words. To get an execution token for a
compiling word @var{X}, use @code{COMP' @var{X} drop} or @code{[COMP']
@var{X} drop}.

The compilation semantics are represented by a @dfn{compilation token}
consisting of two cells: @var{w xt}. The top cell @var{xt} is an
execution token. The compilation semantics represented by the
compilation token can be performed with @code{execute}, which consumes
the whole compilation token, with an additional stack effect determined
by the represented compilation semantics.

doc-[comp']
doc-comp'

You can compile the compilation semantics with @code{postpone,}. I.e.,
@code{COMP' @var{word} POSTPONE,} is equivalent to @code{POSTPONE
@var{word}}.

doc-postpone,

At present, the @var{w} part of a compilation token is an execution
token, and the @var{xt} part represents either @code{execute} or
@code{compile,}. However, don't rely on that kowledge, unless necessary;
we may introduce unusual compilation tokens in the future (e.g.,
compilation tokens representing the compilation semantics of literals).

Named words are also represented by the @dfn{name token}. The abstract
data type @emph{name token} is implemented as NFA (name field address).

doc-find-name
doc-name>int
doc-name?int
doc-name>comp
doc-name>string

@node Wordlists, Files, Tokens for 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, Assembler and Code words, Other I/O, Words
@section Programming Tools

* Debugging::                   Simple and quick.
* Assertions::                  Making your programs self-checking.

@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 Assembler and Code words, Threading Words, Programming Tools, Words
@section Assembler and Code words

Gforth provides some words for defining primitives (words written in
machine code), and for defining the the machine-code equivalent of
@code{DOES>}-based defining words. However, the machine-independent
nature of Gforth poses a few problems: First of all, Gforth runs on
several architectures, so it can provide no standard assembler. What's
worse is that the register allocation not only depends on the processor,
but also on the @code{gcc} version and options used.

The words that Gforth offers encapsulate some system dependences (e.g., the
header structure), so a system-independent assembler may be used in
Gforth. If you do not have an assembler, you can compile machine code
directly with @code{,} and @code{c,}.

doc-assembler
doc-code
doc-end-code
doc-;code
doc-flush-icache

If @code{flush-icache} does not work correctly, @code{code} words
etc. will not work (reliably), either.

These words are rarely used. Therefore they reside in @code{code.fs},
which is usually not loaded (except @code{flush-icache}, which is always
present). You can load them with @code{require code.fs}.

In the assembly code you will want to refer to the inner interpreter's
registers (e.g., the data stack pointer) and you may want to use other
registers for temporary storage. Unfortunately, the register allocation
is installation-dependent.

The easiest solution is to use explicit register declarations
(@pxref{Explicit Reg Vars, , Variables in Specified Registers, gcc.info,
GNU C Manual}) for all of the inner interpreter's registers: You have to
compile Gforth with @code{-DFORCE_REG} (configure option
@code{--enable-force-reg}) and the appropriate declarations must be
present in the @code{machine.h} file (see @code{mips.h} for an example;
you can find a full list of all declarable register symbols with
@code{grep register engine.c}). If you give explicit registers to all
variables that are declared at the beginning of @code{engine()}, you
should be able to use the other caller-saved registers for temporary
storage. Alternatively, you can use the @code{gcc} option
@code{-ffixed-REG} (@pxref{Code Gen Options, , Options for Code
Generation Conventions, gcc.info, GNU C Manual}) to reserve a register
(however, this restriction on register allocation may slow Gforth
significantly).

If this solution is not viable (e.g., because @code{gcc} does not allow
you to explicitly declare all the registers you need), you have to find
out by looking at the code where the inner interpreter's registers
reside and which registers can be used for temporary storage. You can
get an assembly listing of the engine's code with @code{make engine.s}.

In any case, it is good practice to abstract your assembly code from the
actual register allocation. E.g., if the data stack pointer resides in
register @code{17}, create an alias for this register called @code{sp}, and use that in your assembly code. Another option for implementing normal and defining words efficiently is: adding the wanted functionality to the source of Gforth. For normal words you just have to edit @file{primitives} (@pxref{Automatic Generation}), defining words (equivalent to @code{;CODE} words, for fast defined words) may require changes in @file{engine.c}, @file{kernal.fs}, @file{prims2x.fs}, and possibly @file{cross.fs}. @node Threading Words, , Assembler and Code words, 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 The code addresses produced by various defining words are produced by the following words: doc-docol: doc-docon: doc-dovar: doc-douser: doc-dodefer: doc-dofield: You can recognize words defined by a @code{CREATE}...@code{DOES>} word with @code{>DOES-CODE}. If the word was defined in that way, the value returned is different from 0 and identifies the @code{DOES>} used by the defining word. @node Tools, ANS conformance, Words, Top @chapter Tools @menu * ANS Report:: Report the words used, sorted by wordset @end menu See also @ref{Emacs and Gforth}. @node ANS Report, , Tools, Tools @section @file{ans-report.fs}: Report the words used, sorted by wordset If you want to label a Forth program as ANS Forth Program, you must document which wordsets the program uses; for extension wordsets, it is helpful to list the words the program requires from these wordsets (because Forth systems are allowed to provide only some words of them). The @file{ans-report.fs} tool makes it easy for you to determine which words from which wordset and which non-ANS words your application uses. You simply have to include @file{ans-report.fs} before loading the program you want to check. After loading your program, you can get the report with @code{print-ans-report}. A typical use is to run this as batch job like this: @example gforth ans-report.fs myprog.fs -e "print-ans-report bye" @end example The output looks like this (for @file{compat/control.fs}): @example The program uses the following words from CORE : : POSTPONE THEN ; immediate ?dup IF 0= from BLOCK-EXT : \ from FILE : ( @end example @subsection Caveats Note that @file{ans-report.fs} just checks which words are used, not whether they are used in an ANS Forth conforming way! Some words are defined in several wordsets in the standard. @file{ans-report.fs} reports them for only one of the wordsets, and not necessarily the one you expect. It depends on usage which wordset is the right one to specify. E.g., if you only use the compilation semantics of @code{S"}, it is a Core word; if you also use its interpretation semantics, it is a File word. @node ANS conformance, Model, Tools, Top @chapter ANS conformance To the best of our knowledge, Gforth is an ANS Forth System @itemize @bullet @item providing the Core Extensions word set @item providing the Block word set @item providing the Block Extensions word set @item providing the Double-Number word set @item providing the Double-Number Extensions word set @item providing the Exception word set @item providing the Exception Extensions word set @item providing the Facility word set @item providing @code{MS} and @code{TIME&DATE} from the Facility Extensions word set @item providing the File Access word set @item providing the File Access Extensions word set @item providing the Floating-Point word set @item providing the Floating-Point Extensions word set @item providing the Locals word set @item providing the Locals Extensions word set @item providing the Memory-Allocation word set @item providing the Memory-Allocation Extensions word set (that one's easy) @item providing the Programming-Tools word set @item providing @code{;CODE}, @code{AHEAD}, @code{ASSEMBLER}, @code{BYE}, @code{CODE}, @code{CS-PICK}, @code{CS-ROLL}, @code{STATE}, @code{[ELSE]}, @code{[IF]}, @code{[THEN]} from the Programming-Tools Extensions word set @item providing the Search-Order word set @item providing the Search-Order Extensions word set @item providing the String word set @item providing the String Extensions word set (another easy one) @end itemize In addition, ANS Forth systems are required to document certain implementation choices. This chapter tries to meet these requirements. In many cases it gives a way to ask the system for the information instead of providing the information directly, in particular, if the information depends on the processor, the operating system or the installation options chosen, or if they are likely to change during the maintenance of Gforth. @comment The framework for the rest has been taken from pfe. @menu * The Core Words:: * The optional Block word set:: * The optional Double Number word set:: * The optional Exception word set:: * The optional Facility word set:: * The optional File-Access word set:: * The optional Floating-Point word set:: * The optional Locals word set:: * The optional Memory-Allocation word set:: * The optional Programming-Tools word set:: * The optional Search-Order word set:: @end menu @c ===================================================================== @node The Core Words, The optional Block word set, ANS conformance, ANS conformance @comment node-name, next, previous, up @section The Core Words @c ===================================================================== @menu * core-idef:: Implementation Defined Options * core-ambcond:: Ambiguous Conditions * core-other:: Other System Documentation @end menu @c --------------------------------------------------------------------- @node core-idef, core-ambcond, The Core Words, The Core Words @subsection Implementation Defined Options @c --------------------------------------------------------------------- @table @i @item (Cell) aligned addresses: processor-dependent. Gforth's alignment words perform natural alignment (e.g., an address aligned for a datum of size 8 is divisible by 8). Unaligned accesses usually result in a @code{-23 THROW}. @item @code{EMIT} and non-graphic characters: The character is output using the C library function (actually, macro) @code{putc}. @item character editing of @code{ACCEPT} and @code{EXPECT}: This is modeled on the GNU readline library (@pxref{Readline Interaction, , Command Line Editing, readline, The GNU Readline Library}) with Emacs-like key bindings. @kbd{Tab} deviates a little by producing a full word completion every time you type it (instead of producing the common prefix of all completions). @item character set: The character set of your computer and display device. Gforth is 8-bit-clean (but some other component in your system may make trouble). @item Character-aligned address requirements: installation-dependent. Currently a character is represented by a C @code{unsigned char}; in the future we might switch to @code{wchar_t} (Comments on that requested). @item character-set extensions and matching of names: Any character except the ASCII NUL charcter can be used in a name. Matching is case-insensitive (except in @code{TABLE}s. The matching is performed using the C function @code{strncasecmp}, whose function is probably influenced by the locale. E.g., the @code{C} locale does not know about accents and umlauts, so they are matched case-sensitively in that locale. For portability reasons it is best to write programs such that they work in the @code{C} locale. Then one can use libraries written by a Polish programmer (who might use words containing ISO Latin-2 encoded characters) and by a French programmer (ISO Latin-1) in the same program (of course, @code{WORDS} will produce funny results for some of the words (which ones, depends on the font you are using)). Also, the locale you prefer may not be available in other operating systems. Hopefully, Unicode will solve these problems one day. @item conditions under which control characters match a space delimiter: If @code{WORD} is called with the space character as a delimiter, all white-space characters (as identified by the C macro @code{isspace()}) are delimiters. @code{PARSE}, on the other hand, treats space like other delimiters. @code{PARSE-WORD} treats space like @code{WORD}, but behaves like @code{PARSE} otherwise. @code{(NAME)}, which is used by the outer interpreter (aka text interpreter) by default, treats all white-space characters as delimiters. @item format of the control flow stack: The data stack is used as control flow stack. The size of a control flow stack item in cells is given by the constant @code{cs-item-size}. At the time of this writing, an item consists of a (pointer to a) locals list (third), an address in the code (second), and a tag for identifying the item (TOS). The following tags are used: @code{defstart}, @code{live-orig}, @code{dead-orig}, @code{dest}, @code{do-dest}, @code{scopestart}. @item conversion of digits > 35 The characters @code{[\]^_'} are the digits with the decimal value 36@minus{}41. There is no way to input many of the larger digits. @item display after input terminates in @code{ACCEPT} and @code{EXPECT}: The cursor is moved to the end of the entered string. If the input is terminated using the @kbd{Return} key, a space is typed. @item exception abort sequence of @code{ABORT"}: The error string is stored into the variable @code{"error} and a @code{-2 throw} is performed. @item input line terminator: For interactive input, @kbd{C-m} (CR) and @kbd{C-j} (LF) terminate lines. One of these characters is typically produced when you type the @kbd{Enter} or @kbd{Return} key. @item maximum size of a counted string: @code{s" /counted-string" environment? drop .}. Currently 255 characters on all ports, but this may change. @item maximum size of a parsed string: Given by the constant @code{/line}. Currently 255 characters. @item maximum size of a definition name, in characters: 31 @item maximum string length for @code{ENVIRONMENT?}, in characters: 31 @item method of selecting the user input device: The user input device is the standard input. There is currently no way to change it from within Gforth. However, the input can typically be redirected in the command line that starts Gforth. @item method of selecting the user output device: @code{EMIT} and @code{TYPE} output to the file-id stored in the value @code{outfile-id} (@code{stdout} by default). Gforth uses buffered output, so output on a terminal does not become visible before the next newline or buffer overflow. Output on non-terminals is invisible until the buffer overflows. @item methods of dictionary compilation: What are we expected to document here? @item number of bits in one address unit: @code{s" address-units-bits" environment? drop .}. 8 in all current ports. @item number representation and arithmetic: Processor-dependent. Binary two's complement on all current ports. @item ranges for integer types: Installation-dependent. Make environmental queries for @code{MAX-N}, @code{MAX-U}, @code{MAX-D} and @code{MAX-UD}. The lower bounds for unsigned (and positive) types is 0. The lower bound for signed types on two's complement and one's complement machines machines can be computed by adding 1 to the upper bound. @item read-only data space regions: The whole Forth data space is writable. @item size of buffer at @code{WORD}: @code{PAD HERE - .}. 104 characters on 32-bit machines. The buffer is shared with the pictured numeric output string. If overwriting @code{PAD} is acceptable, it is as large as the remaining dictionary space, although only as much can be sensibly used as fits in a counted string. @item size of one cell in address units: @code{1 cells .}. @item size of one character in address units: @code{1 chars .}. 1 on all current ports. @item size of the keyboard terminal buffer: Varies. You can determine the size at a specific time using @code{lp@@ tib - .}. It is shared with the locals stack and TIBs of files that include the current file. You can change the amount of space for TIBs and locals stack at Gforth startup with the command line option @code{-l}. @item size of the pictured numeric output buffer: @code{PAD HERE - .}. 104 characters on 32-bit machines. The buffer is shared with @code{WORD}. @item size of the scratch area returned by @code{PAD}: The remainder of dictionary space. You can even use the unused part of the data stack space. The current size can be computed with @code{sp@@ pad - .}. @item system case-sensitivity characteristics: Dictionary searches are case insensitive (except in @code{TABLE}s). However, as explained above under @i{character-set extensions}, the matching for non-ASCII characters is determined by the locale you are using. In the default @code{C} locale all non-ASCII characters are matched case-sensitively. @item system prompt: @code{ ok} in interpret state, @code{ compiled} in compile state. @item division rounding: installation dependent. @code{s" floored" environment? drop .}. We leave the choice to @code{gcc} (what to use for @code{/}) and to you (whether to use @code{fm/mod}, @code{sm/rem} or simply @code{/}). @item values of @code{STATE} when true: -1. @item values returned after arithmetic overflow: On two's complement machines, arithmetic is performed modulo 2**bits-per-cell for single arithmetic and 4**bits-per-cell for double arithmetic (with appropriate mapping for signed types). Division by zero typically results in a @code{-55 throw} (Floating-point unidentified fault), although a @code{-10 throw} (divide by zero) would be more appropriate. @item whether the current definition can be found after @t{DOES>}: No. @end table @c --------------------------------------------------------------------- @node core-ambcond, core-other, core-idef, The Core Words @subsection Ambiguous conditions @c --------------------------------------------------------------------- @table @i @item a name is neither a word nor a number: @code{-13 throw} (Undefined word). Actually, @code{-13 bounce}, which preserves the data and FP stack, so you don't lose more work than necessary. @item a definition name exceeds the maximum length allowed: @code{-19 throw} (Word name too long) @item addressing a region not inside the various data spaces of the forth system: The stacks, code space and name space are accessible. Machine code space is typically readable. Accessing other addresses gives results dependent on the operating system. On decent systems: @code{-9 throw} (Invalid memory address). @item argument type incompatible with parameter: This is usually not caught. Some words perform checks, e.g., the control flow words, and issue a @code{ABORT"} or @code{-12 THROW} (Argument type mismatch). @item attempting to obtain the execution token of a word with undefined execution semantics: @code{-14 throw} (Interpreting a compile-only word). In some cases, you get an execution token for @code{compile-only-error} (which performs a @code{-14 throw} when executed). @item dividing by zero: typically results in a @code{-55 throw} (floating point unidentified fault), although a @code{-10 throw} (divide by zero) would be more appropriate. @item insufficient data stack or return stack space: Not checked. This typically results in mysterious illegal memory accesses, producing @code{-9 throw} (Invalid memory address) or @code{-23 throw} (Address alignment exception). @item insufficient space for loop control parameters: like other return stack overflows. @item insufficient space in the dictionary: Not checked. Similar results as stack overflows. However, typically the error appears at a different place when one inserts or removes code. @item interpreting a word with undefined interpretation semantics: For some words, we defined interpretation semantics. For the others: @code{-14 throw} (Interpreting a compile-only word). @item modifying the contents of the input buffer or a string literal: These are located in writable memory and can be modified. @item overflow of the pictured numeric output string: Not checked. @item parsed string overflow: @code{PARSE} cannot overflow. @code{WORD} does not check for overflow. @item producing a result out of range: On two's complement machines, arithmetic is performed modulo 2**bits-per-cell for single arithmetic and 4**bits-per-cell for double arithmetic (with appropriate mapping for signed types). Division by zero typically results in a @code{-55 throw} (floatingpoint unidentified fault), although a @code{-10 throw} (divide by zero) would be more appropriate. @code{convert} and @code{>number} currently overflow silently. @item reading from an empty data or return stack: The data stack is checked by the outer (aka text) interpreter after every word executed. If it has underflowed, a @code{-4 throw} (Stack underflow) is performed. Apart from that, the stacks are not checked and underflows can result in similar behaviour as overflows (of adjacent stacks). @item unexpected end of the input buffer, resulting in an attempt to use a zero-length string as a name: @code{Create} and its descendants perform a @code{-16 throw} (Attempt to use zero-length string as a name). Words like @code{'} probably will not find what they search. Note that it is possible to create zero-length names with @code{nextname} (should it not?). @item @code{>IN} greater than input buffer: The next invocation of a parsing word returns a string wih length 0. @item @code{RECURSE} appears after @code{DOES>}: Compiles a recursive call to the defining word, not to the defined word. @item argument input source different than current input source for @code{RESTORE-INPUT}: @code{-12 THROW}. Note that, once an input file is closed (e.g., because the end of the file was reached), its source-id may be reused. Therefore, restoring an input source specification referencing a closed file may lead to unpredictable results instead of a @code{-12 THROW}. In the future, Gforth may be able to restore input source specifications from other than the current input soruce. @item data space containing definitions gets de-allocated: Deallocation with @code{allot} is not checked. This typically resuls in memory access faults or execution of illegal instructions. @item data space read/write with incorrect alignment: Processor-dependent. Typically results in a @code{-23 throw} (Address alignment exception). Under Linux on a 486 or later processor with alignment turned on, incorrect alignment results in a @code{-9 throw} (Invalid memory address). There are reportedly some processors with alignment restrictions that do not report them. @item data space pointer not properly aligned, @code{,}, @code{C,}: Like other alignment errors. @item less than u+2 stack items (@code{PICK} and @code{ROLL}): Not checked. May cause an illegal memory access. @item loop control parameters not available: Not checked. The counted loop words simply assume that the top of return stack items are loop control parameters and behave accordingly. @item most recent definition does not have a name (@code{IMMEDIATE}): @code{abort" last word was headerless"}. @item name not defined by @code{VALUE} used by @code{TO}: @code{-32 throw} (Invalid name argument) (unless name was defined by @code{CONSTANT}; then it just changes the constant). @item name not found (@code{'}, @code{POSTPONE}, @code{[']}, @code{[COMPILE]}): @code{-13 throw} (Undefined word) @item parameters are not of the same type (@code{DO}, @code{?DO}, @code{WITHIN}): Gforth behaves as if they were of the same type. I.e., you can predict the behaviour by interpreting all parameters as, e.g., signed. @item @code{POSTPONE} or @code{[COMPILE]} applied to @code{TO}: Assume @code{: X POSTPONE TO ; IMMEDIATE}. @code{X} performs the compilation semantics of @code{TO}. @item String longer than a counted string returned by @code{WORD}: Not checked. The string will be ok, but the count will, of course, contain only the least significant bits of the length. @item u greater than or equal to the number of bits in a cell (@code{LSHIFT}, @code{RSHIFT}): Processor-dependent. Typical behaviours are returning 0 and using only the low bits of the shift count. @item word not defined via @code{CREATE}: @code{>BODY} produces the PFA of the word no matter how it was defined. @code{DOES>} changes the execution semantics of the last defined word no matter how it was defined. E.g., @code{CONSTANT DOES>} is equivalent to @code{CREATE , DOES>}. @item words improperly used outside @code{<#} and @code{#>}: Not checked. As usual, you can expect memory faults. @end table @c --------------------------------------------------------------------- @node core-other, , core-ambcond, The Core Words @subsection Other system documentation @c --------------------------------------------------------------------- @table @i @item nonstandard words using @code{PAD}: None. @item operator's terminal facilities available: After processing the command line, Gforth goes into interactive mode, and you can give commands to Gforth interactively. The actual facilities available depend on how you invoke Gforth. @item program data space available: @code{sp@@ here - .} gives the space remaining for dictionary and data stack together. @item return stack space available: By default 16 KBytes. The default can be overridden with the @code{-r} switch (@pxref{Invocation}) when Gforth starts up. @item stack space available: @code{sp@@ here - .} gives the space remaining for dictionary and data stack together. @item system dictionary space required, in address units: Type @code{here forthstart - .} after startup. At the time of this writing, this gives 70108 (bytes) on a 32-bit system. @end table @c ===================================================================== @node The optional Block word set, The optional Double Number word set, The Core Words, ANS conformance @section The optional Block word set @c ===================================================================== @menu * block-idef:: Implementation Defined Options * block-ambcond:: Ambiguous Conditions * block-other:: Other System Documentation @end menu @c --------------------------------------------------------------------- @node block-idef, block-ambcond, The optional Block word set, The optional Block word set @subsection Implementation Defined Options @c --------------------------------------------------------------------- @table @i @item the format for display by @code{LIST}: First the screen number is displayed, then 16 lines of 64 characters, each line preceded by the line number. @item the length of a line affected by @code{\}: 64 characters. @end table @c --------------------------------------------------------------------- @node block-ambcond, block-other, block-idef, The optional Block word set @subsection Ambiguous conditions @c --------------------------------------------------------------------- @table @i @item correct block read was not possible: Typically results in a @code{throw} of some OS-derived value (between -512 and -2048). If the blocks file was just not long enough, blanks are supplied for the missing portion. @item I/O exception in block transfer: Typically results in a @code{throw} of some OS-derived value (between -512 and -2048). @item invalid block number: @code{-35 throw} (Invalid block number) @item a program directly alters the contents of @code{BLK}: The input stream is switched to that other block, at the same position. If the storing to @code{BLK} happens when interpreting non-block input, the system will get quite confused when the block ends. @item no current block buffer for @code{UPDATE}: @code{UPDATE} has no effect. @end table @c --------------------------------------------------------------------- @node block-other, , block-ambcond, The optional Block word set @subsection Other system documentation @c --------------------------------------------------------------------- @table @i @item any restrictions a multiprogramming system places on the use of buffer addresses: No restrictions (yet). @item the number of blocks available for source and data: depends on your disk space. @end table @c ===================================================================== @node The optional Double Number word set, The optional Exception word set, The optional Block word set, ANS conformance @section The optional Double Number word set @c ===================================================================== @menu * double-ambcond:: Ambiguous Conditions @end menu @c --------------------------------------------------------------------- @node double-ambcond, , The optional Double Number word set, The optional Double Number word set @subsection Ambiguous conditions @c --------------------------------------------------------------------- @table @i @item @var{d} outside of range of @var{n} in @code{D>S}: The least significant cell of @var{d} is produced. @end table @c ===================================================================== @node The optional Exception word set, The optional Facility word set, The optional Double Number word set, ANS conformance @section The optional Exception word set @c ===================================================================== @menu * exception-idef:: Implementation Defined Options @end menu @c --------------------------------------------------------------------- @node exception-idef, , The optional Exception word set, The optional Exception word set @subsection Implementation Defined Options @c --------------------------------------------------------------------- @table @i @item @code{THROW}-codes used in the system: The codes -256@minus{}-511 are used for reporting signals (see @file{errore.fs}). The codes -512@minus{}-2047 are used for OS errors (for file and memory allocation operations). The mapping from OS error numbers to throw code is -512@minus{}@code{errno}. One side effect of this mapping is that undefined OS errors produce a message with a strange number; e.g., @code{-1000 THROW} results in @code{Unknown error 488} on my system. @end table @c ===================================================================== @node The optional Facility word set, The optional File-Access word set, The optional Exception word set, ANS conformance @section The optional Facility word set @c ===================================================================== @menu * facility-idef:: Implementation Defined Options * facility-ambcond:: Ambiguous Conditions @end menu @c --------------------------------------------------------------------- @node facility-idef, facility-ambcond, The optional Facility word set, The optional Facility word set @subsection Implementation Defined Options @c --------------------------------------------------------------------- @table @i @item encoding of keyboard events (@code{EKEY}): Not yet implemeted. @item duration of a system clock tick System dependent. With respect to @code{MS}, the time is specified in microseconds. How well the OS and the hardware implement this, is another question. @item repeatability to be expected from the execution of @code{MS}: System dependent. On Unix, a lot depends on load. If the system is lightly loaded, and the delay is short enough that Gforth does not get swapped out, the performance should be acceptable. Under MS-DOS and other single-tasking systems, it should be good. @end table @c --------------------------------------------------------------------- @node facility-ambcond, , facility-idef, The optional Facility word set @subsection Ambiguous conditions @c --------------------------------------------------------------------- @table @i @item @code{AT-XY} can't be performed on user output device: Largely terminal dependant. No range checks are done on the arguments. No errors are reported. You may see some garbage appearing, you may see simply nothing happen. @end table @c ===================================================================== @node The optional File-Access word set, The optional Floating-Point word set, The optional Facility word set, ANS conformance @section The optional File-Access word set @c ===================================================================== @menu * file-idef:: Implementation Defined Options * file-ambcond:: Ambiguous Conditions @end menu @c --------------------------------------------------------------------- @node file-idef, file-ambcond, The optional File-Access word set, The optional File-Access word set @subsection Implementation Defined Options @c --------------------------------------------------------------------- @table @i @item File access methods used: @code{R/O}, @code{R/W} and @code{BIN} work as you would expect. @code{W/O} translates into the C file opening mode @code{w} (or @code{wb}): The file is cleared, if it exists, and created, if it does not (both with @code{open-file} and @code{create-file}). Under Unix @code{create-file} creates a file with 666 permissions modified by your umask. @item file exceptions: The file words do not raise exceptions (except, perhaps, memory access faults when you pass illegal addresses or file-ids). @item file line terminator: System-dependent. Gforth uses C's newline character as line terminator. What the actual character code(s) of this are is system-dependent. @item file name format System dependent. Gforth just uses the file name format of your OS. @item information returned by @code{FILE-STATUS}: @code{FILE-STATUS} returns the most powerful file access mode allowed for the file: Either @code{R/O}, @code{W/O} or @code{R/W}. If the file cannot be accessed, @code{R/O BIN} is returned. @code{BIN} is applicable along with the retured mode. @item input file state after an exception when including source: All files that are left via the exception are closed. @item @var{ior} values and meaning: The @var{ior}s returned by the file and memory allocation words are intended as throw codes. They typically are in the range -512@minus{}-2047 of OS errors. The mapping from OS error numbers to @var{ior}s is -512@minus{}@var{errno}. @item maximum depth of file input nesting: limited by the amount of return stack, locals/TIB stack, and the number of open files available. This should not give you troubles. @item maximum size of input line: @code{/line}. Currently 255. @item methods of mapping block ranges to files: By default, blocks are accessed in the file @file{blocks.fb} in the current working directory. The file can be switched with @code{USE}. @item number of string buffers provided by @code{S"}: 1 @item size of string buffer used by @code{S"}: @code{/line}. currently 255. @end table @c --------------------------------------------------------------------- @node file-ambcond, , file-idef, The optional File-Access word set @subsection Ambiguous conditions @c --------------------------------------------------------------------- @table @i @item attempting to position a file outside it's boundaries: @code{REPOSITION-FILE} is performed as usual: Afterwards, @code{FILE-POSITION} returns the value given to @code{REPOSITION-FILE}. @item attempting to read from file positions not yet written: End-of-file, i.e., zero characters are read and no error is reported. @item @var{file-id} is invalid (@code{INCLUDE-FILE}): An appropriate exception may be thrown, but a memory fault or other problem is more probable. @item I/O exception reading or closing @var{file-id} (@code{include-file}, @code{included}): The @var{ior} produced by the operation, that discovered the problem, is thrown. @item named file cannot be opened (@code{included}): The @var{ior} produced by @code{open-file} is thrown. @item requesting an unmapped block number: There are no unmapped legal block numbers. On some operating systems, writing a block with a large number may overflow the file system and have an error message as consequence. @item using @code{source-id} when @code{blk} is non-zero: @code{source-id} performs its function. Typically it will give the id of the source which loaded the block. (Better ideas?) @end table @c ===================================================================== @node The optional Floating-Point word set, The optional Locals word set, The optional File-Access word set, ANS conformance @section The optional Floating-Point word set @c ===================================================================== @menu * floating-idef:: Implementation Defined Options * floating-ambcond:: Ambiguous Conditions @end menu @c --------------------------------------------------------------------- @node floating-idef, floating-ambcond, The optional Floating-Point word set, The optional Floating-Point word set @subsection Implementation Defined Options @c --------------------------------------------------------------------- @table @i @item format and range of floating point numbers: System-dependent; the @code{double} type of C. @item results of @code{REPRESENT} when @var{float} is out of range: System dependent; @code{REPRESENT} is implemented using the C library function @code{ecvt()} and inherits its behaviour in this respect. @item rounding or truncation of floating-point numbers: System dependent; the rounding behaviour is inherited from the hosting C compiler. IEEE-FP-based (i.e., most) systems by default round to nearest, and break ties by rounding to even (i.e., such that the last bit of the mantissa is 0). @item size of floating-point stack: @code{s" FLOATING-STACK" environment? drop .}. Can be changed at startup with the command-line option @code{-f}. @item width of floating-point stack: @code{1 floats}. @end table @c --------------------------------------------------------------------- @node floating-ambcond, , floating-idef, The optional Floating-Point word set @subsection Ambiguous conditions @c --------------------------------------------------------------------- @table @i @item @code{df@@} or @code{df!} used with an address that is not double-float aligned: System-dependent. Typically results in a @code{-23 THROW} like other alignment violations. @item @code{f@@} or @code{f!} used with an address that is not float aligned: System-dependent. Typically results in a @code{-23 THROW} like other alignment violations. @item Floating-point result out of range: System-dependent. Can result in a @code{-55 THROW} (Floating-point unidentified fault), or can produce a special value representing, e.g., Infinity. @item @code{sf@@} or @code{sf!} used with an address that is not single-float aligned: System-dependent. Typically results in an alignment fault like other alignment violations. @item BASE is not decimal (@code{REPRESENT}, @code{F.}, @code{FE.}, @code{FS.}): The floating-point number is converted into decimal nonetheless. @item Both arguments are equal to zero (@code{FATAN2}): System-dependent. @code{FATAN2} is implemented using the C library function @code{atan2()}. @item Using ftan on an argument @var{r1} where cos(@var{r1}) is zero: System-dependent. Anyway, typically the cos of @var{r1} will not be zero because of small errors and the tan will be a very large (or very small) but finite number. @item @var{d} cannot be presented precisely as a float in @code{D>F}: The result is rounded to the nearest float. @item dividing by zero: @code{-55 throw} (Floating-point unidentified fault) @item exponent too big for conversion (@code{DF!}, @code{DF@@}, @code{SF!}, @code{SF@@}): System dependent. On IEEE-FP based systems the number is converted into an infinity. @item @var{float}<1 (@code{facosh}): @code{-55 throw} (Floating-point unidentified fault) @item @var{float}=<-1 (@code{flnp1}): @code{-55 throw} (Floating-point unidentified fault). On IEEE-FP systems negative infinity is typically produced for @var{float}=-1. @item @var{float}=<0 (@code{fln}, @code{flog}): @code{-55 throw} (Floating-point unidentified fault). On IEEE-FP systems negative infinity is typically produced for @var{float}=0. @item @var{float}<0 (@code{fasinh}, @code{fsqrt}): @code{-55 throw} (Floating-point unidentified fault). @code{fasinh} produces values for these inputs on my Linux box (Bug in the C library?) @item |@var{float}|>1 (@code{facos}, @code{fasin}, @code{fatanh}): @code{-55 throw} (Floating-point unidentified fault). @item integer part of float cannot be represented by @var{d} in @code{f>d}: @code{-55 throw} (Floating-point unidentified fault). @item string larger than pictured numeric output area (@code{f.}, @code{fe.}, @code{fs.}): This does not happen. @end table @c ===================================================================== @node The optional Locals word set, The optional Memory-Allocation word set, The optional Floating-Point word set, ANS conformance @section The optional Locals word set @c ===================================================================== @menu * locals-idef:: Implementation Defined Options * locals-ambcond:: Ambiguous Conditions @end menu @c --------------------------------------------------------------------- @node locals-idef, locals-ambcond, The optional Locals word set, The optional Locals word set @subsection Implementation Defined Options @c --------------------------------------------------------------------- @table @i @item maximum number of locals in a definition: @code{s" #locals" environment? drop .}. Currently 15. This is a lower bound, e.g., on a 32-bit machine there can be 41 locals of up to 8 characters. The number of locals in a definition is bounded by the size of locals-buffer, which contains the names of the locals. @end table @c --------------------------------------------------------------------- @node locals-ambcond, , locals-idef, The optional Locals word set @subsection Ambiguous conditions @c --------------------------------------------------------------------- @table @i @item executing a named local in interpretation state: @code{-14 throw} (Interpreting a compile-only word). @item @var{name} not defined by @code{VALUE} or @code{(LOCAL)} (@code{TO}): @code{-32 throw} (Invalid name argument) @end table @c ===================================================================== @node The optional Memory-Allocation word set, The optional Programming-Tools word set, The optional Locals word set, ANS conformance @section The optional Memory-Allocation word set @c ===================================================================== @menu * memory-idef:: Implementation Defined Options @end menu @c --------------------------------------------------------------------- @node memory-idef, , The optional Memory-Allocation word set, The optional Memory-Allocation word set @subsection Implementation Defined Options @c --------------------------------------------------------------------- @table @i @item values and meaning of @var{ior}: The @var{ior}s returned by the file and memory allocation words are intended as throw codes. They typically are in the range -512@minus{}-2047 of OS errors. The mapping from OS error numbers to @var{ior}s is -512@minus{}@var{errno}. @end table @c ===================================================================== @node The optional Programming-Tools word set, The optional Search-Order word set, The optional Memory-Allocation word set, ANS conformance @section The optional Programming-Tools word set @c ===================================================================== @menu * programming-idef:: Implementation Defined Options * programming-ambcond:: Ambiguous Conditions @end menu @c --------------------------------------------------------------------- @node programming-idef, programming-ambcond, The optional Programming-Tools word set, The optional Programming-Tools word set @subsection Implementation Defined Options @c --------------------------------------------------------------------- @table @i @item ending sequence for input following @code{;code} and @code{code}: @code{end-code} @item manner of processing input following @code{;code} and @code{code}: The @code{assembler} vocabulary is pushed on the search order stack, and the input is processed by the text interpreter, (starting) in interpret state. @item search order capability for @code{EDITOR} and @code{ASSEMBLER}: The ANS Forth search order word set. @item source and format of display by @code{SEE}: The source for @code{see} is the intermediate code used by the inner interpreter. The current @code{see} tries to output Forth source code as well as possible. @end table @c --------------------------------------------------------------------- @node programming-ambcond, , programming-idef, The optional Programming-Tools word set @subsection Ambiguous conditions @c --------------------------------------------------------------------- @table @i @item deleting the compilation wordlist (@code{FORGET}): Not implemented (yet). @item fewer than @var{u}+1 items on the control flow stack (@code{CS-PICK}, @code{CS-ROLL}): This typically results in an @code{abort"} with a descriptive error message (may change into a @code{-22 throw} (Control structure mismatch) in the future). You may also get a memory access error. If you are unlucky, this ambiguous condition is not caught. @item @var{name} can't be found (@code{forget}): Not implemented (yet). @item @var{name} not defined via @code{CREATE}: @code{;code} behaves like @code{DOES>} in this respect, i.e., it changes the execution semantics of the last defined word no matter how it was defined. @item @code{POSTPONE} applied to @code{[IF]}: After defining @code{: X POSTPONE [IF] ; IMMEDIATE}. @code{X} is equivalent to @code{[IF]}. @item reaching the end of the input source before matching @code{[ELSE]} or @code{[THEN]}: Continue in the same state of conditional compilation in the next outer input source. Currently there is no warning to the user about this. @item removing a needed definition (@code{FORGET}): Not implemented (yet). @end table @c ===================================================================== @node The optional Search-Order word set, , The optional Programming-Tools word set, ANS conformance @section The optional Search-Order word set @c ===================================================================== @menu * search-idef:: Implementation Defined Options * search-ambcond:: Ambiguous Conditions @end menu @c --------------------------------------------------------------------- @node search-idef, search-ambcond, The optional Search-Order word set, The optional Search-Order word set @subsection Implementation Defined Options @c --------------------------------------------------------------------- @table @i @item maximum number of word lists in search order: @code{s" wordlists" environment? drop .}. Currently 16. @item minimum search order: @code{root root}. @end table @c --------------------------------------------------------------------- @node search-ambcond, , search-idef, The optional Search-Order word set @subsection Ambiguous conditions @c --------------------------------------------------------------------- @table @i @item changing the compilation wordlist (during compilation): The word is entered into the wordlist that was the compilation wordlist at the start of the definition. Any changes to the name field (e.g., @code{immediate}) or the code field (e.g., when executing @code{DOES>}) are applied to the latest defined word (as reported by @code{last} or @code{lastxt}), if possible, irrespective of the compilation wordlist. @item search order empty (@code{previous}): @code{abort" Vocstack empty"}. @item too many word lists in search order (@code{also}): @code{abort" Vocstack full"}. @end table @node Model, Integrating Gforth, ANS conformance, Top @chapter Model This chapter has yet to be written. It will contain information, on which internal structures you can rely. @node Integrating Gforth, Emacs and Gforth, Model, Top @chapter Integrating Gforth into C programs This is not yet implemented. Several people like to use Forth as scripting language for applications that are otherwise written in C, C++, or some other language. The Forth system ATLAST provides facilities for embedding it into applications; unfortunately it has several disadvantages: most importantly, it is not based on ANS Forth, and it is apparently dead (i.e., not developed further and not supported). The facilities provided by Gforth in this area are inspired by ATLASTs facilities, so making the switch should not be hard. We also tried to design the interface such that it can easily be implemented by other Forth systems, so that we may one day arrive at a standardized interface. Such a standard interface would allow you to replace the Forth system without having to rewrite C code. You embed the Gforth interpreter by linking with the library @code{libgforth.a} (give the compiler the option @code{-lgforth}). All global symbols in this library that belong to the interface, have the prefix @code{forth_}. (Global symbols that are used internally have the prefix @code{gforth_}). You can include the declarations of Forth types and the functions and variables of the interface with @code{#include <forth.h>}. Types. Variables. Data and FP Stack pointer. Area sizes. functions. forth_init(imagefile) forth_evaluate(string) exceptions? forth_goto(address) (or forth_execute(xt)?) forth_continue() (a corountining mechanism) Adding primitives. No checking. Signals? Accessing the Stacks @node Emacs and Gforth, Internals, Integrating Gforth, Top @chapter Emacs and Gforth Gforth comes with @file{gforth.el}, an improved version of @file{forth.el} by Goran Rydqvist (included 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-u C-x \}) regions and removing debugging tracers (@kbd{C-x ~}, @pxref{Debugging}). 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, @pxref{Select Tags Table,,Selecting a Tags Table,emacs, Emacs Manual}). The TAGS file for the preloaded words is @file{(datadir)/gforth/\$(VERSION)/TAGS} (e.g.,
@file{/usr/local/share/gforth/0.2.0/TAGS}).

To get all these benefits, add the following lines to your @file{.emacs}
file:

@example
(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

The ideas in this section have also been published in the papers
@cite{ANS fig/GNU/??? Forth} (in German) by Bernd Paysan, presented at
the Forth-Tagung '93 and @cite{A Portable Forth Engine} by M. Anton
Ertl, presented at EuroForth '93; the latter is available at
@*@file{http://www.complang.tuwien.ac.at/papers/ertl93.ps.Z}.

* Portability::
* Primitives::
* System Architecture::
* Performance::

@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
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 Forth's
double numbers@footnote{Unfortunately, long longs are not implemented
properly on all machines (e.g., on alpha-osf1, long longs are only 64
bits, the same size as longs (and pointers), but they should be twice as
long according to @ref{Long Long, , Double-Word Integers, gcc.info, GNU
C Manual}). So, we had to implement doubles in C after all. Still, on
most machines we can use long longs and achieve better performance than
with the emulation package.}. 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.

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.

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}.

@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).

* Scheduling::
* DOES>::

@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
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.

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

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.

@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
(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
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
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

* Automatic Generation::
* TOS Optimization::
* Produced code::

@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
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 @bullet
@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 @bullet
@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
look at the resulting file @file{engine.s}.

@node System Architecture, Performance, 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 are considering 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
at run-time. The loader also has to replace tokens standing for
primitive calls with the appropriate code-field addresses (or code

@node  Performance,  , System Architecture, Internals
@section Performance

On RISCs the Gforth engine is very close to optimal; i.e., it is usually
impossible to write a significantly faster engine.

On register-starved machines like the 386 architecture processors
improvements are possible, because @code{gcc} does not utilize the
registers as well as a human, even with explicit register declarations;
e.g., Bernd Beuster wrote a Forth system fragment in assembly language
and hand-tuned it for the 486; this system is 1.19 times faster on the
Sieve benchmark on a 486DX2/66 than Gforth compiled with
@code{gcc-2.6.3} with @code{-DFORCE_REG}.

However, this potential advantage of assembly language implementations
is not necessarily realized in complete Forth systems: We compared
Gforth (direct threaded, compiled with @code{gcc-2.6.3} and
@code{-DFORCE_REG}) with Win32Forth 1.2093, LMI's NT Forth (Beta, May
1994) and Eforth (with and without peephole (aka pinhole) optimization
of the threaded code); all these systems were written in assembly
language. We also compared Gforth with three systems written in C:
PFE-0.9.14 (compiled with @code{gcc-2.6.3} with the default
configuration for Linux: @code{-O2 -fomit-frame-pointer -DUSE_REGS
-DUNROLL_NEXT}), ThisForth Beta (compiled with gcc-2.6.3 -O3
-fomit-frame-pointer; ThisForth employs peephole optimization of the
threaded code) and TILE (compiled with @code{make opt}). We benchmarked
Gforth, PFE, ThisForth and TILE on a 486DX2/66 under Linux. Kenneth
O'Heskin kindly provided the results for Win32Forth and NT Forth on a
486DX2/66 with similar memory performance under Windows NT. Marcel
Hendrix ported Eforth to Linux, then extended it to run the benchmarks,
added the peephole optimizer, ran the benchmarks and reported the
results.

We used four small benchmarks: the ubiquitous Sieve; bubble-sorting and
matrix multiplication come from the Stanford integer benchmarks and have
been translated into Forth by Martin Fraeman; we used the versions
included in the TILE Forth package, but with bigger data set sizes; and
a recursive Fibonacci number computation for benchmarking calling
performance. The following table shows the time taken for the benchmarks
scaled by the time taken by Gforth (in other words, it shows the speedup
factor that Gforth achieved over the other systems).

@example
relative      Win32-    NT       eforth       This-
time  Gforth Forth Forth eforth  +opt   PFE Forth  TILE
sieve     1.00  1.39  1.14   1.39  0.85  1.58  3.18  8.58
bubble    1.00  1.31  1.41   1.48  0.88  1.50        3.88
matmul    1.00  1.47  1.35   1.46  0.74  1.58        4.09
fib       1.00  1.52  1.34   1.22  0.86  1.74  2.99  4.30
@end example

You may find the good performance of Gforth compared with the systems
written in assembly language quite surprising. One important reason for
the disappointing performance of these systems is probably that they are
not written optimally for the 486 (e.g., they use the @code{lods}
instruction). In addition, Win32Forth uses a comfortable, but costly
method for relocating the Forth image: like @code{cforth}, it computes
per NEXT (@pxref{System Architecture}).

Only Eforth with the peephole optimizer performs comparable to
Gforth. The speedups achieved with peephole optimization of threaded
code are quite remarkable. Adding a peephole optimizer to Gforth should
cause similar speedups.

The speedup of Gforth over PFE, ThisForth and TILE can be easily
explained with the self-imposed restriction to standard C, which makes
efficient threading impossible (however, the measured implementation of
PFE uses a GNU C extension: @ref{Global Reg Vars, , Defining Global
Register Variables, gcc.info, GNU C Manual}).  Moreover, current C
compilers have a hard time optimizing other aspects of the ThisForth
and the TILE source.

Note that the performance of Gforth on 386 architecture processors
varies widely with the version of @code{gcc} used. E.g., @code{gcc-2.5.8}
failed to allocate any of the virtual machine registers into real
machine registers by itself and would not work correctly with explicit
register declarations, giving a 1.3 times slower engine (on a 486DX2/66
running the Sieve) than the one measured above.

In @cite{Translating Forth to Efficient C} by M. Anton Ertl and Martin
Maierhofer (presented at EuroForth '95), an indirect threaded version of
Gforth is compared with Win32Forth, NT Forth, PFE, and ThisForth; that
version of Gforth is 2\%@minus{}8\% slower on a 486 than the version
used here. The paper available at
@*@file{http://www.complang.tuwien.ac.at/papers/ertl&maierhofer95.ps.gz};
it also contains numbers for some native code systems. You can find
numbers for Gforth on various machines in @file{Benchres}.

@node Bugs, Origin, Internals, Top
@chapter Bugs

Known bugs are described in the file BUGS in the Gforth distribution.

If you find a bug, please send a bug report to
@code{bug-gforth@@gnu.ai.mit.edu}. A bug report should
describe the Gforth version used (it is announced at the start of an
interactive Gforth session), the machine and operating system (on Unix
systems you can use @code{uname -a} to produce this information), the
installation options (send the @code{config.status} file), and a
complete list of changes you (or your installer) have made to the Gforth
sources (if any); it should contain a program (or a sequence of keyboard
commands) that reproduces the bug and a description of what you think
constitutes the buggy behaviour.

For a thorough guide on reporting bugs read @ref{Bug Reporting, , How
to Report Bugs, gcc.info, GNU C Manual}.

@node Origin, Word Index, Bugs, Top
@chapter Authors and Ancestors of Gforth

@section Authors and Contributors

The Gforth project was started in mid-1992 by Bernd Paysan and Anton
Ertl. The third major author was Jens Wilke.  Lennart Benschop (who was
one of Gforth's first users, in mid-1993) and Stuart Ramsden inspired us
with their continuous feedback. Lennart Benshop contributed
@file{glosgen.fs}, while Stuart Ramsden has been working on automatic
Kleinrubatscher, Christian Pirker, Dirk Zoller, Marcel Hendrix, John
Wavrik, Barrie Stott and Marc de Groot.

Gforth also owes a lot to the authors of the tools we used (GCC, CVS,
and autoconf, among others), and to the creators of the Internet: Gforth
was developed across the Internet, and its authors have not met
physically yet.

@section Pedigree

Gforth descends from BigForth (1993) and fig-Forth. Gforth and PFE (by
Dirk Zoller) will cross-fertilize each other. Of course, a significant
part of the design of Gforth was prescribed by ANS Forth.

Bernd Paysan wrote BigForth, a descendent from TurboForth, an unreleased
32 bit native code version of VolksForth for the Atari ST, written
mostly by Dietrich Weineck.

VolksForth descends from F83. It was written by Klaus Schleisiek, Bernd
Pennemann, Georg Rehfeld and Dietrich Weineck for the C64 (called
UltraForth there) in the mid-80s and ported to the Atari ST in 1986.

Henry Laxen and Mike Perry wrote F83 as a model implementation of the
Forth-83 standard. !! Pedigree? When?

A team led by Bill Ragsdale implemented fig-Forth on many processors in
1979. Robert Selzer and Bill Ragsdale developed the original
implementation of fig-Forth for the 6502 based on microForth.

The principal architect of microForth was Dean Sanderson. microForth was
FORTH, Inc.'s first off-the-shelf product. It was developped in 1976 for
the 1802, and subsequently implemented on the 8080, the 6800 and the
Z80.

All earlier Forth systems were custom-made, usually by Charles Moore,
who discovered (as he puts it) Forth during the late 60s. The first full
Forth existed in 1971.

A part of the information in this section comes from @cite{The Evolution
of Forth} by Elizabeth D. Rather, Donald R. Colburn and Charles
H. Moore, presented at the HOPL-II conference and preprinted in SIGPLAN
Notices 28(3), 1993.  You can find more historical and genealogical

@node Word Index, Node Index, Origin, Top
@chapter Word Index

This index is as incomplete as the manual. Each word is listed with
stack effect and wordset.

@printindex fn

@node Node Index,  , Word Index, Top
@chapter Node Index

This index is even less complete than the manual.

@contents  @contents
@bye  @bye

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