### Diff for /gforth/Attic/gforth.ds between versions 1.2 and 1.17

version 1.2, 1994/11/14 19:01:16 version 1.17, 1995/09/15 14:52:51
Line 1 Line 1
\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 0.1

Copyright @copyright{} 1994 GNU Forth Development Group  Copyright @copyright{} 1994 Gforth Development Group

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
Line 38  Copyright @copyright{} 1994 GNU Forth De Line 38  Copyright @copyright{} 1994 GNU Forth De

@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 0.1
@sp 2  @sp 2
@center Anton Ertl  @center Anton Ertl
@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
Copyright @copyright{} 1994 GNU Forth Development Group  Copyright @copyright{} 1994 Gforth Development Group

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

Line 72  Copyright @copyright{} 1994 GNU Forth De Line 74  Copyright @copyright{} 1994 GNU Forth De

@node Top, License, (dir), (dir)  @node Top, License, (dir), (dir)
@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 0.0.
@end ifinfo  @end ifinfo

@menu  @menu
* License::               * License::
* 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.  * ANS conformance::             Implementation-defined options etc.
* Model::               The abstract machine of GNU Forth  * Model::                       The abstract machine of Gforth
* Emacs and GForth::    The GForth Mode  * Emacs and Gforth::            The Gforth Mode
* Internals::           Implementation details  * Internals::                   Implementation details
* Bugs::                How to report them  * Bugs::                        How to report them
* Pedigree::            Ancestors of GNU Forth  * Pedigree::                    Ancestors of Gforth
* Word Index::          An item for each Forth word  * Word Index::                  An item for each Forth word
* Node Index::          An item for each node  * Node Index::                  An item for each node
@end menu  @end menu

@node License, Goals, Top, Top  @node License, Goals, Top, Top
Line 98  personal machines. This manual correspon Line 100  personal machines. This manual correspon

@iftex  @iftex
@unnumbered Preface  @unnumbered Preface
This manual documents GNU Forth. The reader is expected to know  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 123  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 139  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.

Line 173  other languages should find it ok. Line 173  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 253  the user initialization file @file{.gfor Line 253  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, ANS conformance, Invocation, Top
@chapter Forth Words  @chapter Forth Words

@menu  @menu
* Notation::  * Notation::
* Arithmetic::  * Arithmetic::
* Stack Manipulation::  * Stack Manipulation::
* Memory access::  * Memory access::
* Control Structures::  * Control Structures::
* Local Variables::  * Locals::
* Defining Words::  * Defining Words::
* Vocabularies::  * Wordlists::
* Files::  * Files::
* Blocks::  * Blocks::
* Other I/O::  * Other I/O::
* Programming Tools::  * Programming Tools::
* Threading Words::
@end menu  @end menu

@node Notation, Arithmetic, Words, Words  @node Notation, Arithmetic, Words, Words
Line 277  then in @file{~}, then in the normal pat Line 278  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
Line 314  wordsets. Words that are not defined in Line 315  wordsets. Words that are not defined in
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 353  Wordlist ID, same size as Cell Line 353  Wordlist ID, same size as Cell
Pointer to a name structure  Pointer to a name structure
@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
overflow on addition or multiplication, you may hear about division by  overflow on addition or multiplication, you may hear about division by
Line 363  corresponds to @code{2 1 -}. Forth offer Line 363  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}).

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

@node Single precision, Bitwise operations, Arithmetic, Arithmetic
@subsection Single precision  @subsection Single precision
doc-+  doc-+
doc--  doc--
Line 377  doc-abs Line 386  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 395  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 407  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 421  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+1}. 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
avoid them), you might start with @cite{David Goldberg, What Every
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 496  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).

@menu
* Data stack::
* Floating point stack::
* Return stack::
* Locals stack::
* Stack pointer manipulation::
@end menu

@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 535  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 545  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 556  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 570  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

@menu
* Stack-Memory transfers::
* Address arithmetic::
* Memory block access::
@end menu

@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 596  doc-sf!
doc-df@  doc-df@
doc-df!  doc-df!

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

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 612  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 622  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 cell-aligned; in addition, gforth guarantees that these addresses  are cell-aligned; in addition, Gforth guarantees that these addresses
are aligned for all purposes.  are aligned for all purposes.

Note that the standard defines a word @code{char}, which has nothing to
do with address arithmetic.

doc-chars  doc-chars
doc-char+  doc-char+
doc-cells  doc-cells
Line 540  doc-dfloats Line 646  doc-dfloats
doc-dfloat+  doc-dfloat+
doc-dfalign  doc-dfalign
doc-dfaligned  doc-dfaligned
doc-maxalign
doc-maxaligned
doc-cfalign
doc-cfaligned
doc-address-unit-bits  doc-address-unit-bits

@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 666  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 674  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.

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

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

@example  @example
Line 581  ELSE Line 702  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 608  can avoid using @code{?dup}. Line 729  can avoid using @code{?dup}.
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 738  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 770  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 703  Therefore we recommend avoiding using @c Line 826  Therefore we recommend avoiding using @c
@var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative  @var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative
case behaves symmetrical to the positive case:  case behaves symmetrical to the positive case:

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

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

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

The loop is terminated when the border between @var{limit@minus{}sgn(n)} and  The loop is terminated when the border between @var{limit@minus{}sgn(n)} and
@var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS  @var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS
Line 729  FOR Line 852  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.

@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
ahead  doc-ahead
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 887  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

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-for
loop  doc-loop
s+loop  doc-s+loop
+loop  doc-+loop
next  doc-next
leave  doc-leave
?leave  doc-?leave
unloop  doc-unloop
undo  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.

E.g., instead of writing

@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{BEGIN} 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 808  locals wordset, but also our own, more p Line 996  locals wordset, but also our own, more p
implemented the ANS Forth locals wordset through our locals wordset).  implemented the ANS Forth locals wordset through our locals wordset).

@menu  @menu
* Gforth locals::
* ANS Forth locals::
@end menu  @end menu

@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 1044  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 1064  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:

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

@node Where are locals visible by name?, How long do locals live?, Gforth locals, Gforth locals
@subsubsection Where are locals visible by name?  @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 1123  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
AHEAD  AHEAD
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 1165  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 1181  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 1196  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 @}
AHEAD  AHEAD
ASSUME-LIVE  ASSUME-LIVE
BEGIN  BEGIN
Line 1025  rearranging the loop. E.g., the most i Line 1226  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 1247  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 1259  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
with a traditional implementation of @code{max}).  a traditional implementation of @code{max}).

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 1113  are initialized with the right value for Line 1316  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 1340 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 1354  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 1394  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 1444  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 1465  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
Line 1274  The whole definition must be in one line Line 1479  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{values}). I.e., they are initialized from the stack. Using their  (@xref{Values}). 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{anslocal.fs} to implement the syntax
on the other system.  on the other system.
Line 1292  doc-(local) Line 1497  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, Wordlists, Locals, Words
@section Defining Words

@menu
* Values::
@end menu

@node Values,  , Defining Words, Defining Words
@subsection Values

@node Wordlists, Files, Defining Words, Words
@section Wordlists

@node Files, Blocks, Wordlists, Words
@section Files

@node Blocks, Other I/O, Files, Words
@section Blocks

@node Other I/O, Programming Tools, Blocks, Words
@section Other I/O

@node Programming Tools, Threading Words, Other I/O, Words
@section Programming Tools

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

@node Debugging, Assertions, Programming Tools, Programming Tools
@subsection Debugging

The simple debugging aids provided in @file{debugging.fs}
are meant to support a different style of debugging than the
tracing/stepping debuggers used in languages with long turn-around
times.

A much better (faster) way in fast-compilig languages is to add
printing code at well-selected places, let the program run, look at
the output, see where things went wrong, add more printing code, etc.,
until the bug is found.

The word @code{~~} is easy to insert. It just prints debugging
information (by default the source location and the stack contents). It
is also easy to remove (@kbd{C-x ~} in the Emacs Forth mode to
query-replace them with nothing). The deferred words
@code{printdebugdata} and @code{printdebugline} control the output of
@code{~~}. The default source location output format works well with
Emacs' compilation mode, so you can step through the program at the
source level using @kbd{C-x } (the advantage over a stepping debugger
is that you can step in any direction and you know where the crash has
happened or where the strange data has occurred).

Note that the default actions clobber the contents of the pictured
numeric output string, so you should not use @code{~~}, e.g., between
@code{<#} and @code{#>}.

doc-~~
doc-printdebugdata
doc-printdebugline

@node Assertions,  , Debugging, Programming Tools
@subsection Assertions

It is a good idea to make your programs self-checking, in particular, if
you use an assumption (e.g., that a certain field of a data structure is
never zero) that may become wrong during maintenance. Gforth supports
assertions for this purpose. They are used like this:

@example
assert( @var{flag} )
@end example

The code between @code{assert(} and @code{)} should compute a flag, that
should be true if everything is alright and false otherwise. It should
not change anything else on the stack. The overall stack effect of the
assertion is @code{( -- )}. E.g.

@example
assert( 1 1 + 2 = ) \ what we learn in school
assert( dup 0<> ) \ assert that the top of stack is not zero
assert( false ) \ this code should not be reached
@end example

The need for assertions is different at different times. During
debugging, we want more checking, in production we sometimes care more
for speed. Therefore, assertions can be turned off, i.e., the assertion
becomes a comment. Depending on the importance of an assertion and the
time it takes to check it, you may want to turn off some assertions and
keep others turned on. Gforth provides several levels of assertions for
this purpose:

doc-assert0(
doc-assert1(
doc-assert2(
doc-assert3(
doc-assert(
doc-)

@code{Assert(} is the same as @code{assert1(}. The variable
@code{assert-level} specifies the highest assertions that are turned
on. I.e., at the default @code{assert-level} of one, @code{assert0(} and
@code{assert1(} assertions perform checking, while @code{assert2(} and
@code{assert3(} assertions are treated as comments.

Note that the @code{assert-level} is evaluated at compile-time, not at
run-time. I.e., you cannot turn assertions on or off at run-time, you
have to set the @code{assert-level} appropriately before compiling a
piece of code. You can compile several pieces of code at several
@code{assert-level}s (e.g., a trusted library at level 1 and newly
written code at level 3).

doc-assert-level

If an assertion fails, a message compatible with Emacs' compilation mode
is produced and the execution is aborted (currently with @code{ABORT"}.
If there is interest, we will introduce a special throw code. But if you
intend to @code{catch} a specific condition, using @code{throw} is
probably more appropriate than an assertion).

@node Threading Words,  , Programming Tools, Words
@section Threading Words

These words provide access to code addresses and other threading stuff
in Gforth (and, possibly, other interpretive Forths). It more or less
abstracts away the differences between direct and indirect threading
(and, for direct threading, the machine dependences). However, at
present this wordset is still inclomplete. It is also pretty low-level;
some day it will hopefully be made unnecessary by an internals words set
that abstracts implementation details away completely.

doc->code-address
doc->does-code
doc-code-address!
doc-does-code!
doc-does-handler!
doc-/does-handler

@node ANS conformance, Model, Words, Top
@chapter ANS conformance

To the best of our knowledge, Gforth is an

ANS Forth System
@itemize
@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{AHEAD}, @code{BYE}, @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{putchar}.

@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. 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} and @kbd{C-j} 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:
The user output device is the standard output. It cannot be redirected
from within Gforth, but typically from the command line that starts
Gforth. 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. 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 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} (floatingpoint 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)

@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:
You get an execution token representing the compilation semantics
instead.

@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). Note that this is
checked only by the outer (aka text) interpreter; if the word is
@code{execute}d in some other way, it will typically perform it's
compilation semantics even in interpret state. (We could change @code{'}
and relatives not to give the xt of such words, but we think that would
be too restrictive).

@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 unexepected 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}:
!!???If the argument input source is a valid input source then it gets
restored. Otherwise causes @code{-12 THROW} which unless caught issues
the message "argument type mismatch" and aborts.

@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)

@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} is equivalent to
@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:
!!??

@item program data space available:
@code{sp@ here - .} gives the space remaining for dictionary and data
stack together.

@item return stack space available:
!!??

@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{}@var{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:
Currently, the block words automatically access the file
@file{blocks.fb} in the currend working directory. More sophisticated
methods could be implemented if there is demand (and a volunteer).

@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:
What's the question?!!

@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 an alignment fault like other
alignment violations.

@item @code{f@@} or @code{f!} used with an address that is not float  aligned:
System-dependent. Typically results in an alignment fault 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}:
Not implemented (yet).

@item manner of processing input following @code{;code} and @code{code}:
Not implemented (yet).

@item search order capability for @code{EDITOR} and @code{ASSEMBLER}:
Not implemented (yet). If they were implemented, they would use the
search order wordset.

@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} is not implemented (yet). If it were, it would behave like
@code{DOES>} in this respect, i.e., change 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 definition is put into the wordlist that is the compilation wordlist
when @code{REVEAL} is executed (by @code{;}, @code{DOES>},
@code{RECURSIVE}, etc.).

@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, Emacs and Gforth, ANS conformance, Top
@chapter Model

@node Emacs and Gforth, Internals, Model, Top
@chapter Emacs and Gforth

Gforth comes with @file{gforth.el}, an improved version of
@file{forth.el} by Goran Rydqvist (icluded in the TILE package). The
improvements are a better (but still not perfect) handling of
indentation. I have also added comment paragraph filling (@kbd{M-q}),
commenting (@kbd{C-x \}) and uncommenting (@kbd{C-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).

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

@example
(autoload 'forth-mode "gforth.el")
(setq auto-mode-alist (cons '("\\.fs\\'" . forth-mode) auto-mode-alist))
@end example

@node Internals, Bugs, Emacs and Gforth, Top
@chapter Internals

Reading this section is not necessary for programming with Gforth. It
should be helpful for finding your way in the Gforth sources.

@menu
* Portability::
* Threading::
* Primitives::
* System Architecture::
* Performance::
@end menu

@node Portability, Threading, Internals, Internals
@section Portability

One of the main goals of the effort is availability across a wide range
of personal machines. fig-Forth, and, to a lesser extent, F83, achieved
this goal by manually coding the engine in assembly language for several
then-popular processors. This approach is very labor-intensive and the
results are short-lived due to progress in computer architecture.

Others have avoided this problem by coding in C, e.g., Mitch Bradley
(cforth), Mikael Patel (TILE) and Dirk Zoller (pfe). This approach is
particularly popular for UNIX-based Forths due to the large variety of
architectures of UNIX machines. Unfortunately an implementation in C
does not mix well with the goals of efficiency and with using
traditional techniques: Indirect or direct threading cannot be expressed
in C, and switch threading, the fastest technique available in C, is
significantly slower. Another problem with C is that it's very
cumbersome to express double integer arithmetic.

Fortunately, there is a portable language that does not have these
limitations: GNU C, the version of C processed by the GNU C compiler
(@pxref{C Extensions, , Extensions to the C Language Family, gcc.info,
GNU C Manual}). Its labels as values feature (@pxref{Labels as Values, ,
Labels as Values, gcc.info, GNU C Manual}) makes direct and indirect
threading possible, its @code{long long} type (@pxref{Long Long, ,
Double-Word Integers, gcc.info, GNU C Manual}) corresponds to Forths
double numbers. GNU C is available for free on all important (and many
unimportant) UNIX machines, VMS, 80386s running MS-DOS, the Amiga, and
the Atari ST, so a Forth written in GNU C can run on all these
machines.

Writing in a portable language has the reputation of producing code that
is slower than assembly. For our Forth engine we repeatedly looked at
the code produced by the compiler and eliminated most compiler-induced
inefficiencies by appropriate changes in the source-code.

However, register allocation cannot be portably influenced by the
programmer, leading to some inefficiencies on register-starved
machines. We use explicit register declarations (@pxref{Explicit Reg
Vars, , Variables in Specified Registers, gcc.info, GNU C Manual}) to
improve the speed on some machines. They are turned on by using the
@code{gcc} switch @code{-DFORCE_REG}. Unfortunately, this feature not
only depends on the machine, but also on the compiler version: On some
machines some compiler versions produce incorrect code when certain
explicit register declarations are used. So by default
@code{-DFORCE_REG} is not used.

@node Threading, Primitives, Portability, Internals
@section Threading

GNU C's labels as values extension (available since @code{gcc-2.0},
@pxref{Labels as Values, , Labels as Values, gcc.info, GNU C Manual})
makes it possible to take the address of @var{label} by writing
@code{&&@var{label}}.  This address can then be used in a statement like
@code{goto *@var{address}}. I.e., @code{goto *&&x} is the same as
@code{goto x}.

With this feature an indirect threaded NEXT looks like:
@example
cfa = *ip++;
ca = *cfa;
goto *ca;
@end example
For those unfamiliar with the names: @code{ip} is the Forth instruction
pointer; the @code{cfa} (code-field address) corresponds to ANS Forths
execution token and points to the code field of the next word to be
executed; The @code{ca} (code address) fetched from there points to some
executable code, e.g., a primitive or the colon definition handler
@code{docol}.

Direct threading is even simpler:
@example
ca = *ip++;
goto *ca;
@end example

Of course we have packaged the whole thing neatly in macros called
@code{NEXT} and @code{NEXT1} (the part of NEXT after fetching the cfa).

@menu
* Scheduling::
* Direct or Indirect Threaded?::
* DOES>::
@end menu

@node Scheduling, Direct or Indirect Threaded?, Threading, Threading
@subsection Scheduling

There is a little complication: Pipelined and superscalar processors,
i.e., RISC and some modern CISC machines can process independent
instructions while waiting for the results of an instruction. The
compiler usually reorders (schedules) the instructions in a way that
achieves good usage of these delay slots. However, on our first tries
the compiler did not do well on scheduling primitives. E.g., for
@code{+} implemented as
@example
n=sp[0]+sp[1];
sp++;
sp[0]=n;
NEXT;
@end example
the NEXT comes strictly after the other code, i.e., there is nearly no
scheduling. After a little thought the problem becomes clear: The
compiler cannot know that sp and ip point to different addresses (and
the version of @code{gcc} we used would not know it even if it was
possible), so it could not move the load of the cfa above the store to
the TOS. Indeed the pointers could be the same, if code on or very near
the top of stack were executed. In the interest of speed we chose to
forbid this probably unused feature'' and helped the compiler in
scheduling: NEXT is divided into the loading part (@code{NEXT_P1}) and
the goto part (@code{NEXT_P2}). @code{+} now looks like:
@example
n=sp[0]+sp[1];
sp++;
NEXT_P1;
sp[0]=n;
NEXT_P2;
@end example
This can be scheduled optimally by the compiler.

This division can be turned off with the switch @code{-DCISC_NEXT}. This
switch is on by default on machines that do not profit from scheduling
(e.g., the 80386), in order to preserve registers.

@node Direct or Indirect Threaded?, DOES>, Scheduling, Threading
@subsection Direct or Indirect Threaded?

Both! After packaging the nasty details in macro definitions we
realized that we could switch between direct and indirect threading by
simply setting a compilation flag (@code{-DDIRECT_THREADED}) and
defining a few machine-specific macros for the direct-threading case.
On the Forth level we also offer access words that hide the
differences between the threading methods (@pxref{Threading Words}).

Indirect threading is implemented completely
machine-independently. Direct threading needs routines for creating
jumps to the executable code (e.g. to docol or dodoes). These routines
are inherently machine-dependent, but they do not amount to many source
lines. I.e., even porting direct threading to a new machine is a small
effort.

@node DOES>,  , Direct or Indirect Threaded?, Threading
@subsection DOES>
One of the most complex parts of a Forth engine is @code{dodoes}, i.e.,
the chunk of code executed by every word defined by a
@code{CREATE}...@code{DOES>} pair. The main problem here is: How to find
the Forth code to be executed, i.e. the code after the @code{DOES>} (the
DOES-code)? There are two solutions:

In fig-Forth the code field points directly to the dodoes and the
DOES-code address is stored in the cell after the code address
(i.e. at cfa cell+). It may seem that this solution is illegal in the
Forth-79 and all later standards, because in fig-Forth this address
lies in the body (which is illegal in these standards). However, by
making the code field larger for all words this solution becomes legal
again. We use this approach for the indirect threaded version. Leaving
a cell unused in most words is a bit wasteful, but on the machines we
are targetting this is hardly a problem. The other reason for having a
code field size of two cells is to avoid having different image files
for direct and indirect threaded systems (@pxref{System Architecture}).

The other approach is that the code field points or jumps to the cell
after @code{DOES}. In this variant there is a jump to @code{dodoes} at
this address. @code{dodoes} can then get the DOES-code address by
computing the code address, i.e., the address of the jump to dodoes,
and add the length of that jump field. A variant of this is to have a
call to @code{dodoes} after the @code{DOES>}; then the return address
(which can be found in the return register on RISCs) is the DOES-code
address. Since the two cells available in the code field are usually
used up by the jump to the code address in direct threading, we use
this approach for direct threading. We did not want to add another
cell to the code field.

@node Primitives, System Architecture, Threading, Internals
@section Primitives

@menu
* Automatic Generation::
* TOS Optimization::
* Produced code::
@end menu

@node Automatic Generation, TOS Optimization, Primitives, Primitives
@subsection Automatic Generation

Since the primitives are implemented in a portable language, there is no
longer any need to minimize the number of primitives. On the contrary,
having many primitives is an advantage: speed. In order to reduce the
number of errors in primitives and to make programming them easier, we
provide a tool, the primitive generator (@file{prims2x.fs}), that
automatically generates most (and sometimes all) of the C code for a
primitive from the stack effect notation.  The source for a primitive
has the following form:

@format
@var{Forth-name}        @var{stack-effect}      @var{category}  [@var{pronounc.}]
[@code{""}@var{glossary entry}@code{""}]
@var{C code}
[@code{:}
@var{Forth code}]
@end format

The items in brackets are optional. The category and glossary fields
are there for generating the documentation, the Forth code is there
for manual implementations on machines without GNU C. E.g., the source
for the primitive @code{+} is:
@example
+    n1 n2 -- n    core    plus
n = n1+n2;
@end example

This looks like a specification, but in fact @code{n = n1+n2} is C
code. Our primitive generation tool extracts a lot of information from
the stack effect notations@footnote{We use a one-stack notation, even
though we have separate data and floating-point stacks; The separate
notation can be generated easily from the unified notation.}: The number
of items popped from and pushed on the stack, their type, and by what
name they are referred to in the C code. It then generates a C code
prelude and postlude for each primitive. The final C code for @code{+}
looks like this:

@example
I_plus: /* + ( n1 n2 -- n ) */  /* label, stack effect */
/*  */                          /* documentation */
@{
DEF_CA                          /* definition of variable ca (indirect threading) */
Cell n1;                        /* definitions of variables */
Cell n2;
Cell n;
n1 = (Cell) sp[1];              /* input */
n2 = (Cell) TOS;
sp += 1;                        /* stack adjustment */
NAME("+")                       /* debugging output (with -DDEBUG) */
@{
n = n1+n2;                      /* C code taken from the source */
@}
NEXT_P1;                        /* NEXT part 1 */
TOS = (Cell)n;                  /* output */
NEXT_P2;                        /* NEXT part 2 */
@}
@end example

This looks long and inefficient, but the GNU C compiler optimizes quite
well and produces optimal code for @code{+} on, e.g., the R3000 and the
HP RISC machines: Defining the @code{n}s does not produce any code, and
using them as intermediate storage also adds no cost.

There are also other optimizations, that are not illustrated by this
example: Assignments between simple variables are usually for free (copy
propagation). If one of the stack items is not used by the primitive
(e.g.  in @code{drop}), the compiler eliminates the load from the stack
(dead code elimination). On the other hand, there are some things that
the compiler does not do, therefore they are performed by
@file{prims2x.fs}: The compiler does not optimize code away that stores
a stack item to the place where it just came from (e.g., @code{over}).

While programming a primitive is usually easy, there are a few cases
where the programmer has to take the actions of the generator into
account, most notably @code{?dup}, but also words that do not (always)
fall through to NEXT.

@node TOS Optimization, Produced code, Automatic Generation, Primitives
@subsection TOS Optimization

An important optimization for stack machine emulators, e.g., Forth
engines, is keeping  one or more of the top stack items in
registers.  If a word has the stack effect @var{in1}...@var{inx} @code{--}
@var{out1}...@var{outy}, keeping the top @var{n} items in registers
@itemize
@item
is better than keeping @var{n-1} items, if @var{x>=n} and @var{y>=n},
due to fewer loads from and stores to the stack.
@item is slower than keeping @var{n-1} items, if @var{x<>y} and @var{x<n} and
@var{y<n}, due to additional moves between registers.
@end itemize

In particular, keeping one item in a register is never a disadvantage,
if there are enough registers. Keeping two items in registers is a
disadvantage for frequent words like @code{?branch}, constants,
variables, literals and @code{i}. Therefore our generator only produces
code that keeps zero or one items in registers. The generated C code
covers both cases; the selection between these alternatives is made at
C-compile time using the switch @code{-DUSE_TOS}. @code{TOS} in the C
code for @code{+} is just a simple variable name in the one-item case,
otherwise it is a macro that expands into @code{sp[0]}. Note that the
GNU C compiler tries to keep simple variables like @code{TOS} in
registers, and it usually succeeds, if there are enough registers.

The primitive generator performs the TOS optimization for the
floating-point stack, too (@code{-DUSE_FTOS}). For floating-point
operations the benefit of this optimization is even larger:
floating-point operations take quite long on most processors, but can be
performed in parallel with other operations as long as their results are
not used. If the FP-TOS is kept in a register, this works. If
it is kept on the stack, i.e., in memory, the store into memory has to
wait for the result of the floating-point operation, lengthening the
execution time of the primitive considerably.

The TOS optimization makes the automatic generation of primitives a
bit more complicated. Just replacing all occurrences of @code{sp[0]} by
@code{TOS} is not sufficient. There are some special cases to
consider:
@itemize
@item In the case of @code{dup ( w -- w w )} the generator must not
eliminate the store to the original location of the item on the stack,
if the TOS optimization is turned on.
@item Primitives with stack effects of the form @code{--}
@var{out1}...@var{outy} must store the TOS to the stack at the start.
Likewise, primitives with the stack effect @var{in1}...@var{inx} @code{--}
must load the TOS from the stack at the end. But for the null stack
effect @code{--} no stores or loads should be generated.
@end itemize

@node Produced code,  , TOS Optimization, Primitives
@subsection Produced code

To see what assembly code is produced for the primitives on your machine
with your compiler and your flag settings, type @code{make engine.s} and
look at the resulting file @file{engine.s}.

@node System Architecture, 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
during loading. Address relocation is performed at image load-time, not
at run-time. The loader also has to replace tokens standing for
primitive calls with the appropriate code-field addresses (or code
addresses in the case of direct threading).

@node  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 (compiled with @code{gcc-2.6.3} and @code{-DFORCE_REG}) with
Win32Forth and LMI's NT Forth, two systems written in assembly, and with
two systems written in C: PFE-0.9.11 (compiled with @code{gcc-2.6.3}
with the default configuration for Linux: @code{-O2 -fomit-frame-pointer
-DUSE_REGS}) and ThisForth Beta (compiled with gcc-2.6.3 -O3
-fomit-frame-pointer). We benchmarked Gforth, PFE and ThisForth 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.

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; and a recursive Fibonacci number
computation for benchmark 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               This-
time     Gforth     Forth     Forth       PFE     Forth
sieve        1.00      1.30      1.07      1.67      2.98
bubble       1.00      1.30      1.40      1.66
matmul       1.00      1.40      1.29      2.24
fib          1.00      1.44      1.26      1.82      2.82
@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
the actual addresses at run time, resulting in two address computations
per NEXT (@pxref{System Architecture}).

The speedup of Gforth over PFE and ThisForth can be easily explained
with the self-imposed restriction to standard C (although the measured
implementation of PFE uses a GNU C extension: global register
variables), which makes efficient threading impossible.  Moreover,
current C compilers have a hard time optimizing other aspects of the
ThisForth 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.

@node Bugs, Pedigree, 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 !!. 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 (!! a way to find them out), 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 Pedigree, Word Index, Bugs, Top
@chapter 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 child of VolksForth.

VolksForth descends from F83. !! Authors? When?

Laxen and 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. Dean Sanderson and Bill Ragsdale developed the original
implementation of fig-Forth based on microForth.

!! microForth pedigree

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
information about Forth there.

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

This index is as incomplete as the manual.

@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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