Diff for /gforth/Attic/gforth.ds between versions 1.1 and 1.13

version 1.1, 1994/10/24 19:15:57 version 1.13, 1995/03/20 18:16:24
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 GNU Forth Manual
 @setchapternewpage odd  @comment @setchapternewpage odd
 @comment %**end of header (This is for running Texinfo on a region.)  @comment %**end of header (This is for running Texinfo on a region.)
 @ifinfo  @ifinfo
Line 15  Copyright @copyright{} 1994 GNU Forth De Line 15  Copyright @copyright{} 1994 GNU Forth De
      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 77  personal machines. This manual correspon Line 77  personal machines. This manual correspon
 @end ifinfo  @end ifinfo
 @menu  @menu
 * License::               * License::                     
 * Goals::               About the GNU Forth Project  * Goals::                       About the GNU Forth Project
 * Other Books::         Things you might want to read  * Other Books::                 Things you might want to read
 * Invocation::          Starting GNU Forth  * Invocation::                  Starting GNU Forth
 * Words::               Forth words available in GNU Forth  * Words::                       Forth words available in GNU Forth
 * ANS conformance::     Implementation-defined options etc.  * ANS conformance::             Implementation-defined options etc.
 * Model::               The abstract machine of GNU Forth  * Model::                       The abstract machine of GNU Forth
 * Emacs and GForth::    The GForth Mode  * Emacs and GForth::            The GForth Mode
 * Internals::           Implementation details  * Internals::                   Implementation details
 * Bugs::                How to report them  * Bugs::                        How to report them
 * Pedigree::            Ancestors of GNU Forth  * Pedigree::                    Ancestors of GNU Forth
 * Word Index::          An item for each Forth word  * Word Index::                  An item for each Forth word
 * Node Index::          An item for each node  * Node Index::                  An item for each node
 @end menu  @end menu
 @node License, Goals, Top, Top  @node License, Goals, Top, Top
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
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  
 @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}).
   * 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
 doc-d+  doc-d+
 doc-d-  doc-d-
Line 404  doc-dabs Line 416  doc-dabs
 doc-dmin  doc-dmin
 doc-dmax  doc-dmax
 @node Stack Manipulation,,,  @node Floating Point,  , Double precision, Arithmetic
   @subsection Floating Point
   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}.
   @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,
Line 417  theoretically keep floating point number Line 478  theoretically keep floating point number
 additional difficulty, you don't know how many cells a floating point  additional difficulty, you don't know how many cells a floating point
 number takes. It is reportedly possible to write words in a way that  number takes. It is reportedly possible to write words in a way that
 they work also for a unified stack model, but we do not recommend trying  they work also for a unified stack model, but we do not recommend trying
 it. Also, a Forth system is allowed to keep the local variables on the  it. Instead, just say that your program has an environmental dependency
   on a separate FP stack.
   Also, a Forth system is allowed to keep the local variables on the
 return stack. This is reasonable, as local variables usually eliminate  return stack. This is reasonable, as local variables usually eliminate
 the need to use the return stack explicitly. So, if you want to produce  the need to use the return stack explicitly. So, if you want to produce
 a standard complying program and if you are using local variables in a  a standard complying program and if you are using local variables in a
 word, forget about return stack manipulations in that word (see the  word, forget about return stack manipulations in that word (see the
 standard document for the exact rules).  standard document for the exact rules).
   * Data stack::                  
   * Floating point stack::        
   * Return stack::                
   * Locals stack::                
   * Stack pointer manipulation::  
   @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 517  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 527  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 538  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 552  doc-rp!
 doc-lp@  doc-lp@
 doc-lp!  doc-lp!
 @node Memory access  @node Memory access, Control Structures, Stack Manipulation, Words
 @section Memory access  @section Memory access
   * Stack-Memory transfers::      
   * 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 578  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 522  The standard guarantees that addresses r Line 607  The standard guarantees that addresses r
 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 628  doc-dfloats
 doc-dfloat+  doc-dfloat+
 doc-dfalign  doc-dfalign
 doc-dfaligned  doc-dfaligned
 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 648  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 656  compile state, i.e., in a colon definiti
 limitation, but have not seen a satisfying way around it yet, although  limitation, but have not seen a satisfying way around it yet, although
 many schemes have been proposed.  many schemes have been proposed.
   * Selection::                   
   * Simple Loops::                
   * Counted Loops::               
   * Arbitrary control structures::  
   * Calls and returns::           
   * Exception Handling::          
   @end menu
   @node Selection, Simple Loops, Control Structures, Control Structures
 @subsection Selection  @subsection Selection
 @example  @example
Line 581  ELSE Line 684  ELSE
 @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 711  can avoid using @code{?dup}.
   @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{}
 @end example  @end example
Line 617  Executes the first @var{codei}, where th Line 720  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 752  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 689  There are several variations on the coun Line 794  There are several variations on the coun
 index by @var{n} instead of by 1. The loop is terminated when the border  index by @var{n} instead of by 1. The loop is terminated when the border
 between @var{limit-1} and @var{limit} is crossed. E.g.:  between @var{limit-1} and @var{limit} is crossed. E.g.:
 4 0 ?DO  i .  2 +LOOP   prints 0 2  @code{4 0 ?DO  i .  2 +LOOP}   prints @code{0 2}
 4 1 ?DO  i .  2 +LOOP   prints 1 3  @code{4 1 ?DO  i .  2 +LOOP}   prints @code{1 3}
 The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative:  The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative:
 -1 0 ?DO  i .  -1 +LOOP  prints 0 -1  @code{-1 0 ?DO  i .  -1 +LOOP}  prints @code{0 -1}
  0 0 ?DO  i .  -1 +LOOP  prints nothing  @code{ 0 0 ?DO  i .  -1 +LOOP}  prints nothing
 Therefore we recommend avoiding using @code{@var{n} +LOOP} with negative  Therefore we recommend avoiding using @code{@var{n} +LOOP} with negative
 @var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative  @var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative
 case behaves symmetrical to the positive case:  case behaves symmetrical to the positive case:
 -2 0 ?DO  i .  -1 +LOOP  prints 0 -1  @code{-2 0 ?DO  i .  -1 S+LOOP}  prints @code{0 -1}
 -1 0 ?DO  i .  -1 +LOOP  prints 0  @code{-1 0 ?DO  i .  -1 S+LOOP}  prints @code{0}
  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-sgn(n)} and  The loop is terminated when the border between @var{limit@minus{}sgn(n)} and
 @var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS  @var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS
 Forth standard.  Forth standard.
Line 734  iterates @var{n+1} times; @code{i} produ Line 839  iterates @var{n+1} times; @code{i} produ
 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 Locals  @node Arbitrary control structures, Calls and returns, Counted Loops, Control Structures
   @subsection Arbitrary control structures
   ANS Forth permits and supports using control structures in a non-nested
   way. Information about incomplete control structures is stored on the
   control-flow stack. This stack may be implemented on the Forth data
   stack, and this is what we have done in gforth.
   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
   building any control structure possible (except control structures that
   need storage, like calls, coroutines, and backtracking).
   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
   really good idea to manipulate the control flow stack with
   @code{cs-pick} and @code{cs-roll}, not with data stack manipulation
   Some standard control structure words are built from these words:
   Counted loop words constitute a separate group of words:
   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
   every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path
   through the definition (@code{LOOP} etc. compile an @code{UNLOOP} on the
   fall-through path). Also, you have to ensure that all @code{LEAVE}s are
   resolved (by using one of the loop-ending words or @code{DONE}).
   Another group of control structure words are
   @i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and
   @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
   E.g., instead of writing
   if [ 1 cs-roll ]
   again then
   @end example
   we recommend defining control structure words, e.g.,
   : while ( dest -- orig dest )
    1 cs-roll ; immediate
   : repeat ( orig dest -- )
    POSTPONE again
    POSTPONE then ; immediate
   @end example
   and then using these to create the control structure:
   @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
   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
   @node Exception Handling,  , Calls and returns, Control Structures
   @subsection Exception Handling
   @node Locals, Defining Words, Control Structures, Words
 @section Locals  @section Locals
   Local variables can make Forth programming more enjoyable and Forth
   programs easier to read. Unfortunately, the locals of ANS Forth are
   laden with restrictions. Therefore, we provide not only the ANS Forth
   locals wordset, but also our own, more powerful locals wordset (we
   implemented the ANS Forth locals wordset through our locals wordset).
   * gforth locals::               
   * ANS Forth locals::            
   @end menu
   @node gforth locals, ANS Forth locals, Locals, Locals
   @subsection gforth locals
   Locals can be defined with
   @{ local1 local2 ... -- comment @}
   @end example
   @{ local1 local2 ... @}
   @end example
   : max @{ n1 n2 -- n3 @}
    n1 n2 > if
    endif ;
   @end example
   The similarity of locals definitions with stack comments is intended. A
   locals definition often replaces the stack comment of a word. The order
   of the locals corresponds to the order in a stack comment and everything
   after the @code{--} is really a comment.
   This similarity has one disadvantage: It is too easy to confuse locals
   declarations with stack comments, causing bugs and making them hard to
   find. However, this problem can be avoided by appropriate coding
   conventions: Do not use both notations in the same program. If you do,
   they should be distinguished using additional means, e.g. by position.
   The name of the local may be preceded by a type specifier, e.g.,
   @code{F:} for a floating point value:
   : CX* @{ F: Ar F: Ai F: Br F: Bi -- Cr Ci @}
   \ complex multiplication
    Ar Br f* Ai Bi f* f-
    Ar Bi f* Ai Br f* f+ ;
   @end example
   GNU Forth currently supports cells (@code{W:}, @code{W^}), doubles
   (@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters
   (@code{C:}, @code{C^}) in two flavours: a value-flavoured local (defined
   with @code{W:}, @code{D:} etc.) produces its value and can be changed
   with @code{TO}. A variable-flavoured local (defined with @code{W^} etc.)
   produces its address (which becomes invalid when the variable's scope is
   left). E.g., the standard word @code{emit} can be defined in therms of
   @code{type} like this:
   : emit @{ C^ char* -- @}
       char* 1 type ;
   @end example
   A local without type specifier is a @code{W:} local. Both flavours of
   locals are initialized with values from the data or FP stack.
   Currently there is no way to define locals with user-defined data
   structures, but we are working on it.
   GNU Forth allows defining locals everywhere in a colon definition. This
   poses the following questions:
   * Where are locals visible by name?::  
   * How long do locals live? ::   
   * Programming Style::           
   * Implementation::              
   @end menu
   @node Where are locals visible by name?, How long do locals live?, gforth locals, gforth locals
   @subsubsection Where are locals visible by name?
   Basically, the answer is that locals are visible where you would expect
   it in block-structured languages, and sometimes a little longer. If you
   want to restrict the scope of a local, enclose its definition in
   These words behave like control structure words, so you can use them
   with @code{CS-PICK} and @code{CS-ROLL} to restrict the scope in
   arbitrary ways.
   If you want a more exact answer to the visibility question, here's the
   basic principle: A local is visible in all places that can only be
   reached through the definition of the local@footnote{In compiler
   construction terminology, all places dominated by the definition of the
   local.}. In other words, it is not visible in places that can be reached
   without going through the definition of the local. E.g., locals defined
   in @code{IF}...@code{ENDIF} are visible until the @code{ENDIF}, locals
   defined in @code{BEGIN}...@code{UNTIL} are visible after the
   @code{UNTIL} (until, e.g., a subsequent @code{ENDSCOPE}).
   The reasoning behind this solution is: We want to have the locals
   visible as long as it is meaningful. The user can always make the
   visibility shorter by using explicit scoping. In a place that can
   only be reached through the definition of a local, the meaning of a
   local name is clear. In other places it is not: How is the local
   initialized at the control flow path that does not contain the
   definition? Which local is meant, if the same name is defined twice in
   two independent control flow paths?
   This should be enough detail for nearly all users, so you can skip the
   rest of this section. If you relly must know all the gory details and
   options, read on.
   In order to implement this rule, the compiler has to know which places
   are unreachable. It knows this automatically after @code{AHEAD},
   @code{AGAIN}, @code{EXIT} and @code{LEAVE}; in other cases (e.g., after
   most @code{THROW}s), you can use the word @code{UNREACHABLE} to tell the
   compiler that the control flow never reaches that place. If
   @code{UNREACHABLE} is not used where it could, the only consequence is
   that the visibility of some locals is more limited than the rule above
   says. If @code{UNREACHABLE} is used where it should not (i.e., if you
   lie to the compiler), buggy code will be produced.
   Another problem with this rule is that at @code{BEGIN}, the compiler
   does not know which locals will be visible on the incoming
   back-edge. All problems discussed in the following are due to this
   ignorance of the compiler (we discuss the problems using @code{BEGIN}
   loops as examples; the discussion also applies to @code{?DO} and other
   loops). Perhaps the most insidious example is:
   [ 1 CS-ROLL ] THEN
     @{ x @}
   @end example
   This should be legal according to the visibility rule. The use of
   @code{x} can only be reached through the definition; but that appears
   textually below the use.
   From this example it is clear that the visibility rules cannot be fully
   implemented without major headaches. Our implementation treats common
   cases as advertised and the exceptions are treated in a safe way: The
   compiler makes a reasonable guess about the locals visible after a
   @code{BEGIN}; if it is too pessimistic, the
   user will get a spurious error about the local not being defined; if the
   compiler is too optimistic, it will notice this later and issue a
   warning. In the case above the compiler would complain about @code{x}
   being undefined at its use. You can see from the obscure examples in
   this section that it takes quite unusual control structures to get the
   compiler into trouble, and even then it will often do fine.
   If the @code{BEGIN} is reachable from above, the most optimistic guess
   is that all locals visible before the @code{BEGIN} will also be
   visible after the @code{BEGIN}. This guess is valid for all loops that
   are entered only through the @code{BEGIN}, in particular, for normal
   @code{BEGIN}...@code{WHILE}...@code{REPEAT} and
   @code{BEGIN}...@code{UNTIL} loops and it is implemented in our
   compiler. When the branch to the @code{BEGIN} is finally generated by
   @code{AGAIN} or @code{UNTIL}, the compiler checks the guess and
   warns the user if it was too optimisitic:
     @{ x @}
     \ x ? 
   [ 1 cs-roll ] THEN
   @end example
   Here, @code{x} lives only until the @code{BEGIN}, but the compiler
   optimistically assumes that it lives until the @code{THEN}. It notices
   this difference when it compiles the @code{UNTIL} and issues a
   warning. The user can avoid the warning, and make sure that @code{x}
   is not used in the wrong area by using explicit scoping:
     @{ x @}
   [ 1 cs-roll ] THEN
   @end example
   Since the guess is optimistic, there will be no spurious error messages
   about undefined locals.
   If the @code{BEGIN} is not reachable from above (e.g., after
   @code{AHEAD} or @code{EXIT}), the compiler cannot even make an
   optimistic guess, as the locals visible after the @code{BEGIN} may be
   defined later. Therefore, the compiler assumes that no locals are
   visible after the @code{BEGIN}. However, the useer can use
   @code{ASSUME-LIVE} to make the compiler assume that the same locals are
   visible at the BEGIN as at the point where the item was created.
   @{ x @}
   [ 1 CS-ROLL ] THEN
   @end example
   Other cases where the locals are defined before the @code{BEGIN} can be
   handled by inserting an appropriate @code{CS-ROLL} before the
   @code{ASSUME-LIVE} (and changing the control-flow stack manipulation
   behind the @code{ASSUME-LIVE}).
   Cases where locals are defined after the @code{BEGIN} (but should be
   visible immediately after the @code{BEGIN}) can only be handled by
   rearranging the loop. E.g., the ``most insidious'' example above can be
   arranged into:
     @{ x @}
     ... 0=
   @end example
   @node How long do locals live?, Programming Style, Where are locals visible by name?, gforth locals
   @subsubsection How long do locals live?
   The right answer for the lifetime question would be: A local lives at
   least as long as it can be accessed. For a value-flavoured local this
   means: until the end of its visibility. However, a variable-flavoured
   local could be accessed through its address far beyond its visibility
   scope. Ultimately, this would mean that such locals would have to be
   garbage collected. Since this entails un-Forth-like implementation
   complexities, I adopted the same cowardly solution as some other
   languages (e.g., C): The local lives only as long as it is visible;
   afterwards its address is invalid (and programs that access it
   afterwards are erroneous).
   @node Programming Style, Implementation, How long do locals live?, gforth locals
   @subsubsection Programming Style
   The freedom to define locals anywhere has the potential to change
   programming styles dramatically. In particular, the need to use the
   return stack for intermediate storage vanishes. Moreover, all stack
   manipulations (except @code{PICK}s and @code{ROLL}s with run-time
   determined arguments) can be eliminated: If the stack items are in the
   wrong order, just write a locals definition for all of them; then
   write the items in the order you want.
   This seems a little far-fetched and eliminating stack manipulations is
   unlikely to become a conscious programming objective. Still, the number
   of stack manipulations will be reduced dramatically if local variables
   are used liberally (e.g., compare @code{max} in @ref{gforth locals} with
   a traditional implementation of @code{max}).
   This shows one potential benefit of locals: making Forth programs more
   readable. Of course, this benefit will only be realized if the
   programmers continue to honour the principle of factoring instead of
   using the added latitude to make the words longer.
   Using @code{TO} can and should be avoided.  Without @code{TO},
   every value-flavoured local has only a single assignment and many
   advantages of functional languages apply to Forth. I.e., programs are
   easier to analyse, to optimize and to read: It is clear from the
   definition what the local stands for, it does not turn into something
   different later.
   E.g., a definition using @code{TO} might look like this:
   : strcmp @{ addr1 u1 addr2 u2 -- n @}
    u1 u2 min 0
      addr1 c@ addr2 c@ - ?dup
        unloop exit
      addr1 char+ TO addr1
      addr2 char+ TO addr2
    u1 u2 - ;
   @end example
   Here, @code{TO} is used to update @code{addr1} and @code{addr2} at
   every loop iteration. @code{strcmp} is a typical example of the
   readability problems of using @code{TO}. When you start reading
   @code{strcmp}, you think that @code{addr1} refers to the start of the
   string. Only near the end of the loop you realize that it is something
   This can be avoided by defining two locals at the start of the loop that
   are initialized with the right value for the current iteration.
   : strcmp @{ addr1 u1 addr2 u2 -- n @}
    addr1 addr2
    u1 u2 min 0 
    ?do @{ s1 s2 @}
      s1 c@ s2 c@ - ?dup 
        unloop exit
      s1 char+ s2 char+
    u1 u2 - ;
   @end example
   Here it is clear from the start that @code{s1} has a different value
   in every loop iteration.
   @node Implementation,  , Programming Style, gforth locals
   @subsubsection Implementation
   GNU Forth uses an extra locals stack. The most compelling reason for
   this is that the return stack is not float-aligned; using an extra stack
   also eliminates the problems and restrictions of using the return stack
   as locals stack. Like the other stacks, the locals stack grows toward
   lower addresses. A few primitives allow an efficient implementation:
   In addition to these primitives, some specializations of these
   primitives for commonly occurring inline arguments are provided for
   efficiency reasons, e.g., @code{@@local0} as specialization of
   @code{@@local#} for the inline argument 0. The following compiling words
   compile the right specialized version, or the general version, as
   Combinations of conditional branches and @code{lp+!#} like
   @code{?branch-lp+!#} (the locals pointer is only changed if the branch
   is taken) are provided for efficiency and correctness in loops.
   A special area in the dictionary space is reserved for keeping the
   local variable names. @code{@{} switches the dictionary pointer to this
   area and @code{@}} switches it back and generates the locals
   initializing code. @code{W:} etc.@ are normal defining words. This
   special area is cleared at the start of every colon definition.
   A special feature of GNU Forths dictionary is used to implement the
   definition of locals without type specifiers: every wordlist (aka
   vocabulary) has its own methods for searching
   etc. (@pxref{Wordlists}). For the present purpose we defined a wordlist
   with a special search method: When it is searched for a word, it
   actually creates that word using @code{W:}. @code{@{} changes the search
   order to first search the wordlist containing @code{@}}, @code{W:} etc.,
   and then the wordlist for defining locals without type specifiers.
   The lifetime rules support a stack discipline within a colon
   definition: The lifetime of a local is either nested with other locals
   lifetimes or it does not overlap them.
   At @code{BEGIN}, @code{IF}, and @code{AHEAD} no code for locals stack
   pointer manipulation is generated. Between control structure words
   locals definitions can push locals onto the locals stack. @code{AGAIN}
   is the simplest of the other three control flow words. It has to
   restore the locals stack depth of the corresponding @code{BEGIN}
   before branching. The code looks like this:
   @code{lp+!#} current-locals-size @minus{} dest-locals-size
   @code{branch} <begin>
   @end format
   @code{UNTIL} is a little more complicated: If it branches back, it
   must adjust the stack just like @code{AGAIN}. But if it falls through,
   the locals stack must not be changed. The compiler generates the
   following code:
   @code{?branch-lp+!#} <begin> current-locals-size @minus{} dest-locals-size
   @end format
   The locals stack pointer is only adjusted if the branch is taken.
   @code{THEN} can produce somewhat inefficient code:
   @code{lp+!#} current-locals-size @minus{} orig-locals-size
   <orig target>:
   @code{lp+!#} orig-locals-size @minus{} new-locals-size
   @end format
   The second @code{lp+!#} adjusts the locals stack pointer from the
   level at the @var{orig} point to the level after the @code{THEN}. The
   first @code{lp+!#} adjusts the locals stack pointer from the current
   level to the level at the orig point, so the complete effect is an
   adjustment from the current level to the right level after the
   In a conventional Forth implementation a dest control-flow stack entry
   is just the target address and an orig entry is just the address to be
   patched. Our locals implementation adds a wordlist to every orig or dest
   item. It is the list of locals visible (or assumed visible) at the point
   described by the entry. Our implementation also adds a tag to identify
   the kind of entry, in particular to differentiate between live and dead
   (reachable and unreachable) orig entries.
   A few unusual operations have to be performed on locals wordlists:
   Several features of our locals wordlist implementation make these
   operations easy to implement: The locals wordlists are organised as
   linked lists; the tails of these lists are shared, if the lists
   contain some of the same locals; and the address of a name is greater
   than the address of the names behind it in the list.
   Another important implementation detail is the variable
   @code{dead-code}. It is used by @code{BEGIN} and @code{THEN} to
   determine if they can be reached directly or only through the branch
   that they resolve. @code{dead-code} is set by @code{UNREACHABLE},
   @code{AHEAD}, @code{EXIT} etc., and cleared at the start of a colon
   definition, by @code{BEGIN} and usually by @code{THEN}.
   Counted loops are similar to other loops in most respects, but
   @code{LEAVE} requires special attention: It performs basically the same
   service as @code{AHEAD}, but it does not create a control-flow stack
   entry. Therefore the information has to be stored elsewhere;
   traditionally, the information was stored in the target fields of the
   branches created by the @code{LEAVE}s, by organizing these fields into a
   linked list. Unfortunately, this clever trick does not provide enough
   space for storing our extended control flow information. Therefore, we
   introduce another stack, the leave stack. It contains the control-flow
   stack entries for all unresolved @code{LEAVE}s.
   Local names are kept until the end of the colon definition, even if
   they are no longer visible in any control-flow path. In a few cases
   this may lead to increased space needs for the locals name area, but
   usually less than reclaiming this space would cost in code size.
   @node ANS Forth locals,  , gforth locals, Locals
   @subsection ANS Forth locals
   The ANS Forth locals wordset does not define a syntax for locals, but
   words that make it possible to define various syntaxes. One of the
   possible syntaxes is a subset of the syntax we used in the gforth locals
   wordset, i.e.:
   @{ local1 local2 ... -- comment @}
   @end example
   @{ local1 local2 ... @}
   @end example
   The order of the locals corresponds to the order in a stack comment. The
   restrictions are:
   @itemize @bullet
   Locals can only be cell-sized values (no type specifers are allowed).
   Locals can be defined only outside control structures.
   Locals can interfere with explicit usage of the return stack. For the
   exact (and long) rules, see the standard. If you don't use return stack
   accessing words in a definition using locals, you will we all right. The
   purpose of this rule is to make locals implementation on the return
   stack easier.
   The whole definition must be in one line.
   @end itemize
   Locals defined in this way behave like @code{VALUE}s
   (@xref{Values}). I.e., they are initialized from the stack. Using their
   name produces their value. Their value can be changed using @code{TO}.
   Since this syntax is supported by gforth directly, you need not do
   anything to use it. If you want to port a program using this syntax to
   another ANS Forth system, use @file{anslocal.fs} to implement the syntax
   on the other system.
   Note that a syntax shown in the standard, section A.13 looks
   similar, but is quite different in having the order of locals
   reversed. Beware!
   The ANS Forth locals wordset itself consists of the following word
   The ANS Forth locals extension wordset defines a syntax, but it is so
   awful that we strongly recommend not to use it. We have implemented this
   syntax to make porting to gforth easy, but do not document it here. The
   problem with this syntax is that the locals are defined in an order
   reversed with respect to the standard stack comment notation, making
   programs harder to read, and easier to misread and miswrite. The only
   merit of this syntax is that it is easy to implement using the ANS Forth
   locals wordset.
   @node Defining Words, Wordlists, Locals, Words
   @section Defining Words
   @node Values,  , Defining Words, Defining Words
   @subsection Values
   @node Wordlists, Files, Defining Words, Words
   @section Wordlists
   @node Files, Blocks, Wordlists, Words
   @section Files
   @node Blocks, Other I/O, Files, Words
   @section Blocks
   @node Other I/O, Programming Tools, Blocks, Words
   @section Other I/O
   @node Programming Tools, Threading Words, Other I/O, Words
   @section Programming Tools
   * 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
   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{#>}.
   @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:
   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.
   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:
   @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).
   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.
   @node ANS conformance, Model, Words, Top
   @chapter ANS conformance
   @node Model, Emacs and GForth, ANS conformance, Top
   @chapter Model
   @node Emacs and GForth, Internals, Model, Top
   @chapter Emacs and GForth
   GForth comes with @file{gforth.el}, an improved version of
   @file{forth.el} by Goran Rydqvist (icluded in the TILE package). The
   improvements are a better (but still not perfect) handling of
   indentation. I have also added comment paragraph filling (@kbd{M-q}),
   commenting (@kbd{C-x \}) and uncommenting (@kbd{C-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}
   (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.
   * Portability::                 
   * Threading::                   
   * Primitives::                  
   * System Architecture::         
   @end menu
   @node Portability, Threading, Internals, Internals
   @section Portability
   One of the main goals of the effort is availability across a wide range
   of personal machines. fig-Forth, and, to a lesser extent, F83, achieved
   this goal by manually coding the engine in assembly language for several
   then-popular processors. This approach is very labor-intensive and the
   results are short-lived due to progress in computer architecture.
   Others have avoided this problem by coding in C, e.g., Mitch Bradley
   (cforth), Mikael Patel (TILE) and Dirk Zoller (pfe). This approach is
   particularly popular for UNIX-based Forths due to the large variety of
   architectures of UNIX machines. Unfortunately an implementation in C
   does not mix well with the goals of efficiency and with using
   traditional techniques: Indirect or direct threading cannot be expressed
   in C, and switch threading, the fastest technique available in C, is
   significantly slower. Another problem with C is that it's very
   cumbersome to express double integer arithmetic.
   Fortunately, there is a portable language that does not have these
   limitations: GNU C, the version of C processed by the GNU C compiler
   (@pxref{C Extensions, , Extensions to the C Language Family, gcc.info,
   GNU C Manual}). Its labels as values feature (@pxref{Labels as Values, ,
   Labels as Values, gcc.info, GNU C Manual}) makes direct and indirect
   threading possible, its @code{long long} type (@pxref{Long Long, ,
   Double-Word Integers, gcc.info, GNU C Manual}) corresponds to Forths
   double numbers. GNU C is available for free on all important (and many
   unimportant) UNIX machines, VMS, 80386s running MS-DOS, the Amiga, and
   the Atari ST, so a Forth written in GNU C can run on all these
   machines@footnote{Due to Apple's look-and-feel lawsuit it is not
   available on the Mac (@pxref{Boycott, , Protect Your Freedom---Fight
   ``Look And Feel'', gcc.info, GNU C Manual}).}.
   Writing in a portable language has the reputation of producing code that
   is slower than assembly. For our Forth engine we repeatedly looked at
   the code produced by the compiler and eliminated most compiler-induced
   inefficiencies by appropriate changes in the source-code.
   However, register allocation cannot be portably influenced by the
   programmer, leading to some inefficiencies on register-starved
   machines. We use explicit register declarations (@pxref{Explicit Reg
   Vars, , Variables in Specified Registers, gcc.info, GNU C Manual}) to
   improve the speed on some machines. They are turned on by using the
   @code{gcc} switch @code{-DFORCE_REG}. Unfortunately, this feature not
   only depends on the machine, but also on the compiler version: On some
   machines some compiler versions produce incorrect code when certain
   explicit register declarations are used. So by default
   @code{-DFORCE_REG} is not used.
   @node Threading, Primitives, Portability, Internals
   @section Threading
   GNU C's labels as values extension (available since @code{gcc-2.0},
   @pxref{Labels as Values, , Labels as Values, gcc.info, GNU C Manual})
   makes it possible to take the address of @var{label} by writing
   @code{&&@var{label}}.  This address can then be used in a statement like
   @code{goto *@var{address}}. I.e., @code{goto *&&x} is the same as
   @code{goto x}.
   With this feature an indirect threaded NEXT looks like:
   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
   Direct threading is even simpler:
   ca = *ip++;
   goto *ca;
   @end example
   Of course we have packaged the whole thing neatly in macros called
   @code{NEXT} and @code{NEXT1} (the part of NEXT after fetching the cfa).
   * Scheduling::                  
   * 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
   @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:
   @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
   @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
   * 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:
   @var{Forth-name}        @var{stack-effect}      @var{category}  [@var{pronounc.}]
   [@code{""}@var{glossary entry}@code{""}]
   @var{C 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:
   +    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:
   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
   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
   @item In the case of @code{dup ( w -- w w )} the generator must not
   eliminate the store to the original location of the item on the stack,
   if the TOS optimization is turned on.
   @item Primitives with stack effects of the form @code{--}
   @var{out1}...@var{outy} must store the TOS to the stack at the start.
   Likewise, primitives with the stack effect @var{in1}...@var{inx} @code{--}
   must load the TOS from the stack at the end. But for the null stack
   effect @code{--} no stores or loads should be generated.
   @end itemize
   @node Produced code,  , TOS Optimization, Primitives
   @subsection Produced code
   To see what assembly code is produced for the primitives on your machine
   with your compiler and your flag settings, type @code{make engine.s} and
   look at the resulting file @file{engine.s}.
   @node System Architecture,  , Primitives, Internals
   @section System Architecture
   Our Forth system consists not only of primitives, but also of
   definitions written in Forth. Since the Forth compiler itself belongs
   to those definitions, it is not possible to start the system with the
   primitives and the Forth source alone. Therefore we provide the Forth
   code as an image file in nearly executable form. At the start of the
   system a C routine loads the image file into memory, sets up the
   memory (stacks etc.) according to information in the image file, and
   starts executing Forth code.
   The image file format is a compromise between the goals of making it
   easy to generate image files and making them portable. The easiest way
   to generate an image file is to just generate a memory dump. However,
   this kind of image file cannot be used on a different machine, or on
   the next version of the engine on the same machine, it even might not
   work with the same engine compiled by a different version of the C
   compiler. We would like to have as few versions of the image file as
   possible, because we do not want to distribute many versions of the
   same image file, and to make it easy for the users to use their image
   files on many machines. We currently need to create a different image
   file for machines with different cell sizes and different byte order
   (little- or big-endian)@footnote{We consider adding information to the
   image file that enables the loader to change the byte order.}.
   Forth code that is going to end up in a portable image file has to
   comply to some restrictions: addresses have to be stored in memory with
   special words (@code{A!}, @code{A,}, etc.) in order to make the code
   relocatable. Cells, floats, etc., have to be stored at the natural
   alignment boundaries@footnote{E.g., store floats (8 bytes) at an address
   dividable by~8. This happens automatically in our system when you use
   the ANS Forth alignment words.}, in order to avoid alignment faults on
   machines with stricter alignment. The image file is produced by a
   metacompiler (@file{cross.fs}).
   So, unlike the image file of Mitch Bradleys @code{cforth}, our image
   file is not directly executable, but has to undergo some manipulations
   during loading. Address relocation is performed at image load-time, not
   at run-time. The loader also has to replace tokens standing for
   primitive calls with the appropriate code-field addresses (or code
   addresses in the case of direct threading).
   @node Bugs, Pedigree, Internals, Top
   @chapter Bugs
   @node Pedigree, Word Index, Bugs, Top
   @chapter Pedigree
   @node Word Index, Node Index, Pedigree, Top
   @chapter Word Index
   @node Node Index,  , Word Index, Top
   @chapter Node Index
 @contents  @contents
 @bye  @bye

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