Initial revision

[binutils.git] / gas / doc / gasp.texi

\input texinfo               @c             -*- Texinfo -*-
@setfilename gasp.info
@c
@c This file documents the assembly preprocessor "GASP"
@c
@c Copyright (c) 1994 Free Software Foundation, Inc.
@c
@c This text may be freely distributed under the terms of the GNU
@c General Public License.

@ifinfo
@format
START-INFO-DIR-ENTRY
* gasp: (gasp).                     The GNU Assembler Preprocessor
END-INFO-DIR-ENTRY
@end format
@end ifinfo

@syncodeindex ky cp
@syncodeindex fn cp

@finalout
@setchapternewpage odd
@settitle GASP
@titlepage
@c FIXME boring title
@title GASP, an assembly preprocessor
@subtitle for GASP version 1
@sp 1
@subtitle March 1994
@author Roland Pesch
@page

@tex
{\parskip=0pt \hfill Cygnus Support\par
}
@end tex

@vskip 0pt plus 1filll
Copyright @copyright{} 1994, 1995 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions.
@end titlepage

@ifinfo
Copyright @copyright{} 1994, 1995 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

@ignore
Permission is granted to process this file through TeX and print the
results, provided the printed document carries a copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).
@end ignore

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions.

@node Top
@top GASP

GASP is a preprocessor for assembly programs.

This file describes version 1 of GASP.

Steve Chamberlain wrote GASP; Roland Pesch wrote this manual.

@menu
* Overview::                    What is GASP?
* Invoking GASP::               Command line options.
* Commands::                    Preprocessor commands.
* Index::                       Index.
@end menu
@end ifinfo

@node Overview
@chapter What is GASP?

The primary purpose of the @sc{gnu} assembler is to assemble the output of
other programs---notably compilers.  When you have to hand-code
specialized routines in assembly, that means the @sc{gnu} assembler is
an unfriendly processor: it has no directives for macros, conditionals,
or many other conveniences that you might expect.

In some cases you can simply use the C preprocessor, or a generalized
preprocessor like @sc{m4}; but this can be awkward, since none of these
things are designed with assembly in mind.

@sc{gasp} fills this need.  It is expressly designed to provide the
facilities you need with hand-coded assembly code.  Implementing it as a
preprocessor, rather than part of the assembler, allows the maximum
flexibility: you can use it with hand-coded assembly, without paying a
penalty of added complexity in the assembler you use for compiler
output.

Here is a small example to give the flavor of @sc{gasp}.  This input to
@sc{gasp}

@cartouche
@example
        .MACRO  saveregs from=8 to=14
count   .ASSIGNA \from
        ! save r\from..r\to
        .AWHILE  \&count LE \to
        mov     r\&count,@@-sp
count   .ASSIGNA  \&count + 1
        .AENDW
        .ENDM

        saveregs from=12

bar:    mov     #H'dead+10,r0
foo     .SDATAC "hello"<10>
        .END
@end example
@end cartouche

@noindent
generates this assembly program:

@cartouche
@example
        ! save r12..r14
        mov     r12,@@-sp
        mov     r13,@@-sp
        mov     r14,@@-sp

bar:    mov     #57005+10,r0
foo:    .byte   6,104,101,108,108,111,10
@end example
@end cartouche

@node Invoking GASP
@chapter Command Line Options

@c FIXME!  Or is there a simpler way, calling from GAS option?
The simplest way to use @sc{gasp} is to run it as a filter and assemble
its output.  In Unix and its ilk, you can do this, for example:

@c FIXME! GASP filename suffix convention?
@example
$ gasp prog.asm | as -o prog.o
@end example

Naturally, there are also a few command-line options to allow you to
request variations on this basic theme.  Here is the full set of
possibilities for the @sc{gasp} command line.

@example
gasp  [ -a | --alternate ]
      [ -c @var{char} | --commentchar @var{char} ]
      [ -d | --debug ]  [ -h | --help ] [ -M | --mri ]
      [ -o @var{outfile} | --output @var{outfile} ]
      [ -p | --print ]  [ -s | --copysource ]
      [ -u | --unreasonable ]  [ -v | --version ]
      @var{infile} @dots{}
@end example

@ftable @code
@item @var{infile} @dots{}
@c FIXME! Why not stdin as default infile?
The input file names.  You must specify at least one input file; if you
specify more, @sc{gasp} preprocesses them all, concatenating the output
in the order you list the @var{infile} arguments.

Mark the end of each input file with the preprocessor command
@code{.END}.  @xref{Other Commands,, Miscellaneous commands}.

@item -a
@itemx --alternate
Use alternative macro syntax.  @xref{Alternate,, Alternate macro
syntax}, for a discussion of how this syntax differs from the default
@sc{gasp} syntax.

@cindex comment character, changing
@cindex semicolon, as comment
@cindex exclamation mark, as comment
@cindex shriek, as comment
@cindex bang, as comment
@cindex @code{!} default comment char
@cindex @code{;} as comment char
@item -c '@var{char}'
@itemx --commentchar '@var{char}'
Use @var{char} as the comment character.  The default comment character
is @samp{!}.  For example, to use a semicolon as the comment character,
specify @w{@samp{-c ';'}} on the @sc{gasp} command line.  Since
assembler command characters often have special significance to command
shells, it is a good idea to quote or escape @var{char} when you specify
a comment character.

For the sake of simplicity, all examples in this manual use the default
comment character @samp{!}.

@item -d
@itemx --debug
Show debugging statistics.  In this version of @sc{gasp}, this option
produces statistics about the string buffers that @sc{gasp} allocates
internally.  For each defined buffersize @var{s}, @sc{gasp} shows the
number of strings @var{n} that it allocated, with a line like this:

@example
strings size @var{s} : @var{n}
@end example

@noindent
@sc{gasp} displays these statistics on the standard error stream, when
done preprocessing.

@item -h
@itemx --help
Display a summary of the @sc{gasp} command line options.

@item -M
@itemx --mri
Use MRI compatibility mode.  Using this option causes @sc{gasp} to
accept the syntax and pseudo-ops used by the Microtec Research
@code{ASM68K} assembler.

@item -o @var{outfile}
@itemx --output @var{outfile}
Write the output in a file called @var{outfile}.  If you do not use the
@samp{-o} option, @sc{gasp} writes its output on the standard output
stream.

@item -p
@itemx --print
Print line numbers.  @sc{gasp} obeys this option @emph{only} if you also
specify @samp{-s} to copy source lines to its output.  With @samp{-s
-p}, @sc{gasp} displays the line number of each source line copied
(immediately after the comment character at the beginning of the line).

@item -s
@itemx --copysource
Copy the source lines to the output file.  Use this option
to see the effect of each preprocessor line on the @sc{gasp} output.
@sc{gasp} places a comment character (@samp{!} by default) at
the beginning of each source line it copies, so that you can use this
option and still assemble the result.

@item -u
@itemx --unreasonable
Bypass ``unreasonable expansion'' limit.  Since you can define @sc{gasp}
macros inside other macro definitions, the preprocessor normally
includes a sanity check.  If your program requires more than 1,000
nested expansions, @sc{gasp} normally exits with an error message.  Use
this option to turn off this check, allowing unlimited nested
expansions.

@item -v
@itemx --version
Display the @sc{gasp} version number.
@end ftable

@node Commands
@chapter Preprocessor Commands

@sc{gasp} commands have a straightforward syntax that fits in well with
assembly conventions.  In general, a command extends for a line, and may
have up to three fields: an optional label, the command itself, and
optional arguments to the command.  You can write commands in upper or
lower case, though this manual shows them in upper case.  @xref{Syntax
Details,, Details of the GASP syntax}, for more information.

@menu
* Conditionals::
* Loops::
* Variables::
* Macros::
* Data::
* Listings::
* Other Commands::
* Syntax Details::
* Alternate::
@end menu

@node Conditionals
@section Conditional assembly

The conditional-assembly directives allow you to include or exclude
portions of an assembly depending on how a pair of expressions, or a
pair of strings, compare.

The overall structure of conditionals is familiar from many other
contexts.  @code{.AIF} marks the start of a conditional, and precedes
assembly for the case when the condition is true.   An optional
@code{.AELSE} precedes assembly for the converse case, and an
@code{.AENDI} marks the end of the condition.

@c FIXME! Why doesn't -u turn off this check?
You may nest conditionals up to a depth of 100; @sc{gasp} rejects
nesting beyond that, because it may indicate a bug in your macro
structure.

@c FIXME! Why isn't there something like cpp's -D option?  Conditionals
@c        would be much more useful if there were.
Conditionals are primarily useful inside macro definitions, where you
often need different effects depending on argument values.
@xref{Macros,, Defining your own directives}, for details about defining
macros.

@ftable @code
@item .AIF @var{expra} @var{cmp} @var{exprb}
@itemx .AIF "@var{stra}" @var{cmp} "@var{strb}"

The governing condition goes on the same line as the @code{.AIF}
preprocessor command.  You may compare either two strings, or two
expressions.

When you compare strings, only two conditional @var{cmp} comparison
operators are available: @samp{EQ} (true if @var{stra} and @var{strb}
are identical), and @samp{NE} (the opposite).

When you compare two expressions, @emph{both expressions must be
absolute} (@pxref{Expressions,, Arithmetic expressions in GASP}).  You
can use these @var{cmp} comparison operators with expressions:

@ftable @code
@item EQ
Are @var{expra} and @var{exprb} equal?  (For strings, are @var{stra} and
@var{strb} identical?)

@item NE
Are @var{expra} and @var{exprb} different?  (For strings, are @var{stra}
and @var{strb} different?

@item LT
Is @var{expra} less than @var{exprb}?  (Not allowed for strings.)

@item LE
Is @var{expra} less than or equal to @var{exprb}?  (Not allowed for strings.)

@item GT
Is @var{expra} greater than @var{exprb}?  (Not allowed for strings.)

@item GE
Is @var{expra} greater than or equal to @var{exprb}?  (Not allowed for
strings.)
@end ftable

@item .AELSE
Marks the start of assembly code to be included if the condition fails.
Optional, and only allowed within a conditional (between @code{.AIF} and
@code{.AENDI}).

@item .AENDI
Marks the end of a conditional assembly.
@end ftable

@node Loops
@section Repetitive sections of assembly

Two preprocessor directives allow you to repeatedly issue copies of the
same block of assembly code.

@ftable @code
@item .AREPEAT @var{aexp}
@itemx .AENDR
If you simply need to repeat the same block of assembly over and over a
fixed number of times, sandwich one instance of the repeated block
between @code{.AREPEAT} and @code{.AENDR}.  Specify the number of
copies as @var{aexp} (which must be an absolute expression).  For
example, this repeats two assembly statements three times in succession:

@cartouche
@example
        .AREPEAT        3
        rotcl   r2
        div1    r0,r1
        .AENDR
@end example
@end cartouche

@item .AWHILE @var{expra} @var{cmp} @var{exprb}
@itemx .AENDW
@itemx .AWHILE @var{stra} @var{cmp} @var{strb}
@itemx .AENDW
To repeat a block of assembly depending on a conditional test, rather
than repeating it for a specific number of times, use @code{.AWHILE}.
@code{.AENDW} marks the end of the repeated block.  The conditional
comparison works exactly the same way as for @code{.AIF}, with the same
comparison operators (@pxref{Conditionals,, Conditional assembly}).

Since the terms of the comparison must be absolute expression,
@code{.AWHILE} is primarily useful within macros.  @xref{Macros,,
Defining your own directives}.
@end ftable

@cindex loops, breaking out of
@cindex breaking out of loops
You can use the @code{.EXITM} preprocessor directive to break out of
loops early (as well as to break out of macros).  @xref{Macros,,
Defining your own directives}.

@node Variables
@section Preprocessor variables

You can use variables in @sc{gasp} to represent strings, registers, or
the results of expressions.

You must distinguish two kinds of variables: 
@enumerate
@item
Variables defined with @code{.EQU} or @code{.ASSIGN}.  To evaluate this
kind of variable in your assembly output, simply mention its name.  For
example, these two lines define and use a variable @samp{eg}:

@cartouche
@example
eg     .EQU   FLIP-64
       @dots{}
       mov.l  eg,r0
@end example
@end cartouche

@emph{Do not use} this kind of variable in conditional expressions or
while loops; @sc{gasp} only evaluates these variables when writing
assembly output.

@item
Variables for use during preprocessing.  You can define these
with @code{.ASSIGNC} or @code{.ASSIGNA}.  To evaluate this
kind of variable, write @samp{\&} before the variable name; for example,

@cartouche
@example
opcit  .ASSIGNA  47
       @dots{}
       .AWHILE  \&opcit GT 0
       @dots{}
       .AENDW
@end example
@end cartouche

@sc{gasp} treats macro arguments almost the same way, but to evaluate
them you use the prefix @samp{\} rather than @samp{\&}.
@xref{Macros,, Defining your own directives}.
@end enumerate

@ftable @code
@item @var{pvar} .EQU @var{expr}
@c FIXME!  Anything to beware of re GAS directive of same name?
Assign preprocessor variable @var{pvar} the value of the expression
@var{expr}.  There are no restrictions on redefinition; use @samp{.EQU}
with the same @var{pvar} as often as you find it convenient.

@item @var{pvar} .ASSIGN @var{expr}
Almost the same as @code{.EQU}, save that you may not redefine
@var{pvar} using @code{.ASSIGN} once it has a value.
@c FIXME!!  Supposed to work this way, apparently, but on 9feb94 works
@c          just like .EQU

@item @var{pvar} .ASSIGNA @var{aexpr}
Define a variable with a numeric value, for use during preprocessing.
@var{aexpr} must be an absolute expression.  You can redefine variables
with @code{.ASSIGNA} at any time.

@item @var{pvar} .ASSIGNC "@var{str}"
Define a variable with a string value, for use during preprocessing.
You can redefine variables with @code{.ASSIGNC} at any time.

@item @var{pvar} .REG (@var{register})
Use @code{.REG} to define a variable that represents a register.  In
particular, @var{register} is @emph{not evaluated} as an expression.
You may use @code{.REG} at will to redefine register variables.
@end ftable

All these directives accept the variable name in the ``label'' position,
that is at the left margin.  You may specify a colon after the variable
name if you wish; the first example above could have started @samp{eg:}
with the same effect.

@c pagebreak makes for better aesthetics---ensures macro and expansion together
@page
@node Macros
@section Defining your own directives

The commands @code{.MACRO} and @code{.ENDM} allow you to define macros
that generate assembly output.  You can use these macros with a syntax
similar to built-in @sc{gasp} or assembler directives.  For example,
this definition specifies a macro @code{SUM} that adds together a range of
consecutive registers:

@cartouche
@example
        .MACRO  SUM FROM=0, TO=9
        ! \FROM \TO
        mov     r\FROM,r10
COUNT   .ASSIGNA        \FROM+1
        .AWHILE \&COUNT LE \TO
        add     r\&COUNT,r10
COUNT   .ASSIGNA        \&COUNT+1
        .AENDW
        .ENDM
@end example
@end cartouche

@noindent
With that definition, @samp{SUM 0,5} generates this assembly output:

@cartouche
@example
        ! 0 5
        mov     r0,r10
        add     r1,r10
        add     r2,r10
        add     r3,r10
        add     r4,r10
        add     r5,r10
@end example
@end cartouche

@ftable @code
@item .MACRO @var{macname}
@itemx .MACRO @var{macname} @var{macargs} @dots{}
Begin the definition of a macro called @var{macname}.  If your macro
definition requires arguments, specify their names after the macro name,
separated by commas or spaces.  You can supply a default value for any
macro argument by following the name with @samp{=@var{deflt}}.  For
example, these are all valid @code{.MACRO} statements:

@table @code
@item .MACRO COMM
Begin the definition of a macro called @code{COMM}, which takes no
arguments.

@item .MACRO PLUS1 P, P1
@itemx .MACRO PLUS1 P P1
Either statement begins the definition of a macro called @code{PLUS1},
which takes two arguments; within the macro definition, write
@samp{\P} or @samp{\P1} to evaluate the arguments.

@item .MACRO RESERVE_STR P1=0 P2
Begin the definition of a macro called @code{RESERVE_STR}, with two
arguments.  The first argument has a default value, but not the second.
After the definition is complete, you can call the macro either as
@samp{RESERVE_STR @var{a},@var{b}} (with @samp{\P1} evaluating to
@var{a} and @samp{\P2} evaluating to @var{b}), or as @samp{RESERVE_STR
,@var{b}} (with @samp{\P1} evaluating as the default, in this case
@samp{0}, and @samp{\P2} evaluating to @var{b}).
@end table

When you call a macro, you can specify the argument values either by
position, or by keyword.  For example, @samp{SUM 9,17} is equivalent to
@samp{SUM TO=17, FROM=9}.  Macro arguments are preprocessor variables
similar to the variables you define with @samp{.ASSIGNA} or
@samp{.ASSIGNC}; in particular, you can use them in conditionals or for
loop control.  (The only difference is the prefix you write to evaluate
the variable: for a macro argument, write @samp{\@var{argname}}, but for
a preprocessor variable, write @samp{\&@var{varname}}.)

@item @var{name} .MACRO
@itemx @var{name} .MACRO ( @var{macargs} @dots{} )
@c FIXME check: I think no error _and_ no args recognized if I use form
@c       NAME  .MACRO   ARG ARG
An alternative form of introducing a macro definition: specify the macro
name in the label position, and the arguments (if any) between
parentheses after the name.  Defaulting rules and usage work the same
way as for the other macro definition syntax.

@item .ENDM
Mark the end of a macro definition.

@item .EXITM
Exit early from the current macro definition, @code{.AREPEAT} loop, or
@code{.AWHILE} loop.

@cindex number of macros executed
@cindex macros, count executed
@item \@@
@sc{gasp} maintains a counter of how many macros it has
executed in this pseudo-variable; you can copy that number to your
output with @samp{\@@}, but @emph{only within a macro definition}.

@item LOCAL @var{name} [ , @dots{} ]
@emph{Warning: @code{LOCAL} is only available if you select ``alternate
macro syntax'' with @samp{-a} or @samp{--alternate}.}  @xref{Alternate,,
Alternate macro syntax}.

Generate a string replacement for each of the @var{name} arguments, and
replace any instances of @var{name} in each macro expansion.  The
replacement string is unique in the assembly, and different for each
separate macro expansion.  @code{LOCAL} allows you to write macros that
define symbols, without fear of conflict between separate macro expansions.
@end ftable

@node Data
@section Data output

In assembly code, you often need to specify working areas of memory;
depending on the application, you may want to initialize such memory or
not.  @sc{gasp} provides preprocessor directives to help you avoid
repetitive coding for both purposes.

You can use labels as usual to mark the data areas.

@menu
* Initialized::
* Uninitialized::
@end menu

@node Initialized
@subsection Initialized data

These are the @sc{gasp} directives for initialized data, and the standard
@sc{gnu} assembler directives they expand to:

@ftable @code
@item .DATA @var{expr}, @var{expr}, @dots{}
@itemx .DATA.B @var{expr}, @var{expr}, @dots{}
@itemx .DATA.W @var{expr}, @var{expr}, @dots{}
@itemx .DATA.L @var{expr}, @var{expr}, @dots{}
Evaluate arithmetic expressions @var{expr}, and emit the corresponding
@code{as} directive (labelled with @var{lab}).  The unqualified
@code{.DATA} emits @samp{.long}; @code{.DATA.B} emits @samp{.byte};
@code{.DATA.W} emits @samp{.short}; and @code{.DATA.L} emits
@samp{.long}.

For example, @samp{foo .DATA 1,2,3} emits @samp{foo: .long 1,2,3}.

@item .DATAB @var{repeat}, @var{expr}
@itemx .DATAB.B @var{repeat}, @var{expr}
@itemx .DATAB.W @var{repeat}, @var{expr}
@itemx .DATAB.L @var{repeat}, @var{expr}
@c FIXME! Looks like gasp accepts and ignores args after 2nd.
Make @code{as} emit @var{repeat} copies of the value of the expression
@var{expr} (using the @code{as} directive @code{.fill}).
@samp{.DATAB.B} repeats one-byte values; @samp{.DATAB.W} repeats
two-byte values; and @samp{.DATAB.L} repeats four-byte values.
@samp{.DATAB} without a suffix repeats four-byte values, just like
@samp{.DATAB.L}.

@c FIXME! Allowing zero might be useful for edge conditions in macros.
@var{repeat} must be an absolute expression with a positive value.

@item .SDATA "@var{str}" @dots{}
String data.  Emits a concatenation of bytes, precisely as you specify
them (in particular, @emph{nothing is added to mark the end} of the
string).  @xref{Constants,, String and numeric constants}, for details
about how to write strings.  @code{.SDATA} concatenates multiple
arguments, making it easy to switch between string representations.  You
can use commas to separate the individual arguments for clarity, if you
choose.

@item .SDATAB @var{repeat}, "@var{str}" @dots{}
Repeated string data.  The first argument specifies how many copies of
the string to emit; the remaining arguments specify the string, in the
same way as the arguments to @code{.SDATA}.

@item .SDATAZ "@var{str}" @dots{}
Zero-terminated string data.  Just like @code{.SDATA}, except that
@code{.SDATAZ} writes a zero byte at the end of the string.

@item .SDATAC "@var{str}" @dots{}
Count-prefixed string data.  Just like @code{.SDATA}, except that
@sc{gasp} precedes the string with a leading one-byte count.  For
example, @samp{.SDATAC "HI"} generates @samp{.byte 2,72,73}.  Since the
count field is only one byte, you can only use @code{.SDATAC} for
strings less than 256 bytes in length.
@end ftable

@node Uninitialized
@subsection Uninitialized data

@c FIXME!  .space different on some platforms, notably HPPA.  Config?
Use the @code{.RES}, @code{.SRES}, @code{.SRESC}, and @code{.SRESZ}
directives to reserve memory and leave it uninitialized.  @sc{gasp}
resolves these directives to appropriate calls of the @sc{gnu}
@code{as} @code{.space} directive.

@ftable @code
@item .RES @var{count}
@itemx .RES.B @var{count}
@itemx .RES.W @var{count}
@itemx .RES.L @var{count}
Reserve room for @var{count} uninitialized elements of data.  The
suffix specifies the size of each element: @code{.RES.B} reserves
@var{count} bytes, @code{.RES.W} reserves @var{count} pairs of bytes,
and @code{.RES.L} reserves @var{count} quartets.  @code{.RES} without a
suffix is equivalent to @code{.RES.L}.

@item .SRES @var{count}
@itemx .SRES.B @var{count}
@itemx .SRES.W @var{count}
@itemx .SRES.L @var{count}
@c FIXME!  This is boring.  Shouldn't it at least have a different
@c         default size?  (e.g. the "S" suggests "string", for which .B
@c         would be more appropriate)
@code{.SRES} is a synonym for @samp{.RES}.

@item .SRESC @var{count}
@itemx .SRESC.B @var{count}
@itemx .SRESC.W @var{count}
@itemx .SRESC.L @var{count}
Like @code{.SRES}, but reserves space for @code{@var{count}+1} elements.

@item .SRESZ @var{count}
@itemx .SRESZ.B @var{count}
@itemx .SRESZ.W @var{count}
@itemx .SRESZ.L @var{count}
Like @code{.SRES}, but reserves space for @code{@var{count}+1} elements.
@end ftable

@node Listings
@section Assembly listing control

The @sc{gasp} listing-control directives correspond to
related @sc{gnu} @code{as} directives.

@ftable @code
@item .PRINT LIST
@itemx .PRINT NOLIST
Print control.  This directive emits the @sc{gnu} @code{as} directive
@code{.list} or @code{.nolist}, according to its argument.  @xref{List,,
@code{.list}, as.info, Using as}, for details on how these directives
interact.

@item .FORM LIN=@var{ln}
@itemx .FORM COL=@var{cols}
@itemx .FORM LIN=@var{ln} COL=@var{cols}
Specify the page size for assembly listings: @var{ln} represents the
number of lines, and @var{cols} the number of columns.  You may specify
either page dimension independently, or both together.  If you do not
specify the number of lines, @sc{gasp} assumes 60 lines; if you do not
specify the number of columns, @sc{gasp} assumes 132 columns.
(Any values you may have specified in previous instances of @code{.FORM}
do @emph{not} carry over as defaults.)  Emits the @code{.psize}
assembler directive.

@item .HEADING @var{string}
Specify @var{string} as the title of your assembly listings.  Emits
@samp{.title "@var{string}"}.

@item .PAGE
Force a new page in assembly listings.  Emits @samp{.eject}.
@end ftable

@node Other Commands
@section Miscellaneous commands

@ftable @code
@item .ALTERNATE
Use the alternate macro syntax henceforth in the assembly.
@xref{Alternate,, Alternate macro syntax}.

@item .ORG
@c FIXME!  This is very strange, since _GAS_ understands .org
This command is recognized, but not yet implemented.  @sc{gasp}
generates an error message for programs that use @code{.ORG}.

@item .RADIX @var{s}
@c FIXME no test cases in testsuite/gasp
@sc{gasp} understands numbers in any of base two, eight, ten, or
sixteen.  You can encode the base explicitly in any numeric constant
(@pxref{Constants,, String and numeric constants}).  If you write
numbers without an explicit indication of the base, the most recent
@samp{.RADIX @var{s}} command determines how they are interpreted.
@var{s} is a single letter, one of the following:

@table @code
@item .RADIX B
Base 2.

@item .RADIX Q
Base 8.

@item .RADIX D
Base 10.  This is the original default radix.

@item .RADIX H
Base 16.
@end table

You may specify the argument @var{s} in lower case (any of @samp{bqdh})
with the same effects.

@item .EXPORT @var{name}
@itemx .GLOBAL @var{name}
@c FIXME! No test cases in testsuite/gasp
Declare @var{name} global (emits @samp{.global @var{name}}).  The two
directives are synonymous.

@item .PROGRAM
No effect: @sc{gasp} accepts this directive, and silently ignores it.

@item .END
Mark end of each preprocessor file.  @sc{gasp} issues a warning if it
reaches end of file without seeing this command.

@item .INCLUDE "@var{str}"
Preprocess the file named by @var{str}, as if its contents appeared
where the @code{.INCLUDE} directive does.  @sc{gasp} imposes a maximum
limit of 30 stacked include files, as a sanity check.
@c FIXME!  Why is include depth not affected by -u?

@item .ALIGN @var{size}
@c FIXME! Why is this not utterly pointless?
Evaluate the absolute expression @var{size}, and emit the assembly
instruction @samp{.align @var{size}} using the result.
@end ftable

@node Syntax Details
@section Details of the GASP syntax

Since @sc{gasp} is meant to work with assembly code, its statement
syntax has no surprises for the assembly programmer.

@cindex whitespace
@emph{Whitespace} (blanks or tabs; @emph{not} newline) is partially
significant, in that it delimits up to three fields in a line.  The
amount of whitespace does not matter; you may line up fields in separate
lines if you wish, but @sc{gasp} does not require that.

@cindex fields of @sc{gasp} source line
@cindex label field
The @emph{first field}, an optional @dfn{label}, must be flush left in a
line (with no leading whitespace) if it appears at all.  You may use a
colon after the label if you wish; @sc{gasp} neither requires the colon
nor objects to it (but will not include it as part of the label name).

@cindex directive field
The @emph{second field}, which must appear after some whitespace,
contains a @sc{gasp} or assembly @dfn{directive}.

@cindex argument fields
Any @emph{further fields} on a line are @dfn{arguments} to the
directive; you can separate them from one another using either commas or
whitespace.

@menu
* Markers::
* Constants::
* Symbols::
* Expressions::
* String Builtins::
@end menu

@node Markers
@subsection Special syntactic markers

@sc{gasp} recognizes a few special markers: to delimit comments, to
continue a statement on the next line, to separate symbols from other
characters, and to copy text to the output literally.  (One other
special marker, @samp{\@@}, works only within macro definitions;
@pxref{Macros,, Defining your own directives}.)

@cindex comments
The trailing part of any @sc{gasp} source line may be a @dfn{comment}.
A comment begins with the first unquoted comment character (@samp{!} by
default), or an escaped or doubled comment character (@samp{\!} or
@samp{!!} by default), and extends to the end of a line.  You can
specify what comment character to use with the @samp{-c} option
(@pxref{Invoking GASP,, Command Line Options}).  The two kinds of
comment markers lead to slightly different treatment:

@table @code
@item !
A single, un-escaped comment character generates an assembly comment in
the @sc{gasp} output.  @sc{gasp} evaluates any preprocessor variables
(macro arguments, or variables defined with @code{.ASSIGNA} or
@code{.ASSIGNC}) present.  For example, a macro that begins like this

@example
        .MACRO  SUM FROM=0, TO=9
        ! \FROM \TO
@end example

@noindent
issues as the first line of output a comment that records the
values you used to call the macro.

@c comments, preprocessor-only
@c preprocessor-only comments
@c GASP-only comments
@item \!
@itemx !!
Either an escaped comment character, or a double comment character,
marks a @sc{gasp} source comment.  @sc{gasp} does not copy such comments
to the assembly output.
@end table

@cindex continuation character
@kindex +
To @emph{continue a statement} on the next line of the file, begin the
second line with the character @samp{+}.

@cindex literal copy to output
@cindex copying literally to output
@cindex preprocessing, avoiding
@cindex avoiding preprocessing
Occasionally you may want to prevent @sc{gasp} from preprocessing some
particular bit of text.  To @emph{copy literally} from the @sc{gasp}
source to its output, place @samp{\(} before the string to copy, and
@samp{)} at the end.  For example, write @samp{\(\!)} if you need the
characters @samp{\!} in your assembly output.

@cindex symbol separator
@cindex text, separating from symbols
@cindex symbols, separating from text
To @emph{separate a preprocessor variable} from text to appear
immediately after its value, write a single quote (@code{'}).  For
example, @samp{.SDATA "\P'1"} writes a string built by concatenating the
value of @code{P} and the digit @samp{1}.  (You cannot achieve this by
writing just @samp{\P1}, since @samp{P1} is itself a valid name for a
preprocessor variable.)

@node Constants
@subsection String and numeric constants

There are two ways of writing @dfn{string constants} in @sc{gasp}: as
literal text, and by numeric byte value.  Specify a string literal
between double quotes (@code{"@var{str}"}).  Specify an individual
numeric byte value as an absolute expression between angle brackets
(@code{<@var{expr}>}.  Directives that output strings allow you to
specify any number of either kind of value, in whatever order is
convenient, and concatenate the result.  (Alternate syntax mode
introduces a number of alternative string notations; @pxref{Alternate,,
Alternate macro syntax}.)

@c Details of numeric notation, e.g. base prefixes
You can write @dfn{numeric constants} either in a specific base, or in
whatever base is currently selected (either 10, or selected by the most
recent @code{.RADIX}).

To write a number in a @emph{specific base}, use the pattern
@code{@var{s}'@var{ddd}}: a base specifier character @var{s}, followed
by a single quote followed by digits @var{ddd}.  The base specifier
character matches those you can specify with @code{.RADIX}: @samp{B} for
base 2, @samp{Q} for base 8, @samp{D} for base 10, and @samp{H} for base
16.  (You can write this character in lower case if you prefer.)

@c FIXME! What are rules for recognizing number in deflt base?  Whatever
@c        is left over after parsing other things??

@node Symbols
@subsection Symbols

@sc{gasp} recognizes symbol names that start with any alphabetic character,
@samp{_}, or @samp{$}, and continue with any of the same characters or
with digits.  Label names follow the same rules.

@node Expressions
@subsection Arithmetic expressions in GASP

@cindex absolute expressions
@cindex relocatable expressions
There are two kinds of expressions, depending on their result:
@dfn{absolute} expressions, which resolve to a constant (that is, they
do not involve any values unknown to @sc{gasp}), and @dfn{relocatable}
expressions, which must reduce to the form

@example
@var{addsym}+@var{const}-@var{subsym}
@end example

@noindent
where @var{addsym} and @var{subsym} are assembly symbols of unknown
value, and @var{const} is a constant.

Arithmetic for @sc{gasp} expressions follows very similar rules to C.
You can use parentheses to change precedence; otherwise, arithmetic
primitives have decreasing precedence in the order of the following
list.

@enumerate
@item
Single-argument @code{+} (identity), @code{-} (arithmetic opposite), or
@code{~} (bitwise negation).  @emph{The argument must be an absolute
expression.}

@item
@code{*} (multiplication) and @code{/} (division).  @emph{Both arguments
must be absolute expressions.}

@item
@code{+} (addition) and @code{-} (subtraction).  @emph{At least one argument
must be absolute.}
@c FIXME!  Actually, subtraction doesn't check for this.

@item
@code{&} (bitwise and).  @emph{Both arguments must be absolute.}

@item
@c FIXME!  I agree ~ is a better notation than ^ for xor, but is the
@c         improvement worth differing from C?
@code{|} (bitwise or) and @code{~} (bitwise exclusive or; @code{^} in
C).  @emph{Both arguments must be absolute.}
@end enumerate

@node String Builtins
@subsection String primitives

You can use these primitives to manipulate strings (in the argument
field of @sc{gasp} statements):

@ftable @code
@item .LEN("@var{str}")
Calculate the length of string @code{"@var{str}"}, as an absolute
expression.  For example, @samp{.RES.B .LEN("sample")} reserves six
bytes of memory.

@item .INSTR("@var{string}", "@var{seg}", @var{ix})
Search for the first occurrence of @var{seg} after position @var{ix} of
@var{string}.  For example, @samp{.INSTR("ABCDEFG", "CDE", 0)} evaluates
to the absolute result @code{2}.

The result is @code{-1} if @var{seg} does not occur in @var{string}
after position @var{ix}.

@item .SUBSTR("@var{string}",@var{start},@var{len})
The substring of @var{string} beginning at byte number @var{start} and
extending for @var{len} bytes.
@end ftable

@node Alternate
@section Alternate macro syntax

If you specify @samp{-a} or @samp{--alternate} on the @sc{gasp} command
line, the preprocessor uses somewhat different syntax.  This syntax is
reminiscent of the syntax of Phar Lap macro assembler, but it
is @emph{not} meant to be a full emulation of Phar Lap or similar
assemblers.  In particular, @sc{gasp} does not support directives such
as @code{DB} and @code{IRP}, even in alternate syntax mode.

In particular, @samp{-a} (or @samp{--alternate}) elicits these
differences:

@table @emph
@item Preprocessor directives
You can use @sc{gasp} preprocessor directives without a leading @samp{.}
dot.  For example, you can write @samp{SDATA} with the same effect as
@samp{.SDATA}.

@item LOCAL
One additional directive, @code{LOCAL}, is available.  @xref{Macros,,
Defining your own directives}, for an explanation of how to use
@code{LOCAL}.

@need 2000
@item String delimiters
You can write strings delimited in these other ways besides
@code{"@var{string}"}:

@table @code
@item '@var{string}'
You can delimit strings with single-quote charaters.

@item <@var{string}>
You can delimit strings with matching angle brackets.
@end table

@item single-character string escape
To include any single character literally in a string (even if the
character would otherwise have some special meaning), you can prefix the
character with @samp{!} (an exclamation mark).  For example, you can
write @samp{<4.3 !> 5.4!!>} to get the literal text @samp{4.3 > 5.4!}.

@item Expression results as strings
You can write @samp{%@var{expr}} to evaluate the expression @var{expr}
and use the result as a string.  
@end table

@node Index
@unnumbered Index

@printindex cp

@contents
@bye