This part of the documentation is a modified version of the GNU Assembler Manual.
Therefore it is licensed under the GNU Free Documentation License.
The GNU assembler as
is primarily intended to assemble the output
of the GNU C compiler for use by the linker, so it may be regarded as an internal
part of TIGCC package.
However, it may be called as a standalone program, and the GNU team
tried to make as
assemble everything correctly that other assemblers
for the same machine would assemble. Any exceptions are documented explicitly.
This doesn't mean as
always uses the same syntax as other
assemblers for the same architecture; for example, there exist several
incompatible versions of the MC 68000 assembly language syntax, so the syntax used
in the GNU assembler is not exactly the same as in some other assemblers
(like the A68k Assembler, which is the most frequently
used assembler for the TI-89 and TI-92+, and which is also included in the TIGCC
package as a standalone program).
This documentation will cover as
features which are applicable
to TIGCC. The most frequent use of as
is probably as
an inline assembler, which allows mixing assembly
statements with C code using the asm
keyword.
This documentation is not
intended as an introduction to programming in assembly language. In a similar
vein, you will not find here details about machine architecture: here you can not
expect detailed description of the instruction set, standard mnemonics, registers
or addressing modes. You may want to consult the Motorola manufacturer's machine
architecture manual for such information.
Note: It is possible to use source files for the GNU Assembler
together with C source files in TIGCC projects.
Original author: Free Software Foundation, Inc.
Authors of the modifications: Zeljko Juric, Sebastian Reichelt, and Kevin Kofler
Published by the TIGCC Team.
See the History section for details and copyright information.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or any
later version published by the Free Software Foundation;
with no Invariant Sections, with no Front-Cover Texts, and with no
Back-Cover Texts. A copy of the license is included in the
section entitled "GNU Free Documentation License".
After the program name as
, the command line may contain
options and file names. Options may appear in any order, and may be
before, after, or between file names. The order of file names is
significant.
--
(two hyphens) by itself names the standard input file
explicitly, as one of the files for as
to assemble.
Except for --
any command line argument that begins with a
hyphen (-
) is an option. Each option changes the behavior of
as
. No option changes the way another option works. An
option is a -
followed by one or more letters; the case of
the letter is important. All options are optional.
Some options expect exactly one file name to follow them. The file
name may either immediately follow the option's letter (compatible
with older assemblers) or it may be the next command argument (GNU
standard). These two command lines are equivalent:
as -o my-object-file.o mumble.s as -omy-object-file.o mumble.s
If you are invoking as
via tigcc
,
you can use the '-Wa' option to pass arguments through to the assembler.
The assembler arguments must be separated from each other (and the '-Wa')
by commas. For example:
tigcc -c -g -O -Wa,-alh,-L file.c
This passes two options to the assembler: '-alh' (emit a listing to
standard output with high-level and assembly source) and '-L' (retain
local symbols in the symbol table).
Usually you do not need to use this '-Wa' mechanism, since many compiler
command-line options are automatically passed to the assembler by the compiler.
(You can call the GNU compiler driver with the '-v' option to see
precisely what options it passes to each compilation pass, including the
assembler.)
Here is a brief summary of how to invoke as
.
-a[cdhlmns]
Turn on listings, in any of a variety of ways:
-ac
omit false conditionals
-ad
omit debugging directives
-ah
include high-level source
-al
include assembly
-am
include macro expansions
-an
omit forms processing
-as
include symbols
=file
set the name of the listing file
You may combine these options; for example, use '-aln' for assembly
listing without forms processing. The '=file' option, if used, must be
the last one. By itself, '-a' defaults to '-ahls'.
For more information, see Enabling Listings.
--all-relocs
Output all references to non-absolute symbols in the assembled file as
relocation items in the object file, even if the form of a reference would
permit the assembler to resolve it. This especially affects pc-relative
references to symbols defined in the same section, and certain calculations
with symbols. For some calculations, this requires special TIGCC-specific
support for negative relocation items, which makes object files unusable with
older versions of TIGCC. If a calculation cannot be output without being
resolved, an error message is generated. This option implies
'--keep-locals'. The assembler also outputs a special symbol
__ld_all_relocs
to tell the linker that there are no implicit
dependencies between different locations inside the sections.
-D
Ignored. This option is accepted for script compatibility with calls to other assemblers.
--defsym sym=value
Define the symbol sym to be value before assembling the input file.
value must be an integer constant. As in C, a leading 0x
indicates a hexadecimal value, and a leading 0
indicates an octal value.
-f
"fast" - skip whitespace and comment preprocessing (assume source is
compiler output).
This option should only be used when assembling programs written by a
(trusted) compiler. It stops the assembler from doing whitespace
and comment preprocessing on the input file(s) before assembling them.
See Preprocessing.
Warning: if you use '-f' when the files actually need to be
preprocessed (if they contain comments, for example), as
does
not work correctly.
--gdwarf2
Generate DWARF 2 debugging information for each assembler line. This may help debugging assembler code, if the debugger can handle it.
--gstabs
Generate stabs debugging information for each assembler line. This may help debugging assembler code, if the debugger can handle it.
--help
Print a summary of the command line options and exit.
--target-help
Print a summary of all target specific options and exit.
-I dir
Add directory dir to the search list for .include
directives.
-J
Don't warn about signed overflow.
-K
This option is accepted but has no effect on the 680x0 family.
-L
--keep-locals
Keep (in the symbol table) local symbols. On traditional a.out systems
these start with L
, but different systems have different local
label prefixes. See Including Local Labels.
--listing-lhs-width=number
Set the maximum width, in words, of the output data column for an assembler
listing to number.
For more information, see Configuring Listing Output.
--listing-lhs-width2=number
Set the maximum width, in words, of the output data column for continuation lines in an assembler listing to number.
--listing-rhs-width=number
Set the maximum width of an input source line, as displayed in a listing, to number bytes.
--listing-cont-lines=number
Set the maximum number of lines printed in a listing for a single line of input to number+1.
-M
--mri
Use MRI compatibility mode. See Assembling in MRI Compatibility Mode.
--MD depfile
Generate a dependency file. This file consists of a single rule suitable for
make
describing the dependencies of the main source file. The rule
is written to the file named in its argument. This feature is used in the automatic
updating of makefiles. It is not particulary useful for TIGCC.
-o objfile
Name the object-file output from as
objfile.
See Naming the Output File.
-R
Fold the data section into the text section. See Joining the Data and Text Sections.
--statistics
Print the maximum space (in bytes) and total time (in seconds) used by assembly.
--strip-local-absolute
Remove local absolute symbols from the outgoing symbol table.
--traditional-format
Use a more traditional output format. See Traditional Assembler Output Format.
-v
-version
Print the as
version.
--version
Print the as
version and exit.
-W
--no-warn
Suppress warning messages.
See Controlling Warnings for more information about warning switches.
--fatal-warnings
Treat warnings as errors.
--warn
Don't suppress warning messages or treat them as errors.
-w
Ignored.
-x
Ignored.
-Z
Generate an object file even after errors.
-- | files
Standard input, or source files to assemble.
The Motorola 680x0 version of as
has a few machine
dependent options:
-l
You can use the '-l' option to shorten the size of references to undefined
symbols. If you do not use the '-l' option, references to undefined
symbols are wide enough for a full long
(32 bits). (Since
as
cannot know where these symbols end up, as
can
only allocate space for the linker to fill in later. Since as
does not know how far away these symbols are, it allocates as much space as it
can.) If you use this option, the references are only one word wide (16 bits).
This may be useful if you want the object file to be as small as possible, and
you know that the relevant symbols are always less 32 KB away. This option
implies '--short-jumps'.
--short-jumps
The '--short-jumps' option shortens the size of branches to
undefined symbols. Unlike '-l', other references to undefined symbols
are kept wide enough for a full long
(32 bits), unless an
explicit size is specified. This enables you to optimize a modular program
that is smaller than 32 KB as well as possible, while still being able to
reference an external BSS or data section (since no jumps can point into
these sections). Previously (and in non-TIGCC assemblers), the '-l'
option acted like this, but the documentation did not say this.
--register-prefix-optional
Since the compiler as configured for TIGCC
does not prepend an underscore to the names of user variables, the
assembler requires a %
before any use of a register name. This
is intended to let the assembler distinguish between C variables and
functions named a0
through a7
, and so on.
The '--register-prefix-optional' option may be used
to permit omitting the %
even in TIGCC.
If this is done, it will generally be impossible to
refer to C variables and functions with the same names as register
names.
--bitwise-or
Normally the character |
is treated as a comment character, which
means that it can not be used in expressions. The '--bitwise-or'
option turns |
into a normal character. In this mode, you must
either use C style comments, or start comments with a #
character
at the beginning of a line.
--base-size-default-16
--base-size-default-32
If you use an addressing mode with a base register without specifying
the size, as
will normally use the full 32 bit value.
For example, the addressing mode %a0@(%d0)
is equivalent to
%a0@(%d0:l)
. You may use the '--base-size-default-16'
option to tell as
to default to using the 16 bit value.
In this case, %a0@(%d0)
is equivalent to %a0@(%d0:w)
.
You may use the '--base-size-default-32' option to restore the
default behaviour.
--disp-size-default-16
--disp-size-default-32
If you use an addressing mode with a displacement, and the value of the
displacement is not known, as
will normally assume that
the value is 32 bits. For example, if the symbol disp
has not
been defined, as
will assemble the addressing mode
%a0@(disp,%d0)
as though disp
is a 32 bit value. You may
use the '--disp-size-default-16' option to tell as
to instead assume that the displacement is 16 bits. In this case,
as
will assemble %a0@(disp,%d0)
as though
disp
is a 16 bit value. You may use the
'--disp-size-default-32' option to restore the default behaviour.
--pcrel
Always keep branches PC-relative. In the M680x0 architecture all branches
are defined as PC-relative. However, on some processors
(including the M68000 used in calculators) they are limited
to word displacements maximum. When as
needs a long branch
that is not available, it normally emits an absolute jump instead. This
option disables this substitution. When this option is given and no long
branches are available, only word branches will be emitted. An error
message will be generated if a word branch cannot reach its target.
See Branch Improvement.
-m680x0
as
can assemble code for several different members of the
Motorola 680x0 family. The default in TIGCC is to assemble
code for the 68000 microprocessor. The following options may be used to
change the default. These options control which instructions and
addressing modes are permitted. The members of the 680x0 family are
very similar. For detailed information about the differences, see the
Motorola manuals. (These options are not very useful for TIGCC.)
-m68000
-m68ec000
-m68hc000
-m68hc001
-m68008
-m68302
-m68306
-m68307
-m68322
-m68356
Assemble for the 68000. '-m68008', '-m68302', and so on are synonyms for '-m68000', since the chips are the same from the point of view of the assembler.
-m68010
Assemble for the 68010.
-m68020
-m68ec020
Assemble for the 68020.
-m68030
-m68ec030
Assemble for the 68030.
-m68040
-m68ec040
Assemble for the 68040.
-m68060
-m68ec060
Assemble for the 68060.
-mcpu32
-m68330
-m68331
-m68332
-m68333
-m68334
-m68336
-m68340
-m68341
-m68349
-m68360
Assemble for the CPU32 family of chips.
-m5200
Assemble for the ColdFire family of chips.
-m68881
-m68882
Assemble 68881 floating point instructions. This is the default for the 68020, 68030, and the CPU32. The 68040 and 68060 always support floating point instructions.
-mno-68881
Do not assemble 68881 floating point instructions. This is the default for 68000 and the 68010. The 68040 and 68060 always support floating point instructions, even if this option is used.
-m68851
Assemble 68851 MMU instructions. This is the default for the 68020, 68030, and 68060. The 68040 accepts a somewhat different set of MMU instructions; '-m68851' and '-m68040' should not be used together.
-mno-68851
Do not assemble 68851 MMU instructions. This is the default for the 68000, 68010, and the CPU32. The 68040 accepts a somewhat different set of MMU instructions.
The options starting with '-a' enable listing output from the assembler. By itself,
'-a' requests high-level, assembly, and symbols listing.
You can use other letters to select specific options for the list:
'-ah' requests a high-level language listing,
'-al' requests an output-program assembly listing, and
'-as' requests a symbol table listing.
High-level listings require that a compiler debugging option like
'-g' be used, and that assembly listings ('-al') be requested
also.
Use the '-ac' option to omit false conditionals from a listing. Any lines
which are not assembled because of a false .if
(or .ifdef
, or any
other conditional), or a true .if
followed by an .else
, will be
omitted from the listing.
Use the '-ad' option to omit debugging directives from the
listing.
Once you have specified one of these options, you can further control
listing output and its appearance using the directives .list
,
.nolist
, .psize
, .eject
, .title
, and
.sbttl
.
The '-an' option turns off all forms processing.
If you do not request listing output with one of the '-a' options, the
listing-control directives have no effect.
The letters after '-a' may be combined into one option,
e.g., '-aln'.
Note if the assembler source is coming from the standard input (e.g. because it
is being created by gcc
and the '-pipe' command line switch
is being used) then the listing will not contain any comments or preprocessor
directives. This is because the listing code buffers input source lines from
stdin only after they have been preprocessed by the assembler. This reduces
memory usage and makes the code more efficient.
The listing feature of the assembler can be enabled via the command line switch '-a' (see Enabling Listings). This feature combines the input source file(s) with a hex dump of the corresponding locations in the output object file, and displays them as a listing file. The format of this listing can be controlled by pseudo ops inside the assembler source (see Enabling Listings for details) and also by the following switches:
--listing-lhs-width=number
Sets the maximum width, in words, of the first line of the hex byte dump. This dump appears on the left hand side of the listing output.
--listing-lhs-width2=number
Sets the maximum width, in words, of any further lines of the hex byte dump for a given input source line. If this value is not specified, it defaults to being the same as the value specified for '--listing-lhs-width'. If neither switch is used the default is to one.
--listing-rhs-width=number
Sets the maximum width, in characters, of the source line that is displayed alongside the hex dump. The default value for this parameter is 100. The source line is displayed on the right hand side of the listing output.
--listing-cont-lines=number
Sets the maximum number of continuation lines of hex dump that will be displayed for a given single line of source input. The default value is 4.
There is always one object file output when you run as
. By
default it has the name
a.out
.
You can use the '-o' option (which takes exactly one filename) to give the
object file a different name.
Whatever the object file is called, as
overwrites any
existing file of the same name.
as
should never give a warning or error message when
assembling compiler output. But programs written by people often
cause as
to give a warning that a particular assumption was
made. All such warnings are directed to the standard error file.
If you use the '-W' and '--no-warn' options, no warnings are issued.
This only affects the warning messages: it does not change any particular of
how as
assembles your file. Errors, which stop the assembly,
are still reported.
If you use the '--fatal-warnings' option, as
considers
files that generate warnings to be in error.
You can switch these options off again by specifying '--warn', which
causes warnings to be output as usual.
The '-R' option tells as
to write the object file as if all
data-section data lives in the text section. This is only done at
the very last moment: your binary data are the same, but data
section parts are relocated differently. The data section part of
your object file is zero bytes long because all its bytes are
appended to the text section. (see Sections and Relocation).
When you specify '-R', it would be possible to generate shorter
address displacements (because we do not have to cross between text and
data section). We refrain from doing this simply for compatibility with
older versions of as
. In the future, '-R' may work this way.
When as
is configured for COFF output (which is the case in TIGCC),
this option is only useful if you use sections named .text
and
.data
.
Labels beginning with L
(upper case only) are called local
labels. See Symbol Names. Normally you do not see such labels when
debugging, because they are intended for the use of programs (like
compilers) that compose assembler programs, not for your notice.
Normally both as
and ld
discard such labels, so you do not
normally debug with them.
The '-L' option tells as
to retain those L...
symbols
in the object file. Usually, if you do this, you also tell the linker
ld
to preserve symbols whose names begin with L
.
By default, a local label is any label beginning with L
, but each
target is allowed to redefine the local label prefix.
For some targets, the output of as
is different in some ways
from the output of some existing assembler. The '--traditional-format' switch requests
as
to use the traditional format instead.
For example, it disables the exception frame optimizations which
as
normally does by default on gcc
output.
The '-M' or '--mri' option selects MRI compatibility mode. This
changes the syntax and pseudo-op handling of as
to make it
compatible with the ASM68K
assembler from Microtec Research. The exact nature of the
MRI syntax will not be documented here; see the MRI manuals for more
information. Note in particular that the handling of macros and macro
arguments is somewhat different. The purpose of this option is to permit
assembling existing MRI assembler code using as
.
The MRI compatibility is not complete. Certain operations of the MRI assembler
depend upon its object file format, and can not be supported using other object
file formats. Supporting these would require enhancing each object file format
individually. These are:
global symbols in common section
The m68k MRI assembler supports common sections which are merged by the linker.
Other object file formats do not support this. as
handles
common sections by treating them as a single common symbol. It permits local
symbols to be defined within a common section, but it can not support global
symbols, since it has no way to describe them.
complex relocations The MRI assemblers support relocations against a negated section address, and relocations which combine the start addresses of two or more sections. These are not support by other object file formats.
END
pseudo-op specifying start address
The MRI END
pseudo-op permits the specification of a start address.
This is not supported by other object file formats. The start address may
instead be specified using the '-e' option to the linker, or in a linker
script.
IDNT
, .ident
and NAME
pseudo-ops
The MRI IDNT
, .ident
and NAME
pseudo-ops assign a module
name to the output file. This is not supported by other object file formats.
ORG
pseudo-op
The m68k MRI ORG
pseudo-op begins an absolute section at a given
address. This differs from the usual as
.org
pseudo-op,
which changes the location within the current section. Absolute sections are
not supported by other object file formats. The address of a section may be
assigned within a linker script.
There are some other features of the MRI assembler which are not supported by
as
, typically either because they are difficult or because they
seem of little consequence. Some of these may be supported in future releases.
EBCDIC strings EBCDIC strings are not supported.
packed binary coded decimal
Packed binary coded decimal is not supported. This means that the DC.P
and DCB.P
pseudo-ops are not supported.
FEQU
pseudo-op
The m68k FEQU
pseudo-op is not supported.
NOOBJ
pseudo-op
The m68k NOOBJ
pseudo-op is not supported.
OPT
branch control options
The m68k OPT
branch control options - B
, BRS
, BRB
,
BRL
, and BRW
- are ignored. as
automatically
relaxes all branches, whether forward or backward, to an appropriate size, so
these options serve no purpose.
OPT
list control options
The following m68k OPT
list control options are ignored: C
,
CEX
, CL
, CRE
, E
, G
, I
, M
,
MEX
, MC
, MD
, X
.
other OPT
options
The following m68k OPT
options are ignored: NEST
, O
,
OLD
, OP
, P
, PCO
, PCR
, PCS
, R
.
OPT
D
option is default
The m68k OPT
D
option is the default, unlike the MRI assembler.
OPT NOD
may be used to turn it off.
XREF
pseudo-op.
The m68k XREF
pseudo-op is ignored.
.debug
pseudo-op
The i960 .debug
pseudo-op is not supported.
.extended
pseudo-op
The i960 .extended
pseudo-op is not supported.
.list
pseudo-op.
The various options of the i960 .list
pseudo-op are not supported.
.optimize
pseudo-op
The i960 .optimize
pseudo-op is not supported.
.output
pseudo-op
The i960 .output
pseudo-op is not supported.
.setreal
pseudo-op
The i960 .setreal
pseudo-op is not supported.
We use the phrase source program, abbreviated source, to
describe the program input to one run of as
. The program may
be in one or more files; how the source is partitioned into files
doesn't change the meaning of the source.
The source program is a concatenation of the text in all the files, in the
order specified.
Each time you run as
it assembles exactly one source
program. The source program is made up of one or more files.
(The standard input is also a file.)
You give as
a command line that has zero or more input file
names. The input files are read (from left file name to right). A
command line argument (in any position) that has no special meaning
is taken to be an input file name.
If you give as
no file names it attempts to read one input file
from the as
standard input, which is normally your terminal. You
may have to type Ctrl-D
to tell as
there is no more program
to assemble.
Use --
if you need to explicitly name the standard input file
in your command line.
If the source is empty, as
produces a small, empty object
file.
There are two ways of locating a line in the input file (or files) and
either may be used in reporting error messages. One way refers to a line
number in a physical file; the other refers to a line number in a
"logical" file. See Error and Warning Messages.
Physical files are those files named in the command line given
to as
.
Logical files are simply names declared explicitly by assembler
directives; they bear no relation to physical files. Logical file names help
error messages reflect the original source file, when as
source
is itself synthesized from other files. as
understands the
#
directives emitted by the gcc
preprocessor. See also
.file
.
Every time you run as
, it produces an output file, which is
your assembly language program translated into numbers. This file
is the object file. Its default name is
a.out
.
You can give it another name by using the '-o' option. Conventionally,
object file names end with .o
. The default name is used for historical
reasons: older assemblers were capable of assembling self-contained programs
directly into a runnable program. (For some formats, this isn't currently
possible, but it can be done for the a.out
format.)
The object file is meant for input to the linker ld
. It contains
assembled program code, information to help ld
integrate
the assembled program into a runnable file, and (optionally) symbolic
information for the debugger.
as
may write warnings and error messages to the standard error
file (usually your terminal). This should not happen when a compiler
runs as
automatically. Warnings report an assumption made so
that as
could keep assembling a flawed program; errors report a
grave problem that stops the assembly.
Warning messages have the format
file_name:NNN:Warning Message Text
(where NNN is a line number). If a logical file name has been given
(see .file
) it is used for the filename, otherwise the name of
the current input file is used. If a logical line number was given
(see .line
)
then it is used to calculate the number printed,
otherwise the actual line in the current source file is printed. The
message text is intended to be self explanatory (in the grand Unix
tradition).
Error messages have the format
file_name:NNN:FATAL:Error Message Text
The file name and line number are derived as for warning messages. The actual message text may be rather less explanatory because many of them aren't supposed to happen.
The machine-independent syntax used by the GNU assembler is similar to what many other assemblers use; it is inspired by the BSD 4.2 assembler. Motorola-specific features are explained at the end of this chapter.
The as
internal preprocessor:
adjusts and removes extra whitespace. It leaves one space or tab before the keywords on a line, and turns any other whitespace on the line into a single space.
removes all comments, replacing them with a single space, or an appropriate number of newlines.
converts character constants into the appropriate numeric values.
It does not do macro processing, include file handling, or
anything else you may get from your C compiler's preprocessor. You can
do include file processing with the .include
directive
(see .include
). You can use the GNU C compiler driver
to get other "CPP" style preprocessing by giving the input file a
.S
suffix. See Options Controlling the Kind of
Output, gcc.info, Using GNU CC.
Excess whitespace, comments, and character constants
cannot be used in the portions of the input text that are not
preprocessed.
If the first line of an input file is #NO_APP
or if you use the
'-f' option, whitespace and comments are not removed from the input file.
Within an input file, you can ask for whitespace and comment removal in
specific portions of the by putting a line that says #APP
before the
text that may contain whitespace or comments, and putting a line that says
#NO_APP
after this text. This feature is mainly intend to support
asm
statements in compilers whose output is otherwise free of comments
and whitespace.
Whitespace is one or more blanks or tabs, in any order. Whitespace is used to separate symbols, and to make programs neater for people to read. Unless within character constants (see Character Constants), any whitespace means the same as exactly one space.
There are two ways of rendering comments to as
. In both
cases the comment is equivalent to one space.
Anything from /*
through the next */
is a comment.
This means you may not nest these comments.
/* The only way to include a newline ('\n') in a comment is to use this sort of comment. */ /* This sort of comment does not nest. */
Anything from the line comment character to the next newline
is considered a comment and is ignored. The line comment character is
|
on the 680x0 family of processors.
To be compatible with past assemblers, lines that begin with #
have a
special interpretation. Following the #
should be an absolute
expression (see Expressions): the logical line number of the next
line. Then a string (see Strings) is allowed: if present it is a
new logical file name. The rest of the line, if any, should be whitespace.
If the first non-whitespace characters on the line are not numeric,
the line is ignored. (Just like a comment.)
# This is an ordinary comment. # 42-6 "new_file_name" # New logical file name # This is logical line # 36.
This feature is deprecated, and may disappear from future versions
of as
.
A symbol is one or more characters chosen from the set of all
letters (both upper and lower case), digits and the three characters
_.$
.
No symbol may begin with a digit. Case is significant.
There is no length limit: all characters are significant. Symbols are
delimited by characters not in that set, or by the beginning of a file
(since the source program must end with a newline, the end of a file is
not a possible symbol delimiter). See Symbols.
A statement ends at a newline character (\n
) or at a
semicolon (;
). The newline or semicolon is considered part of
the preceding statement. Newlines and semicolons within character
constants are an exception: they do not end statements.
It is an error to end any statement with end-of-file: the last
character of any input file should be a newline.
An empty statement is allowed, and may include whitespace. It is ignored.
A statement begins with zero or more labels, optionally followed by a
key symbol which determines what kind of statement it is. The key
symbol determines the syntax of the rest of the statement. If the
symbol begins with a dot .
then the statement is an assembler
directive: typically valid for any computer. If the symbol begins with
a letter the statement is an assembly language instruction: it
assembles into a machine language instruction.
A label is a symbol immediately followed by a colon (:
).
Whitespace before a label or after a colon is permitted, but you may not
have whitespace between a label's symbol and its colon. See Labels.
label: .directive followed by something another_label: # This is an empty statement. instruction operand_1, operand_2, ...
A constant is a number, written so that its value is known by inspection, without knowing any context. Like this:
.byte 74, 0112, 092, 0x4A, 0X4a, 'J, '\J # All the same value. .ascii "Ring the bell\7" # A string constant. .octa 0x123456789abcdef0123456789ABCDEF0 # A bignum. .float 0f-314159265358979323846264338327\<PHONE_NUMBER>1.693993751E-40 # - pi, a flonum.
There are two kinds of character constants. A character stands for one character in one byte and its value may be used in numeric expressions. String constants (properly called string literals) are potentially many bytes and their values may not be used in arithmetic expressions.
A string is written between double-quotes. It may contain
double-quotes or null characters. The way to get special characters
into a string is to escape these characters: precede them with
a backslash \
character. For example \\
represents
one backslash: the first \
is an escape which tells
as
to interpret the second character literally as a backslash
(which prevents as
from recognizing the second \
as an
escape character). The complete list of escapes follows.
\b
Mnemonic for backspace; for ASCII this is octal code 010.
\f
Mnemonic for FormFeed; for ASCII this is octal code 014.
\n
Mnemonic for newline; for ASCII this is octal code 012.
\r
Mnemonic for carriage-Return; for ASCII this is octal code 015.
\t
Mnemonic for horizontal Tab; for ASCII this is octal code 011.
\ digit digit digit
An octal character code. The numeric code is 3 octal digits.
For compatibility with other Unix systems, 8 and 9 are accepted as digits:
for example, \008
has the value 010, and \009
the value 011.
\x hex-digits...
A hex character code. All trailing hex digits are combined. Either upper or
lower case x
works.
\\
Represents one \
character.
\"
Represents one "
character. Needed in strings to represent
this character, because an unescaped "
would end the string.
\ anything-else
Any other character when escaped by \
gives a warning, but
assembles as if the \
was not present. The idea is that if
you used an escape sequence you clearly didn't want the literal
interpretation of the following character. However as
has no
other interpretation, so as
knows it is giving you the wrong
code and warns you of the fact.
Which characters are escapable, and what those escapes represent, varies widely among assemblers. The current set is what we think the BSD 4.2 assembler recognizes, and is a subset of what most C compilers recognize. If you are in doubt, do not use an escape sequence.
A single character may be written as a single quote immediately
followed by that character. The same escapes apply to characters as
to strings. So if you want to write the character backslash, you
must write '\\
where the first \
escapes the second
\
. As you can see, the quote is an acute accent, not a
grave accent. A newline
(or semicolon ;
)
immediately following an acute accent is taken as a literal character
and does not count as the end of a statement. The value of a character
constant in a numeric expression is the machine's byte-wide code for
that character. as
assumes your character code is ASCII:
'A
means 65, 'B
means 66, and so on.
as
distinguishes three kinds of numbers according to how they
are stored in the target machine. Integers are numbers that
would fit into an int
in the C language. Bignums are
integers, but they are stored in more than 32 bits. Flonums
are floating point numbers, described below.
A binary integer is 0b
or 0B
followed by zero or more of
the binary digits 01
.
An octal integer is 0
followed by zero or more of the octal
digits (01234567
).
A decimal integer starts with a non-zero digit followed by zero or
more digits<PHONE_NUMBER>
).
A hexadecimal integer is 0x
or 0X
followed by one or
more hexadecimal digits chosen from<PHONE_NUMBER>abcdefABCDEF
.
Integers have the usual values. To denote a negative integer, use
the prefix operator -
discussed under expressions
(see Prefix Operators).
A bignum has the same syntax and semantics as an integer except that the number (or its negative) takes more than 32 bits to represent in binary. The distinction is made because in some places integers are permitted while bignums are not.
A flonum represents a floating point number. The translation is
indirect: a decimal floating point number from the text is converted by
as
to a generic binary floating point number of more than
sufficient precision. This generic floating point number is converted
to a particular computer's floating point format (or formats) by a
portion of as
specialized to that computer.
The version of as
used by TIGCC does not use TI's SMAP II BCD
format; it emits standard IEEE floating point numbers.
It would be pointless to implement the correct behavior, since the appropriate
numbers are easy to write, and converting between base 2 and 10 can
decrease precision.
A flonum is written by writing (in order)
The digit 0
.
A letter, to tell as
the rest of the number is a flonum.
An optional sign: either +
or -
.
An optional integer part: zero or more decimal digits.
An optional fractional part: .
followed by zero
or more decimal digits.
An optional exponent, consisting of:
An E
or e
.
Optional sign: either +
or -
.
One or more decimal digits.
At least one of the integer part or the fractional part must be
present. The floating point number has the usual base-10 value.
as
does all processing using integers. Flonums are computed
independently of any floating point hardware in the computer running
as
.
In this configuration of as
(which does not prepend
an underscore to the names of user variables), the
assembler requires a '%'
before any use of a register name. This
is intended to let the assembler distinguish between C variables and
functions named 'a0'
through 'a7'
, and so on.
Two different syntaxes for the Motorola 680x0 are widely used.
The first one was developed at MIT. The second one is the
standard Motorola syntax for this chip, and it differs from the MIT syntax.
as
can accept Motorola syntax for operands, even if MIT syntax
is used for other operands in the same instruction. The two kinds of syntax are
fully compatible.
The MIT syntax uses instructions names and
syntax compatible with the Sun assembler. Intervening periods are
ignored; for example, movl
is equivalent to mov.l
.
In the following table, apc stands for any of the address registers
(%a0
through %a7
), the program counter (%pc
), the
zero-address relative to the program counter (%zpc
), a suppressed
address register (%za0
through %za7
), or it may be omitted
entirely. The use of size means one of w
or l
, and
it may be omitted, along with the leading colon, unless a scale is also
specified. The use of scale means one of 1
, 2
,
4
, or 8
, and it may always be omitted along with the
leading colon.
The following addressing modes are understood
(note that some of them are valid only on 68020 or later processors,
not on the ordinary 68000 used in TI calculators):
Immediate
#number
Data Register
%d0
through %d7
Address Register
%a0
through %a7
%a7
is also known as %sp
, i.e. the Stack Pointer. %a6
is also known as %fp
, the Frame Pointer.
Address Register Indirect
%a0@
through %a7@
Address Register Postincrement
%a0@+
through %a7@+
Address Register Predecrement
%a0@-
through %a7@-
Indirect Plus Offset
apc@(number)
Index
apc@(number,register:size:scale)
The number may be omitted.
Postindex
apc@(number)@(onumber,register:size:scale)
The onumber or the register, but not both, may be omitted.
Preindex
apc@(number,register:size:scale)@(onumber)
The number may be omitted. Omitting the register produces
the Postindex addressing mode.
Absolute
symbol
, or digits
, optionally followed by
:b
, :w
, or :l
.
In the following table, apc stands for any of the address registers
(%a0
through %a7
), the program counter (%pc
), the
zero-address relative to the program counter (%zpc
), or a
suppressed address register (%za0
through %za7
). The use
of size means one of w
or l
, and it may always be
omitted along with the leading dot. The use of scale means one of
1
, 2
, 4
, or 8
, and it may always be omitted
along with the leading asterisk.
The following additional addressing modes are understood
(note that some of them are valid only on 68020 or later processors,
not on the ordinary 68000 used in TI calculators):
Address Register Indirect
(%a0)
through (%a7)
%a7
is also known as %sp
, i.e. the Stack Pointer. %a6
is also known as %fp
, the Frame Pointer.
Address Register Postincrement
(%a0)+
through (%a7)+
Address Register Predecrement
-(%a0)
through -(%a7)
Indirect Plus Offset
number(%a0)
through number(%a7)
,
or number(%pc)
.
The number may also appear within the parentheses, as in
(number,%a0)
. When used with the pc, the
number may be omitted (with an address register, omitting the
number produces Address Register Indirect mode).
Index
number(apc,register.size*scale)
The number may be omitted, or it may appear within the
parentheses. The apc may be omitted. The register and the
apc may appear in either order. If both apc and
register are address registers, and the size and scale
are omitted, then the first register is taken as the base register, and
the second as the index register.
Postindex
([number,apc],register.size*scale,onumber)
The onumber, or the register, or both, may be omitted.
Either the number or the apc may be omitted, but not both.
Preindex
([number,apc,register.size*scale],onumber)
The number, or the apc, or the register, or any two of
them, may be omitted. The onumber may be omitted. The
register and the apc may appear in either order. If both
apc and register are address registers, and the size
and scale are omitted, then the first register is taken as the
base register, and the second as the index register.
Certain pseudo opcodes are permitted for branch instructions.
They expand to the shortest branch instruction that reach the
target. Generally these mnemonics are made by substituting j
for
b
at the start of a Motorola mnemonic.
The following table summarizes the pseudo-operations for the 68000 processor;
the 68020 has some more possibilites.
Note that the 68000 LONG operations are always absolute and require runtime relocation.
They will not be used if the '--pcrel' option is given.
A (*)
flags
cases that are more fully described after the table:
| Displacement | ||||||
| Pseudo-Op | BYTE | WORD | LONG | |||
| jbsr | bsr.s | bsr | jsr | |||
| jra | bra.s | bra | jmp | |||
jXX (*) | bXX.s | bXX | bNX; jmp | |||
dbXX (*) | dbXX | dbXX | dbXX; bra; jmp |
jbsr
jra
These are the simplest jump pseudo-operations; they always map to one
particular machine instruction, depending on the displacement to the
branch target. This instruction will be a byte or word branch if that
is sufficient. Otherwise, if the '--pcrel' option is not
given, an absolute long jump will be emitted.
If the '--pcrel' option is given and a word
branch cannot reach the target, an error message is generated.
In addition to standard branch operands, as
allows these
pseudo-operations to have all operands that are allowed for jsr and jmp,
substituting these instructions if the operand given is not valid for a
branch instruction.
jXX
Here, jXX
stands for an entire family of pseudo-operations,
where XX is a conditional branch or condition-code test. The full
list of pseudo-ops in this family is:
| jhi | jls | jcc | jcs | jne | jeq | jvc |
| jvs | jpl | jmi | jge | jlt | jgt | jle |
as
issues a longer code fragment in terms of NX, the opposite condition
to XX. For example, under these conditions:
jXX foo
gives
bNXs oof jmp foo oof:
dbXX
The full family of pseudo-operations covered here is:
| dbhi | dbls | dbcc | dbcs | dbne | dbeq | dbvc |
| dbvs | dbpl | dbmi | dbge | dblt | dbgt | dble |
| dbf | dbra | dbt |
dbXX
instructions allow word displacements only. When
a word displacement is sufficient, each of these pseudo-operations expands
to the corresponding Motorola instruction. When a word displacement is not
sufficient and long branches are available, when the source reads
dbXX foo
, as
emits
dbXX oo1 bra.s oo2 oo1: jmp foo oo2:
The immediate character is #
for Sun compatibility. The
line-comment character is |
(unless the '--bitwise-or'
option is used). If a #
appears at the beginning of a line, it
is treated as a comment unless it looks like # line file
, in
which case it is treated normally.
Roughly, a section is a range of addresses, with no gaps; all data
"in" those addresses is treated the same for some particular purpose.
For example there may be a "read only" section.
The linker ld
reads many object files (partial programs) and
combines their contents to form a runnable program. When as
emits an object file, the partial program is assumed to start at address 0.
ld
assigns the final addresses for the partial program, so that
different partial programs do not overlap. This is actually an
oversimplification, but it suffices to explain how as
uses
sections.
ld
moves blocks of bytes of your program to their run-time
addresses. These blocks slide to their run-time addresses as rigid
units; their length does not change and neither does the order of bytes
within them. Such a rigid unit is called a section. Assigning
run-time addresses to sections is called relocation. It includes
the task of adjusting mentions of object-file addresses so they refer to
the proper run-time addresses.
An object file written by as
has at least three sections, any
of which may be empty. These are named text, data and
bss sections.
as
can also generate whatever other named sections you specify
using the .section
directive.
If you do not use any directives that place output in the .text
or .data
sections, these sections still exist, but are empty.
Within the object file, the text section starts at address 0
, the
data section follows, and the bss section follows the data section.
To let ld
know which data changes when the sections are
relocated, and how to change that data, as
also writes to the
object file details of the relocation needed. To perform relocation
ld
must know, each time an address in the object
file is mentioned:
Where in the object file is the beginning of this reference to an address?
How long (in bytes) is this reference?
Which section does the address refer to? What is the numeric value of (address) - (start-address of section)?
Is the reference to an address "Program-Counter relative"?
In fact, every address as
ever uses is expressed as
(section) + (offset into section)
Further, most expressions as
computes have this section-relative
nature.
In this manual we use the notation {secname N} to mean "offset
N into section secname."
Apart from text, data and bss sections you need to know about the
absolute section. When ld
mixes partial programs,
addresses in the absolute section remain unchanged. For example, address
{absolute 0}
is "relocated" to run-time address 0 by
ld
. Although the linker never arranges two partial programs'
data sections with overlapping addresses after linking, by definition
their absolute sections must overlap. Address {absolute 239}
in one
part of a program is always the same address when the program is running as
address {absolute 239}
in any other part of the program.
The idea of sections is extended to the undefined section. Any
address whose section is unknown at assembly time is by definition
rendered {undefined U} - where U is filled in later.
Since numbers are always defined, the only way to generate an undefined
address is to mention an undefined symbol. A reference to a named
common block would be such a symbol: its value is unknown at assembly
time so it has section undefined.
By analogy the word section is used to describe groups of sections in
the linked program. ld
puts all partial programs' text
sections in contiguous addresses in the linked program. It is
customary to refer to the text section of a program, meaning all
the addresses of all partial programs' text sections. Likewise for
data and bss sections.
Some sections are manipulated by ld
; others are invented for
use of as
and have no meaning except during assembly.
ld
deals with just four kinds of sections, summarized below.
These sections hold your program. as
and ld
treat them as
separate but equal sections. Anything you can say of one section is
true of another.
When the program is running, however, it is
customary for the text section to be unalterable. The
text section is often shared among processes: it contains
instructions, constants and the like. The data section of a running
program is usually alterable: for example, C variables would be stored
in the data section.
bss section
This section contains zeroed bytes when your program begins running. It is used to hold uninitialized variables or common storage. The length of each partial program's bss section is important, but because it starts out containing zeroed bytes there is no need to store explicit zero bytes in the object file. The bss section was invented to eliminate those explicit zeros from object files.
absolute section
Address 0 of this section is always "relocated" to runtime address 0.
This is useful if you want to refer to an address that ld
must
not change when relocating. In this sense we speak of absolute
addresses being "unrelocatable": they do not change during relocation.
undefined section
This "section" is a catch-all for address references to objects not in the preceding sections.
An idealized example of three relocatable sections follows. Memory addresses are on the horizontal axis.
+ - --+ - -+--+ partial program # 1: |ttttt|dddd|00| + - --+ - -+--+ text data bss seg. seg. seg. + - + - + - + partial program # 2: |TTT|DDD|000| + - + - + - + +--+ - + - --+--+ - -+ - + - --+~~ linked program: | |TTT|ttttt| |dddd|DDD|00000| +--+ - + - --+--+ - -+ - + - --+~~ addresses: 0 ...
These sections are meant only for the internal use of as
. They
have no meaning at run-time. You do not really need to know about these
sections for most purposes; but they can be mentioned in as
warning messages, so it might be helpful to have an idea of their
meanings to as
. These sections are used to permit the
value of every expression in your assembly language program to be a
section-relative address.
ASSEMBLER-INTERNAL-LOGIC-ERROR!
An internal assembler logic error has been found. This means there is a bug in the assembler.
expr section
The assembler stores complex expression internally as combinations of symbols. When it needs to represent an expression as a symbol, it puts it in the expr section.
You may have separate groups of
data in named sections
that you want to end up near to each other in the object file, even though they
are not contiguous in the assembler source. as
allows you to
use subsections for this purpose. Within each section, there can be
numbered subsections with values from 0 to 8192. Objects assembled into the
same subsection go into the object file together with other objects in the same
subsection. For example, a compiler might want to store constants in the text
section, but might not want to have them interspersed with the program being
assembled. In this case, the compiler could issue a .text 0
before each
section of code being output, and a .text 1
before each group of
constants being output.
Subsections are optional. If you do not use subsections, everything
goes in subsection number zero.
Subsections appear in your object file in numeric order, lowest numbered
to highest. (All this to be compatible with other people's assemblers.)
The object file contains no representation of subsections; ld
and
other programs that manipulate object files see no trace of them.
They just see all your text subsections as a text section, and all your
data subsections as a data section.
To specify which subsection you want subsequent statements assembled
into, use a numeric argument to specify it, in a .text
expression
or a .data expression
statement.
You
can also use an extra subsection
argument with arbitrarily named sections: .section name,
expression
.
Expression should be an absolute expression.
(see Expressions.) If you just say .text
then .text 0
is assumed. Likewise .data
means .data 0
. Assembly
begins in text 0
. For instance:
.text 0 # The default subsection is text 0 anyway. .ascii "This lives in the first text subsection. *" .text 1 .ascii "But this lives in the second text subsection." .data 0 .ascii "This lives in the data section," .ascii "in the first data subsection." .text 0 .ascii "This lives in the first text section," .ascii "immediately following the asterisk (*)."
Each section has a location counter incremented by one for every byte
assembled into that section. Because subsections are merely a convenience
restricted to as
there is no concept of a subsection location
counter. There is no way to directly manipulate a location counter - but the
.align
directive changes it, and any label definition captures its
current value. The location counter of the section where statements are being
assembled is said to be the active location counter.
The bss section is used for local common variable storage.
You may allocate address space in the bss section, but you may
not dictate data to load into it before your program executes. When
your program starts running, all the contents of the bss
section are zeroed bytes.
The .lcomm
pseudo-op defines a symbol in the bss section.
The .comm
pseudo-op may be used to declare a common symbol, which is
another form of uninitialized symbol.
You may switch into the .bss
section and define
symbols as usual (see .section
). You may only assemble
zero values into the section. Typically the section will only contain symbol definitions
and .skip
directives.
Symbols are a central concept: the programmer uses symbols to name
things, the linker uses symbols to link, and the debugger uses symbols
to debug.
Note that as
does not place symbols in the object file in
the same order they were declared. This may break some debuggers.
A label is written as a symbol immediately followed by a colon
:
. The symbol then represents the current value of the
active location counter, and is, for example, a suitable instruction
operand. You are warned if you use the same symbol to represent two
different locations: the first definition overrides any other
definitions.
A symbol can be given an arbitrary value by writing a symbol, followed
by an equals sign =
, followed by an expression
(see Expressions). This is equivalent to using the .set
directive.
Symbol names begin with a letter or with .
, _
, or $
.
That character may be followed by any string of digits, letters, dollar signs, and underscores.
Case of letters is significant: foo
is a different symbol name
than Foo
.
Each symbol has exactly one name. Each name in an assembly language program
refers to exactly one symbol. You may use that symbol name any number of times
in a program.
Local symbols help compilers and programmers use names temporarily.
They create symbols which are guaranteed to be unique over the entire scope of
the input source code and which can be referred to by a simple notation.
To define a local symbol, write a label of the form N:
(where N
represents any positive integer). To refer to the most recent previous
definition of that symbol write Nb
, using the same number as when
you defined the label. To refer to the next definition of a local label, write
Nf
- The b
stands for "backwards" and the f
stands
for "forwards".
There is no restriction on how you can use these labels, and you can reuse them
as well. So it is possible to repeatedly define the same local label (using
the same number N), although you can only refer to the most recently
defined local label of that number (for a backwards reference) or the next
definition of a specific local label for a forward reference. It is also worth
noting that the first 10 local labels (0:
...9:
) are
implemented in a slightly more efficient manner than the others.
Here is an example:
1: jra 1f 2: jra 1b 1: jra 2f 2: jra 1b
Which is the equivalent of:
label_1: jra label_3 label_2: jra label_1 label_3: jra label_4 label_4: jra label_3
Local symbol names are only a notational device. They are immediately transformed into more conventional symbol names before the assembler uses them. The symbol names stored in the symbol table, appearing in error messages and optionally emitted to the object file. The names are constructed using these parts:
L
All local labels begin with L
. Normally both as
and
ld
forget symbols that start with L
. These labels are
used for symbols you are never intended to see. If you use the
'-L' option, as
retains these symbols in the
object file. If you also instruct ld
to retain these symbols,
you may use them in debugging.
N
This is the number that was used in the local label definition. So if the
label is written 55:
, the number is 55
.
\002
This unusual character is included so you do not accidentally invent a symbol of the same name.
ordinal number
This is a serial number to keep the labels distinct. The first definition of
0:
gets the number 1
. The 15th definition of 0:
gets the
number 15
, and so on. Likewise the first definition of 1:
gets
the number 1
and its 15th defintion gets 15
as well.
as
also supports an even more local form of local labels called
dollar labels. These labels go out of scope (i.e. they become undefined) as soon
as a non-local label is defined. Thus they remain valid for only a small
region of the input source code. Normal local labels, by contrast, remain in
scope for the entire file, or until they are redefined by another occurrence of
the same local label.
Dollar labels are defined in exactly the same way as ordinary local labels,
except that instead of being terminated by a colon, they are terminated by a
dollar sign (for example, 55$
).
They can also be distinguished from ordinary local labels by their transformed
name which uses ASCII character \001
(control-A) as the magic character
to distinguish them from ordinary labels.
The special symbol .
refers to the current address that
as
is assembling into. Thus, the expression melvin:
.long .
defines melvin
to contain its own address.
Assigning a value to .
is treated the same as a .org
directive. Thus, the expression .=.+4
is the same as saying
.space 4
.
Every symbol has, as well as its name, the attributes "Value" and
"Type". Depending on output format, symbols can also have auxiliary
attributes.
If you use a symbol without defining it, as
assumes zero for
all these attributes, and probably won't warn you. This makes the
symbol an externally defined symbol, which is generally what you
would want.
The value of a symbol is (usually) 32 bits. For a symbol which labels a
location in the text, data, bss or absolute sections the value is the
number of addresses from the start of that section to the label.
Naturally for text, data and bss sections the value of a symbol changes
as ld
changes section base addresses during linking. Absolute
symbols' values do not change during linking: that is why they are
called absolute.
The value of an undefined symbol is treated in a special way. If it is
0 then the symbol is not defined in this assembler source file, and
ld
tries to determine its value from other files linked into the
same program. You make this kind of symbol simply by mentioning a symbol
name without defining it. A non-zero value represents a .comm
common declaration. The value is how much common storage to reserve, in
bytes (addresses). The symbol refers to the first address of the
allocated storage.
The type attribute of a symbol contains relocation (section) information, any flag settings indicating that a symbol is external, and (optionally), other information for linkers and debuggers. The exact format depends on the object-code output format in use.
The COFF format supports a multitude of auxiliary symbol attributes;
like the primary symbol attributes, they are set between .def
and
.endef
directives.
The symbol name is set with .def
; the value and type,
respectively, with .val
and .type
.
The as
directives .dim
, .line
, .scl
,
.size
, and .tag
can generate auxiliary symbol table
information for COFF.
An expression specifies an address or numeric value.
Whitespace may precede and/or follow an expression.
The result of an expression must be an absolute number, or else an offset into
a particular section. If an expression is not absolute, and there is not
enough information when as
sees the expression to know its
section, a second pass over the source program might be necessary to interpret
the expression - but the second pass is currently not implemented.
as
aborts with an error message in this situation.
An empty expression has no value: it is just whitespace or null.
Wherever an absolute expression is required, you may omit the
expression, and as
assumes a value of (absolute) 0. This
is compatible with other assemblers.
An integer expression is one or more arguments delimited by operators.
Arguments are symbols, numbers or subexpressions. In other
contexts arguments are sometimes called "arithmetic operands". In
this manual, to avoid confusing them with the "instruction operands" of
the machine language, we use the term "argument" to refer to parts of
expressions only, reserving the word "operand" to refer only to machine
instruction operands.
Symbols are evaluated to yield {section NNN} where
section is one of text, data, bss, absolute,
or undefined. NNN is a signed, 2's complement 32 bit
integer.
Numbers are usually integers.
In principle, a number can be a flonum or bignum. In this case, you are warned
that only the low order 32 bits are used, and as
pretends
these 32 bits are an integer. You may write integer-manipulating
instructions that act on exotic constants, compatible with other
assemblers.
Subexpressions are a left parenthesis (
followed by an integer
expression, followed by a right parenthesis )
; or a prefix
operator followed by an argument.
Operators are arithmetic functions, like +
or %
. Prefix
operators are followed by an argument. Infix operators appear
between their arguments. Operators may be preceded and/or followed by
whitespace.
as
has the following prefix operators. They each take
one argument, which must be absolute.
-
Negation. Two's complement negation.
~
Complementation. Bitwise NOT.
Infix operators take two arguments, one on either side. Operators
have precedence, but operations with equal precedence are performed left
to right. Apart from +
or -
, both arguments must be
absolute, and the result is absolute.
Highest Precedence
*
Multiplication.
/
Division. Truncation is the same as the C operator /
.
%
Remainder.
<
<<
Shift Left. Same as the C operator <<
.
>
>>
Shift Right. Same as the C operator >>
.
Intermediate precedence
|
Bitwise Inclusive OR.
&
Bitwise AND.
^
Bitwise Exclusive OR.
!
Bitwise OR NOT.
Low Precedence
+
Addition. If either argument is absolute, the result has the section of the other argument. You may not add together arguments from different sections.
-
Subtraction. If the right argument is absolute, the result has the section of the left argument. If both arguments are in the same section, the result is absolute. You may not subtract arguments from different sections.
==
Is Equal To.
<>
Is Not Equal To.
<
Is Less Than.
>
Is Greater Than.
>=
Is Greater Than Or Equal To.
<=
Is Less Than Or Equal To.
The comparison operators can be used as infix operators. A true results has a
value of -1 whereas a false result has a value of 0. Note, these operators
perform signed comparisons.
&&
Logical AND.
||
Logical OR.
These two logical operations can be used to combine the results of sub
expressions. Note, unlike the comparison operators a true result returns a
value of 1. Also note that the logical
OR operator has a slightly lower precedence than logical AND.
In short, it's only meaningful to add or subtract the offsets in an address; you can only have a defined section in one of the two arguments.
All assembler directives have names that begin with a period (.
).
The rest of the name is letters, usually in lower case.
This chapter mostly discusses directives that are available regardless of the
target machine configuration for the GNU assembler.
This directive stops the assembly immediately. It is for
compatibility with other assemblers. The original idea was that the
assembly language source would be piped into the assembler. If the sender
of the source quit, it could use this directive to tell as
to
quit also. One day .abort
will not be supported.
.ABORT
is accepted as an alternate spelling of .abort
.
Syntax: .align alignment[, [fill][, max]]
Pad the location counter (in the current subsection) to a particular storage
boundary. alignment (which must be absolute) is the alignment
required, as described below.
fill (also absolute) gives the fill value to be stored in the
padding bytes. It (and the comma) may be omitted. If it is omitted, the
padding bytes are normally zero. However, on some systems, if the section is
marked as containing code and the fill value is omitted, the space is filled
with no-op instructions (I didn't checked whether this is the case in TIGCC).
max is also absolute, and is also optional. If it is present,
it is the maximum number of bytes that should be skipped by this alignment
directive. If doing the alignment would require skipping more bytes than the
specified maximum, then the alignment is not done at all. You can omit the
fill value (the second argument) entirely by simply using two commas after the
required alignment; this can be useful if you want the alignment to be filled
with no-op instructions when appropriate.
The way the required alignment is specified varies from system to system.
For the a29k, hppa, m68k, m88k, w65, sparc, Xtensa, and Renesas / SuperH SH,
and i386 using ELF format,
the first expression is the
alignment request in bytes. For example .align 8
advances
the location counter until it is a multiple of 8. If the location counter
is already a multiple of 8, no change is needed.
For other systems, including the i386 using a.out format, and the arm and
strongarm, it is the
number of low-order zero bits the location counter must have after
advancement. For example .align 3
advances the location
counter until it a multiple of 8. If the location counter is already a
multiple of 8, no change is needed.
This inconsistency is due to the different behaviors of the various
native assemblers for these systems which as
must emulate.
as
also provides .balign
and .p2align
directives,
which have a consistent behavior across all
architectures (but are specific to as
).
Syntax: .ascii strings
.ascii
expects zero or more string literals (see Strings)
separated by commas. It assembles each string (with no automatic
trailing zero byte) into consecutive addresses.
Syntax: .asciz strings
.asciz
is just like .ascii
, but each string is followed by
a zero byte. The "z" in .asciz
stands for "zero".
Syntax: .balign[wl] alignment[, [fill][, max]]
Pad the location counter (in the current subsection) to a particular
storage boundary. alignment (which must be absolute) is the
alignment request in bytes. For example .balign 8
advances
the location counter until it is a multiple of 8. If the location counter
is already a multiple of 8, no change is needed.
fill (also absolute) gives the fill value to be stored in the
padding bytes. It (and the comma) may be omitted. If it is omitted, the
padding bytes are normally zero. However, on some systems, if the section is
marked as containing code and the fill value is omitted, the space is filled
with no-op instructions (I didn't checked whether this is the case in TIGCC).
max is also absolute, and is also optional. If it is present,
it is the maximum number of bytes that should be skipped by this alignment
directive. If doing the alignment would require skipping more bytes than the
specified maximum, then the alignment is not done at all. You can omit the
fill value (the second argument) entirely by simply using two commas after the
required alignment; this can be useful if you want the alignment to be filled
with no-op instructions when appropriate.
The .balignw
and .balignl
directives are variants of the
.balign
directive. The .balignw
directive treats the fill
pattern as a two byte word value. The .balignl
directives treats the
fill pattern as a four byte longword value. For example, .balignw
4,0x368d
will align to a multiple of 4. If it skips two bytes, they will be
filled in with the value 0x368d (the exact placement of the bytes depends upon
the endianness of the processor). If it skips 1 or 3 bytes, the fill value is
undefined.
See also: .p2align
Syntax: .byte expressions
.byte
expects zero or more expressions, separated by commas.
Each expression is assembled into the next byte.
Syntax: .comm symbol, length
.comm
declares a common symbol named symbol. When linking, a
common symbol in one object file may be merged with a defined or common symbol
of the same name in another object file. If ld
does not see a
definition for the symbol - just one or more common symbols - then it will
allocate length bytes of uninitialized memory. length must be an
absolute expression. If ld
sees multiple common symbols with
the same name, and they do not all have the same size, it will allocate space
using the largest size.
See also: .lcomm
Syntax: .data [subsection]
.data
tells as
to assemble the following statements onto the
end of the data subsection numbered subsection (which is an
absolute expression). If subsection is omitted, it defaults
to zero.
In order to be compatible with the Sun assembler, the 680x0 assembler
understands the directives .data1
and .data2
as alternatives to .data 1
and .data 2
.
See also: .text
Syntax: .def name
Begin defining debugging information for a symbol name; the
definition extends until the .endef
directive is encountered.
This directive is generated by compilers to include auxiliary debugging
information in the symbol table. It is only permitted inside
.def
/.endef
pairs.
Syntax: .double flonums
.double
expects zero or more flonums, separated by commas. It
assembles floating point numbers.
See also: .single
Force a page break at this point, when generating assembly listings.
.else
is part of the as
support for conditional
assembly; see .if
. It marks the beginning of a section
of code to be assembled if the condition for the preceding .if
was false.
.end
marks the end of the assembly file. as
does not
process anything in the file past the .end
directive.
.elseif
is part of the as
support for conditional
assembly; see .if
. It is shorthand for beginning a new
.if
block that would otherwise fill the entire .else
section.
This directive flags the end of a symbol definition begun with
.def
.
.endfunc
marks the end of a function specified with .func
.
.endif
is part of the as
support for conditional assembly;
it marks the end of a block of code that is only assembled
conditionally. See .if
.
.endm
terminates a .macro
directive.
.endr
can terminate either an .irp
or an .irpc
directive.
Syntax: .equ symbol, expression
This directive sets the value of symbol to expression.
It is synonymous with .set
.
See also: .equiv
Syntax: .equiv symbol, expression
The .equiv
directive is like .equ
and .set
, except that
the assembler will signal an error if symbol is already defined. Note a
symbol which has been referenced but not actually defined is considered to be
undefined.
Except for the contents of the error message, this is roughly equivalent to
.ifdef SYM .err .endif .equ SYM,VAL
If as
assembles a .err
directive, it will print an error
message and, unless the '-Z' option was used, it will not generate an
object file. This can be used to signal error an conditionally compiled code.
This directive is a special case of the .align
directive; it
aligns the output to an even byte boundary. It is 680x0-specific;
introduced in order to be compatible with the Sun assembler.
Exit early from the current macro definition. See .macro
.
.extern
is accepted in the source program - for compatibility
with other assemblers - but it is ignored. as
treats
all undefined symbols as external.
Generates an error or a warning. If the value of the expression is 500
or more, as
will print a warning message. If the value is less
than 500, as
will print an error message. The message will
include the value of expression. This can occasionally be useful inside
complex nested macros or conditional assembly.
Syntax: .file string
.file
tells as
that we are about to start a new logical
file. string is the new file name. In general, the filename is
recognized whether or not it is surrounded by quotes ("
); but if you wish
to specify an empty file name, you must give the quotes ""
.
Syntax: .fill repeat[, size[, value]]
repeat, size and value are absolute expressions.
This emits repeat copies of size bytes. Repeat
may be zero or more. Size may be zero or more, but if it is
more than 8, then it is deemed to have the value 8, compatible with
other people's assemblers. The contents of each repeat bytes
is taken from an 8-byte number. The highest order 4 bytes are
zero. The lowest order 4 bytes are value rendered in the
byte-order of an integer on the computer as
is assembling for
(big-endian for 680x0).
Each size bytes in a repetition is taken from the lowest order
size bytes of this number. Again, this bizarre behavior is
compatible with other people's assemblers.
size and value are optional.
If the second comma and value are absent, value is
assumed zero. If the first comma and following tokens are absent,
size is assumed to be 1.
Syntax: .float flonums
This directive assembles zero or more flonums, separated by commas. It
has the same effect as .single
.
Syntax: .func name[, label]
.func
emits debugging information to denote function name, and
is ignored unless the file is assembled with debugging enabled.
Only '--gstabs' is currently supported.
label is the entry point of the function, and if omitted, name
prepended with the leading character is used (no leading character in TIGCC).
All functions are currently defined to have void
return type.
The function must be terminated with .endfunc
.
Syntax: .global symbol
.global
makes the symbol visible to ld
. If you define
symbol in your partial program, its value is made available to
other partial programs that are linked with it. Otherwise,
symbol takes its attributes from a symbol of the same name
from another file linked into the same program.
Both spellings (.globl
and .global
) are accepted, for
compatibility with other assemblers. .xdef
is also accepted as a
synonym for .global
.
Syntax: .hword expressions
This expects zero or more expressions, and emits
a 16 bit number for each.
On this target, this directive is a synonym for both .short
and .word
.
This directive is used by some assemblers to place tags in object files.
as
simply accepts the directive for source-file
compatibility with such assemblers, but does not actually emit anything
for it.
Syntax: .if absolute expression
.if
marks the beginning of a section of code which is only
considered part of the source program being assembled if the argument
(which must be an absolute expression) is non-zero. The end of
the conditional section of code must be marked by .endif
;
optionally, you may include code for the
alternative condition, flagged by .else
.
If you have several conditions to check, .elseif
may be used to avoid
nesting blocks if/else within each subsequent .else
block.
The following variants of .if
are also supported:
.ifdef symbol
Assembles the following section of code if the specified symbol has been defined. Note a symbol which has been referenced but not yet defined is considered to be undefined.
.ifc string1, string2
Assembles the following section of code if the two strings are the same. The strings may be optionally quoted with single quotes. If they are not quoted, the first string stops at the first comma, and the second string stops at the end of the line. Strings which contain whitespace should be quoted. The string comparison is case sensitive.
.ifeq absolute expression
Assembles the following section of code if the argument is zero.
.ifeqs string1, string2
Another form of .ifc
. The strings must be quoted using double quotes.
.ifge absolute expression
Assembles the following section of code if the argument is greater than or equal to zero.
.ifgt absolute expression
Assembles the following section of code if the argument is greater than zero.
.ifle absolute expression
Assembles the following section of code if the argument is less than or equal to zero.
.iflt absolute expression
Assembles the following section of code if the argument is less than zero.
.ifnc string1, string2.
Like .ifc
, but the sense of the test is reversed: this assembles the
following section of code if the two strings are not the same.
.ifndef symbol
.ifnotdef symbol
Assembles the following section of code if the specified symbol has not been defined. Both spelling variants are equivalent. Note a symbol which has been referenced but not yet defined is considered to be undefined.
.ifne absolute expression
Assembles the following section of code if the argument is not equal to zero
(in other words, this is equivalent to .if
).
.ifnes string1, string2
Like .ifeqs
, but the sense of the test is reversed: this assembles the
following section of code if the two strings are not the same.
Syntax: .include "file"
This directive provides a way to include supporting files at specified
points in your source program. The code from file is assembled as
if it followed the point of the .include
; when the end of the
included file is reached, assembly of the original file continues. You
can control the search paths used with the '-I' command-line option
(see Command-Line Options). Quotation marks are required
around file.
Syntax: .incbin "file"[, skip[, count]]
The incbin
directive includes file verbatim at the current
location. You can control the search paths used with the '-I' command-line
option (see Command-Line Options). Quotation marks are required
around file.
The skip argument skips a number of bytes from the start of the
file. The count argument indicates the maximum number of bytes to
read. Note that the data is not aligned in any way, so it is the user's
responsibility to make sure that proper alignment is provided both before and
after the incbin
directive.
Syntax: .int expressions
Expect zero or more expressions, of any section, separated by commas.
For each expression, emit a number that, at run time, is the value of that
expression. The byte order and bit size of the number depends on what kind
of target the assembly is for (big endian 32-bit for MC 68000;
be aware that in TIGCC, C language int
variables occupy 16 bits by default).
Syntax: .irp symbol[, value[, value][, ...]]
Evaluate a sequence of statements assigning different values to symbol.
The sequence of statements starts at the .irp
directive, and is
terminated by an .endr
directive. For each value, symbol is
set to value, and the sequence of statements is assembled. If no
value is listed, the sequence of statements is assembled once, with
symbol set to the null string. To refer to symbol within the
sequence of statements, use \symbol.
For example, assembling
.irp param,1,2,3 move.l %d\param,-(%sp) .endr
is equivalent to assembling
move %d1,-(%sp) move.l %d2,-(%sp) move.l %d3,-(%sp)
Syntax: .irpc symbol[, value]
Evaluate a sequence of statements assigning different values to symbol.
The sequence of statements starts at the .irpc
directive, and is
terminated by an .endr
directive. For each character in value,
symbol is set to the character, and the sequence of statements is
assembled. If no value is listed, the sequence of statements is
assembled once, with symbol set to the null string. To refer to
symbol within the sequence of statements, use \symbol.
For example, assembling
.irpc param,123 move.l %d\param,-(%sp) .endr
is equivalent to assembling
move.l %d1,-(%sp) move.l %d2,-(%sp) move.l %d3,-(%sp)
Syntax: .lcomm symbol, length
Reserve length (an absolute expression) bytes for a local common
denoted by symbol. The section and value of symbol are
those of the new local common. The addresses are allocated in the bss
section, so that at run-time the bytes start off zeroed. Symbol
is not declared global (see .global
), so is normally
not visible to ld
.
See also: .comm
as
accepts this directive, for compatibility with other
assemblers, but ignores it.
Syntax: .line line-number
Even though this is a directive associated with the a.out
or
b.out
object-code formats, as
still recognizes it
when producing COFF output, and treats .line
as though it
were the COFF .ln
if it is found outside a
.def
/.endef
pair.
Inside a .def
, .line
is, instead, one of the directives
used by compilers to generate auxiliary symbol information for
debugging.
 The next line has that logical line number. Therefore any other
statements on the current line (after a statement separator character) are
reported as on logical line number line-number-1.
See also: .line
Control (in conjunction with the .nolist
directive) whether or
not assembly listings are generated. These two directives maintain an
internal counter (which is zero initially). .list
increments the
counter, and .nolist
decrements it. Assembly listings are
generated whenever the counter is greater than zero.
By default, listings are disabled. When you enable them (with the
'-a' command line option; see Command-Line Options),
the initial value of the listing counter is one.
Syntax: .long expressions
On this target, .long
is the same as .int
.
Syntax: .macro macname [macargs...]
The commands .macro
and .endm
allow you to define macros that
generate assembly output. For example, this definition specifies a macro
sum
that puts a sequence of numbers into memory:
.macro sum from=0, to=5 .long \from .if \to-\from sum "(\from+1)",\to .endif .endm
With that (recursive) definition, SUM 0,5
is equivalent to this assembly input:
.long 0 .long 1 .long 2 .long 3 .long 4 .long 5
.macro macname
.macro macname macargs
Begin the definition of a macro called 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 =deflt
. For
example, these are all valid .macro
statements:
.macro comm
Begin the definition of a macro called comm
, which takes no
arguments.
.macro plus1 p, p1
.macro plus1 p p1
Either statement begins the definition of a macro called plus1
,
which takes two arguments; within the macro definition, write
\p
or \p1
to evaluate the arguments.
.macro reserve_str p1=0 p2
Begin the definition of a macro called 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
reserve_str a,b
(with \p1
evaluating to
a and \p2
evaluating to b), or as reserve_str
,b
(with \p1
evaluating as the default, in this case
0
, and \p2
evaluating to b).
When you call a macro, you can specify the argument values either by
position, or by keyword. For example, sum 9,17
is equivalent to
sum to=17, from=9
.
.endm
Mark the end of a macro definition.
.exitm
Exit early from the current macro definition.
\
as
maintains a counter of how many macros it has
executed in this pseudo-variable; you can copy that number to your
output with \@
, but only within a macro definition.
Syntax: .mri val
If val is non-zero, this tells as
to enter MRI mode. If
val is zero, this tells as
to exit MRI mode. This change
affects code assembled until the next .mri
directive, or until the end
of the file.
See also: MRI Mode
Control (in conjunction with the .list
directive) whether or
not assembly listings are generated. These two directives maintain an
internal counter (which is zero initially). .list
increments the
counter, and .nolist
decrements it. Assembly listings are
generated whenever the counter is greater than zero.
Syntax: .octa bignums
This directive expects zero or more bignums, separated by commas. For each
bignum, it emits a 16-byte integer.
The term "octa" comes from contexts in which a "word" is two bytes;
hence octa-word for 16 bytes.
Syntax: .org new-lc[, fill]
Advance the location counter of the current section to
new-lc. new-lc is either an absolute expression or an
expression with the same section as the current subsection. That is,
you can't use .org
to cross sections: if new-lc has the
wrong section, the .org
directive is ignored. To be compatible
with former assemblers, if the section of new-lc is absolute,
as
issues a warning, then pretends the section of new-lc
is the same as the current subsection.
.org
may only increase the location counter, or leave it
unchanged; you cannot use .org
to move the location counter
backwards.
Because as
tries to assemble programs in one pass, new-lc
may not be undefined. If you really detest this restriction we eagerly await
a chance to share your improved assembler.
Beware that the origin is relative to the start of the section, not
to the start of the subsection. This is compatible with other
people's assemblers.
When the location counter (of the current subsection) is advanced, the
intervening bytes are filled with fill which should be an
absolute expression. If the comma and fill are omitted,
fill defaults to zero.
Syntax: .p2align[wl] alignment[, [fill][, max]]
Pad the location counter (in the current subsection) to a particular
storage boundary. alignment (which must be absolute) is the
number of low-order zero bits the location counter must have after
advancement. For example .p2align 3
advances the location
counter until it a multiple of 8. If the location counter is already a
multiple of 8, no change is needed.
fill (also absolute) gives the fill value to be stored in the
padding bytes. It (and the comma) may be omitted. If it is omitted, the
padding bytes are normally zero. However, on some systems, if the section is
marked as containing code and the fill value is omitted, the space is filled
with no-op instructions (I didn't checked whether this is the case in TIGCC).
max is also absolute, and is also optional. If it is present,
it is the maximum number of bytes that should be skipped by this alignment
directive. If doing the alignment would require skipping more bytes than the
specified maximum, then the alignment is not done at all. You can omit the
fill value (the second argument) entirely by simply using two commas after the
required alignment; this can be useful if you want the alignment to be filled
with no-op instructions when appropriate.
The .p2alignw
and .p2alignl
directives are variants of the
.p2align
directive. The .p2alignw
directive treats the fill
pattern as a two byte word value. The .p2alignl
directives treats the
fill pattern as a four byte longword value. For example, .p2alignw
2,0x368d
will align to a multiple of 4. If it skips two bytes, they will be
filled in with the value 0x368d (the exact placement of the bytes depends upon
the endianness of the processor). If it skips 1 or 3 bytes, the fill value is
undefined.
See also: .balign
Syntax: .print string
as
will print string on the standard output during
assembly. You must put string in double quotes.
Syntax: .psize lines[, columns]
Use this directive to declare the number of lines - and, optionally, the
number of columns - to use for each page, when generating listings.
If you do not use .psize
, listings use a default line-count
of 60. You may omit the comma and columns specification; the
default width is 200 columns.
as
generates formfeeds whenever the specified number of
lines is exceeded (or whenever you explicitly request one, using
.eject
).
If you specify lines as 0
, no formfeeds are generated save
those explicitly specified with .eject
.
Syntax: .purgem macname
Undefine the macro macname, so that later uses of the string will not be
expanded.
See also: .macro
Syntax: .quad bignums
.quad
expects zero or more bignums, separated by commas. For
each bignum, it emits
an 8-byte integer. If the bignum won't fit in 8 bytes, it prints a
warning message; and just takes the lowest order 8 bytes of the bignum.
The term "quad" comes from contexts in which a "word" is two bytes;
hence quad-word for 8 bytes.
Syntax: .rept count
Repeat the sequence of lines between the .rept
directive and the next
.endr
directive count times.
For example, assembling
.rept 3 .long 0 .endr
is equivalent to assembling
.long 0 .long 0 .long 0
Syntax: .sbttl "subheading"
Use subheading as the title (third line, immediately after the
title line) when generating assembly listings.
This directive affects subsequent pages, as well as the current page if
it appears within ten lines of the top of a page.
Syntax: .scl class
Set the storage-class value for a symbol. This directive may only be
used inside a .def
/.endef
pair. Storage class may flag
whether a symbol is static or external, or it may record further
symbolic debugging information.
Syntax: .section name[, "flags"] or .section name[, subsegment]
Use the .section
directive to assemble the following code into a section
named name.
If the optional argument is quoted, it is taken as flags to use for the
section. Each flag is a single character. The following flags are recognized:
b
bss section (uninitialized data)
n
section is not loaded
w
writable section
d
data section
r
read-only section
x
executable section
m
mergeable section (TIGCC extension, symbols in the section are considered mergeable constants)
u
unaligned section (TIGCC extension, the contents of the section need not be aligned)
s
shared section (meaningful for PE targets, useless for TIGCC)
a
ignored (for compatibility with the ELF version)
If no flags are specified, the default flags depend upon the section name. If
the section name is not recognized, the default will be for the section to be
loaded and writable. Note the n
and w
flags remove attributes
from the section, rather than adding them, so if they are used on their own it
will be as if no flags had been specified at all.
If the optional argument to the .section
directive is not quoted, it is
taken as a subsegment number (see Sub-Sections).
Syntax: .set symbol, expression
Set the value of symbol to expression. This
changes symbol's value and type to conform to
expression. If symbol was flagged as external, it remains
flagged (see Symbol Attributes).
You may .set
a symbol many times in the same assembly.
If you .set
a global symbol, the value stored in the object
file is the last value stored into it.
Syntax: .short expressions
On this target, .short
is the same as .word
.
Syntax: .single flonums
This directive assembles zero or more flonums, separated by commas. It
has the same effect as .float
.
See also: .double
Syntax: .size expression
This directive, permitted only within .def
/.endef
pairs,
is used to set the size associated with a symbol.
Syntax: .sleb128 expressions
sleb128 stands for "signed little endian base 128." This is a
compact, variable length representation of numbers used by the DWARF
symbolic debugging format.
See also: .uleb128
.skip
is recognized on the 680x0 platform as a synonym for
.space
.
Syntax: .space size[, fill]
This directive emits size bytes, each of value fill. Both
size and fill are absolute expressions. If the comma
and fill are omitted, fill is assumed to be zero.
Syntax: .stabd type, other, desc
There are three directives that begin .stab
.
All emit symbols (see Symbols), for use by symbolic debuggers.
The symbols are not entered in the as
hash table: they
cannot be referenced elsewhere in the source file.
Up to five fields are required:
string
This is the symbol's name. It may contain any character except
\000
, so is more general than ordinary symbol names. Some
debuggers used to code arbitrarily complex structures into symbol names
using this field.
type
An absolute expression. The symbol's type is set to the low 8 bits of
this expression. Any bit pattern is permitted, but ld
and debuggers choke on silly bit patterns.
other
An absolute expression. The symbol's "other" attribute is set to the low 8 bits of this expression.
desc
An absolute expression. The symbol's descriptor is set to the low 16 bits of this expression.
value
An absolute expression which becomes the symbol's value.
If a warning is detected while reading a .stabd
, .stabn
,
or .stabs
statement, the symbol has probably already been created;
you get a half-formed symbol in your object file. This is
compatible with earlier assemblers!
If .stabd
is used, the "name" of the symbol generated
is not even an empty string.
It is a null pointer, for compatibility. Older assemblers used a
null pointer so they didn't waste space in object files with empty
strings.
The symbol's value is set to the location counter,
relocatably. When your program is linked, the value of this symbol
is the address of the location counter when the .stabd
was
assembled.
If .stabn
is used, the name of the symbol is set to the empty
string ""
.
If .stabs
is used, all five fields are required.
Syntax: .stabn type, other, desc, value
See .stabd.
Syntax: .stabs string, type, other, desc, value
See .stabd.
Syntax: .string "str"
Copy the characters in str to the object file. You may specify more than
one string to copy, separated by commas.
The assembler marks the end of each string with a 0 byte.
You can use any of the escape sequences described in Strings.
Syntax: .struct expression
Switch to the absolute section, and set the section offset to expression,
which must be an absolute expression. You might use this as follows:
.struct 0 field1: .struct field1 + 4 field2: .struct field2 + 4 field3:
This would define the symbol field1
to have the value 0, the symbol
field2
to have the value 4, and the symbol field3
to have the
value 8. Assembly would be left in the absolute section, and you would need to
use a .section
directive of some sort to change to some other section
before further assembly.
Syntax: .tag structname
This directive is generated by compilers to include auxiliary debugging
information in the symbol table. It is only permitted inside
.def
/.endef
pairs. Tags are used to link structure
definitions in the symbol table with instances of those structures.
Syntax: .text [subsection]
Tells as
to assemble the following statements onto the end of
the text subsection numbered subsection, which is an absolute
expression. If subsection is omitted, subsection number zero
is used.
See also: .data
Syntax: .title "heading"
Use heading as the title (second line, immediately after the
source file name and pagenumber) when generating assembly listings.
This directive affects subsequent pages, as well as the current page if
it appears within ten lines of the top of a page.
Syntax: .type int
This directive, permitted only within .def
/.endef
pairs,
records the integer int as the type attribute of a symbol table entry.
Syntax: .uleb128 expressions
uleb128 stands for "unsigned little endian base 128." This is a
compact, variable length representation of numbers used by the DWARF
symbolic debugging format.
See also: .sleb128
Syntax: .val addr
This directive, permitted only within .def
/.endef
pairs,
records the address addr as the value attribute of a symbol table
entry.
Syntax: .vtable_entry table, offset
This directive finds or creates a symbol table and creates a
VTABLE_ENTRY
relocation for it with an addend of offset.
Syntax: .word expressions
This directive expects zero or more expressions, of any section,
separated by commas.
For each expression, as
emits a 16-bit number for this target.
If you have contributed to as
and your name isn't listed here,
it is not meant as a slight. We just don't know about it. Send mail to the
maintainer, and we'll correct the situation. Currently
the maintainer is Ken Raeburn (email address raeburn@cygnus.com).
(Note: Since this is a modified version of the manual, please check the original
version as well before sending a mail.)
Dean Elsner wrote the original GNU assembler for the VAX. Any
more details?
Jay Fenlason maintained GAS for a while, adding support for GDB-specific debug
information and the 68k series machines, most of the preprocessing pass, and
extensive changes in messages.c
, input-file.c
, write.c
.
K. Richard Pixley maintained GAS for a while, adding various enhancements and
many bug fixes, including merging support for several processors, breaking GAS
up to handle multiple object file format back ends (including heavy rewrite,
testing, an integration of the coff and b.out back ends), adding configuration
including heavy testing and verification of cross assemblers and file splits
and renaming, converted GAS to strictly ANSI C including full prototypes, added
support for m680[34]0 and cpu32, did considerable work on i960 including a COFF
port (including considerable amounts of reverse engineering), a SPARC opcode
file rewrite, DECstation, rs6000, and hp300hpux host ports, updated "know"
assertions and made them work, much other reorganization, cleanup, and lint.
Ken Raeburn wrote the high-level BFD interface code to replace most of the code
in format-specific I/O modules.
The original VMS support was contributed by David L. Kashtan. Eric Youngdale
has done much work with it since.
The Intel 80386 machine description was written by Eliot Dresselhaus.
Minh Tran-Le at IntelliCorp contributed some AIX 386 support.
The Motorola 88k machine description was contributed by Devon Bowen of Buffalo
University and Torbjorn Granlund of the Swedish Institute of Computer Science.
Keith Knowles at the Open Software Foundation wrote the original MIPS back end
(tc-mips.c
, tc-mips.h
), and contributed Rose format support
(which hasn't been merged in yet). Ralph Campbell worked with the MIPS code to
support a.out format.
Support for the Zilog Z8k and Renesas H8/300 and H8/500 processors (tc-z8k,
tc-h8300, tc-h8500), and IEEE 695 object file format (obj-ieee), was written by
Steve Chamberlain of Cygnus Support. Steve also modified the COFF back end to
use BFD for some low-level operations, for use with the H8/300 and AMD 29k
targets.
John Gilmore built the AMD 29000 support, added .include
support, and
simplified the configuration of which versions accept which directives. He
updated the 68k machine description so that Motorola's opcodes always produced
fixed-size instructions (e.g., jsr
), while synthetic instructions
remained shrinkable (jbsr
). John fixed many bugs, including true tested
cross-compilation support, and one bug in relaxation that took a week and
required the proverbial one-bit fix.
Ian Lance Taylor of Cygnus Support merged the Motorola and MIT syntax for the
68k, completed support for some COFF targets (68k, i386 SVR3, and SCO Unix),
added support for MIPS ECOFF and ELF targets, wrote the initial RS/6000 and
PowerPC assembler, and made a few other minor patches.
Steve Chamberlain made as
able to generate listings.
Hewlett-Packard contributed support for the HP9000/300.
Jeff Law wrote GAS and BFD support for the native HPPA object format (SOM)
along with a fairly extensive HPPA testsuite (for both SOM and ELF object
formats). This work was supported by both the Center for Software Science at
the University of Utah and Cygnus Support.
Support for ELF format files has been worked on by Mark Eichin of Cygnus
Support (original, incomplete implementation for SPARC), Pete Hoogenboom and
Jeff Law at the University of Utah (HPPA mainly), Michael Meissner of the Open
Software Foundation (i386 mainly), and Ken Raeburn of Cygnus Support (sparc,
and some initial 64-bit support).
Linas Vepstas added GAS support for the ESA/390 "IBM 370" architecture.
Richard Henderson rewrote the Alpha assembler. Klaus Kaempf wrote GAS and BFD
support for openVMS/Alpha.
Timothy Wall, Michael Hayes, and Greg Smart contributed to the various tic*
flavors.
David Heine, Sterling Augustine, Bob Wilson and John Ruttenberg from Tensilica,
Inc. added support for Xtensa processors.
Several engineers at Cygnus Support have also provided many small bug fixes and
configuration enhancements.
Many others have contributed large or small bugfixes and enhancements. If
you have contributed significant work and are not mentioned on this list, and
want to be, let us know. Some of the history has been lost; we are not
intentionally leaving anyone out.
Original Version: Using as
Published by the Free Software Foundation
59 Temple Place - Suite 330
Boston, MA 02111-1307 USA
Copyright © 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999,
2000, 2001, 2002, 2003 Free Software Foundation, Inc.
Modifications for TIGCC: The GNU Assembler
Published by the TIGCC Team
Copyright © 2000, 2001, 2002, 2003 Zeljko Juric, Sebastian Reichelt, Kevin Kofler