This file is a user guide to the GNU assembler AS
version
2.14.
This version of the file describes AS
configured to generate
code for TARGET architectures.
This document is distributed under the terms of the GNU Free Documentation License. A copy of the license is included in the section entitled "GNU Free Documentation License".
Here is a brief summary of how to invoke AS
. For details,
see Command-Line Options.
AS [-a[cdhlns][=file]] [-D] [--defsym sym=val] [-f] [--gstabs] [--gdwarf2] [--help] [-I dir] [-J] [-K] [-L] [--listing-lhs-width=NUM] [--listing-lhs-width2=NUM] [--listing-rhs-width=NUM] [--listing-cont-lines=NUM] [--keep-locals] [-o objfile] [-R] [--statistics] [-v] [-version] [--version] [-W] [--warn] [--fatal-warnings] [-w] [-x] [-Z] [--target-help] [target-options] [--|files ...]
-a[cdhlmns]
-ac
-ad
-ah
-al
-am
-an
-as
=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
.
-D
--defsym sym=value
0x
indicates a hexadecimal value, and a leading 0
indicates an octal value.
-f
--gstabs
--gdwarf2
--help
--target-help
-I dir
.include
directives.
-J
-K
-L
--keep-locals
L
, but different systems have different local
label prefixes.
--listing-lhs-width=number
--listing-lhs-width2=number
--listing-rhs-width=number
--listing-cont-lines=number
-o objfile
AS
objfile.
-R
--statistics
--strip-local-absolute
-v
-version
as
version.
--version
as
version and exit.
-W
--no-warn
--fatal-warnings
--warn
-w
-x
-Z
-- | files ...
This manual is intended to describe what you need to know to use
GNU AS
. We cover the syntax expected in source files, including
notation for symbols, constants, and expressions; the directives that
AS
understands; and of course how to invoke AS
.
We also cover special features in the TARGET
configuration of AS
, including assembler directives.
On the other hand, this manual is not intended as an introduction to programming in assembly language--let alone programming in general! In a similar vein, we make no attempt to introduce the machine architecture; we do not describe the instruction set, standard mnemonics, registers or addressing modes that are standard to a particular architecture.
GNU as
is really a family of assemblers.
This manual describes AS
, a member of that family which is
configured for the TARGET architectures.
If you use (or have used) the GNU assembler on one architecture, you
should find a fairly similar environment when you use it on another
architecture. Each version has much in common with the others,
including object file formats, most assembler directives (often called
pseudo-ops) and assembler syntax.
AS
is primarily intended to assemble the output of the
GNU C compiler GCC
for use by the linker
LD
. Nevertheless, we've tried to make AS
assemble correctly everything that other assemblers for the same
machine would assemble.
Unlike older assemblers, AS
is designed to assemble a source
program in one pass of the source file. This has a subtle impact on the
.org directive (see .org
).
The GNU assembler can be configured to produce several alternative
object file formats. For the most part, this does not affect how you
write assembly language programs; but directives for debugging symbols
are typically different in different file formats. See Symbol Attributes.
For the TARGET target, AS
is configured to produce
OBJ-NAME format object files.
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
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 <ctl-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 TextThe 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.
This chapter describes command-line options available in all versions of the GNU assembler; see Machine Dependencies, for options specific to the TARGET target.
If you are invoking AS
via the GNU C compiler,
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:
gcc -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.)
-a[cdhlns]
These options 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 (eg 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.
-D
This option has no effect whatsoever, but it is accepted to make it more
likely that scripts written for other assemblers also work with
AS
.
-f
-f
should only be used when assembling programs written by a
(trusted) compiler. -f
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.
.include
Search Path: -I
pathUse this option to add a path to the list of directories
AS
searches for files specified in .include
directives (see .include
). You may use -I
as
many times as necessary to include a variety of paths. The current
working directory is always searched first; after that, AS
searches any -I
directories in the same order as they were
specified (left to right) on the command line.
-K
On the TARGET family, this option is allowed, but has no effect. It is
permitted for compatibility with the GNU assembler on other platforms,
where it can be used to warn when the assembler alters the machine code
generated for .word
directives in difference tables. The TARGET
family does not have the addressing limitations that sometimes lead to this
alteration on other platforms.
-L
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.
This 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.
--listing
The listing feature of the assembler can be enabled via the command line switch
-a
(see a). 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 List see Title see Sbttl
see Psize see Eject) and also by the following switches:
--listing-lhs-width=number
--listing-lhs-width2=number
--listing-lhs-width
. If neither
switch is used the default is to one.
--listing-rhs-width=number
--listing-cont-lines=number
-M
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
or the ASM960
(depending upon the
configured target) 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:
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.
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 are not supported.
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
.
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.
--MD
AS
can generate a dependency file for the file it creates. 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.
-o
There is always one object file output when you run AS
. By
default it has the name
a.out
.
You use this 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.
-R
-R
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 future, -R
may work this way.
--statistics
Use --statistics
to display two statistics about the resources used by
AS
: the maximum amount of space allocated during the assembly
(in bytes), and the total execution time taken for the assembly (in CPU
seconds).
--traditional-format
For some targets, the output of AS
is different in some ways
from the output of some existing assembler. This 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.
-v
You can find out what version of as is running by including the
option -v
(which you can also spell as -version
) on the
command line.
-W
, --warn
, --no-warn
, --fatal-warnings
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.
-Z
After an error message, AS
normally produces no output. If for
some reason you are interested in object file output even after
AS
gives an error message on your program, use the -Z
option. If there are any errors, AS
continues anyways, and
writes an object file after a final warning message of the form n
errors, m warnings, generating bad object file.
This chapter describes the machine-independent syntax allowed in a
source file. AS
syntax is similar to what many other
assemblers use; it is inspired by the BSD 4.2
assembler.
The AS
internal preprocessor:
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 Overall Options.
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 see Machine Dependencies.
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\ 95028841971.693993751E-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.
\008
has the value 010, and \009
the value 011.
x
hex-digits...
x
works.
\
character.
"
character. Needed in strings to represent
this character, because an unescaped "
would end the string.
\
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 (0123456789
).
A hexadecimal integer is 0x
or 0X
followed by one or
more hexadecimal digits chosen from 0123456789abcdefABCDEF
.
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.
A flonum is written by writing (in order)
0
.
AS
the rest of the number is a flonum.
+
or -
.
.
followed by zero
or more decimal digits.
E
or e
.
+
or -
.
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
.
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.
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:
(address) - (start-address of section)?
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.
LD
must
not change when relocating. In this sense we speak of absolute
addresses being "unrelocatable": they do not change during relocation.
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.
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.
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; see
.lcomm
.
The .comm
pseudo-op may be used to declare a common symbol, which is
another form of uninitialized symbol; see See .comm
.
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.
Warning: 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. See .set
.
Symbol names begin with a letter or with one of ._
. On most
machines, you can also use $
in symbol names; exceptions are
noted in Machine Dependencies. That character may be followed by any
string of digits, letters, dollar signs (unless otherwise noted in
Machine Dependencies), 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
too. So that 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: branch 1f 2: branch 1b 1: branch 2f 2: branch 1b
Which is the equivalent of:
label_1: branch label_3 label_2: branch label_1 label_3: branch label_4 label_4: branch 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
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 then AS
retains these symbols in the
object file. If you also instruct LD
to retain these symbols,
you may use them in debugging.
number
55:
then the number is 55
.
C-B
\002
(control-B).
ordinal number
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.
So for example, the first 1:
is named L1C-B1
, the 44th
3:
is named L3C-B44
.
AS
also supports an even more local form of local labels called
dollar labels. These labels go out of scope (ie 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. eg 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. Thus the 5th defintion of 6$
is named L6C-A5
.
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.
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.
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.
-
~
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.
*
/
/
%
<
<<
<<
.
>
>>
>>
.
|
&
^
!
+
-
==
<>
<
>
>=
<=
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.
&&
||
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 but a false results does still return 0. 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 discusses directives that are available regardless of the target machine configuration for the GNU assembler.
.abort
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 tells AS
to
quit also. One day .abort
will not be supported.
.align abs-expr, abs-expr, abs-expr
Pad the location counter (in the current subsection) to a particular storage boundary. The first expression (which must be absolute) is the alignment required, as described below.
The second expression (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.
The third expression 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 GAS must emulate.
GAS also provides .balign
and .p2align
directives,
described later, which have a consistent behavior across all
architectures (but are specific to GAS).
.ascii "string"
....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.
.asciz "string"
....asciz
is just like .ascii
, but each string is followed by
a zero byte. The "z" in .asciz
stands for "zero".
.balign[wl] abs-expr, abs-expr, abs-expr
Pad the location counter (in the current subsection) to a particular
storage boundary. The first expression (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.
The second expression (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.
The third expression 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.
.byte expressions
.byte
expects zero or more expressions, separated by commas.
Each expression is assembled into the next byte.
.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.
.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.
.double flonums
.double
expects zero or more flonums, separated by commas. It
assembles floating point numbers.
.eject
Force a page break at this point, when generating assembly listings.
.else
.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.
.elseif
.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.
.end
.end
marks the end of the assembly file. AS
does not
process anything in the file past the .end
directive.
.endfunc
.endfunc
marks the end of a function specified with .func
.
.endif
.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
.
.equ symbol, expression
This directive sets the value of symbol to expression.
It is synonymous with .set
; see .set
.
.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
.err
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.
.exitm
Exit early from the current macro definition. See Macro.
.extern
.extern
is accepted in the source program--for compatibility
with other assemblers--but it is ignored. AS
treats
all undefined symbols as external.
.fail expression
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.
.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-""
. This
statement may go away in future: it is only recognized to be compatible with
old AS
programs.
.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.
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.
.float flonums
This directive assembles zero or more flonums, separated by commas. It
has the same effect as .single
.
.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 char
is used.
leading char
is usually _
or nothing, depending on the target.
All functions are currently defined to have void
return type.
The function must be terminated with .endfunc
.
.global symbol
, .globl 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.
.hword expressions
This expects zero or more expressions, and emits a 16 bit number for each.
.ident
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.
.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
(see .endif
); optionally, you may include code for the
alternative condition, flagged by .else
(see .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
.ifc string1,string2
.ifeq absolute expression
.ifeqs string1,string2
.ifc
. The strings must be quoted using double quotes.
.ifge absolute expression
.ifgt absolute expression
.ifle absolute expression
.iflt absolute expression
.ifnc string1,string2.
.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
.ifne absolute expression
.if
).
.ifnes string1,string2
.ifeqs
, but the sense of the test is reversed: this assembles the
following section of code if the two strings are not the same.
.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.
.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.
.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.
.irp symbol,values
...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 d\param,sp@- .endr
is equivalent to assembling
move d1,sp@- move d2,sp@- move d3,sp@-
.irpc symbol,values
...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 d\param,sp@- .endr
is equivalent to assembling
move d1,sp@- move d2,sp@- move d3,sp@-
.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
.
.lflags
AS
accepts this directive, for compatibility with other
assemblers, but ignores it.
.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.
.linkonce [type]
Mark the current section so that the linker only includes a single copy of it.
This may be used to include the same section in several different object files,
but ensure that the linker will only include it once in the final output file.
The .linkonce
pseudo-op must be used for each instance of the section.
Duplicate sections are detected based on the section name, so it should be
unique.
This directive is only supported by a few object file formats; as of this writing, the only object file format which supports it is the Portable Executable format used on Windows NT.
The type argument is optional. If specified, it must be one of the following strings. For example:
.linkonce same_sizeNot all types may be supported on all object file formats.
discard
one_only
same_size
same_contents
.ln line-number
.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 MRI mode.
.list
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.
.long expressions
.long
is the same as .int
, see .int
.
.macro
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 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 ...
=deflt
. For
example, these are all valid .macro
statements:
.macro comm
comm
, which takes no
arguments.
.macro plus1 p, p1
.macro plus1 p p1
plus1
,
which takes two arguments; within the macro definition, write
\p
or \p1
to evaluate the arguments.
.macro reserve_str p1=0 p2
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
.exitm
\@
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.
.nolist
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.
.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.
.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.
.p2align[wl] abs-expr, abs-expr, abs-expr
Pad the location counter (in the current subsection) to a particular
storage boundary. The first expression (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.
The second expression (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.
The third expression 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.
.print string
AS
will print string on the standard output during
assembly. You must put string in double quotes.
.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
.
.purgem name
Undefine the macro name, so that later uses of the string will not be expanded. See Macro.
.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.
.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
.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.
.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.
.short expressions
.single flonums
This directive assembles zero or more flonums, separated by commas. It
has the same effect as .float
.
.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 .uleb128
.
.skip 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. This is the same as
.space
.
.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. This is the same
as .skip
.
.stabd, .stabn, .stabs
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:
\000
, so is more general than ordinary symbol names. Some
debuggers used to code arbitrarily complex structures into symbol names
using this field.
LD
and debuggers choke on silly bit patterns.
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!
.stabd type , other , desc
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.
.stabn type , other , desc , value
""
.
.stabs string , type , other , desc , value
.string
"str"Copy the characters in str to the object file. You may specify more than one string to copy, separated by commas. Unless otherwise specified for a particular machine, the assembler marks the end of each string with a 0 byte. You can use any of the escape sequences described in Strings.
.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.
.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.
.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.
.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 .sleb128
.
.word expressions
This directive expects zero or more expressions, of any section, separated by commas.
One day these directives won't work. They are included for compatibility with older assemblers.
Your bug reports play an essential role in making AS
reliable.
Reporting a bug may help you by bringing a solution to your problem, or it may
not. But in any case the principal function of a bug report is to help the
entire community by making the next version of AS
work better.
Bug reports are your contribution to the maintenance of AS
.
In order for a bug report to serve its purpose, you must include the information that enables us to fix the bug.
If you are not sure whether you have found a bug, here are some guidelines:
AS
bug. Reliable assemblers never crash.
AS
produces an error message for valid input, that is a bug.
AS
does not produce an error message for invalid input, that
is a bug. However, you should note that your idea of "invalid input" might
be our idea of "an extension" or "support for traditional practice".
AS
are welcome in any case.
A number of companies and individuals offer support for GNU products. If
you obtained AS
from a support organization, we recommend you
contact that organization first.
You can find contact information for many support companies and
individuals in the file etc/SERVICE
in the GNU Emacs
distribution.
In any event, we also recommend that you send bug reports for AS
to bug-binutils@gnu.org
.
The fundamental principle of reporting bugs usefully is this: report all the facts. If you are not sure whether to state a fact or leave it out, state it!
Often people omit facts because they think they know what causes the problem and assume that some details do not matter. Thus, you might assume that the name of a symbol you use in an example does not matter. Well, probably it does not, but one cannot be sure. Perhaps the bug is a stray memory reference which happens to fetch from the location where that name is stored in memory; perhaps, if the name were different, the contents of that location would fool the assembler into doing the right thing despite the bug. Play it safe and give a specific, complete example. That is the easiest thing for you to do, and the most helpful.
Keep in mind that the purpose of a bug report is to enable us to fix the bug if it is new to us. Therefore, always write your bug reports on the assumption that the bug has not been reported previously.
Sometimes people give a few sketchy facts and ask, "Does this ring a bell?" This cannot help us fix a bug, so it is basically useless. We respond by asking for enough details to enable us to investigate. You might as well expedite matters by sending them to begin with.
To enable us to fix the bug, you should include all these things:
AS
. AS
announces it if you start
it with the --version
argument.
Without this, we will not know whether there is any point in looking for
the bug in the current version of AS
.
AS
source.
AS
--e.g.
"gcc-2.7
".
If we were to try to guess the arguments, we would probably guess wrong and then we might not encounter the bug.
-S
option. If you are using GCC
, use
the options -v --save-temps
; this will save the assembler source in a
file with an extension of .s
, and also show you exactly how
AS
is being run.
Of course, if the bug is that AS
gets a fatal signal, then we
will certainly notice it. But if the bug is incorrect output, we might not
notice unless it is glaringly wrong. You might as well not give us a chance to
make a mistake.
Even if the problem you experience is a fatal signal, you should still say so
explicitly. Suppose something strange is going on, such as, your copy of
AS
is out of synch, or you have encountered a bug in the C
library on your system. (This has happened!) Your copy might crash and ours
would not. If you told us to expect a crash, then when ours fails to crash, we
would know that the bug was not happening for us. If you had not told us to
expect a crash, then we would not be able to draw any conclusion from our
observations.
AS
source, send us context
diffs, as generated by diff
with the -u
, -c
, or -p
option. Always send diffs from the old file to the new file. If you even
discuss something in the AS
source, refer to it by context, not
by line number.
The line numbers in our development sources will not match those in your sources. Your line numbers would convey no useful information to us.
Here are some things that are not necessary:
Often people who encounter a bug spend a lot of time investigating which changes to the input file will make the bug go away and which changes will not affect it.
This is often time consuming and not very useful, because the way we will find the bug is by running a single example under the debugger with breakpoints, not by pure deduction from a series of examples. We recommend that you save your time for something else.
Of course, if you can find a simpler example to report instead of the original one, that is a convenience for us. Errors in the output will be easier to spot, running under the debugger will take less time, and so on.
However, simplification is not vital; if you do not want to do this, report the bug anyway and send us the entire test case you used.
A patch for the bug does help us if it is a good one. But do not omit the necessary information, such as the test case, on the assumption that a patch is all we need. We might see problems with your patch and decide to fix the problem another way, or we might not understand it at all.
Sometimes with a program as complicated as AS
it is very hard to
construct an example that will make the program follow a certain path through
the code. If you do not send us the example, we will not be able to construct
one, so we will not be able to verify that the bug is fixed.
And if we cannot understand what bug you are trying to fix, or why your patch should be an improvement, we will not install it. A test case will help us to understand.
Such guesses are usually wrong. Even we cannot guess right about such things without first using the debugger to find the facts.
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
).
Dean Elsner wrote the original GNU assembler for the VAX.1
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.
Version 1.1, March 2000
Copyright (C) 2000, Free Software Foundation, Inc. 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
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#
: Comments
#APP
: Preprocessing
#NO_APP
: Preprocessing
--
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--fatal-warnings
: W
--listing-cont-lines
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--listing-lhs-width
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balignl
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double
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equ
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err
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exitm
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fail
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-f
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\f
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func
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global
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\xd...
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ifc
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ifge
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ifle
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ifndef
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ifne
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ifnes
directive: If
ifnotdef
directive: If
incbin
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include
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include
directive search path: I
int
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irp
directive: Irp
irpc
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:
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lcomm
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ld
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line
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#
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macro
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\n
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org
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path
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.incbin "file"[,skip[,count]]
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...
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...
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