The Free Software Foundation Inc. thanks The Nice Computer
Company of Australia for loaning Dean Elsner to write the
first (Vax) version of as for Project GNU.
The proprietors, management and staff of TNCCA thank FSF for
distracting the boss while they got some work
done.
Copyright (C) 1991, 92, 93, 94, 95, 1996 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.
Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.
Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions.
This manual is a user guide to the GNU assembler as.
Here is a brief summary of how to invoke as. For details,
see section Command-Line Options.
as [ -a[dhlns][=file] ] [ -D ] [ --defsym sym=val ] [ -f ] [ --help ] [ -I dir ] [ -J ] [ -K ] [ -L ] [ -o objfile ] [ -R ] [ --statistics ] [ -v ] [ -version ] [ --version ] [ -W ] [ -w ] [ -x ] [ -Z ] [ -Av6 | -Av7 | -Av8 | -Asparclite | -Av9 | -Av9a ] [ -xarch=v8plus | -xarch=v8plusa ] [ -bump ] [ -ACA | -ACA_A | -ACB | -ACC | -AKA | -AKB | -AKC | -AMC ] [ -b ] [ -no-relax ] [ -nocpp ] [ -EL ] [ -EB ] [ -G num ] [ -mcpu=CPU ] [ -mips1 ] [ -mips2 ] [ -mips3 ] [ -m4650 ] [ -no-m4650 ] [ --trap ] [ --break ] [ --emulation=name ] [ -mpwrx ] [ -mpwr2 ] [ -mpwr ] [ -m601 ] [ -mppc | -mppc32 | -m403 | -m603 | -m604 ] [ -mppc64 | -m620 ] [ -mcom ] [ -many ] [ -mregnames ] [ -mno-regnames ] [ -mrelocatable ] [ -mrelocatable-lib ] [ -memb ] [ -mlittle | -mlittle-endian ] [ -mbig | -mbig-endian ] [ -- | files ... ]
-a[dhlns]
-ad
-ah
-al
-an
-as
=file
-D
--defsym sym=value
-f
--help
-I dir
.include directives.
-J
-K
-L
-o objfile
as objfile.
-R
--statistics
-v
-version
as version.
--version
as version and exit.
-W
-w
-x
-Z
-- | files ...
The following options are available when as is configured for the Intel 80960 processor.
-ACA | -ACA_A | -ACB | -ACC | -AKA | -AKB | -AKC | -AMC
-b
-no-relax
The following options are available when as is configured
for the SPARC architecture:
-Av6 | -Av7 | -Av8 | -Asparclite | -Av9 | -Av9a
-xarch=v8plus | -xarch=v8plusa
-bump
The following options are available when as is configured for a MIPS processor.
-G num
gp register. It is only accepted for targets that
use ECOFF format, such as a DECstation running Ultrix. The default value is 8.
-EB
-EL
-mips1
-mips2
-mips3
-m4650
-no-m4650
-mcpu=CPU
gcc.
--emulation=name
as to emulated as configured
for some other target, in all respects, including output format (choosing
between ELF and ECOFF only), handling of pseudo-opcodes which may generate
debugging information or store symbol table information, and default
endianness. The available configuration names are: `mipsecoff',
`mipself', `mipslecoff', `mipsbecoff', `mipslelf',
`mipsbelf'. The first two do not alter the default endianness from that
of the primary target for which the assembler was configured; the others change
the default to little- or big-endian as indicated by the `b' or `l'
in the name. Using `-EB' or `-EL' will override the endianness
selection in any case.
This option is currently supported only when the primary target
as is configured for is a MIPS ELF or ECOFF target.
Furthermore, the primary target or others specified with
`--enable-targets=...' at configuration time must include support for
the other format, if both are to be available. For example, the Irix 5
configuration includes support for both.
Eventually, this option will support more configurations, with more
fine-grained control over the assembler's behavior, and will be supported for
more processors.
-nocpp
as ignores this option. It is accepted for compatibility with
the native tools.
--trap
--no-trap
--break
--no-break
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.
This manual also describes some of the machine-dependent features of various flavors of the assembler.
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. You may want to consult the manufacturer's machine architecture manual for this information.
GNU as is really a family of assemblers.
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 section .org new-lc , fill).
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 section Symbol Attributes.
On the machine specific, as can be configured to produce either
b.out or COFF format object files.
On the machine specific, as can be configured to produce either
SOM or ELF 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 section 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.
See section .app-file string.
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, or
b.out when as is configured for the Intel 80960.
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 section .app-file string) it is used for the filename,
otherwise the name of the current input file is used. If a logical line
number was given
(see section .line line-number)
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.
This chapter describes command-line options available in all versions of the GNU assembler; see section Machine Dependent Features, for options specific to particular machine architectures.
If you are invoking as via the GNU C compiler (version 2), 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
emits a listing to standard output with high-level and assembly source.
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[dhlns]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 `-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'.
-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 section Preprocessing.
Warning: if you use `-f' when the files actually need to be preprocessed (if they contain comments, for example),
asdoes not work correctly.
.include search path: -I path
Use this option to add a path to the list of directories
as searches for files specified in .include
directives (see section .include "file"). 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
as sometimes alters the code emitted for directives of the form
`.word sym1-sym2'; see section .word expressions.
You can use the `-K' option if you want a warning issued when this
is done.
-L
Labels beginning with `L' (upper case only) are called local
labels. See section 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. On the HPPA local labels begin with `L$'.
-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. 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:
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.
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.
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.
-o
There is always one object file output when you run as. By
default it has the name
`a.out' (or `b.out', for Intel 960 targets only).
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 section 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.
When as is configured for COFF output,
this option is only useful if you use sections named `.text' and
`.data'.
-R is not supported for any of the HPPA targets. Using
-R generates a warning from as.
--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).
-vYou 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
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 this option, no warnings are issued. This option 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.
-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.
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 section .include "file"). You can use the GNU C compiler driver
to get other "CPP" style preprocessing, by giving the input file a
`.S' suffix. See section `Options Controlling the Kind of Output' in 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 section 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 i960; `!' on the SPARC; `;' for the HPPA; `!' for the Z8000; see section Machine Dependent Features.
On some machines there are two different line comment characters. One character only begins a comment if it is the first non-whitespace character on a line, while the other always begins a comment.
To be compatible with past assemblers, lines that begin with `#' have a special interpretation. Following the `#' should be an absolute expression (see section Expressions): the logical line number of the next line. Then a string (see section 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
`_.$'.
On most machines, you can also use $ in symbol names; exceptions
are noted in section Machine Dependent Features.
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 section Symbols.
A statement ends at a newline character (`\n') or an exclamation point (`!'). The newline or exclamation point is considered part of the preceding statement. Newlines and exclamation points within character constants are an exception: they do not end statements. A statement ends at a newline character (`\n') or line separator character. (The line separator is usually `;', unless this conflicts with the comment character; see section Machine Dependent Features.) The newline or separator character is considered part of the preceding statement. Newlines and separators 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.
You may write a statement on more than one line if you put a
backslash (\) immediately in front of any newlines within the
statement. When as reads a backslashed newline both
characters are ignored. You can even put backslashed newlines in
the middle of symbol names without changing the meaning of your
source program.
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.
Different versions of as for different computers
recognize different instructions. In fact, the same symbol may
represent a different instruction in a different computer's assembly
language.
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 section Labels.
For HPPA targets, labels need not be immediately followed by a colon, but the definition of a label must begin in column zero. This also implies that only one label may be defined on each line.
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-digit hex-digit
x works.
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
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 section Prefix Operator).
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)
as the rest of the number is a flonum.
e is recommended. Case is not important.
On the H8/300, H8/500,
Hitachi SH,
and AMD 29K architectures, the letter must be
one of the letters `DFPRSX' (in upper or lower case).
On the Intel 960 architecture, the letter must be
one of the letters `DFT' (in upper or lower case).
On the HPPA architecture, the letter must be `E' (upper case only).
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.
When it generates COFF output,
as can also generate whatever other named sections you specify
using the `.section' directive (see section .section name, subsection).
If you do not use any directives that place output in the `.text'
or `.data' sections, these sections still exist, but are empty.
When as generates SOM or ELF output for the HPPA,
as can also generate whatever other named sections you
specify using the `.space' and `.subspace' directives. See
HP9000 Series 800 Assembly Language Reference Manual
(HP 92432-90001) for details on the `.space' and `.subspace'
assembler directives.
Additionally, as uses different names for the standard
text, data, and bss sections when generating SOM output. Program text
is placed into the `$CODE$' section, data into `$DATA$', and
BSS into `$BSS$'.
Within the object file, the text section starts at address 0, the
data section follows, and the bss section follows the data section.
When generating either SOM or ELF output files on the HPPA, the text
section starts at address 0, the data section at address
0x4000000, 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.
(For some object formats, such as SOM for the HPPA, some expressions are
symbol-relative instead.)
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.
as and ld treat them as
separate but equal sections. Anything you can say of one section is
true 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. The example uses the traditional section names `.text' and `.data'. Memory addresses are on the horizontal axis.
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.
Assembled bytes
conventionally
fall into two sections: text and data.
You may have separate groups of
data in named sections
text or data
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.
Each subsection is zero-padded up to a multiple of four bytes.
(Subsections may be padded a different amount on different flavors
of as.)
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.
When generating COFF output, you
can also use an extra subsection
argument with arbitrary named sections: `.section name,
expression'.
Expression should be an absolute expression.
(See section 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.
Addresses in the bss section are allocated with special directives; you
may not assemble anything directly into the bss section. Hence there
are no bss subsections. See section .comm symbol , length ,
see section .lcomm symbol , length.
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:
asdoes 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.
On the HPPA, the usual form for a label need not be immediately followed by a
colon, but instead must start in column zero. Only one label may be defined on
a single line. To work around this, the HPPA version of as also
provides a special directive .label for defining labels more flexibly.
A symbol can be given an arbitrary value by writing a symbol, followed
by an equals sign `=', followed by an expression
(see section Expressions). This is equivalent to using the .set
directive. See section .set symbol, expression.
Symbol names begin with a letter or with one of `._'. On most
machines, you can also use $ in symbol names; exceptions are
noted in section Machine Dependent Features. That character may be followed by any
string of digits, letters, dollar signs (unless otherwise noted in
section Machine Dependent Features), 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. There are ten local symbol names, which are re-used throughout the program. You may refer to them using the names `0' `1' ... `9'. To define a local symbol, write a label of the form `N:' (where N represents any digit). To refer to the most recent previous definition of that symbol write `Nb', using the same digit as when you defined the label. To refer to the next definition of a local label, write `Nf'---where N gives you a choice of 10 forward references. The `b' stands for "backwards" and the `f' stands for "forwards".
Local symbols are not emitted by the current GNU C compiler.
There is no restriction on how you can use these labels, but remember that at any point in the assembly you can refer to at most 10 prior local labels and to at most 10 forward local labels.
Local symbol names are only a notation 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 have these parts:
L
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.
digit
^A
ordinal number
For instance, the first 1: is named L1^A1, the 44th
3: is named L3^A44.
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.
a.out
This is an arbitrary 16-bit value. You may establish a symbol's
descriptor value by using a .desc statement
(see section .desc symbol, abs-expression). A descriptor value means nothing to
as.
This is an arbitrary 8-bit value. It means nothing to as.
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.
The SOM format for the HPPA supports a multitude of symbol attributes set with
the .EXPORT and .IMPORT directives.
The attributes are described in HP9000 Series 800 Assembly
Language Reference Manual (HP 92432-90001) under the IMPORT and
EXPORT assembler directive documentation.
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.
*
/
%
<
<<
>
>>
|
&
^
!
+
-
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. Some machine configurations provide additional directives. See section Machine Dependent Features.
.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.
.ABORT
When producing COFF output, as accepts this directive as a
synonym for `.abort'.
When producing b.out output, as accepts this directive,
but ignores it.
.align abs-expr , abs-exprPad the location counter (in the current subsection) to a particular storage boundary. The first expression, which must be absolute, specifies the alignment, according to one of two conventions. Which convention is used depends on the machine architecture:
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).
.app-file string
.app-file
(which may also be spelled `.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 is permitted,
you must give the quotes--"". This statement may go away in
future: it is only recognized to be compatible with old as
programs.
.ascii "string"...
.ascii expects zero or more string literals (see section 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 abs-expr , abs-exprPad 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 value to be stored in the padding bytes. It (and the comma) may be omitted. If it is omitted, the padding bytes are zero.
.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 named common area in the bss section. Normally
ld reserves memory addresses for it during linking, so no partial
program defines the location of the symbol. Use .comm to tell
ld that it must be at least length bytes long. ld
allocates space for each .comm symbol that is at least as
long as the longest .comm request in any of the partial programs
linked. length is an absolute expression.
The syntax for .comm differs slightly on the HPPA. The syntax is
`symbol .comm, length'; symbol is optional.
.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.
.def name
Begin defining debugging information for a symbol name; the
definition extends until the .endef directive is encountered.
This directive is only observed when as is configured for COFF
format output; when producing b.out, `.def' is recognized,
but ignored.
.desc symbol, abs-expressionThis directive sets the descriptor of the symbol (see section Symbol Attributes) to the low 16 bits of an absolute expression.
The `.desc' directive is not available when as is
configured for COFF output; it is only for a.out or b.out
object format. For the sake of compatibility, as accepts
it, but produces no output, when configured for COFF.
.dim
This directive is generated by compilers to include auxiliary debugging
information in the symbol table. It is only permitted inside
.def/.endef pairs.
`.dim' is only meaningful when generating COFF format output; when
as is generating b.out, it accepts this directive but
ignores it.
.double flonums
.double expects zero or more flonums, separated by commas. It
assembles floating point numbers.
The exact kind of floating point numbers emitted depends on how
as is configured. See section Machine Dependent Features.
.ejectForce a page break at this point, when generating assembly listings.
.else
.else is part of the as support for conditional
assembly; see section .if absolute expression. It marks the beginning of a section
of code to be assembled if the condition for the preceding .if
was false.
.endef
This directive flags the end of a symbol definition begun with
.def.
`.endef' is only meaningful when generating COFF format output; if
as is configured to generate b.out, it accepts this
directive but ignores it.
.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 section .if absolute expression.
.equ symbol, expression
This directive sets the value of symbol to expression.
It is synonymous with `.set'; see section .set symbol, expression.
The syntax for equ on the HPPA is
`symbol .equ expression'.
.extern
.extern is accepted in the source program--for compatibility
with other assemblers--but it is ignored. as treats
all undefined symbols as external.
.file string
.file (which may also be spelled `.app-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
result, 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.
The exact kind of floating point numbers emitted depends on how
as is configured.
See section Machine Dependent Features.
.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.
On the HPPA, .global is not always enough to make it accessible to other
partial programs. You may need the HPPA-only .EXPORT directive as well.
See section HPPA Assembler Directives.
.hword expressionsThis expects zero or more expressions, and emits a 16 bit number for each.
This directive is a synonym for `.short'; depending on the target architecture, it may also be a synonym for `.word'.
.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 section .endif); optionally, you may include code for the
alternative condition, flagged by .else (see section .else.
The following variants of .if are also supported:
.ifdef symbol
.ifndef symbol
ifnotdef symbol
.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 section Command-Line Options). Quotation marks are required
around file.
.int expressionsExpect 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 section .global symbol, .globl symbol), so is normally
not visible to ld.
The syntax for .lcomm differs slightly on the HPPA. The syntax is
`symbol .lcomm, length'; symbol is optional.
.lflags
as accepts this directive, for compatibility with other
assemblers, but ignores it.
.line line-number
Change the logical line number. line-number must be an absolute
expression. 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. One day
as will no longer support this directive: it is recognized only
for compatibility with existing assembler programs.
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.
.ln line-number`.ln' is a synonym for `.line'.
.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 section Command-Line Options), the initial value of the listing counter is one.
.long expressions
.long is the same as `.int', see section .int expressions.
.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 ...
.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).
.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 bignumsThis 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 abs-expr , abs-exprPad 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 value to be stored in the padding bytes. It (and the comma) may be omitted. If it is omitted, the padding bytes are zero.
.psize lines , columnsUse 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.
.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.
.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.
The `.scl' directive is primarily associated with COFF output; when
configured to generate b.out output format, as
accepts this directive but ignores it.
.section name, subsection
Assemble the following code into end of subsection numbered
subsection in the COFF named section name. If you omit
subsection, as uses subsection number zero.
`.section .text' is equivalent to the .text directive;
`.section .data' is equivalent to the .data directive.
This directive is only supported for targets that actually support arbitrarily
named sections; on a.out targets, for example, it is not accepted, even
with a standard a.out section name as its parameter.
.set symbol, expressionSet 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 section 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.
The syntax for set on the HPPA is
`symbol .set expression'.
.short expressions
.short is normally the same as `.word'.
See section .word expressions.
In some configurations, however, .short and .word generate
numbers of different lengths; see section Machine Dependent Features.
.single flonums
This directive assembles zero or more flonums, separated by commas. It
has the same effect as .float.
The exact kind of floating point numbers emitted depends on how
as is configured. See section Machine Dependent Features.
.size
This directive is generated by compilers to include auxiliary debugging
information in the symbol table. It is only permitted inside
.def/.endef pairs.
`.size' is only meaningful when generating COFF format output; when
as is generating b.out, it accepts this directive but
ignores it.
.space size , fillThis 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.
Warning:
.spacehas a completely different meaning for HPPA targets; use.blockas a substitute. See HP9000 Series 800 Assembly Language Reference Manual (HP 92432-90001) for the meaning of the.spacedirective. See section HPPA Assembler Directives, for a summary.
.stabd, .stabn, .stabs
There are three directives that begin `.stab'.
All emit symbols (see section 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:
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
.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 section Strings.
.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.
`.tag' is only used when generating COFF format output; when
as is generating b.out, it accepts this directive but
ignores it.
.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.
.type int
This directive, permitted only within .def/.endef pairs,
records the integer int as the type attribute of a symbol table entry.
`.type' is associated only with COFF format output; when
as is configured for b.out output, it accepts this
directive but ignores it.
.val addr
This directive, permitted only within .def/.endef pairs,
records the address addr as the value attribute of a symbol table
entry.
`.val' is used only for COFF output; when as is
configured for b.out, it accepts this directive but ignores it.
.word expressionsThis directive expects zero or more expressions, of any section, separated by commas.
The size of the number emitted, and its byte order, depend on what target computer the assembly is for.
Warning: Special Treatment to support Compilers
Machines with a 32-bit address space, but that do less than 32-bit addressing, require the following special treatment. If the machine of interest to you does 32-bit addressing (or doesn't require it; see section Machine Dependent Features), you can ignore this issue.
In order to assemble compiler output into something that works,
as occasionlly does strange things to `.word' directives.
Directives of the form `.word sym1-sym2' are often emitted by
compilers as part of jump tables. Therefore, when as assembles a
directive of the form `.word sym1-sym2', and the difference between
sym1 and sym2 does not fit in 16 bits, as
creates a secondary jump table, immediately before the next label.
This secondary jump table is preceded by a short-jump to the
first byte after the secondary table. This short-jump prevents the flow
of control from accidentally falling into the new table. Inside the
table is a long-jump to sym2. The original `.word'
contains sym1 minus the address of the long-jump to
sym2.
If there were several occurrences of `.word sym1-sym2' before the
secondary jump table, all of them are adjusted. If there was a
`.word sym3-sym4', that also did not fit in sixteen bits, a
long-jump to sym4 is included in the secondary jump table,
and the .word directives are adjusted to contain sym3
minus the address of the long-jump to sym4; and so on, for as many
entries in the original jump table as necessary.
One day these directives won't work. They are included for compatibility with older assemblers.
The machine instruction sets are (almost by definition) different on
each machine where as runs. Floating point representations
vary as well, and as often supports a few additional
directives or command-line options for compatibility with other
assemblers on a particular platform. Finally, some versions of
as support special pseudo-instructions for branch
optimization.
This chapter discusses most of these differences, though it does not include details on any machine's instruction set. For details on that subject, see the hardware manufacturer's manual.
As a back end for GNU CC as has been throughly tested and should
work extremely well. We have tested it only minimally on hand written assembly
code and no one has tested it much on the assembly output from the HP
compilers.
The format of the debugging sections has changed since the original
as port (version 1.3X) was released; therefore,
you must rebuild all HPPA objects and libraries with the new
assembler so that you can debug the final executable.
The HPPA as port generates a small subset of the relocations
available in the SOM and ELF object file formats. Additional relocation
support will be added as it becomes necessary.
as has no machine-dependent command-line options for the HPPA.
The assembler syntax closely follows the HPPA instruction set reference manual; assembler directives and general syntax closely follow the HPPA assembly language reference manual, with a few noteworthy differences.
First, a colon may immediately follow a label definition. This is simply for compatibility with how most assembly language programmers write code.
Some obscure expression parsing problems may affect hand written code which
uses the spop instructions, or code which makes significant
use of the ! line separator.
as is much less forgiving about missing arguments and other
similar oversights than the HP assembler. as notifies you
of missing arguments as syntax errors; this is regarded as a feature, not a
bug.
Finally, as allows you to use an external symbol without
explicitly importing the symbol. Warning: in the future this will be
an error for HPPA targets.
Special characters for HPPA targets include:
`;' is the line comment character.
`!' can be used instead of a newline to separate statements.
Since `$' has no special meaning, you may use it in symbol names.
The HPPA family uses IEEE floating-point numbers.
as for the HPPA supports many additional directives for
compatibility with the native assembler. This section describes them only
briefly. For detailed information on HPPA-specific assembler directives, see
HP9000 Series 800 Assembly Language Reference Manual (HP 92432-90001).
as does not support the following assembler directives
described in the HP manual:
.endm .liston .enter .locct .leave .macro .listoff
Beyond those implemented for compatibility, as supports one
additional assembler directive for the HPPA: .param. It conveys
register argument locations for static functions. Its syntax closely follows
the .export directive.
These are the additional directives in as for the HPPA:
.block n
.blockz n
.call
.callinfo [ param=value, ... ] [ flag, ... ]
.code
.copyright "string"
.copyright "string"
.enter
.entry
.exit
.export name [ ,typ ] [ ,param=r ]
0 to 3, and
indicates one of four one-word arguments); `rtnval' (the procedure's
result); or `priv_lev' (privilege level). For arguments or the result,
r specifies how to relocate, and must be one of `no' (not
relocatable), `gr' (argument is in general register), `fr' (in
floating point register), or `fu' (upper half of float register).
For `priv_lev', r is an integer.
.half n
as directive .short.
.import name [ ,typ ]
.export; make a procedure available to call. The arguments
use the same conventions as the first two arguments for .export.
.label name
.leave
.origin lc
.org.
.param name [ ,typ ] [ ,param=r ]
.export, but used for static procedures.
.proc
.procend
label .reg expr
.equ; define label with the absolute expression
expr as its value.
.space secname [ ,params ]
.spnum secnam
.space directive.)
.string "str"
as strings.
Warning! The HPPA version of .string differs from the
usual as definition: it does not write a zero byte
after copying str.
.stringz "str"
.string, but appends a zero byte after copying str to object
file.
.subspa name [ ,params ]
.nsubspa name [ ,params ]
.space, but selects a subsection name within the
current section. You may only specify params when you create a
subsection (in the first instance of .subspa for this name).
If specified, the list params declares attributes of the subsection,
identified by keywords. The keywords recognized are `quad=expr'
("quadrant" for this subsection), `align=expr' (alignment for
beginning of this subsection; a power of two), `access=expr' (value
for "access rights" field), `sort=expr' (sorting order for this
subspace in link), `code_only' (subsection contains only code),
`unloadable' (subsection cannot be loaded into memory), `common'
(subsection is common block), `dup_comm' (initialized data may have
duplicate names), or `zero' (subsection is all zeros, do not write in
object file).
.nsubspa always creates a new subspace with the given name, even
if one with the same name already exists.
.version "str"
For detailed information on the HPPA machine instruction set, see PA-RISC Architecture and Instruction Set Reference Manual (HP 09740-90039).
-ACA | -ACA_A | -ACB | -ACC | -AKA | -AKB | -AKC | -AMC
as generates code
for any instruction or feature that is supported by some version of the
960 (even if this means mixing architectures!). In principle,
as attempts to deduce the minimal sufficient processor type if
none is specified; depending on the object code format, the processor type may
be recorded in the object file. If it is critical that the as
output match a specific architecture, specify that architecture explicitly.
-b
call increment routine
.word 0 # pre-counter
Label: BR
call increment routine
.word 0 # post-counter
The counter following a branch records the number of times that branch
was not taken; the differenc between the two counters is the
number of times the branch was taken.
A table of every such Label is also generated, so that the
external postprocessor gbr960 (supplied by Intel) can locate all
the counters. This table is always labelled `__BRANCH_TABLE__';
this is a local symbol to permit collecting statistics for many separate
object files. The table is word aligned, and begins with a two-word
header. The first word, initialized to 0, is used in maintaining linked
lists of branch tables. The second word is a count of the number of
entries in the table, which follow immediately: each is a word, pointing
to one of the labels illustrated above.
The first word of the header is used to locate multiple branch tables,
since each object file may contain one. Normally the links are
maintained with a call to an initialization routine, placed at the
beginning of each function in the file. The GNU C compiler
generates these calls automatically when you give it a `-b' option.
For further details, see the documentation of `gbr960'.
-no-relax
as should generate errors instead, if the target displacement
is larger than 13 bits.
This option does not affect the Compare-and-Jump instructions; the code
emitted for them is always adjusted when necessary (depending on
displacement size), regardless of whether you use `-no-relax'.
as generates IEEE floating-point numbers for the directives
`.float', `.double', `.extended', and `.single'.
.bss symbol, length, align
.lcomm symbol , length.
.extended flonums
.extended expects zero or more flonums, separated by commas; for
each flonum, `.extended' emits an IEEE extended-format (80-bit)
floating-point number.
.leafproc call-lab, bal-lab
callj instruction to enable faster calls of leaf
procedures. If a procedure is known to call no other procedures, you
may define an entry point that skips procedure prolog code (and that does
not depend on system-supplied saved context), and declare it as the
bal-lab using `.leafproc'. If the procedure also has an
entry point that goes through the normal prolog, you can specify that
entry point as call-lab.
A `.leafproc' declaration is meant for use in conjunction with the
optimized call instruction `callj'; the directive records the data
needed later to choose between converting the `callj' into a
bal or a call.
call-lab is optional; if only one argument is present, or if the
two arguments are identical, the single argument is assumed to be the
bal entry point.
.sysproc name, index
All Intel 960 machine instructions are supported; see section i960 Command-line Options for a discussion of selecting the instruction subset for a particular 960 architecture.
Some opcodes are processed beyond simply emitting a single corresponding instruction: `callj', and Compare-and-Branch or Compare-and-Jump instructions with target displacements larger than 13 bits.
callj
You can write callj to have the assembler or the linker determine
the most appropriate form of subroutine call: `call',
`bal', or `calls'. If the assembly source contains
enough information--a `.leafproc' or `.sysproc' directive
defining the operand--then as translates the
callj; if not, it simply emits the callj, leaving it
for the linker to resolve.
The 960 architectures provide combined Compare-and-Branch instructions that permit you to store the branch target in the lower 13 bits of the instruction word itself. However, if you specify a branch target far enough away that its address won't fit in 13 bits, the assembler can either issue an error, or convert your Compare-and-Branch instruction into separate instructions to do the compare and the branch.
Whether as gives an error or expands the instruction depends
on two choices you can make: whether you use the `-no-relax' option,
and whether you use a "Compare and Branch" instruction or a "Compare
and Jump" instruction. The "Jump" instructions are always
expanded if necessary; the "Branch" instructions are expanded when
necessary unless you specify -no-relax---in which case
as gives an error instead.
These are the Compare-and-Branch instructions, their "Jump" variants, and the instruction pairs they may expand into:
as has several additional command-line options for the
PowerPC family.
-mpwrx
-mpwr2
-mpwr
-m601
-mppc
-mppc32
-m403
-m603
-m604
-mppc64
-m620
-mcom
-many
-mregnames
-mno-regnames
The following options refer to the ELF object format:
-mrelocatable
-mrelocatable-lib
-memb
-mlittle
-mlittle-endian
-mbig
-mbig-endian
PowerPC operands are just expressions. The only real issue is that a few operand types are optional. All cases which might use an optional operand separate the operands only with commas (in some cases parentheses are used, as in `lwz 1,0(1)' but such cases never have optional operands). There is never more than one optional operand for an instruction.
Before operands are parsed, as checks to see if an
optional operand exists, and, if it does, counts the number of commas to
see whether the operand should be omitted. For example:
lwz r4,[toc].GS.0.static_int(rtoc)
The argument following the ] must be a symbol name, and the register must be the table-of-contents register: `rtoc' or `2'.
The effect is to use the value 0 as the displacement field in the instruction, and issue a relocation against it based on the symbol. The linker builds the table of contents, and insert the resolved toc offset.
Notes:
lwz r4,[toc32].GS.0.static_int(rtoc) lwz r4,[toc64].GS.0.static_int(rtoc)
Each general register has predefined names of the following form (reg_num between 0 and 31, inclusive):
rreg_num, which has the value reg_num.
r.reg_num, which has the value reg_num.
Each floating point register has predefined names of the following form (reg_num between 0 and 31, inclusive):
freg_num, which has the value reg_num.
f.reg_num, which has the value reg_num.
Each condition register has predefined names of the following form (reg_num between 0 and 7, inclusive):
crreg_num which has the value reg_num.
cr.reg_num which has the value reg_num.
There are individual registers as well:
sp
r.sp
rtoc
r.toc
fpscr
xer
lr
ctr
pmr
dar
dsisr
dec
sdr1
srr0
srr1
The PowerPC programming environment defines a set of floating-point instructions. A specific PowerPC processor may implement all, some, or none of the floating-point instructions. For more information on the floating-point capability of a given processor, refer to the user's manual of that particular processor.
The following pseudo-ops are specific to the RS/6000 XCOFF format.
comm eb extern lcomm ec function bb ef lglobl bc ei rename bf es stabx bi text bs toc csect data
The following pseudo-ops are specific to the ELF object format.
long word short rdata rodata lcomm
The following pseudo-ops are specific to the Windows NT PowerPC PE (coff) format.
previous znop pdata comm ydata lcomm reldata section rdata function ualong tocd
This pseudo-op is used even when not generating XCOFF output.
tc
For detailed information on the PowerPC machine instruction set, see The PowerPC Architecture, edited by May, Silha, Simpson, and Warren, ISBN 1-55860-316-6.
The SPARC chip family includes several successive levels (or other variants) of chip, using the same core instruction set, but including a few additional instructions at each level.
By default, as assumes the core instruction set (SPARC
v6), but "bumps" the architecture level as needed: it switches to
successively higher architectures as it encounters instructions that
only exist in the higher levels.
If not configured for SPARC v9 (sparc64-*-*) GAS will not bump
passed sparclite by default, an option must be passed to enable the
v9 instructions.
GAS treats sparclite as being compatible with v8, unless an architecture is explicitly requested. SPARC v9 is always incompatible with sparclite.
-Av6 | -Av7 | -Av8 | -Asparclite | -Av9 | -Av9a
as reports a fatal error if it encounters an instruction
or feature requiring a higher level.
-xarch=v8plus | -xarch=v8plusa
-bump
The Sparc uses IEEE floating-point numbers.
The Sparc version of as supports the following additional
machine directives:
.align
.common
"bss". This behaves somewhat like .comm, but the
syntax is different.
.half
.short.
.proc
.reserve
"bss". This behaves somewhat like .lcomm, but the
syntax is different.
.seg
"text", "data", or
"data1". It behaves like .text, .data, or
.data 1.
.skip
.space directive.
.word
.word directive produces 32 bit values,
instead of the 16 bit values it produces on many other machines.
.xword
.xword directive produces
64 bit values.
The 80386 has no machine dependent options.
In order to maintain compatibility with the output of gcc,
as supports AT&T System V/386 assembler syntax. This is quite
different from Intel syntax. We mention these differences because
almost all 80386 documents used only Intel syntax. Notable differences
between the two syntaxes are:
Opcode names are suffixed with one character modifiers which specify the
size of operands. The letters `b', `w', and `l' specify
byte, word, and long operands. If no suffix is specified by an
instruction and it contains no memory operands then as tries to
fill in the missing suffix based on the destination register operand
(the last one by convention). Thus, `mov %ax, %bx' is equivalent
to `movw %ax, %bx'; also, `mov $1, %bx' is equivalent to
`movw $1, %bx'. Note that this is incompatible with the AT&T Unix
assembler which assumes that a missing opcode suffix implies long
operand size. (This incompatibility does not affect compiler output
since compilers always explicitly specify the opcode suffix.)
Almost all opcodes have the same names in AT&T and Intel format. There are a few exceptions. The sign extend and zero extend instructions need two sizes to specify them. They need a size to sign/zero extend from and a size to zero extend to. This is accomplished by using two opcode suffixes in AT&T syntax. Base names for sign extend and zero extend are `movs...' and `movz...' in AT&T syntax (`movsx' and `movzx' in Intel syntax). The opcode suffixes are tacked on to this base name, the from suffix before the to suffix. Thus, `movsbl %al, %edx' is AT&T syntax for "move sign extend from %al to %edx." Possible suffixes, thus, are `bl' (from byte to long), `bw' (from byte to word), and `wl' (from word to long).
The Intel-syntax conversion instructions
are called `cbtw', `cwtl', `cwtd', and `cltd' in
AT&T naming. as accepts either naming for these instructions.
Far call/jump instructions are `lcall' and `ljmp' in AT&T syntax, but are `call far' and `jump far' in Intel convention.
Register operands are always prefixes with `%'. The 80386 registers consist of
Opcode prefixes are used to modify the following opcode. They are used to repeat string instructions, to provide section overrides, to perform bus lock operations, and to give operand and address size (16-bit operands are specified in an instruction by prefixing what would normally be 32-bit operands with a "operand size" opcode prefix). Opcode prefixes are usually given as single-line instructions with no operands, and must directly precede the instruction they act upon. For example, the `scas' (scan string) instruction is repeated with:
repne
scas
Here is a list of opcode prefixes:
An Intel syntax indirect memory reference of the form
section:[base + index*scale + disp]
is translated into the AT&T syntax
section:disp(base, index, scale)
where base and index are the optional 32-bit base and
index registers, disp is the optional displacement, and
scale, taking the values 1, 2, 4, and 8, multiplies index
to calculate the address of the operand. If no scale is
specified, scale is taken to be 1. section specifies the
optional section register for the memory operand, and may override the
default section register (see a 80386 manual for section register
defaults). Note that section overrides in AT&T syntax must have
be preceded by a `%'. If you specify a section override which
coincides with the default section register, as does not
output any section register override prefixes to assemble the given
instruction. Thus, section overrides can be specified to emphasize which
section register is used for a given memory operand.
Here are some examples of Intel and AT&T style memory references:
Absolute (as opposed to PC relative) call and jump operands must be
prefixed with `*'. If no `*' is specified, as
always chooses PC relative addressing for jump/call labels.
Any instruction that has a memory operand must specify its size (byte, word, or long) with an opcode suffix (`b', `w', or `l', respectively).
Jump instructions are always optimized to use the smallest possible displacements. This is accomplished by using byte (8-bit) displacement jumps whenever the target is sufficiently close. If a byte displacement is insufficient a long (32-bit) displacement is used. We do not support word (16-bit) displacement jumps (i.e. prefixing the jump instruction with the `addr16' opcode prefix), since the 80386 insists upon masking `%eip' to 16 bits after the word displacement is added.
Note that the `jcxz', `jecxz', `loop', `loopz',
`loope', `loopnz' and `loopne' instructions only come in byte
displacements, so that if you use these instructions (gcc does
not use them) you may get an error message (and incorrect code). The AT&T
80386 assembler tries to get around this problem by expanding `jcxz foo'
to
jcxz cx_zero
jmp cx_nonzero
cx_zero: jmp foo
cx_nonzero:
All 80387 floating point types except packed BCD are supported. (BCD support may be added without much difficulty). These data types are 16-, 32-, and 64- bit integers, and single (32-bit), double (64-bit), and extended (80-bit) precision floating point. Each supported type has an opcode suffix and a constructor associated with it. Opcode suffixes specify operand's data types. Constructors build these data types into memory.
Register to register operations do not require opcode suffixes, so that `fst %st, %st(1)' is equivalent to `fstl %st, %st(1)'.
Since the 80387 automatically synchronizes with the 80386 `fwait'
instructions are almost never needed (this is not the case for the
80286/80287 and 8086/8087 combinations). Therefore, as suppresses
the `fwait' instruction whenever it is implicitly selected by one
of the `fn...' instructions. For example, `fsave' and
`fnsave' are treated identically. In general, all the `fn...'
instructions are made equivalent to `f...' instructions. If
`fwait' is desired it must be explicitly coded.
While GAS normally writes only "pure" 32-bit i386 code, it has limited support for writing code to run in real mode or in 16-bit protected mode code segments. To do this, insert a `.code16' directive before the assembly language instructions to be run in 16-bit mode. You can switch GAS back to writing normal 32-bit code with the `.code32' directive.
GAS understands exactly the same assembly language syntax in 16-bit mode as in 32-bit mode. The function of any given instruction is exactly the same regardless of mode, as long as the resulting object code is executed in the mode for which GAS wrote it. So, for example, the `ret' mnemonic produces a 32-bit return instruction regardless of whether it is to be run in 16-bit or 32-bit mode. (If GAS is in 16-bit mode, it will add an operand size prefix to the instruction to force it to be a 32-bit return.)
This means, for one thing, that you can use GNU CC to write code to be run in real mode or 16-bit protected mode. Just insert the statement `asm(".code16");' at the beginning of your C source file, and while GNU CC will still be generating 32-bit code, GAS will automatically add all the necessary size prefixes to make that code run in 16-bit mode. Of course, since GNU CC only writes small-model code (it doesn't know how to attach segment selectors to pointers like native x86 compilers do), any 16-bit code you write with GNU CC will essentially be limited to a 64K address space. Also, there will be a code size and performance penalty due to all the extra address and operand size prefixes GAS has to add to the instructions.
Note that placing GAS in 16-bit mode does not mean that the resulting code will necessarily run on a 16-bit pre-80386 processor. To write code that runs on such a processor, you would have to refrain from using any 32-bit constructs which require GAS to output address or operand size prefixes. At the moment this would be rather difficult, because GAS currently supports only 32-bit addressing modes: when writing 16-bit code, it always outputs address size prefixes for any instruction that uses a non-register addressing mode. So you can write code that runs on 16-bit processors, but only if that code never references memory.
There is some trickery concerning the `mul' and `imul'
instructions that deserves mention. The 16-, 32-, and 64-bit expanding
multiplies (base opcode `0xf6'; extension 4 for `mul' and 5
for `imul') can be output only in the one operand form. Thus,
`imul %ebx, %eax' does not select the expanding multiply;
the expanding multiply would clobber the `%edx' register, and this
would confuse gcc output. Use `imul %ebx' to get the
64-bit product in `%edx:%eax'.
We have added a two operand form of `imul' when the first operand is an immediate mode expression and the second operand is a register. This is just a shorthand, so that, multiplying `%eax' by 69, for example, can be done with `imul $69, %eax' rather than `imul $69, %eax, %eax'.
The Z8000 as supports both members of the Z8000 family: the unsegmented Z8002, with 16 bit addresses, and the segmented Z8001 with 24 bit addresses.
When the assembler is in unsegmented mode (specified with the
unsegm directive), an address takes up one word (16 bit)
sized register. When the assembler is in segmented mode (specified with
the segm directive), a 24-bit address takes up a long (32 bit)
register. See section Assembler Directives for the Z8000,
for a list of other Z8000 specific assembler directives.
as has no additional command-line options for the Zilog
Z8000 family.
`!' is the line comment character.
You can use `;' instead of a newline to separate statements.
The Z8000 has sixteen 16 bit registers, numbered 0 to 15. You can refer to different sized groups of registers by register number, with the prefix `r' for 16 bit registers, `rr' for 32 bit registers and `rq' for 64 bit registers. You can also refer to the contents of the first eight (of the sixteen 16 bit registers) by bytes. They are named `rnh' and `rnl'.
byte registers r0l r0h r1h r1l r2h r2l r3h r3l r4h r4l r5h r5l r6h r6l r7h r7l word registers r0 r1 r2 r3 r4 r5 r6 r7 r8 r9 r10 r11 r12 r13 r14 r15 long word registers rr0 rr2 rr4 rr6 rr8 rr10 rr12 rr14 quad word registers rq0 rq4 rq8 rq12
as understands the following addressing modes for the Z8000:
rn
@rn
addr
address(rn)
rn(#imm)
rn(rm)
#xx
The Z8000 port of as includes these additional assembler directives, for compatibility with other Z8000 assemblers. As shown, these do not begin with `.' (unlike the ordinary as directives).
segm
unsegm
name
.file
global
.global
wval
.word
lval
.long
bval
.byte
sval
sval expects one string literal, delimited by
single quotes. It assembles each byte of the string into consecutive
addresses. You can use the escape sequence `%xx' (where
xx represents a two-digit hexadecimal number) to represent the
character whose ASCII value is xx. Use this feature to
describe single quote and other characters that may not appear in string
literals as themselves. For example, the C statement `char *a =
"he said \"it's 50% off\"";' is represented in Z8000 assembly language
(shown with the assembler output in hex at the left) as
68652073 sval 'he said %22it%27s 50%25 off%22%00' 61696420 22697427 73203530 25206F66 662200
rsect
.section
block
.space
even
.align; aligns output to even byte boundary.
For detailed information on the Z8000 machine instruction set, see Z8000 Technical Manual.
GNU as for MIPS architectures supports the MIPS
R2000, R3000, R4000 and R6000 processors. For information
about the MIPS instruction set, see MIPS RISC Architecture, by Kane
and Heindrich (Prentice-Hall). For an overview of MIPS assembly
conventions, see "Appendix D: Assembly Language Programming" in the same
work.
The MIPS configurations of GNU as support these
special options:
-G num
gp register. It is only accepted for targets
that use ECOFF format. The default value is 8.
-EB
-EL
as can select big-endian or
little-endian output at run time (unlike the other GNU development
tools, which must be configured for one or the other). Use `-EB'
to select big-endian output, and `-EL' for little-endian.
-mips1
-mips2
-mips3
-m4650
-no-m4650
-m4010
-no-m4010
-mcpu=CPU
gcc.
-nocpp
as, there is no need for `-nocpp', because the
GNU assembler itself never runs the C preprocessor.
--trap
--no-break
as automatically macro expands certain division and
multiplication instructions to check for overflow and division by zero. This
option causes as to generate code to take a trap exception
rather than a break exception when an error is detected. The trap instructions
are only supported at Instruction Set Architecture level 2 and higher.
--break
--no-trap
Assembling for a MIPS ECOFF target supports some additional sections
besides the usual .text, .data and .bss. The
additional sections are .rdata, used for read-only data,
.sdata, used for small data, and .sbss, used for small
common objects.
When assembling for ECOFF, the assembler uses the $gp ($28)
register to form the address of a "small object". Any object in the
.sdata or .sbss sections is considered "small" in this sense.
For external objects, or for objects in the .bss section, you can use
the gcc `-G' option to control the size of objects addressed via
$gp; the default value is 8, meaning that a reference to any object
eight bytes or smaller uses $gp. Passing `-G 0' to
as prevents it from using the $gp register on the basis
of object size (but the assembler uses $gp for objects in .sdata
or sbss in any case). The size of an object in the .bss section
is set by the .comm or .lcomm directive that defines it. The
size of an external object may be set with the .extern directive. For
example, `.extern sym,4' declares that the object at sym is 4 bytes
in length, whie leaving sym otherwise undefined.
Using small ECOFF objects requires linker support, and assumes that the
$gp register is correctly initialized (normally done automatically by
the startup code). MIPS ECOFF assembly code must not modify the
$gp register.
MIPS ECOFF as supports several directives used for
generating debugging information which are not support by traditional MIPS
assemblers. These are .def, .endef, .dim, .file,
.scl, .size, .tag, .type, .val,
.stabd, .stabn, and .stabs. The debugging information
generated by the three .stab directives can only be read by GDB,
not by traditional MIPS debuggers (this enhancement is required to fully
support C++ debugging). These directives are primarily used by compilers, not
assembly language programmers!
GNU as supports an additional directive to change the
MIPS Instruction Set Architecture level on the fly: .set
mipsn. n should be a number from 0 to 3. A value from 1 to 3
makes the assembler accept instructions for the corresponding ISA level,
from that point on in the assembly. .set mipsn affects not only
which instructions are permitted, but also how certain macros are expanded.
.set mips0 restores the ISA level to its original level: either the
level you selected with command line options, or the default for your
configuration. You can use this feature to permit specific R4000
instructions while assembling in 32 bit mode. Use this directive with care!
Traditional MIPS assemblers do not support this directive.
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 Hitachi 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, 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).
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.
Jump to: # - - - . - 1 - : - \ - a - b - c - d - e - f - g - h - i - j - l - m - n - o - p - q - r - s - t - u - v - w - x - z
-A options, i960
-b option, i960
-EB option (MIPS)
-EL option (MIPS)
-G option (MIPS)
-no-relax option, i960
-nocpp ignored (MIPS)
. (symbol)
.param on HPPA
.set mipsn
: (label)
\" (doublequote character)
\\ (`\' character)
\b (backspace character)
\ddd (octal character code)
\f (formfeed character)
\n (newline character)
\r (carriage return character)
\t (tab)
\xdd (hex character code)
a.out symbol attributes
abort directive
ABORT directive
align directive
align directive, SPARC
app-file directive
as version
ascii directive
asciz directive
\\)
\b)
balign directive
bss directive, i960
byte directive
callj, i960 pseudo-opcode
\r)
code16 directive, i386
code32 directive, i386
comm directive
common directive, SPARC
data directive
def directive
desc directive
a.out symbol
dim directive
double directive
double directive, i386
\")
eject directive
else directive
endef directive
endif directive
endm directive
equ directive
exitm directive
extended directive, i96
extern directive
-f)
file directive
fill directive
float directive
float directive, i386
\f)
fwait instruction, i386
gbr960, i960 postprocessor
global directive
gp register, MIPS
half directive, SPARC
\xdd)
hword directive
fwait instruction
mul, imul instructions
callj pseudo-opcode
ident directive
if directive
ifdef directive
ifndef directive
ifnotdef directive
imul instruction, i386
include directive
include directive search path
int directive
int directive, i386
as sections
irp directive
irpc directive
:)
lcomm directive
leafproc directive, i960
lflags directive (ignored)
line directive
#
list directive
ln directive
long directive
long directive, i386
macro directive
as
mul instruction, i386
\n)
nolist directive
octa directive
\ddd)
as
org directive
a.out symbol
p2align directive
.include
proc directive, SPARC
psize directive
as
quad directive
quad directive, i386
rept directive
reserve directive, SPARC
sbttl directive
scl directive
.include
section directive
seg directive, SPARC
set directive
short directive
single directive
single directive, i386
size directive
skip directive, SPARC
space directive
stabd directive
stabn directive
stabs directive
stabx directives
as sections
string directive
string directive on HPPA
a.out
sysproc directive, i960
\t)
tag directive
text directive
tfloat directive, i386
title directive
type directive
val directive
as
word directive
word directive, i386
word directive, SPARC
xword directive, SPARC
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