http://www.gnu.org/software/binutils/manual/gas-2.9.1/html_mono/as.html#SEC201
Using as
The GNU Assembler
January 1994
Dean Elsner, Jay Fenlason & friends
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Table of Contents
Overview
Structure of this Manual
The GNU Assembler
Object File Formats
Command Line
Input Files
Output (Object) File
Error and Warning Messages
Command-Line Options
Enable Listings: -a[cdhlns]
-D
Work Faster: -f
.include search path: -I path
Difference Tables: -K
Include Local Labels: -L
Assemble in MRI Compatibility Mode: -M
Dependency tracking: --MD
Name the Object File: -o
Join Data and Text Sections: -R
Display Assembly Statistics: --statistics
Compatible output: --traditional-format
Announce Version: -v
Suppress Warnings: -W
Generate Object File in Spite of Errors: -Z
Syntax
Preprocessing
Whitespace
Comments
Symbols
Statements
Constants
Character Constants
Strings
Characters
Number Constants
Integers
Bignums
Flonums
Sections and Relocation
Background
Linker Sections
Assembler Internal Sections
Sub-Sections
bss Section
Symbols
Labels
Giving Symbols Other Values
Symbol Names
The Special Dot Symbol
Symbol Attributes
Value
Type
Symbol Attributes: a.out
Descriptor
Other
Symbol Attributes for COFF
Primary Attributes
Auxiliary Attributes
Symbol Attributes for SOM
Expressions
Empty Expressions
Integer Expressions
Arguments
Operators
Prefix Operator
Infix Operators
Assembler Directives
.abort
.ABORT
.align abs-expr, abs-expr, abs-expr
.app-file string
.ascii "string"...
.asciz "string"...
.balign[wl] abs-expr, abs-expr, abs-expr
.byte expressions
.comm symbol , length
.data subsection
.def name
.desc symbol, abs-expression
.dim
.double flonums
.eject
.else
.endef
.endif
.equ symbol, expression
.equiv symbol, expression
.err
.extern
.file string
.fill repeat , size , value
.float flonums
.global symbol, .globl symbol
.hword expressions
.ident
.if absolute expression
.include "file"
.int expressions
.irp symbol,values...
.irpc symbol,values...
.lcomm symbol , length
.lflags
.line line-number
.linkonce [type]
.ln line-number
.mri val
.list
.long expressions
.macro
.nolist
.octa bignums
.org new-lc , fill
.p2align[wl] abs-expr, abs-expr, abs-expr
.psize lines , columns
.quad bignums
.rept count
.sbttl "subheading"
.scl class
.section name
.set symbol, expression
.short expressions
.single flonums
.size
.sleb128 expressions
.skip size , fill
.space size , fill
.stabd, .stabn, .stabs
.string "str"
.symver
.tag structname
.text subsection
.title "heading"
.type int
.val addr
.uleb128 expressions
.word expressions
Deprecated Directives
Machine Dependent Features
ARC Dependent Features
Options
Floating Point
ARC Machine Directives
AMD 29K Dependent Features
Options
Syntax
Macros
Special Characters
Register Names
Floating Point
AMD 29K Machine Directives
Opcodes
ARM Dependent Features
Options
Syntax
Special Characters
Register Names
Floating Point
ARM Machine Directives
Opcodes
D10V Dependent Features
D10V Options
Syntax
Size Modifiers
Sub-Instructions
Special Characters
Register Names
Addressing Modes
@WORD Modifier
Floating Point
Opcodes
H8/300 Dependent Features
Options
Syntax
Special Characters
Register Names
Addressing Modes
Floating Point
H8/300 Machine Directives
Opcodes
H8/500 Dependent Features
Options
Syntax
Special Characters
Register Names
Addressing Modes
Floating Point
H8/500 Machine Directives
Opcodes
HPPA Dependent Features
Notes
Options
Syntax
Floating Point
HPPA Assembler Directives
Opcodes
80386 Dependent Features
Options
AT&T Syntax versus Intel Syntax
Opcode Naming
Register Naming
Opcode Prefixes
Memory References
Handling of Jump Instructions
Floating Point
Writing 16-bit Code
Notes
Intel 80960 Dependent Features
i960 Command-line Options
Floating Point
i960 Machine Directives
i960 Opcodes
callj
Compare-and-Branch
M680x0 Dependent Features
M680x0 Options
Syntax
Motorola Syntax
Floating Point
680x0 Machine Directives
Opcodes
Branch Improvement
Special Characters
MIPS Dependent Features
Assembler options
MIPS ECOFF object code
Directives for debugging information
Directives to override the ISA level
Directives for extending MIPS 16 bit instructions
Directive to mark data as an instruction
Directives to save and restore options
Hitachi SH Dependent Features
Options
Syntax
Special Characters
Register Names
Addressing Modes
Floating Point
SH Machine Directives
Opcodes
SPARC Dependent Features
Options
Enforcing aligned data
Floating Point
Sparc Machine Directives
Z8000 Dependent Features
Options
Syntax
Special Characters
Register Names
Addressing Modes
Assembler Directives for the Z8000
Opcodes
VAX Dependent Features
VAX Command-Line Options
VAX Floating Point
Vax Machine Directives
VAX Opcodes
VAX Branch Improvement
VAX Operands
Not Supported on VAX
v850 Dependent Features
Options
Syntax
Special Characters
Register Names
Floating Point
V850 Machine Directives
Opcodes
Reporting Bugs
Have you found a bug?
How to report bugs
Acknowledgements
Index
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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, 96, 97, 1998 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.
Overview
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[cdhlns][=file] ] [ -D ] [ --defsym sym=val ]
[ -f ] [ --gstabs ] [ --help ] [ -I dir ] [ -J ] [ -K ] [ -L ]
[ --keep-locals ] [ -o objfile ] [ -R ] [ --statistics ] [ -v ]
[ -version ] [ --version ] [ -W ] [ -w ] [ -x ] [ -Z ]
[ -mbig-endian | -mlittle-endian ]
[ -m[arm]1 | -m[arm]2 | -m[arm]250 | -m[arm]3 | -m[arm]6 | -m[arm]7[t][[d]m[i]] ]
[ -m[arm]v2 | -m[arm]v2a | -m[arm]v3 | -m[arm]v3m | -m[arm]v4 | -m[arm]v4t ]
[ -mthumb | -mall ]
[ -mfpa10 | -mfpa11 | -mfpe-old | -mno-fpu ]
[ -EB | -EL ]
[ -mapcs-32 | -mapcs-26 ]
[ -O ]
[ -Av6 | -Av7 | -Av8 | -Asparclet | -Asparclite
-Av8plus | -Av8plusa | -Av9 | -Av9a ]
[ -xarch=v8plus | -xarch=v8plusa ] [ -bump ] [ -32 | -64 ]
[ -ACA | -ACA_A | -ACB | -ACC | -AKA | -AKB | -AKC | -AMC ]
[ -b ] [ -no-relax ]
[ -l ] [ -m68000 | -m68010 | -m68020 | ... ]
[ -nocpp ] [ -EL ] [ -EB ] [ -G num ] [ -mcpu=CPU ]
[ -mips1 ] [ -mips2 ] [ -mips3 ] [ -m4650 ] [ -no-m4650 ]
[ --trap ] [ --break ]
[ --emulation=name ]
[ -- | files ... ]
-a[cdhlmns]
Turn on listings, in any of a variety of ways:
-ac
omit false conditionals
-ad
omit debugging directives
-ah
include high-level source
-al
include assembly
-am
include macro expansions
-an
omit forms processing
-as
include symbols
=file
set the name of the listing file
You may combine these options; for example, use `-aln' for assembly listing without forms processing. The `=file' option, if used, must be the last one. By itself, `-a' defaults to `-ahls'.
-D
Ignored. This option is accepted for script compatibility with calls to other assemblers.
--defsym sym=value
Define the symbol sym to be value before assembling the input file. value must be an integer constant. As in C, a leading `0x' indicates a hexadecimal value, and a leading `0' indicates an octal value.
-f
"fast"---skip whitespace and comment preprocessing (assume source is compiler output).
--gstabs
Generate stabs debugging information for each assembler line. This may help debugging assembler code, if the debugger can handle it.
--help
Print a summary of the command line options and exit.
-I dir
Add directory dir to the search list for .include directives.
-J
Don't warn about signed overflow.
-K
Issue warnings when difference tables altered for long displacements.
-L
--keep-locals
Keep (in the symbol table) local symbols. On traditional a.out systems these start with `L', but different systems have different local label prefixes.
-o objfile
Name the object-file output from as objfile.
-R
Fold the data section into the text section.
--statistics
Print the maximum space (in bytes) and total time (in seconds) used by assembly.
--strip-local-absolute
Remove local absolute symbols from the outgoing symbol table.
-v
-version
Print the as version.
--version
Print the as version and exit.
-W
Suppress warning messages.
-w
Ignored.
-x
Ignored.
-Z
Generate an object file even after errors.
-- | files ...
Standard input, or source files to assemble.
The following options are available when as is configured for an ARC processor.
-mbig-endian
Generate "big endian" format output.
-mlittle-endian
Generate "little endian" format output.
The following options are available when as is configured for the ARM processor family.
-m[arm]1 | -m[arm]2 | -m[arm]250 | -m[arm]3 | -m[arm]6 | -m[arm]7[t][[d]m] | -m[arm]v2 | -m[arm]v2a | -m[arm]v3 | -m[arm]v3m | -m[arm]v4 | -m[arm]v4t
Specify which variant of the ARM architecture is the target.
-mthumb | -mall
Enable or disable Thumb only instruction decoding.
-mfpa10 | -mfpa11 | -mfpe-old | -mno-fpu
Select which Floating Point architcture is the target.
-mapcs-32 | -mapcs-26
Select which procedure calling convention is in use.
-EB | -EL
Select either big-endian (-EB) or little-endian (-EL) output.
The following options are available when as is configured for a D10V processor.
-O
Optimize output by parallelizing instructions.
The following options are available when as is configured for the Intel 80960 processor.
-ACA | -ACA_A | -ACB | -ACC | -AKA | -AKB | -AKC | -AMC
Specify which variant of the 960 architecture is the target.
-b
Add code to collect statistics about branches taken.
-no-relax
Do not alter compare-and-branch instructions for long displacements; error if necessary.
The following options are available when as is configured for the Motorola 68000 series.
-l
Shorten references to undefined symbols, to one word instead of two.
-m68000 | -m68008 | -m68010 | -m68020 | -m68030 | -m68040 | -m68060
| -m68302 | -m68331 | -m68332 | -m68333 | -m68340 | -mcpu32 | -m5200
Specify what processor in the 68000 family is the target. The default is normally the 68020, but this can be changed at configuration time.
-m68881 | -m68882 | -mno-68881 | -mno-68882
The target machine does (or does not) have a floating-point coprocessor. The default is to assume a coprocessor for 68020, 68030, and cpu32. Although the basic 68000 is not compatible with the 68881, a combination of the two can be specified, since it's possible to do emulation of the coprocessor instructions with the main processor.
-m68851 | -mno-68851
The target machine does (or does not) have a memory-management unit coprocessor. The default is to assume an MMU for 68020 and up.
The following options are available when as is configured for the SPARC architecture:
-Av6 | -Av7 | -Av8 | -Asparclet | -Asparclite
-Av8plus | -Av8plusa | -Av9 | -Av9a
Explicitly select a variant of the SPARC architecture. `-Av8plus' and `-Av8plusa' select a 32 bit environment. `-Av9' and `-Av9a' select a 64 bit environment. `-Av8plusa' and `-Av9a' enable the SPARC V9 instruction set with UltraSPARC extensions.
-xarch=v8plus | -xarch=v8plusa
For compatibility with the Solaris v9 assembler. These options are equivalent to -Av8plus and -Av8plusa, respectively.
-bump
Warn when the assembler switches to another architecture.
The following options are available when as is configured for a MIPS processor.
-G num
This option sets the largest size of an object that can be referenced implicitly with the gp register. It is only accepted for targets that use ECOFF format, such as a DECstation running Ultrix. The default value is 8.
-EB
Generate "big endian" format output.
-EL
Generate "little endian" format output.
-mips1
-mips2
-mips3
Generate code for a particular MIPS Instruction Set Architecture level. `-mips1' corresponds to the R2000 and R3000 processors, `-mips2' to the R6000 processor, and `-mips3' to the R4000 processor.
-m4650
-no-m4650
Generate code for the MIPS R4650 chip. This tells the assembler to accept the `mad' and `madu' instruction, and to not schedule `nop' instructions around accesses to the `HI' and `LO' registers. `-no-m4650' turns off this option.
-mcpu=CPU
Generate code for a particular MIPS cpu. This has little effect on the assembler, but it is passed by gcc.
--emulation=name
This option causes as to emulate 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
Control how to deal with multiplication overflow and division by zero. `--trap' or `--no-break' (which are synonyms) take a trap exception (and only work for Instruction Set Architecture level 2 and higher); `--break' or `--no-trap' (also synonyms, and the default) take a break exception.
Structure of this Manual
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.
The GNU Assembler
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. Any exceptions are documented explicitly (see section Machine Dependent Features). This doesn't mean as always uses the same syntax as another assembler for the same architecture; for example, we know of several incompatible versions of 680x0 assembly language syntax.
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).
Object File Formats
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 a.out or COFF format object files. 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.
Command Line
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
Input Files
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.
Filenames and Line-numbers
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.
Output (Object) File
Every time you run as it produces an output file, which is your assembly language program translated into numbers. This file is the object file. Its default name is a.out, 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.
Error and Warning Messages
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) (see section .ln 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.
Command-Line Options
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.)
Enable Listings: -a[cdhlns]
These options enable listing output from the assembler. By itself, `-a' requests high-level, assembly, and symbols listing. You can use other letters to select specific options for the list: `-ah' requests a high-level language listing, `-al' requests an output-program assembly listing, and `-as' requests a symbol table listing. High-level listings require that a compiler debugging option like `-g' be used, and that assembly listings (`-al') be requested also.
Use the `-ac' option to omit false conditionals from a listing. Any lines which are not assembled because of a false .if (or .ifdef, or any other conditional), or a true .if followed by an .else, will be omitted from the listing.
Use the `-ad' option to omit debugging directives from the listing.
Once you have specified one of these options, you can further control listing output and its appearance using the directives .list, .nolist, .psize, .eject, .title, and .sbttl. The `-an' option turns off all forms processing. If you do not request listing output with one of the `-a' options, the listing-control directives have no effect.
The letters after `-a' may be combined into one option, e.g., `-aln'.
-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.
Work Faster: -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), as does 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.
Difference Tables: -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.
Include Local Labels: -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$'. `;' for the ARM family;
Assemble in MRI Compatibility Mode: -M
The -M or --mri option selects MRI compatibility mode. This changes the syntax and pseudo-op handling of as to make it compatible with the ASM68K or the ASM960 (depending upon the configured target) assembler from Microtec Research. The exact nature of the MRI syntax will not be documented here; see the MRI manuals for more information. Note in particular that the handling of macros and macro arguments is somewhat different. The purpose of this option is to permit assembling existing MRI assembler code using as.
The MRI compatibility is not complete. Certain operations of the MRI assembler depend upon its object file format, and can not be supported using other object file formats. Supporting these would require enhancing each object file format individually. These are:
global symbols in common section The m68k MRI assembler supports common sections which are merged by the linker. Other object file formats do not support this. as handles common sections by treating them as a single common symbol. It permits local symbols to be defined within a common section, but it can not support global symbols, since it has no way to describe them.
complex relocations The MRI assemblers support relocations against a negated section address, and relocations which combine the start addresses of two or more sections. These are not support by other object file formats.
END pseudo-op specifying start address The MRI END pseudo-op permits the specification of a start address. This is not supported by other object file formats. The start address may instead be specified using the -e option to the linker, or in a linker script.
IDNT, .ident and NAME pseudo-ops The MRI IDNT, .ident and NAME pseudo-ops assign a module name to the output file. This is not supported by other object file formats.
ORG pseudo-op The m68k MRI ORG pseudo-op begins an absolute section at a given address. This differs from the usual as .org pseudo-op, which changes the location within the current section. Absolute sections are not supported by other object file formats. The address of a section may be assigned within a linker script.
There are some other features of the MRI assembler which are not supported by as, typically either because they are difficult or because they seem of little consequence. Some of these may be supported in future releases.
EBCDIC strings EBCDIC strings are not supported.
packed binary coded decimal Packed binary coded decimal is not supported. This means that the DC.P and DCB.P pseudo-ops are not supported.
FEQU pseudo-op The m68k FEQU pseudo-op is not supported.
NOOBJ pseudo-op The m68k NOOBJ pseudo-op is not supported.
OPT branch control options The m68k OPT branch control options---B, BRS, BRB, BRL, and BRW---are ignored. as automatically relaxes all branches, whether forward or backward, to an appropriate size, so these options serve no purpose.
OPT list control options The following m68k OPT list control options are ignored: C, CEX, CL, CRE, E, G, I, M, MEX, MC, MD, X.
other OPT options The following m68k OPT options are ignored: NEST, O, OLD, OP, P, PCO, PCR, PCS, R.
OPT D option is default The m68k OPT D option is the default, unlike the MRI assembler. OPT NOD may be used to turn it off.
XREF pseudo-op. The m68k XREF pseudo-op is ignored.
.debug pseudo-op The i960 .debug pseudo-op is not supported.
.extended pseudo-op The i960 .extended pseudo-op is not supported.
.list pseudo-op. The various options of the i960 .list pseudo-op are not supported.
.optimize pseudo-op The i960 .optimize pseudo-op is not supported.
.output pseudo-op The i960 .output pseudo-op is not supported.
.setreal pseudo-op The i960 .setreal pseudo-op is not supported.
Dependency tracking: --MD
as can generate a dependency file for the file it creates. This file consists of a single rule suitable for make describing the dependencies of the main source file.
The rule is written to the file named in its argument.
This feature is used in the automatic updating of makefiles.
Name the Object File: -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.
Join Data and Text Sections: -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.
Display Assembly Statistics: --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).
Compatible output: --traditional-format
For some targets, the output of as is different in some ways from the output of some existing assembler. This switch requests as to use the traditional format instead.
For example, it disables the exception frame optimizations which as normally does by default on gcc output.
Announce Version: -v
You can find out what version of as is running by including the option `-v' (which you can also spell as `-version') on the command line.
Suppress Warnings: -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.
Generate Object File in Spite of Errors: -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.'
Syntax
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, except that as does not assemble Vax bit-fields.
Preprocessing
The as internal preprocessor:
adjusts and removes extra whitespace. It leaves one space or tab before the keywords on a line, and turns any other whitespace on the line into a single space.
removes all comments, replacing them with a single space, or an appropriate number of newlines.
converts character constants into the appropriate numeric values.
It does not do macro processing, include file handling, or anything else you may get from your C compiler's preprocessor. You can do include file processing with the .include directive (see 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
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.
Comments
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 `;' for the AMD 29K family; `;' on the ARC; `;' for the H8/300 family; `!' for the H8/500 family; `;' for the HPPA; `#' on the i960; `!' for the Hitachi SH; `!' on the SPARC; `#' on the m32r; `|' on the 680x0; `#' on the Vax; `!' for the Z8000; `#' on the V850; 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.
The V850 assembler also supports a double dash as starting a comment that extends to the end of the line.
`--';
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.
Symbols
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.
Statements
A statement ends at a newline character (`\n') or an "at" sign (`@'). The newline or at sign is considered part of the preceding statement. Newlines and at signs within character constants are an exception: they do not end statements. 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 (for the H8/300) a dollar sign (`$'); or (for the Hitachi-SH or the H8/500) a semicolon (`;'). 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. 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, ...
Constants
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.
Character Constants
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.
Strings
A string is written between double-quotes. It may contain double-quotes or null characters. The way to get special characters into a string is to escape these characters: precede them with a backslash `\' character. For example `\\' represents one backslash: the first \ is an escape which tells as to interpret the second character literally as a backslash (which prevents as from recognizing the second \ as an escape character). The complete list of escapes follows.
\b
Mnemonic for backspace; for ASCII this is octal code 010.
\f
Mnemonic for FormFeed; for ASCII this is octal code 014.
\n
Mnemonic for newline; for ASCII this is octal code 012.
\r
Mnemonic for carriage-Return; for ASCII this is octal code 015.
\t
Mnemonic for horizontal Tab; for ASCII this is octal code 011.
\ digit digit digit
An octal character code. The numeric code is 3 octal digits. For compatibility with other Unix systems, 8 and 9 are accepted as digits: for example, \008 has the value 010, and \009 the value 011.
\x hex-digits...
A hex character code. All trailing hex digits are combined. Either upper or lower case x works.
\\
Represents one `\' character.
\"
Represents one `"' character. Needed in strings to represent this character, because an unescaped `"' would end the string.
\ anything-else
Any other character when escaped by \ gives a warning, but assembles as if the `\' was not present. The idea is that if you used an escape sequence you clearly didn't want the literal interpretation of the following character. However as has no other interpretation, so as knows it is giving you the wrong code and warns you of the fact.
Which characters are escapable, and what those escapes represent, varies widely among assemblers. The current set is what we think the BSD 4.2 assembler recognizes, and is a subset of what most C compilers recognize. If you are in doubt, do not use an escape sequence.
Characters
A single character may be written as a single quote immediately followed by that character. The same escapes apply to characters as to strings. So if you want to write the character backslash, you must write '\\ where the first \ escapes the second \. As you can see, the quote is an acute accent, not a grave accent. A newline (or at sign `@') (or dollar sign `$', for the H8/300; or semicolon `;' for the Hitachi SH or H8/500) 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.
Number Constants
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.
Integers
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).
Bignums
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.
Flonums
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)
The digit `0'. (`0' is optional on the HPPA.)
A letter, to tell 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 ARC, the letter must be one of the letters `DFRS' (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).
An optional sign: either `+' or `-'.
An optional integer part: zero or more decimal digits.
An optional fractional part: `.' followed by zero or more decimal digits.
An optional exponent, consisting of:
An `E' or `e'.
Optional sign: either `+' or `-'.
One or more decimal digits.
At least one of the integer part or the fractional part must be present. The floating point number has the usual base-10 value.
as does all processing using integers. Flonums are computed independently of any floating point hardware in the computer running as.
Sections and Relocation
Background
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. For the H8/300 and H8/500, and for the Hitachi SH, as pads sections if needed to ensure they end on a word (sixteen bit) boundary.
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). 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:
Where in the object file is the beginning of this reference to an address?
How long (in bytes) is this reference?
Which section does the address refer to? What is the numeric value of
(address) - (start-address of section)?
Is the reference to an address "Program-Counter relative"?
In fact, every address as ever uses is expressed as
(section) + (offset into section)
Further, most expressions as computes have this section-relative nature. (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.
Linker Sections
ld deals with just four kinds of sections, summarized below.
named sections
text section
data section
These sections hold your program. 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.
bss section
This section contains zeroed bytes when your program begins running. It is used to hold unitialized variables or common storage. The length of each partial program's bss section is important, but because it starts out containing zeroed bytes there is no need to store explicit zero bytes in the object file. The bss section was invented to eliminate those explicit zeros from object files.
absolute section
Address 0 of this section is always "relocated" to runtime address 0. This is useful if you want to refer to an address that ld must not change when relocating. In this sense we speak of absolute addresses being "unrelocatable": they do not change during relocation.
undefined section
This "section" is a catch-all for address references to objects not in the preceding sections.
An idealized example of three relocatable sections follows. The example uses the traditional section names `.text' and `.data'. Memory addresses are on the horizontal axis.
Assembler Internal Sections
These sections are meant only for the internal use of as. They have no meaning at run-time. You do not really need to know about these sections for most purposes; but they can be mentioned in as warning messages, so it might be helpful to have an idea of their meanings to as. These sections are used to permit the value of every expression in your assembly language program to be a section-relative address.
ASSEMBLER-INTERNAL-LOGIC-ERROR!
An internal assembler logic error has been found. This means there is a bug in the assembler.
expr section
The assembler stores complex expression internally as combinations of symbols. When it needs to represent an expression as a symbol, it puts it in the expr section.
Sub-Sections
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.
bss Section
The bss section is used for local common variable storage. You may allocate address space in the bss section, but you may not dictate data to load into it before your program executes. When your program starts running, all the contents of the bss section are zeroed bytes.
The .lcomm pseudo-op defines a symbol in the bss section; see section .lcomm symbol , length.
The .comm pseudo-op may be used to declare a common symbol, which is another form of uninitialized symbol; see See section .comm symbol , length .
When assembling for a target which supports multiple sections, such as ELF or COFF, you may switch into the .bss section and define symbols as usual; see section .section name. You may only assemble zero values into the section. Typically the section will only contain symbol definitions and .skip directives (see section .skip size , fill).
Symbols
Symbols are a central concept: the programmer uses symbols to name things, the linker uses symbols to link, and the debugger uses symbols to debug.
Warning: as does not place symbols in the object file in the same order they were declared. This may break some debuggers.
Labels
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.
Giving Symbols Other Values
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
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. For the AMD 29K family, `?' is also allowed in the body of a symbol name, though not at its beginning.
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 Symbol Names
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
All local labels begin with `L'. Normally both as and ld forget symbols that start with `L'. These labels are used for symbols you are never intended to see. If you use the `-L' option 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
If the label is written `0:' then the digit is `0'. If the label is written `1:' then the digit is `1'. And so on up through `9:'.
C-A
This unusual character is included so you do not accidentally invent a symbol of the same name. The character has ASCII value `\001'.
ordinal number
This is a serial number to keep the labels distinct. The first `0:' gets the number `1'; The 15th `0:' gets the number `15'; etc.. Likewise for the other labels `1:' through `9:'.
For instance, the first 1: is named L1C-A1, the 44th 3: is named L3C-A44.
The Special Dot Symbol
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'.
Symbol Attributes
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.
Value
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.
Type
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.
Symbol Attributes: a.out
Descriptor
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.
Other
This is an arbitrary 8-bit value. It means nothing to as.
Symbol Attributes for COFF
The COFF format supports a multitude of auxiliary symbol attributes; like the primary symbol attributes, they are set between .def and .endef directives.
Primary Attributes
The symbol name is set with .def; the value and type, respectively, with .val and .type.
Auxiliary Attributes
The as directives .dim, .line, .scl, .size, and .tag can generate auxiliary symbol table information for CO
Using as
The GNU Assembler
January 1994
Dean Elsner, Jay Fenlason & friends
--------------------------------------------------------------------------------
Table of Contents
Overview
Structure of this Manual
The GNU Assembler
Object File Formats
Command Line
Input Files
Output (Object) File
Error and Warning Messages
Command-Line Options
Enable Listings: -a[cdhlns]
-D
Work Faster: -f
.include search path: -I path
Difference Tables: -K
Include Local Labels: -L
Assemble in MRI Compatibility Mode: -M
Dependency tracking: --MD
Name the Object File: -o
Join Data and Text Sections: -R
Display Assembly Statistics: --statistics
Compatible output: --traditional-format
Announce Version: -v
Suppress Warnings: -W
Generate Object File in Spite of Errors: -Z
Syntax
Preprocessing
Whitespace
Comments
Symbols
Statements
Constants
Character Constants
Strings
Characters
Number Constants
Integers
Bignums
Flonums
Sections and Relocation
Background
Linker Sections
Assembler Internal Sections
Sub-Sections
bss Section
Symbols
Labels
Giving Symbols Other Values
Symbol Names
The Special Dot Symbol
Symbol Attributes
Value
Type
Symbol Attributes: a.out
Descriptor
Other
Symbol Attributes for COFF
Primary Attributes
Auxiliary Attributes
Symbol Attributes for SOM
Expressions
Empty Expressions
Integer Expressions
Arguments
Operators
Prefix Operator
Infix Operators
Assembler Directives
.abort
.ABORT
.align abs-expr, abs-expr, abs-expr
.app-file string
.ascii "string"...
.asciz "string"...
.balign[wl] abs-expr, abs-expr, abs-expr
.byte expressions
.comm symbol , length
.data subsection
.def name
.desc symbol, abs-expression
.dim
.double flonums
.eject
.else
.endef
.endif
.equ symbol, expression
.equiv symbol, expression
.err
.extern
.file string
.fill repeat , size , value
.float flonums
.global symbol, .globl symbol
.hword expressions
.ident
.if absolute expression
.include "file"
.int expressions
.irp symbol,values...
.irpc symbol,values...
.lcomm symbol , length
.lflags
.line line-number
.linkonce [type]
.ln line-number
.mri val
.list
.long expressions
.macro
.nolist
.octa bignums
.org new-lc , fill
.p2align[wl] abs-expr, abs-expr, abs-expr
.psize lines , columns
.quad bignums
.rept count
.sbttl "subheading"
.scl class
.section name
.set symbol, expression
.short expressions
.single flonums
.size
.sleb128 expressions
.skip size , fill
.space size , fill
.stabd, .stabn, .stabs
.string "str"
.symver
.tag structname
.text subsection
.title "heading"
.type int
.val addr
.uleb128 expressions
.word expressions
Deprecated Directives
Machine Dependent Features
ARC Dependent Features
Options
Floating Point
ARC Machine Directives
AMD 29K Dependent Features
Options
Syntax
Macros
Special Characters
Register Names
Floating Point
AMD 29K Machine Directives
Opcodes
ARM Dependent Features
Options
Syntax
Special Characters
Register Names
Floating Point
ARM Machine Directives
Opcodes
D10V Dependent Features
D10V Options
Syntax
Size Modifiers
Sub-Instructions
Special Characters
Register Names
Addressing Modes
@WORD Modifier
Floating Point
Opcodes
H8/300 Dependent Features
Options
Syntax
Special Characters
Register Names
Addressing Modes
Floating Point
H8/300 Machine Directives
Opcodes
H8/500 Dependent Features
Options
Syntax
Special Characters
Register Names
Addressing Modes
Floating Point
H8/500 Machine Directives
Opcodes
HPPA Dependent Features
Notes
Options
Syntax
Floating Point
HPPA Assembler Directives
Opcodes
80386 Dependent Features
Options
AT&T Syntax versus Intel Syntax
Opcode Naming
Register Naming
Opcode Prefixes
Memory References
Handling of Jump Instructions
Floating Point
Writing 16-bit Code
Notes
Intel 80960 Dependent Features
i960 Command-line Options
Floating Point
i960 Machine Directives
i960 Opcodes
callj
Compare-and-Branch
M680x0 Dependent Features
M680x0 Options
Syntax
Motorola Syntax
Floating Point
680x0 Machine Directives
Opcodes
Branch Improvement
Special Characters
MIPS Dependent Features
Assembler options
MIPS ECOFF object code
Directives for debugging information
Directives to override the ISA level
Directives for extending MIPS 16 bit instructions
Directive to mark data as an instruction
Directives to save and restore options
Hitachi SH Dependent Features
Options
Syntax
Special Characters
Register Names
Addressing Modes
Floating Point
SH Machine Directives
Opcodes
SPARC Dependent Features
Options
Enforcing aligned data
Floating Point
Sparc Machine Directives
Z8000 Dependent Features
Options
Syntax
Special Characters
Register Names
Addressing Modes
Assembler Directives for the Z8000
Opcodes
VAX Dependent Features
VAX Command-Line Options
VAX Floating Point
Vax Machine Directives
VAX Opcodes
VAX Branch Improvement
VAX Operands
Not Supported on VAX
v850 Dependent Features
Options
Syntax
Special Characters
Register Names
Floating Point
V850 Machine Directives
Opcodes
Reporting Bugs
Have you found a bug?
How to report bugs
Acknowledgements
Index
--------------------------------------------------------------------------------
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, 96, 97, 1998 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.
Overview
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[cdhlns][=file] ] [ -D ] [ --defsym sym=val ]
[ -f ] [ --gstabs ] [ --help ] [ -I dir ] [ -J ] [ -K ] [ -L ]
[ --keep-locals ] [ -o objfile ] [ -R ] [ --statistics ] [ -v ]
[ -version ] [ --version ] [ -W ] [ -w ] [ -x ] [ -Z ]
[ -mbig-endian | -mlittle-endian ]
[ -m[arm]1 | -m[arm]2 | -m[arm]250 | -m[arm]3 | -m[arm]6 | -m[arm]7[t][[d]m[i]] ]
[ -m[arm]v2 | -m[arm]v2a | -m[arm]v3 | -m[arm]v3m | -m[arm]v4 | -m[arm]v4t ]
[ -mthumb | -mall ]
[ -mfpa10 | -mfpa11 | -mfpe-old | -mno-fpu ]
[ -EB | -EL ]
[ -mapcs-32 | -mapcs-26 ]
[ -O ]
[ -Av6 | -Av7 | -Av8 | -Asparclet | -Asparclite
-Av8plus | -Av8plusa | -Av9 | -Av9a ]
[ -xarch=v8plus | -xarch=v8plusa ] [ -bump ] [ -32 | -64 ]
[ -ACA | -ACA_A | -ACB | -ACC | -AKA | -AKB | -AKC | -AMC ]
[ -b ] [ -no-relax ]
[ -l ] [ -m68000 | -m68010 | -m68020 | ... ]
[ -nocpp ] [ -EL ] [ -EB ] [ -G num ] [ -mcpu=CPU ]
[ -mips1 ] [ -mips2 ] [ -mips3 ] [ -m4650 ] [ -no-m4650 ]
[ --trap ] [ --break ]
[ --emulation=name ]
[ -- | files ... ]
-a[cdhlmns]
Turn on listings, in any of a variety of ways:
-ac
omit false conditionals
-ad
omit debugging directives
-ah
include high-level source
-al
include assembly
-am
include macro expansions
-an
omit forms processing
-as
include symbols
=file
set the name of the listing file
You may combine these options; for example, use `-aln' for assembly listing without forms processing. The `=file' option, if used, must be the last one. By itself, `-a' defaults to `-ahls'.
-D
Ignored. This option is accepted for script compatibility with calls to other assemblers.
--defsym sym=value
Define the symbol sym to be value before assembling the input file. value must be an integer constant. As in C, a leading `0x' indicates a hexadecimal value, and a leading `0' indicates an octal value.
-f
"fast"---skip whitespace and comment preprocessing (assume source is compiler output).
--gstabs
Generate stabs debugging information for each assembler line. This may help debugging assembler code, if the debugger can handle it.
--help
Print a summary of the command line options and exit.
-I dir
Add directory dir to the search list for .include directives.
-J
Don't warn about signed overflow.
-K
Issue warnings when difference tables altered for long displacements.
-L
--keep-locals
Keep (in the symbol table) local symbols. On traditional a.out systems these start with `L', but different systems have different local label prefixes.
-o objfile
Name the object-file output from as objfile.
-R
Fold the data section into the text section.
--statistics
Print the maximum space (in bytes) and total time (in seconds) used by assembly.
--strip-local-absolute
Remove local absolute symbols from the outgoing symbol table.
-v
-version
Print the as version.
--version
Print the as version and exit.
-W
Suppress warning messages.
-w
Ignored.
-x
Ignored.
-Z
Generate an object file even after errors.
-- | files ...
Standard input, or source files to assemble.
The following options are available when as is configured for an ARC processor.
-mbig-endian
Generate "big endian" format output.
-mlittle-endian
Generate "little endian" format output.
The following options are available when as is configured for the ARM processor family.
-m[arm]1 | -m[arm]2 | -m[arm]250 | -m[arm]3 | -m[arm]6 | -m[arm]7[t][[d]m] | -m[arm]v2 | -m[arm]v2a | -m[arm]v3 | -m[arm]v3m | -m[arm]v4 | -m[arm]v4t
Specify which variant of the ARM architecture is the target.
-mthumb | -mall
Enable or disable Thumb only instruction decoding.
-mfpa10 | -mfpa11 | -mfpe-old | -mno-fpu
Select which Floating Point architcture is the target.
-mapcs-32 | -mapcs-26
Select which procedure calling convention is in use.
-EB | -EL
Select either big-endian (-EB) or little-endian (-EL) output.
The following options are available when as is configured for a D10V processor.
-O
Optimize output by parallelizing instructions.
The following options are available when as is configured for the Intel 80960 processor.
-ACA | -ACA_A | -ACB | -ACC | -AKA | -AKB | -AKC | -AMC
Specify which variant of the 960 architecture is the target.
-b
Add code to collect statistics about branches taken.
-no-relax
Do not alter compare-and-branch instructions for long displacements; error if necessary.
The following options are available when as is configured for the Motorola 68000 series.
-l
Shorten references to undefined symbols, to one word instead of two.
-m68000 | -m68008 | -m68010 | -m68020 | -m68030 | -m68040 | -m68060
| -m68302 | -m68331 | -m68332 | -m68333 | -m68340 | -mcpu32 | -m5200
Specify what processor in the 68000 family is the target. The default is normally the 68020, but this can be changed at configuration time.
-m68881 | -m68882 | -mno-68881 | -mno-68882
The target machine does (or does not) have a floating-point coprocessor. The default is to assume a coprocessor for 68020, 68030, and cpu32. Although the basic 68000 is not compatible with the 68881, a combination of the two can be specified, since it's possible to do emulation of the coprocessor instructions with the main processor.
-m68851 | -mno-68851
The target machine does (or does not) have a memory-management unit coprocessor. The default is to assume an MMU for 68020 and up.
The following options are available when as is configured for the SPARC architecture:
-Av6 | -Av7 | -Av8 | -Asparclet | -Asparclite
-Av8plus | -Av8plusa | -Av9 | -Av9a
Explicitly select a variant of the SPARC architecture. `-Av8plus' and `-Av8plusa' select a 32 bit environment. `-Av9' and `-Av9a' select a 64 bit environment. `-Av8plusa' and `-Av9a' enable the SPARC V9 instruction set with UltraSPARC extensions.
-xarch=v8plus | -xarch=v8plusa
For compatibility with the Solaris v9 assembler. These options are equivalent to -Av8plus and -Av8plusa, respectively.
-bump
Warn when the assembler switches to another architecture.
The following options are available when as is configured for a MIPS processor.
-G num
This option sets the largest size of an object that can be referenced implicitly with the gp register. It is only accepted for targets that use ECOFF format, such as a DECstation running Ultrix. The default value is 8.
-EB
Generate "big endian" format output.
-EL
Generate "little endian" format output.
-mips1
-mips2
-mips3
Generate code for a particular MIPS Instruction Set Architecture level. `-mips1' corresponds to the R2000 and R3000 processors, `-mips2' to the R6000 processor, and `-mips3' to the R4000 processor.
-m4650
-no-m4650
Generate code for the MIPS R4650 chip. This tells the assembler to accept the `mad' and `madu' instruction, and to not schedule `nop' instructions around accesses to the `HI' and `LO' registers. `-no-m4650' turns off this option.
-mcpu=CPU
Generate code for a particular MIPS cpu. This has little effect on the assembler, but it is passed by gcc.
--emulation=name
This option causes as to emulate 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
Control how to deal with multiplication overflow and division by zero. `--trap' or `--no-break' (which are synonyms) take a trap exception (and only work for Instruction Set Architecture level 2 and higher); `--break' or `--no-trap' (also synonyms, and the default) take a break exception.
Structure of this Manual
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.
The GNU Assembler
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. Any exceptions are documented explicitly (see section Machine Dependent Features). This doesn't mean as always uses the same syntax as another assembler for the same architecture; for example, we know of several incompatible versions of 680x0 assembly language syntax.
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).
Object File Formats
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 a.out or COFF format object files. 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.
Command Line
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
Input Files
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.
Filenames and Line-numbers
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.
Output (Object) File
Every time you run as it produces an output file, which is your assembly language program translated into numbers. This file is the object file. Its default name is a.out, 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.
Error and Warning Messages
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) (see section .ln 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.
Command-Line Options
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.)
Enable Listings: -a[cdhlns]
These options enable listing output from the assembler. By itself, `-a' requests high-level, assembly, and symbols listing. You can use other letters to select specific options for the list: `-ah' requests a high-level language listing, `-al' requests an output-program assembly listing, and `-as' requests a symbol table listing. High-level listings require that a compiler debugging option like `-g' be used, and that assembly listings (`-al') be requested also.
Use the `-ac' option to omit false conditionals from a listing. Any lines which are not assembled because of a false .if (or .ifdef, or any other conditional), or a true .if followed by an .else, will be omitted from the listing.
Use the `-ad' option to omit debugging directives from the listing.
Once you have specified one of these options, you can further control listing output and its appearance using the directives .list, .nolist, .psize, .eject, .title, and .sbttl. The `-an' option turns off all forms processing. If you do not request listing output with one of the `-a' options, the listing-control directives have no effect.
The letters after `-a' may be combined into one option, e.g., `-aln'.
-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.
Work Faster: -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), as does 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.
Difference Tables: -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.
Include Local Labels: -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$'. `;' for the ARM family;
Assemble in MRI Compatibility Mode: -M
The -M or --mri option selects MRI compatibility mode. This changes the syntax and pseudo-op handling of as to make it compatible with the ASM68K or the ASM960 (depending upon the configured target) assembler from Microtec Research. The exact nature of the MRI syntax will not be documented here; see the MRI manuals for more information. Note in particular that the handling of macros and macro arguments is somewhat different. The purpose of this option is to permit assembling existing MRI assembler code using as.
The MRI compatibility is not complete. Certain operations of the MRI assembler depend upon its object file format, and can not be supported using other object file formats. Supporting these would require enhancing each object file format individually. These are:
global symbols in common section The m68k MRI assembler supports common sections which are merged by the linker. Other object file formats do not support this. as handles common sections by treating them as a single common symbol. It permits local symbols to be defined within a common section, but it can not support global symbols, since it has no way to describe them.
complex relocations The MRI assemblers support relocations against a negated section address, and relocations which combine the start addresses of two or more sections. These are not support by other object file formats.
END pseudo-op specifying start address The MRI END pseudo-op permits the specification of a start address. This is not supported by other object file formats. The start address may instead be specified using the -e option to the linker, or in a linker script.
IDNT, .ident and NAME pseudo-ops The MRI IDNT, .ident and NAME pseudo-ops assign a module name to the output file. This is not supported by other object file formats.
ORG pseudo-op The m68k MRI ORG pseudo-op begins an absolute section at a given address. This differs from the usual as .org pseudo-op, which changes the location within the current section. Absolute sections are not supported by other object file formats. The address of a section may be assigned within a linker script.
There are some other features of the MRI assembler which are not supported by as, typically either because they are difficult or because they seem of little consequence. Some of these may be supported in future releases.
EBCDIC strings EBCDIC strings are not supported.
packed binary coded decimal Packed binary coded decimal is not supported. This means that the DC.P and DCB.P pseudo-ops are not supported.
FEQU pseudo-op The m68k FEQU pseudo-op is not supported.
NOOBJ pseudo-op The m68k NOOBJ pseudo-op is not supported.
OPT branch control options The m68k OPT branch control options---B, BRS, BRB, BRL, and BRW---are ignored. as automatically relaxes all branches, whether forward or backward, to an appropriate size, so these options serve no purpose.
OPT list control options The following m68k OPT list control options are ignored: C, CEX, CL, CRE, E, G, I, M, MEX, MC, MD, X.
other OPT options The following m68k OPT options are ignored: NEST, O, OLD, OP, P, PCO, PCR, PCS, R.
OPT D option is default The m68k OPT D option is the default, unlike the MRI assembler. OPT NOD may be used to turn it off.
XREF pseudo-op. The m68k XREF pseudo-op is ignored.
.debug pseudo-op The i960 .debug pseudo-op is not supported.
.extended pseudo-op The i960 .extended pseudo-op is not supported.
.list pseudo-op. The various options of the i960 .list pseudo-op are not supported.
.optimize pseudo-op The i960 .optimize pseudo-op is not supported.
.output pseudo-op The i960 .output pseudo-op is not supported.
.setreal pseudo-op The i960 .setreal pseudo-op is not supported.
Dependency tracking: --MD
as can generate a dependency file for the file it creates. This file consists of a single rule suitable for make describing the dependencies of the main source file.
The rule is written to the file named in its argument.
This feature is used in the automatic updating of makefiles.
Name the Object File: -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.
Join Data and Text Sections: -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.
Display Assembly Statistics: --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).
Compatible output: --traditional-format
For some targets, the output of as is different in some ways from the output of some existing assembler. This switch requests as to use the traditional format instead.
For example, it disables the exception frame optimizations which as normally does by default on gcc output.
Announce Version: -v
You can find out what version of as is running by including the option `-v' (which you can also spell as `-version') on the command line.
Suppress Warnings: -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.
Generate Object File in Spite of Errors: -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.'
Syntax
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, except that as does not assemble Vax bit-fields.
Preprocessing
The as internal preprocessor:
adjusts and removes extra whitespace. It leaves one space or tab before the keywords on a line, and turns any other whitespace on the line into a single space.
removes all comments, replacing them with a single space, or an appropriate number of newlines.
converts character constants into the appropriate numeric values.
It does not do macro processing, include file handling, or anything else you may get from your C compiler's preprocessor. You can do include file processing with the .include directive (see 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
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.
Comments
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 `;' for the AMD 29K family; `;' on the ARC; `;' for the H8/300 family; `!' for the H8/500 family; `;' for the HPPA; `#' on the i960; `!' for the Hitachi SH; `!' on the SPARC; `#' on the m32r; `|' on the 680x0; `#' on the Vax; `!' for the Z8000; `#' on the V850; 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.
The V850 assembler also supports a double dash as starting a comment that extends to the end of the line.
`--';
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.
Symbols
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.
Statements
A statement ends at a newline character (`\n') or an "at" sign (`@'). The newline or at sign is considered part of the preceding statement. Newlines and at signs within character constants are an exception: they do not end statements. 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 (for the H8/300) a dollar sign (`$'); or (for the Hitachi-SH or the H8/500) a semicolon (`;'). 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. 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, ...
Constants
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.
Character Constants
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.
Strings
A string is written between double-quotes. It may contain double-quotes or null characters. The way to get special characters into a string is to escape these characters: precede them with a backslash `\' character. For example `\\' represents one backslash: the first \ is an escape which tells as to interpret the second character literally as a backslash (which prevents as from recognizing the second \ as an escape character). The complete list of escapes follows.
\b
Mnemonic for backspace; for ASCII this is octal code 010.
\f
Mnemonic for FormFeed; for ASCII this is octal code 014.
\n
Mnemonic for newline; for ASCII this is octal code 012.
\r
Mnemonic for carriage-Return; for ASCII this is octal code 015.
\t
Mnemonic for horizontal Tab; for ASCII this is octal code 011.
\ digit digit digit
An octal character code. The numeric code is 3 octal digits. For compatibility with other Unix systems, 8 and 9 are accepted as digits: for example, \008 has the value 010, and \009 the value 011.
\x hex-digits...
A hex character code. All trailing hex digits are combined. Either upper or lower case x works.
\\
Represents one `\' character.
\"
Represents one `"' character. Needed in strings to represent this character, because an unescaped `"' would end the string.
\ anything-else
Any other character when escaped by \ gives a warning, but assembles as if the `\' was not present. The idea is that if you used an escape sequence you clearly didn't want the literal interpretation of the following character. However as has no other interpretation, so as knows it is giving you the wrong code and warns you of the fact.
Which characters are escapable, and what those escapes represent, varies widely among assemblers. The current set is what we think the BSD 4.2 assembler recognizes, and is a subset of what most C compilers recognize. If you are in doubt, do not use an escape sequence.
Characters
A single character may be written as a single quote immediately followed by that character. The same escapes apply to characters as to strings. So if you want to write the character backslash, you must write '\\ where the first \ escapes the second \. As you can see, the quote is an acute accent, not a grave accent. A newline (or at sign `@') (or dollar sign `$', for the H8/300; or semicolon `;' for the Hitachi SH or H8/500) 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.
Number Constants
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.
Integers
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).
Bignums
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.
Flonums
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)
The digit `0'. (`0' is optional on the HPPA.)
A letter, to tell 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 ARC, the letter must be one of the letters `DFRS' (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).
An optional sign: either `+' or `-'.
An optional integer part: zero or more decimal digits.
An optional fractional part: `.' followed by zero or more decimal digits.
An optional exponent, consisting of:
An `E' or `e'.
Optional sign: either `+' or `-'.
One or more decimal digits.
At least one of the integer part or the fractional part must be present. The floating point number has the usual base-10 value.
as does all processing using integers. Flonums are computed independently of any floating point hardware in the computer running as.
Sections and Relocation
Background
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. For the H8/300 and H8/500, and for the Hitachi SH, as pads sections if needed to ensure they end on a word (sixteen bit) boundary.
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). 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:
Where in the object file is the beginning of this reference to an address?
How long (in bytes) is this reference?
Which section does the address refer to? What is the numeric value of
(address) - (start-address of section)?
Is the reference to an address "Program-Counter relative"?
In fact, every address as ever uses is expressed as
(section) + (offset into section)
Further, most expressions as computes have this section-relative nature. (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.
Linker Sections
ld deals with just four kinds of sections, summarized below.
named sections
text section
data section
These sections hold your program. 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.
bss section
This section contains zeroed bytes when your program begins running. It is used to hold unitialized variables or common storage. The length of each partial program's bss section is important, but because it starts out containing zeroed bytes there is no need to store explicit zero bytes in the object file. The bss section was invented to eliminate those explicit zeros from object files.
absolute section
Address 0 of this section is always "relocated" to runtime address 0. This is useful if you want to refer to an address that ld must not change when relocating. In this sense we speak of absolute addresses being "unrelocatable": they do not change during relocation.
undefined section
This "section" is a catch-all for address references to objects not in the preceding sections.
An idealized example of three relocatable sections follows. The example uses the traditional section names `.text' and `.data'. Memory addresses are on the horizontal axis.
Assembler Internal Sections
These sections are meant only for the internal use of as. They have no meaning at run-time. You do not really need to know about these sections for most purposes; but they can be mentioned in as warning messages, so it might be helpful to have an idea of their meanings to as. These sections are used to permit the value of every expression in your assembly language program to be a section-relative address.
ASSEMBLER-INTERNAL-LOGIC-ERROR!
An internal assembler logic error has been found. This means there is a bug in the assembler.
expr section
The assembler stores complex expression internally as combinations of symbols. When it needs to represent an expression as a symbol, it puts it in the expr section.
Sub-Sections
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.
bss Section
The bss section is used for local common variable storage. You may allocate address space in the bss section, but you may not dictate data to load into it before your program executes. When your program starts running, all the contents of the bss section are zeroed bytes.
The .lcomm pseudo-op defines a symbol in the bss section; see section .lcomm symbol , length.
The .comm pseudo-op may be used to declare a common symbol, which is another form of uninitialized symbol; see See section .comm symbol , length .
When assembling for a target which supports multiple sections, such as ELF or COFF, you may switch into the .bss section and define symbols as usual; see section .section name. You may only assemble zero values into the section. Typically the section will only contain symbol definitions and .skip directives (see section .skip size , fill).
Symbols
Symbols are a central concept: the programmer uses symbols to name things, the linker uses symbols to link, and the debugger uses symbols to debug.
Warning: as does not place symbols in the object file in the same order they were declared. This may break some debuggers.
Labels
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.
Giving Symbols Other Values
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
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. For the AMD 29K family, `?' is also allowed in the body of a symbol name, though not at its beginning.
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 Symbol Names
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
All local labels begin with `L'. Normally both as and ld forget symbols that start with `L'. These labels are used for symbols you are never intended to see. If you use the `-L' option 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
If the label is written `0:' then the digit is `0'. If the label is written `1:' then the digit is `1'. And so on up through `9:'.
C-A
This unusual character is included so you do not accidentally invent a symbol of the same name. The character has ASCII value `\001'.
ordinal number
This is a serial number to keep the labels distinct. The first `0:' gets the number `1'; The 15th `0:' gets the number `15'; etc.. Likewise for the other labels `1:' through `9:'.
For instance, the first 1: is named L1C-A1, the 44th 3: is named L3C-A44.
The Special Dot Symbol
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'.
Symbol Attributes
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.
Value
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.
Type
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.
Symbol Attributes: a.out
Descriptor
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.
Other
This is an arbitrary 8-bit value. It means nothing to as.
Symbol Attributes for COFF
The COFF format supports a multitude of auxiliary symbol attributes; like the primary symbol attributes, they are set between .def and .endef directives.
Primary Attributes
The symbol name is set with .def; the value and type, respectively, with .val and .type.
Auxiliary Attributes
The as directives .dim, .line, .scl, .size, and .tag can generate auxiliary symbol table information for CO
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