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# 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 ]
     [ -O | -n | -N ]
     [ -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 a D30V processor.

`-O`
  Optimize output by parallelizing instructions.
`-n`
  Warn when nops are generated.
`-N`
  Warn when a nop after a 32-bit multiply instruction is generated.

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.

- Manual: Structure of this Manual
- GNU Assembler: The GNU Assembler
- Object Formats: Object File Formats
- Command Line: Command Line
- Input Files: Input Files
- Object: Output (Object) File
- Errors: Error and Warning Messages

## 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. `as` understands the ``#'` directives emitted by the `gcc` preprocessor. See also section `.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 `.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.

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