Linux as command

Updated: 11/06/2021 by Computer Hope
as command

On Linux operating systems, The as command launches the GNU assembler.

Description

The as program is 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. 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.

Each time you run as it assembles exactly one source program. The source program is made up of one or more files. The standard input is also a file.

You give as a command line that has zero or more input file names. The input files are read (from left file name to right). A command line argument (in any position) that has no special meaning is taken to be an input file name.

If you give as no file names it attempts to read one input file from the as standard input, which is normally your terminal. You may have to type ctrl-D to tell as there is no more program to assemble.

Use "--" if you need to explicitly name the standard input file in your command line.

If the source is empty, as produces a small, empty object file.

The 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.

If you are invoking as via the GNU C compiler, you can use the -Wa option to pass arguments through to the assembler. The assembler arguments must be separated from each other (and the -Wa) by commas. For example:

gcc -c -g -O -Wa,-alh,-L file.c

This passes two options to the assembler: -alh (emit a listing to standard output with high-level and assembly source) and -L (retain local symbols in the symbol table).

Usually you do not need to use this -Wa mechanism, since many compiler command-line options are automatically passed to the assembler by the compiler. You can call the GNU compiler driver with the -v option to see precisely what options it passes to each compilation pass, including the assembler.

Syntax

as [-a[cdghlns][=file]] [--alternate] [-D] [--compress-debug-sections] 
   [--nocompress-debug-sections] [--debug-prefix-map old=new] 
   [--defsym sym=val] [-f] [-g] [--gstabs] [--gstabs+] [--gdwarf-2] 
   [--help] [-I dir] [-J] [-K] [-L] [--listing-lhs-width=NUM] 
   [--listing-lhs-width2=NUM] [--listing-rhs-width=NUM] 
   [--listing-cont-lines=NUM] [--keep-locals] [-o objfile] [-R] 
   [--reduce-memory-overheads] [--statistics] [-v] [-version] 
   [--version] [-W] [--warn] [--fatal-warnings] [-w] [-x] [-Z] 
   [@FILE] [--size-check=[error|warning]] [--target-help] 
   [target-options] [--|files ...]

Arch Syntax

AArch64:

[-EB|-EL] [-mabi=ABI]

Alpha:

[-mcpu] [-mdebug | -no-mdebug] [-replace | -noreplace] [-relax] [-g] 
[-Gsize] [-F] [-32addr]

ARC:

[-marc[5|6|7|8]] [-EB|-EL]

ARM:

[-mcpu=processor[+extension...]] [-march=architecture[+extension...]] 
[-mfpu=floating-point-format] [-mfloat-abi=abi] [-meabi=ver] [-mthumb] 
[-EB|-EL] [-mapcs-32|-mapcs-26|-mapcs-float| -mapcs-reentrant] 
[-mthumb-interwork] [-k]

Blackfin:

[-mcpu=processor[-sirevision]] [-mfdpic] [-mno-fdpic] [-mnopic]

CRIS:

[--underscore | --no-underscore] [--pic] [-N] [--emulation=criself | 
--emulation=crisaout] [--march=v0_v10 | --march=v10 | --march=v32 | 
--march=common_v10_v32]

D10V:

[-O]

D30V:

[-O|-n|-N]

EPIPHANY:

[-mepiphany|-mepiphany16]

H8/300:

[-h-tick-hex]

i386:

[--32|--n32|--64] [-n] [-march=CPU[+EXTENSION...]] [-mtune=CPU]

i960:

[-ACA|-ACA_A|-ACB|-ACC|-AKA|-AKB| -AKC|-AMC] [-b] [-no-relax]

IA-64:

[-mconstant-gp|-mauto-pic] [-milp32|-milp64|-mlp64|-mp64] [-mle|mbe] 
[-mtune=itanium1|-mtune=itanium2] [-munwind-check=warning|-munwind-check=error] 
[-mhint.b=ok|-mhint.b=warning|-mhint.b=error] [-x|-xexplicit] 
[-xauto] [-xdebug]

IP2K:

[-mip2022|-mip2022ext]

M32C:

[-m32c|-m16c] [-relax] [-h-tick-hex]

M32R:

[--m32rx|--[no-]warn-explicit-parallel-conflicts| --W[n]p]

M680X0:

[-l] [-m68000|-m68010|-m68020|...]

M68HC11:

[-m68hc11|-m68hc12|-m68hcs12|-mm9s12x|-mm9s12xg] [-mshort|-mlong] 
[-mshort-double|-mlong-double] [--force-long-branches] [--short-branches] 
[--strict-direct-mode] [--print-insn-syntax] [--print-opcodes] 
[--generate-example]

MCORE:

[-jsri2bsr] [-sifilter] [-relax] [-mcpu=[210|340]]

MIPS:

[-nocpp] [-EL] [-EB] [-O[optimizationlevel]] [-g[debuglevel]] [-G num] 
[-KPIC] [-call_shared] [-non_shared] [-xgot [-mvxworks-pic] [-mabi=ABI] 
[-32] [-n32] [-64] [-mfp32] [-mgp32] [-march=CPU] [-mtune=CPU] [-mips1] 
[-mips2] [-mips3] [-mips4] [-mips5] [-mips32] [-mips32r2] [-mips64] 
[-mips64r2] [-construct-floats] [-no-construct-floats] [-trap] [-no-break] 
[-break] [-no-trap] [-mips16] [-no-mips16] [-mmicromips] [-mno-micromips] 
[-msmartmips] [-mno-smartmips] [-mips3d] [-no-mips3d] [-mdmx] [-no-mdmx] 
[-mdsp] [-mno-dsp] [-mdspr2] [-mno-dspr2] [-mmt] [-mno-mt] [-mmcu] 
[-mno-mcu] [-mfix7000] [-mno-fix7000] [-mfix-vr4120] [-mno-fix-vr4120] 
[-mfix-vr4130] [-mno-fix-vr4130] [-mdebug] [-no-mdebug] [-mpdr] [-mno-pdr]

MMIX:

[--fixed-special-register-names] [--globalize-symbols] [--gnu-syntax] 
[--relax] [--no-predefined-symbols] [--no-expand] [--no-merge-gregs] 
[-x] [--linker-allocated-gregs]

PDP11:

[-mpic|-mno-pic] [-mall] [-mno-extensions] [-mextension|-mno-extension] 
[-mcpu] [-mmachine]

picoJava:

[-mb|-me]

PowerPC:

[-a32|-a64] [-mpwrx|-mpwr2|-mpwr|-m601|-mppc|-mppc32|-m603|-m604|-m403|-m405| 
-m440|-m464|-m476|-m7400|-m7410|-m7450|-m7455|-m750cl|-mppc64| -m620|-me500|
-e500x2|-me500mc|-me500mc64|-me5500|-me6500|-mppc64bridge| -mbooke|-mpower4|
-mpr4|-mpower5|-mpwr5|-mpwr5x|-mpower6|-mpwr6| -mpower7|-mpw7|-ma2|-mcell|
-mspe|-mtitan|-me300|-mcom] [-many] [-maltivec|-mvsx] [-mregnames|
-mno-regnames] [-mrelocatable|-mrelocatable-lib|-K PIC] [-memb] [-mlittle|
-mlittle-endian|-le|-mbig|-mbig-endian|-be] [-msolaris|-mno-solaris] 
[-nops=count]

RX:

[-mlittle-endian|-mbig-endian] [-m32bit-ints|-m16bit-ints] [-m32bit-doubles|
-m64bit-doubles]

s390:

[-m31|-m64] [-mesa|-mzarch] [-march=CPU] [-mregnames|-mno-regnames] 
[-mwarn-areg-zero]

SCORE:

[-EB][-EL][-FIXDD][-NWARN] [-SCORE5][-SCORE5U][-SCORE7][-SCORE3] 
[-march=score7] [-march=score3] [-USE_R1][-KPIC][-O0][-G num][-V]

SPARC:

[-Av6|-Av7|-Av8|-Asparclet|-Asparclite -Av8plus|-Av8plusa|-Av9|-Av9a] 
[-xarch=v8plus|-xarch=v8plusa] [-bump] [-32|-64]

TIC54X:

[-mcpu=54[123589]|-mcpu=54[56]lp] [-mfar-mode|-mf] [-merrors-to-file 
<file name>|-me <file name>]

TIC6X:

[-march=arch] [-mbig-endian|-mlittle-endian] [-mdsbt|-mno-dsbt] 
[-mpid=no|-mpid=near|-mpid=far] [-mpic|-mno-pic]

TILE-Gx:

[-m32|-m64][-EB][-EL]

Xtensa:

[--[no-]text-section-literals] [--[no-]absolute-literals] [--[no-]target-align] 
[--[no-]longcalls] [--[no-]transform] [--rename-section oldname=newname]

Z80:

[-z80] [-r800] [-ignore-undocumented-instructions] [-Wnud] 
[-ignore-unportable-instructions] [-Wnup] [-warn-undocumented-instructions] 
[-Wud] [-warn-unportable-instructions] [-Wup] 
[-forbid-undocumented-instructions] [-Fud] [-forbid-unportable-instructions] 
[-Fup]

Options

@file Read command-line options from file. The options read are inserted in place of the original @file option. If file does not exist, or cannot be read, then the option will be treated literally, and not removed.

Options in file are separated by whitespace. A whitespace character may be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) may be included by prefixing the character to be included with a backslash. The file may itself contain additional @file options; any such options will be processed recursively.
-a[cdghlmns]

Turn on listings, in any of a variety of ways:

-ac: omit false conditionals

-ad: omit debugging directives

-ag: include general information, like as version and options passed

-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 to 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.
--alternate Begin in alternate macro mode.
--compress-debug-sections Compress DWARF debug sections using zlib. The debug sections are renamed to begin with .zdebug, and the resulting object file may not be compatible with older linkers and object file utilities.
--nocompress-debug-sections Do not compress DWARF debug sections. This is the default.
-D Ignored. This option is accepted for script compatibility with calls to other assemblers.
--debug-prefix-map old=new When assembling files in directory old, record debugging information describing them as in new instead.
--defsym sym=value Define the symbol sym to be value before assembling the input file. The value must be an integer constant. As in C, a leading 0x indicates a hexadecimal value, and a leading 0 indicates an octal value. The value of the symbol can be overridden inside a source file via the use of a ".set" pseudo-op.
-f "fast"---skip whitespace and comment preprocessing (assume source is compiler output).
-g, --gen-debug Generate debugging information for each assembler source line using whichever debug format is preferred by the target. This currently means either STABS, ECOFF or DWARF2.
--gstabs Generate stabs debugging information for each assembler line. This may help debugging assembler code, if the debugger can handle it.
--gstabs+ Generate stabs debugging information for each assembler line, with GNU extensions that probably only gdb can handle, and that could make other debuggers crash or refuse to read your program. This may help debugging assembler code. Currently the only GNU extension is the location of the current working directory at assembling time.
--gdwarf-2 Generate DWARF2 debugging information for each assembler line. This may help debugging assembler code, if the debugger can handle it. Note---this option is only supported by some targets, not all of them.
--size-check=error,
--size-check=warning		 Issue an error or warning for invalid ELF .size directive.
--help Print a summary of the command line options and exit.
--target-help Print a summary of all target specific options and exit.
-I dir Add directory dir to the search list for ".include" directives.
-J Don't warn about signed overflow.
-K Issue warnings when difference tables altered for long displacements.
-L, --keep-locals Keep (in the symbol table) local symbols. These symbols start with system-specific local label prefixes, typically .L for ELF systems or L for traditional a.out systems.
--listing-lhs-width=number Set the maximum width, in words, of the output data column for an assembler listing to number.
--listing-lhs-width2=number Set the maximum width, in words, of the output data column for continuation lines in an assembler listing to number.
--listing-rhs-width=number Set the maximum width of an input source line, as displayed in a listing, to number bytes.
--listing-cont-lines=number Set the maximum number of lines printed in a listing for a single line of input to number + 1.
-o objfile Name the object-file output from as objfile.
-R Fold the data section into the text section. Set the default size of GAS's hash tables to a prime number close to number. Increasing this value can reduce the length of time it takes the assembler to perform its tasks, at the expense of increasing the assembler's memory requirements. Similarly reducing this value can reduce the memory requirements at the expense of speed.
--reduce-memory-overheads This option reduces GAS's memory requirements, at the expense of making the assembly processes slower. Currently this switch is a synonym for --hash-size=4051, but in the future it may have other effects as well.
--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, --no-warn Suppress warning messages.
--fatal-warnings Treat warnings as errors.
--warn Don't suppress warning messages or treat them as errors.
-w Ignored.
-x Ignored.
-Z Generate an object file even after errors.
-- | files ... Standard input, or source files to assemble.

ARM options

The following options are available when as is configured for the 64-bit mode of the ARM Architecture (AArch64):

-EB This option specifies that the output generated by the assembler should be marked as being encoded for a big-endian processor.
-EL This option specifies that the output generated by the assembler should be marked as being encoded for a little-endian processor.

Alpha options

The following options are available when as is configured for an Alpha processor:

-mcpu

This option specifies the target processor. If an attempt is made to assemble an instruction which will not execute on the target processor, the assembler may either expand the instruction as a macro or issue an error message. This option is equivalent to the ".arch" directive.

The following processor names are recognized: 21064, "21064a", 21066, 21068, 21164, "21164a", "21164pc", 21264, "21264a", "21264b", "ev4", "ev5", "lca45", "ev5", "ev56", "pca56", "ev6", "ev67", "ev68". The special name "all" may be used to allow the assembler to accept instructions valid for any Alpha processor.

To support existing practice in OSF/1 with respect to ".arch", and existing practice within MILO (the Linux ARC bootloader), the numbered processor names (e.g., 21064) enable the processor-specific PALcode instructions, while the "electro-vlasic" names (e.g., "ev4") do not.
-mdebug,
-no-mdebug		 Enables or disables the generation of ".mdebug" encapsulation for stabs directives and procedure descriptors. The default is to automatically enable ".mdebug" when the first stabs directive is seen.
-relax This option forces all relocations to be put into the object file, instead of saving space and resolving some relocations at assembly time. Note that this option does not propagate all symbol arithmetic into the object file, because not all symbol arithmetic can be represented. However, the option can still be useful in specific applications.
-replace,
-noreplace		 Enables or disables the optimization of procedure calls, both at assemblage and at link time. These options are only available for VMS targets and "-replace" is the default. See section 1.4.1 of the OpenVMS Linker Utility Manual.
-g This option is used when the compiler generates debug information. When gcc is using mips-tfile to generate debug information for ECOFF, local labels must be passed through to the object file. Otherwise, this option has no effect.
-Gsize A local common symbol larger than size is placed in ".bss", while smaller symbols are placed in ".sbss".
-F, -32addr These options are ignored for backward compatibility.

ARM Options

The following options are available when as is configured for the ARM processor family:

-mcpu=processor[+extension...] Specify which ARM processor variant is the target.
-march=architecture[+extension...] Specify which ARM architecture variant is used by the target.
-mfpu=floating-point-format Select which Floating Point architecture is the target.
-mfloat-abi=abi Select which floating point ABI is in use.
-mthumb Enable Thumb only instruction decoding.
-mapcs-32 | -mapcs-26 |
-mapcs-float | -mapcs-reentrant		 Select which procedure calling convention is in use.
-EB | -EL Select either big-endian (-EB) or little-endian (-EL) output.
-mthumb-interwork Specify that the code was generated with interworking between Thumb and ARM code in mind.
-k Specify that PIC code has been generated.

Blackfin Options

The following options are available when as is configured for the Blackfin processor family:

-mcpu=processor[-sirevision] This option specifies the target processor. The optional sirevision is not used in assembler. It's here such that GCC can easily pass down its "-mcpu=" option. The assembler will issue an error message if an attempt is made to assemble an instruction which will not execute on the target processor. The following processor names are recognized: "bf504", "bf506", "bf512", "bf514", "bf516", "bf518", "bf522", "bf523", "bf524", "bf525", "bf526", "bf527", "bf531", "bf532", "bf533", "bf534", "bf535" (not implemented yet), "bf536", "bf537", "bf538", "bf539", "bf542", "bf542m", "bf544", "bf544m", "bf547", "bf547m", "bf548", "bf548m", "bf549", "bf549m", "bf561", and "bf592".
-mfdpic Assemble for the FDPIC ABI.
-mno-fdpic, -mnopic Disable -mfdpic.

D10V Options

The following option is available when as is configured for a D10V processor:

-O Optimize output by parallelizing instructions.

D30V Options

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.

Epiphany Options

The following options are available when as is configured for an Epiphany processor:

-mepiphany Specifies that the both 32 and 16 bit instructions are allowed. This is the default behavior.
-mepiphany16 Restricts the permitted instructions to the 16 bit set.

i386 Options

The following options are available when as is configured for an i386 processor:

--32 | --x32 | --64

Select the word size, either 32 bits or 64 bits. --32 implies Intel i386 architecture, while --x32 and --64 imply AMD x86-64 architecture with 32-bit or 64-bit word-size respectively.

These options are only available with the ELF object file format, and require that the necessary BFD support has been included (on a 32-bit platform you have to add --enable-64-bit-bfd to configure enable 64-bit usage and use x86-64 as target platform).
-n By default, x86 GAS replaces multiple nop instructions used for alignment within code sections with multi-byte nop instructions such as leal 0(%esi,1),%esi. This switch disables the optimization.
--divide On SVR4-derived platforms, the character / is treated as a comment character, which means that it cannot be used in expressions. The --divide option turns / into a normal character. This does not disable / at the beginning of a line starting a comment, or affect using # for starting a comment.
-march=CPU[+EXTENSION...] This option specifies the target processor. The assembler will issue an error message if an attempt is made to assemble an instruction which will not execute on the target processor. The following processor names are recognized:
  • "i8086"
  • "i186"
  • "i286"
  • "i386"
  • "i486"
  • "i586"
  • "i686"
  • "pentium"
  • "pentiumpro"
  • "pentiumii"
  • "pentiumiii"
  • "pentium4"
  • "prescott"
  • "nocona"
  • "core"
  • "core2"
  • "corei7"
  • "l1om"
  • "k1om"
  • "k6"
  • "k6_2"
  • "athlon"
  • "opteron"
  • "k8"
  • "amdfam10"
  • "bdver1"
  • "bdver2"
  • "bdver3"
  • "btver1"
  • "btver2"
  • "generic32"
  • "generic64"
In addition to the basic instruction set, the assembler can be told to accept various extension mnemonics. For example, "-march=i686+sse4+vmx" extends i686 with sse4 and vmx. The following extensions are currently supported:
  • "8087"
  • "287"
  • "387"
  • "no87"
  • "mmx"
  • "nommx"
  • "sse"
  • "sse2"
  • "sse3"
  • "ssse3"
  • "sse4.1"
  • "sse4.2"
  • "sse4"
  • "nosse"
  • "avx"
  • "avx2"
  • "adx"
  • "rdseed"
  • "prfchw"
  • "noavx"
  • "vmx"
  • "vmfunc"
  • "smx"
  • "xsave"
  • "xsaveopt"
  • "aes"
  • "pclmul"
  • "fsgsbase"
  • "rdrnd"
  • "f16c"
  • "bmi2"
  • "fma"
  • "movbe"
  • "ept"
  • "lzcnt"
  • "hle"
  • "rtm"
  • "invpcid"
  • "clflush"
  • "lwp"
  • "fma4"
  • "xop"
  • "syscall"
  • "rdtscp"
  • "3dnow"
  • "3dnowa"
  • "sse4a"
  • "sse5"
  • "svme"
  • "abm"
  • "padlock"
Note that rather than extending a basic instruction set, the extension mnemonics starting with "no" revoke the respective functionality. When the ".arch" directive is used with -march, the ".arch" directive takes precedent.
-mtune=CPU

This option specifies a processor to optimize for. When used in conjunction with the -march option, only instructions of the processor specified by the -march option will be generated.

Valid CPU values are identical to the processor list of -march=CPU.
-msse2avx This option specifies that the assembler should encode SSE instructions with VEX prefix.
-msse-check=none,
-msse-check=warning,
-msse-check=error		 These options control if the assembler should check SSE instructions. -msse-check=none makes the assembler not to check SSE instructions, which is the default. -msse-check=warning will make the assembler issue a warning for any SSE instruction. -msse-check=error will make the assembler issue an error for any SSE instruction.
-mavxscalar=128,
-mavxscalar=256		 These options control how the assembler should encode scalar AVX instructions. -mavxscalar=128 will encode scalar AVX instructions with 128bit vector length, which is the default. -mavxscalar=256 will encode scalar AVX instructions with 256bit vector length.
-mmnemonic=att,
-mmnemonic=intel		 This option specifies instruction mnemonic for matching instructions. The ".att_mnemonic" and ".intel_mnemonic" directives will take precedent.
-msyntax=att,
-msyntax=intel		 This option specifies instruction syntax when processing instructions. The ".att_syntax" and ".intel_syntax" directives will take precedent.
-mnaked-reg This option specifies that registers don't require a % prefix. The ".att_syntax" and ".intel_syntax" directives will take precedent.

Intel 80960 Options

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.

IP2K Options

The following options are available when as is configured for the Ubicom IP2K series:

-mip2022ext Specifies that the extended IP2022 instructions are allowed.
-mip2022 Restores the default behavior, which restricts the permitted instructions to just the basic IP2022 ones.

M32C and M16C Options

The following options are available when as is configured for the Renesas M32C and M16C processors:

-m32c Assemble M32C instructions.
-m16c Assemble M16C instructions (the default).
-relax Enable support for link-time relaxations.
-h-tick-hex Support H'00 style hex constants in addition to 0x00 style.

M32R Options

The following options are available when as is configured for the Renesas M32R (formerly Mitsubishi M32R) series:

--m32rx Specify which processor in the M32R family is the target. The default is normally the M32R, but this option changes it to the M32RX.
--warn-explicit-parallel-conflicts
or --Wp		 Produce warning messages when questionable parallel constructs are encountered.
--no-warn-explicit-parallel-conflicts
or --Wnp		 Do not produce warning messages when questionable parallel constructs are encountered.

68000 Series Options

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.

PD11 Options

The following options are available when as is configured for the DEC PDP-11:

-mpic | -mno-pic Generate position-independent (or position-dependent) code. The default is -mpic.
-mall,
-mall-extensions		 Enable all instruction set extensions. This is the default.
-mno-extensions Disable all instruction set extensions.
-mextension |
-mno-extension		 Enable (or disable) a particular instruction set extension.
-mcpu Enable the instruction set extensions supported by a particular CPU, and disable all other extensions.
-mmachine Enable the instruction set extensions supported by a particular machine model, and disable all other extensions.

PicoJava Options

The following options are available when as is configured for a picoJava processor:

-mb Generate "big endian" format output.
-ml Generate "little endian" format output.

68HC11 and 68HC12 Options

The following options are available when as is configured for the Motorola 68HC11 or 68HC12 series.

-m68hc11 | -m68hc12 |
-m68hcs12 | -mm9s12x |
-mm9s12xg		 Specify what processor is the target. The default is defined by the configuration option when building the assembler.
--xgate-ramoffset Instruct the linker to offset RAM addresses from S12X address space into XGATE address space.
-mshort Specify to use the 16-bit integer ABI.
-mlong Specify to use the 32-bit integer ABI.
-mshort-double Specify to use the 32-bit double ABI.
-mlong-double Specify to use the 64-bit double ABI.
--force-long-branches Relative branches are turned into absolute ones. This concerns conditional branches, unconditional branches and branches to a sub routine.
-S | --short-branches Do not turn relative branches into absolute ones when the offset is out of range.
--strict-direct-mode Do not turn the direct addressing mode into extended addressing mode when the instruction does not support direct addressing mode.
--print-insn-syntax Print the syntax of instruction in case of error.
--print-opcodes Print the list of instructions with syntax and then exit.
--generate-example Print an example of instruction for each possible instruction and then exit. This option is only useful for testing as.

SPARC Options

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.

c54x Options

The following options are available when as is configured for the 'c54x architecture.

-mfar-mode Enable extended addressing mode. All addresses and relocations will assume extended addressing (usually 23 bits).
-mcpu=CPU_VERSION Sets the CPU version being compiled for.
-merrors-to-file FILE NAME Redirect error output to a file, for broken systems which don't support such behavior in the shell.

MIPS Options

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, -mips4,
-mips5, -mips32,
-mips32r2,
-mips64,
-mips64r2		 Generate code for a particular MIPS Instruction Set Architecture level. -mips1 is an alias for -march=r3000, -mips2 is an alias for -march=r6000, -mips3 is an alias for -march=r4000 and -mips4 is an alias for -march=r8000. -mips5, -mips32, -mips32r2, -mips64, and -mips64r2 correspond to generic MIPS V, MIPS32, MIPS32 Release 2, MIPS64, and MIPS64 Release 2 ISA processors, respectively.
-march=CPU Generate code for a particular MIPS cpu.
-mtune=cpu Schedule and tune for a particular MIPS cpu.
-mfix7000,
-mno-fix7000		 Cause nops to be inserted if the read of the destination register of an mfhi or mflo instruction occurs in the following two instructions.
-mdebug,
-no-mdebug		 Cause stabs-style debugging output to go into an ECOFF-style .mdebug section instead of the standard ELF .stabs sections.
-mpdr, -mno-pdr Control generation of ".pdr" sections.
-mgp32, -mfp32 The register sizes are normally inferred from the ISA and ABI, but these flags force a certain group of registers to be treated as 32 bits wide at all times. -mgp32 controls the size of general-purpose registers and -mfp32 controls the size of floating-point registers.
-mips16,
-no-mips16		 Generate code for the MIPS 16 processor. This is equivalent to putting ".set mips16" at the start of the assembly file. -no-mips16 turns off this option.
-mmicromips,
-mno-micromips		 Generate code for the microMIPS processor. This is equivalent to putting ".set micromips" at the start of the assembly file. -mno-micromips turns off this option. This is equivalent to putting ".set nomicromips" at the start of the assembly file.
-msmartmips,
-mno-smartmips		 Enables the SmartMIPS extension to the MIPS32 instruction set. This is equivalent to putting ".set smartmips" at the start of the assembly file. -mno-smartmips turns off this option.
-mips3d,
-no-mips3d		 Generate code for the MIPS-3D Application Specific Extension. This tells the assembler to accept MIPS-3D instructions. -no-mips3d turns off this option.
-mdmx, -no-mdmx Generate code for the MDMX Application Specific Extension. This tells the assembler to accept MDMX instructions. -no-mdmx turns off this option.
-mdsp, -mno-dsp Generate code for the DSP Release 1 Application Specific Extension. This tells the assembler to accept DSP Release 1 instructions. -mno-dsp turns off this option.
-mdspr2,
-mno-dspr2		 Generate code for the DSP Release 2 Application Specific Extension. This option implies -mdsp. This tells the assembler to accept DSP Release 2 instructions. -mno-dspr2 turns off this option.
-mmt, -mno-mt Generate code for the MT Application Specific Extension. This tells the assembler to accept MT instructions. -mno-mt turns off this option.
-mmcu, -mno-mcu Generate code for the MCU Application Specific Extension. This tells the assembler to accept MCU instructions. -mno-mcu turns off this option.
--construct-floats,
--no-construct-floats		 The --no-construct-floats option disables the construction of double width floating point constants by loading the two halves of the value into the two single width floating point registers that make up the double width register. By default, --construct-floats is selected, allowing construction of these floating point constants.
--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 overrides 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.
-n When this option is used, as will issue a warning every time it generates a nop instruction from a macro.

MCore Options

The following options are available when as is configured for an MCore processor.

-jsri2bsr,
-nojsri2bsr		 Enable or disable the JSRI to BSR transformation. By default, this is enabled. The command line option -nojsri2bsr can disable it.
-sifilter, -nosifilter Enable or disable the silicon filter behavior. By default, this is disabled. The default can be overridden by the -sifilter command line option.
-relax Alter jump instructions for long displacements.
-mcpu=[210|340] Select the cpu type on the target hardware. This controls which instructions can be assembled.
-EB Assemble for a big endian target.
-EL Assemble for a little endian target.

PowerPC Options

The following options are available when as is configured for a PowerPC processor.

-a32 Generate ELF32 or XCOFF32.
-a64 Generate ELF64 or XCOFF64.
-K PIC Set EF_PPC_RELOCATABLE_LIB in ELF flags.
-mpwrx | -mpwr2 Generate code for POWER/2 (RIOS2).
-mpwr Generate code for POWER (RIOS1)
-m601 Generate code for PowerPC 601.
-mppc, -mppc32,
-m603, -m604		 Generate code for PowerPC 603/604.
-m403, -m405 Generate code for PowerPC 403/405.
-m440 Generate code for PowerPC 440. BookE and some 405 instructions.
-m464 Generate code for PowerPC 464.
-m476 Generate code for PowerPC 476.
-m7400, -m7410,
-m7450, -m7455		 Generate code for PowerPC 7400/7410/7450/7455.
-m750cl Generate code for PowerPC 750CL.
-mppc64, -m620 Generate code for PowerPC 620/625/630.
-me500, -me500x2 Generate code for Motorola e500 core complex.
-me500mc Generate code for Freescale e500mc core complex.
-me500mc64 Generate code for Freescale e500mc64 core complex.
-me5500 Generate code for Freescale e5500 core complex.
-me6500 Generate code for Freescale e6500 core complex.
-mspe Generate code for Motorola SPE instructions.
-mtitan Generate code for AppliedMicro Titan core complex.
-mppc64bridge Generate code for PowerPC 64, including bridge insns.
-mbooke Generate code for 32-bit BookE.
-ma2 Generate code for A2 architecture.
-me300 Generate code for PowerPC e300 family.
-maltivec Generate code for processors with AltiVec instructions.
-mvsx Generate code for processors with Vector-Scalar instructions.
-mpower4, -mpwr4 Generate code for Power4 architecture.
-mpower5, -mpwr5, -mpwr5x Generate code for Power5 architecture.
-mpower6, -mpwr6 Generate code for Power6 architecture.
-mpower7, -mpwr7 Generate code for Power7 architecture.
-mcell Generate code for Cell Broadband Engine architecture.
-mcom Generate code Power/PowerPC common instructions.
-many Generate code for any architecture (PWR/PWRX/PPC).
-mregnames Allow symbolic names for registers.
-mno-regnames Do not allow symbolic names for registers.
-mrelocatable Support for GCC's -mrelocatable option.
-mrelocatable-lib Support for GCC's -mrelocatable-lib option.
-memb Set PPC_EMB bit in ELF flags.
-mlittle,
-mlittle-endian, -le		 Generate code for a little endian machine.
-mbig,
-mbig-endian, -be		 Generate code for a big endian machine.
-msolaris Generate code for Solaris.
-mno-solaris Do not generate code for Solaris.
-nops=count If an alignment directive inserts more than count nops, put a branch at the beginning to skip execution of the nops.

s390 Options

The following options are available when as is configured for the s390 processor family.

-m31, -m64 Select the word size, either 31/32 bits or 64 bits.
-mesa, -mzarch Select the architecture mode, either the Enterprise System Architecture (esa) or the z/Architecture mode (zarch).
-march=processor Specify which s390 processor variant is the target, g6, g6, z900, z990, z9-109, z9-ec, or z10.
-mregnames,
-mno-regnames		 Allow or disallow symbolic names for registers.
-mwarn-areg-zero Warn whenever the operand for a base or index register has been specified but evaluates to zero.

TMS320C6000 Options

The following options are available when as is configured for a TMS320C6000 processor.

-march=arch Enable (only) instructions from architecture arch. By default, all instructions are permitted.

The following values of arch are accepted: "c62x", "c64x", "c64x+", "c67x", "c67x+", "c674x".
-mdsbt, -mno-dsbt The -mdsbt option causes the assembler to generate the "Tag_ABI_DSBT" attribute with a value of 1, indicating that the code is using DSBT addressing. The -mno-dsbt option, the default, causes the tag to have a value of 0, indicating that the code does not use DSBT addressing. The linker will emit a warning if objects of different types (DSBT and non-DSBT) are linked together.
-mpid=no,
-mpid=near,
-mpid=far		 The -mpid= option causes the assembler to generate the "Tag_ABI_PID" attribute with a value indicating the form of data addressing used by the code. -mpid=no, the default, indicates position-dependent data addressing, -mpid=near indicates position-independent addressing with GOT accesses using near DP addressing, and -mpid=far indicates position-independent addressing with GOT accesses using far DP addressing. The linker will emit a warning if objects built with different settings of this option are linked together.
-mpic, -mno-pic The -mpic option causes the assembler to generate the "Tag_ABI_PIC" attribute with a value of 1, indicating that the code uses position-independent code addressing, The "-mno-pic" option, the default, causes the tag to have a value of 0, indicating position-dependent code addressing. The linker will emit a warning if objects of different type (position-dependent and position-independent) are linked together.
-mbig-endian,
-mlittle-endian		 Generate code for the specified endianness. The default is little-endian.

TILE-Gx Options

The following options are available when as is configured for a TILE-Gx processor:

-m32 | -m64 Select the word size, either 32 bits or 64 bits.
-EB | -EL Select the endianness, either big-endian (-EB) or little-endian (-EL).

Xtensa Options

The following options are available when as is configured for an Xtensa processor:

--text-section-literals |
--no-text-section-literals		 Control the treatment of literal pools. The default is --no-text-section-literals, which places literals in separate sections in the output file. This allows the literal pool to be placed in a data RAM/ROM. With --text-section-literals, the literals are interspersed in the text section to keep them as close as possible to their references. This may be necessary for large assembly files, where the literals would otherwise be out of range of the "L32R" instructions in the text section. These options only affect literals referenced via PC-relative "L32R" instructions; literals for absolute mode "L32R" instructions are handled separately.
--absolute-literals |
--no-absolute-literals		 Indicate to the assembler whether "L32R" instructions use absolute or PC-relative addressing. If the processor includes the absolute addressing option, the default is to use absolute "L32R" relocations. Otherwise, only the PC-relative "L32R" relocations can be used.
--target-align |
--no-target-align		 Enable or disable automatic alignment to reduce branch penalties at some expense in code size. This optimization is enabled by default. Note that the assembler always aligns instructions like "LOOP" that have fixed alignment requirements.
--longcalls |
--no-longcalls		 Enable or disable transformation of call instructions to allow calls across a greater range of addresses. This option should be used when call targets can potentially be out of range. It may degrade both code size and performance, but the linker can generally optimize away the unnecessary overhead when a call ends up within range. The default is --no-longcalls.
--transform |
--no-transform		 Enable or disable all assembler transformations of Xtensa instructions, including both relaxation and optimization. The default is --transform; --no-transform should only be used in the rare cases when the instructions must be exactly as specified in the assembly source. Using --no-transform causes out of range instruction operands to be errors.
--rename-section oldname=newname Rename the oldname section to newname. This option can be used multiple times to rename multiple sections.

Z80 Options

The following options are available when as is configured for a Z80 family processor:

-z80 Assemble for Z80 processor.
-r800 Assemble for R800 processor.
-ignore-undocumented-instructions,
-Wnud		 Assemble undocumented Z80 instructions that also work on R800 without warning.
-ignore-unportable-instructions,
-Wnup		 Assemble all undocumented Z80 instructions without warning.
-warn-undocumented-instructions,
-Wud		 Issue a warning for undocumented Z80 instructions that also work on R800.
-warn-unportable-instructions,
-Wup		 Issue a warning for undocumented Z80 instructions that do not work on R800.
-forbid-undocumented-instructions,
-Fud		 Treat all undocumented instructions as errors.
-forbid-unportable-instructions,
-Fup		 Treat undocumented Z80 instructions that do not work on R800 as errors.

ld — Link editor for object files.