Using as

Table of Contents

Using as

This file is a user guide to the GNU assembler as (Arm GNU Toolchain 13.2.rel1 (Build arm-13.7)) version 2.41.0.

This document is distributed under the terms of the GNU Free Documentation License. A copy of the license is included in the section entitled “GNU Free Documentation License”.

1 Overview

Here is a brief summary of how to invoke as. For details, see Command-Line Options.

as [-a[cdghlns][=file]]
 [–alternate]
 [–compress-debug-sections] [–nocompress-debug-sections]
 [-D]
 [–dump-config]
 [–debug-prefix-map old=new]
 [–defsym sym=val]
 [–elf-stt-common=[no|yes]]
 [–emulation=name]
 [-f]
 [-g] [–gstabs] [–gstabs+]
 [–gdwarf-<N>] [–gdwarf-sections]
 [–gdwarf-cie-version=VERSION]
 [–generate-missing-build-notes=[no|yes]]
 [–gsframe]
 [–hash-size=N]
 [–help] [–target-help]
 [-I dir]
 [-J]
 [-K]
 [–keep-locals]
 [-L]
 [–listing-lhs-width=NUM]
 [–listing-lhs-width2=NUM]
 [–listing-rhs-width=NUM]
 [–listing-cont-lines=NUM]
 [–multibyte-handling=[allow|warn|warn-sym-only]]
 [–no-pad-sections]
 [-o objfile] [-R]
 [–sectname-subst]
 [–size-check=[error|warning]]
 [–statistics]
 [-v] [-version] [–version]
 [-W] [–warn] [–fatal-warnings] [-w] [-x]
 [-Z] [@FILE]
 [target-options]
 [–|files …]

Target AArch64 options:
   [-EB|-EL]
   [-mabi=ABI]

Target Alpha options:
   [-mcpu]
   [-mdebug | -no-mdebug]
   [-replace | -noreplace]
   [-relax] [-g] [-Gsize]
   [-F] [-32addr]

Target ARC options:
   [-mcpu=cpu]
   [-mA6|-mARC600|-mARC601|-mA7|-mARC700|-mEM|-mHS]
   [-mcode-density]
   [-mrelax]
   [-EB|-EL]

Target ARM options:
   [-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]

Target Blackfin options:
   [-mcpu=processor[-sirevision]]
   [-mfdpic]
   [-mno-fdpic]
   [-mnopic]

Target BPF options:
   [-EL] [-EB]

Target CRIS options:
   [–underscore | –no-underscore]
   [–pic] [-N]
   [–emulation=criself | –emulation=crisaout]
   [–march=v0_v10 | –march=v10 | –march=v32 | –march=common_v10_v32]

Target C-SKY options:
   [-march=arch] [-mcpu=cpu]
   [-EL] [-mlittle-endian] [-EB] [-mbig-endian]
   [-fpic] [-pic]
   [-mljump] [-mno-ljump]
   [-force2bsr] [-mforce2bsr] [-no-force2bsr] [-mno-force2bsr]
   [-jsri2bsr] [-mjsri2bsr] [-no-jsri2bsr ] [-mno-jsri2bsr]
   [-mnolrw ] [-mno-lrw]
   [-melrw] [-mno-elrw]
   [-mlaf ] [-mliterals-after-func]
   [-mno-laf] [-mno-literals-after-func]
   [-mlabr] [-mliterals-after-br]
   [-mno-labr] [-mnoliterals-after-br]
   [-mistack] [-mno-istack]
   [-mhard-float] [-mmp] [-mcp] [-mcache]
   [-msecurity] [-mtrust]
   [-mdsp] [-medsp] [-mvdsp]

Target D10V options:
   [-O]

Target D30V options:
   [-O|-n|-N]

Target EPIPHANY options:
   [-mepiphany|-mepiphany16]

Target H8/300 options:
   [-h-tick-hex]

Target i386 options:
   [–32|–x32|–64] [-n]
   [-march=CPU[+EXTENSION…]] [-mtune=CPU]

Target IA-64 options:
   [-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]

Target IP2K options:
   [-mip2022|-mip2022ext]

Target M32C options:
   [-m32c|-m16c] [-relax] [-h-tick-hex]

Target M32R options:
   [–m32rx|–[no-]warn-explicit-parallel-conflicts|
   –W[n]p]

Target M680X0 options:
   [-l] [-m68000|-m68010|-m68020|…]

Target M68HC11 options:
   [-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]

Target MCORE options:
   [-jsri2bsr] [-sifilter] [-relax]
   [-mcpu=[210|340]]

Target Meta options:
   [-mcpu=cpu] [-mfpu=cpu] [-mdsp=cpu]
Target MICROBLAZE options:

Target MIPS options:
   [-nocpp] [-EL] [-EB] [-O[optimization level]]
   [-g[debug level]] [-G num] [-KPIC] [-call_shared]
   [-non_shared] [-xgot [-mvxworks-pic]
   [-mabi=ABI] [-32] [-n32] [-64] [-mfp32] [-mgp32]
   [-mfp64] [-mgp64] [-mfpxx]
   [-modd-spreg] [-mno-odd-spreg]
   [-march=CPU] [-mtune=CPU] [-mips1] [-mips2]
   [-mips3] [-mips4] [-mips5] [-mips32] [-mips32r2]
   [-mips32r3] [-mips32r5] [-mips32r6] [-mips64] [-mips64r2]
   [-mips64r3] [-mips64r5] [-mips64r6]
   [-construct-floats] [-no-construct-floats]
   [-mignore-branch-isa] [-mno-ignore-branch-isa]
   [-mnan=encoding]
   [-trap] [-no-break] [-break] [-no-trap]
   [-mips16] [-no-mips16]
   [-mmips16e2] [-mno-mips16e2]
   [-mmicromips] [-mno-micromips]
   [-msmartmips] [-mno-smartmips]
   [-mips3d] [-no-mips3d]
   [-mdmx] [-no-mdmx]
   [-mdsp] [-mno-dsp]
   [-mdspr2] [-mno-dspr2]
   [-mdspr3] [-mno-dspr3]
   [-mmsa] [-mno-msa]
   [-mxpa] [-mno-xpa]
   [-mmt] [-mno-mt]
   [-mmcu] [-mno-mcu]
   [-mcrc] [-mno-crc]
   [-mginv] [-mno-ginv]
   [-mloongson-mmi] [-mno-loongson-mmi]
   [-mloongson-cam] [-mno-loongson-cam]
   [-mloongson-ext] [-mno-loongson-ext]
   [-mloongson-ext2] [-mno-loongson-ext2]
   [-minsn32] [-mno-insn32]
   [-mfix7000] [-mno-fix7000]
   [-mfix-rm7000] [-mno-fix-rm7000]
   [-mfix-vr4120] [-mno-fix-vr4120]
   [-mfix-vr4130] [-mno-fix-vr4130]
   [-mfix-r5900] [-mno-fix-r5900]
   [-mdebug] [-no-mdebug]
   [-mpdr] [-mno-pdr]

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

Target Nios II options:
   [-relax-all] [-relax-section] [-no-relax]
   [-EB] [-EL]

Target NDS32 options:
    [-EL] [-EB] [-O] [-Os] [-mcpu=cpu]
    [-misa=isa] [-mabi=abi] [-mall-ext]
    [-m[no-]16-bit]  [-m[no-]perf-ext] [-m[no-]perf2-ext]
    [-m[no-]string-ext] [-m[no-]dsp-ext] [-m[no-]mac] [-m[no-]div]
    [-m[no-]audio-isa-ext] [-m[no-]fpu-sp-ext] [-m[no-]fpu-dp-ext]
    [-m[no-]fpu-fma] [-mfpu-freg=FREG] [-mreduced-regs]
    [-mfull-regs] [-m[no-]dx-regs] [-mpic] [-mno-relax]
    [-mb2bb]

Target PDP11 options:
   [-mpic|-mno-pic] [-mall] [-mno-extensions]
   [-mextension|-mno-extension]
   [-mcpu] [-mmachine]

Target picoJava options:
   [-mb|-me]

Target PowerPC options:
   [-a32|-a64]
   [-mpwrx|-mpwr2|-mpwr|-m601|-mppc|-mppc32|-m603|-m604|-m403|-m405|
    -m440|-m464|-m476|-m7400|-m7410|-m7450|-m7455|-m750cl|-mgekko|
    -mbroadway|-mppc64|-m620|-me500|-e500x2|-me500mc|-me500mc64|-me5500|
    -me6500|-mppc64bridge|-mbooke|-mpower4|-mpwr4|-mpower5|-mpwr5|-mpwr5x|
    -mpower6|-mpwr6|-mpower7|-mpwr7|-mpower8|-mpwr8|-mpower9|-mpwr9-ma2|
    -mcell|-mspe|-mspe2|-mtitan|-me300|-mcom]
   [-many] [-maltivec|-mvsx|-mhtm|-mvle]
   [-mregnames|-mno-regnames]
   [-mrelocatable|-mrelocatable-lib|-K PIC] [-memb]
   [-mlittle|-mlittle-endian|-le|-mbig|-mbig-endian|-be]
   [-msolaris|-mno-solaris]
   [-nops=count]

Target PRU options:
   [-link-relax]
   [-mnolink-relax]
   [-mno-warn-regname-label]

Target RISC-V options:
   [-fpic|-fPIC|-fno-pic]
   [-march=ISA]
   [-mabi=ABI]
   [-mlittle-endian|-mbig-endian]

Target RL78 options:
   [-mg10]
   [-m32bit-doubles|-m64bit-doubles]

Target RX options:
   [-mlittle-endian|-mbig-endian]
   [-m32bit-doubles|-m64bit-doubles]
   [-muse-conventional-section-names]
   [-msmall-data-limit]
   [-mpid]
   [-mrelax]
   [-mint-register=number]
   [-mgcc-abi|-mrx-abi]

Target s390 options:
   [-m31|-m64] [-mesa|-mzarch] [-march=CPU]
   [-mregnames|-mno-regnames]
   [-mwarn-areg-zero]

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

Target SPARC options:
   [-Av6|-Av7|-Av8|-Aleon|-Asparclet|-Asparclite
    -Av8plus|-Av8plusa|-Av8plusb|-Av8plusc|-Av8plusd
    -Av8plusv|-Av8plusm|-Av9|-Av9a|-Av9b|-Av9c
    -Av9d|-Av9e|-Av9v|-Av9m|-Asparc|-Asparcvis
    -Asparcvis2|-Asparcfmaf|-Asparcima|-Asparcvis3
    -Asparcvisr|-Asparc5]
   [-xarch=v8plus|-xarch=v8plusa]|-xarch=v8plusb|-xarch=v8plusc
    -xarch=v8plusd|-xarch=v8plusv|-xarch=v8plusm|-xarch=v9
    -xarch=v9a|-xarch=v9b|-xarch=v9c|-xarch=v9d|-xarch=v9e
    -xarch=v9v|-xarch=v9m|-xarch=sparc|-xarch=sparcvis
    -xarch=sparcvis2|-xarch=sparcfmaf|-xarch=sparcima
    -xarch=sparcvis3|-xarch=sparcvisr|-xarch=sparc5
    -bump]
   [-32|-64]
   [–enforce-aligned-data][–dcti-couples-detect]

Target TIC54X options:
 [-mcpu=54[123589]|-mcpu=54[56]lp] [-mfar-mode|-mf]
 [-merrors-to-file <filename>|-me <filename>]

Target TIC6X options:
   [-march=arch] [-mbig-endian|-mlittle-endian]
   [-mdsbt|-mno-dsbt] [-mpid=no|-mpid=near|-mpid=far]
   [-mpic|-mno-pic]

Target TILE-Gx options:
   [-m32|-m64][-EB][-EL]

Target Visium options:
   [-mtune=arch]

Target Xtensa options:
 [–[no-]text-section-literals] [–[no-]auto-litpools]
 [–[no-]absolute-literals]
 [–[no-]target-align] [–[no-]longcalls]
 [–[no-]transform]
 [–rename-section oldname=newname]
 [–[no-]trampolines]
 [–abi-windowed|–abi-call0]

Target Z80 options:
  [-march=CPU[-EXT][+EXT]]
  [-local-prefix=PREFIX]
  [-colonless]
  [-sdcc]
  [-fp-s=FORMAT]
  [-fp-d=FORMAT]

@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

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. See .altmacro.

--compress-debug-sections

Compress DWARF debug sections using zlib with SHF_COMPRESSED from the ELF ABI. The resulting object file may not be compatible with older linkers and object file utilities. Note if compression would make a given section larger then it is not compressed.

--compress-debug-sections=none

--compress-debug-sections=zlib

--compress-debug-sections=zlib-gnu

--compress-debug-sections=zlib-gabi

--compress-debug-sections=zstd

These options control how DWARF debug sections are compressed. --compress-debug-sections=none is equivalent to --nocompress-debug-sections. --compress-debug-sections=zlib and --compress-debug-sections=zlib-gabi are equivalent to --compress-debug-sections. --compress-debug-sections=zlib-gnu compresses DWARF debug sections using the obsoleted zlib-gnu format. The debug sections are renamed to begin with ‘.zdebug’. --compress-debug-sections=zstd compresses DWARF debug sections using zstd. Note - if compression would actually make a section larger, then it is not compressed nor renamed.

--nocompress-debug-sections

Do not compress DWARF debug sections. This is usually the default for all targets except the x86/x86_64, but a configure time option can be used to override this.

-D

Enable denugging in target specific backends, if supported. Otherwise ignored. Even if 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. 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.

--dump-config

Displays how the assembler is configured and then exits.

--elf-stt-common=no

--elf-stt-common=yes

These options control whether the ELF assembler should generate common symbols with the STT_COMMON type. The default can be controlled by a configure option --enable-elf-stt-common.

--emulation=name

If the assembler is configured to support multiple different target configurations then this option can be used to select the desired form.

-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. When the debug format is DWARF then a .debug_info and .debug_line section is only emitted when the assembly file doesn’t generate one itself.

--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.

--gdwarf-3

This option is the same as the --gdwarf-2 option, except that it allows for the possibility of the generation of extra debug information as per version 3 of the DWARF specification. Note - enabling this option does not guarantee the generation of any extra information, the choice to do so is on a per target basis.

--gdwarf-4

This option is the same as the --gdwarf-2 option, except that it allows for the possibility of the generation of extra debug information as per version 4 of the DWARF specification. Note - enabling this option does not guarantee the generation of any extra information, the choice to do so is on a per target basis.

--gdwarf-5

This option is the same as the --gdwarf-2 option, except that it allows for the possibility of the generation of extra debug information as per version 5 of the DWARF specification. Note - enabling this option does not guarantee the generation of any extra information, the choice to do so is on a per target basis.

--gdwarf-sections

Instead of creating a .debug_line section, create a series of .debug_line.foo sections where foo is the name of the corresponding code section. For example a code section called .text.func will have its dwarf line number information placed into a section called .debug_line.text.func. If the code section is just called .text then debug line section will still be called just .debug_line without any suffix.

--gdwarf-cie-version=version

Control which version of DWARF Common Information Entries (CIEs) are produced. When this flag is not specificed the default is version 1, though some targets can modify this default. Other possible values for version are 3 or 4.

--generate-missing-build-notes=yes

--generate-missing-build-notes=no

These options control whether the ELF assembler should generate GNU Build attribute notes if none are present in the input sources. The default can be controlled by the --enable-generate-build-notes configure option.

--gsframe

--gsframe

Create .sframe section from CFI directives.

--hash-size N

Ignored. Supported for command line compatibility with other assemblers.

--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. See Symbol Names.

--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.

--multibyte-handling=allow

--multibyte-handling=warn

--multibyte-handling=warn-sym-only

--multibyte-handling=warn_sym_only

Controls how the assembler handles multibyte characters in the input. The default (which can be restored by using the allow argument) is to allow such characters without complaint. Using the warn argument will make the assembler generate a warning message whenever any multibyte character is encountered. Using the warn-sym-only argument will only cause a warning to be generated when a symbol is defined with a name that contains multibyte characters. (References to undefined symbols will not generate a warning).

--no-pad-sections

Stop the assembler for padding the ends of output sections to the alignment of that section. The default is to pad the sections, but this can waste space which might be needed on targets which have tight memory constraints.

-o objfile

Name the object-file output from as objfile.

-R

Fold the data section into the text section.

--reduce-memory-overheads

Ignored. Supported for compatibility with tools that apss the same option to both the assembler and the linker.

--sectname-subst

Honor substitution sequences in section names. See .section name.

--size-check=error

--size-check=warning

Issue an error or warning for invalid ELF .size directive.

--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.

See AArch64 Options, for the options available when as is configured for the 64-bit mode of the ARM Architecture (AArch64).

See Alpha Options, for the options available when as is configured for an Alpha processor.

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

-mcpu=cpu

This option selects the core processor variant.

-EB | -EL

Select either big-endian (-EB) or little-endian (-EL) output.

-mcode-density

Enable Code Density extension instructions.

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 has been generated with interworking between Thumb and ARM code in mind.

-mccs

Turns on CodeComposer Studio assembly syntax compatibility mode.

-k

Specify that PIC code has been generated.

See Blackfin Options, for the options available when as is configured for the Blackfin processor family.

See BPF Options, for the options available when as is configured for the Linux kernel BPF processor family.

See the info pages for documentation of the CRIS-specific options.

See C-SKY Options, for the options available when as is configured for the C-SKY processor family.

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 Adapteva EPIPHANY series.

See Epiphany Options, for the options available when as is configured for an Epiphany processor.

See i386-Options, for the options available when as is configured for an i386 processor.

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 behaviour, which restricts the permitted instructions to just the basic IP2022 ones.

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.

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.

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.

See Nios II Options, for the options available when as is configured for an Altera Nios II processor.

For details about the PDP-11 machine dependent features options, see PDP-11-Options.

-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.

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.

See PRU Options, for the options available when as is configured for a PRU processor.

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.

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 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 FILENAME

Redirect error output to a file, for broken systems which don’t support such behaviour in the shell.

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

-mips32r3

-mips32r5

-mips32r6

-mips64

-mips64r2

-mips64r3

-mips64r5

-mips64r6

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’, ‘-mips32r3’, ‘-mips32r5’, ‘-mips32r6’, ‘-mips64’, ‘-mips64r2’, ‘-mips64r3’, ‘-mips64r5’, and ‘-mips64r6’ correspond to generic MIPS V, MIPS32, MIPS32 Release 2, MIPS32 Release 3, MIPS32 Release 5, MIPS32 Release 6, MIPS64, MIPS64 Release 2, MIPS64 Release 3, MIPS64 Release 5, and MIPS64 Release 6 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.

-mfix-rm7000

-mno-fix-rm7000

Cause nops to be inserted if a dmult or dmultu instruction is followed by a load instruction.

-mfix-r5900

-mno-fix-r5900

Do not attempt to schedule the preceding instruction into the delay slot of a branch instruction placed at the end of a short loop of six instructions or fewer and always schedule a nop instruction there instead. The short loop bug under certain conditions causes loops to execute only once or twice, due to a hardware bug in the R5900 chip.

-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.

-mgp64

-mfp64

The register sizes are normally inferred from the ISA and ABI, but these flags force a certain group of registers to be treated as 64 bits wide at all times. ‘-mgp64’ controls the size of general-purpose registers and ‘-mfp64’ controls the size of floating-point registers.

-mfpxx

The register sizes are normally inferred from the ISA and ABI, but using this flag in combination with ‘-mabi=32’ enables an ABI variant which will operate correctly with floating-point registers which are 32 or 64 bits wide.

-modd-spreg

-mno-odd-spreg

Enable use of floating-point operations on odd-numbered single-precision registers when supported by the ISA. ‘-mfpxx’ implies ‘-mno-odd-spreg’, otherwise the default is ‘-modd-spreg’.

-mips16

-no-mips16

Generate code for the MIPS 16 processor. This is equivalent to putting .module mips16 at the start of the assembly file. ‘-no-mips16’ turns off this option.

-mmips16e2

-mno-mips16e2

Enable the use of MIPS16e2 instructions in MIPS16 mode. This is equivalent to putting .module mips16e2 at the start of the assembly file. ‘-mno-mips16e2’ turns off this option.

-mmicromips

-mno-micromips

Generate code for the microMIPS processor. This is equivalent to putting .module micromips at the start of the assembly file. ‘-mno-micromips’ turns off this option. This is equivalent to putting .module 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 .module 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.

-mdspr3

-mno-dspr3

Generate code for the DSP Release 3 Application Specific Extension. This option implies ‘-mdsp’ and ‘-mdspr2’. This tells the assembler to accept DSP Release 3 instructions. ‘-mno-dspr3’ turns off this option.

-mmsa

-mno-msa

Generate code for the MIPS SIMD Architecture Extension. This tells the assembler to accept MSA instructions. ‘-mno-msa’ turns off this option.

-mxpa

-mno-xpa

Generate code for the MIPS eXtended Physical Address (XPA) Extension. This tells the assembler to accept XPA instructions. ‘-mno-xpa’ 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.

-mcrc

-mno-crc

Generate code for the MIPS cyclic redundancy check (CRC) Application Specific Extension. This tells the assembler to accept CRC instructions. ‘-mno-crc’ turns off this option.

-mginv

-mno-ginv

Generate code for the Global INValidate (GINV) Application Specific Extension. This tells the assembler to accept GINV instructions. ‘-mno-ginv’ turns off this option.

-mloongson-mmi

-mno-loongson-mmi

Generate code for the Loongson MultiMedia extensions Instructions (MMI) Application Specific Extension. This tells the assembler to accept MMI instructions. ‘-mno-loongson-mmi’ turns off this option.

-mloongson-cam

-mno-loongson-cam

Generate code for the Loongson Content Address Memory (CAM) instructions. This tells the assembler to accept Loongson CAM instructions. ‘-mno-loongson-cam’ turns off this option.

-mloongson-ext

-mno-loongson-ext

Generate code for the Loongson EXTensions (EXT) instructions. This tells the assembler to accept Loongson EXT instructions. ‘-mno-loongson-ext’ turns off this option.

-mloongson-ext2

-mno-loongson-ext2

Generate code for the Loongson EXTensions R2 (EXT2) instructions. This option implies ‘-mloongson-ext’. This tells the assembler to accept Loongson EXT2 instructions. ‘-mno-loongson-ext2’ turns off this option.

-minsn32

-mno-insn32

Only use 32-bit instruction encodings when generating code for the microMIPS processor. This option inhibits the use of any 16-bit instructions. This is equivalent to putting .set insn32 at the start of the assembly file. ‘-mno-insn32’ turns off this option. This is equivalent to putting .set noinsn32 at the start of the assembly file. By default ‘-mno-insn32’ is selected, allowing all instructions to be used.

--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.

--relax-branch

--no-relax-branch

The ‘--relax-branch’ option enables the relaxation of out-of-range branches. By default ‘--no-relax-branch’ is selected, causing any out-of-range branches to produce an error.

-mignore-branch-isa

-mno-ignore-branch-isa

Ignore branch checks for invalid transitions between ISA modes. The semantics of branches does not provide for an ISA mode switch, so in most cases the ISA mode a branch has been encoded for has to be the same as the ISA mode of the branch’s target label. Therefore GAS has checks implemented that verify in branch assembly that the two ISA modes match. ‘-mignore-branch-isa’ disables these checks. By default ‘-mno-ignore-branch-isa’ is selected, causing any invalid branch requiring a transition between ISA modes to produce an error.

-mnan=encoding

Select between the IEEE 754-2008 (-mnan=2008) or the legacy (-mnan=legacy) NaN encoding format. The latter is the default.

--emulation=name

This option was formerly used to switch between ELF and ECOFF output on targets like IRIX 5 that supported both. MIPS ECOFF support was removed in GAS 2.24, so the option now serves little purpose. It is retained for backwards compatibility.

The available configuration names are: ‘mipself’, ‘mipslelf’ and ‘mipsbelf’. Choosing ‘mipself’ now has no effect, since the output is always ELF. ‘mipslelf’ and ‘mipsbelf’ select little- and big-endian output respectively, but ‘-EL’ and ‘-EB’ are now the preferred options instead.

-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.

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 be used to disable it.

-sifilter

-nosifilter

Enable or disable the silicon filter behaviour. 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.

See Meta Options, for the options available when as is configured for a Meta processor.

See the info pages for documentation of the MMIX-specific options.

See NDS32 Options, for the options available when as is configured for a NDS32 processor.

See PowerPC-Opts, for the options available when as is configured for a PowerPC processor.

See RISC-V-Options, for the options available when as is configured for a RISC-V processor.

See the info pages for documentation of the RX-specific 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, ‘g5’ (or ‘arch3’), ‘g6’, ‘z900’ (or ‘arch5’), ‘z990’ (or ‘arch6’), ‘z9-109’, ‘z9-ec’ (or ‘arch7’), ‘z10’ (or ‘arch8’), ‘z196’ (or ‘arch9’), ‘zEC12’ (or ‘arch10’), ‘z13’ (or ‘arch11’), ‘z14’ (or ‘arch12’), ‘z15’ (or ‘arch13’), or ‘z16’ (or ‘arch14’).

-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.

See TIC6X Options, for the options available when as is configured for a TMS320C6000 processor.

See TILE-Gx Options, for the options available when as is configured for a TILE-Gx processor.

See Visium Options, for the options available when as is configured for a Visium processor.

See Xtensa Options, for the options available when as is configured for an Xtensa processor.

See Z80 Options, for the options available when as is configured for an Z80 processor.

1.1 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.

1.2 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 Machine Dependencies). 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 .org).

1.3 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 Symbol Attributes.

1.4 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

1.5 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 Error and Warning Messages.

Physical files are those files named in the command line given to as.

Logical files are simply names declared explicitly by assembler directives; they bear no relation to physical files. Logical file names help error messages reflect the original source file, when as source is itself synthesized from other files. as understands the ‘#’ directives emitted by the gcc preprocessor. See also .file.

1.6 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. 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.

1.7 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 both a logical file name (see .file) and a logical line number (see .line) have been given then they will be used, otherwise the file name and line number in the current assembler source file will be used. The message text is intended to be self explanatory (in the grand Unix tradition).

Note the file name must be set via the logical version of the .file directive, not the DWARF2 version of the .file directive. For example:

  .file 2 "bar.c"
     error_assembler_source
  .file "foo.c"
  .line 30
      error_c_source

produces this output:

  Assembler messages:
  asm.s:2: Error: no such instruction: `error_assembler_source'
  foo.c:31: Error: no such instruction: `error_c_source'

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.

2 Command-Line Options

This chapter describes command-line options available in all versions of the GNU assembler; see Machine Dependencies, for options specific to particular machine architectures.

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.)

2.1 Enable Listings: -a[cdghlns]

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 ‘-ag’ option to print a first section with general assembly information, like as version, switches passed, or time stamp.

Use the ‘-ac’ option to omit false conditionals from a listing. Any lines which are not assembled because of a false .if (or .ifdef, or any other conditional), or a true .if followed by an .else, will be omitted from the listing.

Use the ‘-ad’ option to omit debugging directives from the listing.

Once you have specified one of these options, you can further control listing output and its appearance using the directives .list, .nolist, .psize, .eject, .title, and .sbttl. The ‘-an’ option turns off all forms processing. If you do not request listing output with one of the ‘-a’ options, the listing-control directives have no effect.

The letters after ‘-a’ may be combined into one option, e.g., ‘-aln’.

Note if the assembler source is coming from the standard input (e.g., because it is being created by gcc and the ‘-pipe’ command-line switch is being used) then the listing will not contain any comments or preprocessor directives. This is because the listing code buffers input source lines from stdin only after they have been preprocessed by the assembler. This reduces memory usage and makes the code more efficient.

2.2 --alternate

Begin in alternate macro mode, see .altmacro.

2.3 -D

This option enables debugging, if it is supported by the assembler’s configuration. Otherwise it does nothing as is ignored. This allows scripts designed to work with other assemblers to also work with GAS. as.

2.4 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 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.

2.5 .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 .include). You may use -I as many times as necessary to include a variety of paths. The current working directory is always searched first; after that, as searches any ‘-I’ directories in the same order as they were specified (left to right) on the command line.

2.6 Difference Tables: -K

as sometimes alters the code emitted for directives of the form ‘.word sym1-sym2’. See .word. You can use the ‘-K’ option if you want a warning issued when this is done.

2.7 Include Local Symbols: -L

Symbols beginning with system-specific local label prefixes, typically ‘.L’ for ELF systems or ‘L’ for traditional a.out systems, are called local symbols. See Symbol Names. Normally you do not see such symbols 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 symbols, so you do not normally debug with them.

This option tells as to retain those local symbols in the object file. Usually if you do this you also tell the linker ld to preserve those symbols.

2.8 Configuring listing output: --listing

The listing feature of the assembler can be enabled via the command-line switch ‘-a’ (see a). This feature combines the input source file(s) with a hex dump of the corresponding locations in the output object file, and displays them as a listing file. The format of this listing can be controlled by directives inside the assembler source (i.e., .list (see List), .title (see Title), .sbttl (see Sbttl), .psize (see Psize), and .eject (see Eject) and also by the following switches:

--listing-lhs-width=‘number’

Sets the maximum width, in words, of the first line of the hex byte dump. This dump appears on the left hand side of the listing output.

--listing-lhs-width2=‘number’

Sets the maximum width, in words, of any further lines of the hex byte dump for a given input source line. If this value is not specified, it defaults to being the same as the value specified for ‘--listing-lhs-width’. If neither switch is used the default is to one.

--listing-rhs-width=‘number’

Sets the maximum width, in characters, of the source line that is displayed alongside the hex dump. The default value for this parameter is 100. The source line is displayed on the right hand side of the listing output.

--listing-cont-lines=‘number’

Sets the maximum number of continuation lines of hex dump that will be displayed for a given single line of source input. The default value is 4.

2.9 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 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.

2.10 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.

2.11 Output Section Padding

Normally the assembler will pad the end of each output section up to its alignment boundary. But this can waste space, which can be significant on memory constrained targets. So the --no-pad-sections option will disable this behaviour.

2.12 Name the Object File: -o

There is always one object file output when you run as. By default it has the name a.out. You use this option (which takes exactly one filename) to give the object file a different name.

Whatever the object file is called, as overwrites any existing file of the same name.

2.13 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 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 or ELF 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.

2.14 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).

2.15 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.

2.16 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.

2.17 Control Warnings: -W, --warn, --no-warn, --fatal-warnings

as should never give a warning or error message when assembling compiler output. But programs written by people often cause as to give a warning that a particular assumption was made. All such warnings are directed to the standard error file.

If you use the -W and --no-warn options, no warnings are issued. This only affects the warning messages: it does not change any particular of how as assembles your file. Errors, which stop the assembly, are still reported.

If you use the --fatal-warnings option, as considers files that generate warnings to be in error.

You can switch these options off again by specifying --warn, which causes warnings to be output as usual.

2.18 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.’

3 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.

3.1 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 .include). You can use the GNU C compiler driver to get other “CPP” style preprocessing by giving the input file a ‘.S’ suffix. See the ’Options Controlling the Kind of Output’ section of the GCC manual for more details

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 file 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 intended to support asm statements in compilers whose output is otherwise free of comments and whitespace.

3.2 Whitespace

Whitespace is one or more blanks or tabs, in any order. Whitespace is used to separate symbols, and to make programs neater for people to read. Unless within character constants (see Character Constants), any whitespace means the same as exactly one space.

3.3 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 a line comment character up to the next newline is considered a comment and is ignored. The line comment character is target specific, and some targets support multiple comment characters. Some targets also have line comment characters that only work if they are the first character on a line. Some targets use a sequence of two characters to introduce a line comment. Some targets can also change their line comment characters depending upon command-line options that have been used. For more details see the Syntax section in the documentation for individual targets.

If the line comment character is the hash sign (‘#’) then it still has the special ability to enable and disable preprocessing (see Preprocessing) and to specify logical line numbers:

To be compatible with past assemblers, lines that begin with ‘#’ have a special interpretation. Following the ‘#’ should be an absolute expression (see Expressions): the logical line number of the next line. Then a string (see Strings) is allowed: if present it is a new logical file name. The rest of the line, if any, should be whitespace.

If the first non-whitespace characters on the line are not numeric, the line is ignored. (Just like a comment.)

                          # This is an ordinary comment.
# 42-6 "new_file_name"    # New logical file name
                          # This is logical line # 36.

This feature is deprecated, and may disappear from future versions of as.

3.4 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 Machine Dependencies. No symbol may begin with a digit. Case is significant. There is no length limit; all characters are significant. Multibyte characters are supported, but note that the setting of the --multibyte-handling option might prevent their use. Symbols are delimited by characters not in that set, or by the beginning of a file (since the source program must end with a newline, the end of a file is not a possible symbol delimiter). See Symbols.

Symbol names may also be enclosed in double quote " characters. In such cases any characters are allowed, except for the NUL character. If a double quote character is to be included in the symbol name it must be preceded by a backslash \ character.

3.5 Statements

A statement ends at a newline character (‘\n’) or a line separator character. The line separator character is target specific and described in the Syntax section of each target’s documentation. Not all targets support a line separator character. The newline or line separator character is considered to be 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.

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 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, …

3.6 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.

3.6.1 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.

3.6.1.1 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.

backslash-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.

3.6.1.2 Characters

A single character may be written as a single quote immediately followed by that character. Some backslash escapes apply to characters, \b, \f, \n, \r, \t, and \" with the same meaning as for strings, plus \' for a single quote. So if you want to write the character backslash, you must write '\\ where the first \ escapes the second \. As you can see, the quote is an acute accent, not a grave accent. A newline immediately following an acute accent is taken as a literal character and does not count as the end of a statement. The value of a character constant in a numeric expression is the machine’s byte-wide code for that character. as assumes your character code is ASCII: 'A means 65, 'B means 66, and so on.

3.6.2 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.

3.6.2.1 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 Prefix Operators).

3.6.2.2 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.

3.6.2.3 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 and Renesas / SuperH SH 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 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.

4 Sections and Relocation

4.1 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 for the Renesas / SuperH 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 or ELF output, as can also generate whatever other named sections you specify using the ‘.section’ directive (see .section). 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.

4.2 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 of another. When the program is running, however, it is customary for the text section to be unalterable. The text section is often shared among processes: it contains instructions, constants and the like. The data section of a running program is usually alterable: for example, C variables would be stored in the data section.

bss section

This section contains zeroed bytes when your program begins running. It is used to hold uninitialized 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.

                      +-----+----+--+
partial program # 1:  |ttttt|dddd|00|
                      +-----+----+--+

                      text   data bss
                      seg.   seg. seg.

                      +---+---+---+
partial program # 2:  |TTT|DDD|000|
                      +---+---+---+

                      +--+---+-----+--+----+---+-----+~~
linked program:       |  |TTT|ttttt|  |dddd|DDD|00000|
                      +--+---+-----+--+----+---+-----+~~

    addresses:        0 …

4.3 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 expressions internally as combinations of symbols. When it needs to represent an expression as a symbol, it puts it in the expr section.

4.4 Sub-Sections

Assembled bytes conventionally fall into two sections: text and data. You may have separate groups of data in named sections that you want to end up near to each other in the object file, even though they are not contiguous in the assembler source. as allows you to use subsections for this purpose. Within each section, there can be numbered subsections with values from 0 to 8192. Objects assembled into the same subsection go into the object file together with other objects in the same subsection. For example, a compiler might want to store constants in the text section, but might not want to have them interspersed with the program being assembled. In this case, the compiler could issue a ‘.text 0’ before each section of code being output, and a ‘.text 1’ before each group of constants being output.

Subsections are optional. If you do not use subsections, everything goes in subsection number zero.

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’. When generating ELF output, you can also use the .subsection directive (see SubSection) to specify a subsection: ‘.subsection expression’. Expression should be an absolute expression (see Expressions). If you just say ‘.text’ then ‘.text 0’ is assumed. Likewise ‘.data’ means ‘.data 0’. Assembly begins in text 0. For instance:

.text 0     # The default subsection is text 0 anyway.
.ascii "This lives in the first text subsection. *"
.text 1
.ascii "But this lives in the second text subsection."
.data 0
.ascii "This lives in the data section,"
.ascii "in the first data subsection."
.text 0
.ascii "This lives in the first text section,"
.ascii "immediately following the asterisk (*)."

Each section has a location counter incremented by one for every byte assembled into that section. Because subsections are merely a convenience restricted to as there is no concept of a subsection location counter. There is no way to directly manipulate a location counter—but the .align directive changes it, and any label definition captures its current value. The location counter of the section where statements are being assembled is said to be the active location counter.

4.5 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 .lcomm.

The .comm pseudo-op may be used to declare a common symbol, which is another form of uninitialized symbol; see .comm.

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. You may only assemble zero values into the section. Typically the section will only contain symbol definitions and .skip directives (see .skip).

5 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.

5.1 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.

5.2 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 Expressions). This is equivalent to using the .set directive. See .set. In the same way, using a double equals sign ‘=’‘=’ here represents an equivalent of the .eqv directive. See .eqv.

Blackfin does not support symbol assignment with ‘=’.

5.3 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 Machine Dependencies. That character may be followed by any string of digits, letters, dollar signs (unless otherwise noted for a particular target machine), and underscores. These restrictions do not apply when quoting symbol names by ‘"’, which is permitted for most targets. Escaping characters in quoted symbol names with ‘\’ generally extends only to ‘\’ itself and ‘"’, at the time of writing.

Case of letters is significant: foo is a different symbol name than Foo.

Symbol names do not start with a digit. An exception to this rule is made for Local Labels. See below.

Multibyte characters are supported, but note that the setting of the multibyte-handling option might prevent their use. To generate a symbol name containing multibyte characters enclose it within double quotes and use escape codes. cf See Strings. Generating a multibyte symbol name from a label is not currently supported.

Since multibyte symbol names are unusual, and could possibly be used maliciously, as provides a command line option (--multibyte-handling=warn-sym-only) which can be used to generate a warning message whenever a symbol name containing multibyte characters is defined.

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

A local symbol is any symbol beginning with certain local label prefixes. By default, the local label prefix is ‘.L’ for ELF systems or ‘L’ for traditional a.out systems, but each target may have its own set of local label prefixes. On the HPPA local symbols begin with ‘L$’.

Local symbols are defined and used within the assembler, but they are normally not saved in object files. Thus, they are not visible when debugging. You may use the ‘-L’ option (see Include Local Symbols) to retain the local symbols in the object files.

Local Labels

Local labels are different from local symbols. Local labels help compilers and programmers use names temporarily. They create symbols which are guaranteed to be unique over the entire scope of the input source code and which can be referred to by a simple notation. To define a local label, write a label of the form ‘N:’ (where N represents any non-negative integer). To refer to the most recent previous definition of that label write ‘Nb’, using the same number as when you defined the label. To refer to the next definition of a local label, write ‘Nf’. The ‘b’ stands for “backwards” and the ‘f’ stands for “forwards”.

There is no restriction on how you can use these labels, and you can reuse them too. So that it is possible to repeatedly define the same local label (using the same number ‘N’), although you can only refer to the most recently defined local label of that number (for a backwards reference) or the next definition of a specific local label for a forward reference. It is also worth noting that the first 10 local labels (‘0:’…‘9:’) are implemented in a slightly more efficient manner than the others.

Here is an example:

1:        branch 1f
2:        branch 1b
1:        branch 2f
2:        branch 1b

Which is the equivalent of:

label_1:  branch label_3
label_2:  branch label_1
label_3:  branch label_4
label_4:  branch label_3

Local label names are only a notational device. They are immediately transformed into more conventional symbol names before the assembler uses them. The symbol names are stored in the symbol table, appear in error messages, and are optionally emitted to the object file. The names are constructed using these parts:

local label prefix

All local symbols begin with the system-specific local label prefix. Normally both as and ld forget symbols that start with the local label prefix. These labels are used for symbols you are never intended to see. If you use the ‘-L’ option then as retains these symbols in the object file. If you also instruct ld to retain these symbols, you may use them in debugging.

number

This is the number that was used in the local label definition. So if the label is written ‘55:’ then the number is ‘55’.

C-B

This unusual character is included so you do not accidentally invent a symbol of the same name. The character has ASCII value of ‘\002’ (control-B).

ordinal number

This is a serial number to keep the labels distinct. The first definition of ‘0:’ gets the number ‘1’. The 15th definition of ‘0:’ gets the number ‘15’, and so on. Likewise the first definition of ‘1:’ gets the number ‘1’ and its 15th definition gets ‘15’ as well.

So for example, the first 1: may be named .L1C-B1, and the 44th 3: may be named .L3C-B44.

Dollar Local Labels

On some targets as also supports an even more local form of local labels called dollar labels. These labels go out of scope (i.e., they become undefined) as soon as a non-local label is defined. Thus they remain valid for only a small region of the input source code. Normal local labels, by contrast, remain in scope for the entire file, or until they are redefined by another occurrence of the same local label.

Dollar labels are defined in exactly the same way as ordinary local labels, except that they have a dollar sign suffix to their numeric value, e.g., ‘55$:’.

They can also be distinguished from ordinary local labels by their transformed names which use ASCII character ‘\001’ (control-A) as the magic character to distinguish them from ordinary labels. For example, the fifth definition of ‘6$’ may be named ‘.L6C-A5’.

5.4 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’.

5.5 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.

5.5.1 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.

5.5.2 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.

5.5.3 Symbol Attributes: a.out

5.5.3.1 Descriptor

This is an arbitrary 16-bit value. You may establish a symbol’s descriptor value by using a .desc statement (see .desc). A descriptor value means nothing to as.

5.5.3.2 Other

This is an arbitrary 8-bit value. It means nothing to as.

5.5.4 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.

5.5.4.1 Primary Attributes

The symbol name is set with .def; the value and type, respectively, with .val and .type.

5.5.4.2 Auxiliary Attributes

The as directives .dim, .line, .scl, .size, .tag, and .weak can generate auxiliary symbol table information for COFF.

5.5.5 Symbol Attributes for SOM

The SOM format for the HPPA supports a multitude of symbol attributes set with the .EXPORT and .IMPORT directives.

The attributes are described in HP9000 Series 800 Assembly Language Reference Manual (HP 92432-90001) under the IMPORT and EXPORT assembler directive documentation.

6 Expressions

An expression specifies an address or numeric value. Whitespace may precede and/or follow an expression.

The result of an expression must be an absolute number, or else an offset into a particular section. If an expression is not absolute, and there is not enough information when as sees the expression to know its section, a second pass over the source program might be necessary to interpret the expression—but the second pass is currently not implemented. as aborts with an error message in this situation.

6.1 Empty Expressions

An empty expression has no value: it is just whitespace or null. Wherever an absolute expression is required, you may omit the expression, and as assumes a value of (absolute) 0. This is compatible with other assemblers.

6.2 Integer Expressions

An integer expression is one or more arguments delimited by operators.

6.2.1 Arguments

Arguments are symbols, numbers or subexpressions. In other contexts arguments are sometimes called “arithmetic operands”. In this manual, to avoid confusing them with the “instruction operands” of the machine language, we use the term “argument” to refer to parts of expressions only, reserving the word “operand” to refer only to machine instruction operands.

Symbols are evaluated to yield {section NNN} where section is one of text, data, bss, absolute, or undefined. NNN is a signed, 2’s complement 32 bit integer.

Numbers are usually integers.

A number can be a flonum or bignum. In this case, you are warned that only the low order 32 bits are used, and as pretends these 32 bits are an integer. You may write integer-manipulating instructions that act on exotic constants, compatible with other assemblers.

Subexpressions are a left parenthesis ‘(’ followed by an integer expression, followed by a right parenthesis ‘)’; or a prefix operator followed by an argument.

6.2.2 Operators

Operators are arithmetic functions, like + or %. Prefix operators are followed by an argument. Infix operators appear between their arguments. Operators may be preceded and/or followed by whitespace.

6.2.3 Prefix Operator

as has the following prefix operators. They each take one argument, which must be absolute.

-

Negation. Two’s complement negation.

~

Complementation. Bitwise not.

6.2.4 Infix Operators

Infix operators take two arguments, one on either side. Operators have precedence, but operations with equal precedence are performed left to right. Apart from + or -, both arguments must be absolute, and the result is absolute.

2. Highest Precedence

   *

   Multiplication.

   /

   Division. Truncation is the same as the C operator ‘/’

   %

   Remainder.

   <<

   Shift Left. Same as the C operator ‘<<’.

   >>

   Shift Right. Same as the C operator ‘>>’.

3. Intermediate precedence

   |

   Bitwise Inclusive Or.

   &

   Bitwise And.

   ^

   Bitwise Exclusive Or.

   !

   Bitwise Or Not.

4. Low Precedence

   +

   Addition. If either argument is absolute, the result has the section of the other argument. You may not add together arguments from different sections.

   -

   Subtraction. If the right argument is absolute, the result has the section of the left argument. If both arguments are in the same section, the result is absolute. You may not subtract arguments from different sections.

   ==

   Is Equal To

   <>

   !=

   Is Not Equal To

   <

   Is Less Than

   >

   Is Greater Than

   >=

   Is Greater Than Or Equal To

   <=

   Is Less Than Or Equal To

   The comparison operators can be used as infix operators. A true result has a value of -1 whereas a false result has a value of 0. Note, these operators perform signed comparisons.

5. Lowest Precedence

   &&

   Logical And.

   ||

   Logical Or.

   These two logical operations can be used to combine the results of sub expressions. Note, unlike the comparison operators a true result returns a value of 1 but a false result does still return 0. Also note that the logical or operator has a slightly lower precedence than logical and.

In short, it’s only meaningful to add or subtract the offsets in an address; you can only have a defined section in one of the two arguments.

7 Assembler Directives

All assembler directives have names that begin with a period (‘.’). The names are case insensitive for most targets, and usually written in lower case.

This chapter discusses directives that are available regardless of the target machine configuration for the GNU assembler. Some machine configurations provide additional directives. See Machine Dependencies.

7.1 .abort

This directive stops the assembly immediately. It is for compatibility with other assemblers. The original idea was that the assembly language source would be piped into the assembler. If the sender of the source quit, it could use this directive tells as to quit also. One day .abort will not be supported.

7.2 .ABORT (COFF)

When producing COFF output, as accepts this directive as a synonym for ‘.abort’.

7.3 .align [abs-expr[, abs-expr[, abs-expr]]]

Pad the location counter (in the current subsection) to a particular storage boundary. The first expression (which must be absolute) is the alignment required, as described below. If this expression is omitted then a default value of 0 is used, effectively disabling alignment requirements.

The second expression (also absolute) gives the fill value to be stored in the padding bytes. It (and the comma) may be omitted. If it is omitted, the padding bytes are normally zero. However, on most systems, if the section is marked as containing code and the fill value is omitted, the space is filled with no-op instructions.

The third expression is also absolute, and is also optional. If it is present, it is the maximum number of bytes that should be skipped by this alignment directive. If doing the alignment would require skipping more bytes than the specified maximum, then the alignment is not done at all. You can omit the fill value (the second argument) entirely by simply using two commas after the required alignment; this can be useful if you want the alignment to be filled with no-op instructions when appropriate.

The way the required alignment is specified varies from system to system. For the arc, hppa, i386 using ELF, iq2000, m68k, or1k, s390, sparc, tic4x and xtensa, the first expression is the alignment request in bytes. For example ‘.align 8’ advances the location counter until it is a multiple of 8. If the location counter is already a multiple of 8, no change is needed. For the tic54x, the first expression is the alignment request in words.

For other systems, including ppc, i386 using a.out format, arm and strongarm, it is the number of low-order zero bits the location counter must have after advancement. For example ‘.align 3’ advances the location counter until it is a multiple of 8. If the location counter is already a multiple of 8, no change is needed.

This inconsistency is due to the different behaviors of the various native assemblers for these systems which GAS must emulate. GAS also provides .balign and .p2align directives, described later, which have a consistent behavior across all architectures (but are specific to GAS).

7.4 .altmacro

Enable alternate macro mode, enabling:

LOCAL name [ , … ]

One additional directive, LOCAL, is available. It is used to generate a string replacement for each of the name arguments, and replace any instances of name in each macro expansion. The replacement string is unique in the assembly, and different for each separate macro expansion. LOCAL allows you to write macros that define symbols, without fear of conflict between separate macro expansions.

String delimiters

You can write strings delimited in these other ways besides "string":

'string'

You can delimit strings with single-quote characters.

<string>

You can delimit strings with matching angle brackets.

single-character string escape

To include any single character literally in a string (even if the character would otherwise have some special meaning), you can prefix the character with ‘!’ (an exclamation mark). For example, you can write ‘<4.3 !> 5.4!!>’ to get the literal text ‘4.3 > 5.4!’.

Expression results as strings

You can write ‘%expr’ to evaluate the expression expr and use the result as a string.

7.5 .ascii "string"…

.ascii expects zero or more string literals (see Strings) separated by commas. It assembles each string (with no automatic trailing zero byte) into consecutive addresses.

7.6 .asciz "string"…

.asciz is just like .ascii, but each string is followed by a zero byte. The “z” in ‘.asciz’ stands for “zero”. Note that multiple string arguments not separated by commas will be concatenated together and only one final zero byte will be stored.

7.7 .attach_to_group name

Attaches the current section to the named group. This is like declaring the section with the G attribute, but can be done after the section has been created. Note if the group section does not exist at the point that this directive is used then it will be created.

7.8 .balign[wl] [abs-expr[, abs-expr[, abs-expr]]]

Pad the location counter (in the current subsection) to a particular storage boundary. The first expression (which must be absolute) is the alignment request in bytes. For example ‘.balign 8’ advances the location counter until it is a multiple of 8. If the location counter is already a multiple of 8, no change is needed. If the expression is omitted then a default value of 0 is used, effectively disabling alignment requirements.

The second expression (also absolute) gives the fill value to be stored in the padding bytes. It (and the comma) may be omitted. If it is omitted, the padding bytes are normally zero. However, on most systems, if the section is marked as containing code and the fill value is omitted, the space is filled with no-op instructions.

The third expression is also absolute, and is also optional. If it is present, it is the maximum number of bytes that should be skipped by this alignment directive. If doing the alignment would require skipping more bytes than the specified maximum, then the alignment is not done at all. You can omit the fill value (the second argument) entirely by simply using two commas after the required alignment; this can be useful if you want the alignment to be filled with no-op instructions when appropriate.

The .balignw and .balignl directives are variants of the .balign directive. The .balignw directive treats the fill pattern as a two byte word value. The .balignl directives treats the fill pattern as a four byte longword value. For example, .balignw 4,0x368d will align to a multiple of 4. If it skips two bytes, they will be filled in with the value 0x368d (the exact placement of the bytes depends upon the endianness of the processor). If it skips 1 or 3 bytes, the fill value is undefined.

7.9 .bss subsection

.bss tells as to assemble the following statements onto the end of the bss section. For ELF based targets an optional subsection expression (which must evaluate to a positive integer) can be provided. In this case the statements are appended to the end of the indicated bss subsection.

7.10 Bundle directives

7.10.1 .bundle_align_mode abs-expr

.bundle_align_mode enables or disables aligned instruction bundle mode. In this mode, sequences of adjacent instructions are grouped into fixed-sized bundles. If the argument is zero, this mode is disabled (which is the default state). If the argument it not zero, it gives the size of an instruction bundle as a power of two (as for the .p2align directive, see P2align).

For some targets, it’s an ABI requirement that no instruction may span a certain aligned boundary. A bundle is simply a sequence of instructions that starts on an aligned boundary. For example, if abs-expr is 5 then the bundle size is 32, so each aligned chunk of 32 bytes is a bundle. When aligned instruction bundle mode is in effect, no single instruction may span a boundary between bundles. If an instruction would start too close to the end of a bundle for the length of that particular instruction to fit within the bundle, then the space at the end of that bundle is filled with no-op instructions so the instruction starts in the next bundle. As a corollary, it’s an error if any single instruction’s encoding is longer than the bundle size.

7.10.2 .bundle_lock and .bundle_unlock

The .bundle_lock and directive .bundle_unlock directives allow explicit control over instruction bundle padding. These directives are only valid when .bundle_align_mode has been used to enable aligned instruction bundle mode. It’s an error if they appear when .bundle_align_mode has not been used at all, or when the last directive was .bundle_align_mode 0.

For some targets, it’s an ABI requirement that certain instructions may appear only as part of specified permissible sequences of multiple instructions, all within the same bundle. A pair of .bundle_lock and .bundle_unlock directives define a bundle-locked instruction sequence. For purposes of aligned instruction bundle mode, a sequence starting with .bundle_lock and ending with .bundle_unlock is treated as a single instruction. That is, the entire sequence must fit into a single bundle and may not span a bundle boundary. If necessary, no-op instructions will be inserted before the first instruction of the sequence so that the whole sequence starts on an aligned bundle boundary. It’s an error if the sequence is longer than the bundle size.

For convenience when using .bundle_lock and .bundle_unlock inside assembler macros (see Macro), bundle-locked sequences may be nested. That is, a second .bundle_lock directive before the next .bundle_unlock directive has no effect except that it must be matched by another closing .bundle_unlock so that there is the same number of .bundle_lock and .bundle_unlock directives.

7.11 .byte expressions

.byte expects zero or more expressions, separated by commas. Each expression is assembled into the next byte.

Note - this directive is not intended for encoding instructions, and it will not trigger effects like DWARF line number generation. Instead some targets support special directives for encoding arbitrary binary sequences as instructions such as .insn or .inst.

7.12 CFI directives

7.12.1 .cfi_sections section_list

.cfi_sections may be used to specify whether CFI directives should emit .eh_frame section, .debug_frame section and/or .sframe section. If section_list contains .eh_frame, .eh_frame is emitted, if section_list contains .debug_frame, .debug_frame is emitted, and finally, if section_list contains .sframe, .sframe is emitted. To emit multiple sections, specify them together in a list. For example, to emit both .eh_frame and .debug_frame, use .eh_frame, .debug_frame. The default if this directive is not used is .cfi_sections .eh_frame.

On targets that support compact unwinding tables these can be generated by specifying .eh_frame_entry instead of .eh_frame.

Some targets may support an additional name, such as .c6xabi.exidx which is used by the target.

The .cfi_sections directive can be repeated, with the same or different arguments, provided that CFI generation has not yet started. Once CFI generation has started however the section list is fixed and any attempts to redefine it will result in an error.

7.12.2 .cfi_startproc [simple]

.cfi_startproc is used at the beginning of each function that should have an entry in .eh_frame. It initializes some internal data structures. Don’t forget to close the function by .cfi_endproc.

Unless .cfi_startproc is used along with parameter simple it also emits some architecture dependent initial CFI instructions.

7.12.3 .cfi_endproc

.cfi_endproc is used at the end of a function where it closes its unwind entry previously opened by .cfi_startproc, and emits it to .eh_frame.

7.12.4 .cfi_personality encoding [, exp]

.cfi_personality defines personality routine and its encoding. encoding must be a constant determining how the personality should be encoded. If it is 255 (DW_EH_PE_omit), second argument is not present, otherwise second argument should be a constant or a symbol name. When using indirect encodings, the symbol provided should be the location where personality can be loaded from, not the personality routine itself. The default after .cfi_startproc is .cfi_personality 0xff, no personality routine.

7.12.5 .cfi_personality_id id

cfi_personality_id defines a personality routine by its index as defined in a compact unwinding format. Only valid when generating compact EH frames (i.e. with .cfi_sections eh_frame_entry.

7.12.6 .cfi_fde_data [opcode1 [, …]]

cfi_fde_data is used to describe the compact unwind opcodes to be used for the current function. These are emitted inline in the .eh_frame_entry section if small enough and there is no LSDA, or in the .gnu.extab section otherwise. Only valid when generating compact EH frames (i.e. with .cfi_sections eh_frame_entry.

7.12.7 .cfi_lsda encoding [, exp]

.cfi_lsda defines LSDA and its encoding. encoding must be a constant determining how the LSDA should be encoded. If it is 255 (DW_EH_PE_omit), the second argument is not present, otherwise the second argument should be a constant or a symbol name. The default after .cfi_startproc is .cfi_lsda 0xff, meaning that no LSDA is present.

7.12.8 .cfi_inline_lsda [align]

.cfi_inline_lsda marks the start of a LSDA data section and switches to the corresponding .gnu.extab section. Must be preceded by a CFI block containing a .cfi_lsda directive. Only valid when generating compact EH frames (i.e. with .cfi_sections eh_frame_entry.

The table header and unwinding opcodes will be generated at this point, so that they are immediately followed by the LSDA data. The symbol referenced by the .cfi_lsda directive should still be defined in case a fallback FDE based encoding is used. The LSDA data is terminated by a section directive.

The optional align argument specifies the alignment required. The alignment is specified as a power of two, as with the .p2align directive.

7.12.9 .cfi_def_cfa register, offset

.cfi_def_cfa defines a rule for computing CFA as: take address from register and add offset to it.

7.12.10 .cfi_def_cfa_register register

.cfi_def_cfa_register modifies a rule for computing CFA. From now on register will be used instead of the old one. Offset remains the same.

7.12.11 .cfi_def_cfa_offset offset

.cfi_def_cfa_offset modifies a rule for computing CFA. Register remains the same, but offset is new. Note that it is the absolute offset that will be added to a defined register to compute CFA address.

7.12.12 .cfi_adjust_cfa_offset offset

Same as .cfi_def_cfa_offset but offset is a relative value that is added/subtracted from the previous offset.

7.12.13 .cfi_offset register, offset

Previous value of register is saved at offset offset from CFA.

7.12.14 .cfi_val_offset register, offset

Previous value of register is CFA + offset.

7.12.15 .cfi_rel_offset register, offset

Previous value of register is saved at offset offset from the current CFA register. This is transformed to .cfi_offset using the known displacement of the CFA register from the CFA. This is often easier to use, because the number will match the code it’s annotating.

7.12.16 .cfi_register register1, register2

Previous value of register1 is saved in register register2.

7.12.17 .cfi_restore register

.cfi_restore says that the rule for register is now the same as it was at the beginning of the function, after all initial instruction added by .cfi_startproc were executed.

7.12.18 .cfi_undefined register

From now on the previous value of register can’t be restored anymore.

7.12.19 .cfi_same_value register

Current value of register is the same like in the previous frame, i.e. no restoration needed.

7.12.20 .cfi_remember_state and .cfi_restore_state

.cfi_remember_state pushes the set of rules for every register onto an implicit stack, while .cfi_restore_state pops them off the stack and places them in the current row. This is useful for situations where you have multiple .cfi_* directives that need to be undone due to the control flow of the program. For example, we could have something like this (assuming the CFA is the value of rbp):

        je label
        popq %rbx
        .cfi_restore %rbx
        popq %r12
        .cfi_restore %r12
        popq %rbp
        .cfi_restore %rbp
        .cfi_def_cfa %rsp, 8
        ret
label:
        /* Do something else */

Here, we want the .cfi directives to affect only the rows corresponding to the instructions before label. This means we’d have to add multiple .cfi directives after label to recreate the original save locations of the registers, as well as setting the CFA back to the value of rbp. This would be clumsy, and result in a larger binary size. Instead, we can write:

        je label
        popq %rbx
        .cfi_remember_state
        .cfi_restore %rbx
        popq %r12
        .cfi_restore %r12
        popq %rbp
        .cfi_restore %rbp
        .cfi_def_cfa %rsp, 8
        ret
label:
        .cfi_restore_state
        /* Do something else */

That way, the rules for the instructions after label will be the same as before the first .cfi_restore without having to use multiple .cfi directives.

7.12.21 .cfi_return_column register

Change return column register, i.e. the return address is either directly in register or can be accessed by rules for register.

7.12.22 .cfi_signal_frame

Mark current function as signal trampoline.

7.12.23 .cfi_window_save

SPARC register window has been saved.

7.12.24 .cfi_escape expression[, …]

Allows the user to add arbitrary bytes to the unwind info. One might use this to add OS-specific CFI opcodes, or generic CFI opcodes that GAS does not yet support.

7.12.25 .cfi_val_encoded_addr register, encoding, label

The current value of register is label. The value of label will be encoded in the output file according to encoding; see the description of .cfi_personality for details on this encoding.

The usefulness of equating a register to a fixed label is probably limited to the return address register. Here, it can be useful to mark a code segment that has only one return address which is reached by a direct branch and no copy of the return address exists in memory or another register.

7.13 .comm symbol , length

.comm declares a common symbol named symbol. When linking, a common symbol in one object file may be merged with a defined or common symbol of the same name in another object file. If ld does not see a definition for the symbol–just one or more common symbols–then it will allocate length bytes of uninitialized memory. length must be an absolute expression. If ld sees multiple common symbols with the same name, and they do not all have the same size, it will allocate space using the largest size.

When using ELF or (as a GNU extension) PE, the .comm directive takes an optional third argument. This is the desired alignment of the symbol, specified for ELF as a byte boundary (for example, an alignment of 16 means that the least significant 4 bits of the address should be zero), and for PE as a power of two (for example, an alignment of 5 means aligned to a 32-byte boundary). The alignment must be an absolute expression, and it must be a power of two. If ld allocates uninitialized memory for the common symbol, it will use the alignment when placing the symbol. If no alignment is specified, as will set the alignment to the largest power of two less than or equal to the size of the symbol, up to a maximum of 16 on ELF, or the default section alignment of 4 on PE¹.

The syntax for .comm differs slightly on the HPPA. The syntax is ‘symbol .comm, length’; symbol is optional.

7.14 .data subsection

.data tells as to assemble the following statements onto the end of the data subsection numbered subsection (which is an absolute expression). If subsection is omitted, it defaults to zero.

7.15 .dc[size] expressions

The .dc directive expects zero or more expressions separated by commas. These expressions are evaluated and their values inserted into the current section. The size of the emitted value depends upon the suffix to the .dc directive:

‘.a’

Emits N-bit values, where N is the size of an address on the target system.

‘.b’

Emits 8-bit values.

‘.d’

Emits double precision floating-point values.

‘.l’

Emits 32-bit values.

‘.s’

Emits single precision floating-point values.

‘.w’

Emits 16-bit values. Note - this is true even on targets where the .word directive would emit 32-bit values.

‘.x’

Emits long double precision floating-point values.

If no suffix is used then ‘.w’ is assumed.

The byte ordering is target dependent, as is the size and format of floating point values.

Note - these directives are not intended for encoding instructions, and they will not trigger effects like DWARF line number generation. Instead some targets support special directives for encoding arbitrary binary sequences as instructions such as .insn or .inst.

7.16 .dcb[size] number [,fill]

This directive emits number copies of fill, each of size bytes. Both number and fill are absolute expressions. If the comma and fill are omitted, fill is assumed to be zero. The size suffix, if present, must be one of:

‘.b’

Emits single byte values.

‘.d’

Emits double-precision floating point values.

‘.l’

Emits 4-byte values.

‘.s’

Emits single-precision floating point values.

‘.w’

Emits 2-byte values.

‘.x’

Emits long double-precision floating point values.

If the size suffix is omitted then ‘.w’ is assumed.

The byte ordering is target dependent, as is the size and format of floating point values.

7.17 .ds[size] number [,fill]

This directive emits number copies of fill, each of size bytes. Both number and fill are absolute expressions. If the comma and fill are omitted, fill is assumed to be zero. The size suffix, if present, must be one of:

‘.b’

Emits single byte values.

‘.d’

Emits 8-byte values.

‘.l’

Emits 4-byte values.

‘.p’

Emits values with size matching packed-decimal floating-point ones.

‘.s’

Emits 4-byte values.

‘.w’

Emits 2-byte values.

‘.x’

Emits values with size matching long double precision floating-point ones.