The following documentation discusses the directives that are available, regardless of the target machine configuration for the GNU assembler. Some machine configurations provide additional directives. See also Machine dependent features.
NOTE:
One
day the .abort
and .line
directives won’t work. They are included for compatibility with older assemblers.
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.
When producing COFF output, as accepts this directive as a synonym for .abort.
When producing b.out output, as accepts this directive, but ignores it.
Pad the location counter (in the current subsection) to a particular storage boundary.
The first expression (abs-expr, which must be absolute) is the alignment required, as the following discussion details.
The second expression (also absolute) gives the value to be stored in the padding bytes. It (and the comma) may be omitted. If it is omitted, the padding bytes are zero. However, on some 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 directive. If doing the alignment would require skipping more bytes than the specified maximum, then the alignment is not done at all. To omit the fill value (the second argument) entirely, use 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 a29k, hppa, m86k, m88k, w65, sparc, and Hitachi SH, and i386 using ELF format, 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 other systems, including the i386 using a.out format, 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 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 as must emulate. as also provides .balign and .p2align directives, described later, which have a consistent behavior across all architectures (but are specific to as).
.app-file (which may also be spelled ‘.file’) tells as that we are about to start a new logical file. string is the new file name. In general, the filename is recognized whether or not it is surrounded by quotes ‘"’; but if you wish to specify an empty file name is permitted, you must give the quotes–" ". This statement may go away in future: it is only recognized to be compatible with old as programs.
.ascii expects zero or more string literals (see Strings) separated by commas. It assembles each string (with no automatic trailing zero byte) into consecutive addresses.
.asciz is just like .ascii, but each string is followed by a zero byte. The z in .asciz stands for zero.
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.
The second expression (also absolute) gives the value to be stored in the padding bytes. It (and the comma) may be omitted. If it is omitted, the padding bytes are zero. However, on some 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 directive. If doing the alignment would require skipping more bytes than the specified maximum, then the alignment is not done at all. To omit the fill value (the second argument) entirely, use 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.
.byte expects zero or more expressions, separated by commas. Each expression is assembled into the next byte.
.comm declares a named common area in the bss section. Normally ld reserves memory addresses for it during linking, so no partial program defines the location of the symbol. Use .comm to tell ld that it must be at least length bytes long. ld allocates space for each .comm symbol that is at least as long as the longest .comm request in any of the partial programs linked. length is an absolute expression.
The syntax for .comm differs slightly on the HPPA. The syntax is symbol.comm, length; symbol is optional.
.data 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.
Begin defining debugging information for a symbol name; the definition extends until the .endef directive is encountered.
This directive is only observed when as is configured for COFF format output; when producing b.out, .def is recognized, but ignored.
This directive sets the descriptor of the symbol (see Symbol attributes) to the low 16 bits of an absolute expression.
The .desc directive is not available when as is configured for COFF output; it is only for a.out or b.out object format. For the sake of compatibility, as accepts it, but produces no output, when configured for COFF.
This directive is generated by compilers to include auxiliary debugging information in the symbol table. It is only permitted inside .def/.endef pairs. .dim is only meaningful when generating COFF format output; when as is generating b.out, it accepts this directive but ignores it.
.double expects zero or more flonums, separated by commas. It assembles floating point numbers. The exact kind of floating point numbers emitted depends on how as is configured. See Machine dependent features for documentation specifically discussing a particular architecture.
Force a page break at this point, when generating assembly listings.
.else is part of the as support for conditional assembly; see .if absolute expression. It marks the beginning of a section of code to be assembled if the condition for the preceding .if was false.
.end marks the end of the assembly file. as does not process anything in the file past the .end directive.
This directive flags the end of a symbol definition begun with .def.
.endef is only meaningful when generating COFF format output; if as is configured to generate b.out, it accepts this directive but ignores it.
.endfunc marks the end of a function specified with the .func directive
.endif is part of the as support for conditional assembly; it marks the end of a block of code that is only assembled conditionally. See.if absolute expression.
This directive sets the value of symbol to expression. It is synonymous with .set; see .set symbol, expression . The syntax for equ on the HPPA is symbol.equ expression.
Exit early from the current macro definition.
.extern is accepted in the source program—for compatibility with other assemblers—but it is ignored. as treats all undefined symbols as external.
.file (which may also be spelled .app-file) tells as that we are about to start a new logical file. string is the new file name. In general, the filename is recognized whether or not it is surrounded by quotes ("); but if you wish to specify an empty file name, you must give the quotes– " ". This statement may go away in future; it is only recognized to be compatible with old as programs. In some configurations of as, .file has already been removed to avoid conflicts with other assemblers. For documentation for a particular machine on this subject of .file, see Machine dependent features.
result, size and value are absolute expressions. This emits repeat copies of size bytes. repeat may be zero or more. size may be zero or more, but if it is more than 8, then it is deemed to have the value 8, compatible with other people’s assemblers. The contents of each repeat bytes is taken from an 8-byte number. The highest order 4 bytes are zero. The lowest order 4 bytes are value rendered in the byte-order of an integer on the computer as is assembling for. Each size bytes in a repetition is taken from the lowest order size bytes of this number. Again, this bizarre behavior is compatible with other people’s assemblers.
size and value are optional. If the second comma and value are absent, value is assumed zero. If the first comma and following tokens are absent, size is assumed to be 1.
.func emits debugging information to denote function, name, and is ignored unless the file is assembled with debugging enabled. Only --gstabs is currently supported. label is the entry point of the function and, if omitted, name prepends with leading char.
‘leading char’ is usually an underscore (_) or nothing, depending on the target. All functions are currently defined to have void return type. The function must be terminated with .endfunc.
.float assembles zero or more flonums, separated by commas. It has the same effect as .single. The exact kind of floating point numbers emitted depends on how as is configured. For documentation on a particular architecture's floating point issues, see Machine dependent features.
.global makes the symbol visible to ld. If you define symbol in your partial program, its value is made available to other partial programs that are linked with it. Otherwise, symbol takes its attributes from a symbol of the same name from another file linked into the same program. Both spellings (.globl and .global) are accepted, for compatibility with other assemblers.
On the HPPA, .global is not always enough to make it accessible to other partial programs. You may need the HPPA-only .EXPORT directive as well. See Assembler directives for HPPA.
.hword expects zero or more expressions, and emits a 16 bit number for each.
This directive is a synonym for .short; depending on the target architecture, it may also be a synonym for .word.
.ident is used by some assemblers to place tags in object files. as simply accepts the directive for source-file compatibility with such assemblers, but does not actually emit anything for it.
.if marks the beginning of a section of code which is only considered part of the source program being assembled if the argument (which must be an absolute expression) is non-zero. The end of the conditional section of code must be marked by .endif (see .endif ); optionally, you may include code for the alternative condition, flagged by .else (see .else . The following variants of .if are also supported:
.include provides a way to include supporting files at specified points in your source program. The code from file is assembled as if it followed the point of the .include; when the end of the included file is reached, assembly of the original file continues. You can control the search paths used with the -I command-line option (see Command-line options). Quotation marks are required around file.
Expect zero or more expressions, of any section, separated by commas. For each expression, emit a number that, at run time, is the value of that expression. The byte order and bit size of the number depends on what kind of target the assembly is for.
Evaluate a sequence of statements assigning different values to symbol. The sequence of statements starts at the .irp directive, and is terminated by an .endr directive. For each value, symbol is set to value, and the sequence of statements is assembled.
If no value is listed, the sequence of statements is assembled once, with symbol set to the null string. To refer to symbol within the sequence of statements, use \symbol.
Evaluate a sequence of statements assigning different values to symbol. The sequence of statements starts at the .irpc directive, and is terminated by an .endr directive. For each character in value, symbol is set to the character, and the sequence of statements is assembled. If no value is listed, the sequence of statements is assembled once, with symbol set to the null string. To refer to symbol within the sequence of statements, use \symbol.
.lcomm Reserve length (an absolute expression) bytes for a local common denoted by symbol. The section and value of symbol are those of the new local common. The addresses are allocated in the bss section, so that at run-time the bytes start off zeroed. symbol is not declared global (see .global symbol, .globl symbol ), so is normally not visible to ld.
The syntax for .lcomm differs slightly on the HPPA. The syntax is symbol.lcomm, length; symbol is optional.
as accepts this directive, for compatibility with other assemblers, but ignores it.
Change
the logical line number. line-number
must be an absolute expression. The next line has that logical line number.
Therefore any other statements on the current line (after a statement separator
character) are reported as on logical line number line-number-1.
One day as
will no longer support this directive: it is recognized only for compatibility
with existing assembler programs.
Inside a .def, .line is, instead, one of the directives used by compilers to generate auxiliary symbol information for debugging.
.ln is a synonym for .line.
If val is non-zero, this tells as to enter MRI mode. If val is zero, this tells as to exit MRI mode. This change affects code assembled until the next .mri directive, or until the end of the file. See Assemble in MRI compatibilty mode: -M .
Control (in conjunction with the .nolist directive) whether or not assembly listings are generated. These two directives maintain an internal counter (which is zero initially). .list increments the counter, and .nolist decrements it. Assembly listings are generated whenever the counter is greater than zero. By default, listings are disabled. When you enable them (with the -a command line option; see Command-line options), the initial value of the listing counter is one.
Mark the current section so that the linker only includes a single copy of it. This may be used to include the same section in several different object files, but ensure that the linker will only include it once in the final output file. The .linkonce pseudo-op must be used for each instance of the section. Duplicate sections are detected based on the section name, so it should be unique. This directive is only supported by a few object file formats; as of this writing, the only object file format which supports it is the Portable Executable format used on Windows NT. The type argument is optional. If specified, it must be one of the strings in the following list. Not all types may be supported on all object file formats.
Silently discard duplicate sections. This is the default.
Warn if there are duplicate sections, but still keep only one copy.
Warn if any of the duplicates have different sizes.
Warn if any of the duplicates do not have exactly the same contents.The following example shows the usage.
.linkonce same_size
.long is the same as .int, see .int expressions .
The commands, .macro and .endm, allow you to define macros that generate assembly output. For example, the following definition specifies a macro sum that puts a sequence of numbers into memory.
Control (in conjunction with the .list directive) whether or not assembly listings are generated. These two directives maintain an internal counter (which is zero initially). .list increments the counter, and .nolist decrements it. Assembly listings are generated whenever the counter is greater than zero.
.octa This directive expects zero or more bignums, separated by commas. For each bignum, it emits a 16-byte integer.
The term, octa, comes from contexts in which a word is two bytes; hence octa-word for 16 bytes.
Advance the location counter of the current section to new-lc. new-lc is either an absolute expression or an expression with the same section as the current subsection. That is, you can’t use .org to cross sections: if new-lc has the wrong section, the .org directive is ignored. To be compatible with former assemblers, if the section of new-lc is absolute, as issues a warning, then pretends the section of new-lc is the same as the current subsection. .org may only increase the location counter, or leave it unchanged; you cannot use .org to move the location counter backwards. Because as tries to assemble programs in one pass, new-lc may not be undefined. If you really detest this restriction we eagerly await a chance to share your improved assembler.
NOTE:
The
origin is relative to the start of the section, not to the start of the
subsection. This is compatible with other assemblers.
When the location counter (of the current subsection) is advanced, the intervening bytes are filled with fill which should be an absolute expression. If the comma and fill are omitted, fill defaults to zero.
.p2align[wl] pads the location counter (in the current subsection) to a particular storage boundary.
The first expression (which must be absolute) is the number of low-order zero bits the location counter must have after advancement. For example .p2align 3 advances the location counter until it a multiple of 8. If the location counter is already a multiple of 8, no change is needed.
The second expression (also absolute) gives the value to be stored in the padding bytes. It (and the comma) may be omitted. If it is omitted, the padding bytes are zero.
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. Omit the fill value (the second argument) entirely by 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 .p2alignw and .p2alignl directives are variants of the .p2align directive. The .p2alignw directive treats the fill pattern as a two byte word value. The .p2alignl directives treats the fill pattern as a four byte longword value. For example, .p2alignw2,0x368d 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.
as will print string on the standard output during assembly. You must put string in double quotes.
Use this directive to declare the number of lines—and, optionally, the number of columns—to use for each page, when generating listings.
If you do not use .psize, listings use a default line-count of 60. You may omit the comma and columns specification; the default width is 200 columns.
as generates formfeeds whenever the specified number of lines is exceeded (or whenever you explicitly request one, using .eject).
If you specify lines as 0, no formfeeds are generated save those explicitly specified with .eject.
.quad expects zero or more bignums, separated by commas. For each bignum, it emits an 8-byte integer. If the bignum won’t fit in 8 bytes, it prints a warning message; and just takes the lowest order 8 bytes of the bignum. The term quad comes from contexts in which a word is two bytes; hence quad-word for 8 bytes.
Repeat the sequence of lines between the .rept directive and the next .endr directive count times.
Use subheading as the title (third line, immediately after the title line) when generating assembly listings.
This directive affects subsequent pages, as well as the current page if it appears within ten lines of the top of a page.
Set the storage-class value for a symbol. This directive may only be used inside a .def/.endef pair. Storage class may flag whether a symbol is static or external, or it may record further symbolic debugging information.
The .scl directive is primarily associated with COFF output; when configured to generate b.out output format, as accepts this directive but ignores it.
Assemble the following code into end of subsection numbered subsection in the COFF named section name. If you omit subsection, as uses subsection number zero. .section .text is equivalent to the .text directive; .section .data is equivalent to the .data directive. This directive is only supported for targets that actually support arbitrarily named sections; on a.out targets, for example, it is not accepted, even with a standard a.out section name as its parameter.
Set the value of symbol to expression. This changes symbol’s value and type to conform to expression. If symbol was flagged as external, it remains flagged. (See Symbol attributes.)
You may .set a symbol many times in the same assembly.
If you .set a global symbol, the value stored in the object file is the last value stored into it.
The syntax for set on the HPPA is symbol.set expression.
.short is normally the same as .word. See .word expressions.
In some configurations, however, .short and .word generate numbers of different lengths; for discussion on this subject for particular architectures, see the documentation for the particular machine with Machine dependent features.
.single assembles zero or more flonums, separated by commas. It has the same effect as .float. The exact kind of floating point numbers emitted depends on how as is configured; see the documentation for the particular machine with Machine dependent features.
This directive is generated by compilers to include auxiliary debugging information in the symbol table. It is only permitted inside .def/.endef pairs.
.size is only meaningful when generating COFF format output; when as is generating b.out, it accepts this directive but ignores it.
This
directive emits size
bytes, each of value, fill.
Both size
and fill are
absolute expressions. If the comma and fill
are omitted, fill
is assumed to be zero.
There are three directives that begin .stabs. All emit symbols (see Symbols), for use by symbolic debuggers. The symbols are not entered in the as hash table: they cannot be referenced elsewhere in the source file. Up to five fields are required, as the following descriptions clarify.
The
following directives are three that begin .stabs.
The symbol’s value is set to the location counter, relocatability. When your program is linked, the value of this symbol is the address of the location counter when the .stabd was assembled.
Copy the characters in str to the object file. You may specify more than one string to copy, separated by commas. Unless otherwise specified for a particular machine, the assembler marks the end of each string with a 0 byte. You can use any of the escape sequences described in Strings.
Switch to the absolute section, and set the section offset to expression, which must be an absolute expression. The following example shows the usage.
The previous example’s input would define the symbol, field1, to have the value 0, the symbol, field2, to have the value 4, and the symbol, field3, to have the value 8..struct 0field1:.struct field1 + 4field2:.struct field2 + 4field3:
Assembly would be left in the absolute section, and you would need to use a .section directive of some sort to change to some other section before further assembly.
This directive is generated by compilers to include auxiliary debugging information in the symbol table. It is only permitted inside .def/.endef pairs. Tags are used to link structure definitions in the symbol table with instances of those structures. .tag is only used when generating COFF format output; when as is generating b.out, it accepts this directive but ignores it.
.text tells as to assemble the following statements onto the end of the text subsection numbered subsection, which is an absolute expression. If subsection is omitted, subsection number zero is used.
Use heading as the title (second line, immediately after the source file name and pagenumber) when generating assembly listings. .title affects subsequent pages, as well as the current page if it appears within ten lines of the top of a page.
This directive, permitted only within .def/.endef pairs, records the integer int as the type attribute of a symbol table entry. .type is associated only with COFF format output; when as is configured for b.out output, it accepts this directive but ignores it.
This directive, permitted only within .def/.endef pairs, records the address, addr, as the value attribute of a symbol table entry.
.val is used only for COFF output; when as is configured for b.out, it accepts this directive but ignores it.
This directive expects zero or more expressions, of any section, separated by commas. The size of the number emitted, and its byte order, depend on what target computer the assembly is for.
Machines with a 32-bit address space, but that do less than 32-bit addressing, require the following special treatment. If the machine of interest to you does 32-bit addressing, you can ignore this issue. Also ignore this issue if the machine you use doesn’t require it; see the discussion for the particular machine with Machine dependent features.
In order to assemble compiler output into something that works, as occasionally does strange things to .word directives.
Directives of the form .word sym1-sym2 are often emitted by compilers as part of jump tables. Therefore, when as assembles a directive of the form .word sym1-sym2, and the difference between sym1 and sym2 does not fit in 16 bits, as creates a secondary jump table, immediately before the next label. This secondary jump table is preceded by a short-jump to the first byte after the secondary table. This short-jump prevents the flow of control from accidentally falling into the new table. Inside the table is a long-jump to sym2. The original .word contains sym1 minus the address of the long-jump to sym2. If there were several occurrences of .word sym1-sym2 before the secondary jump table, all of them are adjusted.
If there was a .word sym3-sym4, that also did not fit in sixteen bits, a long-jump to sym4 is included in the secondary jump table, and the .word directives are adjusted to contain sym3 minus the address of the long-jump to sym4; and so on, for as many entries in the original jump table as necessary.
One day these directives won’t work. They are included for compatibility with older assemblers.