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php爱好者> php文档>Assembler Directives

Assembler Directives

时间：2006-06-04 来源：rwen2012

All assembler directives have names that begin with a period (`.'). The rest of the name is letters, usually 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 section Machine Dependent Features.

.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.

.ABORT

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.

.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.

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 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 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 a29k, hppa, m68k, 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 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).

.app-file string

.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 "string"...

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

.asciz "string"...

.asciz is just like .ascii, but each string is followed by a zero byte. The "z" in `.asciz' stands for "zero".

.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.

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 expressions

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

.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, the .comm directive takes an optional third argument. This is the desired alignment of the symbol, specified as a byte boundary (for example, an alignment of 16 means that the least significant 4 bits of the address should be zero). 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.

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

.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.

.def name

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.

.desc symbol, abs-expression

This directive sets the descriptor of the symbol (see section 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.

.dim

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 flonums

.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 section Machine Dependent Features.

.eject

Force a page break at this point, when generating assembly listings.

.else

.else is part of the as support for conditional assembly; see section .if absolute expression. It marks the beginning of a section of code to be assembled if the condition for the preceding .if was false.

.endef

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.

.endif

.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 section .if absolute expression.

.equ symbol, expression

This directive sets the value of symbol to expression. It is synonymous with `.set'; see section .set symbol, expression.

The syntax for equ on the HPPA is `symbol .equ expression'.

.equiv symbol, expression

The .equiv directive is like .equ and .set, except that the assembler will signal an error if symbol is already defined.

Except for the contents of the error message, this is roughly equivalent to

.ifdef SYM

.err

.endif

.equ SYM,VAL

.err

If as assembles a .err directive, it will print an error message and, unless the -Z option was used, it will not generate an object file. This can be used to signal error an conditionally compiled code.

.extern

.extern is accepted in the source program--for compatibility with other assemblers--but it is ignored. as treats all undefined symbols as external.

.file string

.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. See section Machine Dependent Features.

.fill repeat , size , value

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.

.float flonums

This directive 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. See section Machine Dependent Features.

.global symbol, .globl symbol

.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 section HPPA Assembler Directives.

.hword expressions

This 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

This directive 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 absolute expression

.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 section .endif); optionally, you may include code for the alternative condition, flagged by .else (see section .else).

The following variants of .if are also supported:

.ifdef symbol

Assembles the following section of code if the specified symbol has been defined.

.ifndef symbol

.ifnotdef symbol

Assembles the following section of code if the specified symbol has not been defined. Both spelling variants are equivalent.

.include "file"

This directive 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 section Command-Line Options). Quotation marks are required around file.

.int expressions

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.

.irp symbol,values...

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.

For example, assembling

.irp param,1,2,3

move d\param,sp@-

.endr

is equivalent to assembling

move d1,sp@-

move d2,sp@-

move d3,sp@-

.irpc symbol,values...

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.

For example, assembling

.irpc param,123

move d\param,sp@-

.endr

is equivalent to assembling

move d1,sp@-

move d2,sp@-

move d3,sp@-

.lcomm symbol , length

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 section .global symbol, .globl symbol), so is normally not visible to ld.

Some targets permit a third argument to be used with .lcomm. This argument specifies the desired alignment of the symbol in the bss section.

The syntax for .lcomm differs slightly on the HPPA. The syntax is `symbol .lcomm, length'; symbol is optional.

.lflags

as accepts this directive, for compatibility with other assemblers, but ignores it.

.line line-number

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.

Warning: In the AMD29K configuration of as, this command is not available; use the synonym .ln in that context.

Even though this is a directive associated with the a.out or b.out object-code formats, as still recognizes it when producing COFF output, and treats `.line' as though it were the COFF `.ln' if it is found outside a .def/.endef pair.

Inside a .def, `.line' is, instead, one of the directives used by compilers to generate auxiliary symbol information for debugging.

.linkonce [type]

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 following strings. For example:

.linkonce same_size

Not all types may be supported on all object file formats.

discard

Silently discard duplicate sections. This is the default.

one_only

Warn if there are duplicate sections, but still keep only one copy.

same_size

Warn if any of the duplicates have different sizes.

same_contents

Warn if any of the duplicates do not have exactly the same contents.

.ln line-number

`.ln' is a synonym for `.line'.

.mri val

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 section Assemble in MRI Compatibility Mode: -M.

.list

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 section Command-Line Options), the initial value of the listing counter is one.

.long expressions

.long is the same as `.int', see section .int expressions.

.macro

The commands .macro and .endm allow you to define macros that generate assembly output. For example, this definition specifies a macro sum that puts a sequence of numbers into memory:

.macro sum from=0, to=5

.long \from

.if \to-\from

sum "(\from+1)",\to

.endif

.endm

With that definition, `SUM 0,5' is equivalent to this assembly input:

.long 0

.long 1

.long 2

.long 3

.long 4

.long 5

.macro macname

.macro macname macargs ...

Begin the definition of a macro called macname. If your macro definition requires arguments, specify their names after the macro name, separated by commas or spaces. You can supply a default value for any macro argument by following the name with `=deflt'. For example, these are all valid .macro statements:

.macro comm

Begin the definition of a macro called comm, which takes no arguments.

.macro plus1 p, p1

.macro plus1 p p1

Either statement begins the definition of a macro called plus1, which takes two arguments; within the macro definition, write `\p' or `\p1' to evaluate the arguments.

.macro reserve_str p1=0 p2

Begin the definition of a macro called reserve_str, with two arguments. The first argument has a default value, but not the second. After the definition is complete, you can call the macro either as `reserve_str a,b' (with `\p1' evaluating to a and `\p2' evaluating to b), or as `reserve_str ,b' (with `\p1' evaluating as the default, in this case `0', and `\p2' evaluating to b).

When you call a macro, you can specify the argument values either by position, or by keyword. For example, `sum 9,17' is equivalent to `sum to=17, from=9'.

.endm

Mark the end of a macro definition.

.exitm

Exit early from the current macro definition.

as maintains a counter of how many macros it has executed in this pseudo-variable; you can copy that number to your output with `\@', but only within a macro definition.

.nolist

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 bignums

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.

.org new-lc , fill

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.

Beware that the origin is relative to the start of the section, not to the start of the subsection. This is compatible with other people's 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] 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 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 .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, .p2alignw 2,0x368d 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.

.psize lines , columns

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 bignums

.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.

.rept count

Repeat the sequence of lines between the .rept directive and the next .endr directive count times.

For example, assembling

.rept 3

.long 0

.endr

is equivalent to assembling

.long 0

.sbttl "subheading"

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.

.scl class

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.

.section name

Use the .section directive to assemble the following code into a section named name.

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.

For COFF targets, the .section directive is used in one of the following ways:

.section name[, "flags"]

.section name[, subsegment]

If the optional argument is quoted, it is taken as flags to use for the section. Each flag is a single character. The following flags are recognized:

bss section (uninitialized data)

section is not loaded

writable section

data section

read-only section

executable section

If no flags are specified, the default flags depend upon the section name. If the section name is not recognized, the default will be for the section to be loaded and writable.

If the optional argument to the .section directive is not quoted, it is taken as a subsegment number (see section Sub-Sections).

For ELF targets, the .section directive is used like this:

.section name[, "flags"[, @type]]

The optional flags argument is a quoted string which may contain any combintion of the following characters:

section is allocatable

section is writable

section is executable

The optional type argument may contain one of the following constants:

@progbits

section contains data

@nobits

section does not contain data (i.e., section only occupies space)

If no flags are specified, the default flags depend upon the section name. If the section name is not recognized, the default will be for the section to have none of the above flags: it will not be allocated in memory, nor writable, nor executable. The section will contain data.

For ELF targets, the assembler supports another type of .section directive for compatibility with the Solaris assembler:

.section "name"[, flags...]

Note that the section name is quoted. There may be a sequence of comma separated flags:

#alloc

section is allocatable

#write

section is writable

#execinstr

section is executable

.set symbol, expression

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 section 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 expressions

.short is normally the same as `.word'. See section .word expressions.

In some configurations, however, .short and .word generate numbers of different lengths; see section Machine Dependent Features.

.single flonums

This directive 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 section Machine Dependent Features.

.size

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.

.sleb128 expressions

sleb128 stands for "signed little endian base 128." This is a compact, variable length representation of numbers used by the DWARF symbolic debugging format. See section .uleb128 expressions.

.skip size , fill

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. This is the same as `.space'.

.space size , fill

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. This is the same as `.skip'.

Warning: .space has a completely different meaning for HPPA targets; use .block as a substitute. See HP9000 Series 800 Assembly Language Reference Manual (HP 92432-90001) for the meaning of the .space directive. See section HPPA Assembler Directives, for a summary.

On the AMD 29K, this directive is ignored; it is accepted for compatibility with other AMD 29K assemblers.

Warning: In most versions of the GNU assembler, the directive .space has the effect of .block See section Machine Dependent Features.

.stabd, .stabn, .stabs

There are three directives that begin `.stab'. All emit symbols (see section 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:

string

This is the symbol's name. It may contain any character except `\000', so is more general than ordinary symbol names. Some debuggers used to code arbitrarily complex structures into symbol names using this field.

type

An absolute expression. The symbol's type is set to the low 8 bits of this expression. Any bit pattern is permitted, but ld and debuggers choke on silly bit patterns.

other

An absolute expression. The symbol's "other" attribute is set to the low 8 bits of this expression.

desc

An absolute expression. The symbol's descriptor is set to the low 16 bits of this expression.

value

An absolute expression which becomes the symbol's value.

If a warning is detected while reading a .stabd, .stabn, or .stabs statement, the symbol has probably already been created; you get a half-formed symbol in your object file. This is compatible with earlier assemblers!

.stabd type , other , desc

The "name" of the symbol generated is not even an empty string. It is a null pointer, for compatibility. Older assemblers used a null pointer so they didn't waste space in object files with empty strings. The symbol's value is set to the location counter, relocatably. When your program is linked, the value of this symbol is the address of the location counter when the .stabd was assembled.

.stabn type , other , desc , value

The name of the symbol is set to the empty string "".

.stabs string , type , other , desc , value

All five fields are specified.

.string "str"

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 section Strings.

.symver

Use the .symver directive to bind symbols to specific version nodes within a source file. This is only supported on ELF platforms, and is typically used when assembling files to be linked into a shared library. There are cases where it may make sense to use this in objects to be bound into an application itself so as to override a versioned symbol from a shared library.

For ELF targets, the .symver directive is used like this:

.symver name, name2@nodename

In this case, the symbol name must exist and be defined within the file being assembled. The .versym directive effectively creates a symbol alias with the name name2@nodename, and in fact the main reason that we just don't try and create a regular alias is that the @ character isn't permitted in symbol names. The name2 part of the name is the actual name of the symbol by which it will be externally referenced. The name name itself is merely a name of convenience that is used so that it is possible to have definitions for multiple versions of a function within a single source file, and so that the compiler can unambiguously know which version of a function is being mentioned. The nodename portion of the alias should be the name of a node specified in the version script supplied to the linker when building a shared library. If you are attempting to override a versioned symbol from a shared library, then nodename should correspond to the nodename of the symbol you are trying to override.

.tag structname

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 subsection

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.

.title "heading"

Use heading as the title (second line, immediately after the source file name and pagenumber) 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.

.type int

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.

.val addr

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.

.uleb128 expressions

uleb128 stands for "unsigned little endian base 128." This is a compact, variable length representation of numbers used by the DWARF symbolic debugging format. See section .sleb128 expressions.

.word expressions

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.

Warning: Special Treatment to support Compilers

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 (or doesn't require it; see section Machine Dependent Features), you can ignore this issue.

In order to assemble compiler output into something that works, as occasionlly 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.

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.

M680x0 Dependent Features

M680x0 Options

The Motorola 680x0 version of as has a few machine dependent options.

You can use the `-l' option to shorten the size of references to undefined symbols. If you do not use the `-l' option, references to undefined symbols are wide enough for a full long (32 bits). (Since as cannot know where these symbols end up, as can only allocate space for the linker to fill in later. Since as does not know how far away these symbols are, it allocates as much space as it can.) If you use this option, the references are only one word wide (16 bits). This may be useful if you want the object file to be as small as possible, and you know that the relevant symbols are always less than 17 bits away.

For some configurations, especially those where the compiler normally does not prepend an underscore to the names of user variables, the assembler requires a `%' before any use of a register name. This is intended to let the assembler distinguish between C variables and functions named `a0' through `a7', and so on. The `%' is always accepted, but is not required for certain configurations, notably `sun3'. The `--register-prefix-optional' option may be used to permit omitting the `%' even for configurations for which it is normally required. If this is done, it will generally be impossible to refer to C variables and functions with the same names as register names.

Normally the character `|' is treated as a comment character, which means that it can not be used in expressions. The `--bitwise-or' option turns `|' into a normal character. In this mode, you must either use C style comments, or start comments with a `#' character at the beginning of a line.

If you use an addressing mode with a base register without specifying the size, as will normally use the full 32 bit value. For example, the addressing mode `%a0@(%d0)' is equivalent to `%a0@(%d0:l)'. You may use the `--base-size-default-16' option to tell as to default to using the 16 bit value. In this case, `%a0@(%d0)' is equivalent to `%a0@(%d0:w)'. You may use the `--base-size-default-32' option to restore the default behaviour.

If you use an addressing mode with a displacement, and the value of the displacement is not known, as will normally assume that the value is 32 bits. For example, if the symbol `disp' has not been defined, as will assemble the addressing mode `%a0@(disp,%d0)' as though `disp' is a 32 bit value. You may use the `--disp-size-default-16' option to tell as to instead assume that the displacement is 16 bits. In this case, as will assemble `%a0@(disp,%d0)' as though `disp' is a 16 bit value. You may use the `--disp-size-default-32' option to restore the default behaviour.

as can assemble code for several different members of the Motorola 680x0 family. The default depends upon how as was configured when it was built; normally, the default is to assemble code for the 68020 microprocessor. The following options may be used to change the default. These options control which instructions and addressing modes are permitted. The members of the 680x0 family are very similar. For detailed information about the differences, see the Motorola manuals.

`-m68000'

`-m68ec000'

`-m68hc000'

`-m68hc001'

`-m68008'

`-m68302'

`-m68306'

`-m68307'

`-m68322'

`-m68356'

Assemble for the 68000. `-m68008', `-m68302', and so on are synonyms for `-m68000', since the chips are the same from the point of view of the assembler.

`-m68010'

Assemble for the 68010.

`-m68020'

`-m68ec020'

Assemble for the 68020. This is normally the default.

`-m68030'

`-m68ec030'

Assemble for the 68030.

`-m68040'

`-m68ec040'

Assemble for the 68040.

`-m68060'

`-m68ec060'

Assemble for the 68060.

`-mcpu32'

`-m68330'

`-m68331'

`-m68332'

`-m68333'

`-m68334'

`-m68336'

`-m68340'

`-m68341'

`-m68349'

`-m68360'

Assemble for the CPU32 family of chips.

`-m5200'

Assemble for the ColdFire family of chips.

`-m68881'

`-m68882'

Assemble 68881 floating point instructions. This is the default for the 68020, 68030, and the CPU32. The 68040 and 68060 always support floating point instructions.

`-mno-68881'

Do not assemble 68881 floating point instructions. This is the default for 68000 and the 68010. The 68040 and 68060 always support floating point instructions, even if this option is used.

`-m68851'

Assemble 68851 MMU instructions. This is the default for the 68020, 68030, and 68060. The 68040 accepts a somewhat different set of MMU instructions; `-m68851' and `-m68040' should not be used together.

`-mno-68851'

Do not assemble 68851 MMU instructions. This is the default for the 68000, 68010, and the CPU32. The 68040 accepts a somewhat different set of MMU instructions.

Syntax

This syntax for the Motorola 680x0 was developed at MIT.

The 680x0 version of as uses instructions names and syntax compatible with the Sun assembler. Intervening periods are ignored; for example, `movl' is equivalent to `mov.l'.

In the following table apc stands for any of the address registers (`%a0' through `%a7'), the program counter (`%pc'), the zero-address relative to the program counter (`%zpc'), a suppressed address register (`%za0' through `%za7'), or it may be omitted entirely. The use of size means one of `w' or `l', and it may be omitted, along with the leading colon, unless a scale is also specified. The use of scale means one of `1', `2', `4', or `8', and it may always be omitted along with the leading colon.

The following addressing modes are understood:

Immediate

`#number'

Data Register

`%d0' through `%d7'

Address Register

`%a0' through `%a7'
`%a7' is also known as `%sp', i.e. the Stack Pointer. %a6 is also known as `%fp', the Frame Pointer.

Address Register Indirect

`%a0@' through `%a7@'

Address Register Postincrement

`%a0@+' through `%a7@+'

Address Register Predecrement

`%a0@-' through `%a7@-'

Indirect Plus Offset

`apc@(number)'

Index

`apc@(number,register:size:scale)' The number may be omitted.

Postindex

`apc@(number)@(onumber,register:size:scale)' The onumber or the register, but not both, may be omitted.

Preindex

`apc@(number,register:size:scale)@(onumber)' The number may be omitted. Omitting the register produces the Postindex addressing mode.

Absolute

`symbol', or `digits', optionally followed by `:b', `:w', or `:l'.

Motorola Syntax

The standard Motorola syntax for this chip differs from the syntax already discussed (see section Syntax). as can accept Motorola syntax for operands, even if MIT syntax is used for other operands in the same instruction. The two kinds of syntax are fully compatible.

In the following table apc stands for any of the address registers (`%a0' through `%a7'), the program counter (`%pc'), the zero-address relative to the program counter (`%zpc'), or a suppressed address register (`%za0' through `%za7'). The use of size means one of `w' or `l', and it may always be omitted along with the leading dot. The use of scale means one of `1', `2', `4', or `8', and it may always be omitted along with the leading asterisk.

The following additional addressing modes are understood:

Address Register Indirect

`(%a0)' through `(%a7)'
`%a7' is also known as `%sp', i.e. the Stack Pointer. %a6 is also known as `%fp', the Frame Pointer.

Address Register Postincrement

`(%a0)+' through `(%a7)+'

Address Register Predecrement

`-(%a0)' through `-(%a7)'

Indirect Plus Offset

`number(%a0)' through `number(%a7)', or `number(%pc)'. The number may also appear within the parentheses, as in `(number,%a0)'. When used with the pc, the number may be omitted (with an address register, omitting the number produces Address Register Indirect mode).

Index

`number(apc,register.size*scale)' The number may be omitted, or it may appear within the parentheses. The apc may be omitted. The register and the apc may appear in either order. If both apc and register are address registers, and the size and scale are omitted, then the first register is taken as the base register, and the second as the index register.

Postindex

`([number,apc],register.size*scale,onumber)' The onumber, or the register, or both, may be omitted. Either the number or the apc may be omitted, but not both.

Preindex

`([number,apc,register.size*scale],onumber)' The number, or the apc, or the register, or any two of them, may be omitted. The onumber may be omitted. The register and the apc may appear in either order. If both apc and register are address registers, and the size and scale are omitted, then the first register is taken as the base register, and the second as the index register.

Floating Point

Packed decimal (P) format floating literals are not supported. Feel free to add the code!

The floating point formats generated by directives are these.

.float

Single precision floating point constants.

.double

Double precision floating point constants.

.extend

.ldouble

Extended precision (long double) floating point constants.

680x0 Machine Directives

In order to be compatible with the Sun assembler the 680x0 assembler understands the following directives.

.data1

This directive is identical to a .data 1 directive.

.data2

This directive is identical to a .data 2 directive.

.even

This directive is a special case of the .align directive; it aligns the output to an even byte boundary.

.skip

This directive is identical to a .space directive.

Opcodes

Branch Improvement

Certain pseudo opcodes are permitted for branch instructions. They expand to the shortest branch instruction that reach the target. Generally these mnemonics are made by substituting `j' for `b' at the start of a Motorola mnemonic.

The following table summarizes the pseudo-operations. A * flags cases that are more fully described after the table:

          Displacement

          +-------------------------------------------------

          |                68020   68000/10

Pseudo-Op |BYTE    WORD    LONG    LONG      non-PC relative

          +-------------------------------------------------

     jbsr |bsrs    bsr     bsrl    jsr       jsr

      jra |bras    bra     bral    jmp       jmp

*     jXX |bXXs    bXX     bXXl    bNXs;jmpl bNXs;jmp

*    dbXX |dbXX    dbXX        dbXX; bra; jmpl

*    fjXX |fbXXw   fbXXw   fbXXl             fbNXw;jmp

XX: condition

NX: negative of condition XX

*---see full description below

jbsr

jra

These are the simplest jump pseudo-operations; they always map to one particular machine instruction, depending on the displacement to the branch target.

jXX

Here, `jXX' stands for an entire family of pseudo-operations, where XX is a conditional branch or condition-code test. The full list of pseudo-ops in this family is:

 jhi   jls   jcc   jcs   jne   jeq   jvc

 jvs   jpl   jmi   jge   jlt   jgt   jle

For the cases of non-PC relative displacements and long displacements on the 68000 or 68010, as issues a longer code fragment in terms of NX, the opposite condition to XX. For example, for the non-PC relative case:

    jXX foo

gives

     bNXs oof

     jmp foo

 oof:

dbXX

The full family of pseudo-operations covered here is

 dbhi   dbls   dbcc   dbcs   dbne   dbeq   dbvc

 dbvs   dbpl   dbmi   dbge   dblt   dbgt   dble

 dbf    dbra   dbt

Other than for word and byte displacements, when the source reads `dbXX foo', as emits

     dbXX oo1

     bra oo2

 oo1:jmpl foo

 oo2:

fjXX

This family includes

 fjne   fjeq   fjge   fjlt   fjgt   fjle   fjf

 fjt    fjgl   fjgle  fjnge  fjngl  fjngle fjngt

 fjnle  fjnlt  fjoge  fjogl  fjogt  fjole  fjolt

 fjor   fjseq  fjsf   fjsne  fjst   fjueq  fjuge

 fjugt  fjule  fjult  fjun

For branch targets that are not PC relative, as emits

     fbNX oof

     jmp foo

 oof:

when it encounters `fjXX foo'.

Special Characters

The immediate character is `#' for Sun compatibility. The line-comment character is `|' (unless the `--bitwise-or' option is used). If a `#' appears at the beginning of a line, it is treated as a comment unless it looks like `# line file', in which case it is treated normally.

80386 Dependent Features

Options

The 80386 has no machine dependent options.

AT&T Syntax versus Intel Syntax

In order to maintain compatibility with the output of gcc, as supports AT&T System V/386 assembler syntax. This is quite different from Intel syntax. We mention these differences because almost all 80386 documents used only Intel syntax. Notable differences between the two syntaxes are:

AT&T immediate operands are preceded by `$'; Intel immediate operands are undelimited (Intel `push 4' is AT&T `pushl $4'). AT&T register operands are preceded by `%'; Intel register operands are undelimited. AT&T absolute (as opposed to PC relative) jump/call operands are prefixed by `*'; they are undelimited in Intel syntax.
AT&T and Intel syntax use the opposite order for source and destination operands. Intel `add eax, 4' is `addl $4, %eax'. The `source, dest' convention is maintained for compatibility with previous Unix assemblers.
In AT&T syntax the size of memory operands is determined from the last character of the opcode name. Opcode suffixes of `b', `w', and `l' specify byte (8-bit), word (16-bit), and long (32-bit) memory references. Intel syntax accomplishes this by prefixes memory operands (not the opcodes themselves) with `byte ptr', `word ptr', and `dword ptr'. Thus, Intel `mov al, byte ptr foo' is `movb foo, %al' in AT&T syntax.
Immediate form long jumps and calls are `lcall/ljmp $section, $offset' in AT&T syntax; the Intel syntax is `call/jmp far section:offset'. Also, the far return instruction is `lret $stack-adjust' in AT&T syntax; Intel syntax is `ret far stack-adjust'.
The AT&T assembler does not provide support for multiple section programs. Unix style systems expect all programs to be single sections.

Opcode Naming

Opcode names are suffixed with one character modifiers which specify the size of operands. The letters `b', `w', and `l' specify byte, word, and long operands. If no suffix is specified by an instruction and it contains no memory operands then as tries to fill in the missing suffix based on the destination register operand (the last one by convention). Thus, `mov %ax, %bx' is equivalent to `movw %ax, %bx'; also, `mov $1, %bx' is equivalent to `movw $1, %bx'. Note that this is incompatible with the AT&T Unix assembler which assumes that a missing opcode suffix implies long operand size. (This incompatibility does not affect compiler output since compilers always explicitly specify the opcode suffix.)

Almost all opcodes have the same names in AT&T and Intel format. There are a few exceptions. The sign extend and zero extend instructions need two sizes to specify them. They need a size to sign/zero extend from and a size to zero extend to. This is accomplished by using two opcode suffixes in AT&T syntax. Base names for sign extend and zero extend are `movs...' and `movz...' in AT&T syntax (`movsx' and `movzx' in Intel syntax). The opcode suffixes are tacked on to this base name, the from suffix before the to suffix. Thus, `movsbl %al, %edx' is AT&T syntax for "move sign extend from %al to %edx." Possible suffixes, thus, are `bl' (from byte to long), `bw' (from byte to word), and `wl' (from word to long).

The Intel-syntax conversion instructions

`cbw' -- sign-extend byte in `%al' to word in `%ax',
`cwde' -- sign-extend word in `%ax' to long in `%eax',
`cwd' -- sign-extend word in `%ax' to long in `%dx:%ax',
`cdq' -- sign-extend dword in `%eax' to quad in `%edx:%eax',

are called `cbtw', `cwtl', `cwtd', and `cltd' in AT&T naming. as accepts either naming for these instructions.

Far call/jump instructions are `lcall' and `ljmp' in AT&T syntax, but are `call far' and `jump far' in Intel convention.

Register Naming

the 8 32-bit registers `%eax' (the accumulator), `%ebx', `%ecx', `%edx', `%edi', `%esi', `%ebp' (the frame pointer), and `%esp' (the stack pointer).
the 8 16-bit low-ends of these: `%ax', `%bx', `%cx', `%dx', `%di', `%si', `%bp', and `%sp'.
the 8 8-bit registers: `%ah', `%al', `%bh', `%bl', `%ch', `%cl', `%dh', and `%dl' (These are the high-bytes and low-bytes of `%ax', `%bx', `%cx', and `%dx')
the 6 section registers `%cs' (code section), `%ds' (data section), `%ss' (stack section), `%es', `%fs', and `%gs'.
the 3 processor control registers `%cr0', `%cr2', and `%cr3'.
the 6 debug registers `%db0', `%db1', `%db2', `%db3', `%db6', and `%db7'.
the 2 test registers `%tr6' and `%tr7'.
the 8 floating point register stack `%st' or equivalently `%st(0)', `%st(1)', `%st(2)', `%st(3)', `%st(4)', `%st(5)', `%st(6)', and `%st(7)'.

Opcode Prefixes

Opcode prefixes are used to modify the following opcode. They are used to repeat string instructions, to provide section overrides, to perform bus lock operations, and to give operand and address size (16-bit operands are specified in an instruction by prefixing what would normally be 32-bit operands with a "operand size" opcode prefix). Opcode prefixes are usually given as single-line instructions with no operands, and must directly precede the instruction they act upon. For example, the `scas' (scan string) instruction is repeated with:

        repne

        scas

Here is a list of opcode prefixes:

Section override prefixes `cs', `ds', `ss', `es', `fs', `gs'. These are automatically added by specifying using the section:memory-operand form for memory references.
Operand/Address size prefixes `data16' and `addr16' change 32-bit operands/addresses into 16-bit operands/addresses. Note that 16-bit addressing modes (i.e. 8086 and 80286 addressing modes) are not supported (yet).
The bus lock prefix `lock' inhibits interrupts during execution of the instruction it precedes. (This is only valid with certain instructions; see a 80386 manual for details).
The wait for coprocessor prefix `wait' waits for the coprocessor to complete the current instruction. This should never be needed for the 80386/80387 combination.
The `rep', `repe', and `repne' prefixes are added to string instructions to make them repeat `%ecx' times.

Memory References

An Intel syntax indirect memory reference of the form

section:[base + index*scale + disp]

is translated into the AT&T syntax

section:disp(base, index, scale)

where base and index are the optional 32-bit base and index registers, disp is the optional displacement, and scale, taking the values 1, 2, 4, and 8, multiplies index to calculate the address of the operand. If no scale is specified, scale is taken to be 1. section specifies the optional section register for the memory operand, and may override the default section register (see a 80386 manual for section register defaults). Note that section overrides in AT&T syntax must have be preceded by a `%'. If you specify a section override which coincides with the default section register, as does not output any section register override prefixes to assemble the given instruction. Thus, section overrides can be specified to emphasize which section register is used for a given memory operand.

Here are some examples of Intel and AT&T style memory references:

AT&T: `-4(%ebp)', Intel: `[ebp - 4]'

base is `%ebp'; disp is `-4'. section is missing, and the default section is used (`%ss' for addressing with `%ebp' as the base register). index, scale are both missing.

AT&T: `foo(,%eax,4)', Intel: `[foo + eax*4]'

index is `%eax' (scaled by a scale 4); disp is `foo'. All other fields are missing. The section register here defaults to `%ds'.

AT&T: `foo(,1)'; Intel `[foo]'

This uses the value pointed to by `foo' as a memory operand. Note that base and index are both missing, but there is only one `,'. This is a syntactic exception.

AT&T: `%gs:foo'; Intel `gs:foo'

This selects the contents of the variable `foo' with section register section being `%gs'.

Absolute (as opposed to PC relative) call and jump operands must be prefixed with `*'. If no `*' is specified, as always chooses PC relative addressing for jump/call labels.

Any instruction that has a memory operand must specify its size (byte, word, or long) with an opcode suffix (`b', `w', or `l', respectively).

Handling of Jump Instructions

Jump instructions are always optimized to use the smallest possible displacements. This is accomplished by using byte (8-bit) displacement jumps whenever the target is sufficiently close. If a byte displacement is insufficient a long (32-bit) displacement is used. We do not support word (16-bit) displacement jumps (i.e. prefixing the jump instruction with the `addr16' opcode prefix), since the 80386 insists upon masking `%eip' to 16 bits after the word displacement is added.

Note that the `jcxz', `jecxz', `loop', `loopz', `loope', `loopnz' and `loopne' instructions only come in byte displacements, so that if you use these instructions (gcc does not use them) you may get an error message (and incorrect code). The AT&T 80386 assembler tries to get around this problem by expanding `jcxz foo' to

         jcxz cx_zero

         jmp cx_nonzero

cx_zero: jmp foo

cx_nonzero:

Floating Point

All 80387 floating point types except packed BCD are supported. (BCD support may be added without much difficulty). These data types are 16-, 32-, and 64- bit integers, and single (32-bit), double (64-bit), and extended (80-bit) precision floating point. Each supported type has an opcode suffix and a constructor associated with it. Opcode suffixes specify operand's data types. Constructors build these data types into memory.

Floating point constructors are `.float' or `.single', `.double', and `.tfloat' for 32-, 64-, and 80-bit formats. These correspond to opcode suffixes `s', `l', and `t'. `t' stands for temporary real, and that the 80387 only supports this format via the `fldt' (load temporary real to stack top) and `fstpt' (store temporary real and pop stack) instructions.
Integer constructors are `.word', `.long' or `.int', and `.quad' for the 16-, 32-, and 64-bit integer formats. The corresponding opcode suffixes are `s' (single), `l' (long), and `q' (quad). As with the temporary real format the 64-bit `q' format is only present in the `fildq' (load quad integer to stack top) and `fistpq' (store quad integer and pop stack) instructions.

Register to register operations do not require opcode suffixes, so that `fst %st, %st(1)' is equivalent to `fstl %st, %st(1)'.

Writing 16-bit Code

While GAS normally writes only "pure" 32-bit i386 code, it has limited support for writing code to run in real mode or in 16-bit protected mode code segments. To do this, insert a `.code16' directive before the assembly language instructions to be run in 16-bit mode. You can switch GAS back to writing normal 32-bit code with the `.code32' directive.

GAS understands exactly the same assembly language syntax in 16-bit mode as in 32-bit mode. The function of any given instruction is exactly the same regardless of mode, as long as the resulting object code is executed in the mode for which GAS wrote it. So, for example, the `ret' mnemonic produces a 32-bit return instruction regardless of whether it is to be run in 16-bit or 32-bit mode. (If GAS is in 16-bit mode, it will add an operand size prefix to the instruction to force it to be a 32-bit return.)

This means, for one thing, that you can use GNU CC to write code to be run in real mode or 16-bit protected mode. Just insert the statement `asm(".code16");' at the beginning of your C source file, and while GNU CC will still be generating 32-bit code, GAS will automatically add all the necessary size prefixes to make that code run in 16-bit mode. Of course, since GNU CC only writes small-model code (it doesn't know how to attach segment selectors to pointers like native x86 compilers do), any 16-bit code you write with GNU CC will essentially be limited to a 64K address space. Also, there will be a code size and performance penalty due to all the extra address and operand size prefixes GAS has to add to the instructions.

Note that placing GAS in 16-bit mode does not mean that the resulting code will necessarily run on a 16-bit pre-80386 processor. To write code that runs on such a processor, you would have to refrain from using any 32-bit constructs which require GAS to output address or operand size prefixes. At the moment this would be rather difficult, because GAS currently supports only 32-bit addressing modes: when writing 16-bit code, it always outputs address size prefixes for any instruction that uses a non-register addressing mode. So you can write code that runs on 16-bit processors, but only if that code never references memory.

Notes

There is some trickery concerning the `mul' and `imul' instructions that deserves mention. The 16-, 32-, and 64-bit expanding multiplies (base opcode `0xf6'; extension 4 for `mul' and 5 for `imul') can be output only in the one operand form. Thus, `imul %ebx, %eax' does not select the expanding multiply; the expanding multiply would clobber the `%edx' register, and this would confuse gcc output. Use `imul %ebx' to get the 64-bit product in `%edx:%eax'.

We have added a two operand form of `imul' when the first operand is an immediate mode expression and the second operand is a register. This is just a shorthand, so that, multiplying `%eax' by 69, for example, can be done with `imul $69, %eax' rather than `imul $69, %eax, %eax'.