as: i386-Prefixes
9.15.6 Instruction Prefixes
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Instruction prefixes are used to modify the following instruction. They
are used to repeat string instructions, to provide section overrides, to
perform bus lock operations, and to change operand and address sizes.
(Most instructions that normally operate on 32-bit operands will use
16-bit operands if the instruction has an "operand size" prefix.)
Instruction prefixes are best written on the same line as the
instruction they act upon. For example, the 'scas' (scan string)
instruction is repeated with:
repne scas %es:(%edi),%al
You may also place prefixes on the lines immediately preceding the
instruction, but this circumvents checks that 'as' does with prefixes,
and will not work with all prefixes.
Here is a list of instruction 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, while 'data32'
and 'addr32' change 16-bit ones (in a '.code16' section) into
32-bit operands/addresses. These prefixes _must_ appear on the
same line of code as the instruction they modify. For example, in
a 16-bit '.code16' section, you might write:
addr32 jmpl *(%ebx)
* 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 ('%cx' times if the
current address size is 16-bits).
* The 'rex' family of prefixes is used by x86-64 to encode extensions
to i386 instruction set. The 'rex' prefix has four bits -- an
operand size overwrite ('64') used to change operand size from
32-bit to 64-bit and X, Y and Z extensions bits used to extend the
register set.
You may write the 'rex' prefixes directly. The 'rex64xyz'
instruction emits 'rex' prefix with all the bits set. By omitting
the '64', 'x', 'y' or 'z' you may write other prefixes as well.
Normally, there is no need to write the prefixes explicitly, since
gas will automatically generate them based on the instruction
operands.
Info Catalog
as: i386-Regs
as: i386-Dependent
as: i386-Memory