as.info: i386-Prefixes

Instruction Prefixes

   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.