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.