i386-Prefixes - Using as
Description: Using as

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#### 9.13.7 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.