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8.8.5 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.