Opcode prefix

In computing, an opcode prefix is a numeric prefix code that alters the function of a following opcode. On some instruction set architectures multiple opcode prefixes are allowed sequentially, with all combining to alter the subsequent opcode. The opcode prefix is a portion of a larger machine languageinstruction that specifies the operation to be performed.

In addition to the opcode, some instructions specify the operands the operation will act upon. Opcode prefixes may alter the number, size, or addressing mode of the operands.

RISC processors do not use opcode prefixes.