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REX prefix

The byte 0x40-0x4F in front of many x86-64 instructions; it switches to 64-bit operands and lets the instruction name registers r8 to r15.

REX is a one-byte prefix 0100WRXB placed immediately before the opcode: W selects 64-bit operand size, and R, X and B supply a fourth bit for the ModRM reg, SIB index and ModRM r/m or SIB base register numbers.

Examples

  • In 48 01 c8, 48 is a REX prefix with W = 1: it makes the add 64-bit.

See also

SIB byte ModRM byte opcode base register
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