The REX prefix (from "register extensions") and REX coding scheme was introduced as part of AMD's x86-64 instruction set architecture that extended Intel's IA-32 instruction set architecture for microprocessors from Intel, AMD and others.
It provides additional space for encoding 64-bit addressing modes and additional registers present in the x86-64 architecture.
The one-byte REX prefix can be added to new and existing instructions. It provides 4 bits of payload in total and allows addressing 16 registers.
A two-byte REX2 prefix was proposed by Intel's Advanced Performance Extensions in 2023 that would offer 8 bits of payload and allow addressing 32 registers.
| # of bytes | 1, 2 | 1 | 1 | 0, 1 | 0, 1, 2, 4 | 0, 1 | |
|---|---|---|---|---|---|---|---|
| Part | [Prefixes] | [REX] | OPCODE | ModR/M | [SIB] | [DISP] | [IMM] |
The REX coding scheme uses an opcode prefix consisting of one byte, which may be added to existing or new instruction codes. [1]
It has the four high-order bits set to four, which replaces sixteen opcodes numbered 0x40–0x4F. Previously, those opcodes were individual INC and DEC instructions for the eight standard processor registers; x86-64 code must use ModR/M INC and DEC instructions. [2]
In the x86 architecture, instructions with a memory operand almost always use the ModR/M byte which specifies the addressing mode. This byte has three bit fields:
The base-plus-index and scale-plus-index forms of 32-bit addressing (encoded with r/m = 100 and mod ≠ 11) require another addressing byte, the SIB byte. It has the following fields:
| Byte | Bit | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| REX | |||||||||
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | ||
| 0 (0x4_) | 0 | 1 | 0 | 0 | W | R | X | B | |
| REX2 (2-byte REX) | |||||||||
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | ||
| 0 (0xD5) | 1 | 1 | 0 | 1 | 0 | 1 | 0 | 1 | |
| 1 | M0 | R4 | X4 | B4 | W | R3 | X3 | B3 | |
The REX prefix's bit-field W changes the operand size to 64 bits, R expands reg to 4 bits, B expands r/m (or opreg in the few opcodes that encode the register in the 3 lowest opcode bits, such as "POP reg"), and X and B expand index and base in the SIB byte.
The REX2 prefix is a 2-byte variant of the REX prefix, that is proposed with Intel's Advanced Performance Extensions and allows addressing 32 registers.