ModR/M

Opcodes in x86 are generally one-byte, though two-byte instructions and prefixes exist. ModR/M is the byte following the opcode and adds additional information for how the instruction is executed.[1]^: §2.1 The format is:

where REG specifies a register and R/M may contain a register or specify an addressing mode, depending upon the value of MOD.

Some instructions cannot make use of the REG portion of the ModR/M byte. Many of these instructions are "multiplexed" using this field, where a single opcode can refer to multiple instructions, and the REG field determines the instruction. In opcode listings, these are specified by following the opcode with a slash (/) and a digit 0-7.[1]^{: §3.1.1.1}

The ModR/M byte is central to the changes introduced with AMD's 64-bit extension to the original instruction set. In long mode, any opcode whose highest four bits are 0100 (decimal 4) are considered to be a new prefix, the REX prefix.[2]^: §1.2.7 The lowest four bits of the prefix byte serve various purposes, including an extra bit for the REG and R/M fields of the ModR/M byte that follows. Among other changes, expanding these values from three bits to four doubles the number of available processor registers from eight to sixteen.[2]^: §1.4

The SIB byte is an optional post-opcode byte in x86 assembly on the i386 and later, used for complex addressing.

SIB bytes are formatted similarly to ModR/M bytes, and take the form of (scale * index) + base + displacement, where the SCALE is 1, 2, 4, or 8. BASE and INDEX each encode a register.[3] The displacement is a constant offset encoded after the SIB byte which is applied to the final address.[4]

A REX prefix can optionally allow the SIB byte to use SSE registers.[1]

• x86 instruction listings