ModR/M and SIB Bytes

The ModR/M and SIB bytes follow the opcode byte(s) in many of the processor instructions. They contain the following information:

• The indexing type or register number to be used in the instruction

• The register to be used, or more information to select the instruction

• The base, index, and scale information

The ModR/M byte contains three fields of information:

• The mod field, which occupies the two most significant bits of the byte, combines with the r/m field to form 32 possible values: eight registers and 24 indexing modes.

• The reg field, which occupies the next three bits following the mod field, specifies either a register number or three more bits of opcode information. The meaning of the reg field is determined by the first (opcode) byte of the instruction.

• The r/m field, which occupies the three least significant bits of the byte, can specify a register as the location of an operand, or can form part of the addressing-mode encoding in combination with the mod field as described above.

The based indexed forms of 32-bit addressing require the SIB byte. The presence of the SIB byte is indicated by certain encodings of the ModR/M byte. The SIB byte then includes the following fields:

• The ss field, which occupies the two most significant bits of the byte, specifies the scale factor.

• The index field, which occupies the next three bits following the ss field and specifies the register number of the index register.

• The base field, which occupies the three least significant bits of the byte, specifies the register number of the base register.

┌────────────────────────────────────────────────┐
│                  MODR/M BYTE                   │
│                                                │
│     7     6    5     4     3   2    1     0    │
│   ┌──────────┬────────────────┬──────────────┐ │
│   │  MOD     │   REG/OPCODE   │     R/M      │ │
│   └──────────┴────────────────┴──────────────┘ │
│                                                │
│            SIB (SCALE INDEX BASE) BYTE         │
│                                                │
│     7     6    5     4     3   2    1     0    │
│   ┌──────────┬────────────────┬──────────────┐ │
│   │    SS    │     INDEX      │     BASE     │ │
│   └──────────┴────────────────┴──────────────┘ │
│                                                │
└────────────────────────────────────────────────┘