X86-64 Instruction Encoding: Difference between revisions

The ModR/M and SIB bytes is used to encode the source and destination register or memory offset of an instruction.

The ModR/M byte encodes a register or an opcode extension, and a register or a memory address. It is only required when ''/r'' or ''/0'' through ''/7'' is specified at the opcode syntax. It has the following fields:

* A 3-bit opcode extension, which is used by some instructions but has no further meaning other than distincting the instruction from other instructions. When ''/0'' through ''/7'' is specified at the opcode syntax, use that value as the value for the ''MODRM.reg'' field.

* A 3-bit register reference, which can be used as the source or the destination of an instruction (depending on the instruction). This is the case when ''/r'' is specified at the opcode syntax. Which register is meant depends on the [[#Operand-size and address-size override prefix|operand-size]] of the instruction and the instruction itself. See [[#Registers|Registers]] for the values to use for each of the registers. The REX.R, VEX.~R or XOP.~R field can extend this field with 1 most-significant bit to 4 bits total.

! B,R or X!! Value!! 8-bit GP!! 16-bit GP!! 32-bit GP!! 64-bit GP!! 64-bit MMX!! 128-bit XMM!! 256-bit YMM!! 16-bit Segment!! 32-bit Control!! 32-bit Debug

| 0||0||AL||AX||EAX||RAX||MM0||XMM0||YMM0||ES||CR0||DR0

| 0||1||CL||CX||ECX||RCX||MM1||XMM1||YMM1||CS||CR1||DR1

| 0||2||DL||DX||EDX||RDX||MM2||XMM2||YMM2||SS||CR2||DR2

| 0||3||BL||BX||EBX||RBX||MM3||XMM3||YMM3||DS||CR3||DR3

| 0||4||AH, SPL<span style="vertical-align: super">[[#Table2Note1|1]]</span>||SP||ESP||RSP||MM4||XMM4||YMM4||FS||CR4||DR4

| 0||5||CH, BPL<span style="vertical-align: super">[[#Table2Note1|1]]</span>||BP||EBP||RBP||MM5||XMM5||YMM5||GS||CR5||DR5

| 0||6||DH, SIL<span style="vertical-align: super">[[#Table2Note1|1]]</span>||SI||ESI||RSI||MM6||XMM6||YMM6||invalid||CR6||DR6

| 0||7||BH, DIL<span style="vertical-align: super">[[#Table2Note1|1]]</span>||DI||EDI||RDI||MM7||XMM7||YMM7||invalid||CR7||DR7

| 1||0||R8L||R8W||R8D||R8||MM0||XMM8||YMM8||ES||CR8||DR8

| 1||1||R9L||R9W||R9D||R9||MM1||XMM9||YMM9||CS||CR9||DR9

| 1||2||R10L||R10W||R10D||R10||MM2||XMM10||YMM10||SS||CR10||DR10

| 1||3||R11L||R11W||R11D||R11||MM3||XMM11||YMM11||DS||CR11||DR11

| 1||4||R12L||R12W||R12D||R12||MM4||XMM12||YMM12||FS||CR12||DR12

| 1||5||R13L||R13W||R13D||R13||MM5||XMM13||YMM13||GS||CR13||DR13

| 1||6||R14L||R14W||R14D||R14||MM6||XMM14||YMM14||invalid||CR14||DR14

| 1||7||R15L||R15W||R15D||R15||MM7||XMM15||YMM15||invalid||CR15||DR15

! b00||1

! b01||2

! b10||4

! b11||8