x86 instruction listings

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x86 instruction listings

The x86 instruction set refers to the set of instructions that x86-compatible microprocessors support. The instructions are usually part of an executable program, often stored as a computer file and executed on the processor.

The x86 instruction set has been extended several times, introducing wider registers and datatypes as well as new functionality.[1]

x86 integer instructions

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Main article: x86 assembly language

Below is the full 8086/8088 instruction set of Intel (81 instructions total).[2] These instructions are also available in 32-bit mode, they operate instead on 32-bit registers (eax, ebx, etc.) and values instead of their 16-bit (ax, bx, etc.) counterparts. The updated instruction set is grouped according to architecture (i186, i286, i386, i486, i586/i686) and is referred to as (32-bit) x86 and (64-bit) x86-64 (also known as AMD64).

Original 8086/8088 instructions

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This is the original instruction set. In the 'Notes' column, r means register, m means memory address and imm means immediate (i.e. a value).

Original 8086/8088 instruction set
In-
struc-
tion		 Meaning Notes Opcode
AAA ASCII adjust AL after addition used with unpacked binary-coded decimal 0x37
AAD ASCII adjust AX before division 8086/8088 datasheet documents only base 10 version of the AAD instruction (opcode 0xD5 0x0A), but any other base will work. Later Intel's documentation has the generic form too. NEC V20 and V30 (and possibly other NEC V-series CPUs) always use base 10, and ignore the argument, causing a number of incompatibilities 0xD5
AAM ASCII adjust AX after multiplication Only base 10 version (Operand is 0xA) is documented, see notes for AAD 0xD4
AAS ASCII adjust AL after subtraction 0x3F
ADC Add with carry (1) r += (r/m/imm+CF); (2) m += (r/imm+CF); 0x10...0x15, 0x80...0x81/2, 0x83/2
ADD Add (1) r += r/m/imm; (2) m += r/imm; 0x00...0x05, 0x80/0...0x81/0, 0x83/0
AND Logical AND (1) r &= r/m/imm; (2) m &= r/imm; 0x20...0x25, 0x80...0x81/4, 0x83/4
CALL Call procedure push eip; eip points to the instruction directly after the call 0x9A, 0xE8, 0xFF/2, 0xFF/3
CBW Convert byte to word AX = AL ; sign extended 0x98
CLC Clear carry flag CF = 0; 0xF8
CLD Clear direction flag DF = 0; 0xFC
CLI Clear interrupt flag IF = 0; 0xFA
CMC Complement carry flag CF = !CF; 0xF5
CMP Compare operands (1) r - r/m/imm; (2) m - r/imm; 0x38...0x3D, 0x80...0x81/7, 0x83/7
CMPSB Compare bytes in memory. May be used with a REPE or REPNE prefix to test and repeat the instruction CX times. 
if (DF==0) *(byte*)SI++ - *(byte*)ES:DI++; else       *(byte*)SI-- - *(byte*)ES:DI--;
0xA6
CMPSW Compare words. May be used with a REPE or REPNE prefix to test and repeat the instruction CX times. 
if (DF==0) *(word*)SI++ - *(word*)ES:DI++; else       *(word*)SI-- - *(word*)ES:DI--;
0xA7
CWD Convert word to doubleword 0x99
DAA Decimal adjust AL after addition (used with packed binary-coded decimal) 0x27
DAS Decimal adjust AL after subtraction 0x2F
DEC Decrement by 1 0x48...0x4F, 0xFE/1, 0xFF/1
DIV Unsigned divide (1) AX = DX:AX / r/m; resulting DX = remainder (2) AL = AX / r/m; resulting AH = remainder 0xF7/6, 0xF6/6
ESC Used with floating-point unit 0xD8..0xDF
HLT Enter halt state 0xF4
IDIV Signed divide (1) AX = DX:AX / r/m; resulting DX = remainder (2) AL = AX / r/m; resulting AH = remainder 0xF7/7, 0xF6/7
IMUL Signed multiply in One-operand form (1) DX:AX = AX * r/m; (2) AX = AL * r/m 0xF7/5, 0xF6/5
IN Input from port (1) AL = port[imm]; (2) AL = port[DX]; (3) AX = port[imm]; (4) AX = port[DX]; 0xE4, 0xE5, 0xEC, 0xED
INC Increment by 1 0x40...0x47, 0xFE/0, 0xFF/0
INT Call to interrupt 0xCC, 0xCD
INTO Call to interrupt if overflow 0xCE
IRET Return from interrupt 0xCF
Jcc Jump if condition (JA, JAE, JB, JBE, JC, JE, JG, JGE, JL, JLE, JNA, JNAE, JNB, JNBE, JNC, JNE, JNG, JNGE, JNL, JNLE, JNO, JNP, JNS, JNZ, JO, JP, JPE, JPO, JS, JZ) 0x70...0x7F
JCXZ Jump if CX is zero 0xE3
JMP Jump 0xE9...0xEB, 0xFF/4, 0xFF/5
LAHF Load FLAGS into AH register 0x9F
LDS Load DS:r with far pointer 0xC5
LEA Load Effective Address 0x8D
LES Load ES:r with far pointer 0xC4
LOCK Assert BUS LOCK# signal (for multiprocessing) 0xF0
LODSB Load string byte. May be used with a REP prefix to repeat the instruction CX times. if (DF==0) AL = *SI++; else AL = *SI--; 0xAC
LODSW Load string word. May be used with a REP prefix to repeat the instruction CX times. if (DF==0) AX = *SI++; else AX = *SI--; 0xAD
LOOP/
LOOPx		 Loop control (LOOPE, LOOPNE, LOOPNZ, LOOPZ) if (x && --CX) goto lbl; 0xE0...0xE2
MOV Move (1) r = r/m/imm; (2) m = r/imm (3) r/m = sreg; (4) sreg = r/m; 0xA0...0xA3, 0x8C, 0x8E
MOVSB Move byte from string to string. May be used with a REP prefix to repeat the instruction CX times. 
if (DF==0) *(byte*)ES:DI++ = *(byte*)SI++; else       *(byte*)ES:DI-- = *(byte*)SI--;
.		 0xA4
MOVSW Move word from string to string. May be used with a REP prefix to repeat the instruction CX times. 
if (DF==0) *(word*)ES:DI++ = *(word*)SI++; else       *(word*)ES:DI-- = *(word*)SI--;
0xA5
MUL Unsigned multiply (1) DX:AX = AX * r/m; (2) AX = AL * r/m; 0xF7/4, 0xF6/4
NEG Two's complement negation r/m = 0 – r/m; 0xF6/3...0xF7/3
NOP No operation opcode equivalent to XCHG EAX, EAX 0x90
NOT Negate the operand, logical NOT r/m ^= -1; 0xF6/2...0xF7/2
OR Logical OR (1) r ∣= r/m/imm; (2) m ∣= r/imm; 0x08...0x0D, 0x80...0x81/1, 0x83/1
OUT Output to port (1) port[imm] = AL; (2) port[DX] = AL; (3) port[imm] = AX; (4) port[DX] = AX; 0xE6, 0xE7, 0xEE, 0xEF
POP Pop data from stack r/m/sreg = *SP++; 0x07, 0x17, 0x1F, 0x58...0x5F, 0x8F/0
POPF Pop FLAGS register from stack FLAGS = *SP++; 0x9D
PUSH Push data onto stack *--SP = r/m/sreg; 0x06, 0x0E, 0x16, 0x1E, 0x50...0x57, 0xFF/6
PUSHF Push FLAGS onto stack *--SP = FLAGS; 0x9C
RCL Rotate left (with carry) 0xC0...0xC1/2 (186+), 0xD0...0xD3/2
RCR Rotate right (with carry) 0xC0...0xC1/3 (186+), 0xD0...0xD3/3
REPxx Repeat MOVS/STOS/CMPS/LODS/SCAS (REP, REPE, REPNE, REPNZ, REPZ) 0xF2, 0xF3
RET Return from procedure Not a real instruction. The assembler will translate these to a RETN or a RETF depending on the memory model of the target system.
RETN Return from near procedure 0xC2, 0xC3
RETF Return from far procedure 0xCA, 0xCB
ROL Rotate left 0xC0...0xC1/0 (186+), 0xD0...0xD3/0
ROR Rotate right 0xC0...0xC1/1 (186+), 0xD0...0xD3/1
SAHF Store AH into FLAGS 0x9E
SAL Shift Arithmetically left (signed shift left) (1) r/m <<= 1; (2) r/m <<= CL; 0xC0...0xC1/4 (186+), 0xD0...0xD3/4
SAR Shift Arithmetically right (signed shift right) (1) (signed) r/m >>= 1; (2) (signed) r/m >>= CL; 0xC0...0xC1/7 (186+), 0xD0...0xD3/7
SBB Subtraction with borrow (1) r -= (r/m/imm+CF); (2) m -= (r/imm+CF); alternative 1-byte encoding of SBB AL, AL is available via undocumented SALC instruction 0x18...0x1D, 0x80...0x81/3, 0x83/3
SCASB Compare byte string. May be used with a REPE or REPNE prefix to test and repeat the instruction CX times. if (DF==0) AL - *ES:DI++; else AL - *ES:DI--; 0xAE
SCASW Compare word string. May be used with a REPE or REPNE prefix to test and repeat the instruction CX times. if (DF==0) AX - *ES:DI++; else AX - *ES:DI--; 0xAF
SHL Shift left (unsigned shift left) 0xC0...0xC1/4 (186+), 0xD0...0xD3/4
SHR Shift right (unsigned shift right) 0xC0...0xC1/5 (186+), 0xD0...0xD3/5
STC Set carry flag CF = 1; 0xF9
STD Set direction flag DF = 1; 0xFD
STI Set interrupt flag IF = 1; 0xFB
STOSB Store byte in string. May be used with a REP prefix to repeat the instruction CX times. if (DF==0) *ES:DI++ = AL; else *ES:DI-- = AL; 0xAA
STOSW Store word in string. May be used with a REP prefix to repeat the instruction CX times. if (DF==0) *ES:DI++ = AX; else *ES:DI-- = AX; 0xAB
SUB Subtraction (1) r -= r/m/imm; (2) m -= r/imm; 0x28...0x2D, 0x80...0x81/5, 0x83/5
TEST Logical compare (AND) (1) r & r/m/imm; (2) m & r/imm; 0x84, 0x85, 0xA8, 0xA9, 0xF6/0, 0xF7/0
WAIT Wait until not busy Waits until BUSY# pin is inactive (used with floating-point unit) 0x9B
XCHG Exchange data r :=: r/m; A spinlock typically uses xchg as an atomic operation. (coma bug). 0x86, 0x87, 0x91...0x97
XLAT Table look-up translation behaves like MOV AL, [BX+AL] 0xD7
XOR Exclusive OR (1) r ^+= r/m/imm; (2) m ^= r/imm; 0x30...0x35, 0x80...0x81/6, 0x83/6

Added in specific processors

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Added with 80186/80188

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Instruction Opcode Meaning Notes
BOUND 62 /r Check array index against bounds raises software interrupt 5 if test fails
ENTER C8 iw ib Enter stack frame Modifies stack for entry to procedure for high level language. Takes two operands: the amount of storage to be allocated on the stack and the nesting level of the procedure.
INSB/INSW 6C Input from port to string. May be used with a REP prefix to repeat the instruction CX times. equivalent to: 
IN AL, DX MOV ES:[DI], AL INC DI ; adjust DI according to operand size and DF
6D
LEAVE C9 Leave stack frame Releases the local stack storage created by the previous ENTER instruction.
OUTSB/OUTSW 6E Output string to port. May be used with a REP prefix to repeat the instruction CX times. equivalent to: 
MOV AL, DS:[SI] OUT DX, AL INC SI ; adjust SI according to operand size and DF
6F
POPA 61 Pop all general purpose registers from stack equivalent to: 
POP DI POP SI POP BP POP AX ; no POP SP here, all it does is ADD SP, 2 (since AX will be overwritten later) POP BX POP DX POP CX POP AX
PUSHA 60 Push all general purpose registers onto stack equivalent to: 
PUSH AX PUSH CX PUSH DX PUSH BX PUSH SP ; The value stored is the initial SP value PUSH BP PUSH SI PUSH DI
PUSH immediate 6A ib Push an immediate byte/word value onto the stack example: 
PUSH 12h PUSH 1200h
68 iw
IMUL immediate 6B /r ib Signed and unsigned multiplication of immediate byte/word value example: 
IMUL BX,12h IMUL DX,1200h IMUL CX, DX, 12h IMUL BX, SI, 1200h IMUL DI, word ptr [BX+SI], 12h IMUL SI, word ptr [BP-4], 1200h

Note that since the lower half is the same for unsigned and signed multiplication, this version of the instruction can be used for unsigned multiplication as well.

69 /r iw
SHL/SHR/SAL/SAR/ROL/ROR/RCL/RCR immediate C0 Rotate/shift bits with an immediate value greater than 1 example: 
ROL AX,3 SHR BL,3
C1

Added with 80286

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The new instructions added in 80286 add support for x86 protected mode. Some but not all of the instructions are available in real mode as well.

Instruction Opcode Instruction description Real mode Ring
LGDT m16&32[a] 0F 01 /2 Load GDTR (Global Descriptor Table Register) from memory.[b] Yes 0
LIDT m16&32[a] 0F 01 /3 Load IDTR (Interrupt Descriptor Table Register) from memory.[b]
The IDTR controls not just the address/size of the IDT (interrupt Descriptor Table) in protected mode, but the IVT (Interrupt Vector Table) in real mode as well.
LMSW r/m16 0F 01 /6 Load MSW (Machine Status Word) from 16-bit register or memory.[c][d]
CLTS 0F 06 Clear task-switched flag in the MSW.
LLDT r/m16 0F 00 /2 Load LDTR (Local Descriptor Table Register) from 16-bit register or memory.[b] #UD
LTR r/m16 0F 00 /3 Load TR (Task Register) from 16-bit register or memory.[b]

The TSS (Task State Segment) specified by the 16-bit argument is marked busy, but a task switch is not done.

SGDT m16&32[a] 0F 01 /0 Store GDTR to memory. Yes Usually 3[e]
SIDT m16&32[a] 0F 01 /1 Store IDTR to memory.
SMSW r/m16 0F 01 /4 Store MSW to register or 16-bit memory.[f]
SLDT r/m16 0F 00 /0 Store LDTR to register or 16-bit memory.[f] #UD
STR r/m16 0F 00 /1 Store TR to register or 16-bit memory.[f]
ARPL r/m16,r16 63 /r[g] Adjust RPL (Requested Privilege Level) field of selector. The operation performed is:
if (dst & 3) < (src & 3) then    dst = (dst & 0xFFFC) | (src & 3)    eflags.zf = 1 else    eflags.zf = 0
 #UD[h] 3
LAR r,r/m16 0F 02 /r Load access rights byte from the specified segment descriptor.
Reads bytes 4-7 of segment descriptor, bitwise-ANDs it with 0x00FxFF00,[i] then stores the bottom 16/32 bits of the result in destination register. Sets EFLAGS.ZF=1 if the descriptor could be loaded, ZF=0 otherwise. 		#UD
LSL r,r/m16 0F 03 /r Load segment limit from the specified segment descriptor. Sets ZF=1 if the descriptor could be loaded, ZF=0 otherwise.
VERR r/m16 0F 00 /4 Verify a segment for reading. Sets ZF=1 if segment can be read, ZF=0 otherwise.
VERW r/m16 0F 00 /5 Verify a segment for writing. Sets ZF=1 if segment can be written, ZF=0 otherwise.[j]
 LOADALL[k]  0F 05 Load all CPU registers from a 102-byte data structure starting at physical address 800h, including "hidden" part of segment descriptor registers. Yes 0
 STOREALL[k]  F1 0F 04 Store all CPU registers to a 102-byte data structure starting at physical address 800h, then shut down CPU.
  1. ^ a b c d The descriptors used by the LGDT, LIDT, SGDT and SIDT instructions consist of a 2-part data structure. The first part is a 16-bit value, specifying table size in bytes minus 1. The second part is a 32-bit value (64-bit value in 64-bit mode), specifying the linear start address of the table.
    For LGDT and LIDT with a 16-bit operand size, the address is ANDed with 00FFFFFFh. On Intel (but not AMD) CPUs, the SGDT and SIDT instructions with a 16-bit operand size is – as of Intel SDM revision 079, March 2023 – documented to write a descriptor to memory with the last byte being set to 0. However, observed behavior is that bits 31:24 of the descriptor table address are written instead.[3]
  2. ^ a b c d The LGDT, LIDT, LLDT and LTR instructions are serializing on Pentium and later processors.
  3. ^ The LMSW instruction is serializing on Intel processors from Pentium onwards, but not on AMD processors.
  4. ^ On 80386 and later, the "Machine Status Word" is the same as the CR0 control register – however, the LMSW instruction can only modify the bottom 4 bits of this register and cannot clear bit 0. The inability to clear bit 0 means that LMSW can be used to enter but not leave x86 Protected Mode.
    On 80286, it is not possible to leave Protected Mode at all (neither with LMSW nor with LOADALL[4]) without a CPU reset – on 80386 and later, it is possible to leave Protected Mode, but this requires the use of the 80386-and-later MOV to CR0 instruction.
  5. ^ If CR4.UMIP=1 is set, then the SGDT, SIDT, SLDT, SMSW and STR instructions can only run in Ring 0.
    These instructions were unprivileged on all x86 CPUs from 80286 onwards until the introduction of UMIP in 2017.[5] This has been a significant security problem for software-based virtualization, since it enables these instructions to be used by a VM guest to detect that it is running inside a VM.[6][7]
  6. ^ a b c The SMSW, SLDT and STR instructions always use an operand size of 16 bits when used with a memory argument. With a register argument on 80386 or later processors, wider destination operand sizes are available and behave as follows:
    • SMSW: Stores full CR0 in x86-64 long mode, undefined otherwise.
    • SLDT: Zero-extends 16-bit argument on Pentium Pro and later processors, undefined on earlier processors.
    • STR: Zero-extends 16-bit argument.
  7. ^ In 64-bit long mode, the ARPL instruction is not available – the 63 /r opcode has been reassigned to the 64-bit-mode-only MOVSXD instruction.
  8. ^ The ARPL instruction causes #UD in Real mode and Virtual 8086 Mode – Windows 95 and OS/2 2.x are known to make extensive use of this #UD to use the 63 opcode as a one-byte breakpoint to transition from Virtual 8086 Mode to kernel mode.[8][9]
  9. ^ Bits 19:16 of this mask are documented as "undefined" on Intel CPUs.[10] On AMD CPUs, the mask is documented as 0x00FFFF00.
  10. ^ On some Intel CPU/microcode combinations from 2019 onwards, the VERW instruction also flushes microarchitectural data buffers. This enables it to be used as part of workarounds for Microarchitectural Data Sampling security vulnerabilities.[11][12]
  11. ^ a b Undocumented, 80286 only.[4][13][14] (A different variant of LOADALL with a different opcode and memory layout exists on 80386.)

Added with 80386

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The 80386 added support for 32-bit operation to the x86 instruction set. This was done by widening the general-purpose registers to 32 bits and introducing the concepts of OperandSize and AddressSize – most instruction forms that would previously take 16-bit data arguments were given the ability to take 32-bit arguments by setting their OperandSize to 32 bits, and instructions that could take 16-bit address arguments were given the ability to take 32-bit address arguments by setting their AddressSize to 32 bits. (Instruction forms that work on 8-bit data continue to be 8-bit regardless of OperandSize. Using a data size of 16 bits will cause only the bottom 16 bits of the 32-bit general-purpose registers to be modified – the top 16 bits are left unchanged.)

The default OperandSize and AddressSize to use for each instruction is given by the D bit of the segment descriptor of the current code segment - D=0 makes both 16-bit, D=1 makes both 32-bit. Additionally, they can be overridden on a per-instruction basis with two new instruction prefixes that were introduced in the 80386:

  • 66h: OperandSize override. Will change OperandSize from 16-bit to 32-bit if CS.D=0, or from 32-bit to 16-bit if CS.D=1.
  • 67h: AddressSize override. Will change AddressSize from 16-bit to 32-bit if CS.D=0, or from 32-bit to 16-bit if CS.D=1.

The 80386 also introduced the two new segment registers FS and GS as well as the x86 control, debug and test registers.

The new instructions introduced in the 80386 can broadly be subdivided into two classes:

  • Pre-existing opcodes that needed new mnemonics for their 32-bit OperandSize variants (e.g. CWDE, LODSD)
  • New opcodes that introduced new functionality (e.g. SHLD, SETcc)

For instruction forms where the operand size can be inferred from the instruction's arguments (e.g. ADD EAX,EBX can be inferred to have a 32-bit OperandSize due to its use of EAX as an argument), new instruction mnemonics are not needed and not provided.

80386: new instruction mnemonics for 32-bit variants of older opcodes
Type Instruction mnemonic Opcode Description Mnemonic for older 16-bit variant Ring
String instructions[a][b] LODSD AD Load string doubleword: EAX := DS:[rSI±±] LODSW 3
STOSD AB Store string doubleword: ES:[rDI±±] := EAX STOSW
MOVSD A5 Move string doubleword: ES:[rDI±±] := DS:[rSI±±] MOVSW
CMPSD A7 Compare string doubleword: 
temp1 := DS:[rSI±±] temp2 := ES:[rDI±±] CMP temp1, temp2 /* 32-bit compare and set EFLAGS */
 CMPSW
SCASD AF Scan string doubleword: 
temp1 := ES:[rDI±±] CMP EAX, temp1 /* 32-bit compare and set EFLAGS */
 SCASW
INSD 6D Input string from doubleword I/O port:ES:[rDI±±] := port[DX][c] INSW Usually 0[d]
OUTSD 6F Output string to doubleword I/O port:port[DX] := DS:[rSI±±] OUTSW
Other CWDE 98 Sign-extend 16-bit value in AX to 32-bit value in EAX[e] CBW 3
CDQ 99 Sign-extend 32-bit value in EAX to 64-bit value in EDX:EAX.

Mainly used to prepare a dividend for the 32-bit IDIV (signed divide) instruction.

CWD
JECXZ rel8 E3 cb[f] Jump if ECX is zero JCXZ
PUSHAD 60 Push all 32-bit registers onto stack[g] PUSHA
POPAD 61 Pop all 32-bit general-purpose registers off stack[h] POPA
PUSHFD 9C Push 32-bit EFLAGS register onto stack PUSHF Usually 3[i]
POPFD 9D Pop 32-bit EFLAGS register off stack POPF
IRETD CF 32-bit interrupt return. Differs from the older 16-bit IRET instruction in that it will pop interrupt return items (EIP,CS,EFLAGS; also ESP[j] and SS if there is a CPL change; and also ES,DS,FS,GS if returning to virtual 8086 mode) off the stack as 32-bit items instead of 16-bit items. Should be used to return from interrupts when the interrupt handler was entered through a 32-bit IDT interrupt/trap gate.

Instruction is serializing.

IRET
  1. ^ For the 32-bit string instructions, the ±± notation is used to indicate that the indicated register is post-decremented by 4 if EFLAGS.DF=1 and post-incremented by 4 otherwise.
    For the operands where the DS segment is indicated, the DS segment can be overridden by a segment-override prefix – where the ES segment is indicated, the segment is always ES and cannot be overridden.
    The choice of whether to use the 16-bit SI/DI registers or the 32-bit ESI/EDI registers as the address registers to use is made by AddressSize, overridable with the 67 prefix.
  2. ^ The 32-bit string instructions accept repeat-prefixes in the same way as older 8/16-bit string instructions.
    For LODSD, STOSD, MOVSD, INSD and OUTSD, the REP prefix (F3) will repeat the instruction the number of times specified in rCX (CX or ECX, decided by AddressSize), decrementing rCX for each iteration (with rCX=0 resulting in no-op and proceeding to the next instruction).
    For CMPSD and SCASD, the REPE (F3) and REPNE (F2) prefixes are available, which will repeat the instruction, decrementing rCX for each iteration, but only as long as the flag condition (ZF=1 for REPE, ZF=0 for REPNE) holds true AND rCX ≠ 0.
  3. ^ For the INSB/W/D instructions, the memory access rights for the ES:[rDI] memory address might not be checked until after the port access has been performed – if this check fails (e.g. page fault or other memory exception), then the data item read from the port is lost. As such, it is not recommended to use this instruction to access an I/O port that performs any kind of side effect upon read.
  4. ^ I/O port access is only allowed when CPL≤IOPL or the I/O port permission bitmap bits for the port to access are all set to 0.
  5. ^ The CWDE instruction differs from the older CWD instruction in that CWD would sign-extend the 16-bit value in AX into a 32-bit value in the DX:AX register pair.
  6. ^ For the E3 opcode (JCXZ/JECXZ), the choice of whether the instruction will use CX or ECX for its comparison (and consequently which mnemonic to use) is based on the AddressSize, not OperandSize. (OperandSize instead controls whether the jump destination should be truncated to 16 bits or not).
    This also applies to the loop instructions LOOP,LOOPE,LOOPNE (opcodes E0,E1,E2), however, unlike JCXZ/JECXZ, these instructions have not been given new mnemonics for their ECX-using variants.
  7. ^ For PUSHA(D), the value of SP/ESP pushed onto the stack is the value it had just before the PUSHA(D) instruction started executing.
  8. ^ For POPA/POPAD, the stack item corresponding to SP/ESP is popped off the stack (performing a memory read), but not placed into SP/ESP.
  9. ^ The PUSHFD and POPFD instructions will cause a #GP exception if executed in virtual 8086 mode if IOPL is not 3.
    The PUSHF, POPF, IRET and IRETD instructions will cause a #GP exception if executed in Virtual-8086 mode if IOPL is not 3 and VME is not enabled.
  10. ^ If IRETD is used to return from kernel mode to user mode (which will entail a CPL change) and the user-mode stack segment indicated by SS is a 16-bit segment, then the IRETD instruction will only restore the low 16 bits of the stack pointer (ESP/RSP), with the remaining bits keeping whatever value they had in kernel code before the IRETD. This has necessitated complex workarounds on both Linux ("ESPFIX")[15] and Windows.[16] This issue also affects the later 64-bit IRETQ instruction.
80386: new opcodes introduced
Instruction mnemonics Opcode Description Ring
BT r/m, r 0F A3 /r Bit Test.[a]

Second operand specifies which bit of the first operand to test. The bit to test is copied to EFLAGS.CF.

3
BT r/m, imm8 0F BA /4 ib
BTS r/m, r 0F AB /r Bit Test-and-set.[a][b]

Second operand specifies which bit of the first operand to test and set.

BTS r/m, imm8 0F BA /5 ib
BTR r/m, r 0F B3 /r Bit Test and Reset.[a][b]

Second operand specifies which bit of the first operand to test and clear.

BTR r/m, imm8 0F BA /6 ib
BTC r/m, r 0F BB /r Bit Test and Complement.[a][b]

Second operand specifies which bit of the first operand to test and toggle.

BTC r/m, imm8 0F BA /7 ib
BSF r, r/m NFx 0F BC /r[c] Bit scan forward. Returns bit index of lowest set bit in input.[d] 3
BSR r, r/m NFx 0F BD /r[e] Bit scan reverse. Returns bit index of highest set bit in input.[d]
SHLD r/m, r, imm8 0F A4 /r ib Shift Left Double.
The operation of SHLD arg1,arg2,shamt is:
arg1 := (arg1<<shamt) | (arg2>>(operand_size - shamt))[f]
SHLD r/m, r, CL 0F A5 /r
SHRD r/m, r, imm8 0F AC /r ib Shift Right Double.
The operation of SHRD arg1,arg2,shamt is:
arg1 := (arg1>>shamt) | (arg2<<(operand_size - shamt))[f]
SHRD r/m, r, CL 0F AD /r
MOVZX reg, r/m8 0F B6 /r Move from 8/16-bit source to 16/32-bit register with zero-extension. 3
MOVZX reg, r/m16 0F B7 /r
MOVSX reg, r/m8 0F BE /r Move from 8/16-bit source to 16/32/64-bit register with sign-extension.
MOVSX reg, r/m16 0F BF /r
SETcc r/m8 0F 9x /0[g][h] Set byte to 1 if condition is satisfied, 0 otherwise.
Jcc rel16
Jcc rel32 		0F 8x cw
0F 8x cd[g] 		Conditional jump near.

Differs from older variants of conditional jumps in that they accept a 16/32-bit offset rather than just an 8-bit offset.

IMUL r, r/m 0F AF /r Two-operand non-widening integer multiply.
FS: 64 Segment-override prefixes for FS and GS segment registers. 3
GS: 65
PUSH FS 0F A0 Push/pop FS and GS segment registers.
POP FS 0F A1
PUSH GS 0F A8
POP GS 0F A9
LFS r16, m16&16
LFS r32, m32&16		 0F B4 /r Load far pointer from memory.

Offset part is stored in destination register argument, segment part in FS/GS/SS segment register as indicated by the instruction mnemonic.[i]

LGS r16, m16&16
LGS r32, m32&16		 0F B5 /r
LSS r16, m16&16
LSS r32, m32&16		 0F B2 /r
MOV reg,CRx 0F 20 /r[j] Move from control register to general register.[k] 0
MOV CRx,reg 0F 22 /r[j] Move from general register to control register.[k]

Moves to the CR3 control register are serializing and will flush the TLB.[l]

On Pentium and later processors, moves to the CR0 and CR4 control registers are also serializing.[m]

MOV reg,DRx 0F 21 /r[j] Move from x86 debug register to general register.[k]
MOV DRx,reg 0F 23 /r[j] Move from general register to x86 debug register.[k]

On Pentium and later processors, moves to the DR0-DR7 debug registers are serializing.

MOV reg,TRx 0F 24 /r[j] Move from x86 test register to general register.[n]
MOV TRx,reg 0F 26 /r[j] Move from general register to x86 test register.[n]
 ICEBP,
 INT01,
 INT1[o] 		 F1 In-circuit emulation breakpoint.

Performs software interrupt #1 if executed when not using in-circuit emulation.[p]

3
 UMOV r/m, r8  0F 10 /r User Move – perform data moves that can access user memory while in In-circuit emulation HALT mode.

Performs same operation as MOV if executed when not doing in-circuit emulation.[q]

 UMOV r/m, r16/32  0F 11 /r
 UMOV r8, r/m  0F 12 /r
 UMOV r16/32, r/m  0F 13 /r
 XBTS reg,r/m  0F A6 /r Bitfield extract (early 386 only).[r][s]
 IBTS r/m,reg  0F A7 /r Bitfield insert (early 386 only).[r][s]
 LOADALLD,
 LOADALL386[t] 		 0F 07 Load all CPU registers from a 296-byte data structure starting at ES:EDI, including "hidden" part of segment descriptor registers. 0
  1. ^ a b c d For the BT, BTS, BTR and BTC instructions:
    • If the first argument to the instruction is a register operand and/or the second argument is an immediate, then the bit-index in the second argument is taken modulo operand size (16/32/64, in effect using only the bottom 4, 5 or 6 bits of the index.)
    • If the first argument is a memory operand and the second argument is a register operand, then the bit-index in the second argument is used in full – it is interpreted as a signed bit-index that is used to offset the memory address to use for the bit test.
  2. ^ a b c The BTS, BTC and BTR instructions accept the LOCK (F0) prefix when used with a memory argument – this results in the instruction executing atomically.
  3. ^ If the F3 prefix is used with the 0F BC /r opcode, then the instruction will execute as TZCNT on systems that support the BMI1 extension. TZCNT differs from BSF in that TZCNT but not BSR is defined to return operand size if the source operand is zero – for other source operand values, they produce the same result (except for flags).
  4. ^ a b BSF and BSR set the EFLAGS.ZF flag to 1 if the source argument was all-0s and 0 otherwise.
    If the source argument was all-0s, then the destination register is documented as being left unchanged on AMD processors, but set to an undefined value on Intel processors.
  5. ^ If the F3 prefix is used with the 0F BD /r opcode, then the instruction will execute as LZCNT on systems that support the ABM or LZCNT extensions. LZCNT produces a different result from BSR for most input values.
  6. ^ a b For SHLD and SHRD, the shift-amount is masked – the bottom 5 bits are used for 16/32-bit operand size and 6 bits for 64-bit operand size.
    SHLD and SHRD with 16-bit arguments and a shift-amount greater than 16 produce undefined results. (Actual results differ between different Intel CPUs, with at least three different behaviors known.[17])
  7. ^ a b The condition codes supported for the SETcc and Jcc near instructions (opcodes 0F 9x /0 and 0F 8x respectively, with the x nibble specifying the condition) are:
    x cc Condition (EFLAGS)
    0 O OF=1: "Overflow"
    1 NO OF=0: "Not Overflow"
    2 C,B,NAE CF=1: "Carry", "Below", "Not Above or Equal"
    3 NC,NB,AE CF=0: "Not Carry", "Not Below", "Above or Equal"
    4 Z,E ZF=1: "Zero", "Equal"
    5 NZ,NE ZF=0: "Not Zero", "Not Equal"
    6 NA,BE (CF=1 or ZF=1): "Not Above", "Below or Equal"
    7 A,NBE (CF=0 and ZF=0): "Above", "Not Below or Equal"
    8 S SF=1: "Sign"
    9 NS SF=0: "Not Sign"
    A P,PE PF=1: "Parity", "Parity Even"
    B NP,PO PF=0: "Not Parity", "Parity Odd"
    C L,NGE SF≠OF: "Less", "Not Greater Or Equal"
    D NL,GE SF=OF: "Not Less", "Greater Or Equal"
    E LE,NG (ZF=1 or SF≠OF): "Less or Equal", "Not Greater"
    F NLE,G (ZF=0 and SF=OF): "Not Less or Equal", "Greater"
  8. ^ For SETcc, while the opcode is commonly specified as /0 – implying that bits 5:3 of the instruction's ModR/M byte should be 000 – modern x86 processors (Pentium and later) ignore bits 5:3 and will execute the instruction as SETcc regardless of the contents of these bits.
  9. ^ For LFS, LGS and LSS, the size of the offset part of the far pointer is given by operand size – the size of the segment part is always 16 bits. In 64-bit mode, using the REX.W prefix with these instructions will cause them to load a far pointer with a 64-bit offset on Intel but not AMD processors.
  10. ^ a b c d e f For MOV to/from the CRx, DRx and TRx registers, the reg part of the ModR/M byte is used to indicate CRx/DRx/TRx register and r/m part the general-register. Uniquely for the MOV CRx/DRx/TRx opcodes, the top two bits of the ModR/M byte is ignored – these opcodes are decoded and executed as if the top two bits of the ModR/M byte are 11b.
  11. ^ a b c d For moves to/from the CRx and DRx registers, the operand size is always 64 bits in 64-bit mode and 32 bits otherwise.
  12. ^ On processors that support global pages (Pentium and later), global page table entries will not be flushed by a MOV to CR3 − instead, these entries can be flushed by toggling the CR4.PGE bit.
    On processors that support PCIDs, writing to CR3 while PCIDs are enabled will only flush TLB entries belonging to the PCID specified in bits 11:0 of the value written to CR3 (this flush can be suppressed by setting bit 63 of the written value to 1). Flushing pages belonging to other PCIDs can instead be done by toggling the CR4.PGE bit, clearing the CR4.PCIDE bit, or using the INVPCID instruction.
  13. ^ On processors prior to Pentium, moves to CR0 would not serialize the instruction stream – in part for this reason, it is usually required to perform a far jump[18] immediately after a MOV to CR0 if such a MOV is used to enable/disable protected mode and/or memory paging.
    MOV to CR2 is architecturally listed as serializing, but has been reported to be non-serializing on at least some Intel Core-i7 processors.[19]
    MOV to CR8 (introduced with x86-64) is serializing on AMD but not Intel processors.
  14. ^ a b The MOV TRx instructions were discontinued from Pentium onwards.
  15. ^ The INT1/ICEBP (F1) instruction is present on all known Intel x86 processors from the 80386 onwards,[20] but only fully documented for Intel processors from the May 2018 release of the Intel SDM (rev 067) onwards.[21] Before this release, mention of the instruction in Intel material was sporadic, e.g. AP-526 rev 001.[22]
    For AMD processors, the instruction has been documented since 2002.[23]
  16. ^ The operation of the F1(ICEBP) opcode differs from the operation of the regular software interrupt opcode CD 01 in several ways:
      In protected mode, CD 01 will check CPL against the interrupt descriptor's DPL field as an access-rights check, while F1 will not.
    • In virtual-8086 mode, CD 01 will also check CPL against IOPL as an access-rights check, while F1 will not.
    • In virtual-8086 mode with VME enabled, interrupt redirection is supported for CD 01 but not F1.
  17. ^ The UMOV instruction is present on 386 and 486 processors only.[20]
  18. ^ a b The XBTS and IBTS instructions were discontinued with the B1 stepping of 80386.
    They have been used by software mainly for detection of the buggy[24] B0 stepping of the 80386. Microsoft Windows (v2.01 and later) will attempt to run the XBTS instruction as part of its CPU detection if CPUID is not present, and will refuse to boot if XBTS is found to be working.[25]
  19. ^ a b For XBTS and IBTS, the r/m argument represents the data to extract/insert a bitfield from/to, the reg argument the bitfield to be inserted/extracted, AX/EAX a bit-offset and CL a bitfield length.[26]
  20. ^ Undocumented, 80386 only.[27]

Added with 80486

[edit]
Instruction Opcode Description Ring
BSWAP r32 0F C8+r Byte Order Swap. Usually used to convert between big-endian and little-endian data representations. For 32-bit registers, the operation performed is:
r =   (r << 24)     | ((r << 8) & 0x00FF0000)     | ((r >> 8) & 0x0000FF00)     | (r >> 24);

Using BSWAP with a 16-bit register argument produces an undefined result.[a]

3
CMPXCHG r/m8,r8 0F B0 /r[b] Compare and Exchange. If accumulator (AL/AX/EAX/RAX) compares equal to first operand,[c] then EFLAGS.ZF is set to 1 and the first operand is overwritten with the second operand. Otherwise, EFLAGS.ZF is set to 0, and first operand is copied into the accumulator.

Instruction atomic only if used with LOCK prefix.

CMPXCHG r/m,r16
CMPXCHG r/m,r32 		0F B1 /r[b]
XADD r/m,r8 0F C0 /r eXchange and ADD. Exchanges the first operand with the second operand, then stores the sum of the two values into the destination operand.

Instruction atomic only if used with LOCK prefix.

XADD r/m,r16
XADD r/m,r32 		0F C1 /r
INVLPG m8 0F 01 /7 Invalidate the TLB entries that would be used for the 1-byte memory operand.[d]

Instruction is serializing.

0
INVD 0F 08 Invalidate Internal Caches.[e] Modified data in the cache are not written back to memory, potentially causing data loss.[f]
WBINVD NFx 0F 09[g] Write Back and Invalidate Cache.[e] Writes back all modified cache lines in the processor's internal cache to main memory and invalidates the internal caches.
  1. ^ Using BSWAP with 16-bit registers is not disallowed per se (it will execute without producing an #UD or other exceptions) but is documented to produce undefined results – it is reported to produce various different results on 486,[28] 586, and Bochs/QEMU.[29]
  2. ^ a b On Intel 80486 stepping A,[30] the CMPXCHG instruction uses a different encoding - 0F A6 /r for 8-bit variant, 0F A7 /r for 16/32-bit variant. The 0F B0/B1 encodings are used on 80486 stepping B and later.[31][32]
  3. ^ The CMPXCHG instruction sets EFLAGS in the same way as a CMP instruction that uses the accumulator (AL/AX/EAX/RAX) as its first argument would do.
  4. ^ INVLPG executes as no-operation if the m8 argument is invalid (e.g. unmapped page or non-canonical address).
    INVLPG can be used to invalidate TLB entries for individual global pages.
  5. ^ a b The INVD and WBINVD instructions will invalidate all cache lines in the CPU's L1 caches. It is implementation-defined whether they will invalidate L2/L3 caches as well.
    These instructions are serializing – on some processors, they may block interrupts until completion as well.
  6. ^ Under Intel VT-x virtualization, the INVD instruction will cause a mandatory #VMEXIT. Also, on processors that support Intel SGX, if the PRM (Processor Reserved Memory) has been set up by using the PRMRRs (PRM range registers), then the INVD instruction is not permitted and will cause a #GP(0) exception.[33]
  7. ^ If the F3 prefix is used with the 0F 09 opcode, then the instruction will execute as WBNOINVD on processors that support the WBNOINVD extension – this will not invalidate the cache.

Added in P5/P6-class processors

[edit]

Integer/system instructions that were not present in the basic 80486 instruction set, but were added in various x86 processors prior to the introduction of SSE. (Discontinued instructions are not included.)

Instruction Opcode Description Ring Added in
RDMSR 0F 32 Read Model-specific register. The MSR to read is specified in ECX. The value of the MSR is then returned as a 64-bit value in EDX:EAX. 0 IBM 386SLC,[34]
Intel Pentium,
AMD K5,
Cyrix 6x86MX,MediaGXm,
IDT WinChip C6,
Transmeta Crusoe,
DM&P Vortex86DX3
WRMSR 0F 30 Write Model-specific register. The MSR to write is specified in ECX, and the data to write is given in EDX:EAX.[a]

Instruction is, with some exceptions, serializing.[b]

RSM[39] 0F AA Resume from System Management Mode.

Instruction is serializing.

-2
(SMM) 		Intel 386SL,[40][41] 486SL,[c]
Intel Pentium,
AMD 5x86,
Cyrix 486SLC/e,[42]
IDT WinChip C6,
Transmeta Crusoe,
Rise mP6
CPUID 0F A2 CPU Identification and feature information. Takes as input a CPUID leaf index in EAX and, depending on leaf, a sub-index in ECX. Result is returned in EAX,EBX,ECX,EDX.[d]

Instruction is serializing, and causes a mandatory #VMEXIT under virtualization.

Support for CPUID can be checked by toggling bit 21 of EFLAGS (EFLAGS.ID) – if this bit can be toggled, CPUID is present.

Usually 3[e] Intel Pentium,[f]
AMD 5x86,[f]
Cyrix 5x86,[g]
IDT WinChip C6,
Transmeta Crusoe,
Rise mP6,
NexGen Nx586,[h]
UMC Green CPU
CMPXCHG8B m64 0F C7 /1 Compare and Exchange 8 bytes. Compares EDX:EAX with m64. If equal, set ZF[i] and store ECX:EBX into m64. Else, clear ZF and load m64 into EDX:EAX.

Instruction atomic only if used with LOCK prefix.[j]

3 Intel Pentium,
AMD K5,
Cyrix 6x86L,MediaGXm,
IDT WinChip C6,[k]
Transmeta Crusoe,[k]
Rise mP6[k]
RDTSC 0F 31 Read 64-bit Time Stamp Counter (TSC) into EDX:EAX.[l]

In early processors, the TSC was a cycle counter, incrementing by 1 for each clock cycle (which could cause its rate to vary on processors that could change clock speed at runtime) – in later processors, it increments at a fixed rate that doesn't necessarily match the CPU clock speed.[m]

Usually 3[n] Intel Pentium,
AMD K5,
Cyrix 6x86MX,MediaGXm,
IDT WinChip C6,
Transmeta Crusoe,
Rise mP6
RDPMC 0F 33 Read Performance Monitoring Counter. The counter to read is specified by ECX and its value is returned in EDX:EAX.[l] Usually 3[o] Intel Pentium MMX,
Intel Pentium Pro,
AMD K7,
Cyrix 6x86MX,
IDT WinChip C6,
AMD Geode LX,
VIA Nano[p]
CMOVcc reg,r/m 0F 4x /r[q] Conditional move to register. The source operand may be either register or memory.[r] 3 Intel Pentium Pro,
AMD K7,
Cyrix 6x86MX,MediaGXm,
Transmeta Crusoe,
VIA C3 "Nehemiah",[s]
DM&P Vortex86DX3
NOP r/m,
NOPL r/m 		NFx 0F 1F /0[t] Official long NOP.

Other than AMD K7/K8, broadly unsupported in non-Intel processors released before 2005.[u][57]

3 Intel Pentium Pro,[v]
AMD K7, x86-64,[w]
VIA C7[61]
UD2,[x]
UD2A[y] 		0F 0B Undefined Instructions – will generate an invalid opcode (#UD) exception in all operating modes.[z]

These instructions are provided for software testing to explicitly generate invalid opcodes. The opcodes for these instructions are reserved for this purpose.

(3) (80186),[aa]
Intel Pentium[66]
UD1 reg,r/m,[ab]
UD2B reg,r/m[y] 		0F B9 /r[ac]
OIO,
UD0,
UD0 reg,r/m[ad] 		0F FF,
0F FF /r[ac] 		(80186),[aa]
Cyrix 6x86,[71]
AMD K5[73]
SYSCALL 0F 05 Fast System call. 3 AMD K6,[ae]
x86-64[af][ag]
SYSRET 0F 07[ah] Fast Return from System Call. Designed to be used together with SYSCALL. 0[ai]
SYSENTER 0F 34 Fast System call. 3[ai] Intel Pentium II,[aj]
AMD K7,[78][ak]
Transmeta Crusoe,[al]
NatSemi Geode GX2,
VIA C3 "Nehemiah",[am]
DM&P Vortex86DX3
SYSEXIT 0F 35[ah] Fast Return from System Call. Designed to be used together with SYSENTER. 0[ai]
  1. ^ On Intel and AMD CPUs, the WRMSR instruction is also used to update the CPU microcode. This is done by writing the virtual address of the new microcode to upload to MSR 79h on Intel CPUs and MSR C001_0020h[35] on AMD CPUs.
  2. ^ Writes to the following MSRs are not serializing:[36][37]
    Number Name
    48h SPEC_CTRL
    49h PRED_CMD
    10Bh FLUSH_CMD
    122h TSX_CTRL
    6E0h TSC_DEADLINE
    6E1h PKRS
    774h HWP_REQUEST
    (non-serializing only if the FAST_IA32_­HWP_REQUEST bit it set)
    802h to 83Fh (x2APIC MSRs)
    1B01h UARCH_MISC_CTL
    C001_0100h FS_BASE (non-serializing on AMD Zen 4 and later)[38]
    C001_0101h GS_BASE (Zen 4 and later)
    C001_0102h KernelGSbase (Zen 4 and later)
    C001_011Bh Doorbell Register (AMD-specific)
  3. ^ System Management Mode and the RSM instruction were made available on non-SL variants of the Intel 486 only after the initial release of the Intel Pentium in 1993.
  4. ^ On some older 32-bit processors, executing CPUID with a leaf index (EAX) greater than 0 may leave EBX and ECX unmodified, keeping their old values. For this reason, it is recommended to zero out EBX and ECX before executing CPUID.
    Processors noted to exhibit this behavior include Cyrix MII[43] and IDT WinChip 2.[44]

    In 64-bit mode, CPUID will set the top 32 bits of RAX, RBX, RCX and RDX to zero.
  5. ^ On some Intel processors starting from Ivy Bridge, there exists MSRs that can be used to restrict CPUID to ring 0. Such MSRs are documented for at least Ivy Bridge[45] and Denverton.[46]
    The ability to restrict CPUID to ring 0 also exists on AMD processors supporting the "CpuidUserDis" feature (Zen 4 "Raphael" and later).[47]
  6. ^ a b CPUID is also available on some Intel and AMD 486 processor variants that were released after the initial release of the Intel Pentium.
  7. ^ On the Cyrix 5x86 and 6x86 CPUs, CPUID is not enabled by default and must be enabled through a Cyrix configuration register.
  8. ^ On NexGen CPUs, CPUID is only supported with some system BIOSes. On some NexGen CPUs that do support CPUID, EFLAGS.ID is not supported but EFLAGS.AC is, complicating CPU detection.[48]
  9. ^ Unlike the older CMPXCHG instruction, the CMPXCHG8B instruction does not modify any EFLAGS bits other than ZF.
  10. ^ LOCK CMPXCHG8B with a register operand (which is an invalid encoding) will, on some Intel Pentium CPUs, cause a hang rather than the expected #UD exception - this is known as the Pentium F00F bug.
  11. ^ a b c On IDT WinChip, Transmeta Crusoe and Rise mP6 processors, the CMPXCHG8B instruction is always supported, however its CPUID bit may be missing. This is a workaround for a bug in Windows NT.[49]
  12. ^ a b The RDTSC and RDPMC instructions are not ordered with respect to other instructions, and may sample their respective counters before earlier instructions are executed or after later instructions have executed. Invocations of RDPMC (but not RDTSC) may be reordered relative to each other even for reads of the same counter.
    In order to impose ordering with respect to other instructions, LFENCE or serializing instructions (e.g. CPUID) are needed.[50]
  13. ^ Fixed-rate TSC was introduced in two stages:
    Constant TSC
    TSC running at a fixed rate as long as the processor core is not in a deep-sleep (C2 or deeper) mode, but not synchronized between CPU cores. Introduced in Intel Prescott, Yonah and Bonnell. Also present in all Transmeta and VIA Nano[51] CPUs. Does not have a CPUID bit.
    Invariant TSC
    TSC running at a fixed rate, and remaining synchronized between CPU cores in all P-,C- and T-states (but not necessarily S-states).
    Present in AMD K10 and later; Intel Nehalem/Saltwell[52] and later; Zhaoxin WuDaoKou[53] and later. Indicated with a CPUID bit (leaf 8000_0007:EDX[8]).
  14. ^ RDTSC can be run outside Ring 0 only if CR4.TSD=0.
    On Intel Pentium and AMD K5, RDTSC cannot be run in Virtual-8086 mode.[54] Later processors removed this restriction.
  15. ^ RDPMC can be run outside Ring 0 only if CR4.PCE=1.
  16. ^ The RDPMC instruction is not present in VIA processors prior to the Nano.
  17. ^ The condition codes supported for CMOVcc instruction (opcode 0F 4x /r, with the x nibble specifying the condition) are:
    x cc Condition (EFLAGS)
    0 O OF=1: "Overflow"
    1 NO OF=0: "Not Overflow"
    2 C,B,NAE CF=1: "Carry", "Below", "Not Above or Equal"
    3 NC,NB,AE CF=0: "Not Carry", "Not Below", "Above or Equal"
    4 Z,E ZF=1: "Zero", "Equal"
    5 NZ,NE ZF=0: "Not Zero", "Not Equal"
    6 NA,BE (CF=1 or ZF=1): "Not Above", "Below or Equal"
    7 A,NBE (CF=0 and ZF=0): "Above", "Not Below or Equal"
    8 S SF=1: "Sign"
    9 NS SF=0: "Not Sign"
    A P,PE PF=1: "Parity", "Parity Even"
    B NP,PO PF=0: "Not Parity", "Parity Odd"
    C L,NGE SF≠OF: "Less", "Not Greater Or Equal"
    D NL,GE SF=OF: "Not Less", "Greater Or Equal"
    E LE,NG (ZF=1 or SF≠OF): "Less or Equal", "Not Greater"
    F NLE,G (ZF=0 and SF=OF): "Not Less or Equal", "Greater"
  18. ^ In 64-bit mode, CMOVcc with a 32-bit operand size will clear the upper 32 bits of the destination register even if the condition is false.
    For CMOVcc with a memory source operand, the CPU will always read the operand from memory – potentially causing memory exceptions and cache line-fills – even if the condition for the move is not satisfied. (The Intel APX extension defines a set of new EVEX-encoded variants of CMOVcc that will suppress memory exceptions if the condition is false.)
  19. ^ On pre-Nehemiah VIA C3 variants ("Samuel"/"Ezra"), the reg,reg but not reg,[mem] forms of the CMOVcc instructions have been reported to be present as undocumented instructions.[55]
  20. ^ Intel's recommended byte encodings for multi-byte NOPs of lengths 2 to 9 bytes in 32/64-bit mode are (in hex):[56]
    Length Byte Sequence
    2 66 90
    3 0F 1F 00
    4 0F 1F 40 00
    5 0F 1F 44 00 00
    6 66 0F 1F 44 00 00
    7 0F 1F 80 00 00 00 00
    8 0F 1F 84 00 00 00 00 00
    9 66 0F 1F 84 00 00 00 00 00

    For cases where there is a need to use more than 9 bytes of NOP padding, it is recommended to use multiple NOPs.

  21. ^ Unlike other instructions added in Pentium Pro, long NOP does not have a CPUID feature bit.
  22. ^ 0F 1F /0 as long-NOP was introduced in the Pentium Pro, but remained undocumented until 2006.[58] The whole 0F 18..1F opcode range was NOP in Pentium Pro. However, except for 0F 1F /0, Intel does not guarantee that these opcodes will remain NOP in future processors, and have indeed assigned some of these opcodes to other instructions in at least some processors.[59]
  23. ^ Documented for AMD x86-64 since 2002.[60]
  24. ^ While the 0F 0B opcode was officially reserved as an invalid opcode from Pentium onwards, it only got assigned the mnemonic UD2 from Pentium Pro onwards.[62]
  25. ^ a b GNU Binutils have used the UD2A and UD2B mnemonics for the 0F 0B and 0F B9 opcodes since version 2.7.[63]
    Neither UD2A nor UD2B originally took any arguments - UD2B was later modified to accept a ModR/M byte, in Binutils version 2.30.[64]
  26. ^ The UD2 (0F 0B) instruction will additionally stop subsequent bytes from being decoded as instructions, even speculatively. For this reason, if an indirect branch instruction is followed by something that is not code, it is recommended to place an UD2 instruction after the indirect branch.[65]
  27. ^ a b The UD0/1/2 opcodes - 0F 0B, 0F B9 and 0F FF - will cause an #UD exception on all x86 processors from the 80186 onwards (except NEC V-series processors), but did not get explicitly reserved for this purpose until P5-class processors.
  28. ^ While the 0F B9 opcode was officially reserved as an invalid opcode from Pentium onwards, it only got assigned its mnemonic UD1 much later – AMD APM started listing UD1 in its opcode maps from rev 3.17 onwards,[67] while Intel SDM started listing it from rev 061 onwards.[68]
  29. ^ a b For both the 0F B9 and 0F FF opcodes, different x86 implementations are known to differ regarding whether the opcodes accept a ModR/M byte.[69][70]
  30. ^ For the 0F FF opcode, the OIO mnemonic was introduced by Cyrix,[71] while the UD0 menmonic (without arguments) was introduced by AMD and Intel at the same time as the UD1 mnemonic for 0F B9.[67][68] Later Intel (but not AMD) documentation modified its description of UD0 to add a ModR/M byte and take two arguments.[72]
  31. ^ On K6, the SYSCALL/SYSRET instructions were available on Model 7 (250nm "Little Foot") and later, not on the earlier Model 6.[74]
  32. ^ SYSCALL and SYSRET were made an integral part of x86-64 – as a result, the instructions are available in 64-bit mode on all x86-64 processors from AMD, Intel, VIA and Zhaoxin.
    Outside 64-bit mode, the instructions are available on AMD processors only.
  33. ^ The exact semantics of SYSRET differs slightly between AMD and Intel processors: non-canonical return addresses cause a #GP exception to be thrown in Ring 3 on AMD CPUs but Ring 0 on Intel CPUs. This has been known to cause security issues.[75]
  34. ^ a b For the SYSRET and SYSEXIT instructions under x86-64, it is necessary to add the REX.W prefix for variants that will return to 64-bit user-mode code.
    Encodings of these instructions without the REX.W prefix are used to return to 32-bit user-mode code. (Neither of these instructions can be used to return to 16-bit user-mode code.)
  35. ^ a b c The SYSRET, SYSENTER and SYSEXIT instructions are unavailable in Real mode. (SYSENTER is, however, available in Virtual 8086 mode.)
  36. ^ The CPUID flags that indicate support for SYSENTER/SYSEXIT are set on the Pentium Pro, even though the processor does not officially support these instructions.[76]
    Third party testing indicates that the opcodes are present on the Pentium Pro but too buggy to be usable.[77]
  37. ^ On AMD CPUs, the SYSENTER and SYSEXIT instructions are not available in x86-64 long mode (#UD).
  38. ^ On Transmeta CPUs, the SYSENTER and SYSEXIT instructions are only available with version 4.2 or higher of the Transmeta Code Morphing software.[79]
  39. ^ On Nehemiah, SYSENTER and SYSEXIT are available only on stepping 8 and later.[80]

Added as instruction set extensions

[edit]

Added with x86-64

[edit]

These instructions can only be encoded in 64 bit mode. They fall in four groups:

  • original instructions that reuse existing opcodes for a different purpose (MOVSXD replacing ARPL)
  • original instructions with new opcodes (SWAPGS)
  • existing instructions extended to a 64 bit address size (JRCXZ)
  • existing instructions extended to a 64 bit operand size (remaining instructions)

Most instructions with a 64 bit operand size encode this using a REX.W prefix; in the absence of the REX.W prefix, the corresponding instruction with 32 bit operand size is encoded. This mechanism also applies to most other instructions with 32 bit operand size. These are not listed here as they do not gain a new mnemonic in Intel syntax when used with a 64 bit operand size.

Instruction Encoding Meaning Ring
CDQE REX.W 98 Sign extend EAX into RAX 3
CQO REX.W 99 Sign extend RAX into RDX:RAX
CMPSQ REX.W A7 CoMPare String Quadword
CMPXCHG16B m128[a][b] REX.W 0F C7 /1 CoMPare and eXCHanGe 16 Bytes.
Atomic only if used with LOCK prefix.
IRETQ REX.W CF 64-bit Return from Interrupt
JRCXZ rel8 E3 cb Jump if RCX is zero
LODSQ REX.W AD LoaD String Quadword
MOVSXD r64,r/m32 REX.W 63 /r[c] MOV with Sign Extend 32-bit to 64-bit
MOVSQ REX.W A5 Move String Quadword
POPFQ 9D POP RFLAGS Register
PUSHFQ 9C PUSH RFLAGS Register
SCASQ REX.W AF SCAn String Quadword
STOSQ REX.W AB STOre String Quadword
SWAPGS 0F 01 F8 Exchange GS base with KernelGSBase MSR 0
  1. ^ The memory operand to CMPXCHG16B must be 16-byte aligned.
  2. ^ The CMPXCHG16B instruction was absent from a few of the earliest Intel/AMD x86-64 processors. On Intel processors, the instruction was missing from Xeon "Nocona" stepping D,[81] but added in stepping E.[82] On AMD K8 family processors, it was added in stepping F, at the same time as DDR2 support was introduced.[83]
    For this reason, CMPXCHG16B has its own CPUID flag, separate from the rest of x86-64.
  3. ^ Encodings of MOVSXD without REX.W prefix are permitted but discouraged[84] – such encodings behave identically to 16/32-bit MOV (8B /r).

Bit manipulation extensions

[edit]
Main article: X86 Bit manipulation instruction set

Bit manipulation instructions. For all of the VEX-encoded instructions defined by BMI1 and BMI2, the operand size may be 32 or 64 bits, controlled by the VEX.W bit – none of these instructions are available in 16-bit variants.

Bit Manipulation Extension Instruction
mnemonics		 Opcode Instruction description Added in
ABM (LZCNT)[a]
Advanced Bit Manipulation
 POPCNT r16,r/m16
POPCNT r32,r/m32 		F3 0F B8 /r Population Count. Counts the number of bits that are set to 1 in its source argument. K10,
Bobcat,
Haswell,
ZhangJiang,
Gracemont
POPCNT r64,r/m64 F3 REX.W 0F B8 /r
LZCNT r16,r/m16
LZCNT r32,r/m32 		F3 0F BD /r Count Leading zeroes.[b]
If source operand is all-0s, then LZCNT will return operand size in bits (16/32/64) and set CF=1.
LZCNT r64,r/m64 F3 REX.W 0F BD /r
BMI1
Bit Manipulation Instruction Set 1
 TZCNT r16,r/m16
TZCNT r32,r/m32 		F3 0F BC /r Count Trailing zeroes.[c]
If source operand is all-0s, then TZCNT will return operand size in bits (16/32/64) and set CF=1. 		Haswell,
Piledriver,
Jaguar,
ZhangJiang,
Gracemont
TZCNT r64,r/m64 F3 REX.W 0F BC /r
ANDN ra,rb,r/m VEX.LZ.0F38 F2 /r Bitwise AND-NOT: ra = r/m AND NOT(rb)
BEXTR ra,r/m,rb VEX.LZ.0F38 F7 /r Bitfield extract. Bitfield start position is specified in bits [7:0] of rb, length in bits[15:8] of rb. The bitfield is then extracted from the r/m value with zero-extension, then stored in ra. Equivalent to[d]
mask = (1 << rb[15:8]) - 1 ra = (r/m >> rb[7:0]) AND mask
BLSI reg,r/m VEX.LZ.0F38 F3 /3 Extract lowest set bit in source argument. Returns 0 if source argument is 0. Equivalent to
dst = (-src) AND src
BLSMSK reg,r/m VEX.LZ.0F38 F3 /2 Generate a bitmask of all-1s bits up to the lowest bit position with a 1 in the source argument. Returns all-1s if source argument is 0. Equivalent to
dst = (src-1) XOR src
BLSR reg,r/m VEX.LZ.0F38 F3 /1 Copy all bits of the source argument, then clear the lowest set bit. Equivalent to
dst = (src-1) AND src
BMI2
Bit Manipulation Instruction Set 2
 BZHI ra,r/m,rb VEX.LZ.0F38 F5 /r Zero out high-order bits in r/m starting from the bit position specified in rb, then write result to rd. Equivalent to
ra = r/m AND NOT(-1 << rb[7:0]) 		Haswell,
Excavator,[e]
ZhangJiang,
Gracemont
MULX ra,rb,r/m VEX.LZ.F2.0F38 F6 /r Widening unsigned integer multiply without setting flags. Multiplies EDX/RDX with r/m, then stores the low half of the multiplication result in ra and the high half in rb. If ra and rb specify the same register, only the high half of the result is stored.
PDEP ra,rb,r/m VEX.LZ.F2.0F38 F5 /r Parallel Bit Deposit. Scatters contiguous bits from rb to the bit positions set in r/m, then stores result to ra. Operation performed is:
ra=0; k=0; mask=r/m for i=0 to opsize-1 do    if (mask[i] == 1) then        ra[i]=rb[k]; k=k+1
PEXT ra,rb,r/m VEX.LZ.F3.0F38 F5 /r Parallel Bit Extract. Uses r/m argument as a bit mask to select bits in rb, then compacts the selected bits into a contiguous bit-vector. Operation performed is:
ra=0; k=0; mask=r/m for i=0 to opsize-1 do    if (mask[i] == 1) then        ra[k]=rb[i]; k=k+1
RORX reg,r/m,imm8 VEX.LZ.F2.0F3A F0 /r ib Rotate right by immediate without affecting flags.
SARX ra,r/m,rb VEX.LZ.F3.0F38 F7 /r Arithmetic shift right without updating flags.
For SARX, SHRX and SHLX, the shift-amount specified in rb is masked to 5 bits for 32-bit operand size and 6 bits for 64-bit operand size.
SHRX ra,r/m,rb VEX.LZ.F2.0F38 F7 /r Logical shift right without updating flags.
SHLX ra,r/m,rb VEX.LZ.66.0F38 F7 /r Shift left without updating flags.
  1. ^ On AMD CPUs, the "ABM" extension provides both POPCNT and LZCNT. On Intel CPUs, however, the CPUID bit for "ABM" is only documented to indicate the presence of the LZCNT instruction and is listed as "LZCNT", while POPCNT has its own separate CPUID feature bit.
    However, all known processors that implement the "ABM"/"LZCNT" extensions also implement POPCNT and set the CPUID feature bit for POPCNT, so the distinction is theoretical only.
    (The converse is not true – there exist processors that support POPCNT but not ABM, such as Intel Nehalem and VIA Nano 3000.)
  2. ^ The LZCNT instruction will execute as BSR on systems that do not support the LZCNT or ABM extensions. BSR computes the index of the highest set bit in the source operand, producing a different result from LZCNT for most input values.
  3. ^ The TZCNT instruction will execute as BSF on systems that do not support the BMI1 extension. BSF produces the same result as TZCNT for all input operand values except zero – for which TZCNT returns input operand size, but BSF produces undefined behavior (leaves destination unmodified on most modern CPUs).
  4. ^ For BEXTR, the start position and length are not masked and can take values from 0 to 255. If the selected bits extend beyond the end of the r/m argument (which has the usual 32/64-bit operand size), then the excess bits are read out as 0.
  5. ^ On AMD processors before Zen 3, the PEXT and PDEP instructions are quite slow[85] and exhibit data-dependent timing due to the use of a microcoded implementation (about 18 to 300 cycles, depending on the number of bits set in the mask argument). As a result, it is often faster to use other instruction sequences on these processors.[86][87]

Added with Intel TSX

[edit]
Main article: Transactional Synchronization Extensions
TSX Subset Instruction Opcode Description Added in
RTM
Restricted Transactional memory
 XBEGIN rel16
XBEGIN rel32 		C7 F8 cw
C7 F8 cd 		Start transaction. If transaction fails, perform a branch to the given relative offset. Haswell
(Deprecated on desktop/laptop CPUs from 10th generation (Ice Lake, Comet Lake) onwards, but continues to be available on Xeon-branded server parts (e.g. Ice Lake-SP, Sapphire Rapids))
XABORT imm8 C6 F8 ib Abort transaction with 8-bit immediate as error code.
XEND NP 0F 01 D5 End transaction.
XTEST NP 0F 01 D6 Test if in transactional execution. Sets EFLAGS.ZF to 0 if executed inside a transaction (RTM or HLE), 1 otherwise.
HLE
Hardware Lock Elision
 XACQUIRE F2 Instruction prefix to indicate start of hardware lock elision, used with memory atomic instructions only (for other instructions, the F2 prefix may have other meanings). When used with such instructions, may start a transaction instead of performing the memory atomic operation. Haswell
(Discontinued – the last processors to support HLE were Coffee Lake and Cascade Lake)
XRELEASE F3 Instruction prefix to indicate end of hardware lock elision, used with memory atomic/store instructions only (for other instructions, the F3 prefix may have other meanings). When used with such instructions during hardware lock elision, will end the associated transaction instead of performing the store/atomic.
TSXLDTRK
Load Address Tracking suspend/resume
 XSUSLDTRK F2 0F 01 E8 Suspend Tracking Load Addresses Sapphire Rapids
XRESLDTRK F2 0F 01 E9 Resume Tracking Load Addresses

Added with Intel CET

[edit]

Intel CET (Control-Flow Enforcement Technology) adds two distinct features to help protect against security exploits such as return-oriented programming: a shadow stack (CET_SS), and indirect branch tracking (CET_IBT).

CET Subset Instruction Opcode Description Ring Added in
CET_SS
Shadow stack.
When shadow stacks are enabled, return addresses are pushed on both the regular stack and the shadow stack when a function call is made. They are then both popped on return from the function call – if they do not match, then the stack is assumed to be corrupted, and a #CP exception is issued.
The shadow stack is additionally required to be stored in specially marked memory pages which cannot be modified by normal memory store instructions.
 INCSSPD r32 F3 0F AE /5 Increment shadow stack pointer 3 Tiger Lake,
Zen 3
INCSSPQ r64 F3 REX.W 0F AE /5
RDSSPD r32 F3 0F 1E /1 Read shadow stack pointer into register (low 32 bits)[a]
RDSSPQ r64 F3 REX.W 0F 1E /1 Read shadow stack pointer into register (full 64 bits)[a]
SAVEPREVSSP F3 0F 01 EA Save previous shadow stack pointer
RSTORSSP m64 F3 0F 01 /5 Restore saved shadow stack pointer
WRSSD m32,r32 NP 0F 38 F6 /r Write 4 bytes to shadow stack
WRSSQ m64,r64 NP REX.W 0F 38 F6 /r Write 8 bytes to shadow stack
WRUSSD m32,r32 66 0F 38 F5 /r Write 4 bytes to user shadow stack 0
WRUSSQ m64,r64 66 REX.W 0F 38 F5 /r Write 8 bytes to user shadow stack
SETSSBSY F3 0F 01 E8 Mark shadow stack busy
CLRSSBSY m64 F3 0F AE /6 Clear shadow stack busy flag
CET_IBT
Indirect Branch Tracking.
When IBT is enabled, an indirect branch (jump, call, return) to any instruction that is not an ENDBR32/64 instruction will cause a #CP exception.
 ENDBR32 F3 0F 1E FB Terminate indirect branch in 32-bit mode[b] 3 Tiger Lake
ENDBR64 F3 0F 1E FA Terminate indirect branch in 64-bit mode[b]
NOTRACK 3E[c] Prefix used with indirect CALL/JMP near instructions (opcodes FF /2 and FF /4) to indicate that the branch target is not required to start with an ENDBR32/64 instruction. Prefix only honored when NO_TRACK_EN flag is set.
  1. ^ a b The RDSSPD and RDSSPQ instructions act as NOPs on processors where shadow stacks are disabled or CET is not supported.
  2. ^ a b ENDBR32 and ENDBR64 act as NOPs on processors that don't support CET_IBT or where IBT is disabled.
  3. ^ This prefix has the same encoding as the DS: segment override prefix – as of April 2022, Intel documentation does not appear to specify whether this prefix also retains its old segment-override function when used as a no-track prefix, nor does it provide an official mnemonic for this prefix.[88][89] (GNU binutils use "notrack"[90])

Added with XSAVE

[edit]

The XSAVE instruction set extensions are designed to save/restore CPU extended state (typically for the purpose of context switching) in a manner that can be extended to cover new instruction set extensions without the OS context-switching code needing to understand the specifics of the new extensions. This is done by defining a series of state-components, each with a size and offset within a given save area, and each corresponding to a subset of the state needed for one CPU extension or another. The EAX=0Dh CPUID leaf is used to provide information about which state-components the CPU supports and what their sizes/offsets are, so that the OS can reserve the proper amount of space and set the associated enable-bits.

XSAVE Extension Instruction
mnemonics		 Opcode[a] Instruction description Ring Added in
XSAVE
Processor Extended State Save/Restore.
 XSAVE mem
XSAVE64 mem 		NP 0F AE /4
NP REX.W 0F AE /4 		Save state components specified by bitmap in EDX:EAX to memory. 3 Penryn,[b]
Bulldozer,
Jaguar,
Goldmont,
ZhangJiang
XRSTOR mem
XRSTOR64 mem 		NP 0F AE /5
NP REX.W 0F AE /5 		Restore state components specified by EDX:EAX from memory.
XGETBV NP 0F 01 D0 Get value of Extended Control Register.
Reads an XCR specified by ECX into EDX:EAX.[c]
XSETBV NP 0F 01 D1 Set Extended Control Register.[d]
Write the value in EDX:EAX to the XCR specified by ECX. 		0
XSAVEOPT
Processor Extended State Save/Restore Optimized
 XSAVEOPT mem
XSAVEOPT64 mem 		NP 0F AE /6
NP REX.W 0F AE /6 		Save state components specified by EDX:EAX to memory.
Unlike the older XSAVE instruction, XSAVEOPT may abstain from writing processor state items to memory when the CPU can determine that they haven't been modified since the most recent corresponding XRSTOR. 		3 Sandy Bridge,
Steamroller,
Puma,
Goldmont,
ZhangJiang
XSAVEC
Processor Extended State save/restore with compaction.
 XSAVEC mem
XSAVEC64 mem 		NP 0F C7 /4
NP REX.W 0F C7 /4 		Save processor extended state components specified by EDX:EAX to memory with compaction. 3 Skylake,
Goldmont,
Zen 1
XSS
Processor Extended State save/restore, including supervisor state.
 XSAVES mem
XSAVES64 mem 		NP 0F C7 /5
NP REX.W 0F C7 /5 		Save processor extended state components specified by EDX:EAX to memory with compaction and optimization if possible. 0 Skylake,
Goldmont,
Zen 1
XRSTORS mem
XRSTORS64 mem 		NP 0F C7 /3
NP REX.W 0F C7 /3 		Restore state components specified by EDX:EAX from memory.
  1. ^ Under Intel APX, the XSAVE* and XRSTOR* instructions cannot be encoded with the REX2 prefix.
  2. ^ XSAVE was added in steppings E0/R0 of Penryn and is not available in earlier steppings.
  3. ^ On some processors (starting with Skylake, Goldmont and Zen 1), executing XGETBV with ECX=1 is permitted – this will not return XCR1 (no such register exists) but instead return XCR0 bitwise-ANDed with the current value of the "XINUSE" state-component bitmap (a bitmap of XSAVE state-components that are not known to be in their initial state).
    The presence of this functionality of XGETBV is indicated by CPUID.(EAX=0Dh,ECX=1):EAX[bit 2].
  4. ^ The XSETBV instruction will cause a mandatory #VMEXIT if executed under Intel VT-x virtualization.

Added with other cross-vendor extensions

[edit]
Instruction Set Extension Instruction
mnemonics		 Opcode Instruction description Ring Added in
SSE[a]
(non-SIMD)
 PREFETCHNTA m8 0F 18 /0 Prefetch with Non-Temporal Access.
Prefetch data under the assumption that the data will be used only once, and attempt to minimize cache pollution from said data. The methods used to minimize cache pollution are implementation-dependent.[b] 		3 Pentium III,
(K7),[a]
(Geode GX2),[a]
Nehemiah,
Efficeon
PREFETCHT0 m8 0F 18 /1 Prefetch data to all levels of the cache hierarchy.[b]
PREFETCHT1 m8 0F 18 /2 Prefetch data to all levels of the cache hierarchy except L1 cache.[b]
PREFETCHT2 m8 0F 18 /3 Prefetch data to all levels of the cache hierarchy except L1 and L2 caches.[b]
SFENCE NP 0F AE F8+x[c] Store Fence.[d]
SSE2
(non-SIMD)
 LFENCE NP 0F AE E8+x[c] Load Fence and Dispatch Serialization.[e] 3 Pentium 4,
K8,
Efficeon,
C7 Esther
MFENCE NP 0F AE F0+x[c] Memory Fence.[f]
MOVNTI m32,r32
MOVNTI m64,r64 		NP 0F C3 /r
NP REX.W 0F C3 /r 		Non-Temporal Memory Store.
PAUSE F3 90[g] Pauses CPU thread for a short time period.[h]
Intended for use in spinlocks.[i]
CLFSH[j]
Cache Line Flush.
 CLFLUSH m8 NP 0F AE /7 Flush one cache line to memory.
In a system with multiple cache hierarchy levels and/or multiple processors each with their own caches, the line is flushed from all of them. 		3 (SSE2),
Geode LX
MONITOR[k]
Monitor a memory location for memory writes.
 MONITOR[l]
MONITOR EAX,ECX,EDX 		NP 0F 01 C8 Start monitoring a memory location for memory writes. The memory address to monitor is given by DS:AX/EAX/RAX.[m]
ECX and EDX are reserved for extra extension and hint flags, respectively.[n] 		Usually 0[o] Prescott,
Yonah,
Bonnell,
K10,
Nano
MWAIT[l]
MWAIT EAX,ECX 		NP 0F 01 C9 Wait for a write to a monitored memory location previously specified with MONITOR.[p]
ECX and EAX are used to provide extra extension[q] and hint[r] flags, respectively. MWAIT hints are commonly used for CPU power management.
SMX
Safer Mode Extensions.
Load, authenticate and execute a digitally signed "Authenticated Code Module" as part of Intel Trusted Execution Technology.
 GETSEC NP 0F 37[s] Perform an SMX function. The leaf function to perform is given in EAX.[t]
Depending on leaf function, the instruction may take additional arguments in RBX, ECX and EDX. 		Usually 0[u] Conroe/Merom,
WuDaoKou,[103]
Tremont
RDTSCP
Read Time Stamp Counter and Processor ID.
 RDTSCP 0F 01 F9 Read Time Stamp Counter and processor core ID.[v]
The TSC value is placed in EDX:EAX and the core ID in ECX.[w] 		Usually 3[x] K8,[y]
Nehalem,
Silvermont,
Nano
POPCNT[z]
Population Count.
 POPCNT r16,r/m16
POPCNT r32,r/m32 		F3 0F B8 /r Count the number of bits that are set to 1 in its source argument. 3 K10,
Nehalem,
Nano 3000
POPCNT r64,r/m64 F3 REX.W 0F B8 /r
SSE4.2
(non-SIMD)
 CRC32 r32,r/m8 F2 0F 38 F0 /r Accumulate CRC value using the CRC-32C (Castagnoli) polynomial 0x11EDC6F41 (normal form 0x1EDC6F41). This is the polynomial used in iSCSI. In contrast to the more popular one used in Ethernet, its parity is even, and it can thus detect any error with an odd number of changed bits. 3 Nehalem,
Bulldozer,
ZhangJiang
CRC32 r32,r/m16
CRC32 r32,r/m32 		F2 0F 38 F1 /r
CRC32 r64,r/m64 F2 REX.W 0F 38 F1 /r
FSGSBASE
Read/write base address of FS and GS segments from user-mode.
Available in 64-bit mode only.
 RDFSBASE r32
RDFSBASE r64 		F3 0F AE /0
F3 REX.W 0F AE /0 		Read base address of FS: segment. 3 Ivy Bridge,
Steamroller,
Goldmont,
ZhangJiang
RDGSBASE r32
RDGSBASE r64 		F3 0F AE /1
F3 REX.W 0F AE /1 		Read base address of GS: segment.
WRFSBASE r32
WRFSBASE r64 		F3 0F AE /2
F3 REX.W 0F AE /2 		Write base address of FS: segment.
WRGSBASE r32
WRGSBASE r64 		F3 0F AE /3
F3 REX.W 0F AE /3 		Write base address of GS: segment.
MOVBE
Move to/from memory with byte order swap.
 MOVBE r16,m16
MOVBE r32,m32 		NFx 0F 38 F0 /r Load from memory to register with byte-order swap. 3 Bonnell,
Haswell,
Jaguar,
Steamroller,
ZhangJiang
MOVBE r64,m64 NFx REX.W 0F 38 F0 /r
MOVBE m16,r16
MOVBE m32,r32 		NFx 0F 38 F1 /r Store to memory from register with byte-order swap.
MOVBE m64,r64 NFx REX.W 0F 38 F1 /r
INVPCID
Invalidate TLB entries by Process-context identifier.
 INVPCID reg,m128 66 0F 38 82 /r Invalidate entries in TLB and paging-structure caches based on invalidation type in register[aa] and descriptor in m128. The descriptor contains a memory address and a PCID.[ab]

Instruction is serializing on AMD but not Intel CPUs.

0 Haswell,
ZhangJiang,
Zen 3,
Gracemont
PREFETCHW[ac]
Cache-line prefetch with intent to write.
 PREFETCHW m8 0F 0D /1 Prefetch cache line with intent to write.[b] 3 K6-2,
(Cedar Mill),[ad]
Silvermont,
Broadwell,
ZhangJiang
PREFETCH m8[ae] 0F 0D /0 Prefetch cache line.[b]
ADX
Enhanced variants of add-with-carry.
 ADCX r32,r/m32
ADCX r64,r/m64 		66 0F 38 F6 /r
66 REX.W 0F 38 F6 /r 		Add-with-carry. Differs from the older ADC instruction in that it leaves flags other than EFLAGS.CF unchanged. 3 Broadwell,
Zen 1,
ZhangJiang,
Gracemont
ADOX r32,r/m32
ADOX r64,r/m64 		F3 0F 38 F6 /r
F3 REX.W 0F 38 F6 /r 		Add-with-carry, with the overflow-flag EFLAGS.OF serving as carry input and output, with other flags left unchanged.
SMAP
Supervisor Mode Access Prevention.
Repurposes the EFLAGS.AC (alignment check) flag to a flag that prevents access to user-mode memory while in ring 0, 1 or 2.
 CLAC NP 0F 01 CA Clear EFLAGS.AC. 0 Broadwell,
Goldmont,
Zen 1,
LuJiaZui[af]
STAC NP 0F 01 CB Set EFLAGS.AC.
CLFLUSHOPT
Optimized Cache Line Flush.
 CLFLUSHOPT m8 NFx 66 0F AE /7 Flush cache line.
Differs from the older CLFLUSH instruction in that it has more relaxed ordering rules with respect to memory stores and other cache line flushes, enabling improved performance. 		3 Skylake,
Goldmont,
Zen 1
PREFETCHWT1
Cache-line prefetch into L2 cache with intent to write.
 PREFETCHWT1 m8 0F 0D /2 Prefetch data with T1 locality hint (fetch into L2 cache, but not L1 cache) and intent-to-write hint.[b] 3 Knights Landing,
YongFeng
PKU
Protection Keys for user pages.
 RDPKRU NP 0F 01 EE Read User Page Key register into EAX. 3 Skylake-X,
Comet Lake,
Gracemont,
Zen 3,
LuJiaZui[af]
WRPKRU NP 0F 01 EF Write data from EAX into User Page Key Register, and perform a Memory Fence.
CLWB
Cache Line Writeback to memory.
 CLWB m8 NFx 66 0F AE /6 Write one cache line back to memory without invalidating the cache line. 3 Skylake-X,
Zen 2,
Tiger Lake,
Tremont
RDPID
Read processor core ID.
 RDPID r32 F3 0F C7 /7 Read processor core ID into register.[v] 3[ag] Goldmont Plus,
Zen 2,
Ice Lake,
LuJiaZui[af]
MOVDIRI
Move to memory as Direct Store.
 MOVDIRI m32,r32
MOVDIRI m64,r64 		NP 0F 38 F9 /r
NP REX.W 0F 38 F9 /r 		Store to memory using Direct Store (memory store that is not cached or write-combined with other stores). 3 Tiger Lake,
Tremont,
Zen 5
MOVDIR64B
Move 64 bytes as Direct Store.
 MOVDIR64B reg,m512 66 0F 38 F8 /r Move 64 bytes of data from m512 to address given by ES:reg. The 64-byte write is done atomically with Direct Store.[ah] 3 Tiger Lake,
Tremont,
Zen 5
WBNOINVD
Whole Cache Writeback without invalidate.
 WBNOINVD F3 0F 09 Write back all dirty cache lines to memory without invalidation.[ai] Instruction is serializing. 0 Zen 2,
Ice Lake-SP
PREFETCHI
Instruction prefetch.
 PREFETCHIT0 m8 0F 18 /7 Prefetch code to all levels of the cache hierarchy.[aj] 3 Zen 5,
Granite Rapids
PREFETCHIT1 m8 0F 18 /6 Prefetch code to all levels of the cache hierarchy except first-level cache.[aj]
  1. ^ a b c AMD Athlon processors prior to the Athlon XP did not support full SSE, but did introduce the non-SIMD instructions of SSE as part of "MMX Extensions".[91] These extensions (without full SSE) are also present on Geode GX2 and later Geode processors.
  2. ^ a b c d e f g All of the PREFETCH* instructions are hint instructions with effects only on performance, not program semantics. Providing an invalid address (e.g. address of an unmapped page or a non-canonical address) will cause the instruction to act as a NOP without any exceptions generated.
  3. ^ a b c For the SFENCE, LFENCE and MFENCE instructions, the bottom 3 bits of the ModR/M byte are ignored, and any value of x in the range 0..7 will result in a valid instruction.
  4. ^ The SFENCE instruction ensures that all memory stores after the SFENCE instruction are made globally observable after all memory stores before the SFENCE. This imposes ordering on stores that can otherwise be reordered, such as non-temporal stores and stores to WC (Write-Combining) memory regions.[92]
    On Intel CPUs, as well as AMD CPUs from Zen1 onwards (but not older AMD CPUs), SFENCE also acts as a reordering barrier on cache flushes/writebacks performed with the CLFLUSH, CLFLUSHOPT and CLWB instructions. (Older AMD CPUs require MFENCE to order CLFLUSH.)
    SFENCE is not ordered with respect to LFENCE, and an SFENCE+LFENCE sequence is not sufficient to prevent a load from being reordered past a previous store.[93] To prevent such reordering, it is necessary to execute an MFENCE, LOCK or a serializing instruction.
  5. ^ The LFENCE instruction ensures that all memory loads after the LFENCE instruction are made globally observable after all memory loads before the LFENCE.
    On all Intel CPUs that support SSE2, the LFENCE instruction provides a stronger ordering guarantee:[94] it is dispatch-serializing, meaning that instructions after the LFENCE instruction are allowed to start executing only after all instructions before it have retired (which will ensure that all preceding loads but not necessarily stores have completed). The effect of dispatch-serialization is that LFENCE also acts as a speculation barrier and a reordering barrier for accesses to non-memory resources such as performance counters (accessed through e.g. RDTSC or RDPMC) and x2apic MSRs.
    On AMD CPUs, LFENCE is not necessarily dispatch-serializing by default – however, on all AMD CPUs that support any form of non-dispatch-serializing LFENCE, it can be made dispatch-serializing by setting bit 1 of MSR C001_1029.[95]
  6. ^ The MFENCE instruction ensures that all memory loads, stores and cacheline-flushes after the MFENCE instruction are made globally observable after all memory loads, stores and cacheline-flushes before the MFENCE.
    On Intel CPUs, MFENCE is not dispatch-serializing, and therefore cannot be used on its own to enforce ordering on accesses to non-memory resources such as performance counters and x2apic MSRs. MFENCE is still ordered with respect to LFENCE, so if there is a need to enforce ordering between memory stores and subsequent non-memory accesses, then such an ordering can be obtained by issuing an MFENCE followed by an LFENCE.[50][96]
    On AMD CPUs, MFENCE is serializing.
  7. ^ The operation of the PAUSE instruction in 64-bit mode is, unlike NOP, unaffected by the presence of the REX.R prefix. Neither NOP nor PAUSE are affected by the other bits of the REX prefix. A few examples of how opcode 90 interacts with various prefixes in 64-bit mode are:
    • 90 is NOP
    • 41 90 is XCHG R8D,EAX
    • 4E 90 is NOP
    • 49 90 is XCHG R8,RAX
    • F3 90 is PAUSE
    • F3 41 90 is PAUSE
    • F3 4F 90 is PAUSE
  8. ^ The actual length of the pause performed by the PAUSE instruction is implementation-dependent.
    On systems without SSE2, PAUSE will execute as NOP.
  9. ^ Under VT-x or AMD-V virtualization, executing PAUSE many times in a short time interval may cause a #VMEXIT. The number of PAUSE executions and interval length that can trigger #VMEXIT are platform-specific.
  10. ^ While the CLFLUSH instruction was introduced together with SSE2, it has its own CPUID flag and may be present on processors not otherwise implementing SSE2 and/or absent from processors that otherwise implement SSE2. (E.g. AMD Geode LX supports CLFLUSH but not SSE2.)
  11. ^ While the MONITOR and MWAIT instructions were introduced at the same time as SSE3, they have their own CPUID flag that needs to be checked separately from the SSE3 CPUID flag (e.g. Athlon 64 X2 and VIA C7 supported SSE3 but not MONITOR.)
  12. ^ a b For the MONITOR and MWAIT instructions, older Intel documentation[97] lists instruction mnemonics with explicit operands (MONITOR EAX,ECX,EDX and MWAIT EAX,ECX), while newer documentation omits these operands. Assemblers/disassemblers may support one or both of these variants.[98]
  13. ^ For MONITOR, the DS: segment can be overridden with a segment prefix.
    The memory area that will be monitored will be not just the single byte specified by DS:rAX, but a linear memory region containing the byte – the size and alignment of this memory region is implementation-dependent and can be queried through CPUID.
    The memory location to monitor should have memory type WB (write-back cacheable), or else monitoring may fail.
  14. ^ As of April 2024, no extensions or hints have been defined for the MONITOR instruction. As such, the instruction requires ECX=0 and ignores EDX.
  15. ^ On some processors, such as Intel Xeon Phi x200[99] and AMD K10[100] and later, there exist documented MSRs that can be used to enable MONITOR and MWAIT to run in Ring 3.
  16. ^ The wait performed by MWAITmay be ended by system events other than a memory write (e.g. cacheline evictions, interrupts) – the exact set of events that can cause the wait to end is implementation-specific.
    Regardless of whether the wait was ended by a memory write or some other event, monitoring will have ended and it will be necessary to set up monitoring again with MONITOR before using MWAIT to wait for memory writes again.
  17. ^ The extension flags available for MWAIT in the ECX register are:
    Bits MWAIT Extension
    0 Treat interrupts as break events, even when masked (EFLAGS.IF=0). (Available on all non-NetBurst implementations of MWAIT.)
    1 Timed MWAIT: end the wait when the TSC reaches or exceeds the value in EDX:EBX. (Undocumented, reportedly present in Intel Skylake and later Intel processors)[101]
    2 Monitorless MWAIT[102]
    31:3 Not used, must be set to zero.
  18. ^ The hint flags available for MWAIT in the EAX register are:
    Bits MWAIT Hint
    3:0 Sub-state within a C-state (see bits 7:4) (Intel processors only)
    7:4 Target CPU power C-state during wait, minus 1. (E.g. 0000b for C1, 0001b for C2, 1111b for C0)
    31:8 Not used.

    The C-states are processor-specific power states, which do not necessarily correspond 1:1 to ACPI C-states.

  19. ^ For the GETSEC instruction, the REX.W prefix enables 64-bit addresses for the EXITAC leaf function only - REX prefixes are otherwise permitted but ignored for the instruction.
  20. ^ The leaf functions defined for GETSEC (selected by EAX) are:
    EAX Function
    0 (CAPABILITIES) Report SMX capabilities
    2 (ENTERACCES) Enter execution of authenticated code module
    3 (EXITAC) Exit execution of authenticated code module
    4 (SENTER) Enter measured environment
    5 (SEXIT) Exit measured environment
    6 (PARAMETERS) Report SMX parameters
    7 (SMCTRL) SMX Mode Control
    8 (WAKEUP) Wake up sleeping processors in measured environment

    Any unsupported value in EAX causes an #UD exception.

  21. ^ For GETSEC, most leaf functions are restricted to Ring 0, but the CAPABILITIES (EAX=0) and PARAMETERS (EAX=6) leaf functions are available in Ring 3.
  22. ^ a b The "core ID" value read by RDTSCP and RDPID is actually the TSC_AUX MSR (MSR C000_0103h). Whether this value actually corresponds to a processor ID is a matter of operating system convention.
  23. ^ Unlike the older RDTSC instruction, RDTSCP will delay the TSC read until all previous instructions have retired, guaranteeing ordering with respect to preceding memory loads (but not stores). RDTSCP is not ordered with respect to subsequent instructions, though.
  24. ^ RDTSCP can be run outside Ring 0 only if CR4.TSD=0.
  25. ^ Support for RDTSCP was added in stepping F of the AMD K8, and is not available on earlier steppings.
  26. ^ While the POPCNT instruction was introduced at the same time as SSE4.2, it is not considered to be a part of SSE4.2, but instead a separate extension with its own CPUID flag.
    On AMD processors, it is considered to be a part of the ABM extension, but still has its own CPUID flag.
  27. ^ The invalidation types defined for INVPCID (selected by register argument) are:
    Value Function
    0 Invalidate TLB entries matching PCID and virtual memory address in descriptor, excluding global entries
    1 Invalidate TLB entries matching PCID in descriptor, excluding global entries
    2 Invalidate all TLB entries, including global entries
    3 Invalidate all TLB entries, excluding global entries

    Any unsupported value in the register argument causes a #GP exception.

  28. ^ Unlike the older INVLPG instruction, INVPCID will cause a #GP exception if the provided memory address is non-canonical. This discrepancy has been known to cause security issues.[104]
  29. ^ The PREFETCH and PREFETCHW instructions are mandatory parts of the 3DNow! instruction set extension, but are also available as a standalone extension on systems that do not support 3DNow!
  30. ^ The opcodes for PREFETCH and PREFETCHW (0F 0D /r) execute as NOPs on Intel CPUs from Cedar Mill (65nm Pentium 4) onwards, with PREFETCHW gaining prefetch functionality from Broadwell onwards.
  31. ^ The PREFETCH (0F 0D /0) instruction is a 3DNow! instruction, present on all processors with 3DNow! but not necessarily on processors with the PREFETCHW extension.
    On AMD CPUs with PREFETCHW, opcode 0F 0D /0 as well as opcodes 0F 0D /2../7 are all documented to be performing prefetch.
    On Intel processors with PREFETCHW, these opcodes are documented as performing reserved-NOPs[105] (except 0F 0D /2 being PREFETCHWT1 m8 on Xeon Phi only) – third party testing[106] indicates that some or all of these opcodes may be performing prefetch on at least some Intel Core CPUs.
  32. ^ a b c The SMAP, PKU and RDPID instruction set extensions are supported on stepping 2[107] and later of Zhaoxin LuJiaZui, but not on earlier steppings.
  33. ^ Unlike the older RDTSCP instruction which can also be used to read the processor ID, user-mode RDPID is not disabled by CR4.TSD=1.
  34. ^ For MOVDIR64, the destination address given by ES:reg must be 64-byte aligned.
    The operand size for the register argument is given by the address size, which may be overridden by the 67h prefix.
    The 64-byte memory source argument does not need to be 64-byte aligned, and is not guaranteed to be read atomically.
  35. ^ The WBNOINVD instruction will execute as WBINVD if run on a system that doesn't support the WBNOINVD extension.
    WBINVD differs from WBNOINVD in that WBINVD will invalidate all cache lines after writeback.
  36. ^ a b In initial implementations, the PREFETCHIT0 and PREFETCHIT1 instructions will perform code prefetch only when using the RIP-relative addressing mode and act as NOPs otherwise.
    The PREFETCHI instructions are hint instructions only - if an attempt is made to prefetch an invalid address, the instructions will act as NOPs with no exceptions generated. On processors that support Long-NOP but do not support the PREFETCHI instructions, these instructions will always act as NOPs.

Added with other Intel-specific extensions

[edit]
Instruction Set Extension Instruction
mnemonics		 Opcode Instruction description Ring Added in
SSE2 branch hints
Instruction prefixes that can be used with the Jcc instructions to provide branch taken/not-taken hints.
 (HWNT)[a] 2E[b] Instruction prefix: branch hint weakly not taken. 3 Pentium 4,[c]
Meteor Lake[110]
(HST)[a] 3E[b] Instruction prefix: branch hint strongly taken.
SGX
Software Guard Extensions.
Set up an encrypted enclave in which a guest can execute code that a compromised or malicious host cannot inspect or tamper with.
 ENCLS NP 0F 01 CF Perform an SGX Supervisor function. The function to perform is given in EAX[d] - depending on function, the instruction may take additional input operands in RBX, RCX and RDX.

Depending on function, the instruction may return data in RBX and/or an error code in EAX.

0
SGX1
Skylake,[e]
Goldmont Plus
SGX2
Goldmont Plus,
Ice Lake-SP[114]
OVERSUB[111]
Ice Lake-SP,
Tremont
ENCLU NP 0F 01 D7 Perform an SGX User function. The function to perform is given in EAX[f] - depending on function, the instruction may take additional input operands in RBX, RCX and RDX.

Depending on function, the instruction may return data/status information in EAX and/or RCX.

3[g]
ENCLV NP 0F 01 C0 Perform an SGX Virtualization function. The function to perform is given in EAX[h] - depending on function, the instruction may take additional input operands in RBX, RCX and RDX.

Instruction returns status information in EAX.

0[i]
PTWRITE
Write data to a Processor Trace Packet.
 PTWRITE r/m32
PTWRITE r/m64 		F3 0F AE /4
F3 REX.W 0F AE /4 		Read data from register or memory to encode into a PTW packet.[j] 3 Kaby Lake,
Goldmont Plus
PCONFIG
Platform Configuration, including TME-MK ("Total Memory Encryption – Multi-Key") and TSE ("Total Storage Encryption").
 PCONFIG NP 0F 01 C5 Perform a platform feature configuration function. The function to perform is specified in EAX[k] - depending on function, the instruction may take additional input operands in RBX, RCX and RDX.

If the instruction fails, it will set EFLAGS.ZF=1 and return an error code in EAX. If it is successful, it sets EFLAGS.ZF=0 and EAX=0.

0 Ice Lake-SP
CLDEMOTE
Cache Line Demotion Hint.
 CLDEMOTE m8 NP 0F 1C /0 Move cache line containing m8 from CPU L1 cache to a more distant level of the cache hierarchy.[l] 3 (Tremont),
(Alder Lake),
Sapphire Rapids[m]
WAITPKG
User-mode memory monitoring and waiting.
 UMONITOR r16/32/64 F3 0F AE /6 Start monitoring a memory location for memory writes. The memory address to monitor is given by the register argument.[n] 3 Tremont,
Alder Lake
UMWAIT r32
UMWAIT r32,EDX,EAX 		F2 0F AE /6 Timed wait for a write to a monitored memory location previously specified with UMONITOR. In the absence of a memory write, the wait will end when either the TSC reaches the value specified by EDX:EAX or the wait has been going on for an OS-controlled maximum amount of time.[o] Usually 3[p]
TPAUSE r32
TPAUSE r32,EDX,EAX 		66 0F AE /6 Wait until the Time Stamp Counter reaches the value specified in EDX:EAX.[o]

The register argument to the UMWAIT and TPAUSE instructions specifies extra flags to control the operation of the instruction.[q]

SERIALIZE
Instruction Execution Serialization.
 SERIALIZE NP 0F 01 E8 Serialize instruction fetch and execution.[r] 3 Alder Lake
HRESET
Processor History Reset.
 HRESET imm8 F3 0F 3A F0 C0 ib Request that the processor reset selected components of hardware-maintained prediction history. A bitmap of which components of the CPU's prediction history to reset is given in EAX (the imm8 argument is ignored).[s] 0 Alder Lake
UINTR
User Interprocessor interrupt.
Available in 64-bit mode only.
 SENDUIPI reg F3 0F C7 /6 Send Interprocessor User Interrupt.[t] 3 Sapphire Rapids
UIRET F3 0F 01 EC User Interrupt Return.

Pops RIP, RFLAGS and RSP off the stack, in that order.[u]

TESTUI F3 0F 01 ED Test User Interrupt Flag.
Copies UIF to EFLAGS.CF .
CLUI F3 0F 01 EE Clear User Interrupt Flag.
STUI F3 0F 01 EF Set User Interrupt Flag.
ENQCMD
Enqueue Store.

Part of Intel DSA (Data Streaming Accelerator Architecture).[118]

 ENQCMD r32/64,m512 F2 0F 38 F8 /r Enqueue Command. Reads a 64-byte "command data" structure from memory (m512 argument) and writes atomically to a memory-mapped Enqueue Store device (register argument provides the memory address of this device, using ES segment and requiring 64-byte alignment.) Sets ZF=0 to indicate that device accepted the command, or ZF=1 to indicate that command was not accepted (e.g. queue full or the memory location was not an Enqueue Store device.) 3 Sapphire Rapids
ENQCMDS r32/64,m512 F3 0F 38 F8 /r Enqueue Command Supervisor. Differs from ENQCMD in that it can place an arbitrary PASID (process address-space identifier) and a privilege-bit in the "command data" to enqueue. 0
WRMSRNS
Non-serializing Write to Model-specific register.
 WRMSRNS NP 0F 01 C6 Write Model-specific register. The MSR to write is specified in ECX, and the data to write is given in EDX:EAX.

The instruction differs from the older WRMSR instruction in that it is not serializing.

0 Sierra Forest
MSRLIST
Read/write multiple Model-specific registers.
Available in 64-bit mode only.
 RDMSRLIST F2 0F 01 C6 Read multiple MSRs. RSI points to a table of up to 64 MSR indexes to read (64 bits each), RDI points to a table of up to 64 data items that the MSR read-results will be written to (also 64 bits each), and RCX provides a 64-entry bitmap of which of the table entries to actually perform an MSR read for.[v] 0 Sierra Forest
WRMSRLIST F3 0F 01 C6 Write multiple MSRs. RSI points to a table of up to 64 MSR indexes to write (64 bits each), RDI points to a table of up to 64 data items to write into the MSRs (also 64 bits each), and RCX provides a 64-entry bitmap of which of the table entries to actually perform an MSR write for.[v]

The instruction is not serializing.

CMPCCXADD
Atomically perform a compare - and a fetch-and-add if the condition is met.
Available in 64-bit mode only.
 CMPccXADD m32,r32,r32
CMPccXADD m64,r64,r64
  		VEX.128.66.0F38.W0 Ex /r
VEX.128.66.0F38.W1 Ex /r
[w][x]  		Read value from memory, then compare to first register operand. If the comparison passes, then add the second register operand to the memory value. The instruction as a whole is performed atomically.
The operation of CMPccXADD [mem],reg1,reg2 is:
temp1 := [mem] EFLAGS := CMP temp1, reg1 // sets EFLAGS like regular compare reg1 := temp1 if( condition )     [mem] := temp1 + reg2
 3 Sierra Forest,
Lunar Lake
PBNDKB
Platform Bind Key to Binary Large Object.

Part of Intel TSE (Total Storage Encryption), and available in 64-bit mode only.

 PBNDKB NP 0F 01 C7 Bind information to a platform by encrypting it with a platform-specific wrapping key. The instruction takes as input the addresses to two 256-byte-aligned "bind structures" in RBX and RCX, reads the structure pointed to by RBX and writes a modified structure to the address given in RCX.

If the instruction fails, it will set EFLAGS.ZF=1 and return an error code in EAX. If it is successful, it sets EFLAGS.ZF=0 and EAX=0.

0 Lunar Lake
  1. ^ a b The branch hint mnemonics HWNT and HST are listed in early Willamette documentation only[108] - later Intel documentation lists the branch hint prefixes without assigning them a mnemonic.[109]
  2. ^ a b The 2E and 3E prefixes are interpreted as branch hints only when used with the Jcc conditional branch instructions (opcodes 70..7F and 0F 80..8F) - when used with other opcodes, they may take other meanings (e.g. for instructions with memory operands outside 64-bit mode, they will work as segment-override prefixes CS: and DS:, respectively). On processors that don't support branch hints, these prefixes are accepted but ignored when used with Jcc.
  3. ^ Branch hints are supported on all NetBurst (Pentium 4 family) processors - but not supported on any other known processor prior to their re-introduction in "Redwood Cove" CPUs, starting with "Meteor Lake" in 2023.
  4. ^ The leaf functions defined for ENCLS (selected by EAX) are:
    EAX Function
    0 (ECREATE) Create an enclave
    1 (EADD) Add a page
    2 (EINIT) Initialize an enclave
    3 (EREMOVE) Remove a page from EPC (Enclave Page Cache)
    4 (EDBGRD) Read data by debugger
    5 (EDBGWR) Write data by debugger
    6 (EEXTEND) Extend EPC page measurement
    7 (ELDB) Load an EPC page as blocked
    8 (ELDU) Load an EPC page as unblocked
    9 (EBLOCK) Block an EPC page
    A (EPA) Add version array
    B (EWB) Writeback/invalidate EPC page
    C (ETRACK) Activate EBLOCK checks
    Added with SGX2
    D (EAUG) Add page to initialized enclave
    E (EMODPTR) Restrict permissions of EPC page
    F (EMODT) Change type of EPC page
    Added with OVERSUB[111]
    10 (ERDINFO) Read EPC page type/status info
    11 (ETRACKC) Activate EBLOCK checks
    12 (ELDBC) Load EPC page as blocked with enhanced error reporting
    13 (ELDUC) Load EPC page as unblocked with enhanced error reporting
    Other
    18 (EUPDATESVN) Update SVN (Security Version Number) after live microcode update[112]

    Any unsupported value in EAX causes a #GP exception.

  5. ^ SGX is deprecated on desktop/laptop processors from 11th generation (Rocket Lake, Tiger Lake) onwards, but continues to be available on Xeon-branded server parts.[113]
  6. ^ The leaf functions defined for ENCLU (selected by EAX) are:
    EAX Function
    0 (EREPORT) Create a cryptographic report
    1 (EGETKEY) Create a cryptographic key
    2 (EENTER) Enter an Enclave
    3 (ERESUME) Re-enter an Enclave
    4 (EEXIT) Exit an Enclave
    Added with SGX2
    5 (EACCEPT) Accept changes to EPC page
    6 (EMODPE) Extend EPC page permissions
    7 (EACCEPTCOPY) Initialize pending page
    Added with TDX[115]
    8 (EVERIFYREPORT2) Verify a cryptographic report of a trust domain
    Added with AEX-Notify
    9 (EDECCSSA) Decrement TCS.CSSA

    Any unsupported value in EAX causes a #GP exception.
    The EENTER and ERESUME functions cannot be executed inside an SGX enclave – the other functions can only be executed inside an enclave.

  7. ^ ENCLU can only be executed in ring 3, not rings 0/1/2.
  8. ^ The leaf functions defined for ENCLV (selected by EAX) are:
    EAX Function
    Added with OVERSUB[111]
    0 (EDECVIRTCHILD) Decrement VIRTCHILDCNT in SECS
    1 (EINCVIRTCHILD) Increment VIRTCHILDCNT in SECS
    2 (ESETCONTEXT) Set ENCLAVECONTEXT field in SECS

    Any unsupported value in EAX causes a #GP exception.
    The ENCLV instruction is only present on systems that support the EPC Oversubscription Extensions to SGX ("OVERSUB").

  9. ^ ENCLV is only available if Intel VMX operation is enabled with VMXON, and will produce #UD otherwise.
  10. ^ For PTWRITE, the write to the Processor Trace Packet will only happen if a set of enable-bits (the "TriggerEn", "ContextEn", "FilterEn" bits of the RTIT_STATUS MSR and the "PTWEn" bit of the RTIT_CTL MSR) are all set to 1.
    The PTWRITE instruction is indicated in the SDM to cause an #UD exception if the 66h instruction prefix is used, regardless of other prefixes.
  11. ^ The leaf functions defined for PCONFIG (selected by EAX) are:
    EAX Function
    0 MKTME_KEY_PROGRAM:
    Program key and encryption mode to use with an TME-MK Key ID.
    Added with TSE
    1 TSE_KEY_PROGRAM:
    Direct key programming for TSE.
    2 TSE_KEY_PROGRAM_WRAPPED:
    Wrapped key programming for TSE.

    Any unsupported value in EAX causes a #GP(0) exception.

  12. ^ For CLDEMOTE, the cache level that it will demote a cache line to is implementation-dependent.
    Since the instruction is considered a hint, it will execute as a NOP without any exceptions if the provided memory address is invalid or not in the L1 cache. It may also execute as a NOP under other implementation-dependent circumstances as well.
    On systems that do not support the CLDEMOTE extension, it executes as a NOP.
  13. ^ Intel documentation lists Tremont and Alder Lake as the processors in which CLDEMOTE was introduced. However, as of May 2022, no Tremont or Alder Lake models have been observed to have the CPUID feature bit for CLDEMOTE set, while several of them have the CPUID bit cleared.[116]
    As of April 2023, the CPUID feature bit for CLDEMOTE has been observed to be set for Sapphire Rapids.[117]
  14. ^ For UMONITOR, the operand size of the address argument is given by the address size, which may be overridden by the 67h prefix. The default segment used is DS:, which can be overridden with a segment prefix.
  15. ^ a b For the UMWAIT and TPAUSE instructions, the operating system can use the IA32_UMWAIT_CONTROL MSR to limit the maximum amount of time that a single UMWAIT/TPAUSE invocation is permitted to wait. The UMWAIT and TPAUSE instructions will set RFLAGS.CF to 1 if they reached the IA32_UMWAIT_CONTROL-defined time limit and 0 otherwise.
  16. ^ TPAUSE and UMWAIT can be run outside Ring 0 only if CR4.TSD=0.
  17. ^ For the register argument to the UMWAIT and TPAUSE instructions, the following flag bits are supported:
    Bits Usage
    0 Preferred optimization state.
    • 0 = C0.2 (slower wakeup, improves performance of other SMT threads on same core)
    • 1 = C0.1 (faster wakeup)
    31:1 (Reserved)
  18. ^ While serialization can be performed with older instructions such as e.g. CPUID and IRET, these instructions perform additional functions, causing side-effects and reduced performance when stand-alone instruction serialization is needed. (CPUID additionally has the issue that it causes a mandatory #VMEXIT when executed under virtualization, which causes a very large overhead.) The SERIALIZE instruction performs serialization only, avoiding these added costs.
  19. ^ A bitmap of CPU history components that can be reset through HRESET is provided by CPUID.(EAX=20h,ECX=0):EBX.
    As of July 2023, the following bits are defined:
    Bit Usage
    0 Intel Thread Director history
    31:1 (Reserved)
  20. ^ The register argument to SENDUIPI is an index to pick an entry from the UITT (User-Interrupt Target Table, a table specified by the new UINTR_TT and UINT_MISC MSRs.)
  21. ^ On Sapphire Rapids processors, the UIRET instruction always sets UIF (User Interrupt Flag) to 1. On Sierra Forest and later processors, UIRET will set UIF to the value of bit 1 of the value popped off the stack for RFLAGS - this functionality is indicated by CPUID.(EAX=7,ECX=1):EDX[17].
  22. ^ a b For the RDMSRLIST and WRMSRLIST instructions, the addresses specified in the RSI and RDI registers must be 8-byte aligned.
  23. ^ The condition codes supported for the CMPccXADD instructions (opcode VEX.128.66.0F38 Ex /r with the x nibble specifying the condition) are:
    x cc Condition (EFLAGS)
    0 O OF=1: "Overflow"
    1 NO OF=0: "Not Overflow"
    2 B CF=1: "Below"
    3 NB CF=0: "Not Below"
    4 Z ZF=1: "Zero"
    5 NZ ZF=0: "Not Zero"
    6 BE (CF=1 or ZF=1): "Below or Equal"
    7 NBE (CF=0 and ZF=0): "Not Below or Equal"
    8 S SF=1: "Sign"
    9 NS SF=0: "Not Sign"
    A P PF=1: "Parity"
    B NP PF=0: "Not Parity"
    C L SF≠OF: "Less"
    D NL SF=OF: "Not Less"
    E LE (ZF=1 or SF≠OF): "Less or Equal"
    F NLE (ZF=0 and SF=OF): "Not Less or Equal"
  24. ^ Even though the CMPccXADD instructions perform a locked memory operation, they do not require or accept the LOCK (F0h) prefix - attempting to use this prefix results in #UD.

Added with other AMD-specific extensions

[edit]
Instruction Set Extension Instruction
mnemonics		 Opcode Instruction description Ring Added in
AltMovCr8
Alternative mechanism to access the CR8 control register.[a]
 MOV reg,CR8 F0 0F 20 /0[b] Read the CR8 register. 0 K8[c]
MOV CR8,reg F0 0F 22 /0[b] Write to the CR8 register.
MONITORX
Monitor a memory location for writes in user mode.
 MONITORX NP 0F 01 FA Start monitoring a memory location for memory writes. Similar to older MONITOR, except available in user mode. 3 Excavator
MWAITX NP 0F 01 FB Wait for a write to a monitored memory location previously specified with MONITORX.
MWAITX differs from the older MWAIT instruction mainly in that it runs in user mode and that it can accept an optional timeout argument (given in TSC time units) in EBX (enabled by setting bit[1] of ECX to 1.)
CLZERO
Zero out full cache line.
 CLZERO rAX NP 0F 01 FC Write zeroes to all bytes in a memory region that has the size and alignment of a CPU cache line and contains the byte addressed by DS:rAX.[d] 3 Zen 1
RDPRU
Read processor register in user mode.
 RDPRU NP 0F 01 FD Read selected MSRs (mainly performance counters) in user mode. ECX specifies which register to read.[e]

The value of the MSR is returned in EDX:EAX.

Usually 3[f] Zen 2
MCOMMIT
Commit Stores To Memory.
 MCOMMIT F3 0F 01 FA Ensure that all preceding stores in thread have been committed to memory, and that any errors encountered by these stores have been signalled to any associated error logging resources. The set of errors that can be reported and the logging mechanism are platform-specific.
Sets EFLAGS.CF to 0 if any errors occurred, 1 otherwise. 		3 Zen 2
INVLPGB
Invalidate TLB Entries with broadcast.
 INVLPGB NP 0F 01 FE Invalidate TLB Entries for a range of pages, with broadcast. The invalidation is performed on the processor executing the instruction, and also broadcast to all other processors in the system.
rAX takes the virtual address to invalidate and some additional flags, ECX takes the number of pages to invalidate, and EDX specifies ASID and PCID to perform TLB invalidation for. 		0 Zen 3
TLBSYNC NP 0F 01 FF Synchronize TLB invalidations.
Wait until all TLB invalidations signalled by preceding invocations of the INVLPGB instruction on the same logical processor have been responded to by all processors in the system. Instruction is serializing.
  1. ^ The standard way to access the CR8 register is to use an encoding that makes use of the REX.R prefix, e.g. 44 0F 20 07 (MOV RDI,CR8). However, the REX.R prefix is only available in 64-bit mode.
    The AltMovCr8 extension adds an additional method to access CR8, using the F0 (LOCK) prefix instead of REX.R – this provides access to CR8 outside 64-bit mode.
  2. ^ a b Like other variants of MOV to/from the CRx registers, the AltMovCr8 encodings ignore the top 2 bits of the instruction's ModR/M byte, and always execute as if these two bits are set to 11b.
    The AltMovCr8 encodings are available in 64-bit mode. However, combining the LOCK prefix with the REX.R prefix is not permitted and will cause an #UD exception.
  3. ^ Support for AltMovCR8 was added in stepping F of the AMD K8, and is not available on earlier steppings.
  4. ^ For CLZERO, the address size and 67h prefix control whether to use AX, EAX or RAX as address. The default segment DS: can be overridden by a segment-override prefix. The provided address does not need to be aligned – hardware will align it as necessary.
    The CLZERO instruction is intended for recovery from otherwise-fatal Machine Check errors. It is non-cacheable, cannot be used to allocate a cache line without a memory access, and should not be used for fast memory clears.[119]
  5. ^ The register numbering used by RDPRU does not necessarily match that of RDMSR/WRMSR.
    The registers supported by RDPRU as of December 2022 are:
    ECX Register
    0 MPERF (MSR 0E7h: Maximum Performance Frequency Clock Count)
    1 APERF (MSR 0E8h: Actual Performance Frequency Clock Count)

    Unsupported values in ECX return 0.

  6. ^ If CR4.TSD=1, then the RDPRU instruction can only run in ring 0.

x87 floating-point instructions

[edit]

The x87 coprocessor, if present, provides support for floating-point arithmetic. The coprocessor provides eight data registers, each holding one 80-bit floating-point value (1 sign bit, 15 exponent bits, 64 mantissa bits) – these registers are organized as a stack, with the top-of-stack register referred to as "st" or "st(0)", and the other registers referred to as st(1),st(2),...st(7). It additionally provides a number of control and status registers, including "PC" (precision control, to control whether floating-point operations should be rounded to 24, 53 or 64 mantissa bits) and "RC" (rounding control, to pick rounding-mode: round-to-zero, round-to-positive-infinity, round-to-negative-infinity, round-to-nearest-even) and a 4-bit condition code register "CC", whose four bits are individually referred to as C0,C1,C2 and C3). Not all of the arithmetic instructions provided by x87 obey PC and RC.

Original 8087 instructions

[edit]
Instruction description Mnemonic Opcode Additional items
x87 Non-Waiting[a] FPU Control Instructions Waiting
mnemonic[b]
Initialize x87 FPU FNINIT DB E3 FINIT
Load x87 Control Word FLDCW m16 D9 /5 (none)
Store x87 Control Word FNSTCW m16 D9 /7 FSTCW
Store x87 Status Word FNSTSW m16 DD /7 FSTSW
Clear x87 Exception Flags FNCLEX DB E2 FCLEX
Load x87 FPU Environment FLDENV m112/m224[c] D9 /4 (none)
Store x87 FPU Environment FNSTENV m112/m224[c] D9 /6 FSTENV
Save x87 FPU State, then initialize x87 FPU FNSAVE m752/m864[c] DD /6 FSAVE
Restore x87 FPU State FRSTOR m752/m864[c] DD /4 (none)
Enable Interrupts (8087 only)[d] FNENI DB E0 FENI
Disable Interrupts (8087 only)[d] FNDISI DB E1 FDISI
x87 Floating-point Load/Store/Move Instructions precision
control		 rounding
control
Load floating-point value onto stack FLD m32 D9 /0 No —
FLD m64 DD /0
FLD m80 DB /5
FLD st(i) D9 C0+i
Store top-of-stack floating-point value to memory or stack register FST m32 D9 /2 No Yes
FST m64 DD /2
FST st(i)[e] DD D0+i No —
Store top-of-stack floating-point value to memory or stack register, then pop FSTP m32 D9 /3 No Yes
FSTP m64 DD /3
FSTP m80[e] DB /7 No —
FSTP st(i)[e][f] DD D8+i
DF D0+i[g]
DF D8+i[g]
Push +0.0 onto stack FLDZ D9 EE No —
Push +1.0 onto stack FLD1 D9 E8
Push π (approximately 3.14159) onto stack FLDPI D9 EB No 387[h]
Push (approximately 3.32193) onto stack FLDL2T D9 E9
Push (approximately 1.44269) onto stack FLDL2E D9 EA
Push (approximately 0.30103) onto stack FLDLG2 D9 EC
Push (approximately 0.69315) onto stack FLDLN2 D9 ED
Exchange top-of-stack register with other stack register FXCH st(i)[i][j] D9 C8+i No —
DD C8+i[g]
DF C8+i[g]
x87 Integer Load/Store Instructions precision
control		 rounding
control
Load signed integer value onto stack from memory, with conversion to floating-point FILD m16 DF /0 No —
FILD m32 DB /0
FILD m64 DF /5
Store top-of-stack value to memory, with conversion to signed integer FIST m16 DF /2 No Yes
FIST m32 DB /2
Store top-of-stack value to memory, with conversion to signed integer, then pop stack FISTP m16 DF /3 No Yes
FISTP m32 DB /3
FISTP m64 DF /7
Load 18-digit Binary-Coded-Decimal integer value onto stack from memory, with conversion to floating-point FBLD m80[k] DF /4 No —
Store top-of-stack value to memory, with conversion to 18-digit Binary-Coded-Decimal integer, then pop stack FBSTP m80 DF /6 No 387[h]
x87 Basic Arithmetic Instructions precision
control		 rounding
control
Floating-point add
dst <- dst + src
 FADD m32 D8 /0 Yes Yes
FADD m64 DC /0
FADD st,st(i) D8 C0+i
FADD st(i),st DC C0+i
Floating-point multiply
dst <- dst * src
 FMUL m32 D8 /1 Yes Yes
FMUL m64 DC /1
FMUL st,st(i) D8 C8+i
FMUL st(i),st DC C8+i
Floating-point subtract
dst <- dst – src
 FSUB m32 D8 /4 Yes Yes
FSUB m64 DC /4
FSUB st,st(i) D8 E0+i
FSUB st(i),st DC E8+i
Floating-point reverse subtract
dst <- src – dst
 FSUBR m32 D8 /5 Yes Yes
FSUBR m64 DC /5
FSUBR st,st(i) D8 E8+i
FSUBR st(i),st DC E0+i
Floating-point divide[l]
dst <- dst / src
 FDIV m32 D8 /6 Yes Yes
FDIV m64 DC /6
FDIV st,st(i) D8 F0+i
FDIV st(i),st DC F8+i
Floating-point reverse divide
dst <- src / dst
 FDIVR m32 D8 /7 Yes Yes
FDIVR m64 DC /7
FDIVR st,st(i) D8 F8+i
FDIVR st(i),st DC F0+i
Floating-point compare
CC <- result_of( st(0) – src )
Same operation as subtract, except that it updates the x87 CC status register instead of any of the FPU stack registers
 FCOM m32 D8 /2 No —
FCOM m64 DC /2
FCOM st(i)[i] D8 D0+i
DC D0+i[g]
x87 Basic Arithmetic Instructions with Stack Pop precision
control		 rounding
control
Floating-point add and pop FADDP st(i),st[i] DE C0+i Yes Yes
Floating-point multiply and pop FMULP st(i),st[i] DE C8+i Yes Yes
Floating-point subtract and pop FSUBP st(i),st[i] DE E8+i Yes Yes
Floating-point reverse-subtract and pop FSUBRP st(i),st[i] DE E0+i Yes Yes
Floating-point divide and pop FDIVP st(i),st[i] DE F8+i Yes Yes
Floating-point reverse-divide and pop FDIVRP st(i),st[i] DE F0+i Yes Yes
Floating-point compare and pop FCOMP m32 D8 /3 No —
FCOMP m64 DC /3
FCOMP st(i)[i] D8 D8+i
DC D8+i[g]
DE D0+i[g]
Floating-point compare to st(1), then pop twice FCOMPP DE D9 No —
x87 Basic Arithmetic Instructions with Integer Source Argument precision
control		 rounding
control
Floating-point add by integer FIADD m16 DA /0 Yes Yes
FIADD m32 DE /0
Floating-point multiply by integer FIMUL m16 DA /1 Yes Yes
FIMUL m32 DE /1
Floating-point subtract by integer FISUB m16 DA /4 Yes Yes
FISUB m32 DE /4
Floating-point reverse-subtract by integer FISUBR m16 DA /5 Yes Yes
FISUBR m32 DE /5
Floating-point divide by integer FIDIV m16 DA /6 Yes Yes
FIDIV m32 DE /6
Floating-point reverse-divide by integer FIDIVR m16 DA /7 Yes Yes
FIDIVR m32 DE /7
Floating-point compare to integer FICOM m16 DA /2 No —
FICOM m32 DE /2
Floating-point compare to integer, and stack pop FICOMP m16 DA /3 No —
FICOMP m32 DE /3
x87 Additional Arithmetic Instructions precision
control		 rounding
control
Floating-point change sign FCHS D9 E0 No —
Floating-point absolute value FABS D9 E1 No —
Floating-point compare top-of-stack value to 0 FTST D9 E4 No —
Classify top-of-stack st(0) register value.
The classification result is stored in the x87 CC register.[m] 		FXAM D9 E5 No —
Split the st(0) value into two values E and M representing the exponent and mantissa of st(0).
The split is done such that , where E is an integer and M is a number whose absolute value is within the range .  [n]
st(0) is then replaced with E, after which M is pushed onto the stack. 		FXTRACT D9 F4 No —
Floating-point partial[o] remainder (not IEEE 754 compliant): FPREM D9 F8 No —[p]
Floating-point square root FSQRT D9 FA Yes Yes
Floating-point round to integer FRNDINT D9 FC No Yes
Floating-point power-of-2 scaling. Rounds the value of st(1) to integer with round-to-zero, then uses it as a scale factor for st(0):[q] FSCALE D9 FD No Yes[r]
x87 Transcendental Instructions[s] Source operand
range restriction
Base-2 exponential minus 1, with extra precision for st(0) close to 0: F2XM1 D9 F0 8087: 
80387: 
Base-2 Logarithm:followed by stack pop FYL2X[t] D9 F1 no restrictions
Partial Tangent: Computes from st(0) a pair of values X and Y, such thatThe Y value replaces the top-of-stack value, and then X is pushed onto the stack.
On 80387 and later x87, but not original 8087, X is always 1.0 		FPTAN D9 F2 8087: 
80387: 
Two-argument arctangent with quadrant adjustment:[u] followed by stack pop FPATAN D9 F3 8087: 
80387: no restrictions
Base-2 Logarithm plus 1, with extra precision for st(0) close to 0:followed by stack pop FYL2XP1[t] D9 F9 Intel: 
AMD: 
Other x87 Instructions
No operation[v] FNOP D9 D0
Decrement x87 FPU Register Stack Pointer FDECSTP D9 F6
Increment x87 FPU Register Stack Pointer FINCSTP D9 F7
Free x87 FPU Register FFREE st(i) DD C0+i
Check and handle pending unmasked x87 FPU exceptions WAIT,
FWAIT		 9B
Floating-point store and pop, without stack underflow exception FSTPNCE st(i) D9 D8+i[g]
Free x87 register, then stack pop FFREEP st(i) DF C0+i[g]
  1. ^ x87 coprocessors (other than the 8087) handle exceptions in a fairly unusual way. When an x87 instruction generates an unmasked arithmetic exception, it will still complete without causing a CPU fault – instead of causing a fault, it will record within the coprocessor information needed to handle the exception (instruction pointer, opcode, data pointer if the instruction had a memory operand) and set FPU status-word flag to indicate that a pending exception is present. This pending exception will then cause a CPU fault when the next x87, MMX or WAIT instruction is executed.
    The exception to this is x87's "Non-Waiting" instructions, which will execute without causing such a fault even if a pending exception is present (with some caveats, see application note AP-578[120]). These instructions are mostly control instructions that can inspect and/or modify the pending-exception state of the x87 FPU.
  2. ^ For each non-waiting x87 instruction whose mnemonic begins with FN, there exists a pseudo-instruction that has the same mnemonic except without the N. These pseudo-instructions consist of a WAIT instruction (opcode 9B) followed by the corresponding non-waiting x87 instruction. For example:
    • FNCLEX is an instruction with the opcode DB E2. The corresponding pseudo-instruction FCLEX is then encoded as 9B DB E2.
    • FNSAVE ES:[BX+6] is an instruction with the opcode 26 DD 77 06. The corresponding pseudo-instruction FSAVE ES:[BX+6] is then encoded as 9B 26 DD 77 06
    These pseudo-instructions are commonly recognized by x86 assemblers and disassemblers and treated as single instructions, even though all x86 CPUs with x87 coprocessors execute them as a sequence of two instructions.
  3. ^ a b c d On 80387 and later x87 FPUs, FLDENV, F(N)STENV, FRSTOR and F(N)SAVE exist in 16-bit and 32-bit variants. The 16-bit variants will load/store a 14-byte floating-point environment data structure to/from memory – the 32-bit variants will load/store a 28-byte data structure instead. (F(N)SAVE/FRSTOR will additionally load/store an additional 80 bytes of FPU data register content after the FPU environment, for a total of 94 or 108 bytes). The choice between the 16-bit and 32-bit variants is based on the CS.D bit and the presence of the 66h instruction prefix. On 8087 and 80287, only the 16-bit variants are available.
    64-bit variants of these instructions do not exist – using REX.W under x86-64 will cause the 32-bit variants to be used. Since these can only load/store the bottom 32 bits of FIP and FDP, it is recommended to use FXSAVE64/FXRSTOR64 instead if 64-bit operation is desired.
  4. ^ a b In the case of an x87 instruction producing an unmasked FPU exception, the 8087 FPU will signal an IRQ some indeterminate time after the instruction was issued. This may not always be possible to handle,[121] and so the FPU offers the F(N)DISI and F(N)ENI instructions to set/clear the Interrupt Mask bit (bit 7) of the x87 Control Word,[122] to control the interrupt.
    Later x87 FPUs, from 80287 onwards, changed the FPU exception mechanism to instead produce a CPU exception on the next x87 instruction. This made the Interrupt Mask bit unnecessary, so it was removed.[123] In later Intel x87 FPUs, the F(N)ENI and F(N)DISI instructions were kept for backwards compatibility, executing as NOPs that do not modify any x87 state.
  5. ^ a b c FST/FSTP with an 80-bit destination (m80 or st(i)) and an sNaN source value will produce exceptions on AMD but not Intel FPUs.
  6. ^ FSTP ST(0) is a commonly used idiom for popping a single register off the x87 register stack.
  7. ^ a b c d e f g h i Intel x87 alias opcode. Use of this opcode is not recommended.
    On the Intel 8087 coprocessor, several reserved opcodes would perform operations behaving similarly to existing defined x87 instructions. These opcodes were documented for the 8087[124] and 80287,[125] but then omitted from later manuals until the October 2017 update of the Intel SDM.[126]
    They are present on all known Intel x87 FPUs but unavailable on some older non-Intel FPUs, such as AMD Geode GX/LX, DM&P Vortex86[127] and NexGen 586PF.[128]
  8. ^ a b On the 8087 and 80287, FBSTP and the load-constant instructions always use the round-to-nearest rounding mode. On the 80387 and later x87 FPUs, these instructions will use the rounding mode specified in the x87 RC register.
  9. ^ a b c d e f g h i For the FADDP, FSUBP, FSUBRP, FMULP, FDIVP, FDIVRP, FCOM, FCOMP and FXCH instructions, x86 assemblers/disassemblers may recognize variants of the instructions with no arguments. Such variants are equivalent to variants using st(1) as their first argument.
  10. ^ On Intel Pentium and later processors, FXCH is implemented as a register renaming rather than a true data move. This has no semantic effect, but enables zero-cycle-latency operation. It also allows the instruction to break data dependencies for the x87 top-of-stack value, improving attainable performance for code optimized for these processors.
  11. ^ The result of executing the FBLD instruction on non-BCD data is undefined.
  12. ^ On early Intel Pentium processors, floating-point divide was subject to the Pentium FDIV bug. This also affected instructions that perform divide as part of their operations, such as FPREM and FPATAN.[129]
  13. ^ The FXAM instruction will set C0, C2 and C3 based on value type in st(0) as follows:
    C3 C2 C0 Classification
    0 0 0 Unsupported (unnormal or pseudo-NaN)
    0 0 1 NaN
    0 1 0 Normal finite number
    0 1 1 Infinity
    1 0 0 Zero
    1 0 1 Empty
    1 1 0 Denormal number
    1 1 1 Empty (may occur on 8087/80287 only)

    C1 is set to the sign-bit of st(0), regardless of whether st(0) is Empty or not.

  14. ^ For FXTRACT, if st(0) is zero or ±∞, then M is set equal to st(0). If st(0) is zero, E is set to 0 on 8087/80287 but -∞ on 80387 and later. If st(0) is ±∞, then E is set to +∞.
  15. ^ For FPREM, if the quotient Q is larger than , then the remainder calculation may have been done only partially – in this case, the FPREM instruction will need to be run again in order to complete the remainder calculation. This is indicated by the instruction setting C2 to 1.
    If the instruction did complete the remainder calculation, it will set C2 to 0 and set the three bits {C0,C3,C1} to the bottom three bits of the quotient Q.
    On 80387 and later, if the instruction didn't complete the remainder calculation, then the computed remainder Q used for argument reduction will have been rounded to a multiple of 8 (or larger power-of-2), so that the bottom 3 bits of the quotient can still be correctly retrieved in a later pass that does complete the remainder calculation.
  16. ^ The remainder computation done by the FPREM instruction is always exact with no roundoff errors.
  17. ^ For the FSCALE instruction on 8087 and 80287, st(1) is required to be in the range . Also, its absolute value must be either 0 or at least 1. If these requirements are not satisfied, the result is undefined.
    These restrictions were removed in the 80387.
  18. ^ For FSCALE, rounding is only applied in the case of overflow, underflow or subnormal result.
  19. ^ The x87 transcendental instructions do not obey PC or RC, but instead compute full 80-bit results. These results are not necessarily correctly rounded (see Table-maker's dilemma) – they may have an error of up to ±1 ulp on Pentium or later, or up to ±1.5 ulps on earlier x87 coprocessors.
  20. ^ a b For the FYL2X and FYL2XP1 instructions, the maximum error bound of ±1 ulp only holds for st(1)=1.0 – for other values of st(1), the error bound is increased to ±1.35 ulps.
  21. ^ For FPATAN, the following adjustments are done as compared to just computing a one-argument arctangent of the ratio :
    • If both st(0) and st(1) are ±∞, then the arctangent is computed as if each of st(0) and st(1) had been replaced with ±1 of the same sign. This produces a result that is an odd multiple of .
    • If both st(0) and st(1) are ±0, then the arctangent is computed as if st(0) but not st(1) had been replaced with ±1 of the same sign, producing a result of ±0 or .
    • If st(0) is negative (has sign bit set), then an addend of with the same sign as st(1) is added to the result.
  22. ^ While FNOP is a no-op in the sense that will leave the x87 FPU register stack unmodified, it may still modify FIP and CC, and it may fault if a pending x87 FPU exception is present.

x87 instructions added in later processors

[edit]
Instruction description Mnemonic Opcode Additional items
x87 Non-Waiting Control Instructions added in 80287 Waiting
mnemonic
Notify FPU of entry into Protected Mode[a] FNSETPM DB E4 FSETPM
Store x87 Status Word to AX FNSTSW AX DF E0 FSTSW AX
x87 Instructions added in 80387[b] Source operand
range restriction
Floating-point unordered compare.
Similar to the regular floating-point compare instruction FCOM, except will not produce an exception in response to any qNaN operands.		 FUCOM st(i)[c] DD E0+i no restrictions
Floating-point unordered compare and pop FUCOMP st(i)[c] DD E8+i
Floating-point unordered compare to st(1), then pop twice FUCOMPP DA E9
IEEE 754 compliant floating-point partial remainder.[d] FPREM1 D9 F5
Floating-point sine and cosine.
Computes two values and  [e]
Top-of-stack st(0) is replaced with S, after which C is pushed onto the stack.		 FSINCOS D9 FB
Floating-point sine.[e] FSIN D9 FE
Floating-point cosine.[e] FCOS D9 FF
x87 Instructions added in Pentium Pro Condition for
conditional moves
Floating-point conditional move to st(0) based on EFLAGS FCMOVB st(0),st(i) DA C0+i below (CF=1)
FCMOVE st(0),st(i) DA C8+i equal (ZF=1)
FCMOVBE st(0),st(i) DA D0+i below or equal
(CF=1 or ZF=1)
FCMOVU st(0),st(i) DA D8+i unordered (PF=1)
FCMOVNB st(0),st(i) DB C0+i not below (CF=0)
FCMOVNE st(0),st(i) DB C8+i not equal (ZF=0)
FCMOVNBE st(0),st(i) DB D0+i not below or equal
(CF=0 and ZF=0)
FCMOVNU st(0),st(i) DB D8+i not unordered (PF=0)
Floating-point compare and set EFLAGS.
Differs from the older FCOM floating-point compare instruction in that it puts its result in the integer EFLAGS register rather than the x87 CC register.[f]		 FCOMI st(0),st(i) DB F0+i
Floating-point compare and set EFLAGS, then pop FCOMIP st(0),st(i) DF F0+i
Floating-point unordered compare and set EFLAGS FUCOMI st(0),st(i) DB E8+i
Floating-point unordered compare and set EFLAGS, then pop FUCOMIP st(0),st(i) DF E8+i
x87 Non-Waiting Instructions added in Pentium II, AMD K7 and SSE[g] 64-bit mnemonic
(REX.W prefix)
Save x87, MMX and SSE state to 512-byte data structure[h][i][j] FXSAVE m512byte NP 0F AE /0 FXSAVE64 m512byte
Restore x87, MMX and SSE state from 512-byte data structure[h][i] FXRSTOR m512byte NP 0F AE /1 FXRSTOR64 m512byte
x87 Instructions added as part of SSE3
Floating-point store integer and pop, with round-to-zero FISTTP m16 DF /1
FISTTP m32 DB /1
FISTTP m64 DD /1
  1. ^ The x87 FPU needs to know whether it is operating in Real Mode or Protected Mode because the floating-point environment accessed by the F(N)SAVE, FRSTOR, FLDENV and F(N)STENV instructions has different formats in Real Mode and Protected Mode. On 80287, the F(N)SETPM instruction is required to communicate the real-to-protected mode transition to the FPU. On 80387 and later x87 FPUs, real↔protected mode transitions are communicated automatically to the FPU without the need for any dedicated instructions – therefore, on these FPUs, FNSETPM executes as a NOP that does not modify any FPU state.
  2. ^ Not including discontinued instructions specific to particular 80387-compatible FPU models.
  3. ^ a b For the FUCOM and FUCOMP instructions, x86 assemblers/disassemblers may recognize variants of the instructions with no arguments. Such variants are equivalent to variants using st(1) as their first argument.
  4. ^ The 80387 FPREM1 instruction differs from the older FPREM (D9 F8) instruction in that the quotient Q is rounded to integer with round-to-nearest-even rounding rather than the round-to-zero rounding used by FPREM. Like FPREM, FPREM1 always computes an exact result with no roundoff errors. Like FPREM, it may also perform a partial computation if the quotient is too large, in which case it must be run again.
  5. ^ a b c Due to the x87 FPU performing argument reduction for sin/cos with only about 68 bits of precision, the value of k used in the calculation of FSIN, FCOS and FSINCOS is not precisely 1.0, but instead given by[130][131]This argument reduction inaccuracy also affects the FPTAN instruction.
  6. ^ The FCOMI, FCOMIP, FUCOMI and FUCOMIP instructions write their results to the ZF, CF and PF bits of the EFLAGS register. On Intel but not AMD processors, the SF, AF and OF bits of EFLAGS are also zeroed out by these instructions.
  7. ^ The FXSAVE and FXRSTOR instructions were added in the "Deschutes" revision of Pentium II, and are not present in earlier "Klamath" revision.
    They are also present in AMD K7.
    They are also considered an integral part of SSE and are therefore present in all processors with SSE.
  8. ^ a b The FXSAVE and FXRSTOR instructions will save/restore SSE state only on processors that support SSE. Otherwise, they will only save/restore x87 and MMX state.
    The x87 section of the state saved/restored by FXSAVE/FXRSTOR has a completely different layout than the data structure of the older F(N)SAVE/FRSTOR instructions, enabling faster save/restore by avoiding misaligned loads and stores.
  9. ^ a b When floating-point emulation is enabled with CR0.EM=1, FXSAVE(64) and FXRSTOR(64) are considered to be x87 instructions and will accordingly produce an #NM (device-not-available) exception. Other than WAIT, these are the only opcodes outside the D8..DF ESC opcode space that exhibit this behavior. (All opcodes in D8..DF will produce #NM if CR0.EM=1, even for undefined opcodes that would produce #UD otherwise.)
  10. ^ Unlike the older F(N)SAVE instruction, FXSAVE will not initialize the FPU after saving its state to memory, but instead leave the x87 coprocessor state unmodified.

SIMD instructions

[edit]
Main article: x86 SIMD instruction listings

Cryptographic instructions

[edit]
Main article: List of x86 cryptographic instructions

Virtualization instructions

[edit]
Main article: List of x86 virtualization instructions

Other instructions

[edit]
See also: List of discontinued x86 instructions

x86 also includes discontinued instruction sets which are no longer supported by Intel and AMD, and undocumented instructions which execute but are not officially documented.

Undocumented x86 instructions

[edit]

The x86 CPUs contain undocumented instructions which are implemented on the chips but not listed in some official documents. They can be found in various sources across the Internet, such as Ralf Brown's Interrupt List and at sandpile.org

Some of these instructions are widely available across many/most x86 CPUs, while others are specific to a narrow range of CPUs.

Undocumented instructions that are widely available across many x86 CPUs include

[edit]
Mnemonics Opcodes Description Status
AAM imm8 D4 ib ASCII-Adjust-after-Multiply. On the 8086, documented for imm8=0Ah only, which is used to convert a binary multiplication result to BCD.

The actual operation is AH ← AL/imm8; AL ← AL mod imm8 for any imm8 value (except zero, which produces a divide-by-zero exception).[132]

Available beginning with 8086, documented for imm8 values other than 0Ah since Pentium (earlier documentation lists no arguments).
AAD imm8 D5 ib ASCII-Adjust-Before-Division. On the 8086, documented for imm8=0Ah only, which is used to convert a BCD value to binary for a following division instruction.

The actual operation is AL ← (AL+(AH*imm8)) & 0FFh; AH ← 0 for any imm8 value.

SALC,
SETALC 		D6 Set AL depending on the value of the Carry Flag (a 1-byte alternative of SBB AL, AL) Available beginning with 8086, but only documented since Pentium Pro.
ICEBP,
INT1 		F1 Single byte single-step exception / Invoke ICE Available beginning with 80386, documented (as INT1) since Pentium Pro. Executes as undocumented instruction prefix on 8086 and 80286.[133]
TEST r/m8,imm8 F6 /1 ib Undocumented variants of the TEST instruction.[134] Performs the same operation as the documented F6 /0 and F7 /0 variants, respectively. Available since the 8086.

Unavailable on some 80486 steppings.[135][136]

TEST r/m16,imm16,
TEST r/m32,imm32 		F7 /1 iw,
F7 /1 id
SHL, SAL (D0..D3) /6,
(C0..C1) /6 ib 		Undocumented variants of the SHL instruction.[134] Performs the same operation as the documented (D0..D3) /4 and (C0..C1) /4 ib variants, respectively. Available since the 80186 (performs different operation on the 8086)[137]
(multiple) 82 /(0..7) ib Alias of opcode 80h, which provides variants of 8-bit integer instructions (ADD, OR, ADC, SBB, AND, SUB, XOR, CMP) with an 8-bit immediate argument.[138] Available since the 8086.[138] Explicitly unavailable in 64-bit mode but kept and reserved for compatibility.[139]
OR/AND/XOR r/m16,imm8 83 /(1,4,6) ib 16-bit OR/AND/XOR with a sign-extended 8-bit immediate. Available on 8086, but only documented from 80386 onwards.[140][141]
REPNZ MOVS F2 (A4..A5) The behavior of the F2 prefix (REPNZ, REPNE) when used with string instructions other than CMPS/SCAS is officially undefined, but there exists commercial software (e.g. the version of FDISK distributed with MS-DOS versions 3.30 to 6.22[142]) that rely on it to behave in the same way as the documented F3 (REP) prefix. Available since the 8086.
REPNZ STOS F2 (AA..AB)
REP RET F3 C3 The use of the REP prefix with the RET instruction is not listed as supported in either the Intel SDM or the AMD APM. However, AMD's optimization guide for the AMD-K8 describes the F3 C3 encoding as a way to encode a two-byte RET instruction – this is the recommended workaround for an issue in the AMD-K8's branch predictor that can cause branch prediction to fail for some 1-byte RET instructions.[143] At least some versions of gcc are known to use this encoding.[144] Executes as RET on all known x86 CPUs.
NOP 67 90 NOP with address-size override prefix. The use of the 67h prefix for instructions without memory operands is listed by the Intel SDM (vol 2, section 2.1.1) as "reserved", but it is used in Microsoft Windows 95 as a workaround for a bug in the B1 stepping of Intel 80386.[145][146] Executes as NOP on 80386 and later.
NOP r/m 0F 1F /0 Official long NOP.

Introduced in the Pentium Pro in 1995, but remained undocumented until March 2006.[58][147][148]

Available on Pentium Pro and AMD K7[149] and later.

Unavailable on AMD K6, AMD Geode LX, VIA Nehemiah.[150]

NOP r/m 0F 0D /r Reserved-NOP. Introduced in 65 nm Pentium 4. Intel documentation lists this opcode as NOP in opcode tables but not instruction listings since June 2005.[151][152] From Broadwell onwards, 0F 0D /1 has been documented as PREFETCHW, while 0F 0D /0 and /2../7 have been reported to exhibit undocumented prefetch functionality.[106]

On AMD CPUs, 0F 0D /r with a memory argument is documented as PREFETCH/PREFETCHW since K6-2 – originally as part of 3Dnow!, but has been kept in later AMD CPUs even after the rest of 3Dnow! was dropped.

Available on Intel CPUs since 65 nm Pentium 4.

UD1 0F B9 /r Intentionally undefined instructions, but unlike UD2 (0F 0B) these instructions were left unpublished until December 2016.[153][68]

Microsoft Windows 95 Setup is known to depend on 0F FF being invalid[154][155] – it is used as a self check to test that its #UD exception handler is working properly.

Other invalid opcodes that are being relied on by commercial software to produce #UD exceptions include FF FF (DIF-2,[156] LaserLok[157]) and C4 C4 ("BOP"[158][159]), however as of January 2022 they are not published as intentionally invalid opcodes.

All of these opcodes produce #UD exceptions on 80186 and later (except on NEC V20/V30, which assign at least 0F FF to the NEC-specific BRKEM instruction.)
UD0 0F FF

Undocumented instructions that appear only in a limited subset of x86 CPUs include

[edit]
Mnemonics Opcodes Description Status
REP MUL F3 F6 /4, F3 F7 /4 On 8086/8088, a REP or REPNZ prefix on a MUL or IMUL instruction causes the result to be negated. This is due to the microcode using the “REP prefix present” bit to store the sign of the result. 8086/8088 only.[160]
REP IMUL F3 F6 /5, F3 F7 /5
REP IDIV F3 F6 /7, F3 F7 /7 On 8086/8088, a REP or REPNZ prefix on an IDIV (but not DIV) instruction causes the quotient to be negated. This is due to the microcode using the “REP prefix present” bit to store the sign of the quotient. 8086/8088 only.[160]
SAVEALL,

STOREALL

(F1) 0F 04 Exact purpose unknown, causes CPU hang (HCF). The only way out is CPU reset.[161]

In some implementations, emulated through BIOS as a halting sequence.[162]

In a forum post at the Vintage Computing Federation, this instruction (with F1 prefix) is explained as SAVEALL. It interacts with ICE mode.

Only available on 80286.
LOADALL 0F 05 Loads All Registers from Memory Address 0x000800H Only available on 80286.

Opcode reused for SYSCALL in AMD K6 and later CPUs.

LOADALLD 0F 07 Loads All Registers from Memory Address ES:EDI Only available on 80386.

Opcode reused for SYSRET in AMD K6 and later CPUs.

CL1INVMB 0F 0A[163] On the Intel SCC (Single-chip Cloud Computer), invalidate all message buffers. The mnemonic and operation of the instruction, but not its opcode, are described in Intel's SCC architecture specification.[164] Available on the SCC only.
PATCH2 0F 0E On AMD K6 and later maps to FEMMS operation (fast clear of MMX state) but on Intel identified as uarch data read on Intel[165] Only available in Red unlock state (0F 0F too)
PATCH3 0F 0F Write uarch Can change RAM part of microcode on Intel
UMOV r,r/m,
UMOV r/m,r 		0F (10..13) /r Moves data to/from user memory when operating in ICE HALT mode.[166] Acts as regular MOV otherwise. Available on some 386 and 486 processors only.

Opcodes reused for SSE instructions in later CPUs.

NXOP 0F 55 NexGen hypercode interface.[167] Available on NexGen Nx586 only.
(multiple) 0F (E0..FB)[168] NexGen Nx586 "hyper mode" instructions.

The NexGen Nx586 CPU uses "hyper code"[169] (x86 code sequences unpacked at boot time and only accessible in a special "hyper mode" operation mode, similar to DEC Alpha's PALcode and Intel's XuCode[170]) for many complicated operations that are implemented with microcode in most other x86 CPUs. The Nx586 provides a large number of undocumented instructions to assist hyper mode operation.

Available in Nx586 hyper mode only.
PSWAPW mm,mm/m64 0F 0F /r BB Undocumented AMD 3DNow! instruction on K6-2 and K6-3. Swaps 16-bit words within 64-bit MMX register.[171][172]

Instruction known to be recognized by MASM 6.13 and 6.14.

Available on K6-2 and K6-3 only.

Opcode reused for documented PSWAPD instruction from AMD K7 onwards.

Un­known mnemonic 64 D6 Using the 64 (FS: segment) prefix with the undocumented D6 (SALC/SETALC) instruction will, on UMC CPUs only, cause EAX to be set to 0xAB6B1B07.[173][174] Available on the UMC Green CPU only. Executes as SALC on non-UMC CPUs.
FS: Jcc 64 (70..7F) rel8,

64 0F (80..8F) rel16/32

On Intel NetBurst (Pentium 4) CPUs, the 64h (FS: segment) instruction prefix will, when used with conditional branch instructions, act as a branch hint to indicate that the branch will be alternating between taken and not-taken.[175] Unlike other NetBurst branch hints (CS: and DS: segment prefixes), this hint is not documented. Available on NetBurst CPUs only.

Segment prefixes on conditional branches are accepted but ignored by non-NetBurst CPUs.

JMPAI 0F 3F Jump and execute instructions in the undocumented Alternate Instruction Set. Only available on some x86 processors made by VIA Technologies.
(FMA4) VEX.66.0F38 (5C..5F,68..6F,78..7F) /r imm8 On AMD Zen1, FMA4 instructions are present but undocumented (missing CPUID flag). The reason for leaving the feature undocumented may or may not have been due to a buggy implementation.[176] Removed from Zen2 onwards.
(unknown, multiple) 0F 0F /r ?? The whitepapers for SandSifter[177] and UISFuzz[178] report the detection of large numbers of undocumented instructions in the 3DNow! opcode range on several different AMD CPUs (at least Geode NX and C-50). Their operation is not known.

On at least AMD K6-2, all of the unassigned 3DNow! opcodes (other than the undocumented PF2IW, PI2FW and PSWAPW instructions) are reported to execute as equivalents of POR (MMX bitwise-OR instruction).[172]

Present on some AMD CPUs with 3DNow!.
MOVDB,

GP2MEM

Un­known Microprocessor Report's article "MediaGX Targets Low-Cost PCs" from 1997, covering the introduction of the Cyrix MediaGX processor, lists several new instructions that are said to have been added to this processor in order to support its new "Virtual System Architecture" features, including MOVDB and GP2MEM – and also mentions that Cyrix did not intend to publish specifications for these instructions.[179] Unknown. No specification known to have been published.
REP XSHA512 F3 0F A6 E0 Perform SHA-512 hashing.

Supported by OpenSSL[180] as part of its VIA PadLock support, and listed in a Zhaoxin-supplied Linux kernel patch,[181] but not documented by the VIA PadLock Programming Guide.

Only available on some x86 processors made by VIA Technologies and Zhaoxin.
REP XMODEXP F3 0F A6 F8 Instructions to perform modular exponentiation and random number generation, respectively.

Listed in a VIA-supplied patch to add support for VIA Nano-specific PadLock instructions to OpenSSL,[182] but not documented by the VIA PadLock Programming Guide.

XRNG2 F3 0F A7 F8
Un­known mnemonic 0F A7 (C1..C7) Detected by CPU fuzzing tools such as SandSifter[177] and UISFuzz[178] as executing without causing #UD on several different VIA and Zhaoxin CPUs. Unknown operation, may be related to the documented XSTORE (0F A7 C0) instruction.
Un­known mnemonic F2 0F A6 C0 Zhaoxin SM2 instruction. CPUID flags listed in a Linux kernel patch for OpenEuler,[183] description and opcode (but no instruction mnemonic) provided in a Zhaoxin patent application[184] and a Zhaoxin-provided Linux kernel patch.[185] Present in Zhaoxin KX-6000G.[186]
ZXPAUSE F2 0F A6 D0 Pause the processor until the Time Stamp Counter reaches or exceeds the value specified in EDX:EAX. Low-power processor C-state can be requested in ECX. Listed in OpenEuler kernel patch.[187] Present in Zhaoxin KX-7000.
MONTMUL2 Un­known Zhaoxin RSA/"xmodx" instructions. Mnemonics and CPUID flags are listed in a Linux kernel patch for OpenEuler,[183] but opcodes and instruction descriptions are not available. Unknown. Some Zhaoxin CPUs[186] have the CPUID flags for these instructions set.

Undocumented x87 instructions

[edit]
Mnemonics Opcodes Description Status
FENI,

FENI8087_NOP

DB E0 FPU Enable Interrupts (8087) Documented for the Intel 80287.[125]

Present on all Intel x87 FPUs from 80287 onwards. For FPUs other than the ones where they were introduced on (8087 for FENI/FDISI and 80287 for FSETPM), they act as NOPs.

These instructions and their operation on modern CPUs are commonly mentioned in later Intel documentation, but with opcodes omitted and opcode table entries left blank (e.g. Intel SDM 325462-077, April 2022 mentions them twice without opcodes).

The opcodes are, however, recognized by Intel XED.[188]

FDISI,

FDISI8087_NOP

DB E1 FPU Disable Interrupts (8087)
FSETPM,

FSETPM287_NOP

DB E4 FPU Set Protected Mode (80287)
(no mnemonic) D9 D7,  D9 E2,
D9 E7,  DD FC,
DE D8,  DE DA,
DE DC,  DE DD,
DE DE,  DF FC 		"Reserved by Cyrix" opcodes These opcodes are listed as reserved opcodes that will produce "unpredictable results" without generating exceptions on at least Cyrix 6x86,[189] 6x86MX, MII, MediaGX, and AMD Geode GX/LX.[190] (The documentation for these CPUs all list the same ten opcodes.)

Their actual operation is not known, nor is it known whether their operation is the same on all of these CPUs.

See also

[edit]

References

[edit]
  1. ^ "Re: Intel Processor Identification and the CPUID Instruction". Retrieved 2013-04-21.
  2. ^ "Intel 80x86 Instruction Set Summary" (PDF). eecs.wsu.edu.
  3. ^ Michal Necasek, SGDT/SIDT Fiction and Reality, 4 May 2017. Archived on 29 Nov 2023.
  4. ^ a b Intel, Undocumented iAPX 286 Test Instruction. Archived on 20 Dec 2023.
  5. ^ WikiChip, UMIP – x86. Archived on 16 Mar 2023.
  6. ^ Oracle Corp, Oracle® VM VirtualBox Administrator's Guide for Release 6.0, section 3.5: Details About Software Virtualization. Archived on 8 Dec 2023.
  7. ^ MBC Project, Virtual Machine Detection (permanent link) or Virtual Machine Detection (non permanent link)
  8. ^ Andrew Schulman, "Unauthorized Windows 95" (ISBN 1-56884-169-8), chapter 8, p.249,257.
  9. ^ US Patent 4974159, "Method of transferring control in a multitasking computer system" mentions 63h/ARPL.
  10. ^ Intel, Pentium® Processor Family Developer’s Manual, Volume 3, 1995, order no. 241430-004, section 12.7, p. 323
  11. ^ Intel, How Microarchitectural Data Sampling works, see mitigations section. Archived on Apr 22,2022
  12. ^ Linux kernel documentation, Microarchitectural Data Sampling (MDS) mitigation Archived 2020-10-21 at the Wayback Machine
  13. ^ VCF Forums, I found the SAVEALL opcode, jun 21, 2019. Archived on 13 Apr 2023.
  14. ^ rep lodsb, Intel 286 secrets: ICE mode and F1 0F 04, aug 12, 2022. Archived on 8 Dec 2023.
  15. ^ LKML, (PATCH) x86-64, espfix: Don't leak bits 31:16 of %esp returning to 16-bit stack, Apr 29, 2014. Archived on Jan 4, 2018
  16. ^ Raymond Chen, Getting MS-DOS games to run on Windows 95: Working around the iretd problem, Apr 4, 2016. Archived on Mar 15, 2019
  17. ^ sandpile.org, x86 architecture rFLAGS register, see note #7. Archived on 3 Nov 2011.
  18. ^ iPXE, Commit bc35b24: Fix use of writable code segment on 486 and earlier CPUs, Github, Feb 2, 2022 − indicates that when leaving protected mode on 386/486 by writing to CR0, it is specifically necessary to do a far JMP (opcode EA) in order to restore proper real-mode access-rights for the CS segment, and that other far control transfers (e.g. RETF, IRET) will not do this. Archived on 4 Nov 2024.
  19. ^ Can Bölük, Speculating the entire x86-64 Instruction Set In Seconds with This One Weird Trick, Mar 22, 2021. Archived on Mar 23, 2021.
  20. ^ a b Robert Collins, Undocumented OpCodes, 29 july 1995. Archived on 21 feb 2001
  21. ^ Michal Necasek, ICEBP finally documented, OS/2 Museum, May 25, 2018. Archived on 6 June 2018
  22. ^ Intel, AP-526: Optimization For Intel's 32-bit Processors, order no. 242816-001, october 1995 – lists SALC on page 83, INT1 on page 86 and FFREEP on page 114. Archived from the original on 22 Dec 1996.
  23. ^ AMD, AMD 64-bit Technology, vol 2: System Programming, order no. 24593, rev 3.06, aug 2002, page 248
  24. ^ "Intel 80386 CPU Information | PCjs Machines". www.pcjs.org.
  25. ^ Geoff Chappell, CPU Identification before CPUID, 27 Jan 2020.