x86 instruction listings
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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
[edit]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
[edit]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).
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 | r = m; DS = 2 + m; |
0xC5 |
LEA | Load Effective Address | 0x8D | |
LES | Load ES:r with far pointer | r = m; ES = 2 + m; |
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
[edit]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 |
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.
|
- ^ a b c d The descriptors used by the
LGDT
,LIDT
,SGDT
andSIDT
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.
ForLGDT
andLIDT
with a 16-bit operand size, the address is ANDed with 00FFFFFFh. On Intel (but not AMD) CPUs, theSGDT
andSIDT
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] - ^ a b c d The
LGDT
,LIDT
,LLDT
andLTR
instructions are serializing on Pentium and later processors. - ^ The
LMSW
instruction is serializing on Intel processors from Pentium onwards, but not on AMD processors. - ^ 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 thatLMSW
can be used to enter but not leave x86 Protected Mode.
On 80286, it is not possible to leave Protected Mode at all (neither withLMSW
nor withLOADALL
[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-laterMOV
toCR0
instruction. - ^ If
CR4.UMIP=1
is set, then theSGDT
,SIDT
,SLDT
,SMSW
andSTR
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] - ^ a b c The
SMSW
,SLDT
andSTR
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.
- ^ In 64-bit long mode, the
ARPL
instruction is not available – the63 /r
opcode has been reassigned to the 64-bit-mode-onlyMOVSXD
instruction. - ^ 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 the63
opcode as a one-byte breakpoint to transition from Virtual 8086 Mode to kernel mode.[8][9] - ^ Bits 19:16 of this mask are documented as "undefined" on Intel CPUs.[10] On AMD CPUs, the mask is documented as
0x00FFFF00
. - ^ 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] - ^ a b Undocumented, 80286 only.[4][13][14] (A different variant of
LOADALL
with a different opcode and memory layout exists on 80386.)
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 ifCS.D=0
, or from 32-bit to 16-bit ifCS.D=1
.67h
: AddressSize override. Will change AddressSize from 16-bit to 32-bit ifCS.D=0
, or from 32-bit to 16-bit ifCS.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.
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 |
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
|
- ^ 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 the67
prefix. - ^ The 32-bit string instructions accept repeat-prefixes in the same way as older 8/16-bit string instructions.
ForLODSD
,STOSD
,MOVSD
,INSD
andOUTSD
, theREP
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).
ForCMPSD
andSCASD
, theREPE
(F3
) andREPNE
(F2
) prefixes are available, which will repeat the instruction, decrementing rCX for each iteration, but only as long as the flag condition (ZF=1 forREPE
, ZF=0 forREPNE
) holds true AND rCX ≠ 0. - ^ For the
INSB/W/D
instructions, the memory access rights for theES:[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. - ^ 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.
- ^ The
CWDE
instruction differs from the olderCWD
instruction in thatCWD
would sign-extend the 16-bit value in AX into a 32-bit value in the DX:AX register pair. - ^ For the
E3
opcode (JCXZ
/JECXZ
), the choice of whether the instruction will useCX
orECX
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 instructionsLOOP
,LOOPE
,LOOPNE
(opcodesE0
,E1
,E2
), however, unlikeJCXZ
/JECXZ
, these instructions have not been given new mnemonics for their ECX-using variants. - ^ For
PUSHA(D)
, the value of SP/ESP pushed onto the stack is the value it had just before thePUSHA(D)
instruction started executing. - ^ 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. - ^ The
PUSHFD
andPOPFD
instructions will cause a #GP exception if executed in virtual 8086 mode if IOPL is not 3.
ThePUSHF
,POPF
,IRET
andIRETD
instructions will cause a #GP exception if executed in Virtual-8086 mode if IOPL is not 3 and VME is not enabled. - ^ 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 theIRETD
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 theIRETD
. This has necessitated complex workarounds on both Linux ("ESPFIX")[15] and Windows.[16] This issue also affects the later 64-bitIRETQ
instruction.
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 On Pentium and later processors, moves to the | |
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 | |
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 |
- ^ a b c d For the
BT
,BTS
,BTR
andBTC
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.
- ^ a b c The
BTS
,BTC
andBTR
instructions accept theLOCK
(F0
) prefix when used with a memory argument – this results in the instruction executing atomically. - ^ If the
F3
prefix is used with the0F BC /r
opcode, then the instruction will execute asTZCNT
on systems that support the BMI1 extension.TZCNT
differs fromBSF
in thatTZCNT
but notBSR
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). - ^ a b
BSF
andBSR
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. - ^ If the
F3
prefix is used with the0F BD /r
opcode, then the instruction will execute asLZCNT
on systems that support the ABM or LZCNT extensions.LZCNT
produces a different result fromBSR
for most input values. - ^ a b For
SHLD
andSHRD
, 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
andSHRD
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]) - ^ a b The condition codes supported for the
SETcc
andJcc near
instructions (opcodes0F 9x /0
and0F 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" - ^ 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 asSETcc
regardless of the contents of these bits. - ^ For
LFS
,LGS
andLSS
, 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 theREX.W
prefix with these instructions will cause them to load a far pointer with a 64-bit offset on Intel but not AMD processors. - ^ a b c d e f For
MOV
to/from theCRx
,DRx
andTRx
registers, the reg part of the ModR/M byte is used to indicateCRx/DRx/TRx
register and r/m part the general-register. Uniquely for theMOV 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 are11b
. - ^ a b c d For moves to/from the
CRx
andDRx
registers, the operand size is always 64 bits in 64-bit mode and 32 bits otherwise. - ^ On processors that support global pages (Pentium and later), global page table entries will not be flushed by a
MOV
toCR3
− 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 theINVPCID
instruction. - ^ 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 aMOV
toCR0
if such aMOV
is used to enable/disable protected mode and/or memory paging.MOV
toCR2
is architecturally listed as serializing, but has been reported to be non-serializing on at least some Intel Core-i7 processors.[19]MOV
toCR8
(introduced with x86-64) is serializing on AMD but not Intel processors. - ^ a b The
MOV TRx
instructions were discontinued from Pentium onwards. - ^ 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] - ^ The operation of the
F1
(ICEBP
) opcode differs from the operation of the regular software interrupt opcodeCD 01
in several ways:- In protected mode,
- In virtual-8086 mode,
CD 01
will also check CPL against IOPL as an access-rights check, whileF1
will not. - In virtual-8086 mode with VME enabled, interrupt redirection is supported for
CD 01
but notF1
.
CD 01
will check CPL against the interrupt descriptor's DPL field as an access-rights check, whileF1
will not. - In virtual-8086 mode,
- ^ The UMOV instruction is present on 386 and 486 processors only.[20]
- ^ a b The
XBTS
andIBTS
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 theXBTS
instruction as part of its CPU detection ifCPUID
is not present, and will refuse to boot ifXBTS
is found to be working.[25] - ^ a b For
XBTS
andIBTS
, 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] - ^ Undocumented, 80386 only.[27]
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 |
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 | |
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 | |
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. |
- ^ 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] - ^ 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. The0F B0/B1
encodings are used on 80486 stepping B and later.[31][32] - ^ The
CMPXCHG
instruction setsEFLAGS
in the same way as aCMP
instruction that uses the accumulator (AL/AX/EAX/RAX) as its first argument would do. - ^
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. - ^ a b The
INVD
andWBINVD
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. - ^ 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 theINVD
instruction is not permitted and will cause a #GP(0) exception.[33] - ^ If the
F3
prefix is used with the0F 09
opcode, then the instruction will execute asWBNOINVD
on processors that support the WBNOINVD extension – this will not invalidate the cache.
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 |
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 |
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] |
- ^ 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 MSR79h
on Intel CPUs and MSRC001_0020h
[35] on AMD CPUs. - ^ 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
to83Fh
(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) - ^ 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. - ^ 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 executingCPUID
.
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. - ^ 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 restrictCPUID
to ring 0 also exists on AMD processors supporting the "CpuidUserDis" feature (Zen 4 "Raphael" and later).[47] - ^ 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. - ^ On the Cyrix 5x86 and 6x86 CPUs,
CPUID
is not enabled by default and must be enabled through a Cyrix configuration register. - ^ On NexGen CPUs,
CPUID
is only supported with some system BIOSes. On some NexGen CPUs that do supportCPUID
, EFLAGS.ID is not supported but EFLAGS.AC is, complicating CPU detection.[48] - ^ Unlike the older
CMPXCHG
instruction, theCMPXCHG8B
instruction does not modify any EFLAGS bits other than ZF. - ^
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. - ^ 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] - ^ a b The
RDTSC
andRDPMC
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 ofRDPMC
(but notRDTSC
) 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] - ^ 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 (leaf8000_0007:EDX[8]
).
- ^
RDTSC
can be run outside Ring 0 only ifCR4.TSD=0
.
On Intel Pentium and AMD K5,RDTSC
cannot be run in Virtual-8086 mode.[54] Later processors removed this restriction. - ^
RDPMC
can be run outside Ring 0 only ifCR4.PCE=1
. - ^ The
RDPMC
instruction is not present in VIA processors prior to the Nano. - ^ The condition codes supported for
CMOVcc
instruction (opcode0F 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" - ^ 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.
ForCMOVcc
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 ofCMOVcc
that will suppress memory exceptions if the condition is false.) - ^ On pre-Nehemiah VIA C3 variants ("Samuel"/"Ezra"), the
reg,reg
but notreg,[mem]
forms of theCMOVcc
instructions have been reported to be present as undocumented instructions.[55] - ^ 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.
- ^ Unlike other instructions added in Pentium Pro, long NOP does not have a CPUID feature bit.
- ^
0F 1F /0
as long-NOP was introduced in the Pentium Pro, but remained undocumented until 2006.[58] The whole0F 18..1F
opcode range wasNOP
in Pentium Pro. However, except for0F 1F /0
, Intel does not guarantee that these opcodes will remainNOP
in future processors, and have indeed assigned some of these opcodes to other instructions in at least some processors.[59] - ^ Documented for AMD x86-64 since 2002.[60]
- ^ While the
0F 0B
opcode was officially reserved as an invalid opcode from Pentium onwards, it only got assigned the mnemonicUD2
from Pentium Pro onwards.[62] - ^ a b GNU Binutils have used the
UD2A
andUD2B
mnemonics for the0F 0B
and0F B9
opcodes since version 2.7.[63]
NeitherUD2A
norUD2B
originally took any arguments -UD2B
was later modified to accept a ModR/M byte, in Binutils version 2.30.[64] - ^ 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 anUD2
instruction after the indirect branch.[65] - ^ a b The UD0/1/2 opcodes -
0F 0B
,0F B9
and0F 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. - ^ While the
0F B9
opcode was officially reserved as an invalid opcode from Pentium onwards, it only got assigned its mnemonicUD1
much later – AMD APM started listingUD1
in its opcode maps from rev 3.17 onwards,[67] while Intel SDM started listing it from rev 061 onwards.[68] - ^ a b For both the
0F B9
and0F FF
opcodes, different x86 implementations are known to differ regarding whether the opcodes accept a ModR/M byte.[69][70] - ^ For the
0F FF
opcode, theOIO
mnemonic was introduced by Cyrix,[71] while theUD0
menmonic (without arguments) was introduced by AMD and Intel at the same time as theUD1
mnemonic for0F B9
.[67][68] Later Intel (but not AMD) documentation modified its description ofUD0
to add a ModR/M byte and take two arguments.[72] - ^ On K6, the
SYSCALL
/SYSRET
instructions were available on Model 7 (250nm "Little Foot") and later, not on the earlier Model 6.[74] - ^
SYSCALL
andSYSRET
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. - ^ 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] - ^ a b For the
SYSRET
andSYSEXIT
instructions under x86-64, it is necessary to add theREX.W
prefix for variants that will return to 64-bit user-mode code.
Encodings of these instructions without theREX.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.) - ^ a b c The
SYSRET
,SYSENTER
andSYSEXIT
instructions are unavailable in Real mode. (SYSENTER
is, however, available in Virtual 8086 mode.) - ^ The
CPUID
flags that indicate support forSYSENTER
/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] - ^ On AMD CPUs, the
SYSENTER
andSYSEXIT
instructions are not available in x86-64 long mode (#UD). - ^ On Transmeta CPUs, the
SYSENTER
andSYSEXIT
instructions are only available with version 4.2 or higher of the Transmeta Code Morphing software.[79] - ^ On Nehemiah,
SYSENTER
andSYSEXIT
are available only on stepping 8 and later.[80]
Added as instruction set extensions
[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
replacingARPL
) - 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 |
- ^ The memory operand to
CMPXCHG16B
must be 16-byte aligned. - ^ 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. - ^ Encodings of
MOVSXD
without REX.W prefix are permitted but discouraged[84] – such encodings behave identically to 16/32-bitMOV
(8B /r
).
Bit manipulation extensions
[edit]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 |
---|---|---|---|---|
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
| |||
|
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 todst = (-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 todst = (src-1) AND src
| ||
|
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 tora = 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. |
- ^ On AMD CPUs, the "ABM" extension provides both
POPCNT
andLZCNT
. On Intel CPUs, however, the CPUID bit for "ABM" is only documented to indicate the presence of theLZCNT
instruction and is listed as "LZCNT", whilePOPCNT
has its own separate CPUID feature bit.
However, all known processors that implement the "ABM"/"LZCNT" extensions also implementPOPCNT
and set the CPUID feature bit for POPCNT, so the distinction is theoretical only.
(The converse is not true – there exist processors that supportPOPCNT
but not ABM, such as Intel Nehalem and VIA Nano 3000.) - ^ The
LZCNT
instruction will execute asBSR
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 fromLZCNT
for most input values. - ^ The
TZCNT
instruction will execute asBSF
on systems that do not support the BMI1 extension.BSF
produces the same result asTZCNT
for all input operand values except zero – for whichTZCNT
returns input operand size, butBSF
produces undefined behavior (leaves destination unmodified on most modern CPUs). - ^ 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 ther/m
argument (which has the usual 32/64-bit operand size), then the excess bits are read out as 0. - ^ On AMD processors before Zen 3, the
PEXT
andPDEP
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]TSX Subset | Instruction | Opcode | Description | Added in |
---|---|---|---|---|
|
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.
| ||
|
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.
| ||
|
XSUSLDTRK
|
F2 0F 01 E8
|
Suspend Tracking Load Addresses | Sapphire Rapids |
XRESLDTRK
|
F2 0F 01 E9
|
Resume Tracking Load Addresses |
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 |
---|---|---|---|---|---|
|
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 | |||
|
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.
|
- ^ a b The
RDSSPD
andRDSSPQ
instructions act as NOPs on processors where shadow stacks are disabled or CET is not supported. - ^ a b
ENDBR32
andENDBR64
act as NOPs on processors that don't support CET_IBT or where IBT is disabled. - ^ 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 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 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 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 |
|
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. |
- ^ Under Intel APX, the
XSAVE*
andXRSTOR*
instructions cannot be encoded with the REX2 prefix. - ^ XSAVE was added in steppings E0/R0 of Penryn and is not available in earlier steppings.
- ^ On some processors (starting with Skylake, Goldmont and Zen 1), executing
XGETBV
with ECX=1 is permitted – this will not returnXCR1
(no such register exists) but instead returnXCR0
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 ofXGETBV
is indicated by CPUID.(EAX=0Dh,ECX=1):EAX[bit 2]. - ^ 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 |
---|---|---|---|---|---|
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] | |||
|
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] | |||
|
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 [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.
| |||
|
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
|
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 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
| ||||
|
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
| ||||
|
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 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 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 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] | |||
|
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.
| |||
|
CLAC
|
NP 0F 01 CA
|
Clear EFLAGS.AC .
|
0 | Broadwell, Goldmont, Zen 1, LuJiaZui[af] |
STAC
|
NP 0F 01 CB
|
Set EFLAGS.AC .
| |||
|
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 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 |
|
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 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 r32
|
F3 0F C7 /7
|
Read processor core ID into register.[v] | 3[ag] | Goldmont Plus, Zen 2, Ice Lake, LuJiaZui[af] |
|
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 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
|
F3 0F 09
|
Write back all dirty cache lines to memory without invalidation.[ai] Instruction is serializing. | 0 | Zen 2, Ice Lake-SP |
|
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] |
- ^ 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.
- ^ 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. - ^ a b c For the
SFENCE
,LFENCE
andMFENCE
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. - ^ The
SFENCE
instruction ensures that all memory stores after theSFENCE
instruction are made globally observable after all memory stores before theSFENCE
. 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 theCLFLUSH
,CLFLUSHOPT
andCLWB
instructions. (Older AMD CPUs requireMFENCE
to orderCLFLUSH
.)SFENCE
is not ordered with respect toLFENCE
, and anSFENCE+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 anMFENCE
,LOCK
or a serializing instruction. - ^ The
LFENCE
instruction ensures that all memory loads after theLFENCE
instruction are made globally observable after all memory loads before theLFENCE
.
On all Intel CPUs that support SSE2, theLFENCE
instruction provides a stronger ordering guarantee:[94] it is dispatch-serializing, meaning that instructions after theLFENCE
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 thatLFENCE
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
orRDPMC
) 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-serializingLFENCE
, it can be made dispatch-serializing by setting bit 1 of MSRC001_1029
.[95] - ^ The
MFENCE
instruction ensures that all memory loads, stores and cacheline-flushes after theMFENCE
instruction are made globally observable after all memory loads, stores and cacheline-flushes before theMFENCE
.
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 toLFENCE
, 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 anMFENCE
followed by anLFENCE
.[50][96]
On AMD CPUs,MFENCE
is serializing. - ^ The operation of the
PAUSE
instruction in 64-bit mode is, unlikeNOP
, unaffected by the presence of theREX.R
prefix. NeitherNOP
norPAUSE
are affected by the other bits of theREX
prefix. A few examples of how opcode90
interacts with various prefixes in 64-bit mode are:90
isNOP
41 90
isXCHG R8D,EAX
4E 90
isNOP
49 90
isXCHG R8,RAX
F3 90
isPAUSE
F3 41 90
isPAUSE
F3 4F 90
isPAUSE
- ^ The actual length of the pause performed by the
PAUSE
instruction is implementation-dependent.
On systems without SSE2,PAUSE
will execute as NOP. - ^ Under VT-x or AMD-V virtualization, executing
PAUSE
many times in a short time interval may cause a #VMEXIT. The number ofPAUSE
executions and interval length that can trigger #VMEXIT are platform-specific. - ^ 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 supportsCLFLUSH
but not SSE2.) - ^ While the
MONITOR
andMWAIT
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.) - ^ a b For the
MONITOR
andMWAIT
instructions, older Intel documentation[97] lists instruction mnemonics with explicit operands (MONITOR EAX,ECX,EDX
andMWAIT EAX,ECX
), while newer documentation omits these operands. Assemblers/disassemblers may support one or both of these variants.[98] - ^ 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. - ^ 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. - ^ 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
andMWAIT
to run in Ring 3. - ^ The wait performed by
MWAIT
may 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 withMONITOR
before usingMWAIT
to wait for memory writes again. - ^ 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. - ^ 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.
- ^ For the
GETSEC
instruction, theREX.W
prefix enables 64-bit addresses for the EXITAC leaf function only - REX prefixes are otherwise permitted but ignored for the instruction. - ^ 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.
- ^ 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. - ^ a b The "core ID" value read by
RDTSCP
andRDPID
is actually theTSC_AUX
MSR (MSRC000_0103h
). Whether this value actually corresponds to a processor ID is a matter of operating system convention. - ^ 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. - ^
RDTSCP
can be run outside Ring 0 only ifCR4.TSD=0
. - ^ Support for
RDTSCP
was added in stepping F of the AMD K8, and is not available on earlier steppings. - ^ 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. - ^ 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.
- ^ 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] - ^ The
PREFETCH
andPREFETCHW
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! - ^ The opcodes for
PREFETCH
andPREFETCHW
(0F 0D /r
) execute as NOPs on Intel CPUs from Cedar Mill (65nm Pentium 4) onwards, withPREFETCHW
gaining prefetch functionality from Broadwell onwards. - ^ 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, opcode0F 0D /0
as well as opcodes0F 0D /2../7
are all documented to be performing prefetch.
On Intel processors with PREFETCHW, these opcodes are documented as performing reserved-NOPs[105] (except0F 0D /2
beingPREFETCHWT1 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. - ^ 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.
- ^ Unlike the older
RDTSCP
instruction which can also be used to read the processor ID, user-modeRDPID
is not disabled byCR4.TSD=1
. - ^ 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 the67h
prefix.
The 64-byte memory source argument does not need to be 64-byte aligned, and is not guaranteed to be read atomically. - ^ The
WBNOINVD
instruction will execute asWBINVD
if run on a system that doesn't support the WBNOINVD extension.WBINVD
differs fromWBNOINVD
in thatWBINVD
will invalidate all cache lines after writeback. - ^ a b In initial implementations, the
PREFETCHIT0
andPREFETCHIT1
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 |
---|---|---|---|---|---|
|
HWNT ,hint-not-taken [a]
|
2E [b]
|
Instruction prefix: branch hint weakly not taken. | 3 | Pentium 4,[c] Meteor Lake[111] |
HST ,hint-taken [a]
|
3E [b]
|
Instruction prefix: branch hint strongly taken. | |||
|
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 | |
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 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
|
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 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] |
|
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 | |||
|
SERIALIZE
|
NP 0F 01 E8
|
Serialize instruction fetch and execution.[r] | 3 | Alder Lake |
|
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 |
|
SENDUIPI reg
|
F3 0F C7 /6
|
Send Interprocessor User Interrupt.[t] | 3 | Sapphire Rapids |
UIRET
|
F3 0F 01 EC
|
User Interrupt Return. | |||
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 reg,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.[v]) 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 reg,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
|
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 |
0 | Sierra Forest |
|
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.[w] | 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.[w] The MSRs are written in table order. The instruction is not serializing. | |||
|
CMPccXADD m32,r32,r32 CMPccXADD m64,r64,r64 |
VEX.128.66.0F38.W0 Ex /r VEX.128.66.0F38.W1 Ex /r [x][y] |
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
|
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 |
- ^ a b The branch hint mnemonics
HWNT
andHST
are listed in early Willamette documentation only[108] - later Intel documentation lists the branch hint prefixes without assigning them a mnemonic.[109]Intel XED uses the mnemonics
hint-taken
andhint-not-taken
for these branch hints.[110] - ^ a b The
2E
and3E
prefixes are interpreted as branch hints only when used with theJcc
conditional branch instructions (opcodes70..7F
and0F 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 prefixesCS:
andDS:
, respectively). On processors that don't support branch hints, these prefixes are accepted but ignored when used withJcc
. - ^ 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.
- ^ 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[112] 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[113] Any unsupported value in EAX causes a #GP exception.
- ^ 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.[114]
- ^ 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[116] 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. - ^
ENCLU
can only be executed in ring 3, not rings 0/1/2. - ^ The leaf functions defined for
ENCLV
(selected by EAX) are:EAX Function Added with OVERSUB[112] 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.
TheENCLV
instruction is only present on systems that support the EPC Oversubscription Extensions to SGX ("OVERSUB"). - ^
ENCLV
is only available if Intel VMX operation is enabled withVMXON
, and will produce #UD otherwise. - ^ For
PTWRITE
, the write to the Processor Trace Packet will only happen if a set of enable-bits (the "TriggerEn", "ContextEn", "FilterEn" bits of theRTIT_STATUS
MSR and the "PTWEn" bit of theRTIT_CTL
MSR) are all set to 1.
ThePTWRITE
instruction is indicated in the SDM to cause an #UD exception if the 66h instruction prefix is used, regardless of other prefixes. - ^ 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.
- ^ 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. - ^ 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.[117]
As of April 2023, the CPUID feature bit for CLDEMOTE has been observed to be set for Sapphire Rapids.[118] - ^ For
UMONITOR
, the operand size of the address argument is given by the address size, which may be overridden by the67h
prefix. The default segment used is DS:, which can be overridden with a segment prefix. - ^ a b For the
UMWAIT
andTPAUSE
instructions, the operating system can use theIA32_UMWAIT_CONTROL
MSR to limit the maximum amount of time that a singleUMWAIT
/TPAUSE
invocation is permitted to wait. TheUMWAIT
andTPAUSE
instructions will setRFLAGS.CF
to 1 if they reached theIA32_UMWAIT_CONTROL
-defined time limit and 0 otherwise. - ^
TPAUSE
andUMWAIT
can be run outside Ring 0 only ifCR4.TSD=0
. - ^ For the register argument to the
UMWAIT
andTPAUSE
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) - ^ While serialization can be performed with older instructions such as e.g.
CPUID
andIRET
, 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.) TheSERIALIZE
instruction performs serialization only, avoiding these added costs. - ^ 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) - ^ The register argument to
SENDUIPI
is an index to pick an entry from the UITT (User-Interrupt Target Table, a table specified by the newUINTR_TT
andUINT_MISC
MSRs.) - ^ 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 byCPUID.(EAX=7,ECX=1):EDX[17]
. - ^ For
ENQCMD
andEMQCMDS
, the operand-size of the register argument is given by the current address-size, which can be overridden with the67h
prefix. - ^ a b For the
RDMSRLIST
andWRMSRLIST
instructions, the addresses specified in the RSI and RDI registers must be 8-byte aligned. - ^ The condition codes supported for the
CMPccXADD
instructions (opcodeVEX.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" - ^ Even though the
CMPccXADD
instructions perform a locked memory operation, they do not require or accept theLOCK
(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 |
---|---|---|---|---|---|
|
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
|
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 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
|
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
|
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
|
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.
|
- ^ 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, theREX.R
prefix is only available in 64-bit mode.
The AltMovCr8 extension adds an additional method to access CR8, using theF0
(LOCK
) prefix instead ofREX.R
– this provides access to CR8 outside 64-bit mode. - ^ 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 theLOCK
prefix with theREX.R
prefix is not permitted and will cause an #UD exception. - ^ Support for AltMovCR8 was added in stepping F of the AMD K8, and is not available on earlier steppings.
- ^ 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.
TheCLZERO
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.[120] - ^ The register numbering used by
RDPRU
does not necessarily match that ofRDMSR
/WRMSR
.
The registers supported byRDPRU
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.
- ^ If
CR4.TSD=1
, then theRDPRU
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.
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
|
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
|
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
|
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
|
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]
|
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
|
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
|
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] |
- ^ 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[121]). These instructions are mostly control instructions that can inspect and/or modify the pending-exception state of the x87 FPU. - ^ 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 aWAIT
instruction (opcode9B
) followed by the corresponding non-waiting x87 instruction. For example:FNCLEX
is an instruction with the opcodeDB E2
. The corresponding pseudo-instructionFCLEX
is then encoded as9B DB E2
.FNSAVE ES:[BX+6]
is an instruction with the opcode26 DD 77 06
. The corresponding pseudo-instructionFSAVE ES:[BX+6]
is then encoded as9B 26 DD 77 06
- ^ a b c d On 80387 and later x87 FPUs,
FLDENV
,F(N)STENV
,FRSTOR
andF(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 theCS.D
bit and the presence of the66h
instruction prefix. On 8087 and 80287, only the 16-bit variants are available.
64-bit variants of these instructions do not exist – usingREX.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 useFXSAVE64
/FXRSTOR64
instead if 64-bit operation is desired. - ^ 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,[122] and so the FPU offers the
F(N)DISI
andF(N)ENI
instructions to set/clear the Interrupt Mask bit (bit 7) of the x87 Control Word,[123] 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.[124] In later Intel x87 FPUs, theF(N)ENI
andF(N)DISI
instructions were kept for backwards compatibility, executing as NOPs that do not modify any x87 state. - ^ 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. - ^
FSTP ST(0)
is a commonly used idiom for popping a single register off the x87 register stack. - ^ 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[125] and 80287,[126] but then omitted from later manuals until the October 2017 update of the Intel SDM.[127]
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[128] and NexGen 586PF.[129] - ^ 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. - ^ a b c d e f g h i For the
FADDP
,FSUBP
,FSUBRP
,FMULP
,FDIVP
,FDIVRP
,FCOM
,FCOMP
andFXCH
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. - ^ 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. - ^ The result of executing the
FBLD
instruction on non-BCD data is undefined. - ^ 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
andFPATAN
.[130] - ^ 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.
- ^ 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 +∞. - ^ For
FPREM
, if the quotient Q is larger than , then the remainder calculation may have been done only partially – in this case, theFPREM
instruction will need to be run again in order to complete the remainder calculation. This is indicated by the instruction settingC2
to 1.
If the instruction did complete the remainder calculation, it will setC2
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. - ^ The remainder computation done by the
FPREM
instruction is always exact with no roundoff errors. - ^ 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. - ^ For
FSCALE
, rounding is only applied in the case of overflow, underflow or subnormal result. - ^ 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.
- ^ a b For the
FYL2X
andFYL2XP1
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. - ^ 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.
- ^ 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
|
- ^ 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
andF(N)STENV
instructions has different formats in Real Mode and Protected Mode. On 80287, theF(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. - ^ Not including discontinued instructions specific to particular 80387-compatible FPU models.
- ^ a b For the
FUCOM
andFUCOMP
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. - ^ The 80387
FPREM1
instruction differs from the olderFPREM
(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 byFPREM
. LikeFPREM
,FPREM1
always computes an exact result with no roundoff errors. LikeFPREM
, it may also perform a partial computation if the quotient is too large, in which case it must be run again. - ^ 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
andFSINCOS
is not precisely 1.0, but instead given by[131][132]This argument reduction inaccuracy also affects theFPTAN
instruction. - ^ The
FCOMI
,FCOMIP
,FUCOMI
andFUCOMIP
instructions write their results to theZF
,CF
andPF
bits of theEFLAGS
register. On Intel but not AMD processors, theSF
,AF
andOF
bits ofEFLAGS
are also zeroed out by these instructions. - ^ The
FXSAVE
andFXRSTOR
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. - ^ a b The
FXSAVE
andFXRSTOR
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 byFXSAVE
/FXRSTOR
has a completely different layout than the data structure of the olderF(N)SAVE
/FRSTOR
instructions, enabling faster save/restore by avoiding misaligned loads and stores. - ^ a b When floating-point emulation is enabled with
CR0.EM=1
,FXSAVE(64)
andFXRSTOR(64)
are considered to be x87 instructions and will accordingly produce an #NM (device-not-available) exception. Other thanWAIT
, these are the only opcodes outside theD8..DF
ESC opcode space that exhibit this behavior. (All opcodes inD8..DF
will produce #NM ifCR0.EM=1
, even for undefined opcodes that would produce #UD otherwise.) - ^ 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.
Cryptographic instructions
[edit]Virtualization instructions
[edit]Other instructions
[edit]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 |
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 | |
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.[134]
|
TEST r/m8,imm8
|
F6 /1 ib
|
Undocumented variants of the TEST instruction.[135] Performs the same operation as the documented F6 /0 and F7 /0 variants, respectively.
|
Available since the 8086. |
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.[135] 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)[138] |
(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.[139]
|
Available since the 8086.[139] Explicitly unavailable in 64-bit mode but kept and reserved for compatibility.[140] |
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.[141][142] |
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[143]) 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.[144] At least some versions of gcc are known to use this encoding.[145]
|
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.[146][147]
|
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][148][149] |
Available on Pentium Pro and AMD K7[150] and later.
Unavailable on AMD K6, AMD Geode LX, VIA Nehemiah.[151] |
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.[152][153] 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, |
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.[154][68]
Microsoft Windows 95 Setup is known to depend on Other invalid opcodes that are being relied on by commercial software to produce #UD exceptions include |
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.[161] | |
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.[161] | |
SAVEALL ,
|
(F1) 0F 04
|
Exact purpose unknown, causes CPU hang (HCF). The only way out is CPU reset.[162]
In some implementations, emulated through BIOS as a halting sequence.[163] In a forum post at the Vintage Computing Federation, this instruction (with |
Only available on 80286. | |
LOADALL
|
0F 05
|
Loads All Registers from Memory Address 0x000800H | Only available on 80286.
Opcode reused for | |
LOADALLD
|
0F 07
|
Loads All Registers from Memory Address ES:EDI | Only available on 80386.
Opcode reused for | |
CL1INVMB
|
0F 0A [164]
|
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.[165] | 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[166]
|
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.[167] 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.[168] | Available on NexGen Nx586 only. | |
(multiple) | 0F (E0..FB) [169]
|
NexGen Nx586 "hyper mode" instructions.
The NexGen Nx586 CPU uses "hyper code"[170] (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[171]) 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.[172][173]
Instruction known to be recognized by MASM 6.13 and 6.14. |
Available on K6-2 and K6-3 only.
Opcode reused for documented | |
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 .[174][175]
|
Available on the UMC Green CPU only. Executes as SALC on non-UMC CPUs.
| |
FS: Jcc
|
64 (70..7F) rel8 ,
|
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.[176] 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.[177] | Removed from Zen2 onwards. | |
(unknown, multiple) | 0F 0F /r ??
|
The whitepapers for SandSifter[178] and UISFuzz[179] 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 |
Present on some AMD CPUs with 3DNow!. | |
MOVDB ,
|
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.[180]
|
Unknown. No specification known to have been published. | |
REP XSHA512
|
F3 0F A6 E0
|
Perform SHA-512 hashing.
Supported by OpenSSL[181] as part of its VIA PadLock support, and listed in a Zhaoxin-supplied Linux kernel patch,[182] 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,[183] 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[178] and UISFuzz[179] 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,[184] description and opcode (but no instruction mnemonic) provided in a Zhaoxin patent application[185] and a Zhaoxin-provided Linux kernel patch.[186] | Present in Zhaoxin KX-6000G.[187] | |
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.[188] | Present in Zhaoxin KX-7000. | |
MONTMUL2
|
Unknown | Zhaoxin RSA/"xmodx" instructions. Mnemonics and CPUID flags are listed in a Linux kernel patch for OpenEuler,[184] but opcodes and instruction descriptions are not available. | Unknown. Some Zhaoxin CPUs[187] have the CPUID flags for these instructions set. |
Undocumented x87 instructions
[edit]Mnemonics | Opcodes | Description | Status |
---|---|---|---|
FENI ,
|
DB E0
|
FPU Enable Interrupts (8087) | Documented for the Intel 80287.[126]
Present on all Intel x87 FPUs from 80287 onwards. For FPUs other than the ones where they were introduced on (8087 for 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.[189] |
FDISI ,
|
DB E1
|
FPU Disable Interrupts (8087) | |
FSETPM ,
|
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,[190] 6x86MX, MII, MediaGX, and AMD Geode GX/LX.[191] (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]- CLMUL
- RDRAND
- Advanced Vector Extensions 2
- AVX-512
- x86 Bit manipulation instruction set
- CPUID
- List of discontinued x86 instructions
References
[edit]- ^ "Re: Intel Processor Identification and the CPUID Instruction". Retrieved 2013-04-21.
- ^ "Intel 80x86 Instruction Set Summary" (PDF). eecs.wsu.edu.
- ^ Michal Necasek, SGDT/SIDT Fiction and Reality, 4 May 2017. Archived on 29 Nov 2023.
- ^ a b Intel, Undocumented iAPX 286 Test Instruction. Archived on 20 Dec 2023.
- ^ WikiChip, UMIP – x86. Archived on 16 Mar 2023.
- ^ Oracle Corp, Oracle® VM VirtualBox Administrator's Guide for Release 6.0, section 3.5: Details About Software Virtualization. Archived on 8 Dec 2023.
- ^ MBC Project, Virtual Machine Detection (permanent link) or Virtual Machine Detection (non permanent link)
- ^ Andrew Schulman, "Unauthorized Windows 95" (ISBN 1-56884-169-8), chapter 8, p.249,257.
- ^ US Patent 4974159, "Method of transferring control in a multitasking computer system" mentions 63h/ARPL.
- ^ Intel, Pentium® Processor Family Developer’s Manual, Volume 3, 1995, order no. 241430-004, section 12.7, p. 323
- ^ Intel, How Microarchitectural Data Sampling works, see mitigations section. Archived on Apr 22,2022
- ^ Linux kernel documentation, Microarchitectural Data Sampling (MDS) mitigation Archived 2020-10-21 at the Wayback Machine
- ^ VCF Forums, I found the SAVEALL opcode, jun 21, 2019. Archived on 13 Apr 2023.
- ^ rep lodsb, Intel 286 secrets: ICE mode and F1 0F 04, aug 12, 2022. Archived on 8 Dec 2023.
- ^ 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
- ^ Raymond Chen, Getting MS-DOS games to run on Windows 95: Working around the iretd problem, Apr 4, 2016. Archived on Mar 15, 2019
- ^ sandpile.org, x86 architecture rFLAGS register, see note #7. Archived on 3 Nov 2011.
- ^ 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 farJMP
(opcodeEA
) 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. - ^ Can Bölük, Speculating the entire x86-64 Instruction Set In Seconds with This One Weird Trick, Mar 22, 2021. Archived on Mar 23, 2021.
- ^ a b Robert Collins, Undocumented OpCodes, 29 july 1995. Archived on 21 feb 2001
- ^ Michal Necasek, ICEBP finally documented, OS/2 Museum, May 25, 2018. Archived on 6 June 2018
- ^ 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 andFFREEP
on page 114. Archived from the original on 22 Dec 1996. - ^ AMD, AMD 64-bit Technology, vol 2: System Programming, order no. 24593, rev 3.06, aug 2002, page 248
- ^ "Intel 80386 CPU Information | PCjs Machines". www.pcjs.org.
- ^ Geoff Chappell, CPU Identification before CPUID, 27 Jan 2020. Archived on 7 Apr 2023.
- ^ Jeff Parsons, Obsolete 80386 Instructions: IBTS and XBTS, PCjs Machines. Archived on Sep 19, 2020.
- ^ Robert Collins, The LOADALL Instruction. Archived from the original on Jun 5, 1997.
- ^ Toth, Ervin (1998-03-16). "BSWAP with 16-bit registers". Archived from the original on 1999-11-03.
The instruction brings down the upper word of the doubleword register without affecting its upper 16 bits.
- ^ Coldwin, Gynvael (2009-12-29). "BSWAP + 66h prefix". Retrieved 2018-10-03.
internal (zero-)extending the value of a smaller (16-bit) register … applying the bswap to a 32-bit value "00 00 AH AL", … truncated to lower 16-bits, which are "00 00". … Bochs … bswap reg16 acts just like the bswap reg32 … QEMU … ignores the 66h prefix
- ^ Intel "i486 Microprocessor" (April 1989, order no. 240440-001) p.142 lists
CMPXCHG
with0F A6/A7
encodings. - ^ Intel "i486 Microprocessor" (November 1989, order no. 240440-002) p.135 lists
CMPXCHG
with0F B0/B1
encodings. - ^ "Intel 486 & 486 POD CPUID, S-spec, & Steppings".
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RDTSC
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UD2A
andUD2B
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UD1
/UD2B
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0F 0B
and0F B9
. - ^ a b AMD, AMD64 Architecture Programmer’s Manual Volume 3, publication no. 24594, rev 3.17, dec 2011 – see page 416 for
UD0
and page 415 and 419 forUD1
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UD1
(with ModR/M byte) andUD0
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OIO
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opcode. - ^ Intel, Software Developer's Manual, vol 2B, order no. 253667-064, oct 2017 – lists
UD0
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0F FF
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MFENCE;LFENCE
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MONITOR
andMWAIT
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HWNT
/HST
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FSIN
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andFPTAN
in volume 1, section 8.3.8 - ^ Robert Collins, Undocumented OpCodes: AAM. Archived on 21 Feb 2001
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NOP
instruction description.) - ^ Intel Software Developers Manual, volume 2B (order no. 253667-060, September 2016) does not list
UD0
andUD1
. - ^ "PCJS : pcjs/x86op0F.js (two-byte x86 opcode handlers), lines 1647–1651". GitHub. 17 April 2022. Archived from the original on 13 Apr 2023.
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Using the REP or REPNE prefix with a MUL or IMUL instruction negates the product. Using the REP or REPNE prefix with an IDIV instruction negates the quotient.
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External links
[edit]- Free IA-32 and x86-64 documentation, provided by Intel
- AMD64 Architecture Programmer's Manual, Volumes 1-5, provided by AMD
- x86 Opcode and Instruction Reference
- x86 and amd64 instruction reference
- Instruction tables: Lists of instruction latencies, throughputs and micro-operation breakdowns for Intel, AMD and VIA CPUs
- Netwide Assembler Instruction List (from Netwide Assembler)