cmp

Compare Two Operands

CMP r/m, r

Subtracts src from dest and updates flags (dest not modified).

Details

Subtracts the source operand from the destination operand and updates the EFLAGS register without modifying the destination. Sets or clears CF, PF, AF, ZF, SF, and OF based on the subtraction result. Available in 8, 16, 32, and 64-bit variants across all modes.

Pseudocode Operation

temp ← dest - src; CF ← (dest < src); ZF ← (temp == 0); SF ← (temp < 0); OF ← overflow(dest, src); PF ← parity(temp); AF ← half_borrow(dest, src);

Example

CMP EAX, 10   ; Compare EAX with 10
JE label      ; Jump if Equal

Encoding

Binary Layout

Format Legacy

Opcode 39

Extension Base

Operands

dest
Register/Memory
src
Register

Reference (Intel® SDM)

Instruction Forms

Opcode	Instruction	Op/En	64/32-bit Mode	Description
3C ib	CMP AL, imm8	I	Valid Valid	Compare imm8 with AL.
3D iw	CMP AX, imm16	I	Valid Valid	Compare imm16 with AX.
3D id	CMP EAX, imm32	I	Valid Valid	Compare imm32 with EAX.
REX.W + 3D id	CMP RAX, imm32	I	Valid N.E.	Compare imm32 sign-extended to 64-bits with RAX.
80 /7 ib	CMP r/m8, imm8	MI	Valid Valid	Compare imm8 with r/m8.
81 /7 iw	CMP r/m16, imm16	MI	Valid Valid	Compare imm16 with r/m16.
81 /7 id	CMP r/m32, imm32	MI	Valid Valid	Compare imm32 with r/m32.
REX.W + 81 /7 id	CMP r/m64, imm32	MI	Valid N.E.	Compare imm32 sign-extended to 64-bits with r/m64.
83 /7 ib	CMP r/m16, imm8	MI	Valid Valid	Compare imm8 with r/m16.
83 /7 ib	CMP r/m32, imm8	MI	Valid Valid	Compare imm8 with r/m32.
REX.W + 83 /7 ib	CMP r/m64, imm8	MI	Valid N.E.	Compare imm8 with r/m64.
38 /r	CMP r/m8, r8	MR	Valid Valid	Compare r8 with r/m8.
39 /r	CMP r/m16, r16	MR	Valid Valid	Compare r16 with r/m16.
39 /r	CMP r/m32, r32	MR	Valid Valid	Compare r32 with r/m32.
REX.W + 39 /r	CMP r/m64,r64	MR	Valid N.E.	Compare r64 with r/m64.
3A /r	CMP r8, r/m8	RM	Valid Valid	Compare r/m8 with r8.
3B /r	CMP r16, r/m16	RM	Valid Valid	Compare r/m16 with r16.
3B /r	CMP r32, r/m32	RM	Valid Valid	Compare r/m32 with r32.
REX.W + 3B /r	CMP r64, r/m64	RM	Valid N.E.	Compare r/m64 with r64.

Description

Compares the first source operand with the second source operand and sets the status flags in the EFLAGS register according to the results. The comparison is performed by subtracting the second operand from the first operand and then setting the status flags in the same manner as the SUB instruction. When an immediate value is used as an operand, it is sign-extended to the length of the first operand. The condition codes used by the Jcc, CMOVcc, and SETcc instructions are based on the results of a CMP instruction. Appendix B, “EFLAGS Condition Codes,” in the Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1, shows the relationship of the status flags and the condition codes. In 64-bit mode, the instruction’s default operation size is 32 bits. Use of the REX.R prefix permits access to additional registers (R8-R15). Use of the REX.W prefix promotes operation to 64 bits. See the summary chart at the beginning of this section for encoding data and limits. CMP—Compare Two Operands Vol. 2A 3-161

Operation

temp := SRC1 − SignExtend(SRC2);
ModifyStatusFlags; (* Modify status flags in the same manner as the SUB instruction*)

Flags Affected

The CF, OF, SF, ZF, AF, and PF flags are set according to the result.

Exceptions

Protected Mode Exceptions

#GP(0) If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. If the DS, ES, FS, or GS register contains a NULL segment selector. #SS(0) If a memory operand effective address is outside the SS segment limit. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3. #UD If the LOCK prefix is used.

Real-Address Mode Exceptions

#GP If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. #SS If a memory operand effective address is outside the SS segment limit.

Virtual-8086 Mode Exceptions

#GP(0) If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. #SS(0) If a memory operand effective address is outside the SS segment limit. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made. #UD If the LOCK prefix is used.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit Mode Exceptions

#SS(0) If a memory address referencing the SS segment is in a non-canonical form. #GP(0) If the memory address is in a non-canonical form. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3. #UD If the LOCK prefix is used. CMP—Compare Two Operands Vol. 2A 3-162