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Intel Pentium CPU Instruction Set Reference

CMP instruction - Compare Two Operands

Code Mnemonic Description
3C ib CMP AL, imm8 Compare imm8 with AL
3D iw CMP AX, imm16 Compare imm16 with AX
3D id CMP EAX, imm32 Compare imm32 with EAX
80 /7 ib CMP r/m8, imm8 Compare imm8 with r/m8
81 /7 iw CMP r/m16, imm16 Compare imm16 with r/m16
81 /7 id CMP r/m32, imm32 Compare imm32 with r/m32
83 /7 ib CMP r/m16, imm8 Compare imm8 with r/m16
83 /7 ib CMP r/m32, imm8 Compare imm8 with r/m32
38 / r CMP r/m8, r8 Compare r8 with r/m8
39 / r CMP r/m16, r16 Compare r16 with r/m16
39 / r CMP r/m32, r32 Compare r32 with r/m32
3A / r CMP r8, r/m8 Compare r/m8 with r8
3B / r CMP r16, r/m16 Compare r/m16 with r16
3B / r CMP r32, r/m32 Compare r/m32 with r32


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 CMP instruction is typically used in conjunction with a conditional jump (Jcc), condition move (CMOVcc), or SETcc instruction. 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 Architecture Software Developer's Manual, Volume 1, shows the relationship of the status flags and the condition codes.

Operands Bytes Clocks
reg, reg 2 1 UV
mem, reg 2 + d(0, 2) 2 UV
reg, mem 2 + d(0, 2) 2 UV
reg, imm 2 + i(1, 2) 1 UV
mem, imm 2 + d(0, 2) + i(1, 2) 2 UV ( not pairable if there is a displacement and immediate)
acc, imm 1 + i(1, 2) 1 UV


ID unaffected DF unaffected
VIP unaffected IF unaffected
VIF unaffected TF unaffected
AC unaffected SF sets according to the result
VM unaffected ZF sets according to the result
RF unaffected AF sets according to the result
NT unaffected PF sets according to the result
IOPL unaffected CF sets according to the result
OF sets according to the result