Instruction Set CMP
CMP -- Compare Two Operands
CMP -- Compare Two Operands
Opcode Instruction Clocks Description
3C ib CMP AL,imm8 2 Compare immediate byte to AL
3D iw CMP AX,imm16 2 Compare immediate word to AX
3D id CMP EAX,imm32 2 Compare immediate dword to EAX
80 /7 ib CMP r/m8,imm8 2/5 Compare immediate byte to r/m
byte
81 /7 iw CMP r/m16,imm16 2/5 Compare immediate word to r/m
word
81 /7 id CMP r/m32,imm32 2/5 Compare immediate dword to r/m
dword
83 /7 ib CMP r/m16,imm8 2/5 Compare sign extended immediate
byte to r/m word
83 /7 ib CMP r/m32,imm8 2/5 Compare sign extended immediate
byte to r/m dword
38 /r CMP r/m8,r8 2/5 Compare byte register to r/m
byte
39 /r CMP r/m16,r16 2/5 Compare word register to r/m
word
39 /r CMP r/m32,r32 2/5 Compare dword register to r/m
dword
3A /r CMP r8,r/m8 2/6 Compare r/m byte to byte
register
3B /r CMP r16,r/m16 2/6 Compare r/m word to word
register
3B /r CMP r32,r/m32 2/6 Compare r/m dword to dword
register
Operation
LeftSRC - SignExtend(RightSRC);
(* CMP does not store a result; its purpose is to set the flags *)
Description
CMP subtracts the second operand from the first but, unlike the SUB
instruction, does not store the result; only the flags are changed. CMP is
typically used in conjunction with conditional jumps and the SETcc
instruction. (Refer to Appendix D for the list of signed and unsigned flag
tests provided.) If an operand greater than one byte is compared to an
immediate byte, the byte value is first sign-extended.
Flags Affected
OF, SF, ZF, AF, PF, and CF as described in Appendix C
Protected Mode Exceptions
#GP(0) for an illegal memory operand effective address in the CS, DS, ES,
FS, or GS segments; #SS(0) for an illegal address in the SS segment;
#PF(fault-code) for a page fault
Real Address Mode Exceptions
Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH
Virtual 8086 Mode Exceptions
Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault