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

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