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80386 Programmer's Reference Manual -- Opcode CMP

CMP -- Compare Two Operands

Opcode	Instruction	Clocks	Description	Example
3C ib	cmpb imm8,al	2	Compare immediate byte to AL	`cmpb $0x7f,%al`
3D iw	cmpw imm16,ax	2	Compare immediate word to AX	`cmpw $0x7fff,%ax`
3D id	cmpl imm32,eax	2	Compare immediate dword to EAX	`cmpl $0x7fffffff,%eax`
80 /7 ib	cmpb imm8,r/m8	2/5	Compare immediate byte to r/m byte	`cmpb $0x7f,%dl cmpb $0x7f,(%ebx,1) cmpb $0x7f,m8(%ebx,1) cmpb $0x7f,m8(%ebx,%ebp,1)`
81 /7 iw	cmpw imm16,r/m16	2/5	Compare immediate word to r/m word	`cmpw $0x7fff,%cx cmpw $0x7fff,(%ebx,1) cmpw $0x7fff,(%ebx,2) cmpw $0x7fff,(%ebx,%ebp,1)`
81 /7 id	cmpl imm32,r/m32	2/5	Compare immediate dword to r/m dword	`cmpl $0x7fffffff,%ecx cmpl $0x7fffffff,(%ebx,2) cmpl $0x7fffffff,(%ebx,4) cmpl $0x7fffffff,(%ebx,%ebp,1)`
83 /7 ib	cmpw imm8,r/m16	2/5	Compare sign extended immediate byte to r/m word	`cmpw $0x7f,%cx cmpw $0x7f,(%ebx,1) cmpw $0x7f,(%ebx,2) cmpw $0x7f,(%ebx,%ebp,1)`
83 /7 ib	cmpl imm8,r/m32	2/5	Compare sign extended immediate byte to r/m dword	`cmpl $0x7f,%ecx cmpl $0x7f,(%ebx,2) cmpl $0x7f,(%ebx,4) cmpl $0x7f,(%ebx,%ebp,1)`
38 /r	cmpb r8,r/m8	2/5	Compare byte register to r/m byte	`cmpb %bh,%dl cmpb %bh,(%ebx,1) cmpb %bh,m8(%ebx,1) cmpb %bh,m8(%ebx,%ebp,1)`
39 /r	cmpw r16,r/m16	2/5	Compare word register to r/m word	`cmpw %bx,%cx cmpw %bx,(%ebx,1) cmpw %bx,(%ebx,2) cmpw %bx,(%ebx,%ebp,1)`
39 /r	cmpl r32,r/m32	2/5	Compare dword register to r/m dword	`cmpl %ebx,%ecx cmpl %ebx,(%ebx,2) cmpl %ebx,(%ebx,4) cmpl %ebx,(%ebx,%ebp,1)`
3A /r	cmpb r/m8,r8	2/6	Compare r/m byte to byte register	`cmpb %dl,%bh cmpb (%ebx,1),%bh cmpb m8(%ebx,1),%bh cmpb m8(%ebx,%ebp,1),%bh`
3B /r	cmpw r/m16,r16	2/6	Compare r/m word to word register	`cmpw %cx,%bx cmpw (%ebx,1),%bx cmpw (%ebx,2),%bx cmpw (%ebx,%ebp,1),%bx`
3B /r	cmpl r/m32,r32	2/6	Compare r/m dword to dword register	`cmpl %ecx,%ebx cmpl (%ebx,2),%ebx cmpl (%ebx,4),%ebx cmpl (%ebx,%ebp,1),%ebx`

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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