Compare Two Operands (cmp)

cmp{bwl}	 	reg[8|16|32], r/m[8|16|32]
cmp{bwl} 		r/m[8|16|32], reg[8|16|32]
cmp{bwl}	 	imm[8|16|32], r/m[8|16|32]

Operation

r/m[8|16|32] - reg[8|16|32]

reg[8|16|32] - r/m[8|16|32]

r/m[8|16|32] - imm[8|16|32]

Description

Subtracts operand1 from operand2, but does not store the result; only changes the flags. cmp is typically executed in conjunction with conditional jumps and the setcc instruction. If an operand greater than one byte is compared to an immediate byte, the immediate byte value is first sign-extended.

Example

Compare the 8-bit constant, 0xff, with the content of the AL register:

cmpb $0xff, %al

Compare the 16-bit content of the DX register with the effective address (addressed by the EDI register plus an offset of 4):

cmpw %dx, 4(%edi)

Compare the 32-bit content of the effective address (addressed by the EDI register plus an offset of 4) to the EDX register:

cmpl 4(%edi), %edx