CMP
Compare Two Operands
The same operation as SUB, but do not save the result in a register, only keep the flags.
| Instruction | Description |
|---|---|
CMP r/m8, imm8 | Compare imm8 with r/m8 |
CMP r/m16, imm16 | Compare imm16 with r/m16 |
CMP r/m32, imm32 | Compare imm32 with r/m32 |
CMP r/m64, imm32 | Compare sign-extended imm32 with r/m64 |
CMP r/m16, imm8 | Compare sign-extended imm8 with r/m16 |
CMP r/m32, imm8 | Compare sign-extended imm8 with r/m32 |
CMP r/m64, imm8 | Compare sign-extended imm8 with r/m64 |
CMP r/m8, r8 | Compare r8 with r/m8 |
CMP r/m16, r16 | Compare r16 with r/m16 |
CMP r/m32, r32 | Compare r32 with r/m32 |
CMP r/m64, r64 | Compare r64 with r/m64 |
CMP r8, r/m8 | Compare r/m8 with r8 |
CMP r16, r/m16 | Compare r/m16 with r16 |
CMP r32, r/m32 | Compare r/m32 with r32 |
CMP r64, r/m64 | Compare r/m64 with r64 |
Intuition:
Condition codes are read in order of the operands. For example:
cmp rax, 10
jg .label
“jump if RAX is greater than 10”
Description
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 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® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1, shows the relationship of the status flags and the condition codes.
In 64-bit mode, the instruction’s default operation size is 32 bits. Use of the REX.R prefix permits access to additional registers (R8-R15). Use of the REX.W prefix promotes operation to 64 bits. See the summary chart at the beginning of this section for encoding data and limits.
Operation
SRC1: first operand
SRC2: second operand
temp = SRC1 - SignExtend(SRC2);
ModifyStatusFlags(); // modify status flags in the same manner as the SUB instruction
Flags Affected
The CF, OF, SF, ZF, AF, and PF flags are set according to the result.