80386 Programmer's Reference Manual -- Opcode CMPS

CMPS/CMPSB/CMPSW/CMPSD -- Compare String Operands

OpcodeInstructionClocks DescriptionExample
A6cmpsb 10 Compare bytes ES:[(E)DI] with DS:[SI]cmpsb
A7cmpsw 10 Compare words ES:[(E)DI] with DS:[SI]cmpsw
A7cmpsl 10 Compare dwords ES:[(E)DI] with DS:[SI]cmpsl


IF (instruction = CMPSD) OR
   (instruction has operands of type DWORD)
THEN OperandSize := 32;
ELSE OperandSize := 16;
IF AddressSize = 16
   use SI for source-index and DI for destination-index
ELSE (* AddressSize = 32 *)
   use ESI for source-index and EDI for destination-index;
IF byte type of instruction
   [source-index] - [destination-index]; (* byte comparison *)
   IF DF = 0 THEN IncDec := 1 ELSE IncDec := -1; FI;
   IF OperandSize = 16
      [source-index] - [destination-index]; (* word comparison *)
      IF DF = 0 THEN IncDec := 2 ELSE IncDec := -2; FI;
   ELSE (* OperandSize = 32 *)
      [source-index] - [destination-index]; (* dword comparison *)
      IF DF = 0 THEN IncDec := 4 ELSE IncDec := -4; FI;
source-index := source-index + IncDec;
destination-index := destination-index + IncDec;


CMPS compares the byte, word, or doubleword pointed to by the source-index register with the byte, word, or doubleword pointed to by the destination-index register.

If the address-size attribute of this instruction is 16 bits, SI and DI will be used for source- and destination-index registers; otherwise ESI and EDI will be used. Load the correct index values into SI and DI (or ESI and EDI) before executing CMPS.

The comparison is done by subtracting the operand indexed by the destination-index register from the operand indexed by the source-index register.

Note that the direction of subtraction for CMPS is [SI] - [DI] or [ESI] - [EDI]. The left operand (SI or ESI) is the source and the right operand (DI or EDI) is the destination. This is the reverse of the usual Intel convention in which the left operand is the destination and the right operand is the source.

The result of the subtraction is not stored; only the flags reflect the change. The types of the operands determine whether bytes, words, or doublewords are compared. For the first operand (SI or ESI), the DS register is used, unless a segment override byte is present. The second operand (DI or EDI) must be addressable from the ES register; no segment override is possible.

After the comparison is made, both the source-index register and destination-index register are automatically advanced. If the direction flag is 0 (CLD was executed), the registers increment; if the direction flag is 1 (STD was executed), the registers decrement. The registers increment or decrement by 1 if a byte is compared, by 2 if a word is compared, or by 4 if a doubleword is compared.

CMPSB, CMPSW and CMPSD are synonyms for the byte, word, and doubleword CMPS instructions, respectively.

CMPS can be preceded by the REPE or REPNE prefix for block comparison of CX or ECX bytes, words, or doublewords. Refer to the description of the REP instruction for more information on this operation.

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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