Opcode Instruction Clocks Description A6 CMPS m8,m8 10 Compare bytes ES:[(E)DI] (second operand) with [(E)SI] (first operand) A7 CMPS m16,m16 10 Compare words ES:[(E)DI] (second operand) with [(E)SI] (first operand) A7 CMPS m32,m32 10 Compare dwords ES:[(E)DI] (second operand) with [(E)SI] (first operand) A6 CMPSB 10 Compare bytes ES:[(E)DI] with DS:[SI] A7 CMPSW 10 Compare words ES:[(E)DI] with DS:[SI] A7 CMPSD 10 Compare dwords ES:[(E)DI] with DS:[SI]
IF (instruction = CMPSD) OR (instruction has operands of type DWORD) THEN OperandSize := 32; ELSE OperandSize := 16; FI; IF AddressSize = 16 THEN use SI for source-index and DI for destination-index ELSE (* AddressSize = 32 *) use ESI for source-index and EDI for destination-index; FI; IF byte type of instruction THEN [source-index] - [destination-index]; (* byte comparison *) IF DF = 0 THEN IncDec := 1 ELSE IncDec := -1; FI; ELSE IF OperandSize = 16 THEN [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; FI; FI; source-index := source-index + IncDec; destination-index := destination-index + IncDec;
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.
Chapter 17 -- 80386 Instruction Set
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