The CMPS instruction 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 of 16-bits, the SI and DI registers will be used for source- and destination-index registers; otherwise the ESI and EDI registers will be used. Load the correct index values into the SI and DI (or ESI and EDI) registers before running the CMPS instruction.
The comparison is done by subtracting the operand indexed by the destination-index register from the operand indexed by the the source-index register.
Note that the direction of subtraction for the CMPS instruction 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 DF flag is 0 (a CLD instruction was run), the registers increment; if the DF flag is 1 (an STD instruction was run), 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.
The CMPSB, CMPSW and CMPSD instructions are synonyms for the byte, word, and doubleword CMPS instructions, respectively.
The CMPS instruction 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.