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CMPS            Compare Strings                      Flags: O D I T S Z A P C
                                                            * - - - * * * * *
CMPS source_string,destination_string

        Logic   CMP [DS:eSI],[ES:eDI]
                if DF = 0
                   eSI . eSI + n        ; n = 1 for byte, 2 for word,
                   eDI . eDI + n        ;     4 for dword (386+)
                else
                   eSI . eSI - n
                   eDI . eDI - n
                endif

    This instruction compares two values by subtracting the
    destination string element pointed to by ES:eDI, from the source
    string element pointed to by DS:eSI, and sets the flags according
    to the results of the comparison. The operands themselves are not
    altered. After the comparison, eSI and eDI are incremented (if the
    direction flag is cleared) or decremented (if the direction flag
    is set), in preparation for comparing the next element of the
    string. If the address-size attribute of CMPS is 16, the SI and DI
    registers will be used as source and destination indices,
    otherwise ESI and EDI.

    The source segment can be changed with a segment override, the
    destination segment cannot.


    Note:       This instruction is always translated by the
                assembler into CMPSB, Compare String Byte, CMPSW,
                Compare String Word, or CMPSD, Compare String Dword,
                depending upon whether source refers to a string of
                bytes, words or doublewords. In either case, you must
                explicitly load the eSI and eDI registers with the
                offset of the source and destination strings.

                CMPSB, CMPSW, and CMPSD are synonyms for the byte,
                word, and doubleword CMPS instructions that do not
                require any operands. They are simpler to use but
                provide no type or segment checking.


    If the REPE or REPNE prefix modifies this instruction, the CPU
    compares the value of the destination string element to the value
    of the source string element. It then steps eSI and eDI in the
    direction indicated by DF by the indicated size, until either the
    REPE/REPNE condition is false or eCX counts to zero.


    Opcode      Format
    A6          CMPS m8,m8              ; = CMPSB
    A7          CMPS m16,m16            ; = CMPSW
    A7          CMPS m32,m32            ; = CMPSD


    Length and timing
    Variations  Bytes   8088    186     286     386     486     Pentium
    cmpsb         1     30      22       8      10       8       5   NP
    cmpsw         1     -       -        -      10       8       5   NP
    cmpsd         1     -       -        -      10       8       5   NP
    repX cmpsb    2     9+30n   5+22n   5+9n    5+9n    7+7n*   9+4n NP
    repX cmpsw    2     9+30n   5+22n   5+9n    5+9n    7+7n*   9+4n NP
    repX cmpsd    2      -       -       -      5+9n    7+7n*   9+4n NP

    repX = repe, repz, repne, or repnz
       * : 5 if n = 0