STD             Set Direction Flag                   Flags: O D I T S Z A P C
                                                            - 1 - - - - - - -
STD

        Logic   DF ← 1                  ; Auto-decrement eSI, eDI

    STD sets the direction flag. After STD is executed, string
    operations will decrement the index registers that they use
    (SI and/or DI; ESI and/or EDI).


    Opcode      Format
    FD          STD


    Length and timing
    Operands    Bytes   8088    186     286     386     486     Pentium
    -            1       2       2       2       2       2       2   NP