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                Device Command Codes (Device Request Header)

       Code           Function

        0    INIT
        1    MEDIA CHECK        (block devices,character = NOP)
        2    BUILD BPB          (block devices,character = NOP)
        3    IOCTL
        4    INPUT              (read)
        5    NONDESTRUCTIVE INPUT NO WAIT (character devices)
        6    INPUT STATUS       (character devices)
        7    INPUT FLUSH        (character devices)
        8    OUTPUT             (write)
        9    OUTPUT             (write with verify)
        10   OUTPUT STATUS      (character devices)
        11   OUTPUT FLUSH       (character devices)
        12   IOCTL OUTPUT
        13   DEVICE OPEN        (DOS 3.x)
        14   DEVICE CLOSE       (DOS 3.x)
        15   REMOVABLE MEDIA    (DOS 3.x)