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                        Device Request Header Format


      Offset Size             Description

        00   byte       length in bytes of the request header
        01   byte       unit code;  the sub-unit the operation is for (block
                        devices);  meaningless for character devices
        02   word       command code, (see DEVICE COMMANDS)
        04  8bytes      reserved for DOS
        0C  nbytes      request data  (variable length)


        - see  INT 21,44  or        DEVICE ATTRIBUTES  or  DEVICE CODES
        - see also  IOCTL,n where "n" is an IOCTL function