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        DOS Drive Parameter Block (DDPB)

        Offset Size   Contens
        00     Byte   Drive number (0 = A, etc.)
        01     Byte   Unit number within device driver
        02-03  Word   Number of bytes per sector
        04     Byte   Largest sector number in cluster (add one for number
                      of sectors/cluster)
        05     Byte   Log base two of the cluster size
        06     Word   Number of reserved (boot) sectors
        08     Byte   Number of FAT-copies
        09     Word   Number of root directory entries
        0B     Word   First data sector on medium
        0D     Word   Largest possible cluster number (Subtract one for
                      number of data clusters)

        From this point the format differs:
        DOS 3.x
        0F     Byte   Number of sectors in one FAT copy
        10     Word   First sector of root directory
        12     DWord  Address of device driver for this drive
        16     Byte   Media descriptor byte for medium
        17     Byte   Drive parameter block valid (0FFh indicates block must
                      be rebuilt, 00h indicates block accessed)
        18     DWord  Address of next device block in list (offset=FFFFh,
                      if last)
        1C     Word   Cluster at which to start search for free space
                      when writing

        DOS 4.x
        0F     Word   Number of sectors in one FAT copy
        11     Word   First sector of root directory
        13     DWord  Address of device driver for this drive
        17     Byte   Media descriptor byte for medium
        18     Byte   FFh = block must be rebuilt, 00h indicates block accessed
        19     DWord  Address of next device block, offset = FFFFh indicates last
        1D     Word   Cluster at which to start search for free space when writing
        1Fh     WORD    number of free clusters on drive, FFFFh = unknown
.