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  In DOS version 4, the BPB data structure contains an extra 4-byte field
  that can contain the number of logical sectors on the disk. (Compare
  Figure D-7 with Figure 5-9 on page 111.) As in previous DOS versions,
  DOS version 4 stores the number of logical sectors in the field at offset
  13H and the number of hidden sectors in the field at offset 1CH. If,
  however, the sum of these two values is greater than 65,535, DOS stores a
  0 in the field at offset 13H and uses the additional field at offset 20H
  to record the total number of logical sectors on the disk.

  Offset in      Length         Description
  Boot Sector    (bytes)
  --------------------------------------------------------------------------
  03H            8              System ID
  0BH            2              Number of bytes per sector
  0DH            1              Number of sectors per cluster
  0EH            2              Number of sectors in reserved area
  10H            1              Number of copies of FAT
  11H            2              Number of root directory entries
  13H            2              Total number of sectors
  15H            1              DOS media descriptor
  16H            2              Number of sectors per FAT
  18H            2              Number of sectors per track
  1AH            2              Number of heads (sides)
  1CH            4              Number of hidden sectors
  20H            4              Total number of sectors (if field at offset
                                13H contains 0)
  --------------------------------------------------------------------------

  Figure D-7.  The extended BIOS parameter block in the DOS 4.0 boot sector.