

                                The Disk Doctor



         FILE ALLOCATION TABLE STRUCTURE

              Files are allocated in clusters, not sectors. A cluster
         is just two contiguous sectors, or 1K in length. The FAT is a
         record of the clusters associated with a particular file; the
         FAT is a one-to-one correspondence with the disk after the
         first two entries; each subsequent entry refers to a cluster
         of the same location on the disk. The FAT and directory size
         is the same on single and double sided floppies.


              On floppies, each 12 bits in the FAT records a cluster
         belong to a particular file - or every 1.5 bytes. 12 bits
         means an entry can range from zero to 4096 ($FFF). On the
         hard disk, each is 16 bits, or a (byte-swapped) integer
         (which allows a range to 65535 or $FFFF).


              If an entry is zero, then the cluster is unused and
         available. If 4081 to 4087 (