CVF Region: FAT

  The FAT is a variable-length region formatted as a File Allocation Table.

  It is located a variable number of sectors after the end of the
  BootSect region; its location can be calculated as
  MdBpbRec.wResSects+wMdResSects sectors from the start of the CVF.

  It may be formatted as a 12-bit FAT or a 16-bit FAT.

  12-bit: Used for volumes less than 32 MB of uncompressed data (this
          includes compressed diskettes).  A 12-bit FAT uses 1½-bytes-per-
          entry and 0ff7h-0fffh have special meaning, so it maxes out at
          4086 clusters.

  16-bit: Used for volumes greater than 4086 clusters (32 MB).  A 16-bit
          FAT can handle up 65526 16K clusters or 512K of uncompressed data
          (same as the DoubleSpace max volume size).

          See File Allocation Table for details.

  The "cluster numbers" contained in this FAT are actually used as indirect
  pointers into the MDFAT, rather than actual disk locations.

  See Mapping DOS FAT to MDFAT for a full discussion.

  One interesting note: COMMAND.COM accesses both the FAT and the MDFAT when
  you use Dir /c.  In order to obtain the initial FAT entry, it uses the (I
  thought) long-obsolete fn 11H (find first file via FCB).  The advantage of
  this over fn 4eH is that it returns a raw directory entry, and therefore
  supplies a pointer into the FAT.

█▌Virtual 2nd FAT▐█
  Since the beginning of time, all DOS disks have contained two copies
  (presumably with the idea of data integrity and recovery).

  However, DoubleSpace maintains a single FAT for the volume.  Since some
  disk utilities rely on finding the second copy directly after the first,
  DoubleSpace simulates or "virtualizes" the second FAT.  Requests to read
  from those "second FAT" sectors are served up data from the first (only)
  FAT and attempted writes to the second FAT are sent to the "first" FAT.

█▌Standard (FAT) Undelete Operations▐█
  When DOS or a utility writes to the FAT, DoubleSpace compares the old FAT
  to the modified FAT and, based on the changes (and the isUsed bit in the
  MdFatEntryRec), it can infer one of the following operations:

  DOS allocated a cluster:
     isUsed=0 and FAT entry went from 0 to any

  DOS or utility freed a cluster (e.g., a DoubleSpace-unaware defragger):
     isUsed=1 and FAT entry went from any to 0

  Utility resurrected a cluster (e.g. Undelete):
     isUsed=0 and FAT entry went from 0 to any

  Depending upon the inference, DoubleSpace can then take action to rectify
  and propagate the changes through the MDFAT and the BitFAT.

See Also: CVF Layout
          DoubleSpace Overview
          DoubleSpace API
          DOS Functions