FAT 12  August 1981
  Filenames A-z0-9_ 

  Used on floppies and drives < 16MiB
    Floppies 1 or 2 sectors/cluster
    Hard drives 8 sectors/cluster

  1 sector for boot record and Bios parameter block.
    Bios parameter block helps OS to understand structure of disk.

  1st DOS
    based on (ripped of from) QDOS - quick and dirty operating system.
    Also modeled after Unix and OS9 (not Mac). 

  12 bit pointers (4096)
    11 clusters (pointers) reserved.
    0 - free cluster.
      Entry 0 in fat contains media descriptor byte.
    1 - unused.
    FF0h-FF6h - reserved (support.microsoft.com)
    FF7h - marks bad cluster.
    FF8h-FFFh mark end of file fat chains.

  8.3 filename.
  Path limit 128 characters (including filename).
  32 sectors for boot directory.
  Approx. 16MB size limit (2^12 pointers * 4K (8 sector) clusters). 
  
  DOS 1.0 
    2 Meg limit.
    No subdirectories.

  Dos 2.0
    Hard drive supported.
    1 partition per drive.
    Possibly no subdirectories.

  DOS 3.0 - 3.2
    1 cluster - up to 16 sectors (8192 bytes) ?
    32 MB or 4087 clusters - uses fat 16.
    1 partition per drive.