File Allocation Table
FAT
FAT32

    (FAT) The component of an MS-DOS or Windows
   95 file system which describes the files, directories,
   and free space on a hard disk or floppy disk.

   A disk is divided into partitions.  Under the FAT file
   system each partition is divided into clusters, each of
   which can be one or more sectors, depending on the size of
   the partition.  Each cluster is either allocated to a file or
   directory or it is free (unused).  A directory lists the name,
   size, modification time and starting cluster of each file or
   subdirectory it contains.

   At the start of the partition is a table (the FAT) with one
   entry for each cluster.  Each entry gives the number of the
   next cluster in the same file or a special value for "not
   allocated" or a special value for "this is the last cluster in
   the chain".  The first few clusters after the FAT contain the
   root directory.

   The FAT file system was originally created for the CP/M[?]
   operating system where files were catalogued using 8-bit
   addressing.  MS DOS's FAT allows only 8.3 filenames.

   With the introduction of MS-DOS 4 an incompatible 16-bit FAT
   (FAT16) with 32-kilobyte clusters was introduced that
   allowed partitions of up to 2 gigabytes.

   Microsoft later created FAT32 to support partitions larger
   than two gigabytes and pathnames greater that 256
   characters.  It also allows more efficient use of disk space
   since clusters are four kilobytes rather than 32 kilobytes.
   FAT32 was first available in OEM Service Release 2 of
   Windows 95 in 1996.  It is not fully backward compatible
   with the 16-bit and 8-bit FATs.

   <IDG article>.
   <http://home.c2i.net/tkjoerne/os/fat.htm>.
   <http://teleport.com/~brainy/>.
   <http://209.67.75.168/hardware/fatgen.htm>.
   <http://support.microsoft.com/support/kb/articles/q154/9/97.asp>.

   Compare: NTFS.

   [How big is a FAT?  Is the term used outside MS DOS?  How long
   is a FAT16 filename?]

   (2000-02-05)