(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://www.idg.net/idgframes/english/content.cgi?vc=docid_9-62525.html)}. {(http://home.c2i.net/tkjoerne/os/fat.htm)}. {(http://www.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)