(FAT) The component of an MS-DOS
or Windows 95 file system
which describes the files
, 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
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://idg.net/idgframes/english/content.cgi?vc=docid_9-62525.html). (http://home.c2i.net/tkjoerne/os/fat.htm). (http://teleport.com/~brainy/). (http://22.214.171.124/hardware/fatgen.htm). (http://support.microsoft.com/support/kb/articles/q154/9/97.asp).
[How big is a FAT? Is the term used outside MS DOS? How long is a FAT16 filename?]