File Allocation Table

(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 {PC-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.

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