File Allocation Table

File Allocation Table (FAT) is the name of a computer file system architecture and a family of industry standard file systems utilizing it.

The FAT file system is a legacy file system which is simple and robust. It offers good performance even in light-weight implementations, but cannot deliver the same performance, reliability and scalability as some modern file systems. It is however supported for compatibility reasons by virtually all existing operating systems for personal computers, and thus is a well-suited format for data exchange between computers and devices of almost any type and age from the early 1980s up to the present.

Originally designed in the late 1970s for use on floppy disks, it was soon adapted and used almost universally on hard disks throughout the DOS and Windows 9x eras for two decades. With the introduction of more powerful computers and operating systems, and the development of more complex filesystems for them, it is no longer the default filesystem for usage on hard drives by most modern desktop operating systems.

Today, FAT file systems are still commonly found on floppy disks, solid-state memory cards, flash memory cards, and on many portable and embedded devices. It is also utilized in the boot stage of EFI-compliant computers.

The name of the file system originates from the file system's prominent usage of an index table, the FAT, statically allocated at the time of formatting. The table contains entries for each cluster, a contiguous area of disk storage. Each entry contains either the number of the next cluster in the file, or else a marker indicating end of file, unused disk space, or special reserved areas of the disk. The root directory of the disk contains the number of the first cluster of each file in that directory; the operating system can then traverse the FAT table, looking up the cluster number of each successive part of the disk file as a cluster chain until the end of the file is reached. In much the same way, sub-directories are implemented as special files containing the directory entries of their respective files.

As disk drives have evolved, the maximum number of clusters has significantly increased, and so the number of bits used to identify each cluster has grown. The successive major versions of the FAT format are named after the number of table element bits: 12 (FAT12), 16 (FAT16), and 32 (FAT32). Each of these variants is still in use. The FAT standard has also been expanded in other ways while generally preserving backward compatibility with existing software.