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The file system uses an index table stored on the device to identify chains of data storage areas associated with a file, the ''[[Design of the FAT file system#FAT|File Allocation Table]]'' (''FAT''). The FAT is statically allocated at the time of formatting. The table is a [[linked list]] of entries for each ''[[cluster (file system)|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 the end of the 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, 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. ''Sub-directories'' are implemented as special files containing the ''directory entries'' of their respective files.
Each entry in the FAT linked list is a fixed number of bits: 12, 16 or 32. The maximum size of a file or a disk drive that can be accessed is the product of the largest number that can be stored in the entries (less a few values reserved to indicate unallocated space or the end of a list) and the size of the disk cluster. Even if only one byte of storage is needed to extend a file, an entire cluster must be allocated to it.
Originally designed as an 8-bit file system, the maximum number of clusters must increase as disk drive capacity increases, and so the number of bits used to identify each cluster has grown. The successive major variants of the FAT format are named after the number of table element bits: 12 ([[#FAT12|FAT12]]), 16 ([[#FAT16|FAT16]]), and 32 ([[#FAT32|FAT32]]).
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