File Allocation Table

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Related to FAT filesystem: file allocation table, ExFAT, NTFS

file allocation table

[‚fīl ‚al·ə′kā·shən ‚tā·bəl]
(computer science)
A table stored on hard or removable disks used to locate files or sections of files if scattered about the disk. Abbreviated FAT.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

File Allocation Table

(file system)
(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/Moperating 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.

Compare: NTFS.

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(File Allocation Table) The file system used for compatibility. Developed for floppy disks in the late 1970s, FAT was the primary file system for hard drives in DOS and Windows prior to NTFS. FAT is also used for digital camera storage. See file system and NTFS.

Today, FAT32 and exFAT are widely used for USB drives as well as external storage drives because they are supported by all major platforms, including Windows, Mac, Linux and mobile devices. An external drive formatted as FAT is interchangeable between Windows PCs and Macs.

FAT Directory and Table
The FAT directory maintains a list of file names and dates, and the FAT table has an entry for each drive cluster. When a drive is high-level formatted, the FAT table is recorded twice.

The directory points to an entry in the table where the file starts. If the file is larger than one cluster, the first table entry points to the next entry and so on until the end of file (see below). See FAT32 and cluster.

The FAT Table
The file RESUME.DOC is stored in clusters 0, 2, 3 and 7. The directory entry points to cluster 0 where the file begins. The entry for cluster 0 points to cluster 2 and so on. BUDGET.XLS is stored in clusters 1, 4, 8 and 9. If a cluster becomes damaged, it is marked and never used again.
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