Fat (File Access Table
FAT is an acronym for file access (or allocation) table. This is a table that the operating system of a computer uses to locate files on a disk.
The FAT was designed and developed by Bill Gates. Gates developed the FAT for a version of Basic that could store programs and data on floppy disks. The design was incorporated in an early version of the operating system for the Intel 8086 chip. Gates purchased the rights to this operating system and rewrote it to create the first version of DOS. The subsequent ramifications to computing and to Gate's success have been enormous.
A file may not be stored contiguously. Due to fragmentation, a file may be divided into many sections that are scattered around the disk. It is the task of the FAT to keep track of all the information. FATs themselves are files, called FAT files. These are maintained on a hard disk and not typically apparent to the user, in the first 63 sectors of a bootable disk.
As a new file is created in a computer's directory, the operating system creates a FAT entry for the new file. The FAT file records where each cluster of information in the newly created information file is located and records their sequential order. Each cluster of information has a FAT entry that describes its current usage, availability, whether the cluster is reserved or assigned to a file, and whether the cluster has become unusable due to a surface defect. The FAT file also operates in reverse, when a file is read. Then, the operating system locates the file information from their various clusters and accurately reassembles them to produce a readable file.
Originally, the file size that was capable of being accurately handled by a FAT was 128 megabytes. Now, however, some FAT configurations can support a file two terabytes in size (a terabyte is approximately one trillion bytes).
FAT file systems have proven to be robust and reliable. For example, they do not lose data if a computer crashes during an update of a file. A utility called CHKDSK or SCANDISK examines the FAT to determine the staus of every record on the disk. If an unallocated sector is found, the user is prompted as to whether the sector should be turned into a file. Additionally, DOS is supplied with a program known as DEFRAG. This program can reorganize the directory, files and free space, which serves to maximize performance.
A FAT malfunction is termed a disk error. Sometimes, the error is correctable without having to reformat and begin again.
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