File Allocation Table (FAT). A data structure employed in the FILESYSTEMS of Microsoft's MS-DOS and Windows operating systems to locate individual files stored on hard and floppy disks. When a disk is FORMATTED it is divided up into many physical SECTORS of equal size grouped together into CLUSTERS. Whenever a new file is created, the file system allocates a number of these clusters to hold that file's data - the FAT is a table that contains the name of each file and the addresses of the clusters that it occupies. When a file is deleted, only its FAT entry, rather than the data itself, is erased, which is why UNDELETE utilities can usually recover the file.
The FAT is stored on the first track of each disk and is read into memory whenever the disk is in use, to be updated and written back whenever the computer is switched off (or one removable disk is swapped for another). Should the FAT become corrupted during this process, then all the files on that disk may be lost.
MS-DOS and versions of Windows prior to WINDOWS98 used 16-bit pointers in their FAT (hence called FAT16) which restricted the size of the largest disk PARTITION they could address to 2 Gigabytes. Windows 98 introduced the FAT32 system, with 32-bit pointers so it can support partitions larger than 2 Gigabytes, which also makes more economical use of disk space by storing the data in more, but smaller, clusters. However FAT16 and FAT32 are incompatible, and may not be deployed on the same hard disk partition.