File Allocation Table

The File Allocation Table (FAT) is a proprietary file system developed by Microsoft, NCR, SCP, IBM, Compaq, Digital Research, Novell, and Caldera. It was first introduced in 1977 with Microsoft BASIC. Developed for use on floppy disks, it originally supported 8 bits of data per cluster, each of which could be tagged as a bad block in case of problems with the media. It was later updated for use on hard disks and other devices. It is often supported for compatibility reasons by current operating systems for personal computers and many mobile devices and embedded systems, allowing interchange of data between disparate systems.

Updates
Continuing increases in drive capacity required updated variants:
 * FAT12 — 12 bits per cluster, up to 4,068 files
 * FAT16 — 16 bits per cluster, up to 65,460 files
 * FAT32 — 32 bits per cluster, up to 268,173,300 files
 * exFAT — up to 32 MB per cluster, up to 4,294,967,285 files

With Windows NT, FAT32 was replaced by NTFS as the default file system. The Extensible File Allocation Table (exFAT) was intended for removable media.