File Allocation Table (FAT) is a file system developed by Microsoft for MS-DOS and is the primary file system for consumer versions of Microsoft Windows up to and including Windows Me. FAT as it applies to flexible/floppy and optical disc cartridges (FAT12 and FAT16 without long filename support) has been standardized as ECMA-107 and ISO/IEC 9293. The file system is partially patented.

The FAT file system is relatively uncomplicated, and is supported by virtually all existing operating systems for personal computers. This ubiquity makes it an ideal format for floppy disks and solid-state memory cards, and a convenient way of sharing data between disparate operating systems installed on the same computer.
 

FAT12

This initial version of FAT is now referred to as FAT12. Designed as a file system for floppy diskettes, it had the following limitations: Cluster addresses were only 12 bits long, which not only limited the cluster count to 4078, but made FAT manipulation a bit tricky with the PC's 8-bit or 16-bit registers. (Under Linux, FAT12 is limited to 4084 clusters.) The disk size was stored as a 16-bit count of sectors, which limited the size to 32 MiB. FAT12 was used by several manufacturers with different physical formats but a typical floppy diskette at the time was 5.25-inch, single-sided, 40 tracks, with 8 sectors per track, resulting in a capacity of only 160 KiB for both the system areas and files. The FAT12 limitations exceeded this capacity by one or more orders of magnitude. The limits were successively lifted in the following years which increased storage capacity dramatically but eventually rendered FAT12 obsolete.
 

FAT16

In November 1987, Compaq DOS 3.31 introduced what is today called the FAT16 format, with the expansion of the 16-bit disk sector index to 32 bits. The result was initially called the DOS 3.31 Large File System. Although the on-disk changes were apparently minor, the entire DOS disk code had to be converted to use 32-bit sector numbers, a task complicated by the fact that it was written in 16-bit assembly language.

The limit on partition size was now dictated by the 8-bit signed count of sectors-per-cluster, which had a maximum power-of-two value of 64. With the usual hard disk sector size of 512 bytes, this gives 32 KiB clusters, thereby fixing the "definitive" limit for the FAT16 partition size at 2 gibibytes. On magneto-optical media, which can have 1 or 2 KiB sectors, the limit is proportionally greater.
 

Long File Names (VFAT, LFNs)

One of the "user experience" goals for the designers of Windows 95 was the ability to use long filenames (LFNs—up to 255 UTF-16 code points long), in addition to classic 8.3 filenames. LFNs were implemented using a work-around in the way directory entries are laid out. The version of the file system with this extension is usually known as VFAT after the Windows 95 VxD device driver, also known as "Virtual FAT" in Microsoft's old document.

FAT32

In order to overcome the volume size limit of FAT16, while still allowing DOS real-mode code to handle the format without unnecessarily reducing the available conventional memory, Microsoft decided to implement a newer generation of FAT, known as FAT32, with cluster values held in a 32-bit field, of which 28 bits are used to hold the cluster number, for a maximum of approximately 250 million (228) clusters. This would allow for drive sizes of up to 8 tebibytes with 32KiB clusters, but the boot sector uses a 32-bit field for the sector count, limiting volume size to 2TiB on a hard disk with 512 byte sectors.

On Windows 95/98, due to the version of Microsoft's ScanDisk utility included with these operating systems being a 16-bit application, the FAT structure is not allowed to grow beyond around 4 million (< 222) clusters, placing the volume limit at 127.53 gibibytes. A limitation in original versions of Windows 98/98SE's Fdisk causes it to incorrectly report disk sizes over 64GiB. A corrected version is available from Microsoft. These limitations do not apply to Windows 2000/XP except during Setup, in which there is a 32GiB limit. Windows Me supports the FAT32 file system without any limits.