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File Allocation Table File System (FAT, FAT12, FAT16)

The most common file system in the PC world is actually a family of file systems. The basic name for this file system is FAT; the name comes from one of the main logical structures that the file system uses: the file allocation table. This file system is the one that was used by DOS on the first IBM PCs, and it became the standard for the PCs that followed. Today, most PCs still use a variant of the basic FAT file system created for those early machines over two decades ago.

The most common name of the file system, "FAT", is problematic, even though it is still often used. The first FAT file system used 12-bit file allocation tables; this was later expanded to 16 bits, and became the most common file system implementation for hard disks from the late 1980s to the late 1990s. To distinguish these versions of FAT from the 32-bit successor called FAT32, the older FAT variants are now sometimes called FAT12 or FAT16. However, you will still hear just "FAT" used a lot; if so, you need to find out what specifically is being referred to, if it matters in that particular context. For more elaboration on the differences between FAT12, FAT16 and FAT32, see this page.

While designed specifically for MS-DOS and used by all of the Microsoft operating systems that followed (in one way or another) FAT's ubiquitousness has caused support for it to become an important characteristic for other operating systems as well. Virtually all PC operating systems can support the basic FAT file system, even ones that bear little resemblance to Microsoft DOS or Windows. FAT12 and FAT16 partitions are the "least common denominator" of file systems--while they are limited in many ways, they are the easiest file systems to use when compatibility is a concern. Even non-PC platforms can in many cases read disks formatted in the FAT file system.

Note: Most of the other subsections of the Hard Disk Logical Structures and File Systems section describe the structures and operation of the FAT file system family in more detail, so I will not get into a larger discussion of FAT here. Please refer to the other sections for details on how FAT works.

Next: Virtual FAT (VFAT)

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