FAT12

FAT12 is a file system from the File Allocation Table family that uses 12-bit cluster addressing. It was designed for small volumes and is most commonly associated with floppy disks and tiny hard drives in early personal computers. As the simplest member of the FAT family, FAT12 provided a compact, widely compatible method for organizing files on limited storage.

FAT12 emerged in the early days of DOS and PC-compatible systems as a solution for volumes too small to justify the later FAT16 design. It saw broad use on 5.25-inch and early 3.5-inch floppy media, as well as small hard drives, before larger volumes shifted toward FAT16 and other formats. Its simplicity contributed to long-term compatibility across many operating systems.

On disk, a FAT12 volume typically contains a boot sector, a reserved area, and two copies of the File Allocation Table, followed by a fixed-size root directory area and then the data region. File data is stored in clusters, and the FAT represents the allocation as 12-bit entries. End-of-chain markers (0xFF8–0xFFF) designate the last cluster of a file. The root directory in FAT12 has a fixed size, with 8.3 filenames being common, though long file names could be supported via optional VFAT extensions on some systems.

Limitations of FAT12 include a relatively small maximum volume size and number of clusters (about 4086 clusters, corresponding to roughly 16 MB with 512-byte sectors and 4 KiB clusters). The format is also limited by its fixed root directory and lack of modern features such as journaling. Long-term use has declined, though FAT12 remains encountered in legacy systems and certain embedded applications.

See also: File Allocation Table, FAT16, FAT32, VFAT, 8.3 filenames.