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Definition: FAT


(File Allocation Table) The file system used for compatibility. Developed for floppy disks in the late 1970s, FAT was the primary file system for hard drives in DOS and Windows prior to NTFS. FAT is also used for digital camera storage. See file system and NTFS.

Today, FAT32 and exFAT are widely used for USB drives as well as external storage drives because they are supported by all major platforms, including Windows, Mac, Linux and mobile devices. An external drive formatted as FAT is interchangeable between Windows PCs and Macs.

FAT Directory and Table
The FAT directory maintains a list of file names and dates, and the FAT table has an entry for each drive cluster. When a drive is high-level formatted, the FAT table is recorded twice.

The directory points to an entry in the table where the file starts. If the file is larger than one cluster, the first table entry points to the next entry and so on until the end of file (see below). See FAT32 and cluster.




The FAT Table
The file RESUME.DOC is stored in clusters 0, 2, 3 and 7. The directory entry points to cluster 0 where the file begins. The entry for cluster 0 points to cluster 2 and so on. BUDGET.XLS is stored in clusters 1, 4, 8 and 9. If a cluster becomes damaged, it is marked and never used again.





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