A FAT is a table that an OS maintains on a hard disk used to map the clusters (the basic units of logical storage on a hard disk) in which a file has been stored. Windows file allocation table entries were originally 16 bits in length, limiting hard disk size to 128 megabytes, assuming a 2,048-size cluster. Now, with 32-bit FAT entry (FAT32) support in Windows 95 and later, the largest size hard disk that can be supported is 2 terabytes.