 FAT12 used 12-bit entries for the cluster addresses in the FAT; some values were reserved to mark the end of a chain of clusters, to mark unusable areas of the disk, or for other purposes, so the maximum number of clusters was limited to 4078 .To conserve disk space, three 12-bit FAT entries used three consecutive 8-bit bytes on disk, requiring manipulation to unpack the 12-bit values.This was sufficient for the original floppy disk drives and small hard disk up to 32 megabytes.