text: FAT is the index of data storage in any disc (floppy/hard disk or CD ROM. The index or the details of file name , length, date time are the data visible to us. the clusters used or the location of data's are also written which is machine language, not visible to us. The FAT system was formerly 12 bit, then 16 bit and now in  32 bit. XP (NTFS) is having 32 bit and 64 bit format.