 The first byte of the relocated FAT in logical sector 0 was used to determine the disk's capacity.640 KB boot disks began with a miniature ADFS file system containing the boot loader, followed by a single FAT.Also, the 640 KB format differed by using physical CHS sector numbers starting with 0 (not 1, as common) and incrementing sectors in the order sector-track-head (not sector-head-track, as common).