The boot sector

The ``boot sector'' occupies the first sector of a FAT12 and FAT16 versions of a dosfs filesystems, and the first two sectors of a FAT32 version dosfs filesystem. (a ``sector'' is the unit by which disk storage is read and written). The boot sector contains the BPB (BIOS Parameter Block) and the bootstrap routine. The BPB specifies the following parameters for the filesystem, in order:

• total sectors in the filesystem

• bytes per sector

• sectors per cluster (a cluster is the minimum unit of allocation in a DOS filesystem)

• reserved sectors

• number of copies of the FAT (File Allocation Table)

• sectors per copy of the FAT

• total number or root directory entries

• the media descriptor byte

• sectors per track

• number of heads

The last two bytes of the boot sector are always 0xaa55 and serve as the boot sector signature.