Boot Sector (since DOS 2.0)
Offset Size Description 00 3bytes jump to executable code 03 8bytes OEM name and version 0B word bytes per sector 0D byte sectors per cluster (allocation unit size) 0E word number of reserved sectors (starting at 0) 10 byte number of FAT's on disk 11 word number of root directory entries (directory size) 13 word number of total sectors (0 if partition > 32Mb) 15 byte media descriptor byte (see MEDIA DESCRIPTOR) 16 word sectors per FAT 18 word sectors per track (DOS 3.0+) 1A word number of heads (DOS 3.0+) 1C word number of hidden sectors (DOS 3.0+) 20 dword (DOS 4+) number of sectors if offset 13 was 0 24 byte (DOS 4+) physical drive number 25 byte (DOS 4+) reserved 26 byte (DOS 4+) signature byte (29h) 27 dword (DOS 4+) volume serial number 2B 11bytes (DOS 4+) volume label 36 8bytes (DOS 4+) reserved - implementation format not guaranteed in all OEM DOS releases - BIOS expects a boot sector of 512 bytes - DOS 3.2 began reading BIOS Parameter Block (BPB) information from the boot sector, previous versions used only the media byte in FAT - DOS 4.x added offsets 20-3Dh and offset 20h determines the number of sectors if offset 13h is zero - hard disks have a master boot record and partition boot records; the master boot record and Disk Partition Table (DPT) share the same sector