Bootsector of the IBM PC
The bootsector of the IBM PC and compatibles is also the partition table of the disk. It is also called Master Boot Record (MBR)
The MBR is 512 bytes large and is the first sector of a disk.
|0||440 bytes||Executable code|
|440||4 bytes||Disk MBR signature|
|446||64 bytes||Partition table|
|510||2 bytes||Boot signature (55 AA)|
When the signature is valid, the computer's BIOS will boot from the disk. However, many operating systems will not recognize partition tables without signatures. As such, the best way to bypass a disk is to disable it from the BIOS boot sequence. If that is not possible, many BIOSes will bypass the disk if the two first bytes in the MBR are:
INT 18h instruction), which should cause a properly compliant BIOS to start the next bootable device.
More information about the IBM PC bootsector is at
- some links
Microsoft have documented the FAT boot-sector format in KB-140418 Version 3 Dated December 6, 2003.