Bootsector of the IBM PC

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The boot sector of the IBM PC and compatibles may also contain the partition table of the storage device if it has been partitioned. It is also called Master Boot Record (MBR).


The MBR is 512 bytes large and is the first sector of a disk.

Start offset Length Contents
0 440 bytes Executable code
440 4 bytes Disk MBR signature
444 2 bytes Unused
446 64 bytes Partition table
510 2 bytes Boot signature (55 AA)

Disable booting

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:

 CD 18

(an INT 18h instruction), which should cause a properly compliant BIOS to start the next bootable device.

See also

More information about the IBM PC bootsector is at

Microsoft have documented the FAT boot-sector format in KB-140418 Version 3 Dated December 6, 2003.

Back to BootSector, which is also SpinOff