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)

Content

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 BOOTROM will boot from the disk. To disable it, you have to wipe the signature bytes. You have the read the whole sector (512 bytes) and change only the signature and read back the sector.

A code example:

help requested

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More information about the IBM PC bootsector is at

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Microsoft have documented the FAT boot-sector format in their Knowledge-base article KB-140418 Version 3 Dated December 6, 2003 [1]

Back to BootSector, which is also SpinOff