Difference between revisions of "Bootsector of the IBM PC"

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(Disable booting)
(I'm pretty sure partition table is optional; clean up the See also section)
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The bootsector of the IBM PC and compatibles is also the partition table of the disk.
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The [[BootSector|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)
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It is also called Master Boot Record (MBR).
  
 
== Content ==
 
== Content ==
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(an <code>INT 18h</code> instruction), which should cause a properly compliant BIOS to start the next bootable device.
 
(an <code>INT 18h</code> instruction), which should cause a properly compliant BIOS to start the next bootable device.
  
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== See also ==
 
More information about the IBM PC bootsector is at
 
More information about the IBM PC bootsector is at
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* [http://en.wikipedia.org/wiki/Master_boot_record '''Master boot record''' on Wikipedia]
 
* http://www.geocities.com/thestarman3/asm/mbr/95BMEMBR.htm
 
* http://www.geocities.com/thestarman3/asm/mbr/95BMEMBR.htm
 
* http://bootmaster.filerecovery.biz/appnote4.html
 
* http://bootmaster.filerecovery.biz/appnote4.html
* '''http://en.wikipedia.org/wiki/Master_boot_record'''
 
 
* add
 
* add
 
* some links
 
* some links

Revision as of 00:05, 23 March 2010

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).

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 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