Difference between revisions of "Bootsector of the IBM PC"
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| − | The | + | 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. | |
| + | In such case, it is also called Master Boot Record ([[Mbr|MBR]]). | ||
== Content == | == Content == | ||
| − | The MBR is 512 bytes large and is the first sector of a disk. | + | The MBR is 512 bytes large and is located in the first sector of a disk. |
{| class="wikitable" | {| class="wikitable" | ||
| Line 23: | Line 24: | ||
== Disable booting == | == Disable booting == | ||
| − | When the signature is valid, the computer's | + | 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 <code>INT 18h</code> instruction), | |
| − | + | which should cause a properly-compliant BIOS to start the next bootable device. | |
| − | + | ||
| − | + | ||
| − | + | ||
| − | + | ||
| − | + | == See also == | |
| − | More information about the IBM PC bootsector | + | More information about the IBM PC bootsector: |
| − | * | + | * [http://en.wikipedia.org/wiki/Master_boot_record '''Master boot record''' on Wikipedia] |
| − | * | + | * [http://en.wikipedia.org/wiki/Boot_sector '''Boot sector''' on Wikipedia] |
| − | * | + | * [http://en.wikipedia.org/wiki/Design_of_the_FAT_file_system Design of the FAT file system on Wikipedia] |
| + | * [http://thestarman.pcministry.com/asm/mbr/95BMEMBR.htm An Examination of the MBR] | ||
| + | * [http://web.archive.org/web/20120826052224/bootmaster.filerecovery.biz/appnote4.html <!-- | ||
| + | -->Another examination of the MBR], similar to the above (archived) | ||
| − | + | Microsoft documented the FAT boot-sector format in <!-- | |
| + | --><!-- | ||
| + | --><!-- --><!-- | ||
| + | --><!-- alternative and/or older links | ||
| + | --><!-- http://support.microsoft.com/?scid=kb%3Ben-us%3B140418&x=11&y=9 | ||
| + | --><!-- http://support.microsoft.com/kb/140418 | ||
| + | --><!-- https://support.microsoft.com/en-us/help/140418 | ||
| + | --><!-- https://jeffpar.github.io/kbarchive/kb/140/Q140418/ | ||
| + | --><!-- https://social.technet.microsoft.com/wiki/contents/articles/6771.the-fat-file-system.aspx#MS_KB_Articles | ||
| + | --><!-- --><!-- | ||
| + | --><!-- | ||
| + | --> | ||
| + | [http://web.archive.org/web/20101129212356/http://support.microsoft.com/kb/140418 KB-140418] | ||
| + | Version 3 Dated December 6, 2003. | ||
Latest revision as of 00:54, 12 September 2020
The boot sector of the IBM PC and compatibles may also contain the partition table of the storage device if it has been partitioned. In such case, it is also called Master Boot Record (MBR).
Content
The MBR is 512 bytes large and is located in 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:
- Master boot record on Wikipedia
- Boot sector on Wikipedia
- Design of the FAT file system on Wikipedia
- An Examination of the MBR
- Another examination of the MBR, similar to the above (archived)
Microsoft documented the FAT boot-sector format in KB-140418 Version 3 Dated December 6, 2003.
