Master boat record

the term master boat record (MBR) designates into the x86-Architektur the first data block (512 byte), of a boatable storage medium divided in partitions, as for example a non removable disk. The MBR contains a partition table, those thoseAllocation of the data medium describes and a boat Loader, a program, which starts an operating system on one of the partitions.

Storage media, which are not divided into partitions, z. B. Disks or CDs, do not contain MBR. Here the first data block becomes as boat sectoror also boat record marks.

Since it contains a partition table, a MBR has the characteristics of a partition sector.

A deletion or a defect at the MBR leads to the fact that the computer cannot do no more boats and/or. the partition wrongly recognizesand in the consequence the operating system or data any longer does not find.

Table of contents

historical development

first storage media were not divided into partitions and contained at position 0:0: 1 (head 0, trace 0, sector 1) the boat sector. This convention should not with non removable disks with several operating systemsare hurt. Therefore the partition table at the end of the boat sector was platziert. The function of the boat Loaders was supplemented in such a way that it evaluates the partition table.

structure of the MBR

address function/contents of size of
hex December
0x0000 0 boat Loader max. 440
0x01B8 440 disk signature
(since Windows 2000)
0x01BC 444 zero
0x01BE 446 partition table 64
0x01FE 510 55 hex MBR signature
0x01FF 511 AA hex

boat Loader

this is a small program, of BIOS is called. Under normal conditions this looks for active partition in the partition table for one „“, loads the boat sector of the first active partition and implements this. Thus the actual operating system is then loaded.

Also boat managers do not exist themselves,by this convention hold, and instead of whose a selection menu or a like indicates, and so z. B. boats of arbitrary partitions permit.

A missing or defective boat Loader leads to it that by the non removable disk at all cannot be gebootet, even ifin one of the partitions an operating system with intact boat sector is present.

see also to boat Loader

disk signature

by Windows 2000 and XP one uses, in order to identify non removable disks. Situations, in which several MBRs with identical signatures exist, should avoidedbecome, since these systems react possibly incorrectly to it.dd if=/dev/null of=/dev/hd [old] bs=1 count=4 seek=440. old> =b recommended.

partition table

indicates, which partitions on the medium are present. A deleted or defective partition table not possibly makes it, on thoseTo access data of the non removable disk. A reconstruction and/or. Repair of the partition table is aufwändig and often even impossible.

see major item partition table

to MBR signature

this signature also Magic NUMBER one calls and consists of the 2 byte 55 hex and AAhex. On Little Endian - systems this is interpreted as 16-Bit-Zahl AA55 hex. If the signature is present, then the BIOS assumes a valid MBR is present. If the signature is not found, the BIOS assumes a new and/or.deleted data medium. The boat procedure is then broken off and an error message, about “Non system is spent or Non Bootable disk “.< ref name= " MSkb149877 "> Microsoft Knowledge cousin - article 149877: MBR signature AA55 not found< /ref>

A correct signature is natural no warranty for valid boat codein the MBR. It serves only to prevent that empty MBRs or MBRs with coincidence data is implemented.

Also with data media, which are used not to boats, a missing signature can have effects. There are BIOS versions, the data media without valid signatureoperate in a slower mode. Before the speed measurement (bench mark) e.g. to initialize first the MBR of the plate concerned. ref name= " cthotline19981911< " is recommended to a new non removable disk> c't Hotline:Wondrous disk acceleration. out No. c't. 19/1998 S. 218< /ref>

MBR more differentlyOperating systems

each operating system provides and/or. supplemented with the installation according to standard the MBR, so that it can be loaded.

DOS and DOS-based Windows versions

under DOS one can regard the MBR with partitionierten non removable disks with debug. If thoseNon removable disk partitioniert (divided in e.g. C: , D: ,….) is, sees one with debug only the boat records of the respective logical partition.

MS-DOS and the Windowsvarianten developing on DOS (thus Windows 95 to Windows ME) overwrite the boat Loader in the MBR with the installationwithout any consideration for its past contents.

In case of an error in the MBR /mbr “ a new standard master boat record for in each case these operating systems can be written under MS-DOS and Windows 9x with the instruction “fdisk. The MBR becomes completeoverwritten (with exception of the partition table), whereby boat viruses are destroyed.< ref name= " MSkb69013 "> Microsoft Knowledge cousin - article 69013: FDISK /MBR overwrites master boat record< /ref> However also a possibly existing boat manager is destroyed.

Windows versions based on Windows NT

the Microsoft Windows versionsthe NT line (z. B.NT 4, W2K and XP) overwrite the boat Loader (the first 446 bytes of the MBR) with each new installation also, consider however still existing information about another operating system from the house Microsoft, thus MS-DOSand other Windowsversionen. In this case a small boat manager with option between the different installed Microsoft operating systems is then activated.

For W2K and XP there is the re-establishment console, here serves the instructions “fixmbr “and” fixed boat “for the repairthe MBR.


of operating systems such as Linux install usually on request „intelligent “Bootmanger, the also stranger of operating systems (z for one. B. Ms-Windowsversionen) takes up (see also boat Loader).

With the program install mbr, e.g. in the Linux distribution Knoppix is contained,a functional boat Loader can be written into the MBR.

Under Linux one can normally write the MBR by means of “dd if=/dev/hda of= to file bs=512 count=1 “into a file. Restore to: “dd of=/dev/hda if= file bs=512

does not standappropriate backup file for the order, can be written the MBR by means of the Bootloaders LILO with the following instruction again and “brightly”: lilo - M /dev/hda - s /dev/null



See also
Wikibooks: Loader - learning and teaching materials

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