Difference between revisions of "Master Boot Record"

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Revision as of 18:24, 8 September 2011

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The Master Boot Record (MBR) is the first 512 bytes of a storage device. The MBR is not a partition; it is reserved for the operating system's bootloader and the storage device's partition table. The MBR may eventually be replaced by the GUID Partition Table, which is part of the Unified Extensible Firmware Interface specification.

Boot process

Booting is a multi-stage process. Most PCs today initialize system devices with firmware called the BIOS (Basic Input/Output System), which is typically stored in a dedicated ROM chip on the system board. After system devices have been initialized, the BIOS looks for the bootloader on the MBR of the first recognized storage device (hard disk drive, solid state drive, CD/DVD drive, USB drive...) or the first partition of the device. It then executes that program. The bootloader reads the partition table, and is then capable of loading the operating system(s). Common GNU/Linux bootloaders include GRUB and LILO.

History

The MBR consists of a short piece of assembly code (the initial bootloader – 446 bytes), a partition table for the 4 primary partitions (16 bytes each) and a sentinel (0xAA55).

The "Conventional" Windows/DOS MBR bootloader code will check the partition table for one and only one active partition, read X sectors from this partition and then transfer control to the operating system. The Windows/DOS bootloader can not boot an Arch Linux partition because it is not designed to load the Linux kernel, and it can only cater for an active, primary partition (which GRUB safely ignores).

The GRand Unified Bootloader (GRUB) is the de facto standard bootloader for GNU/Linux, and users are recommended to install it on the MBR to allow booting from any partition, whether it be primary or logical.

Backup and restoration

Because the MBR is located on the disk it can be backed up and later recovered.

To backup the MBR:

dd if=/dev/hda of=/path/mbr-backup bs=512 count=1

Restore the MBR:

dd if=/path/mbr-backup of=/dev/hda bs=512 count=1
Warning: Restoring the MBR with a mismatching partition table will make your data unreadable and nearly impossible to recover. If you simply need to reinstall the bootloader see GRUB or LILO.

To erase the MBR (may be useful if you have to do a full reinstall of another operating system) only the first 446 bits are zeroed because the rest of the data contains the partition table:

dd if=/dev/zero of=/dev/hda bs=446 count=1

Restoring a Windows boot record

Windows by convention (and for ease of installation) is usually installed on the first partition and installs its partition table and reference to its bootloader to the first sector of that partition. If you accidentally install a bootloader like GRUB to the Windows partition or damage the boot record in some other way, you will need to use a utility to repair it. Microsoft includes an boot sector fix utility Template:Codeline and MBR fix utility called Template:Codeline on their recovery CDs or sometimes the install CD. Using this method you can fix the reference on the boot sector of the first partition to the bootloader file and fix the reference on the MBR to the first partition respectively. After doing this you will have to reinstall GRUB to the MBR as is intended to in the first place (the GRUB bootloader can be assigned to load the Windows bootloader when defined to).

If you wish to revert back to using Windows, you can use the Template:Codeline command which chains from the MBR to the boot sector of the first partition to restore you normal automatic loading of the Windows operating system.

Of note, there is a Linux utility called Template:Codeline (package Template:Package AUR in AUR) that can install MBR's. However this utility is only currently capable of writing new MBR's (all OS's and file systems supported) and boot sectors (a.k.a. boot record i.e. equivalent to using Template:Codeline) for FAT file systems. Most LiveCDs don't have this utility by default so it will need to be installed first, or you can look at a rescue CD that does have it like Parted Magic.

First, write the partition info (table) again by:

ms-sys --partition /dev/sda1

Next write a Windows 2000/XP/2003 MBR:

ms-sys --mbr /dev/sda  # Read options for different versions

Then write the new boot sector (boot record)

ms-sys -(1-6)          # Read options to discover the correct FAT record type

Template:Codeline can also write Windows 98, ME, Vista, and 7 MBRs as well, see Template:Codeline.

Resources