Master Boot Record
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The Master Boot Record (MBR) is the first sector (first 512 bytes) of a storage device. The MBR is not a partition and it is reserved for the operating systems bootloader and the storage device's partition table.
Booting is a multi-stage process. Most PCs today initialize system devices by firmware called the BIOS (Basic Input/Output System). The BIOS is typically contained within the CMOS (Ceramic Metal Oxide Semiconductor) that is executed upon system power-up. After system devices have been initialized, the BIOS then 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 exectutes 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.
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
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 hence installs its bootloader and partition table on 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 reinstall it. Microsoft includes an MBR fix utility called Template:Codeline on their recovery CDs or sometimes the install CD. Using this method will repair the MBR (i.e. reinstall the Windows bootloader and add again the partition table). After doing this you will have to reinstall GRUB---to the MBR, as you should have in the first place, not to the Windows partition. The GRUB bootloader can start the Windows bootloader when you ask it to.
If by chance you do not have a Windows installation/recovery CD, then you can reinstall the Windows bootloader again from GNU/Linux with a utility called Template:Package AUR. First you will need to boot from a LiveCD and then call Template:Codeline. 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
Then to write a Windows 2000/XP/2003 MBR:
ms-sys --mbr /dev/sda