Master Boot Record
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Most PCs today use firmware called the BIOS (Basic Input/Output System). The BIOS is typically contained within a CMOS (Ceramic Metal Oxide Semiconductor) and executes upon system power-up, proceeding to initialize system devices. After system devices have been initialized, the BIOS then launches a program on the Master Boot Record (MBR). The MBR is the first sector (first 512 bytes) of a storage device (hard disk drive, solid state drive, CD/DVD drive, USB drive...). The BIOS launches the program on the MBR of the first recognized BIOS boot device on your computer. This program (also known as a bootloader) reads the partition table (also contained within the MBR) and eventually boots the operating system(s). Common GNU/Linux bootloaders include GRUB and LILO.
A brief overview of the boot process
Booting is a multi-stage process. The MBR is the first 512-byte sector on any hard drive -- it is not a partition.
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 'original' Windows (or rather MS/DOS) bootloader will check the partition table for one and only one 'active' partition and then read X sectors from this partition and then transfer control to whatever is found. This is why the initial DOS-loader will not be able to boot an Arch Linux partition; it can only cater for an active, primary partition.
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 facilitate booting from any partition, whether it is 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):
dd if=/dev/zero of=/dev/hda bs=446 count=1