Each hard-drive (or Cd or other drive or device) has only 1 Master Boot Record. It is in a fixed position on the hard-drive. The MBR is the sector at cylinder 0, head 0, sector 1 of any hard disk or diskette that identifies how and where an operating system is located so that it can be boot (loaded) into the computer's main storage or random access memory.
The Master Boot Record is also sometimes called the "partition sector" or the "master partition table" because it includes a table that locates each partition that the hard disk has been formatted into. In addition to this table, the MBR also includes a program that reads the boot sector record of the partition containing the operating system to be booted into RAM. In turn, that record contains a program that loads the rest of the operating system into RAM.
The MBR, the most important data structure on the disk, is created when the disk is partitioned. The MBR contains a small amount of executable code called the master boot code, the disk signature, and the partition table for the disk. At the end of the MBR is a 2-byte structure called a signature word or end of sector marker, which is always set to 0x55AA. A signature word also marks the end of an extended boot record (EBR) and the boot sector.
The disk signature, a unique number at offset 0x01B8, identifies the disk to the operating system.
The values in the partition table (contained in the MBR) depend directly on the size of the physical disk and on the logical partitioning on that disk. The DOS Boot Record (DBR) holds the Boot Parameter Block (BPB), which contains the logical mapping information for that particular partition. The values in the Boot Parameter Block are absolutely dependent on the size of the partition and the type of file system.
In the above picture, is an example of what a partitioned hard disk drive may look like. In this case, the MBR is the first section of the hard disk drive the computer looks at after the BIOS hands control to the first bootable drive.
These two sectors contain the information necessary to locate and identify the file system used to access the data on the drive. If either of these are damaged, the data becomes inaccessible, even though there may be no damage to the data or the file system itself.
Contrary to popular belief the Mbr does NOT belong to the operating system, it belongs to the drive or device. Often people may say "the Windows Mbr gets broken" but what this really means is that the drive's Mbr will no longer point at Windows.
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