Master Boot Record (MBR)

The master boot record is the first physical block on the disk. (Note: The terms "block", "record", and "sector" are often used interchangebly to describe the smallest addressable unit of a disk drive.) Since the MBR is only one record, or sector, long it is severely limited in what it can contain. Typcially a sector holds 512 bytes.

Here is a formatted dump of the MBR on a Windows XT system.

The MBR contains three types of information.

Executable code: This code searches the Partition Table (described below) to find the first partition that is marked as an "active" or boot partition. The first block of that partition is read into memory and control is transferred to the first byte of that block in memory which is assumed to contain executable code. This disk block is called the Partition Boot Block. Look here for a disassembly (i.e. translation) of the boot code in the Windows XT boot block.
The Partition Table. The partition table specifies how the disk is divided into partitions.
There are several reasons to divide a disk drive into partitions:
- Some file systems have a maximum size limit, and so, on disks that would otherwise be too large, the disk can be divideded into partitions with each parition being set to the file-system's size limit or smaller.
- Each partition can be used for a different purpose. For example, they can contain different kinds of file systems.
- Each partition can contain a different operating system. By changing the active partition one can change which operating system boots.
Normally each partition contains a file system. Every different operating system supports a different set of types of file systems. It is also possible for a partition to contain other types of data (i.e. not contain a file system). For example Linux swap space is normally in a separate partition; there is no file system in such a partition. Some database management systems (such as Oracle) can also place their data directly into a partition without that partition containing a file system. (A partition that is accessed directly without a file system is often referred to as a "raw partition".)
The partition table records the location of each partiion on the drive. The partition table can be thought of as containing rows and columns. Each row describes one partition. There is room for four rows and thus there can be a maximum of four partitions on the disk. (Actually there can be more partitions by having a secondary partition table, but that functionality is implemented by operating systems and is not part of the boot process. The partitions listed in the MBR are often referred to as "primary partitions".)
Each column in the partition table contains one piece of data about the partition:
- The disk address of the beginning of the partition.
- The disk address of the end of the partition.
- A flag specifying the active partition. The active partition is the partition to be booted.
- A numeric code indicating the use of the partition. (Typically this specifies the type of file system within the partition.)
See this page for a formatted display of the partition table for the MBR from the Windows XT system shown on this page. (Of course the exact partition layout will vary from one system to another; the system shown has a few extra partitions, added by the author.)
The signature. The signature is only two bytes and has a consant value (0x55 0xaa). This value can be used by the BIOS boot routine to determine whether there is a valid MBR present.

Next: Partition Boot Record (PBR)

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