Master Boot Record - Overview

Overview

Support for partitioned media, and thereby the Master Boot Record (MBR), was introduced with IBM PC DOS 2.0 in March 1983 in order to support the 10 MB hard disk of the then-new IBM Personal Computer XT, still using the FAT12 file system. The original version of the MBR was written by David Litton of IBM in 1982. The partition table supported up to four primary partitions, of which DOS could only use one. This did not change when FAT16 was introduced as a new file system with DOS 3.0. Support for an extended partition, a special primary partition type used as a container to hold other partitions, was added with DOS 3.2 and nested logical drives inside an extended partition came with DOS 3.30. Since MS-DOS, PC DOS, OS/2 and Windows were never enabled to boot off them, the MBR format and boot code remained almost unchanged in functionality, except for in some third-party implementations, throughout the DOS and OS/2 eras up to 1996, when support for Logical Block Addressing (LBA) (for disks larger than 8 GB) and disk timestamps was introduced with Windows 95B / DOS 7.10. This also reflects the fact, that the MBR is meant to be operating system and file system independent, although this design rule was partially compromised in more recent Microsoft implementations of the MBR, which enforce CHS access for FAT16B and FAT32 partition types whereas they use LBA for the FAT16X and FAT32X partition types.

Despite sometimes poor documentation of certain intrinsical details of the MBR format (which occasionally can cause compatibility problems), it was widely adopted as a de-facto industry standard due to the broad popularity of PC-compatible computers and its semi-static nature over decades, to the extent of being supported by computer operating systems for other platforms, sometimes in addition to other pre-existing or cross-platform standards for bootstrapping and partitioning.

MBR partition entries and the MBR boot code used in commercial operating systems, however, are limited to 32 bits. Therefore, the maximum disk size supported by the MBR partitioning scheme (without using non-standard methods) is limited to 2 TB. Consequently, a different partitioning scheme must be used for larger disks, as they have become widely available since 2010. The MBR partitioning scheme is therefore in the process of being superseded by the GUID partitioning scheme (GPT). The official approach does little more than ensuring data integrity by employing a protective MBR; specifically, it does not provide backward compatibility with operating systems not enabled to support the GPT scheme as well. In the meanwhile, multiple forms of hybrid MBRs have been designed and implemented by third-parties in order to maintain partitions located in the first physical 2 TB of a disk in both partitioning schemes in parallel and/or to allow older operating systems to boot off GPT partitions as well. The present non-standard nature of these solutions can cause various compatibility problems in certain scenarios, though.

The MBR consists of 512 or more bytes located in the first sector of the drive.

It may contain one or more of:

• A partition table describing the partitions of a storage device. In this context the boot sector may also be called a partition sector.
• Bootstrap code: Instructions to identify the configured bootable partition, then load and execute its Volume Boot Record (VBR) as a chain loader.
• Optional 32-bit disk timestamp.
• Optional 32-bit disk signature.