Master Boot Record

A master boot record (MBR) is a special type of boot sector at the very beginning of partitioned computer mass storage devices like fixed disks or removable drives intended for use with IBM PC-compatible systems and beyond. The concept of MBRs was publicly introduced in 1983 with PC DOS 2.0.

The MBR holds the information on how the disc's sectors are divided into partitions, each partition notionally containing a file system. The MBR also contains executable code to function as a loader for the installed operating system—usually by passing control over to the loader's second stage, or in conjunction with each partition's volume boot record (VBR). This MBR code is usually referred to as a boot loader.

The organization of the partition table in the MBR limits the maximum addressable storage space of a partitioned disk to 2  TiB . Approaches to slightly raise this limit assuming 32-bit arithmetics or 4096-byte sectors are not officially supported, as they fatally break compatibility with existing boot loaders and most MBR-compliant operating systems and system tools, and can cause serious data corruption when used outside of narrowly controlled system environments. Therefore, the MBR-based partitioning scheme is in the process of being superseded by the GUID Partition Table (GPT) scheme in new computers. A GPT can coexist with an MBR in order to provide some limited form of backward compatibility for older systems.

MBRs are not present on non-partitioned media such as floppies, superfloppies or other storage devices configured to behave as such.

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 June 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 eras of DOS and OS/2 up to 1996.

In 1996, support for logical block addressing (LBA) was introduced in Windows 95B and DOS 7.10 in order to support disks larger than 8 GB. ''Disk timestamps'' were also introduced. This also reflected the idea that the MBR is meant to be operating system and file system independent. However, 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 LBA is used for / .

Despite sometimes poor documentation of certain intrinsic details of the MBR format (which occasionally caused compatibility problems), it has been widely adopted as a de facto industry standard, due to the broad popularity of PC-compatible computers and its semi-static nature over decades. This was even to the extent of being supported by computer operating systems for other platforms. Sometimes this was 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 on disks using 512-byte sectors (whether real or emulated) by the MBR partitioning scheme (without 33-bit arithmetic) is limited to 2 TiB. 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 Partition Table (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 that do not support the GPT scheme as well. 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 TiB 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 causes various compatibility problems in certain scenarios.

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''.

Disk partitioning

IBM PC DOS 2.0 introduced the FDISK utility to set up and maintain MBR partitions. When a storage device has been partitioned according to this scheme, its MBR contains a partition table describing the locations, sizes, and other attributes of linear regions referred to as partitions.

The partitions themselves may also contain data to describe more complex partitioning schemes, such as extended boot records (EBRs), BSD disklabels, or Logical Disk Manager metadata partitions.

The MBR is not located in a partition; it is located at a first sector of the device (physical offset 0), preceding the first partition. (The boot sector present on a non-partitioned device or within an individual partition is called a volume boot record instead.) In cases where the computer is running a DDO BIOS overlay or boot manager, the partition table may be moved to some other physical location on the device; e.g., Ontrack Disk Manager often placed a copy of the original MBR contents in the second sector, then hid itself from any subsequently booted OS or application, so the MBR copy was treated as if it were still residing in the first sector.

Sector layout

By convention, there are exactly four primary partition table entries in the MBR partition table scheme, although some operating systems and system tools extended this to five (Advanced Active Partitions (AAP) with PTS-DOS 6.60 and DR-DOS 7.07), eight ( AST and NEC MS-DOS 3.x as well as Storage Dimensions SpeedStor), or even sixteen entries (with Ontrack Disk Manager).

Partition table entries

An artifact of hard disk technology from the era of the PC XT, the partition table subdivides a storage medium using units of cylinders, heads, and sectors (CHS addressing). These values no longer correspond to their namesakes in modern disk drives, as well as being irrelevant in other devices such as solid-state drives, which do not physically have cylinders or heads.

In the CHS scheme, sector indices have (almost) always begun with sector 1 rather than sector 0 by convention, and due to an error in all versions of MS-DOS/PC DOS up to including 7.10, the number of heads is generally limited to 255 instead of 256. When a CHS address is too large to fit into these fields, the tuple (1023, 254, 63) is typically used today, although on older systems, and with older disk tools, the cylinder value often wrapped around modulo the CHS barrier near 8 GB, causing ambiguity and risks of data corruption. (If the situation involves a "protective" MBR on a disk with a GPT, Intel's Extensible Firmware Interface specification requires that the tuple (1023, 255, 63) be used.) The 10-bit cylinder value is recorded within two bytes in order to facilitate making calls to the original/legacy INT 13h BIOS disk access routines, where 16 bits were divided into sector and cylinder parts, and not on byte boundaries.

Due to the limits of CHS addressing, a transition was made to using LBA, or logical block addressing. Both the partition length and partition start address are sector values stored in the partition table entries as 32-bit quantities. The sector size used to be considered fixed at 512 (2⁹) bytes, and a broad range of important components including chipsets, boot sectors, operating systems, database engines, partitioning tools, backup and file system utilities and other software had this value hard-coded. Since the end of 2009, disk drives employing 4096-byte sectors ( 4Kn or Advanced Format) have been available, although the size of the sector for some of these drives was still reported as 512 bytes to the host system through conversion in the hard-drive firmware and referred to as 512 emulation drives ( 512e).

Since block addresses and sizes are stored in the partition table of an MBR using 32 bits, the maximum size, as well as the highest start address, of a partition using drives that have 512-byte sectors (actual or emulated) cannot exceed 2 TiB−512 bytes ( bytes or (2³²−1) sectors × 512 (2⁹) bytes per sector). Alleviating this capacity limitation was one of the prime motivations for the development of the GPT.

Since partitioning information is stored in the MBR partition table using a beginning block address and a length, it may in theory be possible to define partitions in such a way that the allocated space for a disk with 512-byte sectors gives a total size approaching 4 TiB, if all but one partition are located below the 2 TiB limit and the last one is assigned as starting at or close to block 2³²−1 and specify the size as up to 2³²−1, thereby defining a partition that requires 33 rather than 32 bits for the sector address to be accessed. However, in practice, only certain LBA-48-enabled operating systems, including Linux, FreeBSD and Windows 7 that use 64-bit sector addresses internally actually support this. Due to code space constraints and the nature of the MBR partition table to only support 32 bits, boot sectors, even if enabled to support LBA-48 rather than LBA-28, often use 32-bit calculations, unless they are specifically designed to support the full address range of LBA-48 or are intended to run on 64-bit platforms only. Any boot code or operating system using 32-bit sector addresses internally would cause addresses to wrap around accessing this partition and thereby result in serious data corruption over all partitions.

For disks that present a sector size other than 512 bytes, such as USB external drives, there are limitations as well. A sector size of 4096 results in an eight-fold increase in the size of a partition that can be defined using MBR, allowing partitions up to 16 TiB (2³² × 4096 bytes) in size. Versions of Windows more recent than Windows XP support the larger sector sizes, as well as Mac OS X, and Linux has supported larger sector sizes since 2.6.31 or 2.6.32, but issues with boot loaders, partitioning tools and computer BIOS implementations present certain limitations, since they are often hard-wired to reserve only 512 bytes for sector buffers, causing memory to become overwritten for larger sector sizes. This may cause unpredictable behaviour as well, and therefore should be avoided when compatibility and standard conformity is an issue.

Where a data storage device has been partitioned with the GPT scheme, the master boot record will still contain a partition table, but its only purpose is to indicate the existence of the GPT and to prevent utility programs that understand only the MBR partition table scheme from creating any partitions in what they would otherwise see as free space on the disk, thereby accidentally erasing the GPT.

System bootstrapping

On IBM PC-compatible computers, the bootstrapping firmware (contained within the ROM BIOS) loads and executes the master boot record. The PC/XT (type 5160) used an Intel 8088 microprocessor. In order to remain compatible, all x86 architecture systems start with the microprocessor in an operating mode referred to as real mode. The BIOS reads the MBR from the storage device into physical memory, and then it directs the microprocessor to the start of the boot code. Since the BIOS runs in real mode, the processor is in real mode when the MBR program begins to execute, and so the beginning of the MBR is expected to contain real-mode machine code.

Since the BIOS bootstrap routine loads and runs exactly one sector from the physical disk, having the partition table in the MBR with the boot code simplifies the design of the MBR program. It contains a small program that loads the Volume Boot Record (VBR) of the targeted partition. Control is then passed to this code, which is responsible for loading the actual operating system. This process is known as chain loading.

Popular MBR code programs were created for booting PC DOS and MS-DOS, and similar boot code remains in wide use. These boot sectors expect the FDISK partition table scheme to be in use and scans the list of partitions in the MBR's embedded partition table to find the only one that is marked with the ''active flag''. It then loads and runs the volume boot record (VBR) of the active partition.

There are alternative boot code implementations, some of which are installed by boot managers, which operate in a variety of ways. Some MBR code loads additional code for a boot manager from the first track of the disk, which it assumes to be "free" space that is not allocated to any disk partition, and executes it. A MBR program may interact with the user to determine which partition on which drive should boot, and may transfer control to the MBR of a different drive. Other MBR code contains a list of disk locations (often corresponding to the contents of files in a filesystem) of the remainder of the boot manager code to load and to execute. (The first relies on behavior that is not universal across all disk partitioning utilities, most notably those that read and write GPTs. The last requires that the embedded list of disk locations be updated when changes are made that would relocate the remainder of the code.)

On machines that do not use x86 processors, or on x86 machines with non-BIOS firmware such as Open Firmware or Extensible Firmware Interface (EFI) firmware, this design is unsuitable, and the MBR is not used as part of the system bootstrap. EFI firmware is instead capable of directly understanding the GPT partitioning scheme and the FAT filesystem format, and loads and runs programs held as files in the EFI System partition. The MBR will be involved only insofar as it might contain a partition table for compatibility purposes if the GPT partition table scheme has been used.

There is some MBR replacement code that emulates EFI firmware's bootstrap, which makes non-EFI machines capable of booting from disks using the GPT partitioning scheme. It detects a GPT, places the processor in the correct operating mode, and loads the EFI compatible code from disk to complete this task.

Disk identity

In addition to the bootstrap code and a partition table, master boot records may contain a disk signature. This is a 32-bit value that is intended to identify uniquely the disk medium (as opposed to the disk unit—the two not necessarily being the same for removable hard disks).

The disk signature was introduced by Windows NT version 3.5, but it is now used by several operating systems, including the Linux kernel version 2.6 and later. Linux tools can use the NT disk signature to determine which disk the machine booted from.

Windows NT (and later Microsoft operating systems) uses the disk signature as an index to all the partitions on any disk ever connected to the computer under that OS; these signatures are kept in Windows Registry keys, primarily for storing the persistent mappings between disk partitions and drive letters. It may also be used in Windows NT BOOT.INI files (though most do not), to describe the location of bootable Windows NT (or later) partitions. One key (among many), where NT disk signatures appear in a Windows 2000/XP registry, is:

HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices\

If a disk's signature stored in the MBR was (in that order) and its first partition corresponded with logical drive C: under Windows, then the REG_BINARY data under the key value \DosDevices\C: would be:

A8 E1 B9 D2 00 7E 00 00 00 00 00 00

The first four bytes are said disk signature. (In other keys, these bytes may appear in reverse order from that found in the MBR sector.) These are followed by eight more bytes, forming a 64-bit integer, in little-endian notation, which are used to locate the byte offset of this partition. In this case, corresponds to the hexadecimal value (). Under the assumption that the drive in question reports a sector size of 512 bytes, then dividing this byte offset by 512 results in 63, which is the physical sector number (or LBA) containing the first sector of the partition (unlike the ''sector count'' used in the sectors value of CHS tuples, which counts from one, the absolute or LBA sector value starts counting from ''zero'').

If this disk had another partition with the values following the disk signature (under, e.g., the key value \DosDevices\D:), it would begin at byte offset (), which is also the first byte of physical sector .

Starting with Windows Vista, the disk signature is also stored in the Boot Configuration Data (BCD) store, and the boot process depends on it. If the disk signature changes, cannot be found or has a conflict, Windows is unable to boot. Unless Windows is forced to use the overlapping part of the LBA address of the Advanced Active Partition entry as pseudo-disk signature, Windows' usage is conflictive with the Advanced Active Partition feature of PTS-DOS 7 and DR-DOS 7.07, in particular if their boot code is located outside the first 8 GB of the disk, so that LBA addressing must be used.

Programming considerations

The MBR originated in the PC XT. IBM PC-compatible computers are little-endian, which means the processor stores numeric values spanning two or more bytes in memory least significant byte first. The format of the MBR on media reflects this convention. Thus, the MBR signature will appear in a disk editor as the sequence 55 AA.

The bootstrap sequence in the BIOS will load the first valid MBR that it finds into the computer's physical memory at address :. The last instruction executed in the BIOS code will be a "jump" to that address in order to direct execution to the beginning of the MBR copy. The primary validation for most BIOSes is the signature at offset , although a BIOS implementer may choose to include other checks, such as verifying that the MBR contains a valid partition table without entries referring to sectors beyond the reported capacity of the disk.

To the BIOS, removable (e.g. floppy) and fixed disks are essentially the same. For either, the BIOS reads the first physical sector of the media into RAM at absolute address , checks the signature in the last two bytes of the loaded sector, and then, if the correct signature is found, transfers control to the first byte of the sector with a jump (JMP) instruction. The only real distinction that the BIOS makes is that (by default, or if the boot order is not configurable) it attempts to boot from the first removable disk before trying to boot from the first fixed disk. From the perspective of the BIOS, the action of the MBR loading a volume boot record into RAM is exactly the same as the action of a floppy disk volume boot record loading the object code of an operating system loader into RAM. In either case, the program that the BIOS loaded is going about the work of chain loading an operating system.

While the MBR boot sector code expects to be loaded at physical address :, all the memory from physical address : (address : is the last one used by a Phoenix BIOS) to :, later relaxed to : (and sometimes up to :)the end of the first 640 KBis available in real mode. The INT 12h BIOS interrupt call may help in determining how much memory can be allocated safely (by default, it simply reads the base memory size in KB from segment:offset location :, but it may be hooked by other resident pre-boot software like BIOS overlays, RPL code or viruses to reduce the reported amount of available memory in order to keep other boot stage software like boot sectors from overwriting them).

The last 66 bytes of the 512-byte MBR are reserved for the partition table and other information, so the MBR boot sector program must be small enough to fit within 446 bytes of memory or less.

The MBR code examines the partition table, selects a suitable partition and loads the program that will perform the next stage of the boot process, usually by making use of INT 13h BIOS calls. The MBR bootstrap code loads and runs (a boot loader- or operating system-dependent) volume boot record code that is located at the beginning of the "active" partition. The volume boot record will fit within a 512-byte sector, but it is safe for the MBR code to load additional sectors to accommodate boot loaders longer than one sector, provided they do not make any assumptions on what the sector size is. In fact, at least 1 KB of RAM is available at address in every IBM XT- and AT-class machine, so a 1 KB sector could be used with no problem. Like the MBR, a volume boot record normally expects to be loaded at address :. This derives from the fact that the volume boot record design originated on unpartitioned media, where a volume boot record would be directly loaded by the BIOS boot procedure; as mentioned above, the BIOS treats MBRs and volume boot records (VBRs) exactly alike. Since this is the same location where the MBR is loaded, one of the first tasks of an MBR is to relocate itself somewhere else in memory. The relocation address is determined by the MBR, but it is most often : (for MS-DOS/PC DOS, OS/2 and Windows MBR code) or : (most DR-DOS MBRs). (Even though both of these segmented addresses resolve to the same physical memory address in real mode, for Apple Darwin to boot, the MBR must be relocated to : instead of :, since the code depends on the DS:SI pointer to the partition entry provided by the MBR, but it erroneously refers to it via :SI only.) It is important not to relocate to other addresses in memory because many VBRs will assume a certain standard memory layout when loading their boot file.

The ''Status'' field in a partition table record is used to indicate an active partition. Standard-conformant MBRs will allow only one partition marked active and use this as part of a sanity-check to determine the existence of a valid partition table. They will display an error message, if more than one partition has been marked active. Some non-standard MBRs will not treat this as an error condition and just use the first marked partition in the row.

Traditionally, values other than (not active) and (active) were invalid and the bootstrap program would display an error message upon encountering them. However, the Plug and Play BIOS Specification and BIOS Boot Specification (BBS) allowed other devices to become bootable as well since 1994. Consequently, with the introduction of MS-DOS 7.10 (Windows 95B) and higher, the MBR started to treat a set bit 7 as active flag and showed an error message for values .. only. It continued to treat the entry as physical drive unit to be used when loading the corresponding partition's VBR later on, thereby now also accepting other boot drives than as valid, however, MS-DOS did not make use of this extension by itself. Storing the actual physical drive number in the partition table does not normally cause backward compatibility problems, since the value will differ from ''only'' on drives other than the first one (which have not been bootable before, anyway). However, even with systems enabled to boot off other drives, the extension may still not work universally, for example, after the BIOS assignment of physical drives has changed when drives are removed, added or swapped. Therefore, per the BIOS Boot Specification (BBS), it is best practice for a modern MBR accepting bit 7 as active flag to pass on the DL value originally provided by the BIOS instead of using the entry in the partition table.

BIOS to MBR interface

The MBR is loaded at memory location : and with the following CPU registers set up when the prior bootstrap loader (normally the IPL in the BIOS) passes execution to it by jumping to : in the CPU's real mode.
* CS: IP = : (fixed)
: Some Compaq BIOSes erroneously use : instead. While this resolves to the same location in real mode memory, it is non-standard and should be avoided, since MBR code assuming certain register values or not written to be relocatable may not work otherwise.
*
: DL is supported by IBM BIOSes as well as most other BIOSes. The Toshiba T1000 BIOS is known not to support this properly, and some old Wyse 286 BIOSes use DL values greater or equal to 2 for fixed disks (thereby reflecting the logical drive numbers under DOS rather than the physical drive numbers of the BIOS). USB sticks configured as removable drives typically get an assignment of DL = , , etc. However, some rare BIOSes erroneously presented them under DL = , just as if they were configured as superfloppies.
: A standard conformant BIOS assigns numbers greater or equal to exclusively to fixed disk / removable drives, and traditionally only values and were passed on as physical drive units during boot. By convention, only fixed disks / removable drives are partitioned, therefore, the only DL value a MBR could see traditionally was . Many MBRs were coded to ignore the DL value and work with a hard-wired value (normally ), anyway.
: The Plug and Play BIOS Specification and BIOS Boot Specification (BBS) allow other devices to become bootable as well since 1994. The later recommends that MBR and VBR code should use DL rather than internally hardwired defaults. This will also ensure compatibility with various non-standard assignments (see examples above), as far as the MBR code is concerned.
: Bootable CD-ROMs following the El Torito

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