Master boot record
A GPT can coexist with an MBR in order to provide some limited form of backward compatibility for older systems.
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.
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 0x06/0x0B, whereas LBA is used for 0x0E/0x0C.
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.
Specifically, it does not provide backward compatibility with operating systems that do not support the GPT scheme as well.
It may contain one or more of: IBM PC DOS 2.0 introduced the FDISK utility to set up and maintain MBR partitions.
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.
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).
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 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.
However, in practice, only certain LBA-48-enabled operating systems, including Linux, FreeBSD and Windows 7[18] that use 64-bit sector addresses internally actually support this.
On IBM PC-compatible computers, the bootstrapping firmware (contained within the ROM BIOS) loads and executes the master boot record.
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.
It contains a small program that loads the Volume Boot Record (VBR) of the targeted partition.
(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.)
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.
In addition to the bootstrap code and a partition table, master boot records may contain a disk signature.
[30] 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.
[a] The bootstrap sequence in the BIOS will load the first valid MBR that it finds into the computer's physical memory at address 0x7C00 to 0x7FFF.
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.
[k] 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 0x0040:0x0013, 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).
(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 0x0000:0x0600 instead of 0x0060:0x0000, since the code depends on the DS:SI pointer to the partition entry provided by the MBR, but it erroneously refers to it via 0x0000:SI only.
[32][34] 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 0x01..0x7F only.
Some third-party utilities may also be used for directly editing the contents of partition tables (without requiring any knowledge of hexadecimal or disk/sector editors), such as MBRWizard.
[o] dd is a POSIX command commonly used to read or write any location on a storage device, MBR included.
The GRUB Legacy interactive console can write to the MBR, using the setup and embed commands, but GRUB2 currently requires grub-install to be run from within an operating system.
Linux sfdisk (on a SystemRescueCD) is able to save a backup of the primary and extended partition table.
