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. 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. This did not change when FAT16 was introduced as a new file system with DOS 3.0. The partition table supported up to four primary partitions, of which DOS could only use one. The original version of the MBR was written by David Litton of IBM in June 1982. 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. MBRs are not present on non-partitioned media such as floppies, superfloppies or other storage devices configured to behave as such. A GPT can coexist with an MBR in order to provide some limited form of backward compatibility for older systems. Therefore, the MBR-based partitioning scheme is in the process of being superseded by the GUID Partition Table (GPT) scheme in new computers. 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. The organization of the partition table in the MBR limits the maximum addressable storage space of a partitioned disk to 2 TiB (2 32 × 512 bytes). This MBR code is usually referred to as a boot loader. 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).
The MBR holds the information on how the logical partitions, containing file systems, are organized on that medium.
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The concept of MBRs was publicly introduced in 1983 with PC DOS 2.0. For the first sector on non-partitioned media, see volume boot record.Ī 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. This article is about a PC-specific type of boot sector on partitioned media.
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