MBRs

MBRs, or Master Boot Records, are boot sectors located at the beginning of storage devices that use BIOS-based startup. Each disk typically has a single MBR in the first sector, a 512-byte block that combines three components: a 446-byte bootloader, a 64-byte partition table with four 16-byte entries, and a 2-byte boot signature (0x55AA). The partition table allows up to four primary partitions, with the option to use an extended partition to host logical drives via an extended boot record.

During the boot process, the BIOS loads the MBR into memory and transfers control to the bootloader contained there. The bootloader then loads the appropriate volume boot record of the active partition, which in turn begins the operating system’s loader sequence. On modern systems that use UEFI, GPT is commonly employed, but BIOS-based booting from MBR disks remains common for compatibility with older systems and some operating systems.

MBRs are subject to corruption and certain forms of malware, including bootkits that overwrite the bootloader portion. Damage to the MBR can render a disk unbootable or misreport partitions. Recovery and repair tools exist to address these issues, such as Windows’ fixmbr and fixboot commands, and open-source utilities like TestDisk or disk-imaging tools. In GPT disks, a protective MBR (with a single 0xEE partition) is sometimes present to shield the disk from being misinterpreted by legacy tools.

Overall, the MBR is a legacy yet still relevant element in the boot process for BIOS-based systems, operating in conjunction with partition tables and, when needed, the extended partition mechanism.