mbr

MBR stands for Master Boot Record. It is a special boot sector located in the first sector (sector 0) of a storage device, and it is used by BIOS-based systems to begin the boot process. The MBR combines two main functions: a small piece of machine code known as the bootloader and a partition table that describes up to four primary partitions on the disk.

The 512-byte MBR includes a bootloader program, a partition table consisting of four 16-byte entries, and a 2-byte boot signature at the end (0x55 0xAA). Each partition entry specifies attributes such as bootable status, starting and ending addresses, and partition type. The four entries enable up to four primary partitions, or three primary partitions plus one extended partition that can contain multiple logical drives. Extended partitions are managed through additional structures called extended boot records (EBRs).

Booting on a BIOS-based system proceeds by the firmware loading the MBR into memory and transferring control to its bootloader code. The bootloader then locates an active partition, loads its boot sector, and hands control to the operating system loader. The MBR and its partition table are specific to the disk’s layout and are independent of the OS, though an active boot partition must contain a valid boot sector for the OS.

Limitations of the MBR scheme include a maximum disk size of around 2 TB for 512-byte sectors and a maximum of four primary partitions (with extended partitions allowing more logical drives). With the advent of UEFI, GUID Partition Table (GPT) has become common, offering larger disk support and additional reliability features, though MBR remains in use for legacy BIOS booting.