MBRBootsektor

MBRBootsektor, or the MBR boot sector, refers to the first sector of a storage device used by BIOS-based computers to start the operating system. It is located in the Master Boot Record (MBR) at the very beginning of the disk and is 512 bytes in size. During boot, the system firmware loads this sector into memory and executes its boot code.

The MBR contains three parts: a 446-byte bootloader program, a 64-byte partition table describing up to four primary partitions, and a 2-byte boot signature (0x55AA) at the end of the sector. Each partition table entry contains information such as bootable flags, starting address, and partition type.

Booting with the MBR follows a sequence typical of legacy BIOS systems. The BIOS reads the MBR from the active disk and transfers control to the bootloader code it contains. The bootloader then examines the partition table to locate an active partition, loads the boot sector (volume boot record) of that partition, and transfers control to it to continue the operating system startup. If no valid boot sector is found, a boot error is typically displayed.

Limitations and variants: The MBR supports a maximum of four primary partitions, with additional partitions accommodated via an extended partition containing logical drives. The disk size limit is about 2 terabytes with 512-byte sectors. In modern systems, MBR usage is often replaced by GPT and UEFI, though many setups retain MBR for compatibility or to support legacy bootloaders. Some GPT disks include a protective MBR to prevent accidental damage from non-GPT-aware tools.

In contemporary contexts, the MBR boot sector remains relevant for backward compatibility, recovery tools, and boot managers, but it is increasingly supplanted by more modern boot mechanisms.