Master Boot Record

The Master Boot Record (MBR), which is normally located on a conventional hard disc drive, is a tiny but crucial component of a computer's storage device. It includes crucial data required to initiate the computer's starting procedure and boot the operating system. The boot code, the partition table, and the disc signature are the three essential parts of the MBR.

MBR Structure and Organization

A crucial element of managing storage devices and the startup process of a computer is the Master startup Record (MBR). It has four primary components and is situated at the start of a storage device like a hard disc or solid-state drive.

Bootloader Code − The bootloader code is responsible for launching the operating system. The first 446 bytes of the MBR are used to store this bootloader code. For this to happen, the computer's firmware runs this program during boot-up. This puts the bootloader for the operating system into memory.

Partition table − The partition table is a data structure that takes up the next 64 bytes of the MBR. It includes details on the physical arrangement of the hard drive's partitions, including the starting and ending sector numbers for each partition. The computer can identify and find the various partitions on the disc with the use of this information.

Disc Signature − The disc signature, which is a special identification code given to the hard drive, is kept in the next four bytes of the MBR. It allows the hard drive to be distinguished from other connected storage devices in a unique fashion.

MBR boot process and loading sequence

An essential part of a computer's boot process is the Master Boot Record (MBR) startup procedure and the loading sequence. At the start of a storage device, such as a hard drive or solid-state drive, lies a short 512-byte sector known as the MBR. The following steps can be used to describe the MBR boot process and loading sequence −

BIOS firmware initialization − Initialization of the Basic Input/Output System (BIOS) firmware occurs when a computer is turned on. The first hardware verifications and preparation of the system environment for booting are handled by the BIOS firmware.

MBR loading − The BIOS firmware loads the MBR, which is located in the boot device's first sector, into memory. The bootloader code in the MBR is then executed by the BIOS firmware.

Execution of bootloader code − The MBR's bootloader code is in charge of launching the operating system into memory. The active partition that houses the bootable operating system is located by the bootloader code by searching the partition table in the MBR. The bootloader then loads the active partition's boot sector into memory and gives the boot sector code control.

Execution of boot sector code − The boot sector code is in charge of putting the remainder of the operating system into memory. Usually, the boot sector code configures system resources, loads extra code and data from the operating system's file system, and initializes hardware.

Operating system loading − After completing its initialization duties, the boot sector code loads the operating system kernel into memory and hands control over to the kernel. After initializing system services, device drivers, and other components required for the operating system to run, the kernel continues the boot process.

Partition table and partitioning schemes

An essential component of the Master Boot Record (MBR) and the overall setup of a hard disc are the partition table and partitioning schemes. The starting and ending sector numbers of each partition are listed in the partition table, which also explains the hard drive's partition layout. The hard drive's partitioning and organization into logical storage units is controlled by the partitioning scheme.

With MBR, the partition table can only include a maximum of four primary partitions, or three primary partitions and one extended partition. This is because the partition table is kept in the MBR itself. There can therefore be more than four partitions on the hard drive by dividing the expanded partition into logical partitions. Each partition has its own distinct partition type identifier and can be set up to store data using a file system like NTFS or FAT32.

With MBR, a number of partitioning techniques are possible, such as −

The MBR partitioning scheme is the common partitioning method used with MBR. Four primary partitions, or three primary partitions plus one extended partition, is the maximum number.

The extended partitioning method goes beyond the MBR partitioning scheme's restriction of four primary partitions. It makes it possible to create a single extended partition that can later be divided up into several logical sections.

The more current GUID Partition Table (GPT) partitioning strategy has taken the place of the MBR partitioning system. It supports larger hard drives and allows for the creation of up to 128 partitions. However, it only supports UEFI-based systems; it does not support conventional BIOS-based systems.


The Master Boot Record (MBR) is one of the crucial steps in a computer's boot process. In this article, we learn that the tiny 512-byte sector at the start of a storage device like a solid-state drive or a hard drive contains the bootloader code, partition table, disc signature, and MBR signature. Understanding the structure and organization of the MBR is important to carry out our operations like troubleshooting and setting the boot process of a computer. This is because the Master Boot Record loads the operating system into memory.

Updated on: 04-May-2023


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