The Master Boot Record (MBR) is a crucial component of a computer’s storage system, particularly on devices that use the traditional Basic Input/Output System (BIOS) firmware. It is located in the first sector, usually the first 512 bytes of a storage device, of a hard drive or solid-state drive (SSD).

The MBR contains essential information for the system to boot, including the partition table and a small piece of executable code called the “bootloader.” The partition table describes the disk’s layout, specifying each partition’s size and location. The bootloader, meanwhile, is responsible for loading the operating system (OS) ’s main boot code from the designated bootable partition.

Read More about the Master Boot Record

A computer can’t run without the MBR. Find out why by looking at its components.

What Are the Components of the Master Boot Record?

Here’s a breakdown of the critical components of the MBR.

  • Bootstrap code: This is a small program, usually 446 bytes in size, that the BIOS firmware executes during the boot process. Its primary function is to locate and load the OS’s bootloader.
  • Partition table: The next 64 bytes of the MBR are reserved for the partition table, which contains entries for up to four primary partitions or three primary partitions and one extended partition. Each entry provides information about the size and starting location of a partition.
  • MBR signature: The final two bytes of the MBR contain a signature (0x55AA) that indicates the presence of a valid MBR. The BIOS checks for this signature before considering the MBR valid.

How Does the Master Boot Record Work?

The MBR plays a crucial role in booting an OS on a computer. Here’s a step-by-step explanation of how the MBR works.

  1. BIOS initialization: When you power on your computer, the BIOS is activated. The BIOS is the firmware stored on a chip on the motherboard. It performs a Power-On Self-Test (POST) to check the hardware’s integrity and functionality.
  1. Boot device selection: After the POST, the BIOS looks for a bootable device to load the OS from. It searches for the MBR in the first sector of the bootable device, usually a hard drive or SSD.
  1. MBR loading: Once the bootable device is identified, the BIOS loads the device’s first sector (512 bytes) into memory. This sector contains the MBR.
  1. MBR execution: The BIOS transfers control to the MBR by executing the 446 bytes of bootstrap code within the MBR. This code is responsible for finding and loading the active partition’s bootloader.
  1. Partition table examination: The MBR contains a partition table that describes the layout of the disk and the locations of the partitions. The bootstrap code examines the partition table to identify the active (bootable) partition. The active partition is the one that contains the OS’s bootloader.
  1. Bootloader loading: Once the active partition is identified, the bootstrap code loads that partition’s first sector (the boot sector) into memory. This sector contains the OS’s bootloader.
  1. Bootloader execution: Control is transferred to the bootloader, a more substantial piece of code than the bootstrap code in the MBR. The bootloader is responsible for further loading the OS kernel and initiating the boot process.
  1. OS startup: The bootloader continues the boot process by loading the necessary components of the OS into memory. Finally, the OS kernel takes control, and the user is presented with the login screen or desktop, depending on the OS.
Steps in the MBR Loading Process

In sum, the MBR acts as a small but critical piece of code that facilitates the boot process by locating and loading the bootloader of the active partition, which then starts the OS. It’s worth noting that modern systems often use the Globally Unique Identifier (GUID) Partition Table (GPT) instead of the MBR for partitioning due to its advantages in handling larger storage capacities and providing additional features.

It’s important to note that the MBR has limitations, such as supporting a maximum of four primary or three primary and one extended partition. In addition, the MBR uses 32-bit addressing, which can limit the maximum addressable storage capacity to 2 terabytes. As a result, newer systems often use the GPT instead of the MBR for partitioning, as the GPT addresses some of the MBR’s limitations and supports larger storage capacities.

Key Takeaways