Initially, the BIOS program evaluates the system hardware and checks the available boot devices containing an MBR. The BIOS contains the code to locate and execute the MBR in order to locate various hard disk partitions and load the OS.

When the system is powered on, it runs the BIOS program stored in the read-only memory. The Master Boot Record includes a program that reads the boot sector record of the partition containing the operating system being booted. Other common names include sector zero, master boot block and master partition boot sector. The MBR is sometimes called the partition sector or the master partition table because it includes a table that locates each partition on the storage media or hard disk drive. The MBR can function as a chain boot loader independently of the OS. However, it is not located within a partition, which is why nonpartitioned storage mediums, like floppy disks, do not contain an MBR. It is created in Windows and DOS systems when the hard drive is partitioned. As the first sector of the hard disk, it has a specific address: Cylinder 0, Head 0, Sector 1. Without the MBR, the system is unable to start. The MBR contains programs that determine which partition on the hard disk is used for the system boot. It also provides information about the OS so it can be loaded for the system boot. Simply put, the MBR is a boot sector category that provides information about the hard disk partitions. In turn, that record contains a program that loads the rest of the OS into RAM. The MBR also includes a program that reads the boot sector record of the partition containing the OS to be booted.

It identifies how and where the system's operating system ( OS) is located in order to be booted (loaded) into the computer's main storage or random access memory ( RAM ). The Master Boot Record (MBR) is the information in the first sector of a hard disk or a removable drive.

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