Master Boot Record

Located in the first sector at the beginning of a hard disk, the Master Boot Record (MBR) combines a program code and data that the program uses (the so-called partition table).

The master boot record code examines the partition table to determine how many logical drives (or additional partitions) the hard disk drive contains—even if just one—and which one to load the operating system.

The master boot record is the first piece of code that a PC BIOS executes after performing its P.O.S.T. (power-on self-test) and, as such, can be the target of viruses that replace it to gain control of the computer before the operating system loads.