Master Boot Record
Located in the very first sector at the beginning of a hard disk, the Master Boot Record (MBR) is a combination of a program code and data used by that program (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 it is just one—and which one to go to in order 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 which replace it in order to gain control of the computer before the operating system loads.