Master Boot Record
From Just Solve the File Format Problem
The Master Boot Record is the first sector of a PC disk. It contains the first stage of the boot loader and a partition table. A valid MBR ends with the byte sequence 55 aa.
During booting the PC BIOS loads the MBR at linear address 0x7c00 and passes control to it (at real mode address 07c0:0000, or sometimes 0000:7c00). The MBR code is responsible for loading the rest of the boot loader, which then loads the operating system.
