Foundation stone #4 - BIOS part 3 - Master Boot Record (MBR)
After completing the hardware verification and loading the hardware interrupts, the BIOS needs to hand over the control to a OS. Since when you start up a computer the memory is clean, the BIOS needs to find the OS code that can take control from not the memory but at another storage device such as the hard disk, floppy disk or a CD-ROM. The boot sequence is a configuration option in the BIOS setup. This configuration lists the media to look for, to load an OS. The BIOS looks up each of the device in the order listed and if it finds one then it loads up the OS. Typically the media would be the hard disk of your PC. Sometimes you would want to change this to a CD-ROM in case if this is the first time you are setting up your machine.
The boot code of any OS is available in what is called a Master Boot Record(MBR). In case of a hard disk it will be cylinder 0, sector 0 and track 0. The MBR contains information about where to look for the starting point of the OS in the disk. The BIOS picks up this information and loads the kernel or the core of the OS into the memory and handles control to that program. After that point the OS takes in charge of your PC. A detailed explanation on the MBR and the entire BIOS sequence is wonderfully explained here in DEW Associates knowledge base
