The BIOS And POST

The System BIOS serves 3 major purposes:

POST - Power On Self Test. in this step it checks that different hardware connected to the system, in order to make sure that everything is working properly. On some kind of errors the BIOS would halt the system. For example, if there is no RAM, the system will be halted, with some kind of indication.

OS Loader - The BIOS is programmed to scan a set of devices (floppy disks, hard drives, CDROMs) to look for a boot program. This program is to be found on sector '0' of a partition on the device marked as the 'active' partition.