Loading The OS Loader

The OS Loader may be located on any of a set of devices. The BIOS looks at each of the devices in order.

A Hard drive normally has an MBR (Master Boot Record). This contains information about the drive's geometry, a master bootstrap program (whose size is approx. 446 bytes) and the drive's partition table.

Normally, the bootstrap program, as taken from the days of DOS, searches for a partition marked as 'active', loads the OS loader found on sector '0' of that partition, and executes it.

Anyone may write their own bootstrap program to do whatever they want. One example is "PC-Vault" - a program that adds security checkup that cannot be overridden even if one tries to restore the original MBR. Another example is, well, LILO.