BIOS or the Basic Input/ Output System is nothing but a simple software code which firsts runs when the computer is turned on. It has all the necessary information that is required to initialize almost every hardware component of Computer.
Once the computer is turned on BIOS performs a POST or Power on Self Test. POST is primarily a diagnostic test that are run on hardware components like RAM.
The BIOS is also responsible for the initialization of the hardware components like hard drives, video and other hardware. It reserves memory addresses for all the IRQs (Interrupt Requests) and the ports available on the mother board. BIOS finally calls an OS program known as the boot loader which uses information from BIOS and other resources to get those programs running which will finally load the operating system.
Once this is done the Operating systems takes control of all the hardware devices by using the information from BIOS.
The different BIOS software codes and all the other computer settings are stored on a chip that derives power from a battery. This battery also provides power to a clock that keeps the accurate time. It is because of this battery that all the settings are retained even if you power off your PC
