BIOS (Basic Input Output System): BIOS is the first software which is run in a PC when it is turned on and booted. Here are the functions of BIOS:
1) It checks if the keyboard and other devices are working and how much RAM is installed in the system.
2) It scans ISA, PCI and USB buses to check the devices attached to these buses and it records the devices.
3) After that boot device is determined from a list of devices saved in CMOS memory. This list can be changed by the user from a BIOS configuration program.
4) First a bootable CD drive is checked for booting. If that fails then system is booted from hard drive. First sector from hard drive which has a program to find which partition is active is read into memory and executed.
5) After that secondary boot loader is read from the partition.
6) Then, operating system from the active partition is read and started.
7) After that, BIOS is queried by operating system for configuration info. For every device, a device driver is checked. If there’s no device driver for that particular device it asks the user to insert a CD containing the particular driver. When all device drivers are present, operating system loads the device drivers in to kernel.
8) After that background processes are initialized and GUI (Graphical user interface) is started.