Whenever you switch on your PC you are welcomed by windows screen and after that you just logon to your system. But have you ever think how all this work i.e. from switching on the computer to getting logged in.
BIOS is responsible to load the process which initiate the operation system. BIOS or Basic Input Output System is small bit of software which a computer uses to test and configure its memory, disk drives, and other hardware. The BIOS is directly mounted on the computer motherboard which communicates further with the chipset.
Phoenix and AMI (American Megatrends) are the two companies which produce the BIOS code for almost all computers. You can see the BIOS name on the chip itself.
Functions of BIOS
BIOS first reads the instructions in CMOS (complementary metal oxide semiconductor) chip that holds the information about the computer’s current configuration settings like current date and time and other optional features. Information on CMOS helps BIOS to configure the chipset to work with the computer’s hardware. This includes the timing and latency of RAM Modules along with it searches for the disk drives and other storage devices for the boot loader software.
Device Drivers and Interrupt handlers are loaded by the BIOS. These handlers are used by the CPU to exchange the data with limited number of input/output devices like keyboards and disk drives. After loading the device drivers and interrupt handlers the BIOS take hold of graphics controller that control the video monitor. If you have more than one video card BIOS will load that one which is chosen in the CMOS code as the primary.
After these BIOS runs the Power on Self Test. This test is important because it verifies that computer’s essential components and processes are working properly. The BIOS will stop if it finds any error in-between.
Following are the main test performed by POST or power on self test
1. Power supply and voltage
2. Error in the BIOS itself
3. CMOS checksum
4. Instructs the CPU to read and write every address in the computer’s memory
5. BIOS test the I/O controller
6. BIOS test the video controller
After the POST ends the BIOS start the graphic controller and after these BIOS read some information about your computer and display it on the screen. Finally BIOS look for the boot loader program on a disk drive which is listed on the CMOS in order of Boot sequence. By default the first item in boot sequence is floppy disk or CDROM. When BIOS finds the boot loader it automatically runs set of programs which make the operating system to load.