BIOS stands for Basic Input Output System. When a computer is first powered on, the operating system (Windows, OSX, Linux, etc.) has yet to be loaded. The BIOS is a type of ROM. ROM stands for Read Only Memory, and this means that the BIOS can’t be written to with new information (at least it is very difficult in the BIOS case). It has a set instructions which first tell it to look at a specific memory address on another chip. This chip is a CMOS RAM chip. RAM (Random Access Memory) can be written to the chip and read off of the chip, which can hold information as long as power is supplied. The importance of using a CMOS chip is its low power benefits. The memory of some CMOS chips can be maintained by a small watch battery for up to ten years. The CMOS memory contains user information about how the computer should turn on. After reading the information on the CMOS chip the BIOS then performs a series of steps which may vary a little, but in general it does the following:

Handlers and Drivers
The BIOS has stored in it the instructions needed to interact with the computer screen, keyboard, mouse, hard drive, and several other basic hardware components. The interrupt handlers are pieces of software specifically designed to tell the processor when the keyboard or other user input device gives a signal. They are called interrupts because user input usually takes priority above other processes, so it is “interrupting” the processes.

POST (Power on Self Test)
Once the BIOS reaches this stage it checks to make sure all of the hardware is functioning properly and whether there is any input. At this point in the setup, many times words will flash on the screen. This is just showing what the BIOS is checking as it is checking it. It goes through memory to make sure it is still operational, checks the hard disk, makes sure the processor is working correctly, checks for input devices (like the keyboard for which it has already loaded the driver), and the system power supply. The BIOS also goes through a select portion of memory to see if any other ROMs are in the system. Sometimes the video card has its own BIOS as might be the case with several other system chips.

Look for Boot Devices
Once the BIOS has made sure everything is ready for the computer it finally looks through the bootable devices in the computer in an order specified by the user. The most common bootable device is the hard drive because that is where the operating system is stored, however, it is also possible to store a bootable system on removable media. Some operating systems can be found on CDs or flash drives, and some of the earliest could be repaired from floppy disks too.
The Operating System
When the BIOS finds the operating system it is to boot from it starts it loading and the operating system takes over.