When starts, the computer system has no idea what drives it has and can not even load its operating system. To solve this problem, a computer uses the Basic Input/Output System (BIOS) to turn it on. This small program is permanently stored in a read-only memory (ROM) chip. Since it is a small program embedded in a chip, it occupies a place between the software and hardware. For this reason it is often called firmware. BIOS performs a number of important functions. It interprets key strokes, displays characters on the screen, handles communication through the computer's ports, and tests the system while turning it on. When a computer is turned on, the CPU hands over control to the BIOS, and the BIOS runs the power on self test of POST. The POST makes sure that all chips on the system board works well. Results of such tests are shown on the monitor at the time of turning on. Finally when the BIOS finds the operating system, in hard disk or floppy disk, it hands over control to the operating system.
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