In its engine starter role, the BIOS performs three tasks. The first is the POST. This test checks out your PC to make sure everything is running properly. If not, you get a POST error message.

If everything is fine, the BIOS moves on to the second task. That is to give you a chance to run the CMOS setup program. The BIOS usually displays a message instructing you to press a few keys in a certain combination to start the program. The BIOS usually will give you a few seconds to start this program.

If you choose not to run the CMOS setup program, the BIOS moves on to its third task of loading the operating system from your hard drive into RAM. After this task is finished, the BIOS hands control of your PC to the CPU, and jumps into its second role, the cookbook.