Built-in firmware stored on a ROM that determines what a computer can do without accessing programs from a disk. On PCs, the BIOS contains all the code required to control the keyboard, display screen, disk drives, serial communications, and a number other functions. Existing on ROM ensures that the BIOS will always be available and will not be damaged bt disk failures. It also makes it possible for a computer to boot iteself.
More: Basic Input/Output System