BIOS (pronounced BI-ose) stands for Basic Input/Output System. It consists of a set of very low-level software routines that are stored in a computer's read-only memory (ROM). These routines play an essential role in a computer's operation. First, they perform the tasks necessary to start or boot the computer. Second, they provide an interface between software and hardware, allowing hardware to respond to commands issued by software. And third, they support various internal services (such as the real-time clock). The Basic Input/Output System is therefore "basic" in the sense that it is fundamental to the operation of a typical computer. Without it, most computers would not work. All popular general-purpose computers, including both Macintoshes and computers embodying the "PC" architecture designed by IBM, contain a BIOS.

When a computer is started, its BIOS routines perform all the tasks necessary to initialize it. These tasks include running...