PC Hardware in a Nutshell, 3rd Edition

BIOS

By themselves, the processor, memory, chipset, and other motherboard components are a useless collection of silicon, plastic, and metal. None of them individually can do much without help from other components, and none of them knows how to talk to other components. Turning this random collection of parts into a functioning computer requires a BIOS, or Basic Input/Output System.

The BIOS is a real-mode program that is more or less permanently stored on a chip that resides on the motherboard. Originally, the BIOS was stored on a ROM chip, whence the term ROM BIOS. The only way to upgrade a ROM BIOS is to physically remove that chip and install a new one. Nowadays, the BIOS resides on a Programmable ROM (PROM), which allows the BIOS code to be updated without physically replacing the chip.

When power is applied to the system board, the CPU initializes. The final step in its hardcoded initialization routine is to jump to a predefined memory address and execute the program that it finds there. That address is the entry point for the BIOS code, so as the CPU finishes initializing, the BIOS code begins executing the system boot sequence. To begin, the BIOS initializes installed hardware and performs the power-on self test (POST). At the conclusion of the system boot sequence, the BIOS loads the operating system and passes control to it.

Once the operating system loads, the BIOS may also perform various low-level system services at the request of the operating system or applications. ...