Getting to Know the BIOS

The BIOS (Basic Input/Output System) is a collection of software utilities and programs that can be invoked by the operating system or application software to perform a variety of hardware-related tasks.

Although many operating systems now contain their own device-oriented programs to improve performance, the BIOS contains a program for just about every activity associated with accessing hardware, including programs for starting the system, testing the hardware, reading and writing to and from storage devices, and moving data between devices.

It boils down to this: The BIOS is the opening act of the PC. It ensures that the hardware is alive, well, and ready for the operating system and then gets the operating system up and running. If you're like most PC users, you probably give the system BIOS of your PC very little notice each time it does its magic when you power it up. That is, until there's a problem and then you'd probably like to shoot the messenger.

Remember The BIOS performs three primary functions, all vital to the usefulness and function of the computer:

l It boots the PC.

l It verifies the configuration data that tells it the internal and peripheral devices that are supposed to be connected to the PC.

l It provides the interface between the hardware (the attached devices) and the software (such as the operating system, device drivers, and application software).

Booting the PC

The instructions that start up the PC and load the operating system into memory and keep it running are part of the group of instructions that are collectively referred to as the system BIOS. The process of starting up the computer and loading the operating system is commonly called booting the computer, or simply the boot sequence. Booting and boot do not refer to any part of kicking or stomping, but are derivatives of the phrase "pulling oneself up by one's own bootstraps," or being able to self-start. When the computer boots, the BIOS is in charge.

When the PC is powered on, the BIOS supplies the PC with its first set of instructions. The instructions supplied by the BIOS are what the PC executes during its power on or boot up sequences until it is able to fetch and execute instructions on its own.

Verifying the hardware

The configuration of a PC is stored in a special type of nonvolatile memory, called Complementary Metal-Oxide Semiconductor, or CMOS (pronounced "sea-moss"), which requires very little power to hold onto its contents. CMOS is the technology used in nearly all memory and processor chips today.

However, in early PCs, it was used only to store the configuration data of the PC. So, although it's used throughout the PC's circuits, CMOS is synonymous with the storage of the PC's configuration data.

CMOS runs on about 1 millionth of an amp of electrical current. This efficiency allows it to store configuration data for a long time, perhaps even years, powered only from low voltage dry cell or lithium batteries. The CMOS battery is located on the motherboard; in older systems it's located in a battery pack attached to the sidewall of the system case.

Instant Answer When the system starts the boot sequence, the BIOS starts a program called the Power- On Self-Test, or POST, that verifies the data in the CMOS to the physical devices it can detect on the system. More on this later in the chapter in the "Running the POST process" section.

Getting input in and output out

After the PC is up and running, its peripheral devices communicate with the system through their device drivers. The system BIOS allows your old PC AT software to run on your Pentium III PC. The BIOS interacts with the hardware to carry out the actions demanded of it. After the PC is booted, the BIOS becomes part of a four-layer software operating environment that allows software to run on a variety of PC platforms without too much trouble. shows the four layers of the PC's operating environment.

The operating layers of a PC.

The application layer (for example, a word processing application) interacts with the operating system, such as Windows, to process its inputs and outputs. The operating system, which can't possibly be created to be exactly compatible with every configuration of PC, interacts with the BIOS, which in turn interacts with the PC's hardware. The BIOS allows the operating system and the application to be created for a general class of hardware because it's specifically created to work with certain types and configurations of hardware. The layers use a standard interface, supplied by the BIOS, to interact with the layer below (or above) it.