April 17, 2011, 6:26 p.m.
posted by shadow
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 many hardware-related tasks. Although many operating systems now contain their own device-oriented programs to improve performance, the BIOS contains a program for almost 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.
The BIOS is the PC’s opening act. It ensures that the hardware is alive, well, and ready for the operating system, and then gets the operating system 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 vital functions for the computer:
Boot the PC.
Verify the configuration data that tells it the internal and peripheral devices that are supposed to be connected to the PC.
Provide the interface between the hardware (the attached devices) and the software (such as the operating system, device drivers, and application software).
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. When the computer boots, the BIOS is in charge.
Booting and boot are derivatives of the phrase “pulling oneself up by one’s own bootstraps,” or being able to self-start.
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.
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 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 PC’s configuration data. 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 (maybe years), powered only from either low-voltage dry cell or lithium batteries. On newer PCs, the CMOS battery is located on the motherboard; however, in many older systems, it may actually be a pair of AA batteries mounted in a plastic battery pack that is attached to the sidewall of the system case.
Instant Answer When the system starts the boot sequence, the BIOS starts a Power-On Self-Test, or POST, program 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.
After the PC is 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 many PC platforms without too much trouble.
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.