Hardware Reference
In-Depth Information
Chapter 5. BIOS
BIOS Basics
It is often difficult for people to understand the difference between hardware and software
in a PC system. The differences can be difficult to understand because both are very much
intertwined in the system design, construction, and operation. However, understanding
these differences is essential to understanding the role of the BIOS in the system.
BIOS stands for basic input/output system , which consists of low-level software that con-
trols the system hardware and acts as an interface between the operating system (OS) and
the hardware. Most people know the term BIOS by another name— device drivers , or just
drivers . In other words, the BIOS is drivers, meaning all of them. It's essentially the link
between hardware and software in a system.
The BIOS consists of software that interfaces the hardware to the OS. The BIOS is unique
compared to normal software in that some of it is preloaded into read-only memory (or
ROM), and some is loaded into RAM from disk.
The BIOS in a running PC is loaded during the system startup from three possible sources:
• Motherboard ROM
• Adapter card ROMs (such as that found on a video card)
• Loaded into RAM from disk (device drivers)
When the PC was introduced, the BIOS software containing all the device drivers for
the entire system was collectively burned into one or more nonvolatile read-only memory
(ROM) chips ( nonvolatile means they retain their data even when the power is turned off)
and placed on the motherboard. In essence, the drivers were self-contained, preloaded into
memory, and accessible any time the PC was powered on.
ThisROMchipalsocontainedapower-onselftest(POST)programandabootstraploader.
The bootstrap program was designed to initiate the loading of an OS by checking for and
loading the boot sector from a floppy disk or, if one was not present, a hard disk. After the
OS was loaded, it could call on the low-level routines (device drivers) in the BIOS to in-
teract with the system hardware. In the early days, all the necessary device drivers were in
the BIOS stored in the motherboard ROM. This included drivers for the keyboard, MDA/
CGA video adapters, serial/parallel ports, floppy controller, hard disk controller, joystick,
and clock.
When the OS loaded, you didn't have to load a driver to interact with those pieces of hard-
warebecause thedriverswerealready preloaded intheROM.That workedgreat aslongas
Search WWH ::




Custom Search