Page 86 - A Practical Guide from Design Planning to Manufacturing

62 Chapter Two

Basic Input Output System

Today Microsoft Windows comes with dozens of built-in applications from
Internet Explorer to Minesweeper, but at its core the primary function of
the operating system is still to load and run programs. However, the oper-
ating system itself is a program, which leads to a “chicken-and-egg” prob-
lem. If the operating system is used to load programs, what loads the
operating system? After the system is powered on the processor’s memory
state and main memory are both blank. The processor has no way of
knowing what type of motherboard it is in or how to load an operating
system. The Basic Input Output System (BIOS) solves this problem.
After resetting itself, the very first program the processor runs is the
BIOS. This is stored in a flash memory chip on the motherboard called
the BIOS ROM. Using flash memory allows the BIOS to be retained even
when the power is off. The first thing the BIOS does is run a Power-On
Self-Test (POST) check. This makes sure the most basic functions of the
motherboard are working. The BIOS program then reads the CMOS
RAM configuration information and allows it to be modified if prompted.
Finally, the BIOS runs a bootstrap loader program that searches for an
operating system to load.
In order to display information on the screen during POST and be able
to access storage devices that might hold the operating system, the
BIOS includes device drivers. These are programs that provide a stan-
dard software interface to different types of hardware. The drivers are
stored in the motherboard BIOS as well as in ROM chips built into
hardware that may be used during the boot process, such as video
adapters and disk drives.
As the operating system boots, one of the first things it will do is load
device drivers from the hard drive into main memory for all the hard-
ware that did not have device drivers either in the motherboard BIOS
or built-in chips. Most operating systems will also load device drivers
to replace all the drivers provided by the BIOS with more sophisticated
higher-performance drivers. As a result, the BIOS device drivers are typ-
ically only used during the system start-up but still play a crucial role.
The drivers stored on a hard drive couldn’t be loaded without at least a
simple BIOS driver that allows the hard drive to be read in the first place.
In addition to the first few seconds of start-up, the only time Windows
XP users will actually be using the BIOS device drivers is when boot-
ing Windows in “safe” mode. If a malfunctioning driver is loaded by the
operating system, it may prevent the user from being able to load the
proper driver. Booting in safe mode causes the operating system to not
load it own drivers and to rely upon the BIOS drivers instead. This
allows problems with the full boot sequence to be corrected before return-
ing to normal operation.