2. Overview
BIOS is a term that stands for basic
input/output system, which at the most basic
level consists of lowlevel software that controls
the system hardware.
BIOS is essentially the link between hardware
and software in a system.
BIOS code is burned or flashed into a ROM
chip that is both nonvolatile and read-only.
3. PC System Layers
A PC system can be described as a series of layers—some hardware
and some software—that interface with each other. In the most basic
sense, you can break a PC down into four primary layers, each of
which can be broken down further into subsets.
•The purpose of the layered design is to enable a
given operating system and applications to run on
different hardware. The figure shows how two
different machines with different hardware can
each use different drivers (BIOS) to interface the
unique hardware to a common operating system
and applications.
•The hardware layer is where most differences lie
between various systems. It is up to the BIOS to
mask the differences between unique hardware so
that the given operating system (and subsequently
the application) can be run.
4. BIOS Hardware/Software
The BIOS itself is software running in memory that consists of all the
various drivers that interface the hardware to the operating system.
The BIOS in a PC comes from three possible sources:
■ Motherboard ROM
■ Adapter card ROM (such as that found on a video card)
■ Loaded into RAM from disk (device drivers)
The motherboard BIOS usually includes drivers for all the basic system
components, including the keyboard, floppy drive, hard drive, serial and
parallel ports, and more.
As systems became more complex, new hardware was added for which no
motherboard BIOS drivers existed.
Rather than requiring a new motherboard BIOS that would specifically
support the new devices, it was far simpler and more practical to copy any
new drivers that were necessary onto the system hard disk and configure
the operating system to load them at boot time.
This is how most CD-ROM drives, sound cards, scanners, printers, and so
on are supported.
5. BIOS and CMOS RAM
Some people confuse BIOS with the CMOS RAM in a system.
The BIOS on the motherboard is stored in a fixed ROM chip.
Also on the motherboard there is a chip called the RTC/NVRAM chip,
which stands for real-time clock/nonvolatile memory. This is where the
BIOS Setup information is stored, and it is actually a digital clock chip
with a few extra bytes of memory.
It is usually called the CMOS chip because it is made using CMOS
(complimentary metal-oxide semiconductor) technology.
When you enter your BIOS Setup, configure your hard disk
parameters or other BIOS Setup settings, and save them, these
settings are written to the storage area in the RTC/NVRAM (otherwise
called CMOS RAM) chip.
Every time your system boots up, it reads the parameters stored in the
CMOS RAM chip to determine how the system should be configured.
A relationship exists between the BIOS and CMOS RAM, but they are
two distinctly different parts of the system.
6. Motherboard BIOS
The BIOS is a collection of programs embedded in one or more
chips, depending on the design of your computer. That
collection of programs is the first thing loaded when you start
your computer, even before the operating system.
The BIOS in most PCs has four main functions:
■ POST (power on self test). The POST tests your computer’s processor,
memory, chipset, video adapter, disk controllers, disk drives, keyboard,
and other crucial components.
■ Setup. The system configuration and setup program is usually a menu-
driven program activated by pressing a special key during the POST,
and it enables you to configure basic system settings.
■ Bootstrap loader. A routine that reads the disk drives looking for a valid
master boot sector. This master boot sector program then continues
the boot process by loading an operating system boot sector, which
then loads the operating system core files.
■ BIOS (basic input/output system). This refers to the collection of actual
drivers used to act as a basic interface between the operating system
and your hardware when the system is booted and running.
7. ROM BIOS Manufacturers
Several popular BIOS manufacturers in the market today supply the
majority of motherboard and system manufacturers with the code for
their ROMs.
Several companies have specialized in the development of a
compatible ROM BIOS product.
The three major companies that come to mind in discussing ROM
BIOS software are American Megatrends, Inc. (AMI), Phoenix
Technologies, and Award Software (now owned by Phoenix
Technologies).
Many OEMs (original equipment manufacturer) have developed their
own compatible ROMs independently. Companies such as Compaq,
AT&T, and Acer have developed their own BIOS products that are
comparable to those offered by AMI, Phoenix, Award, and others.
Most OEMs have their BIOS written for them by a third-party
company. For example, Hewlett-Packard contracts with Phoenix to
develop the motherboard BIOSes for some HP PCs.
8. EEPROM/Flash ROM
Modern BIOS is made of EEPROM.
By using an EEPROM, or flash ROM, you can erase and
reprogram the motherboard ROM in a PC without
removing the chip from the system or even opening up
the system chassis.
In most cases, you download the updated ROM from the
motherboard manufacturer’s Web site and then run a
special program it provides to update the ROM.
An updated BIOS might contain bug fixes or enable new
features not originally found in your system.
9. Upgrading the BIOS
A simple BIOS upgrade can often give your computer better performance
and more features.
The following list shows the primary functions of a ROM BIOS upgrade:
■ Adding LS-120 (120MB) floppy drive support (also known as a SuperDisk)
■ Adding support for hard drives greater than 8GB
■ Adding support for Ultra-DMA/33, UDMA/66, or UDMA/100 IDE hard drives
■ Adding support for bootable ATAPI CD-ROM drives
■ Adding or improving Plug and Play (PnP) support and compatibility
■ Correcting calendar-related and leap-year bugs
■ Correcting known bugs or compatibility problems with certain hardware and
application or operating system software
■ Adding support for newer-type and -speed processors
■ Adding support for ACPI power management
If you install newer hardware or software and follow all the instructions
properly, but you can’t get it to work, specific problems might exist with
the BIOS that an upgrade can fix.
10. Beep Codes
Whenever a POST fails the BIOS may indicate the error through a blank
screen, or a visual error message on the video display, or through an audio
response (beep codes) produced by the system’s speaker.
Different beep codes are generated depending on the device which has failed
to respond to the POST. This translation between beep codes and the device
involved is primarily referred from the mainboard’s manual.
If every device tested is operating correctly a single beep sound is heard.
In PCs, you can use the single beep that most PCs produce between the end
of the POST and the beginning of the boot-up process to separate hardware
problems from software problems.
Errors that occur, or are displayed, before this beep indicate that a hardware
problem of some type exists. This conclusion should be easy to understand
because up to this time, only the BIOS and the basic system hardware have
been active. The operating system side of the system does not come into play
until after the beep occurs.
If the system produces an error message or a beep code before the single
beep, for example, the system has found a problem with the RAM hardware.
11. Example
Here’s an example of
beep codes produced
by a particular BIOS
version from American
Megatrends (AMI)
12. Review
1. What is the function of POST?
2. Does the BIOS supply all drivers for your
hardware?
3. BIOS is a type of ROM, is it modifiable?
4. What are the well-known BIOS manufacturers?
5. How can you distinguish between hardware and
software problems during boot-up?
6. You have disabled the floppy drive in the BIOS.
How can you make sure no one can turn it on?
7. How are beep codes important?
