How does the BIOS work? - by Jeff Tyson
Taken from http://computer.howstuffworks.com/bios.htm
One of the most common uses of Flash
memory is for the basic input/output
system of your computer, commonly known as the BIOS (pronounced
"bye-ose"). On virtually every computer available, the BIOS makes sure all
the other chips, hard drives, ports and CPU function together.
Every desktop and laptop computer in common use today contains a
microprocessor as its central processing unit. The microprocessor is the
hardware component. To get its work done, the microprocessor executes a
set of instructions known as software. You are probably very familiar with two different types of
The operating system - The operating system provides a set of
services for the applications running on your computer, and it also
provides the fundamental user interface for your computer. Windows 98 and
Linux are examples of operating systems.
The applications - Applications are pieces of software that are
programmed to perform specific tasks. On your computer right now you
probably have a browser application, a word processing application, an
e-mail application and so on. You can also buy new applications and
It turns out that the BIOS is the third type of software your computer
needs to operate successfully. In this article, you'll learn all about
BIOS -- what it does, how to configure it and what to do if your BIOS
What BIOS Does
The BIOS software has a number of different roles, but its most important
role is to load the operating system. When you turn on your computer and
the microprocessor tries to execute its first instruction, it has to get
that instruction from somewhere. It cannot get it from the operating
system because the operating system is located on a hard disk, and the
microprocessor cannot get to it without some instructions that tell it
how. The BIOS provides those instructions. Some of the other common tasks
that the BIOS performs include:
A power-on self-test (POST) for all of the different hardware
components in the system to make sure everything is working properly
Activating other BIOS chips on different cards installed in the
computer - For example, SCSI and graphics cards often have their own BIOS
Providing a set of low-level routines that the operating system
to interface to different hardware devices - It is these routines that
give the BIOS its name. They manage things like the keyboard, the screen,
and the serial and parallel ports, especially when the computer is
Managing a collection of settings for the hard disks, clock, etc.
The BIOS is special software that interfaces the major hardware components
of your computer with the operating system. It is usually stored on a
Flash memory chip on the motherboard, but sometimes the chip is another
type of ROM.
BIOS uses Flash memory, a type of ROM.
When you turn on your computer, the BIOS does several things. This is its
1. Check the CMOS Setup for custom settings
2. Load the interrupt handlers and device drivers
3. Initialize registers and power management
4. Perform the power-on self-test (POST)
5. Display system settings
6. Determine which devices are bootable
7. Initiate the bootstrap sequence
The first thing the BIOS does is check the information stored in a tiny
(64 bytes) amount of RAM located on a complementary metal oxide
semiconductor (CMOS) chip. The CMOS Setup provides detailed information
particular to your system and can be altered as your system changes. The
BIOS uses this information to modify or supplement its default programming
as needed. We will talk more about these settings later.
Interrupt handlers are small pieces of software that act as translators
between the hardware components and the operating system. For example,
when you press a key on your keyboard, the signal is sent to the keyboard
interrupt handler, which tells the CPU what it is and passes it on to the
operating system. The device drivers are other pieces of software that
identify the base hardware components such as keyboard, mouse, hard drive
and floppy drive. Since the BIOS is constantly intercepting signals to and
from the hardware, it is usually copied, or shadowed, into RAM to run
Booting the Computer
Whenever you turn on your computer, the first thing you see is the BIOS
software doing its thing. On many machines, the BIOS displays text
describing things like the amount of memory installed in your computer,
the type of hard disk and so on. It turns out that, during this boot
sequence, the BIOS is doing a remarkable amount of work to get your
computer ready to run. This section briefly describes some of those
activities for a typical PC.
After checking the CMOS Setup and loading the interrupt handlers, the BIOS
determines whether the video card is operational. Most video cards have a
miniature BIOS of their own that initializes the memory and graphics
processor on the card. If they do not, there is usually video driver
information on another ROM on the motherboard that the BIOS can load.
Next, the BIOS checks to see if this is a cold boot or a reboot. It does
this by checking the value at memory address 0000:0472. A value of 1234h
indicates a reboot, and the BIOS skips the rest of POST. Anything else is
considered a cold boot.
If it is a cold boot, the BIOS verifies RAM by performing a read/write
test of each memory address. It checks the PS/2 ports or USB ports for a
keyboard and a mouse. It looks for a peripheral component interconnect
(PCI) bus and, if it finds one, checks all the PCI cards. If the BIOS
finds any errors during the POST, it will notify you by a series of beeps
or a text message displayed on the screen. An error at this point is
almost always a hardware problem.
The BIOS then displays some details about your system. This typically
includes information about:
The floppy drive and hard drive
BIOS revision and date
Any special drivers, such as the ones for small computer system interface
(SCSI) adapters, are loaded from the adapter, and the BIOS displays the
information. The BIOS then looks at the sequence of storage devices
identified as boot devices in the CMOS Setup. "Boot" is short for
"bootstrap," as in the old phrase, "Lift yourself up by your bootstraps."
Boot refers to the process of launching the operating system. The BIOS
will try to initiate the boot sequence from the first device. If the BIOS
does not find a device, it will try the next device in the list. If it
does not find the proper files on a device, the startup process will halt.
If you have ever left a floppy disk in the drive when you restarted your
computer, you have probably seen this message.
This is the message you get if a floppy disk is in the drive when you
restart your computer.
The BIOS has tried to boot the computer off of the floppy disk left in the
drive. Since it did not find the correct system files, it could not
continue. Of course, this is an easy fix. Simply pop out the disk and
press a key to continue.
In the previous list, you saw that the BIOS checks the CMOS Setup for
custom settings. Here's what you do to change those settings.
To enter the CMOS Setup, you must press a certain key or combination of
keys during the initial startup sequence. Most systems use "Esc," "Del,"
"F1," "F2," "Ctrl-Esc" or "Ctrl-Alt-Esc" to enter setup. There is usually
a line of text at the bottom of the display that tells you "Press ___ to
Once you have entered setup, you will see a set of text screens with a
number of options. Some of these are standard, while others vary according
to the BIOS manufacturer. Common options include:
System Time/Date - Set the system time and date
Boot Sequence - The order that BIOS will try to load the
Plug and Play - A standard for auto-detecting connected devices;
should be set to "Yes" if your computer and operating system both support
Mouse/Keyboard - "Enable Num Lock," "Enable the Keyboard,"
Drive Configuration - Configure hard drives, CD-ROM and floppy
Memory - Direct the BIOS to shadow to a specific memory address
Security - Set a password for accessing the computer
Power Management - Select whether to use power management, as
as set the amount of time for standby and suspend
Exit - Save your changes, discard your changes or restore default
Be very careful when making changes to setup. Incorrect settings may keep
your computer from booting. When you are finished with your changes, you
should choose "Save Changes" and exit. The BIOS will then restart your
computer so that the new settings take effect.
The BIOS uses CMOS technology to save any changes made to the computer's
settings. With this technology, a small lithium or Ni-Cad battery can
supply enough power to keep the data for years. In fact, some of the newer
chips have a 10-year, tiny lithium battery built right into the CMOS chip!