BIOS

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 (see How Microprocessors Work for details). You
are probably very familiar with two different types of
software:

* 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. (See How Operating Systems Work for lots
of details.)
* 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 install them.

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 needs updating.

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 chips.

* Providing a set of low-level routines that the
operating system uses 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
booting.

* 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.

When you turn on your computer, the BIOS does several things. This is its usual sequence:

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 faster.

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 processor
* The floppy drive and hard drive
* Memory
* BIOS revision and date
* Display

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 disk when you restarted your computer, you have
probably seen this message.

The BIOS has tried to boot the computer off of the 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.