BIOS Components and Features



There are several components and features that are commonly found 
these days as part of a modern BIOS. These are examined in the 
sections below. 


The BIOS ROM

The main hardware component of the system BIOS is the system BIOS 
ROM itself. This is normally located in an electrically-erasable 
read-only memory (EEPROM) chip, which allows it to be updated 
through software control. This is commonly called a flash BIOS.

The BIOS ROM is located in a socket on the motherboard and is 
relatively easy to locate because it is usually labelled with 
the name of the BIOS manufacturer. Most times this is Award, 
American Megatrends (AMI) or Phoenix. There is also often a 
version number on the chip, although the actual BIOS version 
within the chip may be different from what is labelled, because 
of the ability to flash the BIOS mentioned above.

Under normal circumstances, the BIOS ROM is permanent and there 
is normally no reason to need to deal with it in any way. If for 
some reason the BIOS ROM were to become corrupted due to an 
aborted flash update, for example, you might find your PC left 
in a state where it could not be booted. In this situation, you 
might have to physically replace the BIOS ROM, but this is a 
very rare happening.


BIOS CMOS Memory

The BIOS settings that you use to control how your PC works must 
be saved in non-volatile memory so that they are preserved even 
when the machine is off. This is as opposed to regular system 
memory, which is cleared each time you turn off the PC. A special 
type of memory is used to store this information, called CMOS 
memory, and a very small battery is used to trickle a small 
charge to it to make sure that the data it holds is always 
preserved. These memories are very small, typically 64 bytes, 
and the batteries that they use typically last for years. 
This non-volatile memory is sometimes called NVRAM.

CMOS stands for "Complementary Metal Oxide Semiconductor". This 
is one type of technology used to make semiconductors 
(integrated circuits) such as processors, chipset chips, DRAM, 
etc. CMOS has the advantage of requiring very little power, 
compared to some other semiconductor technologies. This is why 
it was chosen for this use, so that the amount of power required 
from the battery would be minimal, and the battery would be able 
to last a long time. This memory came to be called just "CMOS" 
since in the early days most parts of the computer did not use 
CMOS. Ironically, with today's processors having to do more and 
more and needing to do it with lower power consumption, they 
themselves are typically made entirely with CMOS technology. 
However, "CMOS" by itself usually still refers to the BIOS 
settings memory. Old habits die hard in the computer world.

The system uses something called a CMOS checksum as an 
error-detecting code. Each time you change the BIOS settings, 
the checksum is generated by adding together all the bytes in 
the CMOS memory and then storing the lowest byte of the sum. 
Then, each time the system booted, the system recomputes the 
checksum and compares it to the stored value. If they are 
different, then the system knows that the CMOS has been corrupted 
somehow and will warn you with an error, typically something like 
"CMOS Checksum Error" .

There are many different types of batteries used to power the 
CMOS; mostly, these have changed over time as technology has 
evolved. These batteries are discussed here . You will not 
normally have to deal with the CMOS memory directly; it holds 
the settings you enter in the BIOS setup program. Over time, it 
is possible you will have CMOS problems; for example, you may 
find that the machine may begin to forget its settings when it 
is booted. These are usually signs of trouble with the 
motherboard battery.

In addition to the standard CMOS memory used to hold system 
settings, Plug and Play BIOSes use an additional non-volatile 
memory to hold extended system configuration data (ESCD) . This 
is used to record the resource configurations of system devices 
when Plug and Play is used.


BIOS Updates and The Flash BIOS

As you can see by looking at the rest of the information on the 
BIOS, it is a very important program indeed. Its quality and 
modernness determine the features and capabilities of your 
machine to a large degree. The most famous example of this is 
support for IDE/ATA hard disks over 504 MB; the only change 
necessary to support these larger drives in most cases is a BIOS 
capable of doing the geometry translation . There are other 
examples as well; many motherboard manufacturers are able to 
expand the capabilities of their boards, or fix problems with 
them, by making changes to the BIOS and either giving free 
downloads or selling upgrades, just as software application 
houses like Microsoft, Lotus, etc. do.

The BIOS program in your PC is programmed into a read-only memory 
(ROM). ROMs are, of course, not rewriteable the way RAM is; that 
is why they are called "read-only". This presents a problem when 
you want to update your BIOS. In "the old days" when you wanted 
to update your BIOS, the manufacturer sent you a new BIOS chip; 
you opened the box, pulled out the old chip, and put the new one 
in. Needless to say, this is a pain. Fortunately, technology came 
to the rescue through the invention of the flash BIOS. Some 
machines still require the physical upgrade but by using the 
flash BIOS, most newer machines can upgrade using special 
software without having to open the case at all.

Machines with flash BIOS capability use a special type of BIOS 
ROM called an EEPROM; this stands for "Electrically Erasable 
Programmable Read-Only Memory". As you can probably tell by the 
name, this is a ROM that can be erased and re-written using a 
special program. This procedure is called flashing the BIOS and 
a BIOS that can do this is called a flash BIOS. The advantages 
of this capability are obvious; no need to open the case to pull 
the chip, and much lower cost.

Many motherboards have a special "safety feature" to prevent 
accidental (or malicious) changes to the flash BIOS--a jumper 
that must be changed before performing a flash BIOS upgrade. 
While this is a security feature, it also obviates one of the 
great advantages of the flash BIOS, namely not having to open the 
case! Your motherboard manual will tell you whether you have a 
jumper or not. With the increasing commonality of viruses that 
can change flash BIOS code, this may soon be a feature on 
every motherboard. 

There is one big disadvantage to using the flash process to 
upgrade your BIOS. While the BIOS is actually being flashed, it 
is in a very vulnerable state. If you are really unlucky and 
something very bad happens in the middle of the upgrade (for 
example, a power outage), it is possible to end up with a 
corrupted BIOS chip. You can also end up with a corrupted system 
if you boot the wrong flash BIOS image into the chip--motherboard 
manufacturers usually use an unintelligent, universal flash 
program that will happily program the entirely wrong BIOS image 
into your system if you tell it to! Fortunately, some 
manufacturers are now making these programs smarter. For example, 
Award's flash program checks the name of the image file against 
the model of motherboard and will complain if there is a 
mismatch. (Good idea, guys!)

If your system becomes corrupted to the point where it will not 
boot, you are in a catch-22. Your BIOS is corrupted, so you can't 
boot the PC, and since you can't boot the PC there's no 
conventional way to re-flash the BIOS. See this section in the 
troubleshooter for ideas on recovering from this problem.

Fortunately, BIOS chip corruption is quite rare; the BIOS is a 
small program and flashing it usually takes only a few seconds. 
Still, it's not something to try in the middle of a 
thunderstorm, and I try to use a UPS when doing this type of 
work, if possible. And be sure you are using the right 
BIOS image.


The Boot Block

The preceding section discusses updating the program code in 
the system BIOS by flashing it (replacing it under software 
control). If the flash procedure is done improperly, the BIOS 
code can become corrupted, which will cause the system to go 
into an unbootable state. Many newer systems come with a feature 
where a 4 KB "boot block" program is included as part of the 
BIOS. This is a tiny piece of code whose job it is to recover 
from a situation where the BIOS code is incorrect or corrupted.

If your motherboard supports this feature, when the PC tries to 
boot and finds the BIOS code corrupted, the boot block will try 
to recover the BIOS code, usually by reading it from a 
specially-prepared floppy disk. You may have to change a jumper 
on the motherboard to enable this capability, and you may need 
to make use of a "plain vanilla" ISA video card. The boot block 
will load the BIOS code and then when you next reboot, the 
regular BIOS code should be in place and the problem resolved.

You will need to follow the special recovery procedure outlined 
by your motherboard manufacturer if you want to take advantage 
of this feature.


Plug and Play

The BIOS is one of the four major parts of the system whose 
cooperation is required in order to implement Plug and Play 
features on a PC. In short, Plug and Play (also called "PnP" for 
short, I won't get into its sarcastic nicknames here :^) ) is a 
system whereby your PC hardware, BIOS and operating system work 
together to identify and configure resource usage by hardware 
devices automatically. The intention is to reduce the number of 
resource conflicts, jumper settings, and manual driver setups 
required to get the system to work.

The BIOS is a key part of this; in fact it does much of the work 
of identifying and configuring expansion cards, and passing 
configuration information on to the operating system. Plug and 
Play in general is discussed in detail here .
