Difference between CMOS and BIOS

Dennis Faas's picture

CMOS or BIOS? What's the difference?

The CMOS (pronounced "sea moss") is a computer chip responsible for storing settings of a computer system. The word "CMOS" is actually an acronym which stands for Complementary Metal-Oxide Semiconductor.

Try and say that 10 times without slipping!

Most users have undoubtedly witnessed a message stating "Press DEL to enter Setup", or something similar when a computer has been powered on. If the DEL key was pressed at this instance, something called a CMOS utility screen would appear on the screen. Various hardware settings can be manipulated on this screen, such as the geometry of a hard drive, which device to boot first, and so on.

BIOS stands for Basic Input Output System. The BIOS holds the most basic computer instructions required to boot a computer into an operating system such as DOS or Windows. You can think of the BIOS as a mini pre-operating system.

Since the CMOS and BIOS are so tightly intertwined, they are commonly referred to as one in the same. Just recently, I received a question which asked:

" I'm trying to add a hard drive to a computer. When I go to the CMOS setup, a screen comes up asking me for a Password. I can't get into the CMOS to tell the computer the new hard drive is present. Is there a way that I can somehow disable the CMOS password? "

The first thing I would do is open up the computer and look on the main board for a CMOS jumper reset. Typically, the CMOS jumper is located close to a lithium battery which keeps track of the computer's time clock.

There are different methods used to reset a CMOS using a jumper. For this reason, I will not explain the procedure since it may be hazardous to some motherboards. In this case, consult the motherboard manual and read how to reset the CMOS using the jumper on the main board.

What if the computer does not have a CMOS jumper?

Some older computer systems do not have CMOS jumpers. So what do you do if the CMOS is password protected and you can't get in? Answer: killcmos.COM

Killcmos.com is a utility that is used to add an extra "bit" to the CMOS memory which essentially corrupts the settings. When the CMOS settings are unusable, the BIOS intervenes and resets all values to default -- which also removes any CMOS password protection. Once the password protection is removed, all CMOS settings (including the time clock) must be reconfigured. At this point, hard drive geometries can be configured and boot devices can be set to start.

Is killcmos.com a virus? Since the intent of killcmos.com is to reset the CMOS by corrupting the values, some virus scanners will provide a "false positive" when killcmos.com is encountered.

Do not attempt to quarantine or clean the file unless the virus scanner states that killcmos.com is a virus other than the killcmos.com executable. Attemping to clean killcmos.com with a virus scanner will most likely damage the executable and render it useless.

A virus-free copy of killcmos.com can be downloaded from Utility Geek:


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