Difference Between CMOS and BIOS


BIOS and CMOS are two terms that many people think they know but cannot differentiate between. BIOS and CMOS are two different and distinct things in a computer, but being closely related, they are spoken of almost interchangeably. BIOS (basic Input Output System) is a program containing instructions for the start up of a computer, whereas CMOS (complimentary metal oxide semiconductor) is where all this BIOS information such as date, time, and system configuration details that are needed to start the computer are stored. Yes, they are closely related and perform very important functions at the startup of a computer but they cannot be more dissimilar also. Let us find the difference between BIOS and CMOS that seems to be so very confusing.

In the simplest of terms, BIOS is a computer program that controls the computer when you switch on the power until the time the operating system of the computer takes over. Since BIOS is a firmware, it cannot store even this small bit of information, and CMOS it is that is used by BIOS to store the variable data. Thus BIOS initializes and controls the computer on its startup with the help of variable data stored in a memory chip called CMOS.

CMOS stores the information for as long as it receives power. This power is provided to it through a small battery. When you start the computer, it is BIOS that tests the system and readies it for operation searching for all the variable data that is stored on CMOS. After this, it also loads the OS and then passes control to it. What makes the situation confusing for people is the fact that as BIOS information is stored on a CMOS chip, the set up is also sometimes referred to as CMOS set up. But now you know the real difference between BIOS and CMOS, don’t you.

One of the reasons why CMOS was preferred to store the information of BIOS was that it consumes very little power. The CMOS chip is constantly powered and even when the system is off, there is a small battery (CR-2032) that keeps it powered and the information is intact. On the contrary, the important information about start up is stored in the form of a code in a non volatile memory thus it is not lost. There is also the fact that BIOS needs to run for just seconds before OS is on and takes over the control of the system from it.

Though both BIOS and CMOS are important during start up, there is nothing to worry even if this vital information from CMOS is lost. It can easily be recovered by booting the system. BIOS is more important as without it, computer wouldn’t start up at all. Though there are many ways to prevent corruption of BIOS, once it does get corrupt, BIOS chip has to be removed and reprogrammed.


• BIOS is basic input output system while CMOS is complimentary metal oxide semi conductor chip

• BIOS is a program that contains information about the configuration of a computer and this information is stored in a chip called CMOS

• CMOS needs to be constantly powered while BIOS does not

• Even if information from CMOS is lost, it can be recovered while if BIOS becomes corrupt, it needs to be replaced

  • anonymous

    nice explaination!