What is the difference between BIOS and CMOS?

Updated: 06/02/2020 by Computer Hope

The terms BIOS and CMOS both refer to essential parts of your computer's motherboard. They work together and they're both important, but they are not the same thing.

Let's take a look at what the BIOS and CMOS are, and how they're different.


The BIOS (basic input/output system) is firmware stored in a chip on your computer's motherboard. It is the first program that runs when you turn on your computer.


The BIOS performs the POST (power-on self-test), which initializes and tests your computer's hardware. Then it locates and runs your boot loader, or loads your operating system directly.

The BIOS also provides a simple interface for configuring your computer's hardware. When you start your computer, you may see a message like "Press F2 for setup." This setup is your BIOS configuration interface.


When you make changes to your BIOS configuration, the settings are not stored on the BIOS chip itself. Instead, they are stored on a special memory chip, which is called "the CMOS." CMOS stands for "Complementary Metal-Oxide-Semiconductor." It's the name of a manufacturing process used to create processors, RAM (random-access memory), and digital logic circuits, and is also the name for chips created using that process.

Like most RAM chips, the chip that stores your BIOS settings is manufactured using the CMOS process. It holds a small amount of data, usually 256 bytes. The information on the CMOS chip includes types of disk drives are installed, the current date and time of your system clock, and your computer's boot sequence.

On some motherboards, the CMOS is a separate chip. However, on most modern motherboards, it is integrated with the RTC (real-time clock) on the southbridge.

Your BIOS memory is non-volatile: it retains its information even when your computer has no power because your computer needs to remember its BIOS settings even when it's turned off. That's why the CMOS has its own dedicated power source, which is the CMOS battery.

CMOS battery

CMOS battery

The CMOS battery is a lithium-ion battery about the size of a coin. It can hold a charge for up to ten years before needing to be replaced. If your CMOS battery dies, your BIOS settings reset to their defaults when your computer is turned off.


If you turn on your computer and your settings were reset or forgotten, your CMOS battery may be dead. For example, your hard drive may no longer detected correctly, or your system clock may be reset to a date in the past, such as January 1st, 1970. If so, you can replace the battery, see: How to replace the CMOS battery.

Some motherboards, including most modern ATX motherboards, continue to provide power to the CMOS if the battery is replaced while the computer is actively powered.


To replace your CMOS battery, make sure to consult your motherboard documentation for details and safety information. Visit your motherboard manufacturer's support website if you need to download a new copy of your manual.