A CMOS battery backup for BIOS on a motherboard.

If you are having boot or display issues on your computer, you are likely to come across recommendations about clearing the CMOS. But what does CMOS do, and how is it related to BIOS? Here’s everything you need to know.

BIOS Configuration Storage

The term CMOS, pronounced as “sea-moss,” is commonly used to describe a small memory on a computer motherboard that stores BIOS or UEFI settings, including system date, time, and hardware configuration.

CMOS is short for Complementary Metal-Oxide-Semiconductor, which is a type of fabrication process used in the construction of various integrated circuit (IC) chips, including the memory chip being used to store the BIOS settings.

It is also referred to as CMOS RAM, BIOS RAM, and non-volatile BIOS memory. While CMOS is most commonly associated with Windows computers, Macs have a similar memory, and it’s called NVRAM or PRAM.

How Are BIOS and CMOS Related?

A computer chip on a motherboard

To understand the relation between BIOS and CMOS, it helps to know the role of a BIOS in your computer. BIOS, short for Basic Input-Output System, is a firmware that comes with your computer’s motherboard. It’s responsible for initializing your computer’s hardware, testing it, and booting up the operating system when you hit the power button.

However, as BIOS is stored in read-only memory, it can’t save any changes you make to its configuration, such as tweaks to the boot order or overclock settings. These changes, along with the system date and time, are saved in CMOS. So CMOS can be essential to your computer’s stable functioning. And if your CMOS is cleared or its battery fails, you lose all your BIOS tweaks, and it reverts to the default settings.

What Is a CMOS Battery?

Person installing a lithium CR2025 battery as a backup on a motherboard.

The CMOS battery is usually a CR2032 lithium coin battery on your computer’s motherboard. It was traditionally used to power the CMOS as older motherboards used a volatile memory to store the BIOS settings. Volatile memory loses data when it loses power. So a battery was used to provide power to the memory even after you had shut down the computer or removed the power cord.

However, the modern UEFI motherboards use a non-volatile memory or NVRAM to store UEFI configuration data. The non-volatile memory can retain data even without applied power. That said, CMOS batteries are still used to keep time in your computer. And since the removal or failure of CMOS battery has traditionally been associated with clearing CMOS data, modern motherboards also reset UEFI settings when CMOS battery fails.

Most CMOS batteries last anywhere between two to 10 years, depending on the computer usage. But if your computer is showing the wrong system time or date after a reboot or you are facing hardware compatibility issues, it may be time to replace the CMOS battery. Other key symptoms of a failed CMOS battery include problems in the boot process and constant beeping noise from the motherboard. You may also get errors, such as “CMOS Read Error,” “CMOS Battery Failure,” and “CMOS Checksum Error.”

How to Clear CMOS

There are occasions when you may have to clear your computer’s CMOS to reset BIOS or UEFI settings. However, it’s a relatively straightforward process, and depending on whether or not you have access to BIOS or UEFI menu, there are a few ways to achieve this.

One of the easiest ways to clear CMOS is by going to the BIOS or UEFI settings and choosing the reset option. It’s typically called “Load factory defaults” or “Load setup defaults.” You can also use the CMOS clear button if available on your motherboard. This button can either be found on the IO plate or near the CMOS battery.

Clear CMOS button.

In addition, reseating the CMOS battery is another way to clear the CMOS data. But you will need to open your computer and access the motherboard to reseat or replace the battery. But don’t worry. You can find detailed instructions and more ways to clear your computer CMOS in our detailed guide.

Small but Important Part

CMOS is a nifty little component that most computer users never have to worry about. Also, there is no benefit in tinkering with it unless you are troubleshooting your computer and need to revert BIOS or UEFI to its default settings. So you can forget about CMOS until you need it.

Profile Photo for Gaurav Shukla Gaurav Shukla
Gaurav Shukla is a technology journalist with over a decade’s experience reporting and writing about consumer technology. His work has appeared in Android Police, XDA Developers, and NDTV Gadgets 360.
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