What is CMOS?


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If you’re having boot or display problems on your computer, you’ll likely find recommendations on how to clear the CMOS. But what does CMOS do and how is it related to BIOS? Here is everything you need to know.

BIOS settings storage

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

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

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

How are BIOS and CMOS related?

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

However, because the BIOS is stored in read-only memory, it cannot save any changes you make to its settings, such as boot order adjustments or overclock settings. These changes, along with the system date and time, are saved to CMOS. So CMOS can be essential for the stable operation of your computer. And if your CMOS clears or your battery fails, you lose all your BIOS settings and revert to default settings.

What is a CMOS battery?

Person installing a CR2025 lithium battery as a backup on a motherboard.
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The CMOS battery is usually a CR2032 lithium coin battery on your computer’s motherboard. Traditionally, it was used to power the CMOS, as older motherboards used volatile memory to store BIOS settings. Volatile memory loses data when it loses power. Therefore, a battery was used to provide power to the memory even after the computer was turned off or the power cord was removed.

However, modern UEFI motherboards use nonvolatile memory, or NVRAM, to store UEFI configuration data. Non-volatile memory can retain data even with no power applied. That said, CMOS batteries are still used to keep time in your computer. And since CMOS battery removal or failure has traditionally been associated with deleting CMOS data, modern motherboards also reset the UEFI settings when the CMOS battery fails.

Most CMOS batteries last between two and 10 years, depending on the use of the computer. But if your computer displays the wrong system time or date after a reboot or if you have hardware compatibility issues, it may be time to replace the CMOS battery. Other key symptoms of a bad CMOS battery include problems with the boot process and a constant beeping sound 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 times when you may need to clear your computer’s CMOS to reset BIOS or UEFI settings. However, it is a relatively straightforward process, and depending on whether or not you have access to the BIOS or UEFI menu, there are a few ways to accomplish this.

One of the easiest ways to clear CMOS is to go to the BIOS or UEFI settings and choose the reset option. It is usually called “Load Factory Defaults” or “Load Configuration Defaults”. You can also use the Clear CMOS button if it is available on your motherboard. This button can be found on the IO board or near the CMOS battery.

Clear CMOS button.

Also, reseating the CMOS battery is another way to clear CMOS data. But you will need to open up your computer and access the motherboard to reseat or replace the battery. But do not worry. You can find detailed instructions and more ways to clear your computer’s 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 to messing 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.

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