CMOS is a small chip that stores your computer's basic settings when the power is off

CMOS stands for Complementary Metal-Oxide-Semiconductor. It is a type of computer chip that holds information about your hardware — things like the date and time, hard drive size, and boot order — even after you shut down your machine. Without CMOS, your computer would forget these settings every time you powered it off.

CMOS lives on your motherboard and runs on a small battery called a CMOS battery. That battery is why the chip can keep working when your computer is unplugged. The battery typically lasts three to ten years, depending on how often your computer is on and how much power the CMOS draws.

You will not see CMOS doing anything flashy. It does not speed up your games or make programs load faster. Instead, it handles the behind-the-scenes work that lets your computer start up correctly and remember what you told it to do.

Key Takeaways

  • CMOS is a chip on your motherboard that stores hardware settings and the system date and time, powered by a small battery.
  • When the CMOS battery dies, your computer forgets its settings and may fail to start or show the wrong date every time you turn it on.
  • You can reset CMOS by removing the battery for a few seconds, which clears all stored settings and returns them to factory defaults.
  • CMOS settings are accessed through the BIOS or UEFI menu, which you reach by pressing a key like Delete or F2 during startup.

What information does CMOS actually store

CMOS holds a small but essential set of data. The most obvious is the system date and time — if you have ever seen your computer clock reset to 2000 or 1980 after a power loss, that is a sign the CMOS battery is failing. CMOS also stores the order in which your computer checks drives to find the operating system, called the boot order.

It records hardware information like the size and type of your hard drives, the amount of RAM installed, and CPU speed. CMOS also keeps track of power settings — whether your computer should wake from sleep when you move the mouse, for example, or how long to wait before the screen goes dark.

Some CMOS chips store security settings, such as a password that prevents someone from changing the boot order or accessing the BIOS menu without permission. This is why resetting CMOS is sometimes used to bypass a forgotten BIOS password, though modern computers have made this harder to do.

How the CMOS battery works and when it fails

The CMOS battery is a small coin-shaped cell, usually a CR2032, that sits in a socket on the motherboard. It draws very little power — just enough to keep the CMOS chip awake and storing data. Because CMOS uses so little electricity, the battery can last for years even if your computer sits unplugged in a closet.

When the battery weakens, your computer will start to show signs. The date and time will reset to a default value every time you restart. You might see error messages during startup about the system date being incorrect. In some cases, the computer may refuse to start at all, or it may not recognize your hard drives.

A dead CMOS battery is not a hardware failure — it is a normal part of aging. Replacing it is straightforward and costs between five and fifteen dollars for the battery itself. You do not need to replace the entire motherboard.

How to access and change CMOS settings

CMOS settings are managed through a program called the BIOS or UEFI, depending on your computer's age and manufacturer. To enter this menu, you restart your computer and press a specific key during the startup process — usually Delete, F2, F10, or F12. The correct key appears briefly on the screen, or you can check your motherboard manual.

Once inside the BIOS or UEFI menu, you will see options to change the boot order, adjust power settings, enable or disable hardware components, and set the system date and time. The menu looks different on every motherboard, but the layout is usually straightforward — you navigate with arrow keys and press Enter to select.

Most people never need to change CMOS settings after the computer leaves the factory. The main reason to enter the menu is to change the boot order if you want to start from a USB drive or external hard drive, or to troubleshoot a hardware problem.

Resetting CMOS to factory defaults

If your computer is behaving strangely — refusing to start, showing incorrect hardware information, or locked by a forgotten BIOS password — you can reset CMOS to wipe all stored settings and return them to what the manufacturer set.

The most common method is to remove the CMOS battery. Shut down your computer completely, unplug the power cable, and wait at least 30 seconds. Open the case, locate the coin-shaped battery on the motherboard, and gently pop it out of its socket. Wait another 30 seconds, then reinsert the battery and close the case. When you power on, CMOS will be reset.

Some motherboards have a jumper — a small plastic bridge that connects two pins — that you can move to reset CMOS without removing the battery. Check your motherboard manual to see if yours has one. A few newer computers do not allow CMOS resets this way for security reasons.

CMOS versus RAM: why they are different

CMOS and RAM are both types of computer memory, but they do completely different jobs. RAM holds the programs and data your computer is actively using right now — it is fast and holds a lot of information, but it loses everything when you power off. CMOS holds only a small amount of permanent settings and runs on a battery so it survives a power loss.

Think of RAM as your desk — it holds everything you are working on today. CMOS is like a filing cabinet in the corner — it holds only the basic information your desk needs to function, and it stays locked up even when you leave the office.

If your computer is slow or freezing, the problem is almost never CMOS. It is almost always RAM, your hard drive, or a software issue. CMOS problems show up as startup failures, incorrect dates, or missing hardware — not performance problems.

Signs your CMOS battery is dying

The most obvious sign is the system date resetting to a default value — usually January 1, 2000, or the year your motherboard was made — every time you restart. You might also see error messages during startup that mention the date, time, or CMOS checksum.

Some computers will not start at all if the CMOS battery is completely dead, or they will start but fail to recognize hard drives or other hardware. If you have not changed any settings and your computer suddenly cannot find your operating system, a dead CMOS battery is worth checking.

A failing CMOS battery does not cause data loss — your files are safe on your hard drive. It only affects the settings that tell your computer how to start up and what hardware is installed.

Frequently Asked Questions

Can I use my computer if the CMOS battery is dead?

It depends on how dead it is. A weakening battery might only cause the date to reset. A completely dead battery can prevent your computer from starting or recognizing your hard drives. You can still use the computer while you wait for a replacement battery, but you will have to reset the date and time every time you restart.

How much does it cost to replace a CMOS battery?

The battery itself costs between five and fifteen dollars. If you replace it yourself, that is your only cost. If you take the computer to a repair shop, labor charges will add another thirty to eighty dollars depending on the shop and your location.

Will resetting CMOS delete my files or programs?

No. Resetting CMOS only clears the settings stored on the motherboard chip. Your files, programs, and operating system remain on your hard drive untouched. After a reset, your computer will boot normally, but you may need to re-enter settings like the system date or boot order.

What happens if I remove the CMOS battery while the computer is on?

Do not do this. Always shut down completely and unplug the power cable before removing the battery. Removing it while the computer is running can cause data corruption or hardware damage.

Is CMOS the same as the BIOS?

No. CMOS is the chip that stores settings. BIOS or UEFI is the program that lets you view and change those settings. BIOS reads from CMOS when your computer starts up, then uses that information to initialize your hardware.