What Is CMOS and What Is It For?

CMOS and CMOS Batteries: Everything you need to know

CMOS Battery on a Motherboard

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CMOS (short for complementary metal-oxide-semiconductor) is the term usually used to describe the small amount of memory on a computer motherboard that stores the BIOS settings. Some of these BIOS settings include the system time and date as well as hardware settings.

A CMOS sensor is different—it's used by digital cameras to convert images into digital data.

Other Names for CMOS

CMOS (pronounced see-moss) is sometimes referred to as Real-Time Clock (RTC), CMOS RAM, Non-Volatile RAM (NVRAM), Non-Volatile BIOS memory, or complementary-symmetry metal-oxide-semiconductor (COS-MOS).

CMOS is also an abbreviation for other terms that are unrelated to what's talked about on this page, like cellular management operation system and comparison mean opinion score.

Clearing CMOS

Most talk of CMOS involves clearing CMOS, which means to reset the BIOS settings to their default levels. This is a really easy task that's a great troubleshooting step for many types of computer problems.

For example, maybe your computer is freezing up during the POST, in which case clearing the CMOS to reset the BIOS settings to factory default levels, might be the easiest solution.

Or maybe you need to clear CMOS to reset misconfigured BIOS settings to fix certain hardware-related error messages, such as Code 29 errors.

How BIOS and CMOS Work Together

The BIOS is a computer chip on the motherboard like CMOS except that its purpose is to communicate between the processor and other hardware components like the hard drive, USB ports, sound card, video card, and more. A computer without a BIOS wouldn't understand how these pieces of the computer work together.

The BIOS firmware is also what performs the Power On Self Test to test those pieces of hardware, and what ultimately runs the boot loader to launch the operating system.

CMOS is also a computer chip on the motherboard, or more specifically a RAM chip, which means it would normally lose the settings it's storing when the computer is shut down (just like how the contents of RAM are not maintained each time you restart your computer). However, the CMOS battery is used to provide constant power to the chip.

When the computer first boots up, BIOS pulls information from the CMOS chip to understand the hardware settings, time, and anything else that's stored in it. The chip typically stores as little as 256 bytes of information.

What Is a CMOS Battery?

The CMOS is usually powered by a coin-sized CR2032 cell battery, referred to as the CMOS battery.

Most CMOS batteries will last the lifetime of a motherboard, up to 10 years in most cases, but will sometimes need to be replaced.

Incorrect or slow system date and time, and loss of BIOS settings, are major signs of a dead or dying CMOS battery. Replacing them is as easy as swapping out the dead one for a new one.

More About CMOS & CMOS Batteries

While most motherboards have a spot for a CMOS battery, some smaller computers, like many tablets and laptops, have a small external compartment for the CMOS battery that connects to the motherboard via two small wires.

Some devices that use CMOS include microprocessors, microcontrollers, and static RAM (SRAM).

It's important to understand that CMOS and BIOS are not interchangeable terms for the same thing. While they work together for a specific function within the computer, they are two entirely different components.

When the computer is first starting up, there's an option to boot into BIOS or CMOS. Opening the CMOS setup is how you can change the settings it's storing, like the date and time and how the different computer components are first started up. You can also use CMOS setup to disable/enable some hardware devices.

CMOS chips are desirable for battery-powered devices like laptops because they use less power than other types of chips. Although they use both negative polarity circuits and positive polarity circuits (NMOS and PMOS), only one circuit type is powered on at a time.

The Mac equivalent to CMOS is PRAM, which stands for Parameter RAM. You can also reset your Mac's PRAM.