Computers, Laptops & Tablets > Accessories & Hardware 632 632 people found this article helpful What Is CMOS and What Is It For? CMOS and CMOS batteries: Everything you need to know By Tim Fisher Tim Fisher Senior Vice President & Group General Manager, Tech & Sustainability Emporia State University Tim Fisher has more than 30 years' of professional technology experience. He's been writing about tech for more than two decades and serves as the SVP and General Manager of Lifewire. lifewire's editorial guidelines Updated on July 31, 2023 Reviewed by Ryan Perian Reviewed by Ryan Perian Western Governors University Ryan Perian is a certified IT specialist who holds numerous IT certifications and has 12+ years' experience working in the IT industry support and management positions. lifewire's editorial guidelines In This Article Expand Jump to a Section Other Names for CMOS Clearing CMOS How BIOS and CMOS Work Together What Is a CMOS Battery? More About CMOS & CMOS Batteries Frequently Asked Questions Trending Videos Close this video player CMOS 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 image sensor is different—it's used by digital cameras to convert images into digital data. Other Names for CMOS CMOS Battery on a Computer Motherboard. Steve Gschmeissner / Getty Images CMOS (pronounced see-moss) is hardly every described by its full name: complementary metal-oxide-semiconductor. However, it 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. Other CMOS errors revolve around low battery voltage, CMOS checksum, battery failure, and read error. Easy Ways to Clear CMOS 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 bootloader 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 aren't 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 depending on how the device is being used. Incorrect or slow system date and time, and loss of BIOS settings, are major signs of a dead or dying CMOS battery. Another indication of a dead CMOS battery could be a red light on the motherboard. Replacing the CMOS battery is as easy as swapping out the dead one for a new one. You can get a new CMOS battery on Amazon and through other retailers that sell computer replacement parts. 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 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. FAQ What are typical symptoms of CMOS battery failure? Several issues can be related to CMOS failure. For instance, if a laptop has difficulty booting up, can't connect to the internet, or is constantly beeping. Other symptoms include drivers disappearing, peripherals not responding, and the date and time resetting. What is a CMOS Checksum error? A CMOS Checksum error is a conflict between the CMOS and BIOS when booting. You can fix this error by following several troubleshooting steps, including restarting the computer, downloading and flashing a BIOS update, resetting the BIOS, and possibly replacing the CMOS battery. Was this page helpful? Thanks for letting us know! Get the Latest Tech News Delivered Every Day Subscribe Tell us why! Other Not enough details Hard to understand Submit