Full Form of CMOS: What is CMOS in Computer?

The term CMOS might sound unfamiliar but you probably may have heard it frequently. Have you ever thought about how the computer keeps time and date even when it has turned off! The reason for this is CMOS. So, what is CMOS?

In this article, we have briefly covered the most important topics about CMOS, such as the full form of CMOS, What is CMOS, Who invented CMOS, and more. 

What You Will Learn

What does CMOS stand for?

CMOS stands for “Complementary Metal Oxide Semiconductor”. Other terms commonly used for CMOS are CMOS RAM, RTC (Real-Time Clock), NVRAM (Non-Volatile RAM), Non-Volatile BIOS memory, and COS-MOS (Complementary-Symmetry Metal-Oxide-Semiconductor).

Full Form of CMOS

The full form of CMOS can be explained as:

C Complementary
M Metal
O Oxide
S Semiconductor

Now, let’s talk about what CMOS is:

What is CMOS?

CMOS is a battery-powered integrated circuit that is assembled on a computer motherboard. It is a special kind of memory chip used to store basic information on the computer. The information that is stored can be BIOS settings, system time and date, as well as hardware settings of the system. 

Who invented CMOS?

Frank Wanlass first invented CMOS at ‘Fairchild Semiconductor’ in 1963. Later in 1968, the CMOS Integrated Circuit was invented by Albert Medwin and his team.

The first CMOS RAM chip was used in early IBM computers. The chip was named as the Motorola 146818. It was capable of storing 64 bytes of data. 

What is CMOS used for?

As stated above, the main function of CMOS is to store start-up data that BIOS uses to boot the computer system. The CMOS chip is generally powered by the CMOS battery.

For example, there are several functionalities in computers that keep on running even when the computer is turned off. They require very less power consumption. The clock system in computers is one such functionality that consumes a significant amount of power. The clock system keeps on working and this is possible only because of CMOS. 

Besides, we can reinsert the CMOS battery to reset the BIOS settings to the default configuration. This is one of the easiest troubleshooting steps for most of the complex computer errors. 

What is CMOS Battery?

The CMOS battery is a coin-sized cell battery that is responsible for powering up the CMOS memory chip. The CMOS battery is found on the motherboard and can be easily replaced if it is dead. However, replacing a CMOS battery will change the BIOS settings as well as the date and the time of the computer system to default. 

The assembled CMOS battery looks like the following image:

Full Form of CMOS - CMOS in Computer

Note: It should be noted that the CMOS battery and CMOS sensor are the two different things. CMOS sensors are generally used in digital cameras to convert images into digital data.

Devices that use CMOS

CMOS technology is generally used in the following devices:

SRAM (Static RAM)
Digital Logic Circuits

Life Span of CMOS

CMOS is a technology that includes both negative and positive metal-oxide semiconductors. Because CMOS uses one charge at a time, it consumes very less power.  The main reason for this is that the charges can stay in the same state for a longer time. Therefore, energy is consumed only when it is required. Due to this, CMOS based transistors do not generally heat up and provides high-speed. It also helps CMOS last longer.

The standard lifespan of a Lithium-based CMOS battery is around 10 years. However, it may vary depending on the usage and the atmosphere where the computer system resides.

How to know if CMOS battery is failing?

It is very easy to know if the CMOS battery is failing. In such a case, the computer clock system will behave inappropriately. This means that the date of the computer will change to January 1, 1990, and the time will reset to 12:00 pm. This is the common sign that indicates the CMOS battery has failed.

Besides, if there are the following errors as soon the computer boots up, then this might also be an indication for the CMOS battery failure:

CMOS Read Error
CMOS Battery Failure
CMOS Checksum Error
System battery voltage is low

Can we recharge a dead CMOS battery?

It actually depends on the types of battery your computer has. There are both types of batteries. But, most computers are using Lithium batteries and they cannot be recharged. CMOS CR2032 is an example of a non-chargeable battery while CMOS ML2023 is an example of a rechargeable battery. However, the rechargeable batteries cannot be recharged from the computer itself.

How CMOS and BIOS work together?

BIOS and CMOS are the chips on the computer motherboard, but they are two different components with different functionalities. They both work together for a specific purpose on computers. The purpose of the BIOS chip is to communicate between the CPU and other hardware components. Besides, BIOS firmware is responsible for Power-On-Self-Test that makes sure that all the connected components or devices are working.

On the other side, the CMOS chip is similar to a RAM chip, which means it will lose the settings once the computer is turned off. But, CMOS battery is used to keep the CMOS chip working even when the computer is turned off.

When a computer is started, BIOS pulls the data from CMOS chip and synchronizes the time, date, and hardware settings, etc. Then it creates communication between hardware components and boots up normally. The CMOS chip usually stores as little as 256 bytes of data.


CMOS (abbreviation for 'Complementary Metal Oxide Semiconductor') is one of the most important components of a computer system, and turning on the computer without CMOS will be problematic. Besides, if there is a dead CMOS in a computer, the system will boot normally but the time and date will be changed to default every-time when it is turned off. The computer won’t be able to keep the current time and date.

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