What is CMOS?

What is CMOS?

You have probably heard the word CMOS, but do you know what it is? Let's figure it out with this article.

First, let's find out what the BIOS is and what it does in a computer or laptop. The abbreviation BIOS stands for Basic Input Output System. This is a program that ensures that the computer starts and runs until the operating system loads. By itself, it is just a set of instructions stored in a computer's read-only memory (ROM) located on the motherboard. However, if the BIOS had only permanent memory, then its capabilities would be limited to the parameters that were hardwired during the manufacture of the BIOS chip, and users would not be able to make changes to the BIOS settings.

Not being able to change settings on your computer would be very inconvenient. In order to solve this problem, there is a special BIOS-related CMOS memory that stores system settings, in particular, entered by the user through the BIOS Setup interface. At the same time, the total amount of BIOS CMOS memory is extremely small - only 256 bytes!

CMOS manufacturing technology and its history

What is the difference between CMOS memory and other types of memory? The abbreviation CMOS stands for Complementary Metal Oxide Semiconductor. This memory features low power consumption and therefore can store data for a long time.

CMOS technology has a long history and this type of memory called CMOS RAM was first introduced in 1963. It was relatively expensive at that time, but had many advantages. Although this type of memory is slower than conventional RAM, it consumes less electrical energy and generates less thermal energy (heat) during operation.

BIOS data storage does not require high speed, but the amount of energy consumed during this task should ideally be as small as possible, so memory made using CMOS technology is most suitable in this case.

In the half century since its discovery, CMOS technology has been greatly improved. Now CMOS RAM chips are used in most elements of the computer, even in the processor itself. Moreover, this technology is used not only in computers. Chips of this type are also widely used in photosensitive elements (matrices) of devices such as scanners and digital cameras.


Although this memory consumes little power, a reliable power source is still required. In particular, a special RTC CMOS battery is used to power the BIOS memory during those periods when the computer is turned off. The battery lasts about 3-5 years and needs to be changed in order to save the contents of the memory. Most manufacturers provide easy access to the battery, so it can be replaced within minutes.


CMOS memory is a small but very important element of the BIOS system. Smooth operation and longevity of the entire computer depends on this one tiny battery. When the battery fails, the BIOS loses its configuration settings and defaults to the factory settings. This can cause a number of problems, including: an incorrect time and date, difficulty booting up the computer, and random resets. In some cases, a failed CMOS battery can also lead to data loss. If you suspect that your CMOS battery has failed, it's important to replace it as soon as possible. Fortunately, CMOS batteries are relatively inexpensive and easy to replace, but keep in mind that manufacturers have different connectors, wire length, battery types and part numbers. For example, the RTC battery for Lenovo ThinkPad Yoga has the part number 00HN933 and most likely this battery will not work with another laptop. In each case, it is best to look at the documentation for the motherboard or check your original battery for a part number. You can also contact and specialized CMOS battery manufacturer, such as this one, and have them cross-refernce the battery by the computer’s model number. Once the new battery is in place, your computer should function normally again.

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