What does CMOS stand For And What It’s For?

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Last updated on June 24th, at 09:20 am

what CMOS stands for

What is CMOS stands for and what it does on a computer motherboard ? Alternatively referred to as a “Real-time Clock” (RTC). CMOS is short for “Complimentary Metal-Oxide Semiconductor”.

CMOS is a term used to describe the small amount of memory on a computer’s motherboard that stores the BIOS settings and information’s. CMOS is an onboard semiconductor chip powered by a CMOS battery, as shown in the picture above.

The CMOS is also a computer chip but more specifically, it is a RAM chip, which stores information’s about the computer components, and also the various settings for those components and other variables.

However, normal RAM chips lose the information’s stored in them when the power supply is no longer supplied to the chip. So to retain the information’s in the CMOS chip, which stores all information’s about the BIOS settings CMOS battery is provided.

The CMOS setup lets you change the time and date, and settings for how devices are loaded at start up for example, the hard drives, CD and DVD drives and floppy drives.

The CMOS setup lets you enable and disable various hardware devices, including USB ports, the onboard video card, and sound card (if present), parallel and serial ports, and other devices, etc.

A CMOS battery on the motherboard supplies constant power to the CMOS chip to save these BIOS settings. If the battery is removed or the battery dies, the CMOS chip loses all information’s stored on it, thereby all settings are reset back to manufactured date and time and all BIOS settings reset to default settings.

You may have come across messages like “CMOS checksum error” when the system is booting, this happens when your CMOS battery is no longer able to maintain a constant power supply to the CMOS chip. After Changing the CMOS battery and resetting the date and time and BIOS settings will be necessary.

The BIOS program basically reads the information stored in the CMOS chip when the computer starts up. During the POST (Power on self test ) you get the option to enter the BIOS setup either by pressing “Delete” or the “F12” key, by doing so you are actually entering the CMOS setup and not the BIOS setup.

The BIOS chip and the program can not be updated by the user directly. The only way to update the BIOS is by using a BIOS Flash program and update the BIOS, which is provided by the motherboard manufacturer.

The CMOS setup on the other hand allows you to change the time and date, other settings like how a devices are loaded at startup, you can set your boot device preferences, you can also disable certain hardware from the CMOS settings itself like the USB ports, video cards, sound card, parallel or serial ports etc.

Most CMOS batteries usually last the lifetime of a motherboard (up to 8 to10 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. Time to change your CMOS battery.

You may also like to read :

What is BIOS ? How BIOS works.

How to Clear BIOS or CMOS Password

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2 thoughts on “What does CMOS stand For And What It’s For?”

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