The terms BIOS and also CMOS both to express to crucial parts of your computer"s motherboard. They job-related together and they"re both important, yet they are not the very same thing.

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Let"s take it a look at what the BIOS and also CMOS are, and also how they"re different.


The BIOS (Basic Input/Output System) is firmware save in a chip on her computer"s motherboard. It is the an initial program that runs when you revolve on her computer.


The BIOS performs the POST, which initializes and also tests your computer"s hardware. Then it locates and runs her boot loader, or tons your operating system directly.

The BIOS also provides a simple interface because that configuring your computer"s hardware. As soon as you begin your computer, you might see a message like "Press F2 for setup." This setup is your BIOS configuration interface.


When girlfriend make transforms to your BIOS configuration, the settings are not stored on the BIOS chip itself. Instead, they are stored ~ above a one-of-a-kind memory chip, i m sorry is referred to as "the CMOS."CMOS stands for "Complementary Metal-Oxide-Semiconductor." It"s the surname of a manufacturing procedure used to create processors, RAM, and digital reasonable circuits, and is likewise the surname for chips created using the process.

Like many RAM chips, the chip that stores her BIOS settings is made using the CMOS process. That holds a tiny amount the data, normally 256 bytes. The information on the CMOS chip includes types of disc drives room installed, the current date and also time the your device clock, and also your computer"s boots sequence.

On part motherboards, the CMOS is a separate chip. However, on most modern-day motherboards, that is combined with the RTC (real-time clock) top top the southbridge.

Your BIOS storage is non-volatile: it retains its information also when your computer has no power since your computer needs to remember its BIOS settings also when it"s turned off. That"s why the CMOS has its own devoted power source, which is the CMOS battery.

CMOS battery


The CMOS battery is a lithium-ion battery around the size of a coin. It have the right to hold a charge for as much as ten years prior to needing to it is in replaced. If your CMOS battery dies, your BIOS settings reset to your defaults once your computer system is turn off.


If you rotate on your computer and your setups were reset or forgotten, her CMOS battery might be dead. Because that example, your hard drive might no longer detected correctly, or your mechanism clock might be reset to a date in the past, such together January 1st, 1970. If so, you deserve to replace the battery, see: exactly how to replace the CMOS battery.

Some motherboards, consisting of most contemporary ATX motherboards, proceed to provide power to the CMOS if the battery is changed while the computer is proactively powered.

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If you desire to replace your CMOS battery, make sure to consult her motherboard documentation because that details and also safety information. Visit your motherboard manufacturer"s assistance website if you must download a brand-new copy of your manual.