The state BIOS and also CMOS both to express to crucial parts of her computer"s motherboard. They work-related together and also they"re both important, yet they room not the 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 your computer"s motherboard. It is the first program the runs once you rotate on her computer.


The BIOS performs the POST, i m sorry initializes and also tests her computer"s hardware. Climate it locates and also runs your boot loader, or tons your operating device directly.

The BIOS also provides a simple interface for configuring her computer"s hardware. As soon as you start your computer, you might see a blog post like "Press F2 for setup." This setup is her BIOS construction interface.


When you make alters to her BIOS configuration, the settings are no stored ~ above the BIOS chip itself. Instead, they are stored top top a special memory chip, which is referred to as "the CMOS."CMOS represents "Complementary Metal-Oxide-Semiconductor." It"s the surname of a manufacturing process used to develop processors, RAM, and also digital logic circuits, and also is likewise the surname for chips produced using that process.

Like most RAM chips, the chip the stores your BIOS settings is made using the CMOS process. It holds a tiny amount of data, usually 256 bytes. The details on the CMOS chip includes types of disc drives space installed, the present date and time of 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, the 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 system has no power since your computer system needs come remember that is BIOS settings also when it"s turned off. That"s why the CMOS has its own specialized 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 fee for as much as ten years before needing to be replaced. If her CMOS battery dies, your BIOS settings reset to their defaults when your computer system is turn off.


If you revolve on your computer and your settings were reset or forgotten, her CMOS battery may be dead. For example, your difficult drive may no longer detected correctly, or your mechanism clock might be reset come a date in the past, such together January 1st, 1970. If so, you deserve to replace the battery, see: exactly how to change the CMOS battery.

Some motherboards, including most modern-day ATX motherboards, proceed to administer power to the CMOS if the battery is changed while the computer is actively powered.

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If you desire to replace your CMOS battery, make certain 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 her manual.