What Is CMOS and What Is It For

CMOS (complementary metal-oxide-semiconductor) is the term usually used to describe the small amount of memory on a computer motherboard that stores the BIOS settings. Some of these BIOS settings include the system time and date as well as hardware settings.

Most talk of CMOS involves clearing CMOS, which means to reset the BIOS settings to their default levels. This is a really easy task that's a great troubleshooting step for many types of computer problems.

Note: A CMOS sensor is different - it's used by digital cameras to convert images into digital data.
 
Other Names For CMOS
CMOS is sometimes referred to as Real-Time Clock (RTC), CMOS RAM, Non-Volatile RAM (NVRAM), Non-Volatile BIOS memory, or complementary-symmetry metal-oxide-semiconductor (COS-MOS).

How BIOS and CMOS Work Together
The BIOS is a computer chip on the motherboard like CMOS except that it's purpose is to communicate between the processor and other hardware components like the hard drive, USB ports, sound card, video card, and more. A computer without a BIOS wouldn't understand how these pieces of the computer work together.

See my What Is the BIOS? piece for more information on the BIOS.

CMOS is also a computer chip on the motherboard, or more specifically a RAM chip, which means it would normally lose the settings it's storing when the computer is shut down.

However, the CMOS battery is used to provide constant power to the chip.

When the computer first boots up, BIOS pulls information from the CMOS chip to understand the hardware settings, time, and anything else that's stored in it.

What Is a CMOS Battery?
The CMOS is usually powered by a CR2032 cell battery, referred to as the CMOS battery.

Most CMOS batteries will last the lifetime of a motherboard, up to 10 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. Replacing them is as easy as swapping out the dead one for a new one.

More About CMOS & CMOS Batteries
While most motherboards have a spot for a CMOS battery, some smaller computers, like many tablets and laptops, have a small external compartment for the CMOS battery that connects to the motherboard via two small wires.

Some devices that use CMOS include microprocessors, microcontrollers, and static RAM (SRAM).

It's important to understand that CMOS and BIOS are not interchangeable terms for the same thing. While they work together for a specific function within the computer, they are two entirely different components.

When the computer is first starting up, there's an option to boot into BIOS or CMOS. Opening the CMOS setup is how you can change the settings it's storing, like the date and time and how the different computer components are first started up. You can also use CMOS setup to disable/enable some hardware devices.

CMOS chips are desirable for battery-powered devices like laptops because they use less power than other types of chips.

Although they use both negative polarity circuits and positive polarity circuits (NMOS and PMOS), only one circuit type is powered on at a time.

The Mac equivalent to CMOS is PRAM, which stands for Parameter RAM.

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How to do clear a laptop BIOS or CMOS password

A laptop BIOS password is more secure than a traditional desktop to help protect the data if it was stolen. If you are the original owner of the laptop, we highly recommend contacting the manufacturer of the laptop. Companies such as Dell, Lenovo, and Toshiba can reset the password if you are the original owner using the Service Tag, disk, or give you exact steps for your laptop.

If you are not the original owner of the laptop, there are some ways to bypass the password depending on the manufacturer and model of the laptop.

5 to 8 character code on System Disabled screen

Laptop password system disabled codeYou can attempt to get a 5 to 8 character code from the computer, which may be usable to clear the BIOS password.

1. As the computer is booting, press F1, F2, or Del to enter CMOS setup.
2. In the CMOS setup, when prompted for a password, guess three times until you're given the code. If, after entering the wrong password three times, the computer is displaying a 5 to 8 character code, as shown in the picture, continue to the next step.
3. Write down the code and visit Dogbert's BIOS Password Backdoors scripts page to download Python and the free Python scripts to clear many of these types of passwords.

Note: If no code is displayed, try the next suggested solution.
 

Clear by dip switches, jumpers, jumping BIOS, or replacing BIOS
If the above suggestions do not apply to your laptop, you need to open the laptop and clear the password using dip switches or jumpers. You may also be able to jump the BIOS using a paperclip, by soldering a wire to pins on the BIOS, or replacing the BIOS chip.

The location of dip switches or jumpers on the motherboard depends on the brand and model of your laptop. Check the technical documentation for your laptop and contact the manufacturer to determine which motherboard is in your laptop. Search the Internet for that motherboard model for a technical diagram of where components are located, including the dip switches or jumpers for resetting the BIOS.

Which step you take depends on the manufacturer and model of your laptop computer. For most end users, we highly recommend having the laptop serviced by a qualified technician who can properly disassemble a laptop.

Contact laptop manufacturer

If the above steps do not help remove the CMOS password for your laptop, we suggest contacting the laptop manufacturer for steps on how to clear the password. Many manufacturers have strict policies in place to help protect their customers' laptops and only they have the information on how to clear CMOS passwords.

Additional information

See the CMOS definition for related links and information.

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