BIOS (basic input output system) and CMOS (complementary metal-oxide-semiconductor) both are essential parts of every computer’s motherboard. Let us have a look at BIOS and CMOS for their different tasks.

BIOS Full Form is Basic Input Output System

CMOS Full Form is Complementary Metal-Oxide-Semiconductor


This is an important piece of software which is stored in a ROM (read-only memory) chip inside the computer motherboard. It is the first program that runs every time when we turn on our computer. It stores all instructions related to controlling the keyboard, serial/parallel ports, hard disk drive, and other various storage drives.


The BIOS is also responsible for storing all the specifications of the system in the battery backed RAM (which is also known as CMOS RAM) and provides a special setup program to change this information.

The BIOS performs the POST (power-on self-test), which initialises and tests the computer’s hardware. After that, it searches the boot loader and executes it. After that, the operating system gets loaded on the computer memory. BIOS creates an environment for configuring our computer hardware. You can see a message like ‘Press F2 for setup’ as you start your computer. It enables to see the BIOS interface or change its setting accordingly.


As you modify the setting in BIOS configuration, the changes you have made are not stored in the chip of the BIOS. Basically these changes are stored in another special chip memory known as CMOS. Almost all chips like RAM and others (whichever store the BIOS setting) are manufactured in the same way as CMOS chips are manufactured. They can store data of a small size like 256 bytes.

They store the information like disk drives installed, date and time of system, and booting sequence of our system. They may be present in a few motherboards. There is a separate chip for CMOS. The BIOS chip memory is non-volatile. It is capable of storing data even if powered off from computer. This is due to a separate power source of CMOS known as CMOS battery.

CMOS Battery

This is a dedicated power source via a Lithium-ion whose size is equal to a coin. The life of a CMOS battery is approximately 10 years before it requires replacement.

CMOS Battery

As soon as the battery gets expired, your CMOS setting will reset. If your CMOS battery dies, your computer BIOS settings will also reset to their default as soon as you shut down your PC.

BIOS/CMOS setting

You can open BIOS Setup in many ways. Every motherboard make and model has a different way to open it.

1.Start the computer
2. Enter BIOS by pressing [F2 or Delete].
3. Press [F10] to enter BIOS setup.
4. Press [F10 and holding F10 key] to access BIOS.
6. Press [Window] to access system configuration.

Depending on the motherboard manufacturers, BIOS setup utility may have different types of options to set in different ways.

BIOS utility

BIOS hardware configuration has a lot of setting options. You can change these settings as per your need and requirement. After modification and saving, the computer will restart with the respected changes. These changes instruct the hardware to function as per the new settings.

There are a few common things which you can do in almost all BIOS systems:
• change the boot sequence/order (To install operating system).
• default BIOS setup loading.
• set a new BIOS password or remove it.
• adjust the date and time.
• change HDD, FDD, CD/DVD settings.
• display or hide the computer logo.
• activate or deactivate the quick power-on self-test (POST).
• enable or disable the CPU internal cache.
• change CPU settings.
• change memory settings.
• enable or disable onboard USB, IEEE1394, audio.
• change power-on settings.

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