What is CMOS and BIOS in depth ?

👤 Diwas Poudel    🕒 Aug 31 2021    📁 Fix    📜 0 comment

What is CMOS?

CMOS stands for "Complementary Metal Oxide Semiconductor".CMOS comes within a chipset in the modern motherboard and is powered by a CMOS battery.CMOS is also known as Real-Time-Clock(RTC), Non-Volatile RAM (NVRAM), or CMOS RAM. In Apple Motherboard, PRAM is equivalent to CMOS.

CMOS chip consists of millions of transistors.MOS(Metal Oxide Semiconductor) in CMOS referred to the metal gate, oxide insulation, and silicon-based semiconductor. But the term Metal is a misleading term because the CMOS gate is a layer of polysilicon. Complementary in CMOS refers to two types of semiconductors ie. N-type and P-Type semiconductor. This all-metal gate, oxide insulation, and semiconductors work together and forms a CMOS.

History of CMOS

CMOS circuits have around 6 decades of history.CMOS circuits were invented in 1963 by Frank Wanlass at Fairchild Semiconductor and patented in 1967.RCA made the first CMOS integrated circuits in 1968 by Albert Medwin and his team.CMOS was found early adoption in the watch industry. 25 years later, CMOS was used in digital integrated circuits. Gary Kildall developed the first BIOS in PC and it was integrated in 1974 into the CP operating system. 

CMOS Location

CMOS chip is located inside the chipset and CMOS battery is a small circular battery generally located at the bottom right-hand corner in the motherboard. The generally used CMOS battery in PCs is Panasonic CR 2032 3V.

Devices using CMOS

CMOS technology is used in chips such as microprocessors, microcontrollers, Static RAM (SRAM), microcontrollers digital camera image sensors, and other digital logic circuits. Also used in analog circuits like data converters, highly integrated transceivers.

BIOS vs CMOS Chip Vs CMOS Battery

CMOS and BIOS are not the same chip.BIOS stands for Basic Input/Output System which is the system firmware or program that comes pre-installed on the motherboard from the manufacturer.BIOS is stored in the BIOS chip(also called ROM chip). It is non-volatile firmware which means its information will not be erased even when the computer is shut down.

The main function of BIOS are :

1)POST: POST stands for Power ON Self Test. This checks the computer hardware for any error and these checks before loading the OS.
2)Bootstrap Loader: Finds Operating System and if successful then pass control to OS
3)BIOS drivers: Checks available drivers
4)CMOS Setup: Check CMOS Setup and apply it.

5) Resuming from hibernating and sleep state.

Now comes the CMOS chip.
Setting that we have made with BIOS ie. user hardware-level settings like CPU fan speed, DateTime, boot sequences are stored in CMOS chip. In the older motherboard, it used to be a separate chip but now they are being integrated on the southbridge chipset or normally called the chipset. On modern motherboards, the CMOS chip is integrated with the RTC ie. a part of the southbridge chipset.

Also in a modern motherboard, there will not be northbridge. The function of Northbridge is integrated onto the CPU. Onboard Graphics is also located in the CPU itself.

Now comes CMOS Battery

Settings that are made in the CMOS chip are volatile and required continuous power to save those data so this CMOS battery is used.CMOS remembers the setting even though the computer is turned off. The CMOS battery is always in the ON stage unless it is damaged.

cmos chip cmos battery bios chip

Types of BIOS Chip

  • Legacy BIOS: It is an old boot process where the process of booting the computer using BIOS firmware.


  • UEFI BIOS: It is the new boot process used in a modern computer. It is introduced by Microsoft. It has better security features and has a user-friendly interface than Legacy BIOS. This booting process also loads faster than the legacy BIOS booting process.



Check BIOS Types 

To check whether your BIOS is legacy one or UEFI follow these basic steps.

  1. Goto run and Type: msinfo32
  2. Then under System Summary, you will find your BIOS types.


When to Update and Upgrade BIOS

For upgrading the BIOS, we have to remove the old BIOS chip from the motherboard and add a new advanced BIOS chip.

For updating the BIOS, your BIOS must be flashable type BIOS. People update BIOS if there are a security flaws and major bugs in the system if any new component is added to the computer, and your BIOS does not recognize it and not working properly.

How to Update BIOS

There are actually two popular methods of Updating BIOS.

  1. Software Method (preferred one)
  2. Hardware Method

Problem When CMOS Battery Fails

The voltage of the CMOS battery range from 3 to 6V.But generally available CMOS battery is 3 V.If CMOS battery goes below 2.6 V then CMOS battery has more chances of showing the following errors.

  • Date/Time will automatically reset to default settings

IF this happens then recorrect the date-time by going to the BIOS setting and it will work until the computer is running. When you shut down then again Date/Time will reset to default settings.

This default date can be BIOS manufacturer date, epoch, or some date like (1980 or 1990)
  • Sometimes your system will not get the boot

This happens because boot device priority will also get reset and so, the computer cannot understand from where to read OS. So you have to get these types of errors.


  • Cannot detect input/output devices properly

Eg. PCI, mouse, keyboard, and other peripherals do not respond properly

  • Driver fails

Hard Disk Driver, printer driver not working properly and fails to work and will be asked for re-download and re-install every time you boot PC.

  • May get "CMOS checksum error" or "CMOS read error".


  • There will be constant beeping noise in some old motherboard.


How long does CMOS Battery Last?

Ans: According to HP, most batteries last up to 10 years from the date of manufacture.

What happens if you remove a battery?

Ans: First shut down your computer then remove the CMOS battery from the motherboard this will reset CMOS date and time to default and other bios setting to factory default.

Where are the BIOS settings stored?

Ans: BOIS Settings are stored in a flash memory chip that doesn't needs any battery to hold information. but can be erased as needed.

Why is the BIOS stored on ROM instead of RAM?

Ans: As soon as you turn off your computer or restart a computer then information stored in RAM will get erased. So, we need nonvolatile memory termed ROM for storing BOIS data without being erased even your system is turned off.

What is the BIOS date in the computer?

Ans: BIOS system dates are the installation dates of your BIOS in the motherboard by the manufacturer.

How do I check my motherboard BIOS version ?

Ans: Go to Run and type "msinfo32" and press Enter. Then at the right pane search BIOS Version/Date and there, you will find your BIOS date.