Basic Input Output System

BIOS Setup | BIOS Functions | Booting Process

CMOS Battery | CMOS Reset

BIOS stands for Basic Input Output System. It is a fundamental component of a computer’s firmware, providing the necessary instructions for the system to boot up and initialize hardware components during the startup process. The BIOS is an integral part of the motherboard and is stored in a non-volatile memory chip.

Application Software , Operating System , BIOS , How Computer Works

The BIOS stands for Basic Input Output System. The BIOS ( also referred as ROMBIOS ) is a system utility program which loads the operating system when computer is switched on.

Both BIOS And CMOS work together to load the operating system. The CMOS is a small battery powered RAM memory  which retains the user settings and other data required during the booting process.

In this article , we will discuss in detail what is BIOS , Functions Of BIOS , Booting Process , BIOS configuration , CMOS Battery , CMOS Reset and other related topics.

BIOS stands for “Basic Input/Output System.” It is a firmware (software embedded in hardware) that is present on the motherboard of a computer. The BIOS is responsible for initializing and configuring various hardware components during the boot process, ensuring they work together harmoniously to allow the operating system to load and run.

It’s important to note that with the advent of newer technologies like UEFI (Unified Extensible Firmware Interface), the traditional BIOS is being phased out in modern computers. UEFI offers a more advanced and flexible firmware interface, but it still serves the same fundamental purposes as the traditional BIOS.

Operating System Booting Process , BIOS , How Computer Starts

In recent years, the Unified Extensible Firmware Interface (UEFI) has become a modern replacement for the traditional BIOS. UEFI offers several advantages, including support for larger storage devices, faster boot times, a graphical interface, and enhanced security features. While the term “BIOS” is still commonly used, many modern systems use UEFI instead.

Understanding the role of BIOS or UEFI is essential for troubleshooting hardware issues, configuring system settings, and performing system maintenance on a computer.

What Is BIOS ?

The operating system is a system software and an essential component of every computer system. The operating system  acts as an interface between computer system hardware components  and the application programs.

And therefore , the  user  cannot  operate the computer system,  till the  time  operating  system  is  completely loaded into the main memory RAM and fully operational.

The CPU ( Central Processing Unit ) is the brain of the computer system. The  CPU’s  first  job  is to locate and  initiate the process of loading the  operating  system,  every time the computer system  is switched  on.

BIOS Chip , what is UEFI
BIOS Chip , what is UEFI

This is done with the help of  system  utility program called  BIOS – Basic Input Output System. The BIOS also performs some initial diagnostic  tests ( POST ) and initialize the hardware during the booting process.

How BIOS Works ?

The  BIOS.  gets activated when the computer is powered on . The BIOS ( ROMBIOS ) is a system utility software  that is placed in a ROM chip ( Read only ) on the computer system  motherboard  as  a  firmware .

A firmware is software that is embedded into a ROM chip . This protects the BIOS software from any virus attack or getting corrupted due to any changes.

The  BIOS  is  the  first start-up  program  executed by  the  CPU – Central Processing Unit when the  computer system  is  switched on .

The main job  of the  BIOS  is  to  locate  and  load the operating  system  in  to the  main  system memory  RAM ( Random Access Memory ).

Computer Motherboard, what is motherboard

Functions Of BIOS

The BIOS includes a setup utility that allows users to configure various system settings. This utility is accessed by pressing a specific key (often Del, Esc, F2, or F10) during the early stages of the boot process. In the BIOS setup, users can modify settings related to hardware, boot order, security, and more.

The process of loading the computer operating system into the main memory RAM is referred as booting process. The CPU initiates the booting process by invoking the first start-up the BI OS program. The BI OS then initiates the booting sequence.

The booting  process ends when the operating system is completely loaded into the main memory RAM  and fully operational.

The main function of the ROMBIOS is to locate and load the  operating system . During the booting process the ROMBIOS needs information such as date , time and other information about user system settings preferences. This information is provided by CMOS .

The  BIOS  performs  following important functions during the computer system   booting process : 

1. Power-on self-test (POST)

 When you turn on your computer, the BIOS performs a self-test to check if the essential hardware components like CPU, RAM, storage drives, and graphics card are functioning correctly.

2. Boot Device Selection

 The BIOS determines the order in which devices are checked for an operating system to load. It looks for bootable media like hard drives, solid-state drives, optical drives, USB devices, etc., in a specific order until it finds one with a bootable operating system.

3. Hardware Configuration

BIOS configures the settings and parameters of various hardware components to ensure they work efficiently. This includes setting up the CPU clock speed, managing system memory, and configuring peripheral devices.

The BIOS performs low-level initialization of essential hardware components, ensuring that they are in a known and operational state. This includes initializing the motherboard chipset, detecting and configuring connected peripherals, and preparing the system for the OS.

4. CMOS Setup Utility

The BIOS provides access to a configuration screen known as the CMOS setup utility. This utility allows users to modify BIOS settings, such as changing the system date and time, enabling or disabling specific hardware components, and adjusting various system parameters.

5. Low-level System Control

BIOS acts as an intermediary between the hardware and the operating system. Once the boot process is complete, the BIOS hands over control to the operating system, allowing it to take over the management of the computer.

6. System Clock

The BIOS provides the system clock, often referred to as the real-time clock (RTC). The system clock keeps track of the current date and time and is essential for time-sensitive operations and accurate timestamps.

7. BIOS Updates

Manufacturers periodically release BIOS updates to provide bug fixes, improvements, and support for new hardware. Users can update the BIOS, a process known as “flashing the BIOS,” to benefit from these updates.

8. System Security

The BIOS may include basic security features such as password protection to restrict access to the BIOS setup and prevent unauthorized changes to system settings.

          Functions Of  ROMBIOS

  • To  Check the power supply  to  the computer system.
  • To  check that  essential  internal components such  as  RAM  and  external  peripheral devices are connected  and  working.
  • To  initiate  the  computer  system  booting sequence .
  • To  provide  and load the initial  basic  set of device drivers .
  • To  locate the operating system  and load  into the RAM.
  • To  handover the system control  to the operating system  once  the  OS  is  fully loaded and  operational .

BIOS Features

Computer Operating System

          Features Of  BIOS

  • BIOS – Basic Input Output System.
  • ROMBIOS main Function Is to Load the Operating System.
  • BIOS is Placed Inside ROM Chip Called ROMBIOS.
  • ROMBIOS Also Loads Initial Set Of Drivers.
  • BIOS Chip Is Powered By CMOS Battery.
  • ROMBIOS Configuration Is Saved Due To CMOS Battery.
  • BIOS Finally Handover Control To the Operating System.

Types Of BIOS

The BIOS Chip is an essential component placed on every motherboard in a PC . There are many companies that manufacture BIOS firmware chips . 

Since the BIOS is embedded in a ROM Chip , it is also commonly referred to as ROMBIOS. Some commonly used  types of BIOS brands include  :

What is Booting ?

Computer Booting Process

The computer system booting process starts when the user switch on the computer system .

The system power supply unit carries out self-test called POST. The “Power On Self Test” checks the computer systems internal hardware components for compatibility , connection  and  ensures that no errors exist before loading the operating system .  

Only after  successful completion of the POST ,  the bootstrap loader  routine  on the  BI OS  chip is  invoked  to  load  the  operating  system  . If the POST fails then the computer system  will  beep for different   beep codes .

What Is POST ?

Power On Self Test

After successfully completing the POST (  Power On Self Test ) , the BI OS  then  attempts to load the operating system ( OS )  into the main system memory  RAM  ( Random Access Memory ).

The OS is located either on the internal hard disk  attached to the computer system  OR  on the bootable OS disk in the CD ROM drive. The BI OS will pass the control to the operating  system  once  the OS  is  fully  loaded into the memory.

Every time you switch on your computer , a message is displayed on your monitor in first few seconds to press a specified key to enter ROMBIOS menu .

All computers provide access to the  BIOS /  CMOS  setup utility system program which has a menu-based user interface ( UI ) accessed by pressing a certain key on the keyboard  for  first  few seconds  when the  computer starts.

 Usually the key is displayed on the monitor for short time during the early start up, for example “Press F1 to enter CMOS setup“. The actual key depends on  the  specific computer hardware brand .

BIOS CMOS Configuration Setup

By accessing this CMOS utility program , the user can edit and modify the BI OS configuration settings . The  BI OS  and CMOS  interface  provides the   user  the options to  edit  , modify  the  hardware  configuration  settings  and  save  the changes  made  before the exit  .

Features present in the BIOS setup utility  typically  include  :

The options to configure the hardware components, including setting for their various operating modes and frequencies. For example, selecting how the storage controllers are visible to the operating system.

The user can also  adjust  the  microprocessor ( CPU )  clocking speed ,   Settings for  the System  Clock  and enabling or disabling  the system  components .

The CMOS setup gives options  for selecting which devices are potential boot devices, and in which  order  booting   from  them will  be attempted .    


The CMOS. Stands for “Complementary Metal Oxide Semiconductor.”  The CMOS is type of technology used to produce integrated circuits ( IC Chips ).

The CMOS circuits are used in many electronic components, including microprocessors, batteries, and digital camera image sensors.

In computer architecture , the CMOS chip is used to store the critical data used by the operating system during the booting process.

The BIOS and CMOS work together during the computer booting process . The CMOS is also alternatively referred to as a Real-Time Clock ( RTC ).  

The CMOS is a Non-Volatile RAM ( NVRAM ) or CMOS RAM because it is powered by the CMOS battery when system is switched off .

And therefore , due to the CMOS battery , it retains the stored data ( BI OS settings ) even after the computer is switched off . The CMOS stands  for Complementary Metal-Oxide Semiconductor.

The CMOS is placed on the motherboard generally next to ROMBIOS Chip . The CMOS is a  battery powered semiconductor chip ( memory ) inside computer that stores the information needed during the computer booting process.

This information includes the system time , date , system hardware settings , user preferences and password  for  your  computer.

What If BIOS Fails ?

What happens when CMOS battery fails ?

The average life of the CMOS battery is around five years depending upon the usage . The CMOS battery needs to be replaced when it is exhausted.

The CMOS memory stores the BI OS settings, boot order , current date, time and other data used during the system booting process.

When the CMOS memory fails due to any technical issues , the BI OS does not get the data required during the booting process. In such cases the booting fails flashing an error message.

However , the most common cause of this problem is the CMOS battery failure. But if battery is good then other causes need to be examined. 

When CMOS battery fails , the computer system may experience following problems :

Current Date And Time Problem

Due to CMOS battery failure , the system fails to store the current date and time.  As a result , the system will show wrong date and time each time the system is powered off.

This problem can be easily corrected  by replacing the old CMOS battery with new one.

BIOS Fails To Boot The System

The CMOS memory loss due to battery failure may cause a more serious problem. And the BI OS may fail to boot the system. In such case the system will display an error message .

The CMOS memory contains the boot order information and therefore the BI OS fails to detect the operating system.

This problem can be solved  by replacing the old CMOS battery with new one and reentering the boot drive information in the BIOS setup menu.

CMOS Battery Failure

Hardware Failure

The BIOS or COMS memory loss may cause some hardware related issues in the system. In some cases, the system may fail to recognize the hardware peripheral devices connected to the system.

This happens when the BIOS fails to load the initial set of drivers which gives basic hardware control to the user. In such cases the  system display an error message.

How To Reset CMOS ?

The BIOS plays a critical role in booting the computer system . However sometimes  due to some problem , the  BIOS  may  fail  to  boot  the system.

And therefore ,  the operating  system  is  not  loaded into the RAM , necessary make the system operational  . In the event of this situation the user cannot operate the computer .

 This can happen due to change of major component  which is incompatible with existing hardware configuration settings . In such situations . the  user can neither   access  the  ROMBIOS  menu  nor he can  operating system . 

If the BIOS is unable to boot the system  due to  hardware compatibility issue or any other  problem then  such problem  can be resolved  by  resetting  the  CMOS. 

CMOS Rest By Using Motherboard Jumpers

  • Switch off  the computer system  .
  • Locate the  reset  jumper  on the motherboard  near  the CMOS  battery .
  • Now  place the  jumper pins from first  two  pins ( pin 1 & 2 )   to other two pins ( pin 2 & 3 ) to  clear  CMOS   memory  . 
  • Wait for 20 seconds to allow CMOS memory to clear.
  • Now , set  the jumper back  to the   original  position ( pin 1 & 2 ) on the first  two  pins .
  • This  resets  the  CMOS  memory .
  • The Checksum  Error message will be displayed.
BIOS Reset

We can reset the CMOS memory , by  clearing the CMOS memory and restoring settings back to their factory default state.

In most cases, The CMOS memory can be cleared from within the ROMBIOS menu. However , in some cases, this can be done by opening computer’s case and with  the help  of  jumpers on the motherboard , we  can reset  the CMOS  memory .

Some  latest  advanced  motherboards  provide the separate  reset  button   on the motherboard  which  can be used   to  reset  the  BIOS  settings .

How To Reset CMOS ?

CMOS Reset By Removing CMOS Battery

This method is used to clear the CMOS memory when  the motherboard does not have a CLEAR CMOS jumper pins . In this method, the CMOS memory is cleared by removing the CMOS battery .

The CMOS battery provides power to    the  CMOS  volatile memory ( Static RAM ) which  needs constant  power  source  to retain the data  such as current  date , time  and  BIOS settings  values . 

Once the battery is removed , the CMOS memory data will be lost . After some time , the battery is again placed in to the socket thereby restoring the power supply to CMOS memory .

How To Fix Checksum Error ?

How To Replace The CMOS Battery ?

The CMOS battery failure is the most common cause for the BI OS failure and the system fails to boot the system.

What Is Checksum Error ?

The CMOS battery provides continuous power supply to the CMOS memory. If the CMOS battery fails then the CMOS memory also fails to store the data required during the booting process.

The  CMOS battery failure is the most common cause for the BIOS failure and the system fails to boot. In such cases the system will display “Checksum Error” message that needs to be fixed.

The checksum error can be easily fixed by replacing the CMOS battery and then updating the  system data using the BIOS setup.

CMOS Checksum Error , BIOS

How To Replace CMOS Battery ?

The average life of the  CMOS battery can anywhere between two to five years depending upon the usage. The CMOS battery needs to be replaced if it fails to provide the required power to the CMOS memory.

The CMOS battery is located on the motherboard . In case of personal computers , the CMOS battery can be easily reached and replaced . However , in case of laptops , this operation more invasive to access and replace the CMOS battery.

How To Fix Checksum Error ?

The system will show checksum error message in case of BIOS reset and also when the old CMOS battery is replaced with new one.

In such cases the CMOS memory is cleared and BI OS fails to boot the system showing the checksum error. 

In order to fix the checksum error , we need to once again update the system information ( current date , time and boot order ) by using the BIOS setup

Restart the computer after replacing the CMOS battery and get into the BIOS setup by pressing the key to enter the BIOS setup.

Now , restart the computer after replacing the CMOS battery and get into the BIOS setup by pressing the key to enter the BIOS setup.

To enter the BIOS setup Or UEFI setup , you need to keep pressing the key ( Del , F1 Or as prompted by your system ) . You will be prompted by the system for the key to be used to enter the BIOS setup.

The next step is to first update the current date and time . The second step is to change the boot order to select the drive that contains the operating system. This will allow the BIOS easily locate the operating system .

The final step is to SAVE and EXIT the BIOS setup and restart the system.

The UEFI is a new avatar of the BIOS that is soon going to replace the BIOS. As we all know , the BIOS is a small piece of system software that sits on the ROM chip present on the motherboard.  The UEFI has some major advantages as compared to BIOS.

In this article , you will learn what is UEFI , what was the need to introduce UEFI , what is BIOS and its functions, new system security features and finally what are the advantages of UEFI.

Read More

What Is UEFI ?

The system software is an essential component of every computer system . The computer needs different types of system software components to communicate with the various hardware components connected to the system.

The operating system itself is a type of system software  designed to operate the computer system . However , the operating system internally make use of another set of system software used to manage and control the various hardware components .

In this article , you will learn what is system software , why computer needs system software , different types ,  its functions and other related important topics .

Read More

What Is System Software ?

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