Basic Input Output System (BIOS) Explained

What is Basic Input Output System (BIOS)?

Basic Input Output System or BIOS refers to the specific software program used by the microprocessor of a computer to boot up or start the system by passing the control to it when it is powered on.

Technically, BIOS is included in the computer systems as a firmware. It exists in the same place on an Erasable Programmable Read Only Memory or EPROM chip placed on the motherboard.


  • BIOS is a specific type of firmware stored in the EPROM that boots up the system and tells the operating system how to function and communicate with other parts and devices connected to it.
  • There are two types of this firmware in use such as UEFI which can accommodate larger hard drives and Legacy BIOS that are usually found in older motherboards and cannot support drives larger than 2.1 TB.
  • A lot of useful information is made available to the users and the operating system of the computer through this particular firmware.
  • This specific program is designed with an intention to function with all the other components that make the complimentary system chipset.

Understanding Basic Input Output System (BIOS)

What is Basic Input Output System (BIOS)

Also known as the ROM BIOS, System BIOS, PC BIOS or BIOS ROM, the Basic Input Output System is the software program in the EPROM of a computer system that helps in the startup process.

It can also be stored in a flash memory but certainly not in a larger hard drive because it will be unable to boot it from there.

Storing the BIOS in the flash drive helps in rewriting it and making easy updates or adding new features and even fixing bugs by the end user without removing the chip from the motherboard.

However, this may infect the computer system with BIOS rootkits.

Though it is responsible for the basic or initial functions, this particular firmware plays a significant role in the overall functioning of the computer system because it will not boot at all if the program itself is wrong in the first place.

The most common errors that may be related to this firmware are:

  • Being unable to overclock
  • Failed device
  • Erroneous boot device and
  • CPU fan errors.

In such situations, changes may be required in the BIOS settings or it should be upgraded. However, it should be done very carefully because any error may not boot the system at all.

Also, while upgrading, you must make sure that it is compatible with the operating system installed or else it will corrupt the software program and the system will not boot.

Typically, there is no need for a user to mess with the BIOS of their computer system because if the upgrade does not work, it will brick the motherboard rendering the system useless.

The BIOS library contains a few specific functions to operate and/or control the peripherals of the system. These can be set off via external software.

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The users can even perform several functions using the user interface of this software which include and are not limited to:

  • Hardware configuration
  • Selecting boot drives
  • Setting the system clock and date
  • Enabling or disabling specific components of the system
  • Changing the boot order
  • Loading setup defaults
  • Remove or creating a BIOS password
  • Changing drive settings
  • Viewing memory amount
  • Changing the boot up NumLock status
  • Enabling or disabling the computer logo, POST, CPU and BIOS caching
  • Changing CPU, memory and voltage settings and doing much, much more.

There are lots of benefits offered by this firmware but in spite of the good, there are some limitations of the Basic I/O system such as it cannot boot from larger storage drives and when it boots, it does so in the Legacy Mode which is actually a 16-bit real mode and therefore is much slower than UEFI.

Also, offering an option to the end users to upgrade the BIOS may destroy its memory due to carelessness.

Features of BIOS

The features of the firmware display a lot of important information to the users as well as to the operating system of the computer.

Typically, BIOS would include information of the manufacturers of different components of the computer system such as:

  • The CPU and its speed along with the number of processors that are installed
  • The RAM and its speed
  • The hard drive along with its size, type and other specs and
  • The optical drive and its type.

In addition to that, the manufacturer will also set the attribute values related to the BIOS in particular such as:

  • Its description
  • The version string
  • The name of the manufacturer of it
  • The release date in the Coordinated Universal Time format of YYYYMMDDHHMMSS
  • The serial number which may be the same as the computer ID number if the manufacturer of both the computer system and the BIOS is the same
  • The name of the managed system usually in the form hostname: agent_code especially when there is a need to customize a situation and
  • The timestamp when the data is sampled.

The information may also contain the generic codes required to control the display of the computer, the keyboard and others.

And, to the operating system the BIOS tells how it should function with the devices that it has detected.


The main function of the BIOS is to start a computer system when the users switch it on. It also checks and changes the boot order if necessary and even displays any errors that may occur during the boot up process.

The basic need of this program pre-installed on the motherboard is to help in loading the operating system after checking that all necessary components connected to and available in the system are correct and functioning properly.

It also tells the computer how to perform the basic functions such as booting or keyboard control, and identify hardware and configure CPU, memory, hard drive, optical drive, floppy drive, and other related components before loading the operating system.

The program works quickly and performs these specific functions during the boot up process:

  • It first finds out whether or not the boot devices and the attachments are optional or necessary
  • It then determines whether or not the necessary attachments are in the right place
  • Next up it tests them whether or not they are functioning properly and
  • It then cleans up all the registers of the CPU to make them ready for use.
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Finally, it loads the operating system, the whole or part of it, into the Random Access Memory or RAM from the hard disk or the diskette drive or any other type of boot device.

Therefore, ideally, the functions typically involve a combination of a few specific steps such as identifying, configuring, testing and connecting the hardware of the computer to the operating system when the system is turned on.

All these functions together are called and complete the boot process.

In order to explain the functions of this firmware in a more precise way, it can be said that it performs four specific roles. This will make it even easier to break down the entire process and BIOS functions in a much better way.

The roles involve the following:

  • Conducting POST or Power On Self Test to examine the hardware of the computer system before loading the operating system
  • Locating the operating system using the Bootstrap Loader or BSL which is a small program that is activated as soon as the microcontroller is powered up
  • Finding the software and drivers to interface with the operating system once it is running and
  • Setting up the CMOS or the Complementary Metal Oxide Semiconductor which is the non-volatile memory of the BIOS and a configuration program that allows the users to change the system and hardware settings.

In addition to the above, the function of the BIOS also involves managing the flow of data between the operating system of the computer system and the devices attached to it such as the hard disk, keyboard, mouse, video adapter, and printer.

How Many Types of BIOS are There?

Ideally, there are two types of Basic I/O System such as UEFI or Unified Extensible Firmware Interface and Legacy.

UEFI – This BIOS type can accommodate larger drives of the size of 2.2 TB or more with the help of the MBR or the Master Boot Record approach instead of the more modern GPT or GUID Partition Table technology.

Legacy BIOS – This firmware is found in the older motherboards which works just like the UEFI in governing the communication between the CPU and the other components of the computer.

However, it has a few limitations such as it cannot identify any drive that is larger than 2.1 TB. Also, the setup programs in it contain text-only menus.

Both these boot modes come with their characteristic benefits which you should consider if you are given an option to choose one between the two.

The advantages of installing the UEFI boot mode include:

  • No address constraints that are usually associated with Legacy boot mode
  • Larger boot partition than 2 TB in size supported for the operating system
  • Integrated configuration utilities of PCIe or Peripheral Component Interconnect Express device into the Setup Utility menus
  • Displaying images of bootable operating system as labeled entities in the boot list
  • Faster booting
  • Support for over four partitions in the drive
  • Better system management
  • Efficient power management
  • Improved fault management and
  • Robust reliability.
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Why Do You Need a BIOS?

As said earlier, you will need the BIOS to boot up the computer system once you switch it on. Without the BIOS the system will not work.

The microprocessor in the computer needs someone to act as a middleman between the hardware and the operating system to run on and to carry and control the flow of data and information to the I/O devices. This is actually done by the BIOS.

Apart from that, this firmware also plays an important role in ensuring the security of the computer system. Ideally, most of the versions of this particular software come with an option to protect the boot process with a password.

This means that the booting process will not be initiated if the correct password is not entered in the first place. Since this performs all the initial functions virtually, it helps in protecting the entire computer system and its operation.

Does BIOS Use RAM?

Yes, just like all other codes and programs such as operating systems and user programs, the BIOS also needs to use the RAM of a computer system to be loaded into it so that it can run.

All the parts in the motherboard need to go to the BIOS to check the necessary parts and if something is not right or not present the system will stop working.

Moreover, manufacturers design their system in such a way that the Basic Input Output System is copied to the RAM from the ROM every time the computer is booted since the RAM is faster than the ROM.

When the computer is powered on, the processor loads the BIOS code in a particular address of the system memory. It then jumps to the head of that memory address and performs the necessary functions.

This means that the CPU, after switching the system on, will search for the RAM and it is only after that the BIOS firmware will initiate the boot up process looking for the operating system that is stored in the hard drive and then load it into the RAM.

So, RAM is essential for a computer to function.

Questions & Answers:

Where is the Basic Input Output System Stored?

The BIOS is located in the motherboard stored in the EEPROM chip. This chip is typically non-volatile in nature. This means that you even rewrite or update the BIOS.

Why is BIOS Called Firmware?

Typically, firmware signifies the generic name given to all those software programs that reside on the non-volatile memory. And, since BIOS is stored in the EPROM, it is therefore called firmware that ‘talks’ to the devices and tells the operating system how exactly it is supposed to function with them.


The BIOS is a very important program for a computer which helps the system to boot and start functioning.

Apart from that, it also ensures security of the system and also helps in controlling data flow and setting up the CMOS.

Simply said, without this particular firmware, a computer system is as good as defunct.

About Puja Chatterjee

Puja ChatterjeePuja Chatterjee is a technical writer with extensive knowledge about computers. She graduated from BIMS. Her expertise includes technology writing and client relationship management gained through over 12 years of experience. Follow Her at Linkedin.

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