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BIOS (Basic Input Output System) – What is BIOS, Functions, Limitations

BIOS (Basic Input Output System) is a fundamental component in all computers. This post will discuss about what is BIOS (Basic Input Output System), its functions, What does it do, configuring Basic I/O System and its limitations.

What is BIOS

Basic Input Output System is abbreviated as BIOS and was first developed by Gary Kildall, an American scientist. Every computer has a special chip on the Motherboard called as BIOS. All the personal computer comes preinstalled with BIOS. Once a computer is turned on, it needs to perform certain operations. The basic function of Basic I/O System is to govern the early stages of the startup process. It enables the operating system to communicate with the hardware of the computer. Basic I/O System provides the hardware initialization during the booting process.

Introduction to BIOS

Fig. 1 – Introduction to Basic I/O System

To enter the BIOS Setup Screen also called as CMOS Setup, simply power up the computer and while the BIOS is performing Power On Self Test (POST) press the BIOS key which could be F2, F10, Del, F12 etc. This depends on the manufacturer. BIOS setup screen would look like the image shown in the Fig. 2. The instructions to load a basic computer hardware is included in BIOS. The “Power ON Self Test” (POST) helps to verify that boot up requirements are in place. If the computer is unable to pass the POST test, it is indicated through a series of beeps, informing the user about the malfunctioning of the computer.

However nowadays, BIOS has slowly been replaced by UEFI (Unified Extensible Firmware Interface) in new computers which offers benefits like a better user interface and a built in Pre-OS platform for accessing the web. If we want to add additional memory to the chip, it is only possible through physical removal and replacing it with more advanced BIOS. The data on the  chip can be updated using specially designed software, which could fix problems or add additional features to the motherboard.

Major manufactures of Basic I/O System include:

  • American Megatrends Inc (AMI)
  • Phoenix Technologies
  • Ali
  • Winbond

Basic I/O System Setup Screen

Fig. 2 – Basic I/O System Setup Screen

Functions of BIOS

There are four main functions of Basic I/O System. They are:

  • POST
  • Bootstrap Loader
  • BIOS Drivers
  • BIOS Setup


Testing of computer hardware and also making sure that no error is there while loading the operating system.

Bootstrap Loader

Once the capable operating system is loaded, the BIOS passes the control to the operating system.

BIOS Drivers

Low level drivers that give computer basic control over computer’s basic hardware.

BIOS Setup

Hardware settings can be configured using this setup including system settings such as system password, time and date.

BIOS Functions

Fig. 3 – Basic I/O System Functions

What BIOS does on My Computer

There are various functions that Basic I/O System performs but its most important function is to load the operating system. The first instruction that a microprocessor needs, once the PC is switched on is provided by Basic I/O System. The usual sequence it follows are-

  • Check the CMOS setup for custom settings
  • Load the interrupt handlers and device drivers.
  • Initialize registers and power management.
  • Performs “Power On Self Test” (POST)
  • Display system settings.
  • Determine which devices are bootable.
  • Initiate the bootstrap sequence.

The moment the computer is turned on, BIOS checks for RAM stored on Complementary Metal Oxide Semiconductor (CMOS). It provides detailed information pertaining to the system like current Date and Time of the system clock, Boot Sequence, types of Disk Drives. It uses this information to modify or supplement its default programming.

The Operating System (OS) cannot load without a BIOS. OS is stored in hard disk and BIOS loads the drivers for the hard disks and also loads some segments of OS like MBR (Master Boot Record), FAT (File Allocation Table), GPT (GUID Partition Table) into the memory. Once the drivers are loaded, OS loads and controls computer.

Boot Screen

The first thing that appears on the screen as soon as the computer is turned on is the display text describing things like the amount of memory installed on the computer, type of hard disk & so on during the booting process. Fig.4 shows the image of Boot Screen.

Once CMOS is set up & interrupt handlers are checked, it’s time to check whether video card is operational, it initializes the memory & graphic processor on the card. Next it displays some details about the system which includes information about:

  • CPU Specifications
  • The Floppy Drive & Hard Drive
  • Memory Information
  • BIOS Version
  • Chipset, Motherboard Model Information
  • Storage Device Information

Other special drivers, such as Small System Computers Interface (SCSI) are loaded into the adapter & the information is displayed. It, then looks for sequence of storage devices identified as boot devices. Launching the operating system is called booting. If the first device on the list is not located, it checks for the next device on the list. If proper files are not located, the start-up process will be halted.

Computer Boot Screen

Fig. 4 – Computer Boot Screen

Configuring BIOS (Basic Input Output System)

There are different options for its configuration, they vary from one BIOS manufacturer to another. In order to configure BIOS, you must first enter the set-up window. Once that is done, you can enter BIOS configuration window, where we get to see different sections listed at the top of the screen.

The options that appear in all Basic I/O System configuration windows are as following:

  • System Time and Date
  • Boot Sequence
  • Plug and Play
  • Mouse and Keyboard
  • Drive Configuration
  • Memory
  • Security
  • Power Management

System Time and Date

 To set date and time for the system.

Boot Sequence

It refers to the order of devices that a computer search for bootable data.

Plug and Play

This feature is used to detect auto-connected devices. It should be set to ‘YES’, if your operating system and computer supports it.

Mouse and Keyboard

Enable ‘Numlock’ on startup, enable the keyboard and auto detect menus.

Drive Configuration

Configures hard drive, CD/DVD ROM


Direct the BIOS to shadow to a specific memory address.


Setting up the password for accessing the computer.

Power Management

Select the amount of time for standby and suspend.

Limitations of BIOS (Basic Input Output System)

The limitations include:

  • It boots in 16-bit real mode (Legacy Mode) and hence is slower than UEFI.
  • End Users may destroy Basic I/O System Memory while updating it.
  • It cannot boot from large storage drives.
Also Read:
Optical Computer - Components, Working Principle and Why We Need It
Wearable Computing Devices - Technology Features, Working and Evolution History
What is Operating System (OS) - Functions, Types & Resource Management
Ankita Khare
Ankita Khare
Ankita is a Masters in English Literature and has extensive work experience in teaching. She also conducts "Brain Development program for kids. She is an Author, Editor and Partner at Electricalfundablog.


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