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BIOS Full Form
  • Last Updated : 30 Apr, 2020

BIOS stands for Basic Input/Output System. BIOS is the first program to run when the computer starts. It is a ROM chip (EEPROM chip) found on motherboards and is used for hardware initialization during the booting process. It is pre-installed in the computer system. When BIOS boots up the computer system, it checks if all of the components are in place and working and then it loads the operating system into the computer’s RAM from the hard disk of the system.
Nowadays BIOS is written on flash memory, to ease the process of rewriting the BIOS chip, whenever needed.


History of BIOS

BIOS was first introduced in 1975, in CP/M Operating System. Control Program for Microcomputers, was developed for Intel 8080/85-based microcomputers by Gary Kildall of Digital Research, Inc. The system had a simple boot loader in its ROM.
IBM classified the system-bios in two categories when PS/2 machines were introduced:

  • CBIOS: CBIOS stands for Compatibility BIOS. The real-mode portion provided backward compatibility with current operating systems, thus was named CBIOS.
  • ABIOS: The ABIOS stands for Advanced BIOS. It provided new interfaces specically the one suited for multitasking OS.

Functions of BIOS

  • POST: Power on Self Test is the process performed by BIOS immediately after the computer system starts to check any hardware related issues. if any error is found in this process, the system alerts through flashing LEDs, audible beeps, or error messages on the monitor, all of which are technically referred to as POST codes, beep codes, and on-screen POST error messages, respectively.
  • Bootstrap Loader: The bootstrap program stored in ROM is loaded in hard drive, as soon as the computer starts. The bootstrap loader locates the operating system and passes the control to it.
  • Next, Bootstrap Driver configures basic hardware so that system can run. They are low-level drivers and gives basic information about computer hardware.
  • CMOS Setup: Complementary Metal-Oxide Semiconductor(CMOS) chip, saves some system’s information, such as the time, date, and other essential system settings. CMOS chip is powered by the battery located in the motherboard. It also stores some information about hardware. The chip’s content is copied to RAM(main memory) of the system to increase the speed, since CMOS chip is quite slow.
    It is because the BIOS and CMOS, setup configure the system’s crucial information like the password, date, and time.
  • Bootdevice selection: One of the CMOS’s most important role is, it can alter the device boot process. This is important for system restoration because the CMOS may need to change boot priority from the hard drive to the optical drive or flash drive to launch the operating system installer or adjust which hard drive to load the operating system from.

Flowchart of BIOS

Features of BIOS

  • Every BIOS implementation is specifically designed for a particular type of motherboard model or particular computer.
  • Originally, BIOS firmware was stored in the ROM memory but nowadays it is stored in Flash memory so that it can be rewritten without removing the chip from the motherboard.
  • The other reason for doing this is that it allows easy updates of BIOS firmware, so new features can be added and bugs can be fixed.
  • The latest advancement of BIOS is UEFI(Unified Extensible Firmware Interface).


  • The computer system will gain better compatibility.
  • The booting time of system decreases.
  • The overall performance of system improves


  • The disadvantag is that it is a time consuming process. BIOS often boots in 16-bit real mode (not 32-bit nor 64-bit protected mode), and because of the type of chip used, it is not always the fastest.
  • If something goes wrong during the process, the motherboard might get affected. Solving this will not be an easy process.

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