In IBM PC compatible computers, the Basic Input/Output System (BIOS), also known as the system BIOS or ROM BIOS ( /ˈbaɪ.oʊs/), is a de facto standard defining a firmware interface. The name originated from the Basic Input Output System used in the CP/M operating system (released in 1976), where the BIOS was loaded from disk, with only a small boot loader program stored in read-only memory.

The BIOS software is built into the PC, and is the first code run by a PC when powered on ('boot firmware'). When the PC starts up, the first job for the BIOS is the power-on self-test, which initializes and identifies system devices such as the CPU, RAM, video display card, keyboard and mouse, hard disk drive, optical disc drive and other hardware. The BIOS then locates boot loader software held on a peripheral device (designated as a 'boot device'), such as a hard disk or a CD/DVD, and loads and executes that software, giving it control of the PC. This process is known as booting, or booting up, which is short for bootstrapping.

BIOS software is stored on a non-volatile ROM chip on the motherboard. It is specifically designed to work with each particular model of computer, interfacing with various devices that make up the complementary chipset of the system. In modern computer systems, the BIOS contents is stored on an EEPROM chip so that the contents can be rewritten without removing it from the motherboard. This allows BIOS software to be easily upgraded to add new features or fix bugs.

A BIOS has a user interface (UI), typically a menu system accessed by pressing a certain key on the keyboard when the PC starts. In the BIOS UI, a user can:

  • configure hardware
  • set the system clock
  • enable or disable system components
  • select which devices are eligible to be a potential boot device
  • set various password prompts, such as a password for securing access to the BIOS user interface functions itself and preventing malicious users from booting the system from unauthorized peripheral devices.

The BIOS provides a small library of basic input/output functions used to operate and control the peripherals (such as the keyboard, text display functions and so forth), and these software library functions are callable by external software. In the IBM PC and AT, certain peripheral cards, such as hard-drive controllers and video display adapters, carried their own BIOS extension Option ROM, which provided additional functionality. Operating systems and executive software, designed to supersede this basic firmware functionality, provide replacement software interfaces to applications.

The role of the BIOS has changed over time. As of 2011, the BIOS is being replaced by the more complex Extensible Firmware Interface (EFI) in many new machines, but BIOS remains in widespread use. EFI booting has been supported in only Microsoft Windows versions supporting GPT, the Linux kernel 2.6.1 and later, and Mac OS X on Intel-based Macs. However, the distinction between BIOS and EFI is rarely made in terminology by the average computer user, making BIOS a catch-all term for both systems.

Read more about BIOS:  Terminology, IBM PC-compatible BIOS Chips, Flashing The BIOS, BIOS Chip Vulnerabilities, Overclocking, Virus Attacks, Firmware On Adapter Cards, BIOS Boot Specification, Changing Role of The BIOS, The BIOS Business, Comparison