In IBM PC compatible computers, the basic input/output system (BIOS), also known as the System BIOS or ROM BIOS (pronounced /ˈbaɪ.oʊs/), is a de facto standard defining a firmware interface.
Phoenix AwardBIOS CMOS (non-volatile memory) Setup utility on a standard PCThe 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 to initialize and identify system devices such as the 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 chip's contents can be rewritten without removing it from the motherboard, allowing BIOS software to be upgraded in place.
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:
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 UI 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, will 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, and EFI booting has only been supported in Microsoft's operating system products supporting GPT and Linux kernels 2.6.1 and greater builds (and in 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.