(Basic Input Output System) An essential set of routines that sets up the hardware in a PC and boots the operating system. Prior to loading the operating system, the BIOS provides basic software drivers for all peripheral technologies that are part of the PC's motherboard, including the keyboard, mouse, monitor and hard disk. The drivers enable the user to edit configuration settings and allow the hardware to access the hard disk, optical disc or floppy disk to obtain the operating system.

After the operating system is loaded, more elaborate drivers are typically loaded, which replace the BIOS routines used to boot the system. The BIOS also supports internal services such as the real-time clock (time and date).

The BIOS Sets Up the Computer
On startup, the BIOS tests the system and prepares the computer for operation based on the installed hardware and the configuration settings from the manufacturer and user. For example, it initializes memory and caches and assigns resources to all devices connected to the PCI bus. The BIOS searches for BIOS extensions (option ROMs) on plug-in cards and sets up pointers (interrupt vectors) in main memory to access those routines. It then loads the operating system and passes control to it.

A BIOS Can Get Out-Of-Date
The BIOS used to be stored on a ROM chip (the ROM BIOS), which had to be replaced in order to be upgraded. Later, the BIOS was stored on a flash memory chip. In the early days of PCs, the addition of a larger disk drive or new type of peripheral sometimes required a BIOS change. It is less common today, but if a BIOS change is necessary, a flash memory BIOS can be upgraded in place with a software update. See BIOS upgrades, BIOS setup, beep codes and UEFI.