BIOS

The Basic Input/Output System (BIOS) is part of the firmware of IBM PC compatibles that provides the lowest level interface to peripheral devices and controls the first stage of the booting process, including installing the operating system. The BIOS is stored in ROM, or equivalent, in every PC. Its main task is to load and execute the operating system which is usually stored on the computer's hard disk, but may be loaded from a CD-ROM or floppy disk drive at install time.

History
In order to provide better performance (e.g. for screen display), some PC software developers accessed routines directly in the BIOS, rather than using the higher level operating system calls. Thus, the BIOS in the PC compatible computer had to be completely compatible with IBM's original BIOS.

Many older application programs bypassed even the BIOS to address the screen hardware directly just as the BIOS would normally. Consequently, register level compatibility was required in the compatible's display electronics, which meant that upgrades from original hardware may break compatiblity.