Source : Free On-Line Dictionary of Computing

Basic Input/Output System
     
         (BIOS, ROM BIOS) The part of the {system
        software} of the {IBM PC} and compatibles that provides the
        lowest level interface to {peripheral} devices and controls
        the first stage of the {bootstrap} 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 {CD-ROM} or
        {floppy disk} at install time.
     
        In order to provide acceptable performance (e.g. for screen
        display), some software vendors access the routines in the
        BIOS directly, rather than using the higher level operating
        system calls.  Thus, the BIOS in the compatible computer must
        be 100% compatible with the IBM BIOS.
     
        As if that wasn't bad enough, many {application programs}
        bypass even the BIOS and address the screen hardware directly
        just as the BIOS does.  Consequently, {register} level
        compatibility is required in the compatible's display
        electronics, which means that it must provide the same storage
        locations and identification as the original IBM hardware.
     
        (1999-06-09)