(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.