(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.