(BIOS) The part of the operating system of the IBM PC that provides the lowest level interface to peripheral devices. The BIOS is stored in ROM in every IBM PC. In order to provide acceptable performance (e.g. for screen display), software vendors directly access the routines in the BIOS, 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.