The BIOS is definitely a software API, as it is an item of software which has a carefully defined set of interfaces for the OS to call: http://www.ctyme.com/int

The BIOS is definitely a software API, as it is an item of software which has a carefully defined set of interfaces for the OS to call: http://www.ctyme.com/intr/cat-003 http://www.ctyme.com/intr/cat-003. In fact the only thing the clean room reimplementations of the IBM PC BIOS had in common with the original was exposing the same API.