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.
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.
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.