It may have been possible to use them as an API, but it was different in the sense that you were interacting with the hardware directly. There were some nasty consequences to that (it was difficult to maintain compatibility between hardware models, nevermind hardware vendors). There were also some cool consequences to that (you could bend the hardware to do a lot of undocumented stuff).
That said, even firmware calls (the closest thing to a modern software API) were a different beast back then. You could buy documented assembly dumps of the ROM for some computers. Not only was it useful for figuring out how things worked, but you could figure out alternate entry points to use that firmware in undocumented ways.
A memory mapped API isn't inherently more (or less) subject to compatibility issues and versions than any other kind of API. With the C-64 it was hard-wired but that's not any more necessary than burning the OS on to ROM. (Which is to say: totally necessary at the time, for a machine of that capability and cost, but not at all necessary for systems that have the APIs we're comparing it to.
Comments
It may have been possible to use them as an API, but it was different in the sense that you were interacting with the hardware directly. There were some nasty consequences to that (it was difficult to maintain compatibility between hardware models, nevermind hardware vendors). There were also some cool consequences to that (you could bend the hardware to do a lot of undocumented stuff).
That said, even firmware calls (the closest thing to a modern software API) were a different beast back then. You could buy documented assembly dumps of the ROM for some computers. Not only was it useful for figuring out how things worked, but you could figure out alternate entry points to use that firmware in undocumented ways.
A memory mapped API isn't inherently more (or less) subject to compatibility issues and versions than any other kind of API. With the C-64 it was hard-wired but that's not any more necessary than burning the OS on to ROM. (Which is to say: totally necessary at the time, for a machine of that capability and cost, but not at all necessary for systems that have the APIs we're comparing it to.