Well, if you know how the processor accesses memory then you already know the answer. Memory and other hardware are accessed in (almost) the same way, depending on wether the processor uses "memory mapped IO" or "IO mapped IO". In the first case, the processor just tries to read and write to a memory address - but there are no memory there - instead it's some other hardware device, but the processor really can't tell the difference. The latter case is very similar.
On the other hand, if you have no clue on how the processor accesses the memory, you should look up what an "address bus" and a "data bus" is to get you started.
