Device driver software traditionally consists of a lot of "bit-bashing and register-twiddling". The bits, the sequences, and the timing, all have to be just right—or else that chunk of hardware will just refuse to operate properly. But it is also possible to take a higher architectural-level view of device input/output driver software. This class begins with the fundamental driver architectural issues of mutual exclusion in device access, and synchronous vs. asynchronous relations between application tasks and hardware devices. This will be followed by a series of examples of driver architectures which progress in complexity: synchronous input driver; basic asynchronous input driver; free-running asynchronous input driver (queued inputs); latest value only asynchronous driver.