comp.lang.c FAQ list · Question 19.9b

Q: How can I access an I/O board directly?

A: At one level, at least, it's quite simple: you have a device register which is actually wired up so that the bits written to it get coverted to actual voltage levels in the real world that you can do interesting things with. In general, there are two ways to get the bits in and out. (A particular I/O board will use one method or the other; you'll need to consult its documentation for details.)

1. If the device is accessed via a dedicated ``I/O port'', use system-specific functions to communicate with it. Under MS-DOS, for example, there were quasistandard ``inport'' and ``outport'' instructions.
2. If the device uses ``memory-mapped I/O'', that is, if the device register(s) are accessed as if they were normal memory at particular, known locations within the processor's addressing space, use contrived pointer variables to access those locations. See question 19.25.

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