The only C I've really written was on microcontrollers, where you usually change register values directly and it doesn't get much more global than that. I suppose I'm ruined now for "big iron".
Yeah, I worked on the cert for an O/S board support, trying to reverse engineer requirements.
Most of the functions wrote a series of values to the
same register location, to program the I/O device. So there was no way to test the software because there was no way to determine a single result value for a register.
I think most of the registers returned to zero after accepting the input anyway