At the National Museum of Computing in the UK (https://www.tnmoc.org/ https://www.tnmoc.org/) I saw an old minicomputer where, if I recall correctly, the low or

At the National Museum of Computing in the UK (https://www.tnmoc.org/ https://www.tnmoc.org/) I saw an old minicomputer where, if I recall correctly, the low order bit(s?) of the program counter had been hooked up to a speaker for some very simple auditory debugging. My memory is that e.g. infinite loops would be easy to identify as a persistent tone, etc as repeated small changes to the program counter happened.