*I understand that computer operators learned to extract a lot of relevant infor...

I understand that computer operators learned to extract a lot of relevant information from the row of lights on the front of the computer that formed a binary representation of the contents of the program counter.

Parenthetically, these "out of band" debugging cues still work. Try holding an AM radio next to your PC's motherboard. :)

This was occasionally useful for debugging before CPU clock frequencies started to look less like AM radio and more like microwave ovens...