Comment by budman1

If the system was used to control something it was obvious.

There would be some kind of main / idle loop. When this was running, you could see the pattern in the data buss and in the address buss. The loop of code would not be many instructions, they would all be together, so the pattern on the address buss would be quickly repeating.

If you had a slow peripheral, you could see the ISR getting called. It would be at a different address than the main loop, and you could see it flash periodically, like every time a line went to the printer.

As an operator of the system, you had a unique perspective of watching the program run as the system operated; if it was data processing it would not mean much but in a control system it was uncanny.