Comment by mnw21cam

The idea of listening to hardware running a program to tell what it is doing is surprisingly old. On the EDSAC computer[0] a little speaker was connected across one of the serial data lines, which allowed the progress of a program to be listened to. Skilled operators could immediately tell when a program had triggered a bug and either gone off into the weeds or entered a tight loop.

[0]: https://en.wikipedia.org/wiki/EDSAC