Programmer Humor

It was also common to have a single step mode, where the CPU advances one cycle per switch press. Very useful for debugging.

And you could frequently read out the contents of registers directly on rows of lights. This led to the trope of the blinky light computer in Star Trek (original series) and elsewhere. Because the lights would flash in various patterns when the computer was running, as the register contents changed. But in the single step mode you could interpret the values.