The clock module synchronises the working inside the CPU. It triggers changes in state and timing of most if not all of the computer.
The clock module consists of 3x 555 Timer IC’s which are used to generate a timing pulse and also to de-bounce input from the user when taking single steps or changing the clock from run to step.
The variable resistor allows for variable clock speeds. The second and third 555 timers debounces the step input and the step/run select switch. Denouncing removes the tendency for the switch/button to “double tap” itself when the user triggers it. This happens in a fraction of a second which humans can’t see but computers are just that fast.