At the end of the axle that's moved by the rubberball, is a plastic wheel attached. The wheel have a hub withmany, thin spokes, almost like a bicycle wheel. Four optical decoders are then converting the movement

into electrical signals.

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The output of those signals go straight to the DB9 connector on pre-ADB mice. One of the outputs from each axis is wire-OR'd to an interrupt line, so when there's movement, it interrupts the processor which then polls the state of all 4 lines and translates that into how many pixels the mouse should move and which direction. The pre-ADB mouse was super simple mechanically, but annoying to replicate logically in code.