you can race the beam, I've written many demos that use this. On the original Apple II you have to do something called "vapor lock" or "reading the floating bu

you can race the beam, I've written many demos that use this. On the original Apple II you have to do something called "vapor lock" or "reading the floating bus" where you draw a pattern to memory scanned by the video circuitry, then read an unmapped memory address and you get a "ghost" of the last value read by the video circuitry due to capacitance on the bus. You can then find the beam location, but from then on you have to cycle-count everything. Sather describes this in his book.