3 ms·

That sounds even more terrible, with the screen flickering intensely to be able to read the mouse?

• a1369209993 5y ago
  The idea is that you do the calibration once (eg on program start), then the timer interrupt handles it until and unless it get desynced. So only for one frame, total (per run of the mouse-using program).
  • anyfoo 5y ago
    Ah, that makes more sense, thank you. I thought this was constant synchronization, but IIRC the Apple II only has one oscillator from which all clocks are derived (as was common for 8 bit machines).
    • a1369209993 5y ago
      IIUC, it still only has one clock (oscillator), but the mouse card has a seperate timer (counter) in it's VIA, which needs to be synchronized with the video hardware because the VIA is not initialized (with the same number-of-clock-ticks-per-(vblank)-interrupt as the video hardware runs at) at power-on.
      • anyfoo 5y ago
        Yes, I understand. The point is, with a single oscillator (emphasis, I mean oscillator) being the source of all derived clocks, synchronizing once is sufficient, as all clocks will keep a fixed relation. If, say, the video card had its own oscillator (comparably how modern video cards do or did), the clock domains would drift from each other and frequent resynchronization were necessary (but you'd just use the interrupt instead that most likely would be added in this scenario). But I think you helped me understand the scheme. I'd add that according to the article, a single frame is not necessarily enough, given the description on how synchronization can be missed at the first attempt, but this is hardly relevant, given that the synchronization still likely happens only once and within a brief timeframe.