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Yes, the Apple II MouseCard IRQ is synced to the VBL
Some people thought it was not, but experience showed it is. Now theory joins practice and we can collectively be sure. A deep dive into synchronisation via cycle counting.
I did very good art to explain myselfWhile the bug in MAME is not yet fixed, the investigations into it by Robert Justice and R. Belmont (and a bit of myself) made one thing very clear: This part explains the “how does the AppleMouse card know about VBL when it doesn’t have that signal available”: it does it like we programmers do, via software. So this method as one advantage: easy, clean VBL sync on a ][+ with a mouse card, and two inconveniences: it requires a MouseCard, and eats a few hundred cycles that you could otherwise use to draw on the screen.
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