I'd think interrupts would be ideal...

"interrupts have crummy timing anyway (a horizontal scan takes tens of microseconds, you might wait that long just to get the interrupt handler, especially if more priviledged I/O is going on)"

What are you talking about? Interrupts have great timing. BANG and your handler is running. Even if more privileged I/O is going on, *a polling routine isn't going to be running anyway*. An interrupt handler has the advantage of being able to wait even after the trigger has gone away.


...buuuut as far as I know there is no way for the horizontal retrace to trigger an interrupt.