Re: 6551 driver questions

From: Rainer Buchty (
Date: 2003-08-23 20:10:08

> Since the 6551 is connected to the NMI, it is not possible to just
> sei/cli to disable interrupts while modifying the register from outside
> the interrupt handler.

If I recall correctly, the NMI is level driven like a latch not
level-change triggered like a flip-flop. To prevent false triggers from
the 6551, wouldn't it be possible to "lock" the NMI using the CIA's
timer IRQ, then do the changes and finally release the timer IRQ?

So the NMI routine would have to check for the timer IRQ first (and return
in that case), otherwise processing the 6551 IRQ.


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