Mon, Oct 03, 2022 at 09:55:56AM -0500, Jim Brain wrote: >Others know more about NMI behavior, but if NMI is low, I believe the >vector is referenced, the CPU goes to that location, but NMI vector >will not be pulled again until the RTI. If I'm right, the NMI routine >merely has to stay engaged for > 1/60th of a second. NMI can only be triggered on a falling edge of the signal. The RTI instruction does not matter. (In an IRQ handler, RTI would restore flags, which would typically clear the I flag and thus cause another jump to the IRQ handler if the IRQ line is asserted.) You can simply disable NMIs and the Restore key by pulling the NMI line low with the CIA 2 ($dd00). On the Vic-20, you could use the VIA 2. Badly written software would get upset if you hammer on the Restore key. In fact, on the Vic-20, it is possible to overflow the stack by hitting the key. On the C64 and C128, the denoising circuit appears to be better. MarkoReceived on 2022-10-03 18:00:10
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