RE: IRQ for 6502
Date: 2001-08-21 03:08:14

>Ruud Baltissen wrote:
>> The SRC revealed that the unused port A is read to clear the IRQ caused
>> the ATN line. I can disable IRQ with the instruction SEI. If during the
>> IRQ that is disabled an ATN shows up, the IRQ-input of the 6502 is
>> Question: if I enable IRQ again with CLI, will this IRQ still be handled?
>> AFAIK the answer is "Yes", but I want a conformation please.

>The IRQ is not "cached" in any way, but if the IRQ line is still active
>(i.e. negated) when you do the CLI (as it would be unless you explicitly
>cleared the ATN condition by reading port A before the CLI), the IRQ
>will be taken of course.

Also the CPU has to be "awake" to record the fact that an IRQ has occurred.
It is not latched in hardware in any way.

This is different to an NMI which can occur whilst the CPU is supsended (eg
during a DMA) and will be acted upon once it is restarted.


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