In trying to track down detailed references to this bug (where the IRQ that happens as BRK is fetched will cause BRK to be ignored), I found little beyond the acknowledgment that it occurs.

I was hoping to give the 6502 simulator folks some links, or at least some detail on the bug. They can't seem to get the sim to show the behavior at all, no matter where in BRK execution the IRQ line is dropped.

Any links are helpful, of course, but I'm wondering if there a NMOS 6502 user out them with a logic analyzer and a way to trigger IRQ at precise moments during the 6502 cycle who could provide a trace of the event happening.

Questions to answer:

Are we absolutely sure the bug exists? (numerous texts claim it, but they say only that it occurs during BRK fetch, no detail). I'd hate to think it's just an often repeated rumor that has been elevated to truth
What cycle (or portion of the cycle) must the IRQ go low to show the behavior?
Does the same bug happen for NMI during BRK?

If there someone out there who can help, it would be most appreciated.

Jim

We discussed it recently. Take a look at this topic, starting at about the tenth post. It seems like there was a better one. Maybe someone else can find it.