Posted By
Bionic
on 2007-01-09
08:01:32
|  Re: Plus/4 Expansion port...
The timing in the Plus/4 is fairly conservative. Snooping for $FFFX an the address bus and then forcing A9 low should work. You have a couple of 100ns for that. Note that the address bus is active before the data bus so you have plenty of margin to avoid a collision.
If you don't wait for an IRQ before the snooping you may also trigger on a normal memory read. (Which is fairly unlikely though..)
I like the idea with the BRK, because it would also work without the IRQ being active. Forcing $00 into the databus should be possible in the Plus/4 since it is NMOS only. But I think it is quite difficult to detect actual opcode reads. What if an JSR is executed?
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