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Hi guys,

I was fiddling with the a2pic and testing it out with this software called "Combined Enhanced Graphics" (030b_Combined_Enhanced_Graphics.dsk) with the ap2000 and it would just hang reading $c1c1.




It looks like the a6 line going into the prom depends on a6,a7 and ack.

So if ack is low, reading $c1c1 actually gets $c181 which is a value < 128 and if ack is high, reading $c1c1 gets $c1c1 which is >=128. Or maybe the other way around.

>print b@c1c1
FE
[MAME]>

>print b@c181
27


I think for some reason it doesn't ever clear the ack.




Interestingly, the Epson APL card has a very similar looking circuit where a6 depends on a6,a7 and ack. Schematic is from "Epson MX Series DMP Option User Manual [for] Apple II".