Thanks for the disassembly. I traced the code through the OS1.2 disassembly so I got a reasonable idea what was going on (I did it the old school way of turning op-codes into mnemonics and then into intentions)NMI is edge triggered, so I would expect to see NMI toggling pretty rapidly (~17KHz?)
What does NMI look like on the scope?
Can you trace it back through IC27 and IC91?
If the 6845 INT is high (inactive), then one of those must be at fault, or some other signal is shorted.
The problem is the IC97 and IC27 combination. The NMI is being generated by IC97 and it should be blocked by IC27 but IC27 has a weird failure (I think) so ROMSEL sends it off into interrupt again.
Statistics: Posted by maniacminer — Tue Feb 13, 2024 2:17 am