Yes, I can understand the reasoning for this.
On the other hand, less changes to the NitrOS-9 might be nice as well.
A long time ago I had a processor card with both the (standard) 6502 and 6809 on it. And it could just be switched between the processors.
Both processors were almost piggy-backed.
After looking at the data-sheet for the 6809 (Again just now) I conclude that it is not very difficult to have both processors in the system.
The most difficult part, the /Halt, is already there. Keep one of the CPU's halted.
Just figure out the IRQ line and take care of the NMI. (Different reaction, specially combined with /Halt condition.)
If you choose this approach, it would also be possible to have two different memory configurations.
Just replace the MMU with a Pal22V10 and you're done. The extra inputs can be used to switch between two memory-maps.
I would start with fixing the clock generation part. The 6502 creates the Q1 and Q2 signals from Q0.
This is not present in the 6809. And the system needs a double frequency for the D-Ram's to function.
Generally, just inverting the Q0 to get Q1 and using Q0 for Q2 will usually work fine. (Mind the cautious way I expess this, it's not this simple on all Atari's.)
Edited by guus.assmann, Tue Jan 24, 2012 8:45 AM.