Thanks. I hope to try some of that tomorrow. In the meantime, I looked back through the Power Central and Power Demo code, and both of them show the following flow:
A condition is tested. One branch writes $520F 0000, then $5206 0000 -- this must be the "cache off" condition.
The other branch writes $5207 0000, then tests a different condition and writes either $5205 0000 or $5204 0000.
I can post code listings if that's useful, but basically it appears there are two more functional addresses in the GALs ($5207 and $5204). Could this have anything to do with enabling or disabling the FPU? There is a switch for that in Power Central, but I don't know if that needs any bits flipped in hardware.
Out of curiosity, what is happening when one removes jumpers C17 and B23, and why would it affect the floppy drive? Interested to know what's going on with the clocks and cache.
One final issue I didn't mention in the first post is that the bad P33, once it heats up for a half hour or so, becomes unreliable and generates random crashes. There's no obvious pattern I can see in MacsBug, but it looks like it might be loading instructions incorrectly from memory (one or more bits randomly flipped). I think this problem might be on the P33 logic rather than in the CPU itself, but I'm not sure.
Yeah, hopefully it can go to someone who can rescue the special chips. The 68882 that took the brunt of it is relatively easily replaced. If that Mac SE/30 schematic project gets off the ground, someone could whip up a new PCB to give those chips, and many more from bombed boards, a new home.