Charlie, that was a good start, thanks. I know there is no Nubus Controller in these old macs, the CPU does all the work (I think). Dennis, the stack is intact, I mean that something other than the CPU is manipulating data in memory, and generally, by using the ISP and MSP, and keeping an eye on what the int level in the SR is, and values of various registers, you can see where interrupts are coming from, and who services them, etc. But I have this sixth sense that something other than the CPU is doing the work of sorting out what goes to who. I can't quite put my finger on it. Macsbug is a weird and wonderful program, and generally is very good about stepping through instructions and updating registers and memory, while letting VBL tasks and interrupt handlers and page faulting go on in the background. In this case though, just on this machine, stepping through some instructions gets you a hang or bus error. I wonder if it has to do with VM? Maybe the page fault handler is getting interrupted, and can't page memory that the debugger is using? I'll trying disabling VM and trying again, but I would still like to get my hands on the pinout/description for the GLU chip, it's the only thing on the board I know nothing about.

EDIT: I actually might have answered my own question. It seems that upon entry to the debugger, the page fault exception handler is in one of two places. So sometimes when I break into the debugger, im in real memory, sometimes I'm in the application's VM, and sometimes I am in the Debugger's VM, possibly even its own exception handler. I can;t step through that because if I need a page of memory, I'm currently stepping through the program that is doing the paging for me which could be bad.