Got it, I have an idea how to implement the address modifier thing with only one address space that I will try out. It will not support all the corner cases but I'll make sure to check for those and and insert proper warnings when seen.
I figure emulation performance is mostly an issue for op code fetches and intensive data handling which ralrely occured over the VME bus, nothing stops a poor system design in that respect. However there are support for DMA reads which is used reading data buffers from disk/tape controller boards etc but it was quite slow in reality too.
