Thanks for the descriptions above, which essentially define the 4MB RAM limit for most 68000 Macs. Presumably the Portable and some others were able to map the ROM to a higher memory location (and that when this occurred, the system knew the upper RAM bounds). Which leads to the question whether any third party manufactured an add-on for the Plus or earlier that remapped memory for the ROM to permit more than 4MB system RAM. I'm familiar with accelerators with 8MB that allowed 4MB system RAM plus 4MB RAM disk, but not 8MB system RAM. If it had been easy, lots of people would have done it...

Similar limitations applied to boards based on the LC/LCII design but one of the third party accelerator manufacturers cracked this. It's in my notes somewhere but before I look it up I'd appreciate other thoughts.