There was an entire promised-for-years Mac OS 8 codenamed Copland that was to, among other things, add preemptive multitasking and process memory protection to the "classic" Mac OS. That project was a disaster on the order of Apple deciding it needed to cancel it and start over working from an OS it would acquire (BeOS and NeXTstep being the big publicly known candidates with the latter being what they went with). A few things from Copland appeared to survive into what became the much less ambitious actual Mac OS 8 and Mac OS 9. Namely the full-filesystem content indexing system, the platinum UI, and a kind of intermediate "nanokernel" that offered some improvements to multitasking, like not locking up the whole system when the mouse button was held down and allowing other stuff to happen while intensive disk/peripheral IO were happening. From memory I think it make have also added some basic improvements to memory protection, but by way of padding around the address spaces that applications had allocated to them rather than real memory protection. I can't quite remember anymore. I was a teenager at the time.