15 MHz m68030 with a 16 bit memory bus and 10 megabytes of RAM -- A Mac LC II, by any chance? :)
> towards the late '80s and early '90s
By the late 1960s, really. It would probably be possible to port Linux to the IBM Model 67 [1]; might even be easy since GCC can already target the instruction set. The MMU is sufficient. Maybe a tight squeeze with the max of 2 MB of fast core. Would be in a similar ballpark, a bit slower, to that 68030 machine.
Full virtualization, and hardware enforced memory and IO boundaries, were invented early on, too. It took a while for such features to trickle down to mini- and then micro- computers. And then much longer for popular software to take advantage.