On Mon, Jun 2, 2014 at 10:24 AM, John Cowan <cowan@mercury.ccil.org> wrote:
The original Mac 128K was a 68000 processor, and IIRC memory protection
didn't arrive until the 68020.

​Sort of.  Did not arrive to >>Apple<< until the '020 based Mac-II.

Les Crudele (one the 68k's designers) tells a great story about this.   The original device had a PDP-11 base/limit register MMU as an external chip who's number I forget (and could not do demand paging by itself -- Masscomp and Apollo would use 2 of them and build their own MMU).    According to Les, Moto offered to give Apple the MMU chip at a substantial discount (maybe even free) if they would use it for the Mac.  But Jobs famously said it was a personal computer and did not need it (remember the Alto's did not have an MMU either).

Later, Moto would release the '010  ​which could do demanding paging with help of an external MMU.  The Stanford University Network Terminal (aka "SUN" board) could use the '10.   I don't think Apple did themselves, but my memory is that the was an after market '010 mod for some of the Macs.

At Masscomp, we retrofited the original CPU board to take a '010.   In this mode, the 'fixer' 68K and the '010 could run in parallel during a page fault, which was slightly faster.   But functionally, to the end user it was the same as two 68K's.  I'm not sure if Apollo did a retrofit.