I think I may have found the reason why macbsd dies so horribly when you
don't set the memory size to one meg lower than actual memory. It seems
that some of Apple's macos MMU tabes are hosed. On my 20 Meg IIvx, the
tables for the first invalid page look like this:

level 0, dtype 3, y[0]/y[1]: 7FFF0003/013FF9A0 offset 0, new desc:
013FF9A0
level 1, dtype 2, y[0]/y[1]: 004FFC0A/013FF860 offset 140, new desc:
013FF9A0
level 2, dtype 2, y[0]/y[1]: 004FFC0A/013FF860 offset 0, new desc:
004FFC00

(I aborted the output here, since it was going to go into lala land.)

If you notice what descriptor level 1 points to, 013ff860, it's itself.
whoops. This is, shall we say, "bad".