> (1) Wouldn't it be easier to make a version of MacBSD that runs on *top* of
> the Mac OS?

No.  MachTen, MachMac, and MacMach all use the MacOS to varying extents.
The obvious advantage is that you get compatibility pretty much across the
board, as accesses to the hardware are done by the MacOS.  The MacOS,
however, is hardly designed to multitask.  It's not designed with a lick of
memory _protection_.  The whole bloody thing runs in "supervisor" mode
instead of "user" mode...  If I remember right, a number of MacOS variables
are global to the OS, so any MacOS-type process would have to have access
to the memory where they're kept.  Someone who knows more about the MacOS
might be able to come up with more reasons of the top of his/her head.  We
are more interested in running Unix than running Mac, and we know more about
Unix than about the MacOS, so we approached it this way...

> (2) What's keeping MacBSD from being able to run on old 68000 Macs?

Similar to the above...  Unix is a multitasking OS, so there are a number of
processes running at once.  Each process essentially thinks that it runs by
itself, though...  It sees it's own memory space which doesn't get trashed
by anyone else.  It never (almost never ;) gives up the processor
voluntarily.  It's switched out by the Unix kernel, but its state is saved
to be restored later--exactly how it left it.  This state includes its
memory space.  A processes memory space is made up of virtual addresses.
Each virtual address is mapped to a physical address in chunks called pages.
A given page might be a physical address, or it might be on disk somewhere in
which case it needs to be "paged in."  The PMMU handles the
virtual<=>physical mapping in hardware.  It would be prohibitively slow to
try to handle the mapping without it.

It can obviously be faked to some degree, but you lose some of the nicer
(IMO) benefits of Unix...

-allen

-- 
Allen Briggs         Find the cost of freedom / Buried in the ground
- end                Mother Earth will swallow you / Lay your body down
      killing -    
allen.briggs@vt.edu          ***** MacBSD == NetBSD/Mac *****

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