Xah Lee's Unixism

Andreas Krey yyx-nospam at gmx.de
Thu Sep 2 22:29:11 CEST 2004

* Karl A. Krueger (kkrueger at example.edu)
> It was my impression that the Motorola 68000 CPU, upon which the
> original Macintosh was based, did not support memory management in
> hardware.

That is not the problem; one can do memory management and multiple
address spaces in external hardware as well. But the MacOS architecture
obviously wanted to be all in one address space, as did the early
windows versions. This makes GUI easier and networking and fault
isolation harder, but it's a valid tradeoff. :-)

What you can't do with the 68000 is virtual memory management
because that requires the processor to save the state of
execution in the middle of an instruction when needed data
is not physically in memory. 68020 and upwards provided that
feature, and the Sun 3/50 used a 68020 and a proprietary memory
management unit mainly consisting of two fast SRAMs to get
virtual memory support.

I don't know whether the 68000 already had user and supervisor
mode which is also (besides an MMU) a prerequisite for completely
jailing user programs.


np: 4'33

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