Re: Unix without an MMU

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2 total messages Started by Dan Franklin Tue, 07 Jan 1986 16:24

#1917

Author: Dan Franklin
Date: Tue, 07 Jan 1986 16:24

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Mark Crispin's message did indeed list all the practical or semi-practical ways
to implement Unix w/o an MMU.  The people who responded to his message don't
seem to understand what the Unix fork() system call does; the Amiga TOS method
will not work in Unix.  The fork() system call, remember, generates a new
process which is an EXACT COPY of the old process.  Any pointers in the old
process's data are copied to the new process.  But the semantics of fork()
require that those copied pointers now magically point to the COPY of the data
in the NEW PROCESS.  Mark's message listed all the possible ways this could be
accomplished w/o MMU, save one, which is to somehow figure out which random
words of data in the data space are pointers and readjust them at fork time.
This is quite impractical, as anyone who knows something about C will realize.

As an aside, the Sage IV 68000 system had no MMU, and did run Unix.  Or rather,
it ran IDRIS, the Unix look-alike (which does have a Unix-style fork()).  I
don't know which of the methods Mark listed they used, but I suspect the first
one: running only one process at a time in memory, and swapping it out before
starting another one.  At any rate, let the Sage IV be the existence proof that
you CAN run Unix w/o an MMU, and let Mark's message be the explanation of how,
and let's get on with topics more closely related to the 520 ST, which I don't
believe will ever run Unix (though something else from Atari might).

	Dan Franklin (dan@bbn-prophet.arpa)

Re: Unix without an MMU

#2011

Author: Doug Collinge
Date: Sat, 11 Jan 1986 21:21

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In article <8601071639.AA03270@ucbvax.berkeley.edu> dan@BBN-PROPHET.ARPA (Dan Franklin) writes:
>As an aside, the Sage IV 68000 system had no MMU, and did run Unix.  Or rather,
>it ran IDRIS, the Unix look-alike (which does have a Unix-style fork()).  I
>don't know which of the methods Mark listed they used, but I suspect the first
>one: running only one process at a time in memory, and swapping it out before
>starting another one.  

I have IDRIS. It does have fork(). It can run more than one process in memory
at once and usually does but after a fork it runs the parent and child in the
same space by swapping.  Of course, as soon as you do an exec (most of the time
right away) it gives you a new space and the swapping stops.  This is entirely
practical and works just fine.  I see no reason why a similar system couldn't
work on the ST - but it couldn't be SysV or 4.2 because some of the calls
require memory mapping, e.g. realloc().
-- 
		Doug Collinge
		School of Music, University of Victoria,
		PO Box 1700, Victoria, B.C.,
		Canada,  V8W 2Y2  
		decvax!nrl-css!uvicctr!collinge
		decvax!uw-beaver!uvicctr!collinge
		ubc-vision!uvicctr!collinge

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