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From: vandys@hpcupt1.HP.COM (Andrew Valencia(Seattle))
Newsgroups: comp.os.minix
Subject: Re: Minix virtual memory
Message-ID: <5820010@hpcupt1.HP.COM>
Date: 9 Nov 89 16:53:05 GMT
References: <1989Nov6.070745.273@polyslo.CalPoly.EDU>
Organization: Hewlett Packard, Cupertino
Lines: 20

/ hpcupt1:comp.os.minix / jerry@altos86.Altos.COM (Jerry Gardner) /  9:53 am  Nov  8, 1989 /
> ... The 80286 does,
>however, support *segmented* virtual memory.  Each 80286 segment descriptor
>has a present bit and an accessed bit necessary to support virtual management
>of segments.  Granted, it doesn't have a 'dirty' bit to indicate that a segment
>has been written to, but neither does the VAX architecture, and it has gotten
>along fine without it.

	Quite a while ago somebody who had worked on a proprietary UNIX
for Daisy posted a description of the gotchas involved with the 286 when
trying to provide virtual memory.  In particular, I believe there were
significant problems in restarting memory writes after faulting on a segment.
Because of this, you couldn't provide dynamically growable stacks, and you
couldn't use the VAX trick of emulating the modified bit (set the segment
read-only, then flag it modified on a write fault and turn off the read-
only bit).  But with a large-model program I'm sure it could still be used
to provide much better memory utilization compared to a straight swap model.

						Andy
						...!hplabs!hpisoa1!vandys