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From: norsk@sequent.UUCP (Doug Thompson)
Newsgroups: comp.os.minix
Subject: Re: Minix virtual memory
Message-ID: <24609@sequent.UUCP>
Date: 9 Nov 89 17:35:49 GMT
References: <670@crash.cts.com> <3745@altos86.Altos.COM>
Reply-To: norsk@crg2.UUCP (Doug Thompson)
Organization: Sequent Computer Systems, Inc.
Lines: 40

In article <3745@altos86.Altos.COM> jerry@altos86.UUCP (Jerry Gardner) writes:
>In article <670@crash.cts.com> jca@pnet01.cts.com (John C. Archambeau) writes:
>>I would love to hear how you're going to do it.  I've seen the memory
>>management scheme of the 80286 in protected mode.  It isn't pretty.  On a 386
>>though, it would be child's play.  If you're going to implement VM, you might
> (deleted)
>
>Wait a minute here!  It looks like it's time to clear up a big misunderstanding
>many people have with respect to virtual memory on the 80286.  We've all heard
>all the talk of the 80286 being brain damaged and not being able to support
>virtual memory, but this just isn't true.  What is true is that the 80286
>does not support *paged* virtual memory as the 80386 does.  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.
>
>Virtual memory on the 80286 is certainly possible (OS/2 does it :-); it may
>not be "child's play", but it certainly is possible.  BTW, I disagree with
>your assessment that implementing Minix VM on the 80386 would be child's play.
>

One of the problems with the 286 is that is CANNOT restart some of its 
instructions when it gets a segment fault. This is one of the reasons it
is called brain-damaged. When a page or segment fault occurs, its upto the
OS to bring in that page or segment and then restart the faulting instruction
as if no fault occurred. The 386 was fixed in this regard, and does restart
instructions after a fault. BTW upto 6 page faults can occur on one instruction
on a 386, given instruction streams and operand fields.

OS/2 (286) doesn't implement virtual memory, but rather swapping of threads.
I do agree that 386 VM is NOT child's play. But it is easier than trying to
get a 286 to go virtual when the Hardware doesn't support the theory.

-- 
Douglas Thompson		UUCP: ..{tektronix,ogcvax,uunet}!sequent!norsk
Sequent Computer Systems	Phone: (503) 526-5727
15450 SW Koll Parkway	!"The scientist builds to learn;the engineer learns in
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