Received: by 10.68.190.104 with SMTP id gp8mr8376980pbc.4.1340488715470;
        Sat, 23 Jun 2012 14:58:35 -0700 (PDT)
Path: l9ni12111pbj.0!nntp.google.com!news1.google.com!postnews.google.com!j9g2000vbk.googlegroups.com!not-for-mail
From: Michael S <already5cho...@yahoo.com>
Newsgroups: comp.arch
Subject: Re: Virtual address Memory Protection Unit
Date: Sat, 23 Jun 2012 14:58:35 -0700 (PDT)
Organization: http://groups.google.com
Lines: 34
Message-ID: <3e7feda4-a75e-4d7f-a2e0-67c9119bd8bc@j9g2000vbk.googlegroups.com>
References: <892a59b6-d516-4fda-8b52-6c6e3d0dfe30@x39g2000yqx.googlegroups.com>
 <s2jdb9-ntp.ln1@ntp6.tmsw.no> <js1abu$98c$1@needham.csi.cam.ac.uk>
 <wcctxy3f1bk.fsf@shell01.TheWorld.com> <js3g86$kg2$1@needham.csi.cam.ac.uk>
NNTP-Posting-Host: 109.64.108.251
Mime-Version: 1.0
X-Trace: posting.google.com 1340488715 5662 127.0.0.1 (23 Jun 2012 21:58:35 GMT)
X-Complaints-To: groups-abuse@google.com
NNTP-Posting-Date: Sat, 23 Jun 2012 21:58:35 +0000 (UTC)
Complaints-To: groups-abuse@google.com
Injection-Info: j9g2000vbk.googlegroups.com; posting-host=109.64.108.251; posting-account=yxhsVAkAAAAd1ExczpaZ2Ptm4WnXJOwS
User-Agent: G2/1.0
X-HTTP-UserAgent: Mozilla/5.0 (Windows NT 5.1; rv:12.0) Gecko/20100101 Firefox/12.0,gzip(gfe)
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable

On Jun 23, 7:21=A0am, n...@cam.ac.uk wrote:
> In article <wcctxy3f1bk....@shell01.TheWorld.com>,
> Robert A Duff =A0<bobd...@shell01.TheWorld.com> wrote:
>
>
>
> >> Simply abandoning the whole demand paging / TLB model in favour of
> >> segment authorities (not x86 segments, of course) eliminates the
> >> whole horrible mess for essentially no downside.
>
> >Could you please explain what you mean by "segment authorities"?
> >Putting it in google does not enlighten me.
>
> Essentially TLBs, but referring to Unix-like segments, with
> start addresses and lengths; they have had many names over
> the years. =A0So a whole segment would be contiguous.
>
> Regards,
> Nick Maclaren.

Which granularity?
Do virtual address ranges of different segments allowed to overlap?
How do you propose to implement the analog of "hardware page walker"
or do you think that pure software-managed TLB is sufficient?
Do you realize that range checks are significantly more expensive than
equality checks and that it means that you "segment TLB" will have
significantly fewer entries (10x fewer? what hW experts are thinking)
than page TLBs today? Or, may be, you are willing to give up full
associativity?

Of course, all those questions are relatively insignificant. More
significant is OS designers love to manage fixed-size pages and hate
to manage variable-sized segments. So you idea is doomed due this
single reason alone, regardless of any other difficulties.