Re: luckily, there are no dumb questions (dan () geer org)

["Ed Schaller" <schallee () gmail com>]

VMS on the vax did use more than two rings. The lack of that feature
caused some control systems to never upgrade from vms on vax.

Xen virtualization, and I imagine others, utilizes multiple rings as well.



On 6/7/07, johnny cache <johnycsh () gmail com> wrote:
> Wouldn't a better question be: "how is it that -no- mainstream OS uses
> more than 2 rings on x86?" Or  "How come nobody uses x86
> segmentation(by default)?"
>
> I think the simple answer is that most operating system developers
> view these features as baggage that have no analogy on other platforms
> and therefore are to be avoided. Segmentation (by-and-large) got the
> axe on 64-bit x86 chips. Who's to say 4-rings wasn't next on the
> chopping block? If the features have been there and haven't been used
> in over a decade, its probably not a good idea to dust them off and
> start depending on them now. Writing an OS that made effective use of
> all 4 rings would not only be difficult, forward compatability on more
> "sane" CPUs is almost certain not to happen.
>
> Just my 2c.
> -jc
>
>
> > Date: Thu, 07 Jun 2007 15:30:58 -0400
> > From: dan () geer org
> > Subject: [Dailydave] luckily, there are no dumb questions
> > Luckily, there are no dumb questions or this would
> > likely be one.
> >
> > How is it so that MS Windows uses only Rings 0 & 3?
> > An engineering answer, a marketing answer, and/or
> > an historical answer would be welcome.  Don't know
> > why I never thought to ask before, but I'm asking
> > now.  (And if I'm really wrong, please tell me what
> > uses 1|2.)
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