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

["johnny cache" <johnycsh () gmail com>]

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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