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Message-ID:	<43b77b23$0$175$cc7c7865@news.luth.se>
From:	Martin Str|mberg <ams AT speedy DOT ludd DOT ltu DOT se>
Subject:	Re: segmentaion protection
Newsgroups:	comp.os.msdos.djgpp
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Date:	01 Jan 2006 06:48:03 GMT
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senthil <senthilvelan DOT gowthaman AT wipro DOT com> wrote:
>   Can anyone explain me regarding the difference between RPL and CPL.

It's been a while, so it might not be 100% accurate.

RPL is the Privilege Level (PL) of the selector you're calling unless
it's a conforming (segment of a code) selector. So if you execute the
(far) call/jmp (without any violations) the CPL turns into this
RPL. This can only be called/jmped/ireted to lower or equal PL (higher
or equal numbers).

A (interrupt/call/task) gate can point to another code selector. This
is how you (can) raise the PL (lower numbers) as the gate can be PL 3
but pointing to PL 0.

If it's a conforming selector it's something else but the main thing
to remember is that CPL remains the same. And I've never quite
understood when it would be good or advisable to have a conforming
segment. The best so far I've come up with is having the kernel and
userland sharing the same far routines (like a far_strcmp()). Perhaps
it might be useful for microkernels?


Right,

						MartinS

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