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From:	"senthil" <senthilvelan DOT gowthaman AT wipro DOT com>
Newsgroups:	comp.os.msdos.djgpp
Subject:	segmentaion protection
Date:	27 Dec 2005 22:25:05 -0800
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HI

  Can anyone explain me regarding the difference between RPL and CPL.

 The  definition of RPL (requested privilege level) is given as  below

 The RPL is an override privilege level that is assigned to segment
selectors.
  and the requestor's privilege level (RPL) field of a segment
selector is intended to carry the privilege level of a calling
procedure (the calling procedure's CPL) to a called procedure.
My doubt if the RPL is privilege level of calling program  then in case
of transferring control to the non-conforming code segment , the
condition is given as RPL must be lesser than or equal to CPL (
numerically) for  the control transfer to take place.
       So the calling program is at higher privilege  than the called
program, which  against conception that a higher  privileged program
can access only the code segments on its privilege level

The general consideration that I took for accessing the code and data
segments with respect to its privilege level is given below


 A task can access data only on its own level of protection or on that
which is numerically higher (i.e.) on outer one (ring) as related to
current). This results in ring 0 being the most protected in terms of
data access. From the point of view of code segments, ring 0 is the
most privileged as one having access to data areas in entire system.
Reverse situation exist s when looking at the code access: ring 0 can
invoke code that lives on its own level only, while ring 3 can call all
(unless restricted otherwise) code in entire OS

Is the consideration  correct ..

2.In ARM processor we call change  from one mode to to another mode so
that privilege level are varied by  doing so

  How can we program the privilege level  of AMD64 processor.

 or we allowed to set the privilege level  by means of assembly
programming ..
 
Kindly clarify my doubts.

Regards,
senthilvelan

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