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From: | "senthil" <senthilvelan DOT gowthaman AT wipro DOT com> |
Newsgroups: | comp.os.msdos.djgpp |
Subject: | segmentation protection |
Date: | 27 Dec 2005 22:25:56 -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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