Newsgroups: comp.arch
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: PEP: Page Execution Priviledge
Message-ID: <1988Sep30.170503.19191@utzoo.uucp>
Organization: U of Toronto Zoology
References: <2550@sultra.UUCP>
Date: Fri, 30 Sep 88 17:05:03 GMT

In article <2550@sultra.UUCP> dtynan@sultra.UUCP (Der Tynan) writes:
>Rather than have a standard U/S bit in the CPU status register, it might make
>more sense to assign it to each I-page in a paged-MMU system...

Things at least vaguely along those lines have been done.  There is one
major problem that has to be solved:  how do you prevent a user from
branching to some well-chosen place in the *middle* of a privileged
routine?  Say, for example, bypassing some of the legality checks at
the beginning?  One needs some hardware-enforced notion of entry points,
so that transitions from lower privilege to greater privilege get done
only in authorized ways.
-- 
The meek can have the Earth;    |    Henry Spencer at U of Toronto Zoology
the rest of us have other plans.|uunet!attcan!utzoo!henry henry@zoo.toronto.edu