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