Path: utzoo!utgpu!attcan!uunet!ubvax!ames!lamaster
From: lamaster@ames.arc.nasa.gov (Hugh LaMaster)
Newsgroups: comp.arch
Subject: Re: PEP: Page Execution Priviledge
Message-ID: <16017@ames.arc.nasa.gov>
Date: 5 Oct 88 15:43:14 GMT
References: <2550@sultra.UUCP> <76700049@p.cs.uiuc.edu>
Reply-To: lamaster@ames.arc.nasa.gov.UUCP (Hugh LaMaster)
Organization: NASA Ames Research Center, Moffett Field, Calif.
Lines: 27

In article <76700049@p.cs.uiuc.edu> gillies@p.cs.uiuc.edu writes:

>do you prevent a user-call from transferring to some random
>non-instruction on one of these protected pages?  In Multics, you
>could only jump to the beginning of a protected segment, I believe
>(ENTER).
>
>
>

May I assume that the mechanism permitted unrestricted branching WITHIN
a particular code segment, and only restricted accesses OUTSIDE of the
current segment via a protected CALL/ENTER?

This mechanism would seem to be a very low overhead means of providing
"top half of the kernel" type services to a process, since it provides
a protection mechanism which doesn't require a context switch.  It also
extends nicely to a multiprocessor environment: anything which doesn't
require single threaded access could be in the protected system segments.
So, how come everybody doesn't do it?


-- 
  Hugh LaMaster, m/s 233-9,  UUCP ames!lamaster
  NASA Ames Research Center  ARPA lamaster@ames.arc.nasa.gov
  Moffett Field, CA 94035     
  Phone:  (415)694-6117