Path: utzoo!attcan!uunet!ubvax!ames!lamaster
From: lamaster@ames.arc.nasa.gov (Hugh LaMaster)
Newsgroups: comp.arch
Subject: Re: PEP: Page Execution Priviledge
Message-ID: <16149@ames.arc.nasa.gov>
Date: 7 Oct 88 15:53:57 GMT
References: <2550@sultra.UUCP> <76700049@p.cs.uiuc.edu> <16017@ames.arc.nasa.gov> <10435@tekecs.TEK.COM>
Reply-To: lamaster@ames.arc.nasa.gov.UUCP (Hugh LaMaster)
Organization: NASA Ames Research Center, Moffett Field, Calif.
Lines: 40

In article <10435@tekecs.TEK.COM> andrew@frip.gwd.tek.com (Andrew Klossner) writes:
>>	"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 doesn't give you any of the benefits of a context switch.  If
>all you want is naked entry to kernel code, you can usually get it on a
>conventional system with little overhead.  For example, the "trap to
>kernel" instructions on the M88k take just three cycles.  Then the
>kernel is responsible for saving registers, etc.
>
>
>
>

It doesn't give you ANY of the benefits of a context switch?

The idea is to provide system level services that a user process
cannot be trusted to provide for itself, such as figuring out what block of
data to read on the disk, but which do not require single threaded access,
such as allocating/deallocating a block on the disk.  This is a very
important distinction in a multiprocessor system, since it is not a very
good idea to single thread the entire set of system services if you have
more than two processors.

Now, since most of the kernel services provided, even in Unix, are in doing
things that do not require single-threading, there is a very definite 
advantage in splitting things up this way in a multiprocessor system.  And,
there may be no disadvantage on a single processor system, since you have
avoided adding a context switch.




-- 
  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