Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!sdd.hp.com!think.com!barmar
From: barmar@think.com (Barry Margolin)
Newsgroups: comp.os.misc
Subject: Re: Call Gates vs. traps
Message-ID: <1991Apr5.075057.16322@Think.COM>
Date: 5 Apr 91 07:50:57 GMT
References: <1991Apr3.073617.1167@ibmpcug.co.uk>
Sender: news@Think.COM
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 24

In article <1991Apr3.073617.1167@ibmpcug.co.uk> dylan@ibmpcug.CO.UK (Matthew Farwell) writes:
>Multics and OS/2 use Call Gates to implement system calls, whereas the
>rest of the universe uses traps, yes?  What are the advantages/
>disadvantages of using Call Gates over traps?

The main advantage is that the same mechanism is used to call system
routines as library routines.  It also generalizes to multiple protection
levels, whereas system calls are normally only good enough when there are
only two protection levels (user and system).  Multics has eight nested
rings of protection, and gates are used whenever a program wants to call
into a more privileged level.

>p.s. Can anyone tell me how to get any references on Multics?

The only easy-to-find reference is the book on Multics by Organick,
published by the MIT Press.  Other than that, you'll have to find papers
from technical conferences in the mid- to late-60's.  The book probably
contains a bibliography that points out these papers.

--
Barry Margolin, Thinking Machines Corp.

barmar@think.com
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