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From: drd@siia.mv.com (David Dick)
Newsgroups: comp.sys.mac.system
Subject: Re: Protected-mode snake oil
Message-ID: <1990Aug17.133833.9024@siia.mv.com>
Date: 17 Aug 90 13:38:33 GMT
References: <1204.26c2fb48@waikato.ac.nz> <1210.26c694ed@waikato.ac.nz>
Organization: Software Innovations, Inc.
Lines: 39

In <1210.26c694ed@waikato.ac.nz> ccc_ldo@waikato.ac.nz (Lawrence D'Oliveiro, Waikato University) writes:

>Thanks to all those people who pointed out how reliable their UNIX systems
>were. Can any of them tell me: does your system run anything like
>Adobe Type Manager, or After Dark? Or (rummaging through this pile of
>software that I haven't looked at yet) QuicKeys, FileMagic or POWERmenus?
>Can you think of a way of implementing hooks for any of these so that a
>bug in them can't render the system entirely unusable?

The research operating system MULTICS included the concept of rings.
Take the idea of two modes--user and system--and generalize it
so that there is some number of modes > 2.  The innermost ring
would contain super-critical things like process scheduling and
perhaps memory management. The next ring out might contain
device drivers; further out rings containing things closer to
user application programs.  Outer-ring code would invoke
inner-ring code in somewhat the same way system calls are done.
(On MULTICS, ordinary subroutine calls would be done and the 
system would conspire to provide mode-changes the same way
system-mode changes happen with system calls on most operating systems.)

ATM or After Dark would operate in some intermediate ring.  If they
misbehaved, they could trash things in outer rings, but never those
in inner rings.  Some inner-ring code could be responsible for
cleaning up after unfortunate events in outer rings.

Even if a sophisticated facility like rings is not provided (and why
they aren't I don't know--MULTICS had them almost 20 years ago!),
a kernel can still provide protection by manipulating the address
spaces while special-purpose things such as those you mention.  In
UNIX it could be done with special device drivers or additional 
system calls (e.g. to attach video ram to the code's address space).
It's not as elegant and probably isn't practical for all uses, but it's
still FAR better than the totally uncontrolled Mac environment
of today.

David Dick
Software Innovations, Inc. [the Software Moving Company (sm)]
"moving software to UNIX since 1980"