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From: jer@peora.UUCP (J. Eric Roskos)
Newsgroups: net.arch
Subject: Re: inside the OS vs. outside the OS
Message-ID: <762@peora.UUCP>
Date: Wed, 27-Mar-85 09:05:11 EST
Article-I.D.: peora.762
Posted: Wed Mar 27 09:05:11 1985
Date-Received: Thu, 28-Mar-85 03:09:11 EST
References: <501@harvard.ARPA> <5329@utzoo.UUCP>
Organization: Perkin-Elmer SDC, Orlando, Fl.
Lines: 42

> Because if it isn't done in a central place, history shows programmers
> don't bother doing it at all.

To a certain extent, all the discussions over the past week or so about
what should be "in the kernel" or not miss the point made by the above
comment.  There are only a handful of reasons for having something "in the
kernel":

	1) So it will be accessible to all users.  This, of course, is
	   what a central library of subroutines does, but putting it in
	   the kernel encourages all the various compiler-writers
	   to use it instead of inventing their own.

	2) So all users will have to use it.  This is a stronger form of
	   (1), provided to force a stronger degree of uniformity.  It fails
	   when the facility "in the kernel" is not sufficiently general,
	   or is so general that performance suffers.

        3) So that it will have access to resources that are inaccessible
           to code "outside the kernel," e.g., privileged instructions,
           access to device control facilities, higher-priority execution,
           etc., in order to mediate and protect allocation of these
	   resources to the code "outside".

Only #3 above (of the items I've listed) HAS to be in the kernel, if the
kernel is that portion of the code running on a machine that has access to
restricted resources.  There are other reasons related to (3) that make it
beneficial to put the other two in the kernel on a particular machine --
for instance, the PDP-11 front ends on the PDP-10-based operating systems
mentioned before (TOPS20; also TOPS10) are limited resources whose
performance would be degraded if user processes were allowed to be loaded
into them, eventhough you could conceivably allow ordinary users to
download their TTY handling routines into them (assuming proper protection)
-- but those are matters of optimization.
-- 
Full-Name:  J. Eric Roskos
UUCP:       ..!{decvax,ucbvax,ihnp4}!vax135!petsd!peora!jer
US Mail:    MS 795; Perkin-Elmer SDC;
	    2486 Sand Lake Road, Orlando, FL 32809-7642

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