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From: glenn@extro.ucc.su.OZ.AU (Glenn Geers)
Newsgroups: comp.unix.xenix,comp.unix.i386
Subject: Re: Using GCC/GAS Xenix on AT&T Unix V/386.3.2
Message-ID: <1990May4.043925.12686@metro.ucc.su.OZ.AU>
Date: 4 May 90 04:39:25 GMT
References: <2496@crash.cts.com>
Sender: johnt@metro.ucc.su.OZ.AU (Louise Rowling(see rossc))
Organization: Uni Computing Service, Uni of Sydney, Australia
Lines: 29

From article <2496@crash.cts.com>, by jca@pnet01.cts.com (John C. Archambeau):
> 
> Maybe I'd better refine the question, what's the difference between ordinary
> swapping and demand paging?  I've heard of systems having both and the demand
> pager kicking in when the swapper is heavily loaded.  Or am I listening to
> misinformation?

The swapper kicks in when the pager can't handle the load. The fundamental
difference between a purely swapped system (e.g. U*IX on a 286) and a demand
paged system is that the entire text and/or data does not need to be resident
in core in the demand paged case. On the 386 the page size is 4k so you could
conceivably run your machine with 8k of user memory available but you'd really
pay for it by having a slooooooooooooooooow system since every data or text
reference more than ~4k away would require a page fetch from disk. The proof
that xenix pages is purely empirical - take nethack (nice and large), when
you get the initial map of the first level the movement code is not in core
(at least on my machine !) so when I hit a movement key the OS demand pages
in the relevant bits of code (yep, the disk light goes on and there is a slight
delay).

For more coherent info read an OS book - a good one is "Operating Systems:
Design and Implementation" by Andrew S. Tannenbaum. This is the book that
gave rise to comp.os.minix.

Hope this helps - but you should keep in mind that I'm a physicist not a
computer scientist !

						Cheers,
							Glenn