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From: john@datacube.UUCP
Newsgroups: net.arch
Subject: Re: Virtual memory and stuff
Message-ID: <3200003@datacube.UUCP>
Date: Thu, 7-Nov-85 23:32:00 EST
Article-I.D.: datacube.3200003
Posted: Thu Nov  7 23:32:00 1985
Date-Received: Wed, 20-Nov-85 01:06:09 EST
References: <232@polaris.UUCP>
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Nf-ID: #R:polaris:-23200:datacube:3200003:000:2078
Nf-From: datacube!john    Nov  7 23:32:00 1985


    Vertual memory or something like it will always be necessary.
Vertual memory is just a particular solution to the memory hierarchy
problem at a certian level. Memory hierarchys will always be with
us, why? Because for any given memory technology there always will
be a faster more expensive one, and a slower less expensive one. This
is despite the fact that rotating magnetic storage may become obsolete
because semiconductor memories become less expensive per bit and are 
already known to be faster.
    Vertual memory solves the memory hierarchy problem at the level
where we wish to have more in "core" for what ever reason, than we
have core. This will always be true, because of one of the fundamental
laws of computing is that any application or system will expand to 
use all the availabel resources and then demand some more. Imagine
a 1K X 1K X1k array representing a cube of 3D image data, at low 
resolution I might add. Suppose I wish to add a temporal dimension to
this, now prehaps 1K X 1K X 1K X1K in core would be adequate but I 
seriously doubt it. 
    Vertual memory is just a way of dealing with the core to disk
transition in the memory hiearchy occuring at an inconvienent place.
Systems can be built that move this transition to a more convienent 
place, for manny applications. However there always will be
applications where the active code size is greater than the "core"
size and so therefore vertual memory implemented by the operating
system and hardware, or VM implemented by the application in the 
form of paging will be necessary.
    Another thing to consider is the next higher break in the 
memory hierarchy, that between cache and "main memory". Fundamentaly
it is very similar to that between main memory and disk, but nobody
complains about it because cache is seen as an improvement, and 
because the bandwidth mismatch is much less than that between core
and disk.
    The names of the levels of the hierarchy will change but the 
functions will not.

				John Bloomfield

these are opinions and may not be mine or any one else's