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From: mash@mips.COM (John Mashey)
Newsgroups: comp.arch
Subject: Re: 68xxx memory addressing
Keywords: tag 68000
Message-ID: <45000@mips.mips.COM>
Date: 17 Jan 91 23:59:25 GMT
References: <1991Jan16.210201.7962@nstn.ns.ca>
Sender: news@mips.COM
Reply-To: mash@mips.COM (John Mashey)
Distribution: comp
Organization: MIPS Computer Systems, Inc.
Lines: 40

In article <1991Jan16.210201.7962@nstn.ns.ca> maceache@fox.nstn.ns.ca (Tim Maceachern) writes:
>This is an idea I had a while ago.  I believe that most applications do
>not need a 32 bit address space, and most computers won't have that much
>real RAM anyway.  Of course there will be those who want to run huge
>applications in a 32 bit virtual memory, but...
>Why not have a tagged jump instruction in the 68000 set.  It would take
>the top 4 bits of the address register (or memory value) and use it as 
....

I don't have any idea how much the specific feature would be worth,
but I would raise doubts about the first part of this, at least over
time.

Here's a quiz: Q1: during what year would you expect somebody to build & ship
microprocessor-based systems with 4GB-maximum of physical memory?
(And without being lunatic-fringe types)

A: looks like 1993, +/- one year.

If you graph MIPS & Sun servers (and some other people sometimes build
even bigger memories), on a chart with linear-years on hortizontal
access, and log-2 in MB on vertical, you see that this is basically
a straight line, increasing by factor of 4 every 3 years.
This is hardly astonishing, as the stand rule-of-thumb says
DRAMs get 4X bigger every 3 years....

Of course, 4GB == 32-bits, although various traditional hacks are certainly
possible to extend this somewhat, although pain mounts quickly.

Here's another proposed rule of thumb: if you have X bytes of physical
memory, important applications will exist that need 4X that amount
of virtual memory.  Hennessy claims I'm low, that increasing use of
file-mapping and object-oriented programming will tend to increase
the sparseness of use of virtual memory, but lots of other people
agree with the 4X, right now.  (COMMENTS? DATA?)
-- 
-john mashey	DISCLAIMER: <generic disclaimer, I speak for me only, etc>
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