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From: gnu@sun.uucp (John Gilmore)
Newsgroups: net.micro.16k,net.micro.68k
Subject: Re: Is 24 Bits Enough?
Message-ID: <2010@sun.uucp>
Date: Mon, 25-Feb-85 21:23:47 EST
Article-I.D.: sun.2010
Posted: Mon Feb 25 21:23:47 1985
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Firstly I should point out that what we are really talking about is 24
or 32 bits of VIRTUAL address space, not physical.  You can have as few
or as many bits on the other side of the MMU as you like; what comes out
of the (320xx or 680xx) CPU chip is a virtual address.

Now, Chuqui sez:
>                                                We DO have 32bit addressing
> silicon on the boards and on its way to reality, and I expect it will be a
> purchasable commodity long before the megabit chips that will be neccessary
> to make those kind of address spaces truly useful.
AT&T is advertising that they will have production 1MB rams for sale
before the end of 1985.  (Of course, they haven't mentioned price,
pinouts, or anything else...and have never sold chips on the market
before.)
						     32bit addressing makes a
> nice marketing tool, granted, but there really isn't much that a 32bit
> address gives you that a 24bit address doesn't also give you in a
> manufactured product EXCEPT a marketing tool. 

There are a few kinds of applications that really need more than 16
megabytes of address space.  Large, highly correlated databases are the
obvious example, e.g. CAD simulation, linear programming, heavy image
processing.  A friend who consults to Intel reports that their
production scheduling system has hit the limits of the IBM 370's 16-meg
address space -- mostly due to misdesign of the software -- and
requires constant tweaking, or a complete redesign, to make it run.  I
was hearing similar complaints (I don't remember who from) about large
electronic simulations 3 to 4 years ago.

This is not to say that a 16MB virtual space is very limiting.  It's
not.  Our systems provide the full 16MB per process on the 68010, and
nobody uses it.  (They don't want to allocate that much swap space
anyway!)  It's just that when you need it, you really need it, since
recoding to deal with the limit will take months to years and/or slow
down the (already slow, since it's dealing with 16MB of data) program
by a good factor.  The people who need that capability soon will buy
Motorola.  National can certainly afford to ignore that small segment
of the market for the next year or two.

In other words, Chuqui is right...