Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!pasteur!helios.ee.lbl.gov!nosc!logicon.arpa!Makey
From: Makey@LOGICON.ARPA (Jeff Makey)
Newsgroups: comp.society.futures
Subject: Re: Memories...
Message-ID: <254@logicon.arpa>
Date: 21 Dec 88 05:54:04 GMT
References: <8812200325.AA12574@multimax.encore.com>
Organization: Future Procrastinators of America
Lines: 53

In article <8812200325.AA12574@multimax.encore.com> bzs@ENCORE.COM (Barry Shein) writes:
>The address space on most CPUs today is 32 bits (hmm, for arguable
>values of "most", well, very common anyhow.) In fact, it's usually a
>little less, 31 bits is often all you can use. That's 2GB or two or
>four of those boards, not a whole lot. Then bango, you're full up,
>can't use anymore physical memory.

Be careful not to confuse virtual addresses with physical addresses.
For example, the VAX architecture gives you the use of 1.5 * 2**31
bytes of virtual space, but only 2**30 bytes of physical memory.

>The remaining question is, 48 bits or 64 bits for the next
>generation?

Virtual addresses:

Given that very few computer programs (I said, "few."  I don't want to
hear about *your* monster program!) are capable of making good use of
an entire 32-bit virtual address space, I suspect that 32-bit
addresses are a good tradeoff between addressing capability and
program size (change virtual addresses to 64 bits and *every* pointer
in your program doubles in length).  Certain applications (generally,
scientific ones I would imagine) will want larger virtual addresses;
that's what supercomputers are for.

Physical addresses:

It would be nice to have an architecture that would support really
large amounts (48 to 64 bits worth) of physical memory, but -- once
again -- most applications won't need them so I would expect different
hardware configurations of the same architecture to support different
amounts of physical memory, with the larger capacities costing more.

One practical limit to physical memory size is the time required to
transfer data to/from mass storage (e.g., disk).  Demand paging helps
keep this under control, but you're going to be annoyed when it takes
minutes (!) to write your gigabytes of memory out to disk at the end
of the day (battery backup of memory should be standard to avoid
problems with powerfailures).  Also (and I hope this is obvious!),
more memory than mass storage space is a questionable configuration,
at best.  Will mass storage tecnology be the computing bottleneck of
the future?

To summarize, I feel that 32-bit demand-paged virtual addresses are a
central part of an architecture that will satisfy at least 95% of all
applications for the next 10-15 years.  Systems that run lots of *big*
programs will need to be able to map these 32-bit virtual addresses
into maybe 48 bits worth of physical memory.

                           :: Jeff Makey

Department of Tautological Pleonasms and Superfluous Redundancies Department
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