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From: firth@sei.cmu.edu (Robert Firth)
Newsgroups: comp.arch
Subject: Re: Segmented Architectures ( formerly Re: 48-bit computers)
Message-ID: <24004@as0c.sei.cmu.edu>
Date: 12 Apr 91 17:45:47 GMT
References: <1991Apr04.023845.3501@kithrup.COM> <1991Apr6.211320.18594@athena.mit.edu> <7YKAMBE@xds13.ferranti.com> <1991Apr12.021609.5340@athena.mit.edu>
Reply-To: firth@sei.cmu.edu (Robert Firth)
Organization: Software Engineering Institute, Pittsburgh, PA
Lines: 30

In article <1991Apr12.021609.5340@athena.mit.edu> jfc@athena.mit.edu (John F Carr) writes:
>I don't think the tradeoffs between segments and a flat address space are
>the same now for >32 bit machines than they were for >16 bit machines.
>
>In the past decade, memory cost has dropped by about 2^8.  The 32 bit
>address space that some find too small costs 2^8 times as much to fill as
>the 16 bit address space did 12 years ago.

Your figures are approximate, but let's take them as a starting point.
As I recall (having lived through it) by about 1975 we were bashing
into the 16-bit limit often enough to leave major bruises.  At least,
that's when the place I worked for started looking seriously at
segmented machines, flat 20-bit machines, software tricks with
separate I and D spaces, and so on.  That's the vintage of the
Interdata 7/32 and the Algol-68C compiler with separate I and D
segments.

So, if memory cost drops at 2^8 per decade, it will be as cheap
to fill 32 bits in 1995 as it was to fill 16 bits in 1975.  Now,
cheapness isn't everything, but the figure does suggest that, by
1995, we'll be hitting the 32-bit limit in the same way that we
were hitting the 16-bit limit in 1975.

So, if you are designing a machine today (April 1991), to be
shipped in, say, 1994Q1 - not an unreasonable lead time - then,
if you hardwire a 32-bit object limit, your machine will be
constraining an appreciable fraction of potential users within
18 months of first release.

Not, one feels, a prudent business strategy.