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From: "Stephen Fuld" <SFuld@alumni.cmu.edu.invalid>
Newsgroups: comp.arch
Subject: Re: backward architecture, The Design of Design
Date: Thu, 9 May 2024 13:10:42 -0000 (UTC)
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Michael S wrote:

> On Thu, 9 May 2024 08:19:39 -0000 (UTC)
> Thomas Koenig <tkoenig@netcologne.de> wrote:
> 
> > Michael S <already5chosen@yahoo.com> schrieb:
> > 
> > > Can you, please, define the meaning of upward and downward
> > > compatibility? I had never seen this terms before this thread, so
> > > it is possible that I don't understand the meaning.  
> > 
> > The term comes from Brooks.  Specifically, he applied it to the
> > S/360 line of computers which had a very wide performance and
> > price range, and programs (including operating systems) were
> > binary compatible from the lowest to the highest performance and
> > price machine.
> 
> 
> I suppose, it means that my old home PC (Core-i5 3550) is downward
> compatible with my old work PC (Core-i7 3770). And my old work PC is
> upward compatible with my old home PC.
> 
> But I still don't know if it would be correct to say that my old work
> PC is downward compatible with with my just a little newer small FOGA
> development server (E3 1271 v3). My guess that it would be incorrect,
> but it's just guess.
> 
> If Brook was still alive, we could have tried to ask him. But since he
> is not, and since I have no plans to read his books by myself, my only
> chance of knowing is for you or for John Levine to find  the
> definition it in his writings and then tell me. 

Perhaps this interpretation will help clear things up.  Think of
compatibility as a two dimensional graph.  On the Y axis is some
measure of compute power.  The X  axis is time.  So upward/downward
compatibility is among models announced at the same time and delivered
within a small time of each other.  Backward compatibility is along the
X axis, that is, between models announced/delivered at a different
points in time.  So under this scheme, the S/360 model 30 was upward
compatible with the model /65 ( different Y values, but the same x
values) , but the S370s (not counting the /155 and /165) were backward
compatible with the S/260 models (different x values)

The key innovation that IBM made with the S/360 was to announce systems
with a wide range of performance *at the same time*, i.e. different Y
values and the same X value.





-- 
 - Stephen Fuld 
(e-mail address disguised to prevent spam)