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From: Michael S <already5chosen@yahoo.com>
Newsgroups: comp.arch
Subject: Re: backward architecture, The Design of Design
Date: Thu, 9 May 2024 19:52:34 +0300
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On Thu, 9 May 2024 13:10:42 -0000 (UTC)
"Stephen Fuld" <SFuld@alumni.cmu.edu.invalid> wrote:

> 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.
> 
> 
> 
> 
> 

So, when two models are pretty close on time scale, but from the
software perspective one of them is a superset of the other then they
are not upward/downward compatible?