Re: backward architecture, The Design of Design

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?