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From: jgd@cix.co.uk (John Dallman)
Newsgroups: comp.arch
Subject: Re: backward architecture, The Design of Design
Date: Thu, 9 May 2024 20:28 +0100 (BST)
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In article <20240509195234.000000c5@yahoo.com>, already5chosen@yahoo.com
(Michael S) wrote:

> 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?

There's one-way compatibility, from the subset machine to the superset
machine. That might be upwards or downwards, depending on the vagaries of
marketing and their control of features. 

One may, of course, also restrict oneself to the common subset. Some
compilers make that easier than others. 

John