Yeah, the entire MS business depends on backward compatibility. Check Raymond Chen's book 'The Old New Thing' for really fun stories of the great lengths MS developers go to maintain such compatibility.
Also Check the mentioned book 'extras' online chapters on the publisher's page, where the author maintains backward compatibility for Win3.1 games in his youth days. At the time where hardware protection wasn't used by Windows and the code Windows was trying to be compatible to was doing amazingly silly stuff with the x86 processors and PC architecture nuances (GDT, IDT, PIT, etcetera).
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