In looking at some discussions in the iDevices areas of MRF, I periodically see people treating UI features as OS capabilities. This kind of perspective seems a little off to me. I have always thought of the OS as the primary things that abstract the hardware to the object code and UI features as sort of like API extensions (like the boundary between Foundation and AppKit). Obviously, it is a little fuzzy: some graphical features amount to hardware abstraction, and I think there are a few things in Foundation that could be seen as mostly UI support. Mainly, I perceive "OS" as being an extensible base upon which processes are laid, and the UI capabilities should generally not be seen as strictly OS. Granted, mobile devices tend to have less UI flexibility, so treating UI elements as OS might make more sense. As far as Apple is concerned, it is sort of a strange environment where everything is so very modular, but the updates arrive in large, indivisible chunks, and pulling the UI apart from the OS gies against the paradigm of the consistent user experience: it almost seems as though the beauty of modularity goes to waste here. Any thoughts? What is the definition of "OS"?