There was hardware (not sure if it was CPU state or state in the memory controller) to zero-out all of the address bits above the 24th, allowing backward compatibility. I forget the shortcut (option-I? option-M?), but if you selected an executable in Finder, you could go in and change the hardware compatibility mode used to run it.
That's interesting. I don't think the address hack was widely used on the Amiga, so we didn't get anything like that. Amiga Basic was the main one and that was ditched for Arexx for the newer OS versions (a shame, because as much as I liked to mock Microsoft at the time, Amiga Basic was quite decent). I don't think the incompatibility had anything to do with ditching it - people have patched it and it wasn't a big deal; I'm assuming it didn't get an official patch because Commodore had already decided to replace it.
