To clarify, it does not need to be executed cycle exact on the IBM PC, only on the C64.

I was going to say, smooth scrolling is one place where VGA was fairly simple to work with, since it didn’t have to deal with chunky graphics/bitplanes or specific timing tricks.

In fact, Carmack specifically states that packed pixels were the impetus for id’s side scrolling experiments that led to Commander Keen and its ilk.