Earlier quoted context omitted.
Fun fact for C64 guys: The underlying mechanics of Carmack's technique is very similar to the full screen smooth scrolling effect on C64 at any speed and distance. It is nowadays referred to as DMA delay. ELIF: You trick the CPU to display screen data at a different starting point than as designed by the hardware. This is tricky and need to be executed cycle exact. Here is the explanation in detail together with all…
To clarify, it does not need to be executed cycle exact on the IBM PC, only on the C64.
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.