The Wikipedia version is simplified, but it bears mentioning that as written, it's going to be slower than the trivial version for most N's on 1983 compilers and systems. Fixed:

   register n = (count + 7) >> 3;
   switch (count - (n << 3))

I love Duff's Device. I used it to great effect back in the day, and still use it from time to time in my personal projects, just for old time's sake,