Duff's Device - Original Version

Original Version

Straightforward code to copy items from an array to a memory-mapped output register might look like this:

do { /* count > 0 assumed */ *to = *from++; /* Note that the 'to' pointer is NOT incremented */ } while(--count > 0);

Note that this is not a memory-to-memory copy, in which you would see *to++.

While optimizing this, Duff realized that an unrolled version of his loop could be implemented by interlacing the structures of a switch and a loop.

send(to, from, count) register short *to, *from; register count; { register n = (count + 7) / 8; switch(count % 8) { case 0: do { *to = *from++; case 7: *to = *from++; case 6: *to = *from++; case 5: *to = *from++; case 4: *to = *from++; case 3: *to = *from++; case 2: *to = *from++; case 1: *to = *from++; } while(--n > 0); } }

Notice that Duff's device can just as easily be applied with any other size for the unrolled loop, not just 8.