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.
Read more about this topic: Duff's Device
Famous quotes containing the words original and/or version:
““Mother” has always been a generic term synonymous with love, devotion, and sacrifice. There’s always been something mystical and reverent about them. They’re the Walter Cronkites of the human race . . . infallible, virtuous, without flaws and conceived without original sin, with no room for ambivalence.”
—Erma Bombeck (20th century)
“It is never the thing but the version of the thing:
The fragrance of the woman not her self,
Her self in her manner not the solid block,
The day in its color not perpending time,
Time in its weather, our most sovereign lord,
The weather in words and words in sounds of sound.”
—Wallace Stevens (1879–1955)