register n = (count + 7) / 8; /* count > 0 assumed */ switch (count % 8) { case 0: do { *to = *from++;} { *to = *from++; case 7: *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); } Shocking though it appears to all who encounter it for the first time, the device is actually perfectly valid, legal C. C's default {fall through} in case statements has long been its most controversial single feature; Duff observed that "This code forms some sort of argument in that debate, but I'm not sure whether it's for or against." Duff has discussed the device in detail at `http://www.lysator.liu.se/c/duffs-device.html'. Note that the omission of postfix `++' from `*to' was intentional (though confusing). Duff's device can be used to implement memory copy, but the original aim was to copy values serially into a magic IO register.
[For maximal obscurity, the outermost pair of braces above could actually be removed -- GLS]