Question

I’ve read the article on Wikipedia on the Duff’s device, and I don’t get it. I am really interested, but I’ve read the explanation there a couple of times and I still don’t get it how the Duff’s device works.

What would a more detailed explanation be?

Answer 1

There are some good explanations elsewhere, but let me give it a try. (This is a lot easier on a whiteboard!) Here’s the Wikipedia example with some notations.

Let’s say you’re copying 20 bytes. The flow control of the program for the first pass is:

int count;                        // Set to 20 {     int n = (count + 7) / 8;      // n is now 3.  (The 'while' is going                                   //              to be run three times.)      switch (count % 8) {          // The remainder is 4 (20 modulo 8) so                                   // jump to the case 4      case 0:                       // [skipped]              do {                 // [skipped]                  *to = *from++;   // [skipped]     case 7:      *to = *from++;   // [skipped]     case 6:      *to = *from++;   // [skipped]     case 5:      *to = *from++;   // [skipped]     case 4:      *to = *from++;   // Start here.  Copy 1 byte  (total 1)     case 3:      *to = *from++;   // Copy 1 byte (total 2)     case 2:      *to = *from++;   // Copy 1 byte (total 3)     case 1:      *to = *from++;   // Copy 1 byte (total 4)            } while (--n > 0);     // N = 3 Reduce N by 1, then jump up                                   //       to the 'do' if it's still     }                             //        greater than 0 (and it is) } 

Now, start the second pass, we run just the indicated code:

int count;                        // {     int n = (count + 7) / 8;      //                                   //      switch (count % 8) {          //                                   //      case 0:                       //              do {                 // The while jumps to here.                  *to = *from++;   // Copy 1 byte (total 5)     case 7:      *to = *from++;   // Copy 1 byte (total 6)     case 6:      *to = *from++;   // Copy 1 byte (total 7)     case 5:      *to = *from++;   // Copy 1 byte (total 8)     case 4:      *to = *from++;   // Copy 1 byte (total 9)     case 3:      *to = *from++;   // Copy 1 byte (total 10)     case 2:      *to = *from++;   // Copy 1 byte (total 11)     case 1:      *to = *from++;   // Copy 1 byte (total 12)            } while (--n > 0);     // N = 2 Reduce N by 1, then jump up                                   //       to the 'do' if it's still     }                             //       greater than 0 (and it is) } 

Now, start the third pass:

int count;                        // {     int n = (count + 7) / 8;      //                                   //      switch (count % 8) {          //                                   //      case 0:                       //              do {                 // The while jumps to here.                  *to = *from++;   // Copy 1 byte (total 13)     case 7:      *to = *from++;   // Copy 1 byte (total 14)     case 6:      *to = *from++;   // Copy 1 byte (total 15)     case 5:      *to = *from++;   // Copy 1 byte (total 16)     case 4:      *to = *from++;   // Copy 1 byte (total 17)     case 3:      *to = *from++;   // Copy 1 byte (total 18)     case 2:      *to = *from++;   // Copy 1 byte (total 19)     case 1:      *to = *from++;   // Copy 1 byte (total 20)            } while (--n > 0);     // N = 1  Reduce N by 1, then jump up                                   //       to the 'do' if it's still     }                             //       greater than 0 (and it's not, so bail) }                                 // continue here... 

20 bytes are now copied.

Note: The original Duff’s Device (shown above) copied to an I/O device at the to address. Thus, it wasn’t necessary to increment the pointer *to. When copying between two memory buffers you’d need to use *to++.