How does Duff's device work?

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 do

Answer 1

The explanation in Dr. Dobb's Journal is the best that I found on the topic.

This being my AHA moment:

for (i = 0; i < len; ++i) {
    HAL_IO_PORT = *pSource++;
}

becomes:

int n = len / 8;
for (i = 0; i < n; ++i) {
    HAL_IO_PORT = *pSource++;
    HAL_IO_PORT = *pSource++;
    HAL_IO_PORT = *pSource++;
    HAL_IO_PORT = *pSource++;
    HAL_IO_PORT = *pSource++;
    HAL_IO_PORT = *pSource++;
    HAL_IO_PORT = *pSource++;
    HAL_IO_PORT = *pSource++;
}

n = len % 8;
for (i = 0; i < n; ++i) {
    HAL_IO_PORT = *pSource++;
}

becomes:

int n = (len + 8 - 1) / 8;
switch (len % 8) {
    case 0: do { HAL_IO_PORT = *pSource++;
    case 7: HAL_IO_PORT = *pSource++;
    case 6: HAL_IO_PORT = *pSource++;
    case 5: HAL_IO_PORT = *pSource++;
    case 4: HAL_IO_PORT = *pSource++;
    case 3: HAL_IO_PORT = *pSource++;
    case 2: HAL_IO_PORT = *pSource++;
    case 1: HAL_IO_PORT = *pSource++;
               } while (--n > 0);
}