How Duff’s Device Works

I like C, but I have to admit that, sometimes, “The Old Man of Programming” can be a bit of a killjoy. This is one of the most exciting eras in computer history, but lately, C’s acting like he doesn’t even want to have a good time. While the cool kids like Ruby and Haskell are living it up, C’s over in the corner obsessing over bits and bytes and memory alignment and pointers and the stack and machine architecture and unreachable allocations and multiple indirections…and it’s…kind of a buzzkill. You want to tell him, “Dude! Lighten up! This is supposed to be fun!”

But of course C can’t relax. He’s holding the entire computing universe on his shoulders, after all. It’s not turtles all the way down — it’s turtles all the way down, then C. Not a lot of room for fun underneath that, is there?

Is there?

Well. Here’s the secret: C does let loose sometimes. And after being bottled up for so long, when the release finally does come, it’s pretty much an F5 of crazy.

Take for example a little thing called Duff’s Device:

send(short *to, short *from, int count)
{

int 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);

}

}
Okay, what you have here is your basic switch statement merged with a do loop. Pretty messed up, right? And just to be clear: this is totally valid C.

Huh?

If we look at the Jargon File entry, we see that the purpose of Duff’s Device is to continuously copy data into a memory-mapped video register.

Read more: Tenacious C

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