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

Here's a non-detailed explanation which is what I feel to be the crux of Duff's device:

The thing is, C is basically a nice facade for assembly language (PDP-7 assembly to be specific; if you studied that you would see how striking the similarities are). And, in assembly language, you don't really have loops - you have labels and conditional-branch instructions. So the loop is just a part of the overall sequence of instructions with a label and a branch somewhere:

        instruction
label1: instruction
        instruction
        instruction
        instruction
        jump to label1  some condition

and a switch instruction is branching/jumping ahead somewhat:

        evaluate expression into register r
        compare r with first case value
        branch to first case label if equal
        compare r with second case value
        branch to second case label if equal
        etc....
first_case_label: 
        instruction
        instruction
second_case_label: 
        instruction
        instruction
        etc...

In assembly it's easily conceivable how to combine these two control structures, and when you think of it that way, their combination in C doesn't seem so weird anymore.