Originally Posted by rickholio

No, it's not. All you have to do is follow the logic. If it were an adder, S1 would have to be affected by inputs on A2/B2, and it quite clearly is not.

I don't know what that is but it sure as hell isn't an adder.

Run through the states (plug in 0000 through 1111 on the A1/2 B1/2 inputs) and you'll see what I mean.