I recreated and trying to understand for the sake of fun and learning the circuit that can be seen here on second page: Link . This is basically a 4-bit carry-lookahead adder, but there is one thing that I cannot understand. If there will be 1111 and carry bit will be 1, it looks like there is no way to pass a carry to the next unit. Turning carry bit on and off is irrelevant for the output (I mean here output information about carry). Maybe I'm missing something/recreated it wrong or some logic is in the "carry output" unit, anyone could explain it, and show how should connections between that kind of adders should look like? :)
You can download this circuit recreated in CEDAR Logic Simulator Here .