Carry-select

Another solution: carry-select adder

Design trick: When all else fails, GUESS! (precompute)

To build 8-bit adder:

Lower 4 bits: any adder (ripple-carry, carry-lookahead)

Upper 4 bits: 2 adders

First adder has carry-in of 1

Second adder has carry-in of 0

Select between 2 upper results based on carry-out from lower result

Reference:

http://www-inst.eecs.berkeley.edu/~cs152/

Time required:

4-bit add time

Multiplexor to select