Carry Look-Ahead Adder

From the Ripple Carry Adder, we saw that . Thus we write:

Note that:

• and are generated after 1 gate delay
• Each can be implemented using a 2-level SOP using , , and . Hence all can be computed with 2 more gate delays.
• Since , one more gate delay is incurred to produce all

Unfortunately, for a large number of bits, carry look-ahead adders become too expensive, but they are faster than ripple adders.

Hierarchical Carry Look-Ahead Adder

A hierarchical carry look-ahead adder uses a number of smaller carry look-ahead adders in a cascaded fashion to build a larger adder. It's much more practical, combining the Ripple Carry Adder with the carry look-ahead adder.