Rechnerorganisation und Rechnerentwurf, 5th Edition

These equations just represent common sense: CarryIni is a 1 if some earlier adder generates a carry and all intermediary adders propagate a carry. Figure B.6.1 uses plumbing to try to explain carry lookahead.

Even this simplified form leads to large equations and, hence, considerable logic even for a 16-bit adder. Let’s try moving to two levels of abstraction.

Fast Carry Using the Second Level of Abstraction

First, we consider this 4-bit adder with its carry-lookahead logic as a single building block. If we connect them in ripple carry fashion to form a 16-bit adder, the add will be faster than the original with a little more hardware.

To go ...