TABLE OF CONTENTS The most commonly used scheme for accelerating carry propagation is the carry- look-ahead scheme. The main idea behind carry-look-ahead addition is an attempt to generate all incoming carries in parallel (for all the n ?1 high order FAs) and avoid the need to wait until the correct carry propagates from the stage (FA) of the adder where it has been generated. This can be accomplished in principle, since the carries generated and the way they propagate depend only on the digits of the original numbers x n ?1 x n ? 2 x 0 and y n ?1 y n ?2 y 0. These digits are available simultaneously to all stages of the adder and, consequently, each stage can have all the information it needs in order to calculate the correct value of the incoming carry and compute the sum bit accordingly. This, however, would require an inordinately large number of inputs to each stage of the adder, rendering this approach impractical.
One may reduce the number of inputs at each stage by extracting the information needed from the input digits to determine whether new carries will be generated and whether they will be propagated. To this end, we will study in detail the generation and propagation of carries.
There are stages in the adder in which a carry-out is generated regardless of the incoming carry, and as a result, no additional information on previous input digits...
