Multilevel reverse-carry adder

Abstract

The multilevel reverse-carry approach has been proposed previously for fast computation of the most-significant carry of an adder. We extend this approach to generate several carries and apply it to the implementation of the complete adder. Specifically, the operands are split into blocks and each block is added to produce the sum and the sum plus one. Concurrently with these additions the multilevel reverse-carry approach is used to generate the input carries of these blocks. Finally, these carries are used to select among the sum and the sum plus one. We have evaluated the resulting architecture for a 64-bit adder, considering the load introduced by long connections, and we estimate a reduction of about 15% in the critical path delay with respect to traditional implementations of prefix-tree based adders.