A Fast VLSI Adder Architecture

In this paper we present an architecture for performing fixed-point, high-speed, two’s-complement, bit-parallel addition by using the carry-free property of redundant arithmetic and a fast parallel redundant-to-binary conversion scheme. In this architecture, the internal numbers are represented in radix-2 redundant digit form and the inputs and the output of the adder are represented in two’s-complement binary form. The adder operands are added first in a radix-2 redundant adder to produce the result in radix-2 digit (-1, 0, 1) form. This result is then converted to two’s-complement binary form by using the proposed fast parallel conversion scheme. The high-speed conversion for long words is achieved through the use of a novel sign-select operation. This proposed adder is referred to as the sign-select conversion adder and is faster than all previous high-speed two’s-complement binary adders for large word lengths (although it requires larger area). For word lengths between 16 and 64, the proposed design is 20-28% faster than the fastest known binary lookahead adder. The implementation is highly regular with repeated modules and is very well suited for VLSI implementation.