The 20-bit carry-select adder is a modified version of that described in [13]. It consists of 5 stages of 2, 3, 4, 5 and 6 bits (lsb to msb) respectively. Each stage contains two n-bit carry-propagate adders and a 2-input, n + 1 bit multiplexer. In this configuration, the carry propagation time through the stages matches the
propagation time through the bits of the most significant stage.