Fast adder chain

An adder chain for adding together at least first, second, and third digital words, each digital word having n bits, the adder chain comprising:

• an initial block including:
• n/2 two-bit adders, each two-bit adder receiving a two-bit portion of the first digital word and a two-bit portion of the second digital word for computing pseudosums and a pseudocarry for the received portions of the first and second digital words, and
• storing means, coupled to the two-bit adders, for storing the pseudosums and pseudocarries, and for transmitting the pseudosums and the pseudocarries to a subsequent block of more significant order, wherein the storing means includes two sets of storage elements cascade connected to one another and coupled to the adders, one of the sets being coupled to a control signal for selective overriding;
• and
• at least one i-th adder block arranged in an order from a least significant block coupled to the initial block to a most significant block, where i is a natural number greater than 1, for receiving pseudosums and pseudocarries from a (i-1)th block and for receiving an (i+2)th digital word, for computing pseudosums and pseudocarries from the received pseudosums and pseudocarries and the received (i+2)th digital word;
• and
• a final block, receiving pseudosums and pseudocarries from the most significant adder block, including a plurality of adders and storage elements for selectively combining and storing the received pseudosums and pseudocarries and for providing an output word.