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Data Path design: Carry Save Adder

Carry Save Adder (CSA) is useful when more than two operands are to be added. It is based on the idea that full-adder has three inputs and produces two outputs. Third input i.e. carry-in is used as the “regular” value. Therefore, the idea is to take 3 numbers that we want to add together, x + y + z, and convert it into 2 numbers carry (c) + Sum (s) such that x + y + z = c + s. Carry and Sum for the respective bits of the given numbers are computed independently and parallel using full adders of CSA and the delay can be reduced further in CSA. Ripple carry adder or carry look ahead adder is used in the final stage for the final sum and carry word addition. The name “carry save” arises from the fact that we save the carry-out word instead of using it immediately to calculate the final sum. CSAs are used in the calculation of addition of partial products in integer multiplication,