Adders:Carry Save Adder

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Carry Save Adder

When we add up several numbers sequentially, it is not necessary to propagate the carries during each addition. Instead, the carries generated during an addition may be saved as partial carries and added with the next operand during the next addition. Namely, we can accelerate each addition by postponing the carry propagation. This leads to the concept of carry save addition. We may add up numbers by a series of carry save additions, followed by a carry propagate addition. Namely, for multiple-operand addition, only one carry propagate addition is required.

An adder for carry save addition is referred to as a carry save adder, while the adders mentioned in the previous section are called carry propagate adders. A carry save adder sums up a partial sum and a partial carry from the previous stage as well as an operand and produces a new partial sum and partial carry. An n-bit carry save adder consists of just n full adders without interconnections among them.

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