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Carry save adder.
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Carry save adder is based on the idea that a full adder really has three inputs and produces two outputs. While we usually associate the third input with a carry-in. The full adder can be used as a 3-2 reduction network where it starts with bits from 3 words, adds them, and then has an output that is 2 bits wide. We can build an n-bit carry-save adder by using n separate adders .The name ‘carry save’ arises from the fact that it saves the carry-out word instead of using it immediately to calculate a final sum.

Carry save adders are useful in situations where we need to add more than two numbers. Carry save adders found their application in building larger multipliers in which there are trees of carry save adders (Wallace tree) which are used to add the generated partial product terms faster and in an efficient way. Since the design automatically avoids the delay in carry-out bits, a CSA chain may be faster than using standard adders. An example shown above consists 7-2 reduction scheme, this starts with 7 n-bit words a, b,…..,g and uses five carry save adder units to reduce it down to two words. If we want a final sum, then a normal CPA(carry propagate adder)can be used at the bottom of the chain to add the two values together.

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