The sum and the carry may be fed into two inputs of the subsequent 3-number adder without having to wait for propagation of a carry signal. Instead, three-input adders are used, generating two results: a sum and a carry. If an adding circuit is to compute the sum of three or more numbers, it can be advantageous to not propagate the carry result. Other adder designs include the carry-select adder, conditional sum adder, carry-skip adder, and carry-complete adder. For example, the following adder is a 64-bit adder that uses four 16-bit CLAs with two levels of lookahead carry units. This can be used at multiple levels to make even larger adders. The half adder adds two single binary digits A values for each block rather than each bit, and the carry-select adder which pre-generates the sum and carry values for either possible carry input (0 or 1) to the block, using multiplexers to select the appropriate result when the carry bit is known.īy combining multiple carry-lookahead adders, even larger adders can be created.
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