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A block diagram of a 4-bit carry lookahead adder. For each bit in a binary sequence to be added, the carry-lookahead logic will determine whether that bit pair will generate a carry or propagate a carry. This allows the circuit to "pre-process" the two numbers being added to determine the carry ahead of time.
An example of a 4-bit Kogge–Stone adder is shown in the diagram. Each vertical stage produces a "propagate" and a "generate" bit, as shown. The culminating generate bits (the carries) are produced in the last stage (vertically), and these bits are XOR'd with the initial propagate after the input (the red boxes) to produce the sum bits. E.g., the first (least-significant) sum bit is ...
A lookahead carry unit (LCU) is a logical unit in digital circuit design used to decrease calculation time in adder units and used in conjunction with carry look-ahead adders (CLAs). 4-bit adder [ edit ]
If an adding circuit is to compute the sum of three or more numbers, it can be advantageous to not propagate the carry result. Instead, three-input adders are used, generating two results: a sum and a carry. 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.
The Brent–Kung adder is a parallel prefix adder (PPA) form of carry-lookahead adder (CLA). Proposed by Richard Peirce Brent and Hsiang Te Kung in 1982 it introduced higher regularity to the adder structure and has less wiring congestion leading to better performance and less necessary chip area to implement compared to the Kogge–Stone adder (KSA).
A ripple carry adder is a simple adder circuit, but slow because the carry signal has to propagate through each stage of the adder: This diagram shows a 5-bit ripple carry adder in action. There is a five-stage long carry path, so every time two numbers are added with this adder, it needs to wait for the carry to propagate through all five stages.
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A 16-bit carry-select adder with a uniform block size of 4 can be created with three of these blocks and a 4-bit ripple-carry adder. Since carry-in is known at the beginning of computation, a carry select block is not needed for the first four bits. The delay of this adder will be four full adder delays, plus three MUX delays.