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A carry-skip adder [nb 1] also known as a carry-bypass adder is an adder implementation that improves on the delay of a ripple-carry adder with little effort compared to other adders. The improvement of the worst-case delay is achieved by using several carry-skip adders to form a block-carry-skip adder.

The n -bit-carry-skip adder consists of a n -bit-carry-ripple-chain, a n -input AND-gate and one multiplexer. This greatly reduces the latency of the adder through its critical path, since the carry bit for each block can now "skip" over blocks with a group propagate signal set to logic 1 as opposed to a long ripple-carry chain, which would require the carry to ripple through each bit in the adder.

The number of inputs of the AND-gate is equal to the width of the adder. For a large width, this becomes impractical and leads to additional delays, because the AND-gate has to be built as a tree. A good width is achieved, when the sum-logic has the same depth like the n -input AND-gate and the multiplexer.

As the propagate signals are computed in parallel and are early available, the critical path for the skip logic in a carry-skip adder consists only of the delay imposed by the multiplexer conditional skip. Block-carry-skip adders are composed of a number of carry-skip adders. The critical path consists of the ripple path and the skip element of the first block, the skip paths that are enclosed between the first and the last block, and finally the ripple-path of the last block. The performance can be improved, i.

Accordingly the initial blocks of the adder are made smaller so as to quickly detect carry generates that must be propagated the furthers, the middle blocks are made larger because they are not the problem case, and then the most significant blocks are again made smaller so that the late arriving carry inputs can be processed quickly. By using additional skip-blocks in an additional layer, the block-propagate signals p [ i: The problem of determining the block sizes and number of levels required to make the physically fastest carry skip adder is known as the 'carry-skip adder optimization problem'.

This problem is made complex by the fact that a carry-skip adders are implemented with physical devices whose size and other parameters also affects addition time.

The carry-skip optimization problem for variable block sizes and multiple levels for an arbitrary device process node was solved by Thomas W. Breaking this down into more specific terms, in order to build a 4-bit carry-bypass adder, 6 full adders would be needed.

The first two full adders would add the first two bits together. From Wikipedia, the free encyclopedia. Algorithms and Hardware Designs. Archived PDF from the original on Proceedings 8th Symposium on Computer Arithmetic.

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