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A Spanning Tree Carry Lookahead Adder
August 1992 (vol. 41 no. 8)
pp. 931-939

The design of the 56-b significant adder used in the Advanced Micro Devices Am29050 microprocessor is described. Originally implemented in a 1- mu m design role CMOS process, it evaluates 56-b sums in well under 4 ns. The adder employs a novel method for combining carries which does not require the back propagation associated with carry lookahead, and is not limited to radix-2 trees, as is the binary lookahead carry tree of R.P. Brent and H.T. Kung (1982). The adder also utilizes a hybrid carry lookahead-carry select structure which reduces the number of carriers that need to be derived in the carry lookahead tree. This approach produces a circuit well suited for CMOS implementation because of its balanced load distribution and regular layout.

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Index Terms:
spanning tree carry lookahead adder; significant adder; Advanced Micro Devices; Am29050 microprocessor; hybrid carry lookahead-carry select; carry lookahead tree; CMOS; load distribution; layout; 56 bit; adders; carry logic; CMOS integrated circuits; integrated logic circuits; trees (mathematics).
T. Lynch, E.E. Swartzlander, Jr., "A Spanning Tree Carry Lookahead Adder," IEEE Transactions on Computers, vol. 41, no. 8, pp. 931-939, Aug. 1992, doi:10.1109/12.156535
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