A Spanning Tree Carry Lookahead Adder

  • Authors:
  • Thomas Lynch;Earl E. Swartzlander, Jr.

  • Affiliations:
  • Univ. of Texas at Austin, Austin;Univ. of Texas at Austin, Austin

  • Venue:
  • IEEE Transactions on Computers - Special issue on computer arithmetic
  • Year:
  • 1992

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Abstract

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.