One solution: Carry lookahead logic

• Basic idea: Compute all the carries in parallel

  • Derive carries from the data inputs
    • Not from intermediate carries
  • Use combinational two-level logic
  • Compute all sum outputs in parallel
  • Cascade simple (4-bit) adders to make large adders

• Benefits

  • 16-bit ripple-carry adder: ~32 gate delays
  • 16-bit carry-lookahead adder: ~8 gate delays

• Details are in the text

• Issues

  • Complex combinational logic

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