PPT Slide

4 – Combinational Logic Design Examples

© 1996 Gaetano Borriello

Carry-lookahead logic

• Carry generate: Gi = Ai Bi

  • must generate carry when A = B = 1

• Carry propagate: Pi = Ai xor Bi

  • carry-in will equal carry-out here

• Sum and Cout can be re-expressed in terms of generate/propagate:

  • Si = Ai xor Bi xor Ci = Pi xor Ci
  • Ci+1 = Ai Bi + Ai Ci + Bi Ci = Ai Bi + Ci (Ai + Bi) = Ai Bi + Ci (Ai xor Bi) = Gi + Ci Pi

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