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Design and Implementation of VLSI Systems (EN1600) Lecture 26: Datapath Subsystems 2/4

Design and Implementation of VLSI Systems (EN1600) Lecture 26: Datapath Subsystems 2/4. Prof. Sherief Reda Division of Engineering, Brown University Spring 2008. [sources: Weste/Addison Wesley – Rabaey/Pearson]. For a full adder, define what happens to carry

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Design and Implementation of VLSI Systems (EN1600) Lecture 26: Datapath Subsystems 2/4

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  1. Design and Implementation of VLSI Systems (EN1600) Lecture 26: Datapath Subsystems 2/4 Prof. Sherief Reda Division of Engineering, Brown University Spring 2008 [sources: Weste/Addison Wesley – Rabaey/Pearson]

  2. For a full adder, define what happens to carry Generate: Cout = 1 independent of C G = A • B Propagate: Cout = C P = A  B Kill: Cout = 0 independent of C K = ~A • ~B Last lecture we designed a carry-ripple adder PG summary

  3. i k j k-1 The carry into bit i is the carry-out of bit i-1 The sum is equal to Group carry calculations

  4. i k j k-1 Group generate

  5. Carry-ripple adder revisited

  6. Carry-ripple adder revisited The critical path now proceeds through a chain of AND-OR gates rather than a chain of majority gates

  7. 8-bit adder/subtractor add/subt C0=Cin • Subtraction – complement all subtrahend bits (xor gates) and set the low order carry-in A0 1-bit FA S0 B0 C1 A1 1-bit FA S1 B1 C2 A2 1-bit FA S2 B2 C3 . . . C7 A7 1-bit FA S7 B7 C8=Cout

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