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Design and Implementation of VLSI Systems (EN0160) Lecture 28: Datapath Subsystems 4/4

Design and Implementation of VLSI Systems (EN0160) Lecture 28: Datapath Subsystems 4/4. Prof. Sherief Reda Division of Engineering, Brown University Spring 2008. [sources: Weste/Addison Wesley – Rabaey/Pearson]. Shifting a data word by a constant amount is trivial

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Design and Implementation of VLSI Systems (EN0160) Lecture 28: Datapath Subsystems 4/4

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  1. Design and Implementation of VLSI Systems (EN0160) Lecture 28: Datapath Subsystems 4/4 Prof. Sherief Reda Division of Engineering, Brown University Spring 2008 [sources: Weste/Addison Wesley – Rabaey/Pearson]

  2. Shifting a data word by a constant amount is trivial A programmable shifter is more complex D. Shifters Multibit shifter can be cascaded together

  3. Barrel Shifter • Signal passes through at most one transmission gate • Total transistors = N2 • Dominated by wiring

  4. Logarithmic shifter • Total shift is decomposed into stages of 2 • Speed of shifting N bits depends on log N • Number of transistor = 2(log N)* N

  5. E. Multipliers

  6. Array multiplier Where is the critical path?

  7. Critical path of MxN multiplier

  8. Carry Save Multiplier Tmult = tand+(N-1)tcarry+tmerge • The carry bits are not immediately added, but rather are “saved” for the next adder stage • The vector merge adder should be a fast carry lookahead adder

  9. Adders 0/1 detectors Comparators Shifters Multipliers Summary

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