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Sahithyan's S2
Sahithyan's S2 — Computer Organization and Digital Design

Lookup Tables

A technique for implementing combinational logic digital circuits instead of using AND, OR, NOT gates. For each input combination, the output is precomputed and stored in memory. Later, the table is used to output the values based on the applied input combination. Can be cascaded or combined to implement larger and more complex functions. Aka. LUT.

A 4-input LUT can implement any logic function with 4 inputs by storing 16 outputs.

Faster than gate-based logic.

Multiplexers can be used as LUTs.

Usage

  • Core building blocks of FGPAs
  • Used to store precomputed values of complex mathematical functions
  • Used in control units of CPUs to define micro-operations
  • Used to reduce number of gates in complex logic