Logic circuits

Why is a carry-lookahead adder fast?

  1. AIt reduces the number of bits being added
  2. BIt doubles the clock frequency
  3. CIt computes the carry for each bit in parallel from the inputs without waiting for propagation from the lower bits
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Answer: C. It computes the carry for each bit in parallel from the inputs without waiting for propagation from the lower bits

A carry-lookahead adder uses the generate (G = A·B) and propagate (P = A + B, etc.) signals of each bit to compute the carry for each position directly from the inputs, in parallel. Because it does not wait for carries to ripple through in sequence, it avoids delay that grows in proportion to the number of bits.

  • A. The number of bits is unchanged; the method of computation differs.
  • B. It has nothing to do with clock frequency; it shortens the delay of the combinational logic.
  • C. Correct. It computes the carries in parallel by looking ahead.
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