Logic circuits

What is the drawback of a ripple-carry adder made by chaining full adders?

  1. AIt cannot handle negative numbers
  2. BBecause the carry propagates from the lower bits in sequence, the delay grows in proportion to the number of bits
  3. CThe circuit size does not change even as the number of bits increases
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Answer: B. Because the carry propagates from the lower bits in sequence, the delay grows in proportion to the number of bits

In a ripple-carry adder, each full adder must wait for the carry from below before computing, so the carry propagates in sequence from the least significant to the most significant bit. The delay therefore grows in proportion to the number of bits. Faster designs use structures such as carry-lookahead adders.

  • A. With two's complement, negative numbers can be handled by the same circuit.
  • B. Correct. Carry propagation makes the delay proportional to the number of bits.
  • C. More bits means more full adders, so the circuit grows.
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More Logic circuits questions

  1. Which correctly describes the hold time of a flip-flop?
  2. Which correctly describes clock skew in a synchronous circuit?
  3. What is the third state that the output of a tri-state buffer can take?
  4. What range of integers can be represented in 8-bit two's complement?
  5. Why is a carry-lookahead adder fast?