N-bit adder

• Adders for arbitrarily large binary numbers can be constructed by cascading full adders.
• The carry bit ``ripples'' from one stage to the next.
• A drawback of this circuit is that the carry information has to propagate through all stages.
• I.e., there is an electrical path from A₀ and B₀ to S₅ in the diagram below.
• This path traverses 2N-1 gates in an N-bit adder.
• This may lead to undesirably long delays before the output stabilizes.
• The problem can be overcome by more complex carry computations.

  

Figure 3.9: 4-Bit Adder