2-Bit Adder

• Now, we can combine the half adder and the full adder to construct a 2-bit adder.
• The least significant bit does not require a full adder as there is no carry input.
• The half adder determines the least significant bit of the sum as the XOR of the least significant bits of the inputs.
• The carry output of the half adder becomes the carry input of the full adder.
• The full adder computes the sum of the inputs A₁ and A₂ and the carry bit.
• Example: A=3 (11) and B=1 (01):
  • S₀=0
  • $\mbox{carry}_0=1$
  • S₁=0
  • $\mbox{carry}_1=1$
  • Result: 100, which is the binary representation of 4.

  

Figure 3.8: 2-Bit Adder