1-Bit Adder (Half Adder)

• The simplest case arises when two one bit numbers are to be added.
• With one bit, only the numbers 0 and 1 can be represented.
• All possible scenarios can be summarized by the following table:

  Binary/Decimal Inputs		Decimal Result	Binary Result
  A	B	D	Y1 Y0
  0	0	0	0 0
  0	1	1	0 1
  1	0	1	0 1
  1	1	2	1 0

• The least significant bit of the output, Y₀, is the result of XOR-ing the inputs.
• The other bit, Y₁, results from AND-ing A and B.
• The bit Y₁ is called the carry bit.
• Hence, the circuit below can be used to construct a half-adder.

  

Figure 3.6: Half Adder