• The logical gates perform arithmetical functions in electronics devices like calculators and digital instruments.
  • The central processing unit (CPU) of a computer does these arithmetic functions with the help of arithmetic and logic unit(ALU).
  • The logic functions used generally are XOR, OR and AND.
  • Half Adder – it is a 1-bit adder and carries out binary addition with the help of XOR and AND gates. It has two inputs and two outputs.
  • Full Adder – it has three inputs and can add three bits at a time. It is made up of two half adders and one OR gate.
  • Half Subtractor – it uses one XOR and one AND gate.
  • Full Subtrctor – it employs two half subtractors  and one OR gate.


  • It can add 2 binary digits at a time and produce a 2-bit data (sum and carry according to the binary addition rules)
  • It has two inputs for applying the two binary digits to be added.
  • As we know binary addition of two bits always produces 2-bit output data, one SUM and one CARRY.
  • So the adder has two outputs.


  • It has three inputs and two outputs.
  • It can add 3 digits or bits at a time.
  • The bits A and B which are to be added come from the two registers and the third input comes from the carry generated by the previous addition.
  • It produces tow outputs , SUM and CARRY OUT.


  • The first adder CARRY-IN lead is not connected.
  • Different bits are fed to the four adders from two parallel registers which hold these bits.
  • The final SUM appears as a 5-digit display.


  • It can subtract only two binary digits at a time and produce an output of a difference and a borrow.
  • It has two inputs and two outputs.
  • The operation of a half subtractor is based on the rules of binary subtraction.


  • It has three inputs and two outputs.
  • Half subtractor can handle only 2 bits at a time and can be used for the least significant column of a subtractor problem.
  • A full subtractor can take care of higher-order columns.

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