ADDERS & SUBTRACTORS

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.

HALF ADDER

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.

FULL ADDER

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.

PARALLEL BINARY ADDER

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.

HALF SUBTRACTOR

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.

FULL SUBTRACTOR

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.