Q. How to input to circuit?

• Register A bits as a0, a1, a2 and a3 in the corresponding X bits of the Full Adder (FA).
• Register B bits as given in the Figure above as in the corresponding Y bits of FA.
• Please consider every bit of register A and register B is fed to separate full adder unit.
• Please also note that every one of the four inputs from A are applied to X inputs of binary adder and every of the four inputs from B are associated to the data inputs of multiplexers. It means that A input directly goes to adder though B input can be manipulated by the Multiplexer to create a number of different input values as shown in the figure above. B inputs through multiplexers are controlled by two selection lines S₁ and S₀. So using various combinations of S and S we can select data bits of B, complement of B, 0word, or word having All 1's.

The input carry Cin that can be equal to 0 or 1 goes to carry input of the full adder in least significant position. Other carries are cascaded from one phase to the subsequent.  Logically it is same as that of addition performed by us. We do pass carry of lower digits addition to higher digits. The output ofbinary adder is determined from the below arithmetic sum:

 D = X + Y + C_in

  OR 

 D = A + Y + Cin

By controlling value of Y with the two selection lines S₁and S₀ and making C_in equal to 0 or 1 it's possible to implement eight arithmetic micro-operations listed in truth table.

Figure: Arithmetic Circuit Function Table