UGC NET CSE | Junet 2015 | Part 2 | Question: 7

Question

Consider a full-adder with the following input values :

1. $x=1, y=0$ and $C_i$(carry input)$=0$
2. $x=0, y=1$ and $C_i =1$

Compute the value of S(sum) and $C_o$ (carry output) for the above input values :

• A. $S=1, C_o=0$ and $S=0, C_o=1$
• B. $S=0, C_o=0$ and $S=1, C_o=1$
• C. $S=1, C_o=1$ and $S=0, C_o=0$
• D. $S=0, C_o=1$ and $S=1, C_o=0$

Answer 1

Answer A)

Full adder takes 3 bit input and gives sum and carry as output.

Answer 2

In full adder:
For input x, y and C_i
S_o = x ⊕ y ⊕ C_i
C_o = xy + C_i(x + y)

For x = 1, y = 0 and C_i(carry input) = 0
i.e. S_o = 1 ⊕ 0 ⊕ 0 = 1.
C_o = 1*0 + 0(1 + 0) = 0.
For x = 0, y = 1 and C_i = 1
i.e. S_o = 0 ⊕ 1 ⊕ 1 = 0
C_o = 0*1 + 1(0 + 1) = 1.

So, option (A) is correct.

Answer 3

Answer: A

For Full Adder 

SUM = A ⊕ B ⊕ $C_{i}$ 

CARRY = AB + B$C_{i}$  + $C_{i}$ A

a : SUM := 1 ⊕ 0 ⊕ 0 = 1       CARRY := 1*0 + 0*0 + 0*1 = 0

b : SUM := 0 ⊕ 1 ⊕ 1 = 0       CARRY := 0*1 + 1*1 + 1*0 = 1