ISRO | ISRO CS 2016 | Question 6

For a binary half-subtractor having two inputs A and B, the correct set of logical expressions for the outputs D (= A minus B) and X (=borrow) are
(A) D = AB + A’B , X = A’B
(B) D = A’B + AB’ , X = AB’
(C) D = A’B + AB’ , X = A’B

(D) D = AB + A’B , X = AB’

Answer: (C)
Explanation: The Truth Table of half subtractor is:

A       B      OUTPUT       BORROW
0       0        0            0
0       1        1            1
1       0        1            0 
1       1        0            0

D = A’B + AB’
X = A’B
Option (D) is correct.
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