digital logic

Question

Explain this question

Comments

The boxes consist of some circuit which generates the results given by the equation, we need to find what output it generates.

Answer 1

we can easily identified that each box is a full subtractor.

where Y is difference bit and Z is Borrow bit.

if A,B,C is a Inputs to Full-Subtractor, and we are doing A-B

Difference = A ⊕  B ⊕ C

Borrow = A' . B + A' .C + BC

But we have to identify is it doing P-Q or Q-P ?

By seeing Z = R . Q +  P'. R + P' . Q =  P'. R + P' . Q + R . Q ===> we can say it is P - Q

Option B is right