GATE CSE 2003 | Question: 46

Question

Consider the ALU shown below.

 

If the operands are in $2’s$ complement representation, which of the following operations can be performed by suitably setting the control lines $K$ and $C_0$ only (+ and – denote addition and subtraction respectively)?

• A. $A + B$, and $A – B$, but not $A + 1$
• B. $A + B$, and $A + 1$, but not $A – B$
• C. $A + B$, but not $A – B$ or $A + 1$
• D. $A + B$, and $A – B$, and $A + 1$

Comments

@Deepak Poonia sir, here operands are given in 2’s complement already . so for k=0 c0=0 it do A-B ??

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someone please reply to @anirudhkumar18’s query, i also think k=0 c0=0 must do A-B and A+B both as i/p already in 2’s compliment, but most answers here take k=1 for A-B

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@anirudhkumar18

Not necessarily,  if both A and B are positive it will add.

If A and B both are negative, then it will again add.

If A and B are of opposite signs then it will subtract. 

Hope this helps!!

Answer 1

If k=0 and c0 = 0 then A+B

If k=1 and c0=1 then A-B

If k=0 and c0=1 and B =1 then A+1

So, Answer is D.

(If K is connected to c0, then manipulating k and c0 Simultaneously is not be possible, in that case answer is be A)
 

Comments

In the question it is given that you can only change the K and C₀ but setting B is not possible.

To perform A+1 with given circuit, its necessary to make B side input to be 0 & C₀ = 1. Since there is bit wise XOR is taking place before input to the full adder so we can only get B's 1's complement or B as it is. This the reason we can not perform A+1.