GATE CSE 2015 | Set 1 | Question: 9

Question

The following two functions $P1$ and $P2$ that share a variable $B$ with an initial value of $2$ execute concurrently.
$$\begin{array}{|l|l|}\hline  \text{P1() \{ } & \text{P2()\{} \\  \text{ C = B - 1;} & \text{ D = 2 * B; } \\  \text{ B = 2 * C;} & \text{ B = D - 1;} \\ \text{\}} & \text{\}} \\\hline \end{array}$$
The number of distinct values that $B$ can possibly take after the execution is______________________.

Comments

read question care fully it is asked for distict value of B not whole program distinct value this cane be the silly mistake i made this miskake during test

toatl 4 distinct value that is 1,2,3,4

and B having 3 distinct value that is 4,3,2

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Similar question instead of two functions it is given two threads:

Operating System: GATE CSE 2024 | Set 1 | Question: 30

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It's so Simple with cases :

Case 1 : P1 then P2 (executes serially). In this order B will be 3.

Case 2 : P2 then P1 (serially). In this order B will be 4.

Case 3 : when both P1, P2 read the same value B=2, but write one after the other.

 1) P1 writes first then P2. In this case P1's B value(B=2) will be overwrite by P2(B=3). So at last B will be 3.

 2) P2 writes first then P2. In this case P2's B value(B=3) will be overwrite by P1(B=2). So at last B will be 2.

hence B can be 2,3, or 4.

So 3 unique values of B possible.

Answer 1

    The 4 Quick Math Tracks

1. Track 1 ($P_1 \rightarrow P_2$ Completely):
   • $P_1$ finishes: $C = 2 - 1 = 1 \rightarrow B = 2 \times 1 = 2$.
   • $P_2$ takes over with $B = 2$: $D = 2 \times 2 = 4 \rightarrow \mathbf{B = 3}$.
2. Track 2 ($P_2 \rightarrow P_1$ Completely):
   • $P_2$ finishes: $D = 2 \times 2 = 4 \rightarrow B = 4 - 1 = 3$.
   • $P_1$ takes over with $B = 3$: $C = 3 - 1 = 2 \rightarrow \mathbf{B = 4}$.
3. Track 3 ($P_1$ Starts $\rightarrow P_2$ Finishes $\rightarrow P_1$ Overwrites):
   • $P_1$ copies $B=2$ to compute $C=1$ and pauses.
   • $P_2$ runs completely and changes $B$ to $3$.
   • $P_1$ wakes up, ignores the change, and applies its cached $C=1$: $B = 2 \times 1 = \mathbf{2}$.
4. Track 4 ($P_2$ Starts $\rightarrow P_1$ Finishes $\rightarrow P_2$ Overwrites):
   • $P_2$ copies $B=2$ to compute $D=4$ and pauses.
   • $P_1$ runs completely and keeps $B$ at $2$.
   • $P_2$ wakes up and applies its cached $D=4$: $B = 4 - 1 = \mathbf{3}$.

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Unique Set: The final values can only be --- {2, 3, 4}.

 

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