Gate2019 cs Q33

0. Consider three concurrent processes P1, P2 and P3 as shown below, which access a shared variable D that has been initialized to

P1	P2	P3
:	:	:
:	:	:
D = D + 20	D = D – 50	D = D + 10
:	:	:
:	:	:

The processes are executed on a uniprocessor system running a time-shared operating system. If the minimum and maximum possible values of D after the three processes have completed execution are X and Y respectively, then the value of Y – X is _____________.

Note – Numerical Type question

Answer
Comment
Array

Option : A
Explanation :
Minimum value (X) of D will possible when,

P2 reads D=100, preempted.
P1 executes D=D+20, D=120.
P3 executes D=D+10, D=130.
Now, P2 has D=100, executes, D = D-50 = 100-50 = 50. P2 writes D=50 final value.
So, minimum value (X) of D is 50.
Maximum value (Y) of D will possible when,

P1 reads D=100, preempted.
P2 reads D=100, executes, D = D-50 = 100-50 = 50.
Now, P1 executes, D = D+20 = 100+20 = 120.
And now, P3 reads D=120, executes D=D+10, D=130. P3 writes D=130 final value.
So, maximum value (Y) of D is 130.

Therefore,
= Y - X
= 130 - 50
= 80
So, option (A) is correct.

Forgot your password?

I agree with the Terms.