Operating System | Gate-2020| Previous year question|Set-1

Set-15 Operating System Questions

  1. Consider allocation of memory to a new process. Assume that none of the existing holes in the memory will exactly fit the process’s memory requirement. Hence, a new hole of smaller size will be created if allocation is made in any of the existing holes. Which one of the following statements is TRUE? [GATE – 2020]

a. The hole created by worst fit is always larger than the hole created by first fit.
b. The hole created by best fit is never larger than the hole created by first fit
c. The hole created by first fit is always larger than the hole created by next fit.
d. The hole created by next fit is never larger than the hole created by best fit.

Answer :b)


  1. Consider the following statements about process state transitions for a system using preemptive scheduling. [GATE – 2020]

    I. A running process can move to ready state.
    II. A ready process can move to ready state.
    III. A blocked process can move to running state.
    IV. A blocked process can move to ready state.
    Which of the above statements are TRUE?

a. II and III only
b. I, II and III only
c. I, II, III and IV
d. I, II and IV only

Answer : d)


  1. Consider the following set of processes, assumed to have arrived at time 0. Consider the CPU scheduling algorithms Shortest Job First (SJF) and Round Robin (RR). For RR, assume that the processes are scheduled in the order P1,P2,P3,P4.
ProcessesP1P2P3P4
Burst time (in ms)8724

If the time quantum for RR is 4 ms, then the absolute value of the difference between the average turnaround times (in ms) of SJF and RR (round off to 2 decimal places) is _____.  [GATE – 2020]

a. 5.25
b. 5.26
c. 6.7
d. 7.9

Answer : a)
Operating System | Gate-2020|


  1. Each of a set of n processes executes the following code using two semaphores a and b initialized to 1 and 0, respectively. Assume that count is a shared variable
CODE SECTION P

Initialized to 0 and not used in CODE SECTION P

wait (a); count = count+1;
if (count==n) signal (b);
signal (a); wait (b); signal (b);

CODE SECTION Q

What does the code achieve? [GATE – 2020]

a. It ensures that all processes execute CODE SECTION P mutually exclusively.
b. It ensures that at most two processes are in CODE SECTION Q at any time.
c. It ensures that no process executes CODE SECTION Q before every process has finished CODE SECTION P.
d. It ensures that at most n-1 processes are in CODE SECTION P at any time.

Answer : c)
Operating System | Gate-2020|


5. Consider the following five disk access requests of the form (request id, cylinder number) that are present in the disk scheduler queue at a given time.
(P, 155), (Q, 85), (R, 110), (S, 30), (T, 115)
Assume the head is positioned at cylinder 100. The scheduler follows Shortest Seek Time First scheduling to service the requests.
Which one of the following statements is FALSE? [GATE – 2020]

a. The head reverses its direction of movement between servicing of Q and P.
b. T is serviced before P.
c. R is serviced before P.
d. Q is serviced after S, but before T.

Answer : d)


6. Consider a paging system that uses a 1-level page table residing in main memory and a TLB for address translation. Each main memory access takes 100 ns and TLB lookup takes 20 ns. Each page transfer to/from the disk takes 5000 ns. Assume that the TLB hit ratio is 95%, page fault rate is 10%. Assume that for 20% of the total page faults, a dirty page has to be written back to disk before the required page is read in from disk. TLB update time is negligible. The average memory access time in ns (round off to 1 decimal places) is ______. [GATE – 2020]

a. 154.5ns
b. 145.5ns
c. 157.5ns
d. 165.2ns

Answer : a)


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