List of top Computer Science and IT Engineering Questions on Operating Systems asked in GATE CS

Which one or more of the following need to be saved on a context switch from one thread (T1) of a process to another thread (T2) of the same process?

Which one or more of the following CPU scheduling algorithms can potentially cause starvation?

A keyboard connected to a computer is used at a rate of 1 keystroke per second. The computer polls the keyboard every 10 ms, each poll takes 100 \(\mu s\). If a key is detected, an additional 200 \(\mu s\) is used for processing. Let \(T_1\) be the fraction of a second spent in polling and processing a keystroke. In an alternative implementation with interrupts, each keystroke requires 1 ms to service and process. Let \(T_2\) be the fraction of a second in the interrupt method. Find the ratio \(T_1/T_2\).

Consider the two functions incr and decr below. Five threads each call incr once and three threads each call decr once on the same shared variable \(X\) (initially \(10\)). There are two semaphore implementations:

I-1: \(s\) is a binary semaphore initialized to \(1\).
I-2: \(s\) is a counting semaphore initialized to \(2\).

Let \(V_1, V_2\) be the values of \(X\) at the end of all threads for I-1 and I-2, respectively. Which choice gives the minimum possible values of \(V_1, V_2\)?

incr(){              
  wait(s);           
  X = X + 1;         
  signal(s);         
}

decr(){
  wait(s);
  X = X - 1;
  signal(s);
}

Consider the two-dimensional array D[128][128] in C, stored in row-major order. Each physical page frame holds 512 elements of D. There are 30 physical page frames. The following code executes:

for (int i = 0; i<128; i++)
    for (int j = 0; j<128; j++)
        D[j][i] *= 10;

The number of page faults generated during this execution is:

A computer uses 57-bit virtual addresses with multi-level tree-structured page tables (L levels). Page size = 4 KB and each page-table entry (PTE) = 8 bytes. Find \(L\).

A keyboard connected to a computer is used at a rate of 1 keystroke per second. The computer polls the keyboard every 10 ms, each poll takes 100 \(\mu s\). If a key is detected, an additional 200 \(\mu s\) is used for processing. Let \(T_1\) be the fraction of a second spent in polling and processing a keystroke. In an alternative implementation with interrupts, each keystroke requires 1 ms to service and process. Let \(T_2\) be the fraction of a second in the interrupt method. Find the ratio \(T_1/T_2\).

Consider the two-dimensional array D[128][128] in C, stored in row-major order. Each physical page frame holds 512 elements of D. There are 30 physical page frames. The following code executes:

for (int i = 0; i<128; i++)
    for (int j = 0; j<128; j++)
        D[j][i] *= 10;

The number of page faults generated during this execution is:

A computer uses 57-bit virtual addresses with multi-level tree-structured page tables (L levels). Page size = 4 KB and each page-table entry (PTE) = 8 bytes. Find \(L\).