Operating System Quiz 1

Calculate average waiting time using FCFS for P1(0,6), P2(2,8), P3(4,7), P4(5,3).

• 5.75 ms
• 8.5 ms
• 7.5 ms
• 7.25 ms

Answer explanation

FCFS Order: P1 (0–6), P2 (6–14), P3 (14–21), P4 (21–24). Waiting Times: P1: 0 ms P2: 6 - 2 = 4 ms P3: 14 - 4 = 10 ms P4: 21 - 5 = 16 ms Average: (0 + 4 + 10 + 16)/4 = 7.5 ms

In RR (quantum=4ms), P1(10ms), P2(5ms), P3(8ms). What is turnaround time for P3?

• 19 ms
• 21 ms
• 22 ms
• 20 ms

Answer explanation

Execution Order: T0–4: P1 (4ms done, 6ms left) T4–8: P2 (4ms done, 1ms left) T8–12: P3 (4ms done, 4ms left) T12–16: P1 (4ms done, 2ms left) T16–17: P2 (1ms done, finishes) T17–21: P3 (4ms done, finishes). P3 completes at T=21 ms.

SRTF Scheduling: Processes P1(0,7), P2(2,4), P3(4,1). Compute number of context switches.

• 2
• 3
• 4
• 5

Answer explanation

Execution Order: T0–2: P1 T2–4: P2 (preempts P1) T4–5: P3 (preempts P2, shortest remaining time) T5–8: P2 T8–14: P1 Context Switches: P1→P2, P2→P3, P3→P2, P2→P1 → 4 switches.

Priority Scheduling (Preemptive): P1(0,10,3), P2(2,5,1), P3(3,2,2). Compute average turnaround time.

• 14 ms
• 10 ms
• 13 ms
• 15 ms

Answer explanation

Execution Order: T0–2: P1 T2–7: P2 (higher priority, preempts P1) T7–9: P3 T9–19: P1 Turnaround Times: P1: 19 - 0 = 19 ms P2: 7 - 2 = 5 ms P3: 9 - 3 = 6 ms Average: (19 + 5 + 6)/3 = 10 ms

CPU spends 80% in user mode, 10% system calls, 5% I/O wait, 5% idle. CPU Utilization is?

• 0.85
• 0.9
• 0.8
• 0.95

Answer explanation

CPU Utilization = User Mode + System Calls = 80% + 10% = 90%. I/O wait and idle time are excluded.

Which type of OS ensures critical tasks meet deadlines?

• Time-sharing OS
• Distributed OS
• Real-time OS
• Multi-user OS

Answer explanation

Real-time OS guarantees timely execution for time-sensitive tasks.

Which is TRUE about Microkernel architecture?

• Device drivers run in kernel space
• Kernel is small and modular
• Kernel handles all services
• Process management is in user space

Answer explanation

Microkernel minimizes kernel functionality (e.g., only IPC, scheduling), moving other services (e.g., drivers) to user space.

Which scheduling algorithm ensures minimum average waiting time if all processes arrive at time zero?

• FCFS
• SJF (Non-preemptive)
• Round Robin
• Priority Scheduling

Answer explanation

SJF is provably optimal for minimizing average waiting time when all processes arrive simultaneously.

Which scheduling algorithm may cause starvation?

• FCFS
• SJF (Preemptive)
• Round Robin
• None of the above

Answer explanation

SJF (Preemptive) can starve long processes if shorter ones keep arriving.

Context switch involves saving and loading of:

• Only CPU registers
• CPU registers and PCB
• Only Memory contents
• File descriptors only

Answer explanation

Context switches save the CPU state (registers) and PCB of the current process, then load the next process’s PCB and registers.