Operating Systems
Operating Systems tests process management, scheduling, synchronization, deadlock, memory management, file systems and basic commands. The exam favors definitions plus small scheduling or page-count calculations.
Core rules
- Process states: new, ready, running, waiting, terminated. The scheduler picks from the ready queue; the dispatcher performs context switching.
- FCFS can cause the convoy effect; SJF gives optimal average waiting time but needs burst prediction; round robin uses a time quantum; priority scheduling can starve low-priority processes.
- Deadlock needs four Coffman conditions together: mutual exclusion, hold and wait, no preemption, circular wait. Breaking any one prevents deadlock.
- Paging divides logical memory into pages and physical memory into frames; the page table maps them; the TLB caches translations; thrashing is excessive paging.
- A semaphore is a signalling/counting mechanism; a mutex is a locking mechanism with ownership. Both help solve critical-section race conditions.
Worked example
Three processes arrive at time 0 with bursts P1 = 5, P2 = 3, P3 = 2 under FCFS. Waiting times are P1 = 0, P2 = 5, P3 = 8; average = 13/3 = 4.33 ms. Completion times are 5, 8, 10; turnaround times are 5, 8, 10; average = 7.67 ms. Reasoning: FCFS runs in arrival order, so each process waits for all earlier bursts.
Traps
Virtual memory does not increase physical RAM; it extends the logical address space using disk. Round robin is not always optimal; its average waiting time depends on the quantum. Deadlock is not the same as starvation.