Operating Systems for GATE CS — Complete Study Guide
Processes, CPU scheduling, synchronisation, deadlock, memory management, virtual memory, file systems — every OS topic in the GATE CS syllabus with worked examples.
Last updated: April 2026 | GATE CS 2024–2026 syllabus
Why Operating Systems for GATE CS?
- 8–12 marks in GATE CS — second highest after DS+Algorithms. Never skip this subject.
- CPU scheduling questions appear every year — compute average waiting time, turnaround time, and identify the algorithm.
- Deadlock: Banker’s algorithm, resource allocation graphs, necessary conditions — at least 1 question per GATE.
- Memory management: paging, segmentation, TLB, page replacement algorithms (LRU, FIFO, Optimal) — numerical questions every year.
- Synchronisation: mutex, semaphore, monitors, classic problems (Dining Philosophers, Readers-Writers) — conceptual + code tracing.
- Virtual memory: demand paging, page fault rate, effective access time (EAT) — standard numerical formula questions.
GATE Weightage — OS Topics
| Topic | Avg Marks | Frequency |
|---|---|---|
| CPU Scheduling | 2–3 | Every year |
| Memory Management & Paging | 2–3 | Every year |
| Deadlock | 1–2 | Every year |
| Virtual Memory & Page Replacement | 1–2 | Very high |
| Synchronisation | 1–2 | High |
| Processes & Threads | 1 | High |
| File Systems & I/O | 1 | Moderate |
Topic Pages
| # | Topic | Key Concepts |
|---|---|---|
| 1 | Processes & Threads | Process states, PCB, context switching, threads vs processes, user-level vs kernel threads |
| 2 | CPU Scheduling | FCFS, SJF, SRTF, Round Robin, Priority, Multilevel; waiting time, turnaround time, Gantt chart |
| 3 | Process Synchronisation | Race condition, mutex, semaphore, monitors, Dining Philosophers, Readers-Writers, critical section |
| 4 | Deadlock | Coffman conditions, RAG, Banker’s algorithm, detection, prevention, avoidance |
| 5 | Memory Management | Paging, segmentation, internal/external fragmentation, TLB, page table, address translation |
| 6 | Virtual Memory | Demand paging, page fault, EAT, LRU, FIFO, Optimal, Belady’s anomaly, thrashing |
| 7 | File Systems | File allocation (contiguous, linked, indexed), directory structure, FAT, inode |
| 8 | I/O & Disk Scheduling | FCFS, SSTF, SCAN, C-SCAN, LOOK, seek time, rotational latency, disk access time |
| 9 | Formula Sheet | All OS formulas: scheduling, EAT, fragmentation, disk access time, Banker’s algorithm |
Quick Formula Reference
CPU Scheduling:
Turnaround Time (TAT) = Completion Time − Arrival Time
Waiting Time (WT) = TAT − Burst Time
Response Time = First CPU time − Arrival Time
CPU Utilisation = CPU busy time / Total time
Turnaround Time (TAT) = Completion Time − Arrival Time
Waiting Time (WT) = TAT − Burst Time
Response Time = First CPU time − Arrival Time
CPU Utilisation = CPU busy time / Total time
Virtual Memory:
EAT = (1−p) × memory access time + p × page fault time (p = page fault rate)
EAT with TLB = h × (tTLB + tmem) + (1−h) × (tTLB + 2tmem) (h = hit ratio)
Disk:
Disk access time = Seek time + Rotational latency + Transfer time
Avg rotational latency = 1 / (2 × RPM / 60)