Day 42 – Evening Session: OS – Virtual Memory & Page Replacement – ECET 2026

In ECET 2026 CSE, Operating System questions on Virtual Memory and Page Replacement are very common. If you understand these concepts and practice a few problems, you can score easy marks.

---

📘 Concept Notes

🌐 What is Virtual Memory?

• Virtual Memory is a memory management technique that gives an illusion of a very large main memory.
• It allows programs to execute even if they are larger than the actual physical RAM.
• Implemented using demand paging and page replacement.

---

⚙️ Key Concepts

1. Pages & Frames
   • Logical memory is divided into pages.
   • Physical memory is divided into frames.
   • A page from logical memory is mapped to a free frame in physical memory.
2. Page Table
   • Maintains mapping between logical pages and physical frames.
3. Page Fault
   • Occurs when a requested page is not present in memory.
   • The OS loads it from disk into a free frame.
4. Effective Access Time (EAT) Formula

If:

• = page fault rate
• = memory access time
• = page fault service time

Then:

---

🔄 Page Replacement Algorithms

When a page fault occurs and no free frame is available, page replacement is needed.

1. FIFO (First In First Out)
   • Replace the oldest page in memory.
   • Simple but may suffer from Belady’s Anomaly (more frames may cause more faults).
2. LRU (Least Recently Used)
   • Replace the page that has not been used for the longest time.
   • Better than FIFO but needs tracking history.
3. Optimal (OPT)
   • Replace the page that will not be used for the longest time in the future.
   • Theoretical best but not practical.

---

📐 Example – Page Replacement

Reference String = [7, 0, 1, 2, 0, 3, 0, 4], Frames = 3

• Using FIFO, page faults = 6
• Using LRU, page faults = 5

---

🔟 10 Expected MCQs – ECET 2026

Q1. Virtual memory gives an illusion of:
A) Infinite secondary storage
B) Very large main memory
C) CPU speedup
D) None

Q2. A page fault occurs when:
A) Page is in main memory
B) Page is in secondary memory
C) Page is in CPU cache
D) None

Q3. Virtual memory is implemented using:
A) Segmentation
B) Paging
C) Demand Paging
D) Contiguous allocation

Q4. Which algorithm may suffer from Belady’s anomaly?
A) FIFO
B) LRU
C) Optimal
D) None

Q5. Formula for Effective Access Time (EAT) is:
A)
B)
C)
D) None

Q6. Optimal page replacement is:
A) Practical
B) Theoretical best
C) Same as FIFO
D) None

Q7. If , , , then
A) ≈ 220 ns
B) ≈ 82 μs
C) ≈ 200 ns
D) ≈ 1 ms

Q8. Which replacement policy uses “not used for longest time”?
A) FIFO
B) LRU
C) Optimal
D) Clock

Q9. Which is the best practical algorithm?
A) FIFO
B) LRU
C) Optimal
D) Random

Q10. Thrashing is caused by:
A) Too many processes, high page faults
B) Large cache
C) Low I/O
D) CPU scheduling

---

✅ Answer Key

Q.No	Answer
Q1	B
Q2	B
Q3	C
Q4	A
Q5	B
Q6	B
Q7	B
Q8	B
Q9	B
Q10	A

---

🧠 Explanations

• Q1 → B: Virtual memory extends main memory size.
• Q2 → B: Page fault = page not in RAM.
• Q3 → C: Virtual memory is implemented with demand paging.
• Q4 → A: FIFO may cause Belady’s anomaly.
• Q5 → B: Standard formula for EAT.
• Q6 → B: Optimal is theoretical best.
• Q7 → B: Calculation:
  • 
  • 
• Q8 → B: LRU uses longest unused replacement.
• Q9 → B: LRU is best practical approach.
• Q10 → A: Thrashing = too many page faults.

---

🎯 Why Practice Matters

• Virtual Memory and Page Replacement are repeated topics in ECET exams.
• Questions are often direct formulas (EAT) or conceptual (FIFO vs LRU).
• Easy scoring if practiced well.

---

📲 Join Our ECET Prep Community on WhatsApp

👉 Join WhatsApp Group – Click Here