What is Virtual Memory?
uses a section of secondary storage (hard drive or SSD) to act as an
extension of RAM when RAM is full.
Virtual Memory = A portion of secondary storage used by the OS to
simulate extra RAM, allowing more programs to run than physical RAM alone
would permit.
2. Why is Virtual Memory Needed?
as RAM could physically hold.
3. How Virtual Memory Works
Key Terms in this Process
| Term | Definition |
|---|---|
| Page | A fixed-size block of data that RAM and virtual memory are divided into |
| Page File | The area on secondary storage used as virtual memory (also called swap space) |
| Paging | The process of moving pages between RAM and the page file |
| Page Swap | When a page is moved from RAM to the page file, or vice versa |
| Page Fault | When the CPU requests data that is not currently in RAM and must be fetched from the page file |
4. The Paging Process — Step by Step
5. Virtual Memory and the OS
6. Thrashing
between RAM and the page file than actually executing program instructions.
When does thrashing happen?
How to fix thrashing:
7. Advantages and Disadvantages of Virtual Memory
| ✅ Advantages | ❌ Disadvantages |
|---|---|
| Allows more programs to run simultaneously | Much slower than real RAM |
| Prevents the system from crashing when RAM is full | Can cause thrashing if overused |
| Allows large programs to run on systems with limited RAM | Uses up secondary storage space |
| Managed automatically by the OS — no user effort needed | Repeated page swaps increase wear on SSDs |
| Cost-effective — uses existing storage hardware | Overall system performance is reduced |
8. Virtual Memory vs RAM
| Feature | RAM | Virtual Memory |
|---|---|---|
| Location | Physical chips on motherboard | Section of HDD/SSD |
| Speed | Very fast | Very slow |
| Cost | Expensive per GB | Cheap (uses existing storage) |
| Managed by | Hardware | Operating System |
| Access time | Nanoseconds | Milliseconds |
| Volatile? | ✅ Yes | ✅ Yes (page file cleared at shutdown) |
9. Effect of Virtual Memory on Performance
10. Virtual Memory in Context — Full Memory Hierarchy
The further down the hierarchy, the slower, larger, and cheaper the storage.
Virtual memory sits between RAM and full secondary storage in practice,
because it is secondary storage used as extra RAM.
11. Key Vocabulary
| Term | Definition |
|---|---|
| Virtual Memory | A technique using secondary storage as an extension of RAM |
| Page | A fixed-size block that memory is divided into for management |
| Page File | The area on secondary storage reserved for virtual memory |
| Paging | Moving pages of data between RAM and the page file |
| Page Fault | When required data is not in RAM and must be fetched from the page file |
| Page Swap | The act of moving a page between RAM and the page file |
| Thrashing | Severe slowdown caused by excessive, constant paging activity |
| Page Table | An OS data structure tracking the location of each page |
| Operating System | The software responsible for managing virtual memory |
12. Exam Tips ✅
used by the OS using secondary storage.
secondary storage (HDD/SSD) has a much longer access time than RAM.
always define it as spending more time swapping pages than executing instructions.
memory is being used heavily?"
→ Because secondary storage is much slower than RAM, and frequent
page swaps cause significant delays.
installing more RAM — state this clearly.
secondary storage (HDD/SSD), not any form of primary storage chip.
Modified at 2026-04-03 10:00:20