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Intermediate Topic 30 of 51 Operating Systems

Page Faults

Traps when a needed page is not resident in memory.

How it works

A page fault is an exception raised by the MMU when a program accesses a virtual page that is not currently mapped to physical memory or violates its permissions. The OS handler checks validity: if the page is valid but not in memory, it loads it from disk, updates the page table and restarts the instruction, a major fault; if it is in memory but unmapped, a minor fault; if the access is illegal, the process gets a segmentation fault signal.

Step-by-step walkthrough

  1. The program touches an address whose page is not in RAM.
  2. The MMU traps to the kernel with a page fault.
  3. The kernel finds the page on disk and loads it into a free frame.
  4. It updates the page table.
  5. The instruction restarts and succeeds.

Where it is used

Demand paging, lazy allocation, copy-on-write, memory-mapped files and stack growth all work through page faults. Monitoring fault rates reveals memory pressure.

Common pitfall

Major faults cost milliseconds because of disk access, so frequent ones indicate thrashing or insufficient RAM. Page-fault handlers must be fast and carefully written. A program touching memory randomly faults more than one accessing it sequentially. Pre-faulting or locking critical pages can help real-time code.

Where it fits in the course

Page Faults is topic 30 of 51 in Operating Systems, rated Intermediate. It follows Virtual Memory. The next topic is Page Replacement Algorithms.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Operating Systems quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is Page Faults?
Traps when a needed page is not resident in memory. It belongs to Operating Systems and is an intermediate-level topic.
Where do people use Page Faults?
Demand paging, lazy allocation, copy-on-write, memory-mapped files and stack growth all work through page faults.
What is the most common mistake with Page Faults?
Major faults cost milliseconds because of disk access, so frequent ones indicate thrashing or insufficient RAM.
What should I learn before this topic?
Start with Virtual Memory, the topic just before it in Operating Systems.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Operating Systems quiz.
Is Page Faults hard to learn?
It is an Intermediate topic. Be comfortable with the Beginner topics in this area first, then read the common pitfall carefully, since it is where most mistakes happen.
How long does it take to learn Page Faults?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to Page Faults?
Virtual Memory, Page Replacement Algorithms, Segmentation are closely connected. The full list is in the related topics section of this page.
What should I study after Page Faults?
Continue with Page Replacement Algorithms, the next topic in Operating Systems.

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