Page fault handling in Operating Systems - Step-by-Step Execution

Concept Flow - Page fault handling

Process accesses memory

↓

Is page in RAM?

Yes→Access memory successfully

No↓

Trigger page fault interrupt

↓

OS checks page table

↓

Find free frame or select victim page

↓

Load required page from disk to RAM

↓

Update page table

↓

Resume process execution

When a process tries to access a page not in RAM, a page fault occurs. The OS then loads the page from disk into RAM and updates the page table before resuming the process.

Execution Sample

Operating Systems

1. Process tries to read address X
2. Check if page containing X is in RAM
3. If not, trigger page fault
4. OS loads page from disk
5. Update page table
6. Resume process

This sequence shows how the OS handles a page fault step-by-step.

Analysis Table

Step	Action	Condition/Check	Result/Next Step
1	Process accesses memory address X	Is page in RAM?	No, page fault triggered
2	OS receives page fault interrupt	Check page table for page location	Page on disk, not in RAM
3	OS finds free frame or selects victim page	Is free frame available?	Yes, free frame found
4	OS loads page from disk to RAM frame	Page loaded successfully?	Yes
5	OS updates page table entry	Page table updated	Ready to resume process
6	Process resumes execution	Access memory again	Page now in RAM, access successful
7	Process continues normal execution	No further page faults	Execution continues

💡 Process resumes after page is loaded into RAM and page table updated

State Tracker

Variable	Start	After Step 2	After Step 4	After Step 5	Final
Page in RAM	No	No	Yes (loaded)	Yes	Yes
Page table entry	Invalid	Invalid	Invalid	Valid (updated)	Valid
Free frame available	Unknown	Checked - Yes	Used	Used	Used

Key Insights - 3 Insights

Why does the process stop and wait during a page fault?

Because the required page is not in RAM, the OS must load it from disk which takes time. This is shown in execution_table steps 2 to 5 where the OS handles loading before resuming.

What happens if no free frame is available in RAM?

The OS selects a victim page to remove (not shown in this simple trace), frees that frame, then loads the needed page. This is part of step 3 in the execution_table.

How does the OS know where to find the page on disk?

The OS uses the page table to find the disk location of the page, as shown in step 2 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the page actually loaded into RAM?

A Step 2

B Step 4

C Step 5

D Step 6

💡 Hint

Check the 'Action' and 'Result' columns in step 4 where the page is loaded from disk to RAM.

According to variable_tracker, when does the page table entry become valid?

A After Step 5

B After Step 4

C After Step 2

D At Start

💡 Hint

Look at the 'Page table entry' row and see when it changes from 'Invalid' to 'Valid (updated)'.

If there was no free frame available, which step in the execution_table would change?

A Step 5

B Step 4

C Step 3

D Step 6

💡 Hint

Step 3 checks for free frame availability; if none, OS must select a victim page here.

Concept Snapshot

Page fault handling:
- Occurs when process accesses page not in RAM
- OS triggers interrupt, checks page table
- Loads page from disk to RAM
- Updates page table
- Resumes process execution
- May need to free RAM frame if none available

Full Transcript

Page fault handling happens when a process tries to access memory not currently in RAM. The operating system detects this and pauses the process. It checks the page table to find the page on disk, then loads it into RAM. The page table is updated to reflect the new location. Once done, the process resumes and accesses the page successfully. If RAM is full, the OS selects a page to remove before loading the new one. This ensures processes can use more memory than physically available by swapping pages between RAM and disk.

Practice

1. What is a page fault in an operating system?

easy

A. A hardware failure causing the computer to restart

B. A user error when typing commands

C. A type of virus that affects memory pages

D. An event when a program tries to access a page not currently in physical memory

Solution

1. Step 1: Understand what a page is

   A page is a fixed-length block of virtual memory used by the operating system to manage memory efficiently.

2. Step 2: Define page fault

   A page fault occurs when a program tries to access a page that is not currently loaded into physical memory, triggering the OS to load it.

3. Final Answer:

   An event when a program tries to access a page not currently in physical memory -> Option D

4. Quick Check:

   Page fault = Access missing page [OK]

Hint: Page fault means missing page in memory, OS loads it [OK]

Common Mistakes:

• Confusing page fault with hardware failure
• Thinking page fault is a virus
• Assuming user error causes page faults

2. Which of the following is the correct sequence of steps when a page fault occurs?

easy

A. Check page table, load page from disk, update page table, resume process

B. Resume process, load page from disk, update page table, check page table

C. Update page table, resume process, check page table, load page from disk

D. Load page from disk, resume process, check page table, update page table

Solution

1. Step 1: Identify the correct order of handling a page fault

   First, the OS checks the page table to confirm the page is missing.

2. Step 2: Load the missing page and update the page table

   The OS loads the page from disk into memory, updates the page table, then resumes the process.

3. Final Answer:

   Check page table, load page from disk, update page table, resume process -> Option A

4. Quick Check:

   Page fault steps = Check, load, update, resume [OK]

Hint: Page fault handling always starts with checking page table [OK]

Common Mistakes:

• Resuming process before loading page
• Updating page table before loading page
• Ignoring page table check

3. Consider this simplified pseudocode for page fault handling:

if (page not in memory) {
  load page from disk;
  update page table;
  resume process;
} else {
  continue execution;
}

What happens if the page is already in memory?

medium

A. The page table is updated unnecessarily

B. The page is loaded again from disk

C. The process continues execution without delay

D. The process is paused indefinitely

Solution

1. Step 1: Analyze the condition for page fault

   The code checks if the page is not in memory to handle the fault.

2. Step 2: Understand the else branch

   If the page is already in memory, the else branch executes, allowing the process to continue without loading or updating.

3. Final Answer:

   The process continues execution without delay -> Option C

4. Quick Check:

   Page in memory means continue execution [OK]

Hint: If page present, no loading needed, just continue [OK]

Common Mistakes:

• Assuming page reload even if present
• Thinking page table updates always happen
• Believing process pauses unnecessarily

4. A programmer notices that their system crashes during page fault handling. Which of the following is the most likely cause?

medium

A. The page fault handler tries to load a page but does not update the page table

B. The page fault handler correctly loads the page and updates the page table

C. The process never triggers a page fault

D. The system has enough physical memory

Solution

1. Step 1: Understand the role of the page table update

   After loading a page, the page table must be updated to reflect the new page location.

2. Step 2: Identify the problem if page table is not updated

   If the page table is not updated, the system will repeatedly try to load the same page, causing crashes or infinite loops.

3. Final Answer:

   The page fault handler tries to load a page but does not update the page table -> Option A

4. Quick Check:

   Missing page table update causes crash [OK]

Hint: Always update page table after loading page to avoid crashes [OK]

Common Mistakes:

• Ignoring page table updates
• Assuming enough memory prevents faults
• Thinking no page faults means no errors

5. In a system using virtual memory, a program accesses a large dataset that does not fit entirely in physical memory. How does page fault handling help the system run this program efficiently?

hard

A. By deleting unused files from disk to free space

B. By loading only the needed pages into memory when accessed, reducing memory use

C. By preventing the program from running if memory is insufficient

D. By loading the entire dataset into memory at once to avoid faults

Solution

1. Step 1: Understand virtual memory and page faults

   Virtual memory allows programs to use more memory than physically available by loading pages on demand.

2. Step 2: Explain how page fault handling supports efficiency

   When a program accesses a page not in memory, a page fault triggers loading only that page, saving memory and improving performance.

3. Final Answer:

   By loading only the needed pages into memory when accessed, reducing memory use -> Option B

4. Quick Check:

   Page fault loads needed pages only [OK]

Hint: Page faults load pages on demand, saving memory [OK]

Common Mistakes:

• Thinking entire data loads at once
• Believing program stops if memory is low
• Confusing page fault with disk cleanup