Page fault handling in Operating Systems - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a page fault in an operating system?

A page fault happens when a program tries to access a part of memory that is not currently in the physical RAM. The operating system then needs to bring the required data from disk to RAM.

beginner

What is the first step the OS takes when a page fault occurs?

The OS checks if the memory access is valid and if the page is actually on disk or if it is an invalid access causing an error.

beginner

Why does the OS bring a page from disk to RAM during page fault handling?

Because the program needs that page to continue running, and it must be in RAM for the CPU to access it quickly.

intermediate

What happens if the page fault is caused by an invalid memory access?

The OS usually terminates the program or raises an error because the program tried to access memory it shouldn't.

intermediate

How does the OS update the page table after handling a page fault?

The OS updates the page table to mark the page as present in RAM and sets the correct frame address so future accesses do not cause faults.

What triggers a page fault in an operating system?

A Network failure

B Accessing a page not in physical memory

C Running out of disk space

D CPU overheating

A page fault occurs when a program tries to access a page that is not currently loaded in RAM.

What does the OS do first when a page fault occurs?

A Clears the RAM

B Immediately terminates the program

C Checks if the access is valid

D Restarts the computer

The OS first checks if the memory access causing the fault is valid before taking further action.

Where does the OS load the missing page from during page fault handling?

A From disk storage

B From the CPU cache

C From the network

D From another program

The OS loads the missing page from disk storage into RAM.

What happens if the page fault is due to an invalid memory access?

A The OS increases RAM size

B The OS loads the page anyway

C The OS ignores the fault

D The OS raises an error or terminates the program

Invalid memory access causes the OS to raise an error or stop the program to prevent damage.

After handling a page fault, how does the OS prevent the same fault from happening again for that page?

A Updates the page table to mark the page as present

B Deletes the page from disk

C Disables paging

D Restarts the program

The OS updates the page table so the page is marked as present in RAM for future accesses.

Explain the steps the operating system takes when a page fault occurs.

Think about what the OS must do to get the needed data into RAM and continue the program.

Describe what happens if a program tries to access memory it is not allowed to during page fault handling.

Consider how the OS protects memory and handles errors.

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