Page fault handling in Operating Systems - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What happens immediately after a page fault is detected?

When a program tries to access a page not currently in physical memory, a page fault occurs. What is the very first action the operating system takes after detecting this page fault?

A The OS locates the page on disk and loads it into a free frame in physical memory.

B The OS terminates the program immediately due to illegal memory access.

C The OS swaps out a page from physical memory without checking which page to remove.

D The OS ignores the fault and lets the program continue running.

💡 Hint

Think about what the OS must do to make the requested data available to the program.

When a page fault occurs, the OS must bring the missing page from disk into physical memory before the program can continue. It does not terminate the program immediately or ignore the fault. Also, it chooses which page to swap out carefully, not randomly.

📋 Factual

intermediate

Which component is responsible for detecting a page fault?

In a computer system, which hardware or software component detects that a page fault has occurred?

A The Central Processing Unit (CPU) arithmetic logic unit

B The Memory Management Unit (MMU)

C The Disk Controller

D The Network Interface Card (NIC)

💡 Hint

Consider which part handles virtual to physical address translation.

The MMU translates virtual addresses to physical addresses. If the page is not in memory, the MMU triggers a page fault. The CPU ALU handles calculations, the disk controller manages disk operations, and the NIC handles network communication.

🔍 Analysis

advanced

Analyzing page fault handling sequence

Consider the following sequence of steps in handling a page fault. Which step correctly follows the previous one?

1. Page fault is detected by the MMU.
2. Operating system saves the state of the interrupted process.
3. ?
4. Page is loaded into physical memory.

A The OS clears the page table entry without loading the page.

B The OS sends an interrupt to the network controller.

C The OS immediately resumes the interrupted process without loading the page.

D The OS selects a victim page to swap out if no free frame is available.

💡 Hint

Think about what the OS must do if there is no free space in memory.

If no free frame is available, the OS must choose a page to remove (victim page) to free space before loading the requested page. Resuming the process without loading the page or clearing the page table entry would cause errors. Sending an interrupt to the network controller is unrelated.

❓ Comparison

advanced

Difference between minor and major page faults

Which statement correctly distinguishes a minor page fault from a major page fault?

A A minor page fault causes the program to terminate; a major page fault does not.

B A minor page fault requires disk access; a major page fault does not.

C A minor page fault occurs when the page is in memory but not mapped; a major page fault requires loading the page from disk.

D A minor page fault happens only in virtual memory systems without paging.

💡 Hint

Consider whether disk access is needed in each case.

A minor page fault happens when the page is already in physical memory but not yet mapped to the process's page table, so no disk access is needed. A major page fault requires loading the page from disk into memory. The other options are incorrect descriptions.

❓ Reasoning

expert

Why does excessive page faulting degrade system performance?

Excessive page faults can cause a system to slow down drastically, a condition known as thrashing. Why does this happen?

A Because the CPU spends most of its time waiting for pages to be loaded from disk instead of executing instructions.

B Because the system runs out of disk space and cannot save pages.

C Because the network bandwidth is saturated by page fault interrupts.

D Because the operating system disables multitasking during page faults.

💡 Hint

Think about the speed difference between memory and disk.

Page faults require loading data from disk, which is much slower than memory. If page faults happen too often, the CPU waits a lot for disk I/O, reducing overall performance. Disk space or network bandwidth are not the main causes, and multitasking is not disabled during page faults.

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