Page fault handling in Operating Systems - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the cause of a page fault.

Operating Systems

if (error_code & [1]) {
    // Page fault caused by a write operation
}

Drag options to blanks, or click blank then click option'

A0x1

B0x4

C0x2

D0x8

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the bit for read and write operations.

Using the wrong hexadecimal value for the error code bit.

The error code bit 1 (0x2) indicates if the page fault was caused by a write operation.

2fill in blank

medium

Complete the code to check if the page fault was caused by a protection violation on a present page.

Operating Systems

if (error_code & [1]) {
    // Page present: protection fault
}

Drag options to blanks, or click blank then click option'

A0x1

B0x2

C0x4

D0x8

Attempts:

3 left

💡 Hint

Common Mistakes

Using the bit for write fault instead of page presence.

Misinterpreting the error code bits.

The error code bit 0 (0x1) indicates if the page was present (protection violation).

3fill in blank

hard

Fix the error in the code to correctly handle a page fault caused by user mode access.

Operating Systems

if (error_code & [1]) {
    // Fault caused in user mode
}

Drag options to blanks, or click blank then click option'

A0x4

B0x1

C0x2

D0x8

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing user mode bit with write or present bits.

Using incorrect hexadecimal values.

The error code bit 2 (0x4) indicates if the fault happened in user mode.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps page numbers to their status if the page is present and accessed in user mode.

Operating Systems

page_status = {page_num: status for page_num, status in pages.items() if (status & [1]) and (status & [2])}

Drag options to blanks, or click blank then click option'

A0x1

B0x2

C0x4

D0x8

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up bits for write and user mode.

Using bits that do not represent presence or user mode.

Bit 0 (0x1) checks if the page is present, and bit 2 (0x4) checks if it was accessed in user mode.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps virtual addresses to page numbers for pages that are present, writable, and accessed in user mode.

Operating Systems

virtual_to_page = {va: pa for va, (pa, flags) in memory_map.items() if (flags & [1]) and (flags & [2]) and (flags & [3])}

Drag options to blanks, or click blank then click option'

A0x1

B0x2

C0x4

D0x8

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing write bit with other bits.

Forgetting to check all three conditions.

Bit 0 (0x1) means page present, bit 1 (0x2) means writable, and bit 2 (0x4) means accessed in user mode.

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