Demand paging in Operating Systems - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is demand paging in operating systems?

Demand paging is a memory management technique where pages of data are loaded into memory only when they are needed, rather than loading the entire program at once.

beginner

How does demand paging improve system efficiency?

It reduces the amount of memory used by loading only necessary pages, which saves memory and speeds up program startup.

intermediate

What happens when a program tries to access a page not currently in memory during demand paging?

A page fault occurs, causing the operating system to load the required page from disk into memory before continuing execution.

intermediate

Explain the role of the page fault in demand paging.

A page fault signals that the requested page is not in memory. The OS then pauses the program, loads the page from disk, updates the page table, and resumes the program.

advanced

What is the difference between demand paging and pre-paging?

Demand paging loads pages only when needed, while pre-paging loads pages in advance based on predicted needs to reduce page faults.

What triggers a page to be loaded into memory in demand paging?

A When the operating system boots

B When the program starts

C When the page is accessed by the program

D When the user requests it

In demand paging, pages are loaded only when the program tries to access them.

What is a page fault?

A A signal that a page is not in memory and must be loaded

B An error in the program code

C A hardware failure

D A type of virus

A page fault occurs when a program accesses a page not currently in memory, prompting the OS to load it.

Which of the following is a benefit of demand paging?

A Avoiding page faults completely

B Loading all pages at once

C Using more memory

D Faster program startup

Demand paging loads pages only when needed, which helps programs start faster.

What does the operating system do after a page fault occurs?

A Terminates the program

B Loads the required page from disk into memory

C Ignores the fault

D Restarts the computer

The OS loads the missing page from disk to memory to continue program execution.

How does demand paging affect memory usage?

A Decreases memory usage by loading only needed pages

B Increases memory usage by loading all pages

C Has no effect on memory usage

D Uses memory randomly

Demand paging saves memory by loading only the pages that the program actually uses.

Describe how demand paging works and why it is useful in operating systems.

Think about what happens when a program tries to use a page not in memory.

Explain the difference between demand paging and pre-paging.

Consider when pages are loaded in each method.

Practice

1. What is the main purpose of demand paging in operating systems?

easy

A. To load pages into memory only when they are needed

B. To load all program pages into memory before execution

C. To increase the size of physical memory

D. To prevent any page faults during program execution

Solution

1. Step 1: Understand demand paging concept

   Demand paging means loading pages only when the program tries to use them, not before.

2. Step 2: Compare options with concept

   To load pages into memory only when they are needed matches this idea, while others describe different or incorrect behaviors.

3. Final Answer:

   To load pages into memory only when they are needed -> Option A

4. Quick Check:

   Demand paging = load pages on demand [OK]

Hint: Demand paging loads pages only when accessed [OK]

Common Mistakes:

• Thinking all pages load before execution
• Confusing demand paging with increasing memory size
• Assuming demand paging prevents all page faults

2. Which of the following is the correct description of a page fault in demand paging?

easy

A. When all pages are loaded into memory at once

B. When the CPU finishes executing all instructions

C. When the operating system shuts down

D. When a program tries to access a page not currently in memory

Solution

1. Step 1: Define page fault in demand paging

   A page fault occurs when a program accesses a page that is not in physical memory.

2. Step 2: Match definition with options

   When a program tries to access a page not currently in memory correctly describes this event; others describe unrelated events.

3. Final Answer:

   When a program tries to access a page not currently in memory -> Option D

4. Quick Check:

   Page fault = access missing page [OK]

Hint: Page fault happens on missing page access [OK]

Common Mistakes:

• Confusing page fault with program completion
• Thinking page fault happens during shutdown
• Believing page fault means all pages loaded

3. Consider a system using demand paging. If a program accesses pages in this order: 2, 4, 2, 5, 4, which pages cause page faults assuming none are initially in memory?

medium

A. No page faults occur because pages are preloaded

B. Pages 2, 4, and 5 cause page faults

C. Pages 2 and 5 cause page faults, but not 4

D. Only page 2 causes a page fault

Solution

1. Step 1: Identify initial memory state and page accesses

   No pages are in memory initially. Access order is 2, 4, 2, 5, 4.

2. Step 2: Determine which accesses cause page faults

   First access to page 2 causes a fault (not in memory). Then page 4 causes a fault. The second access to page 2 is in memory, no fault. Access to page 5 causes a fault. Last access to page 4 is in memory, no fault.

3. Final Answer:

   Pages 2, 4, and 5 cause page faults -> Option B

4. Quick Check:

   First access to new pages = page fault [OK]

Hint: First access to each new page causes fault [OK]

Common Mistakes:

• Assuming repeated pages cause faults
• Thinking pages preload automatically
• Ignoring initial empty memory state

4. A system using demand paging is experiencing frequent page faults. Which of the following could be a likely cause?

medium

A. The program accesses many pages not yet loaded

B. The CPU clock speed is too high

C. The hard disk is disconnected

D. The program is very small and fits in memory

Solution

1. Step 1: Understand causes of frequent page faults

   Frequent page faults happen when a program accesses many pages not currently in memory.

2. Step 2: Evaluate options for likely cause

   The program accesses many pages not yet loaded matches this cause. CPU speed or disk disconnection do not directly cause page faults. A small program fitting in memory would cause fewer faults.

3. Final Answer:

   The program accesses many pages not yet loaded -> Option A

4. Quick Check:

   Many new page accesses = frequent faults [OK]

Hint: Many new pages accessed cause faults [OK]

Common Mistakes:

• Blaming CPU speed for page faults
• Confusing hardware issues with paging faults
• Assuming small programs cause many faults

5. In a demand paging system, if the page replacement algorithm always removes the page that was least recently used, what is the main benefit of this approach?

hard

A. It increases the total number of pages in memory

B. It prevents any page faults from occurring

C. It reduces the number of page faults by keeping frequently used pages in memory

D. It loads all pages into memory before execution

Solution

1. Step 1: Understand least recently used (LRU) page replacement

   LRU removes the page not used for the longest time, keeping frequently used pages in memory.

2. Step 2: Identify benefit of LRU in demand paging

   This reduces page faults by avoiding removal of pages likely to be used soon.

3. Final Answer:

   It reduces the number of page faults by keeping frequently used pages in memory -> Option C

4. Quick Check:

   LRU keeps frequent pages, reduces faults [OK]

Hint: LRU keeps frequent pages, lowers faults [OK]

Common Mistakes:

• Thinking LRU increases memory size
• Believing LRU prevents all faults
• Confusing LRU with preloading all pages