Demand paging in Operating Systems - Step-by-Step Execution

Concept Flow - Demand paging

Process starts

↓

Page needed?

No↓

Use page in RAM

↓

Continue execution

↓

Process ends

Yes↓

Page fault occurs

↓

Load page from disk to RAM

↓

Update page table

↓

Resume process execution

Demand paging loads pages into RAM only when the process needs them, handling page faults by loading missing pages from disk.

Execution Sample

Operating Systems

Process requests page 5
Is page 5 in RAM?
No -> Page fault
Load page 5 from disk to RAM
Update page table
Resume process

This simulates a process requesting a page not in RAM, causing a page fault and loading the page on demand.

Analysis Table

Step	Action	Page Requested	Page in RAM?	Page Fault?	Result
1	Process requests page	5	No	Yes	Page fault occurs
2	Load page from disk	5	No	Yes	Page 5 loaded into RAM
3	Update page table	5	Yes	No	Page table updated
4	Resume execution	5	Yes	No	Process continues using page 5
5	Process requests page	3	Yes	No	Page 3 used directly
6	Process ends	-	-	-	Execution complete

💡 Process ends after all needed pages are accessed

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	Final
Page Requested	-	5	5	5	5	3	-
Page in RAM	No pages	No	Yes (page 5)	Yes (page 5)	Yes (page 5)	Yes (pages 3,5)	Yes (pages 3,5)
Page Fault	No	Yes	Yes	No	No	No	No

Key Insights - 3 Insights

Why does a page fault happen even though the process requested a page?

A page fault happens because the requested page is not currently loaded in RAM, as shown in execution_table step 1 where 'Page in RAM?' is 'No'.

What happens after the page fault occurs?

After the page fault, the system loads the page from disk into RAM and updates the page table, as seen in steps 2 and 3 of the execution_table.

Does the process always cause a page fault when requesting a page?

No, if the page is already in RAM, no page fault occurs. For example, in step 5, page 3 is in RAM and used directly without fault.

Visual Quiz - 3 Questions

Look at the execution_table at step 1. What is the status of the requested page in RAM?

A The page is not in RAM

B The page is already in RAM

C The page is being removed from RAM

D The page is corrupted

💡 Hint

Check the 'Page in RAM?' column at step 1 in the execution_table

At which step does the page table get updated to include the new page?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for the action 'Update page table' in the execution_table

If the process requests a page already in RAM, what changes in the execution_table?

A A page fault occurs

B The page is loaded from disk

C The process uses the page directly without fault

D The page table is updated

💡 Hint

Compare step 5 where 'Page in RAM?' is 'Yes' and 'Page Fault?' is 'No'

Concept Snapshot

Demand paging loads pages only when needed.
If a page is not in RAM, a page fault occurs.
The OS loads the page from disk into RAM.
Page table is updated to reflect the new page.
Process resumes using the loaded page.
This saves memory and speeds up execution.

Full Transcript

Demand paging is a memory management technique where pages are loaded into RAM only when the process requests them. When a process tries to access a page not in RAM, a page fault occurs. The operating system then loads the required page from disk into RAM and updates the page table. After this, the process resumes execution using the newly loaded page. If the page is already in RAM, the process uses it directly without causing a page fault. This method helps save memory and improves efficiency by loading only necessary pages.

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