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.