Page fault handling in Operating Systems - Step-by-Step Execution

Concept Flow - Page fault handling

Process accesses memory

↓

Is page in RAM?

Yes→Access memory successfully

No↓

Trigger page fault interrupt

↓

OS checks page table

↓

Find free frame or select victim page

↓

Load required page from disk to RAM

↓

Update page table

↓

Resume process execution

When a process tries to access a page not in RAM, a page fault occurs. The OS then loads the page from disk into RAM and updates the page table before resuming the process.

Execution Sample

Operating Systems

1. Process tries to read address X
2. Check if page containing X is in RAM
3. If not, trigger page fault
4. OS loads page from disk
5. Update page table
6. Resume process

This sequence shows how the OS handles a page fault step-by-step.

Analysis Table

Step	Action	Condition/Check	Result/Next Step
1	Process accesses memory address X	Is page in RAM?	No, page fault triggered
2	OS receives page fault interrupt	Check page table for page location	Page on disk, not in RAM
3	OS finds free frame or selects victim page	Is free frame available?	Yes, free frame found
4	OS loads page from disk to RAM frame	Page loaded successfully?	Yes
5	OS updates page table entry	Page table updated	Ready to resume process
6	Process resumes execution	Access memory again	Page now in RAM, access successful
7	Process continues normal execution	No further page faults	Execution continues

💡 Process resumes after page is loaded into RAM and page table updated

State Tracker

Variable	Start	After Step 2	After Step 4	After Step 5	Final
Page in RAM	No	No	Yes (loaded)	Yes	Yes
Page table entry	Invalid	Invalid	Invalid	Valid (updated)	Valid
Free frame available	Unknown	Checked - Yes	Used	Used	Used

Key Insights - 3 Insights

Why does the process stop and wait during a page fault?

Because the required page is not in RAM, the OS must load it from disk which takes time. This is shown in execution_table steps 2 to 5 where the OS handles loading before resuming.

What happens if no free frame is available in RAM?

The OS selects a victim page to remove (not shown in this simple trace), frees that frame, then loads the needed page. This is part of step 3 in the execution_table.

How does the OS know where to find the page on disk?

The OS uses the page table to find the disk location of the page, as shown in step 2 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the page actually loaded into RAM?

A Step 2

B Step 4

C Step 5

D Step 6

💡 Hint

Check the 'Action' and 'Result' columns in step 4 where the page is loaded from disk to RAM.

According to variable_tracker, when does the page table entry become valid?

A After Step 5

B After Step 4

C After Step 2

D At Start

💡 Hint

Look at the 'Page table entry' row and see when it changes from 'Invalid' to 'Valid (updated)'.

If there was no free frame available, which step in the execution_table would change?

A Step 5

B Step 4

C Step 3

D Step 6

💡 Hint

Step 3 checks for free frame availability; if none, OS must select a victim page here.

Concept Snapshot

Page fault handling:
- Occurs when process accesses page not in RAM
- OS triggers interrupt, checks page table
- Loads page from disk to RAM
- Updates page table
- Resumes process execution
- May need to free RAM frame if none available

Full Transcript

Page fault handling happens when a process tries to access memory not currently in RAM. The operating system detects this and pauses the process. It checks the page table to find the page on disk, then loads it into RAM. The page table is updated to reflect the new location. Once done, the process resumes and accesses the page successfully. If RAM is full, the OS selects a page to remove before loading the new one. This ensures processes can use more memory than physically available by swapping pages between RAM and disk.