Operating Systems: Page Replacement Quiz

Questions and Answers

What is the primary purpose of page replacement in an operating system?

To load pages from the disk into memory when there are no free frames (correct)

To terminate user processes immediately to free memory

To swap out all active processes to make space

To increase the level of multiprogramming without page faults

What happens if there is a page fault and no free frames are available?

The operating system kills all running processes

A page replacement algorithm is invoked to select a victim frame (correct)

The user is notified of memory issues

The hardware automatically allocates additional frames

Which of the following steps is NOT part of the modified page fault service routine for page replacement?

Find a free frame or select a victim frame

Find the location of the desired page on the disk

Change the page and frame tables after reading the page

Use a hash table to manage memory (correct)

What is the function of a modify bit in relation to page replacement?

To show whether a page has been modified since it was loaded into memory Signup and view all the answers

Why is it not ideal for the operating system to terminate a user process during a page fault?

It reduces system throughput and user experience Signup and view all the answers

What is one potential outcome if the operating system swaps out a process during page replacement?

Decreased level of multiprogramming Signup and view all the answers

What is the first step taken when a page fault occurs?

Check if the page is genuine or an illegal access Signup and view all the answers

How does the operating system indicate that a page is no longer in memory after replacement?

By updating the page table to reflect the change Signup and view all the answers

What happens if the modify bit is set when selecting a page for replacement?

The page must be written to disk before replacement. Signup and view all the answers

How is the effectiveness of a page replacement algorithm generally evaluated?

By running it on a reference string and counting page faults. Signup and view all the answers

What is the primary goal when selecting a page replacement algorithm?

To minimize the number of page faults. Signup and view all the answers

In a FIFO page replacement algorithm, which page is selected for replacement?

The oldest page in memory. Signup and view all the answers

What can be concluded about the relationship between the number of free frames available and the occurrence of page faults?

More free frames lead to fewer page faults. Signup and view all the answers

What does the modify bit indicate about a page?

The page has been modified since it was loaded into memory. Signup and view all the answers

What structure can be used to implement the FIFO page replacement algorithm?

A FIFO queue. Signup and view all the answers

What happens to the page when there are no free frames and a page fault occurs?

A page from memory must be replaced. Signup and view all the answers

Study Notes

Page Fault Handling and Replacement

A page fault occurs when a process attempts to access memory that is not currently in physical memory.
The operating system verifies that the page request is legitimate and checks the location on the disk.
If all memory frames are occupied, several options exist for handling the page fault:
- Terminating the user process is not preferred, as it undermines system efficiency and user experience.
- Swapping out a process reduces multiprogramming, which decreases overall efficiency.

Page Replacement Strategy

Page replacement allows swapping out an existing page in memory to load the desired page.
The service routine for page faults is modified to implement page replacement when no free frames are available:
- Locate the desired page on disk.
- Check for a free frame:
  - Use it if available.
  - If not, apply a page replacement algorithm to select a victim frame.
- Read the desired page into the now-available frame and update the page/frame tables.
- Restart the user process.

Modify Bit (Dirty Bit)

Each page/frame can have a modify bit indicating if it has been changed since it was loaded into memory.
This bit helps reduce overhead during page replacement:
- If the modify bit is set, the page must be written back to disk before replacement.
- If not set, the page can be discarded without writing to disk, optimizing efficiency.

Page Replacement Algorithms

Goal is to minimize the page-fault rate using various algorithms that evaluate performance based on memory reference patterns.
More available frames typically lead to fewer page faults.

FIFO Page Replacement

The First-In-First-Out (FIFO) algorithm is the simplest method for page replacement.
It tracks the order of pages in memory without needing precise timestamps.
The oldest page is replaced when a new page needs to be loaded:
- A FIFO queue structure is maintained, where new pages are added at the tail and pages are removed from the head.

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Description

Test your knowledge about page replacement in operating systems. This quiz covers key concepts including page faults, free frames, the modified page fault service routine, and the implications of terminating user processes during faults. Dive into the mechanics that keep systems running smoothly.