Operating System Dr. Hamada Lecture 4: Virtual Memory

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Questions and Answers

What is the purpose of modifying the dirty bit in the page frame?

To reduce the overhead of page transfers (correct)

To keep track of which pages have been accessed

To increase the overhead of page transfers

To ensure that all pages are written to disk

Which page replacement algorithm is considered the most optimal, but impractical?

Random Page Replacement

First-In-First-Out (FIFO)

Optimal Page (OPT) Algorithm (correct)

Least Recently Used (LRU)

What is the number of page faults for the Optimal Page (OPT) Algorithm given the sequence of page references: 6, 7, 8, 9, 6, 7, 1, 6, 7, 8, 9, 1, 7, 9, 6?

8 (correct)

What is the number of page faults for the First-In-First-Out (FIFO) Algorithm given the sequence of page references: 6, 7, 8, 9, 6, 7, 1, 6, 7, 8, 9, 1?

9 Signup and view all the answers

What is the number of page faults for the Least Recently Used (LRU) Algorithm given the sequence of page references: 6, 7, 8, 9, 6, 7, 1, 6, 7, 8, 9, 1, 7, 9, 6?

12 Signup and view all the answers

What is the purpose of the page and frame tables in the page replacement process?

To keep track of which pages are currently in memory Signup and view all the answers

What is the purpose of the segment table in segmentation?

To map logical addresses to physical addresses Signup and view all the answers

What does the limit field in a segment table entry represent?

The length of the segment in memory Signup and view all the answers

Which register is used to point to the location of the segment table in memory?

Segment-table base register (STBR) Signup and view all the answers

What is the main problem associated with segmentation as a memory management scheme?

External fragmentation Signup and view all the answers

In the logical address for a segment, what does the segment-number represent?

The index of the segment in the segment table Signup and view all the answers

What is the first step in the paging memory management scheme?

Divide physical memory into frames Signup and view all the answers

What is the purpose of the valid-invalid bit associated with each page table entry?

To indicate whether the page is currently in memory or not Signup and view all the answers

What happens when the MMU encounters a page table entry with the valid-invalid bit set to 'i' during address translation?

The MMU will generate a page fault Signup and view all the answers

What is the purpose of the lazy swapper strategy?

To only swap pages into memory when they are needed Signup and view all the answers

What is the key difference between global and local page replacement strategies?

Global replacement allows one process to take a frame from another, while local replacement does not Signup and view all the answers

What are the main steps the operating system takes when handling a page fault?

<ol> <li>Find a free frame, 2. Swap the page into the frame, 3. Update the page table, 4. Restart the faulting instruction</li> </ol> Signup and view all the answers

What is the goal of page replacement algorithms?

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

What is the purpose of the page offset in the address translation scheme?

To combine with the base address to define the physical memory address Signup and view all the answers

If a process has a size of 72,766 bytes and the page size is 2,048 bytes, how many pages will be required to load the process?

36 pages Signup and view all the answers

What is the purpose of the valid-invalid bit associated with each entry in the page table?

To identify if the page is part of the process' logical address space Signup and view all the answers

Which type of fragmentation is associated with paging?

Internal fragmentation Signup and view all the answers

In the provided example, what is the amount of internal fragmentation for the process with a size of 72,766 bytes and a page size of 2,048 bytes?

962 bytes Signup and view all the answers

What is the purpose of the protection bit associated with each frame in memory protection?

To indicate if the page is read-only or read-write Signup and view all the answers

Study Notes

Page Replacement Algorithm

Victim frame selection is made using page replacement algorithms to reduce overhead of page transfers
Algorithms: FIFO, OPT, LRU

FIFO (First-In-First-Out) Algorithm

Replaces the oldest page that has been present in the main memory for the longest time
Number of Page Faults = 9

OPT (Optimal Page) Algorithm

Replaces page that is farthest to come in the upcoming sequence
Not practical because it is impossible to predict upcoming page references
Number of Page Faults = 8

LRU (Least Recently Used) Algorithm

Replaces page that has not been used in the most amount of time
Associate time of last use with each page
Number of Page Faults = 12

Memory Management Schemes

Segmentation: a memory-management scheme that supports user view of memory
Segmentation allows physical address space of a process to be non-contiguous and not equally divided
Suffers from external fragmentation problem

Segmentation Architecture

Segment table has a separate entry for each segment
Logical address of a segment consists of a two-tuple: base and limit
Used to map two-dimensional user-defined addresses into one-dimensional physical addresses

Paging

Paging steps: divide physical memory into fixed-sized blocks called frames, divide logical memory into blocks of same size called pages
Swapping: pager guesses which pages will be used before swapping out again
Page table entry has a valid-invalid bit associated with it

Page Fault

Occurs when a program tries to access a page that is not in physical memory
Steps to resolve page fault: find free frame, swap page into frame, reset table to indicate page now in memory, set validation bit = v, restart the instruction that caused the page fault

Page Replacement

Finds a page in memory, but not really in use, and pages it out
Goal: minimize number of page faults
Steps: find location of desired page on disk, find a free frame (or select a victim frame if no free frame), set up a page table to translate logical to physical addresses

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Description

This quiz covers memory-management schemes, specifically focusing on segmentation in the context of virtual memory. Segmentation allows for a user view of memory by organizing a program into logical segments such as main program, functions, and variables. Learn about how segmentation permits non-contiguous physical address spaces for processes.

Operating System Dr. Hamada Lecture 4: Virtual Memory - Segmentation

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