Virtual memory allows a program to use more memory than the physical memory available by storing inactive memory pages on disk. It divides programs into pages of equal size and maps pages to frames using a page table. When a page is accessed that is not in memory, a page fault occurs and an algorithm like FIFO or LRU selects a frame to replace based on what page has been in memory longest or least recently used. This allows more programs to run simultaneously by swapping pages in and out of physical memory.

1. Virtual Memory in Windows
Operating System

2. Contents
• Virtual Memory
• Basic concepts
• Page Table
• Page replacement algorithms
• Advantages
• Disadvantages

3. Virtual Memory
• A technique using main memory as a “cache”
for secondary storage (disk).
• He virtual memory technique allows users to
use more memory for a program than the real
memory of a computer.

4. Virtual Memoryvirtualpages
physicalpages

5. Basic Concepts
• Address space(Logical & Physical)
• Pages
• Frames
• Mapping

6. Pages
• Continuous range of addresses
• Logical concept
• Equal size
• Present in any order

7. Frames
• Counterpart of pages
• Physical concept
• Reality
• Equal size

8. Mapping
• Mapping between pages and frames(CPU
generates page addresses and page addresses
are converted in frame addresses called
mapping.)
• Heart of Virtual Memory
• MMU performs mapping

9. Mapping

10. Example
• Lets suppose
• A program having 8 statements
• Processor divides them in to 8 pages
• 1 statement per page
• These pages can be present any where
• Not necessary in sequence

11. Example

12. Address Generation
• After dividing the program into pages
• Processor generates logical address
• Logical address is received by MMU
• Conversion is done to physical address
• Incase if page is not present in the memory
• Page fault occur

13. Address Generation

14. Page Table
• The operating system (OS) maps the virtual to any
physical page (associative placement of pages).
• A page table stored in memory, indexed by the
virtual page number, contains the virtual to physical
address translations.
• An entry contains the physical page number for that
virtual page if the page is currently in memory.

15. Page Table(cont…)
Each process
has such
register.
OS algorithm
determines the
physical page
number.

16. Page Replacement Algorithms
1) FIFO (first in first out)
2) LRU (Least Recently used)
3) OPT (Optimal)

17. 1) FIFO:-
• The FIFO algorithm select the page for
replacement that has been in memory the
longest time
•

18. 2) LRU:-
• The least recently used page replacement algorithm keeps
track page uses over a short period of time.
• The LRU algorithm can be implemented by associating a
counter with every page that is n main memory.

19. 3) OPT:-
• The optimal policy selects that page for replacement for
which the time to the next reference is longest.
• This algorithm result is fewest number of page faults.

20. Advantage :-
• Allows processes whose aggregate memory
requirement is greater than the amount of
physical memory, as infrequently used pages
can reside on the disk.
• Virtual memory allows speed gain when only
a particular segment of the program is
required for the execution of the program.
• This concept is very helpful in implementing
multiprogramming environment.

21. Disadvantage:-
• Applications run slower if the system is using
virtual memory.
• It Takes more time to switch between
applications.
• Less hard drive space for your use.
• It reduces system stability.