Virtual memory allows a computer to use more memory (called the address space) than is physically installed in the system (the memory space) by storing rarely used data on disk. When data is needed, it is moved back into memory. This allows for multiprogramming and for individual programs to be larger than physical memory. Common page replacement algorithms that determine what data to remove from memory and store on disk include first-in, first-out (FIFO), least recently used (LRU), and optimal (OPT) which removes the page not used for the longest time.

1. VIRTUAL MEMORY
Presented by: -
Aaina

2. CONTENTS
 virtual memory
 Needs of virtual memory
 Importance of virtual memory
 Advantage of virtual memory
 Disadvantage of virtual memory
 Address space & memory space
 Page replacement algorithms
 FIFO
 LRU
 OPT

3. VIRTUAL MEMORY
 Virtual memory is a common part of operating system on desktop
computers.
 The term virtual memory refers to something which appears to be present
but actually it is not.
 He virtual memory technique allows users to use more memory for a
program than the real memory of a computer.
Page frames
CPU
Logical
page + x
Mapper
Virtual
address
(1)
Not
present
(4)
Page
frames
(3) Page
frame + x
present
(5)
Secondary
storage

4. NEED OF VIRTUAL MEMORY
 Virtual memory is a imaginary memory which we are
assuming. If we have a material that exceed your
memory at that time we need to use the concept of
virtual memory.
 virtual memory is temporary memory which is used
along with the ram of the system.

5. IMPORTANCE OF VIRTUAL MEMORY
 When your computer runs out of physical memory it
writes what it needs to remember to the hard disc in a
swap file as virtual memory.
 If a computer running Windows requires more memory/
RAM then there is installed in the system to run a
program, etc, it uses a small section of the hard drive for
this purpose

6. ADVANTAGES OF VIRTUAL MEMORY
 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.

7. DISADVANTAGES OF VIRTUAL MEMORY
 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.

8. ADDRESS SPACE AND MEMORY SPACE
 Virtual memory is the address used by the programmer
and the set of such addresses is called address space.
 An address in main memory is called a physical address.
 The set of such locations in main memory is called the
memory space.
 Thus the memory space consist of the actual main
memory locations directly addressable for processing.

9. PAGE FRAME AND HIT RATIO
 Page Frame: - The main memory is divided into fixed
sized portions called page frame.
 The size of page frame is same as the size of page
because pages have to reside in page frames.
 Hit Ratio: - The ratio of the total number of hit divided
by the total CPU access to memory (i.e. hits plus misses)
is called hit ratio.
 Hit ratio=total number of hits
total number of hits + total number of mis

10. PAGE REPLACEMENT ALGORITHMS
 In a computer operating system that uses paging for virtual memory
management, page replacement algorithm decide which memory
pages to page out. When a page of memory need to be allocated.
 Paging happens when a page fault occurs.
 When a program starts execution, one or more pages are
transferred into main memory.
 FIFO (first in first out)
 LRU (Least Recently used)
 OPT (Optimal)

11. FIFO
hit hit hit
 First in first out is very easy to implement
 The fifo algorithm select the page for replacement that has been in memory
the longest time
 The page to be removed is easily determined because its identification
number is at the top of the fifo stack.
 It has the disadvantage that under certain circumstances pages are removed
and loaded from memory to frequently.
p2* p2*
p3
P3*
P5
p2
P3
P5
P4*
P3
P2*
p4
P3
p2*
p4
p5*
P2
p4
p5*
P2
p4
p5
P2
P1*
P5
p3*
p1
p2*
P3
P1
p2*
p3
1 2 3 4 5 6 7 8 9 10 11 12
time
P2 p3 p2 p1 p5 p2 p4 p5 p3 p2 p5 p2
page

12. 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.
p2* p2*
p3
P3*
P5
p2
P3*
P5
P2
P3
P5*
P2
P3
P5
p4*
p2*
P5
P4
P2
p5*
p4
P2
P5
P1*
p2*
P5
P1
P2
p3*
P1
P2
p3*
1 2 3 4 5 6 7 8 9 10 11 12
time
P2 p3 p2 p1 p5 p2 p4 p5 p3 p2 p5 p2
page
hit hit hit hit hit

13. 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.
p2 p2
p3
P2
P3
P5
P2
P3
P5
P2
P3
P5
P4
P3
p5
P4
P3
P5
P4
P3
p5
P2
P3
P5
P2
P3
P5
P2
p3
P1
P2
p3
1 2 3 4 5 6 7 8 9 10 11 12
P2 p3 p2 p1 p5 p2 p4 p5 p3 p2 p5 p2
hit hit hit hit hit
time
page
hit