Virtual memory

1. BRAINWARE UNIVERSITY
S T U D E N T N A M E : - T W I N D A S
S T U D E N T C O D E : - B W U / B T D / 2 1 / 0 1 8
PA P E R N A M E : - P C C - C S D 3 0 1
PA P E R C O D E : - C O M P U T E R O R G A N I Z A T I O N
A N D A R C H I T E C H T U R E
T O P I C N A M E : - V I R T UA L M E M O R Y
C O U R S E : - B . T E C H I N C S E ( D S )
S E M E S T E R : - 3 R D
D E PA R T M E N T : - C O M P U T E R S C I E N C E A N D
E N G I N E E R I N G

2. CONTENTS
 Introduction
 Needs of virtual memory
 Importance of virtual memory
 Address space and memory space
 Page Replacement Algorithms
 FIFO
 LRU
 OPT
 Advantages of Virtual Memory
 Disadvantage of Virtual Memory

3. INTRODUCTION
Virtual memory as an alternate set of memory addresses.
Programs use these virtual addresses rather than real addresses to
store instructions and data.
When the program is actually executed, the virtual addresses are
converted into real memory addresses.

4. NEEDS 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. 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.

7. PAGE REPLACEMENT ALGORITHMS
It is the technique used by operating system to decide which memory
pages swap out .
It is also decided that is memory , how much frames to allocate
to each process.

8. FIRST IN FIRST OUT(FIFO)
Very simple to implement.
Oldest page is replaced for replacement.
Performance is not always good.
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9. OPTIMAL PAGE REPLACEMENT(OPT)
Looks for page in future.
Lowest page fault.
Difficult to implement.
1) Reference String:-7,0,1,2,0,3,0,4,2,3,0,3,2,1,2,0,1,7,0,1
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10. LEAST RECENTLY USED(LRU)
Page which has not been used for the longest time in main memory
the one which will be selected for replacement.
It is like optimal page-replacement algorithm looking
backwards in time.
1) Reference String:-7,0,1,2,0,3,0,4,2,3
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11. ADVANTAGES OF VIRTUAL MEMORY
o Only part of the program needs to be in memory for execution.
o Logical address space is much larger than physical address space.
o Need to allow pages to be swapped in and out.
o Virtual memory allows speed gain when only a particular segment of
the program is required for the execution of the program.
o This concept is very helpful in implementing
multiprogramming environment.

12. DISADVANTAGE OF VIRTUAL MEMORY
o Applications run slower if the system is using virtual memory.
o It takes more time to switch between applications.
o It reduces system stability.

13. REFERANCE
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