Virtual memory is a technique that allows for more memory to be available to programs than the physical memory installed on the computer. When physical memory is full, infrequently used memory pages are written to secondary storage like a hard disk. This allows processes to access more memory than is physically available. Page replacement algorithms like FIFO, LRU, and OPT are used to determine which memory pages should be removed from physical memory and written to secondary storage when space is needed. Virtual memory provides advantages like allowing processes to exceed physical memory limits and improving performance when only parts of programs are actively being used. However, it can reduce performance and system stability when disk access is frequently required.

1. VIRTUAL MEMORY Present by: - Manpreet Kaur

2. CONTENTS virtual memory Needs of virtual memory Importance of virtual memory Memory mapping 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. Virtual address (1) Not present (4) Page frames (3) Page frame + x present (5) Secondary storage Page frames CPU Logical page + x Mapper

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. Memory Mapping Memory mapping is a common concept in POSIX and Windows systems. It enables a process to map disk files to its address space, treating them as memory blocks rather than files. the mapping consists of translating a disk location to a virtual

7. 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.

8. 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.

9. 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.

10. PAGE FRAME AND HIT RATIO Page Frame : - The main memory is divided into fixed sized portions called page frame. 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

11. 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. When a program starts execution, one or more pages are transferred into main memory. Paging happens when a page fault occurs.

12. FIFO 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 too frequently. hit hit hit time page 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 P2 p3 p2 p1 p5 p2 p4 p5 p3 p2 p5 p2

13. 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. time page hit hit hit hit hit 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 P2 p3 p2 p1 p5 p2 p4 p5 p3 p2 p5 p2

14. 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. hit hit hit hit hit time page hit 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