Virtual memory
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Virtual memory allows programs to access more memory than the physical memory available on a computer by storing unused portions of memory on disk. It was first developed in 1959-1962 at the University of Manchester. Key aspects of virtual memory include: dividing memory into pages that can be swapped between disk and physical memory as needed, using page tables to map virtual to physical addresses, and page replacement algorithms like LRU to determine which pages to swap out. Virtual memory provides benefits like running more programs simultaneously but can reduce performance due to disk access times.
Virtual memory
- 1. Virtual Memory Muhammad Umer Farooq 12024119-015@uog.edu.pk 0331-0331-666
- 2. What is Virtual Memory 1 • The term virtual memory refers to something which appears to be present but actually it is not. • Virtual memory allows users to use more memory for a program than the real memory of a computer.
- 3. History 2 • Virtual Memory was developed in approximately 1959 – 1962, at the University of Manchester for the Atlas Computer, completed in 1962. • In 1961, Burroughs released the B5000, the first commercial computer with virtual memory.
- 4. Uses of Virtual Memory 3 • The virtual addresses are used rather than real addresses to store instructions and data. • It is temporary memory which is used along with ram of the system. • When the program is actually executed, the virtual addresses are converted into real memory addresses again.
- 5. Address Space & Memory Space 4 • Virtual memory is the address used by the programmer and the set of such addresses is called address space. • The set of actual main memory locations in main memory is called the memory space.
- 6. Why is it needed…? 5 • Before the development of the virtual memory technique, programmers have to manage directly two-level storage such as main memory or ram and secondary memory . • Without virtual memory, It is too much expensive to execute / manage the program as a whole. • Enlarge the address space, the set of addresses a program can utilize.
- 7. Objectives… 6 • When a computer is executing many programs at the same time, Virtual memory make the computer to share memory efficiently. • Eliminate a restriction that a computer works in memory which is small and be limited. • When many programs are running at the same time, by distributing each suitable memory area to each program, VM protect the programs to interfere each other in each memory area.
- 8. Paging… 7 • To facilitate copying virtual memory into real memory, the operating system divides virtual memory into pages, each of which contains a fixed number of addresses. • Each page is stored on a disk until it is needed. • When the page is needed, the operating system copies it from disk to main memory, translating the virtual addresses into real addresses.
- 9. Paging……(cont’d) 8 • The operating system copies a certain number of pages from your storage device to main memory. • When a program needs a page that is not in maim memory, the operating system copies the required page into memory and copies another page back to the disk.
- 10. Virtual Memory (Paging) 9 Page table Address Space Page table Address Space Physical Memory
- 11. Page fault 10 • An interrupt to the software raised by the hardware when a program accesses a page that is not mapped in physical memory. • It occurs when a program accesses a memory location in its memory and the page corresponding to that memory is not loaded. • when a program accesses a memory location in its memory and the program does not have rights to access the page corresponding to that memory.
- 12. Paging replacement algorithms 11 • OPT(MIN) : eliminate the page that be not expected to be used. • FIFO(first input/first output) : rather than choosing the victim page at random, the oldest page is the first to be removed. • LRU(Least Recently used) : move out the page that is the least rarely used. • LFU(Least Frequently used) : move out the page that is not used often in the past.
- 13. Advantages… 12 • It has become so common because it provides a big benefit for users at a very low cost. • program is no longer constrained by the amount of physical memory. • user would be able to write programs for an extremely large virtual address space. • more programs could be run at the same time • increase CPU utilization and throughput. • less I/O would be needed to load or swap each user program
- 14. Disadvantages 13 • Applications run slower if the system is using virtual memory. • It take more time to switch between applications. • Less hard drive space for your use. • It reduces system stability.