Virtual Memory in Windows
- 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.
- 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
- 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.