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Operating Systems: Virtual Memory
The document discusses how modern operating systems use paging to manage memory. Programs are divided into chunks called pages which are loaded into frames of main memory as needed. This allows more programs to reside partially in memory than would fit entirely, through demand paging which loads only currently used parts of a program. This creates the illusion of larger memory usage via virtual memory.
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Operating Systems: Virtual Memory
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- 5. HARD DISK (MAIN MEMORY) (SECONDARY MEMORY) 2 CACHE 1 Computer programs arestored here Until they need to be executed
- 6. HARD DISK (MAIN MEMORY) (SECONDARY MEMORY) 2 CACHE 1 Computer programs arestored here Until they need to be executedThen they are moved to here
- 7. In modernoperating systems, before a program is loaded into main memory, it is divided into chunks, called PAGES.
- 8. In modernoperating systems, before a program is loaded into main memory, it is divided into chunks, called PAGES. Program 3
- 9. In modernoperating systems, before a program is loaded into main memory, it is divided into chunks, called PAGES. Program 3 Page 2 Page 3 Page 4 Page 5 Page 6 Page 1 Page 7
- 10. Each PAGEis loaded into memory locations called PAGE FRAMES.
- 11. Each PAGEis loaded into memory locations called PAGE FRAMES. 200K available MAIN MEMORY
- 12. Each PAGEis loaded into memory locations called PAGE FRAMES. MAIN MEMORY Page Frame 1 Page Frame 2 Page Frame 3 Page Frame 4 Page Frame 5 Page Frame 6 Page Frame 7 Page Frame 8 Page Frame 9 Page Frame 10 200K available
- 13. If thePAGES are the exact same size as the PAGE FRAMES (and the same size as the disk sectors), this scheme works very well. MAIN MEMORY Page Frame 1 Page Frame 2 Page Frame 3 Page Frame 4 Page Frame 5 Page Frame 6 Page Frame 7 Page Frame 8 Page Frame 9 Page Frame 10 200K available
- 14. The MemoryManager prepares a program for execution by doing the following: 1. Determine the number of pages in the program 2. Locate enough empty page frames in main memory 3. Load all the program’s pages into them The empty page frame does not have to be contagious.
- 15. Consider aprogram that 350 bytes, and the page size is 100 bytes.
- 16. Consider aprogram that 350 bytes, and the page size is 100 bytes. Program 4: 350 bytes
- 17. Consider aprogram that 350 bytes, and the page size is 100 bytes. Program 4: 350 bytes Page 0
- 18. Program 4: 350 bytes Consider a program that 350 bytes, and the page size is 100 bytes. Page 0 Page 1
- 19. Program 4: 350 bytes Consider a program that 350 bytes, and the page size is 100 bytes. Page 0 Page 1 Page 2
- 20. Consider aprogram that 350 bytes, and the page size is 100 bytes. Page 3 Program 4: 350 bytes Page 0 Page 1 Page 2
- 21. Consider aprogram that 350 bytes, and the page size is 100 bytes. Main Memory Page 3 Program 4: 350 bytes Page 0 Page 1 Page 2
- 22. Consider aprogram that 350 bytes, and the page size is 100 bytes. Main Memory Operating System Page 3 Program 4: 350 bytes Page 0 Page 1 Page 2
- 23. Consider aprogram that 350 bytes, and the page size is 100 bytes. Main Memory Operating System Page 2 Page 0 Page 1 Page 3 Page 3 Program 4: 350 bytes Page 0 Page 1 Page 2
- 24. Consider aprogram that 350 bytes, and the page size is 100 bytes. Main Memory Operating System Page 2 Page 0 Page 1 Page 3 A little bit of internal fragmentation Page 3 Program 4: 350 bytes Page 0 Page 1 Page 2
- 25. A usefulextension to the notion of PAGING is DEMAND PAGING. Demand Paging introduces the notion that you don’t have to load the whole program into memory, just part of it. Because not all of the program needs to be in memory at the same time.
- 26. This meansthat lots of programs can be run at the same time, and there is an illusion of a significantly larger amount of memory than with regular paging.
- 27. To makethis work, pages have to be moved very quickly from Secondary Storage to Main Memory and back again (this is called “swapping”).
- 28. This leadsto the concept of VIRTUAL MEMORY The size of main memory appears much larger than the actual size, since many programs can appear to be fully loaded into main memory at the same time, when in actual fact, only part of many programs are loaded into main memory.