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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(MAINMEMORY)(SECONDARYMEMORY)2CACHE 1ComputerprogramsarestoredhereUntil theyneed tobeexecuted
- 6.HARD DISK(MAINMEMORY)(SECONDARYMEMORY)2CACHE 1ComputerprogramsarestoredhereUntil theyneed tobeexecutedThen theyare movedto here
- 7. In modernoperating systems, before aprogram is loaded into main memory, it isdivided into chunks, called PAGES.
- 8. In modernoperating systems, before aprogram is loaded into main memory, it isdivided into chunks, called PAGES.Program 3
- 9. In modernoperating systems, before aprogram is loaded into main memory, it isdivided into chunks, called PAGES.Program 3Page 2Page 3Page 4Page 5Page 6Page 1Page 7
- 10. Each PAGEis loaded into memory locationscalled PAGE FRAMES.
- 11. Each PAGEis loaded into memory locationscalled PAGE FRAMES.200KavailableMAINMEMORY
- 12. Each PAGEis loaded into memory locationscalled PAGE FRAMES.MAINMEMORYPage Frame 1Page Frame 2Page Frame 3Page Frame 4Page Frame 5Page Frame 6Page Frame 7Page Frame 8Page Frame 9Page Frame 10200Kavailable
- 13. If thePAGES are the exactsame size as the PAGEFRAMES (and the samesize as the disk sectors),this scheme works verywell. MAINMEMORYPage Frame 1Page Frame 2Page Frame 3Page Frame 4Page Frame 5Page Frame 6Page Frame 7Page Frame 8Page Frame 9Page Frame 10200Kavailable
- 14. The MemoryManager prepares a program forexecution by doing the following:1. Determine the number of pages in the program2. Locate enough empty page frames in mainmemory3. Load all the program’s pages into themThe empty page frame does not have to becontagious.
- 15. Consider aprogram that 350 bytes, and thepage size is 100 bytes.
- 16. Consider aprogram that 350 bytes, and thepage size is 100 bytes.Program 4:350 bytes
- 17. Consider aprogram that 350 bytes, and thepage size is 100 bytes.Program 4:350 bytesPage 0
- 18.Program 4:350 bytesConsider a program that 350 bytes, and thepage size is 100 bytes.Page 0Page 1
- 19.Program 4:350 bytesConsider a program that 350 bytes, and thepage size is 100 bytes.Page 0Page 1Page 2
- 20. Consider aprogram that 350 bytes, and thepage size is 100 bytes.Page 3Program 4:350 bytesPage 0Page 1Page 2
- 21. Consider aprogram that 350 bytes, and thepage size is 100 bytes.MainMemoryPage 3Program 4:350 bytesPage 0Page 1Page 2
- 22. Consider aprogram that 350 bytes, and thepage size is 100 bytes.MainMemoryOperatingSystemPage 3Program 4:350 bytesPage 0Page 1Page 2
- 23. Consider aprogram that 350 bytes, and thepage size is 100 bytes.MainMemoryOperatingSystemPage 2Page 0Page 1Page 3Page 3Program 4:350 bytesPage 0Page 1Page 2
- 24. Consider aprogram that 350 bytes, and thepage size is 100 bytes.MainMemoryOperatingSystemPage 2Page 0Page 1Page 3A little bit ofinternalfragmentationPage 3Program 4:350 bytesPage 0Page 1Page 2
- 25. A usefulextension to the notion of PAGING isDEMAND PAGING. Demand Paging introduces the notion thatyou don’t have to load the whole programinto memory, just part of it. Because not all of the program needs to be inmemory at the same time.
- 26. This meansthat lots of programs can be runat the same time, and there is an illusion of asignificantly larger amount of memory thanwith regular paging.
- 27. To makethis work, pages have to be movedvery quickly from Secondary Storage to MainMemory and back again (this is called“swapping”).
- 28. This leadsto the concept of VIRTUALMEMORY The size of main memory appears muchlarger than the actual size, since manyprograms can appear to be fully loaded intomain memory at the same time, when inactual fact, only part of many programs areloaded into main memory.