18_ Virtual_Memory_I

18_ Virtual_Memory_I - CSC 4103 Operating Systems Fall 2009...

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CSC 4103 - Operating Systems Fall 2009 Tevfik Ko ! ar Louisiana State University October 29 th , 2009 Lecture - XVIII Virtual Memory 2 Roadmap Virtual Memory Demand Paging Page Faults Page Replacement Page Replacement Algorithms (FIFO, LRU, Optimal etc) Performance of Demand Paging Background Virtual memory – separation of user logical memory from physical memory. Only part of the program needs to be in memory for execution. Logical address space can therefore be much larger than physical address space. Allows address spaces to be shared by several processes. Allows for more efficient process creation. Virtual memory can be implemented via: Demand paging Demand segmentation Demand Paging Bring a page into memory only when it is needed Less I/O needed Less memory needed Faster response More users Page is needed ! reference to it invalid reference ! abort – not-in-memory ! bring to memory Valid-Invalid Bit With each page table entry a valid–invalid bit is associated (1 ! in-memory and legal, 0 ! not-in-memory or invalid) Initially valid–invalid bit is set to 0 on all entries Example of a page table snapshot: During address translation, if valid–invalid bit in page table entry is 0 !
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