Chapter_9

Chapter_9 - Chapter 9: Virtual Memory s Background s Demand...

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9.1 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Chapter 9: Virtual Memory Chapter 9: Virtual Memory Background Demand Paging Process Creation Page Replacement Allocation of Frames Thrashing Demand Segmentation Operating System Examples
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9.2 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Background 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
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9.3 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Virtual Memory That is Larger Than Physical Memory Virtual Memory That is Larger Than Physical Memory
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9.4 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Virtual-address Space Virtual-address Space
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9.5 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Shared Library Using Virtual Memory Shared Library Using Virtual Memory
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9.6 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Demand Paging Demand Paging Bring a page into memory only when it is needed Less I/O needed Less memory needed Faster response More users, processes Page is needed reference to it invalid reference abort not-in-memory bring to memory
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9.7 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Transfer of a Paged Memory to Contiguous Disk Space Transfer of a Paged Memory to Contiguous Disk Space
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9.8 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Valid-Invalid Bit Valid-Invalid Bit With each page table entry a valid–invalid bit is associated 1 legal and in-memory, 0 (legal not-in-memory) or (not legal) 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 page fault 1 1 1 1 0 0 0 Frame # valid-invalid bit page table
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9.9 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Page Table When Some Pages Are Not in Main Memory Page Table When Some Pages Are Not in Main Memory
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9.10 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Page Table When Some Pages Are Not in Main Memory Page Table When Some Pages Are Not in Main Memory
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9.11 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Steps in Handling Page Fault Steps in Handling Page Fault If there is ever a reference to a page, first reference will trap to OS page fault OS looks at another table (usually kept in PCB) to decide: Invalid reference abort. Valid but
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Chapter_9 - Chapter 9: Virtual Memory s Background s Demand...

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