ch09 - Chapter 9: Virtual Memory Chapter Chapter 9: Virtual...

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Chapter 9: Virtual Memory Chapter 9: Virtual Memory
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9.2 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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9.3 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Background Background Ch. 8 requires the instructions being executed must be in physical memory. Dynamic loading eases the restriction, but need special care by the programmer Not good, limiting the size of a program 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. Why? Virtual memory can be implemented via: Demand paging Demand segmentation
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9.4 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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9.5 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Virtual-address Space Virtual-address Space
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9.6 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts Shared Library Using Virtual Memory Shared Library Using Virtual Memory
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9.7 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 Page is needed reference to it invalid reference abort not-in-memory bring to memory
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9.8 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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9.9 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 in-memory, 0 not-in-memory) Initially valid–invalid but 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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9.10 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 Page Fault Page Fault If there is ever a reference to a page, first reference will trap to OS page fault by paging hardware OS looks at another table ( usually kept with PCB ) to decide: Invalid reference abort. Just not in memory.
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This note was uploaded on 03/01/2011 for the course CSSE 250 taught by Professor Dr.yingwuzhu during the Spring '11 term at UH Clear Lake.

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ch09 - Chapter 9: Virtual Memory Chapter Chapter 9: Virtual...

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