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9 Cards in this Set
- Front
- Back
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Demand Paging (what is demand paging) |
-the commonly used virtual memory system -a paging system with swapping -the whole program is stored into a swap disk and only those pages needed are brought into memory. |
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Demand Paging (What is a valid bit) |
-uses a "valid" bit in the page table to to indicate the status of a page. |
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Demand Paging (if statements for valid bit) |
if valid bit =1 ----> the page is in the memory(mapped)
if valid bit =0 ----> the page is not in the memory(unmapped) |
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Demand Paging (page fault) |
when an unmapped page is referenced(called "page fault"), the OS must bring the page into memory. |
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Demand Paging (page replacement) |
in some cases, we want to replace an old page in memory in order to bring in the new page(called "page replacement"). |
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Demand Paging Page Fault Service Routine |
When a page fault occurs: -if there is a free frame, use it, otherwise use page replacement algorithm to select a "victim" page. -performs swap operations -update the page table |
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Demand Paging (performance of demand paging) |
-Performance of demand paging heavily depends on the page fault rate. |
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Demand Paging (When is page replacement necessary?) |
When is page replacement necessary? -when a page fault occurs and no free frame is available |
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Demand Paging (How to keep the page fault rate low?) |
How to keep the page fault rate low? -1.allocate more frames to programs 2.use a good page replacement algorithm. |
