Virtual Memory

Cleaning Policy / Virtual Memory Interface
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Cleaning Policy / Virtual Memory Interface

Paging works best when there are lots of free page frames that can be claimed as page faults take place. If every page frame is full, and moreover customized, before a new page can be brought in, an old page must first be written to disk. To make sure a plentiful supply of free

IMPLEMENTATION ISSUES
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IMPLEMENTATION ISSUES

Implementers of virtual memory systems have to make selections among the major theoretical algorithms, such as second chance versus aging, local versus global page allocation, and demand paging versus prepaging. But they also have to be aware of a number of practical

Instruction Backup
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Instruction Backup

When a program references a page that is not in memory, the instruction causing the fault is stopped partway through and a trap to the operating system takes place. After the operating system has fetched the page required, it must restart the instruction causing the trap. This

SEGMENTATION
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SEGMENTATION

The virtual memory discussed so far is one-dimensional because the virtual addresses go from 0 to some maximum address, one address after another. For various problems, having two or more separate virtual address spaces may be much better than having only one. For

Segmentation with Paging: MULTICS
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Segmentation with Paging: MULTICS

If the segments are large, it may be inconvenient, or even impossible, to keep them in main memory in their entirety. This leads to the idea of paging them, so that only those pages that are really required have to be around. Many significant systems have supported paged

Segmentation with Paging: The Intel Pentium
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Segmentation with Paging: The Intel Pentium

In many ways, the virtual memory on the Pentium resembles that of MULTICS, including the presence of both segmentation and paging. Whereas MULTICS has 256K independent segments, each up to 64K 36-bit words, the Pentium has 16K independent segments, each holding

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