lecture12 - More Fun With Virtual Memory Paging Page...

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More Fun With Virtual Memory: Paging, Page Replacement Strate- gies & Analysis, Page Table Management Backing Store All pages of the memory must be copied to backing store, i.e., disk, so that if their space is needed in memory they can be restored from disk. The analogy to swap space should be pretty obvious: both are copies of a runnable process on disk. The distinction is that paging backing store can contain copies of memory of processes that are currently executing, while swapped processes are always stopped. Paging systems use backing store, and allocate and use it on a per-page basis; swapping systems use swap space and allocate it in units of entire processes’ address spaces. 1 That said, people frequently say swap when they mean backing store; UNIX ® certainly does. It’s expensive (in time) to move pages onto and off of backing store, so OSes take various steps to avoid it. Some systems page executables and other read-only memory from the filesystem directly, avoid- ing the copy to backing store. Others overcommit backing store by only copying a page to backing store if it must be removed from memory for some reason. The tradeoffs here are similar to those for swap spaces. Page Faults When a page is not present, the OS faces a page fault. The following are the basic steps in servicing a page fault: The MMU interrupts the CPU (with a paging interrupt or exception depending on the CPU model) The paging ISR (usually called a pager or page fault handler ) loads the required page into an available frame.
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