L21 - Virtual Memory ECS 154B Computer Architecture II...

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1 ECS 154B Computer Architecture II Winter 2008 Virtual Memory and Memory Hierarchy 7.4-7.5 Adapted from slides by Mary Jane Irwin, Penn State 2 Virtual Memory • Uses main memory as a “cache” – Provides paging for the program – Allows memory protection – Enables code relocation • What makes it work? Locality • Two address spaces – Virtual addresses known to program – Physical addresses used to access memory 3 Paging • Virtual memory allows a process to access more memory than is actually present through paging – Pages currently used by a process are located in main memory – When main memory becomes full, some pages are written to disk • The swap space on disk holds pages temporarily • When accessed again the pages are moved from the swap space to main memory – The Page Table tracks each page’s location • A limited memory may cause thrashing – Pages frequently swapped between memory and disk – Decreases performance drastically 4 Page Replacement • What page is written to disk when memory becomes full? – LRU used most often • Reference bit in the TLB/Page Table set when a page is referenced by the process • OS periodically reads and clears the Reference bits and tracks page usage – Extra overhead for true LRU acceptable because of large access latency for secondary memory
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2 5 Virtual Memory Writes • What happens on writes? – Write back used for pages in memory • Secondary memory very slow • Large page size means it is highly likely page will be referenced again • Large block transfers to secondary memory better than small (few word) transfers – Include dirty bit in TLB and page table • Set when the process writes to the page • When the pages is removed from memory, it is only written if the dirty bit is set 6 Relocation • Virtual memory allows the OS to move a process anywhere in memory – Process only knows the virtual address
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L21 - Virtual Memory ECS 154B Computer Architecture II...

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