Lecture17-virtual-memory - Announcements CS 3214 Computer...

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1 CS 3214 Computer Systems Godmar Back Lecture 17 Announcements Project 4 due Nov 2 Exercise 6 due today CS 3214 Fall 2011 MEMORY MANAGEMENT Part 3 CS 3214 Fall 2011 Virtual Memory Is not a “kind” of memory Is a technique that combines one or more of the following concepts: – Address translation (always) CS 3214 Fall 2011 Address translation (always) – Paging from/to disk (usually) – Protection (usually) Can make storage that isn’t physical DRAM appear as though it were Key goals for Virtual Memory • Virtualization 1. Maintain illusion that each process has entire memory to itself Per-process address spaces 2. Allow processes access to more memory than is CS 3214 Fall 2011 really in the machine (or: sum of all memory used by all processes > physical memory) Makes DRAM a cache for disk • Protection 1. make sure there’s no way for any process to access another process’s data unintentionally 2. protect system-internal data/kernel data Address Translation Provides a way for OS to interpose on memory accesses OS maintains for each process a mapping { virtual addresses } { physical addresses } in a per-process page table – Which virtual addresses are valid (depends on process Which virtual addresses are valid (depends on process memory layout) Where they map to (depends on availability of physical memory) What kind of accesses are allowed (read/write/execute) OS manages page tables Based on input/commands from user processes Based on resource management decisions CS 3214 Fall 2011
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