Lecture 9 - Virtual Memory (2010-02-09)

Lecture 9 - Virtual Memory (2010-02-09) - 1 Main Memory...

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Unformatted text preview: 1 Main Memory and Virtual Memory Main Memory and Virtual Memory • Use of virtual memory – Main memory becomes another level in the memory hierarchy – Enables programs with address space or working set that exceed physically available memory • No need for programmer to manage overlays, etc. • Sparse use of large address space is OK – Allows multiple users or programs to timeshare limited amount of physical memory space and address space 104 • Bottom line: efficient use of expensive resource, and ease of programming Virtual Memory Virtual Memory • Enables – Use more memory than system has – Think program is only one running – Think program is only one running • Don’t have to manage address space usage across programs • E.g. think it always starts at address 0x0 – Memory protection • Each program has private VA space: no-one else can clobber it 105 – Better performance • Start running a large program before all of it has been loaded from disk 2 Virtual Memory Virtual Memory – Placement Placement • Main memory managed in larger blocks – Page size typically 4K – 16K • Fully flexible placement; fully associative • Fully flexible placement; fully associative – Operating system manages placement – Indirection through page table – Maintain mapping between: • Virtual address (seen by programmer) • Physical address (seen by main memory) 106 Virtual Memory Virtual Memory – Placement Placement • Fully associative implies expensive lookup? – In caches, yes: check multiple tags in parallel • In virtual memory expensive lookup is avoided by using • In virtual memory, expensive lookup is avoided by using a level of indirection – Lookup table or hash table – Called a page table 107 3 Virtual Memory Virtual Memory – Identification Identification Virtual Address Physical Address Dirty bit 0x20004000 0x2000 Y/N • Similar to cache tag array...
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This note was uploaded on 08/22/2010 for the course CDA 5106 taught by Professor Staff during the Spring '08 term at University of Central Florida.

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Lecture 9 - Virtual Memory (2010-02-09) - 1 Main Memory...

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