SJSU CS 147 - Virtual Memory (18 pages)

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Virtual Memory



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Virtual Memory

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Lecture Notes


Pages:
18
School:
San Jose State University
Course:
Cs 147 - Computer Architecture
Computer Architecture Documents

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CS 147 Virtual Memory Prof Sin Min Lee Anthony Palladino VIRTUAL MEMORY Why Virtual Memory When computers were first being developed memory was very costly and hard to come by Computers were the size of warehouses yet often only had memory which amounted to 128 kilobytes The complication of running out of memory space during run time began to surface and a solution was needed in order to alleviate this problem A way to provide nearly limitless cheap memory was sought by programmers What was developed was virtual memory Initial Problems Many early programmers thought it too strange a concept to be pursued and argued against it Addressing initially was a problem before a stable process was developed Many thought that virtual memory would simply be to expensive to implement correctly as well as use too large a portion of the processors power It was unclear at first how to mass produce the process for home use Some Terminology Virtual Memory VRAM Virtual Random Access Memory Physical Memory Actually hardware memory MMU Memory management Unit Real Address Physical address in memory Virtual address address of virtual memory Memory address address of physical memory Virtual Memory Features Virtual memory is much larger than real memory Today s Primary job for virtual memory is to avoid software conflicts Conversions are done on the Page tables MMU Controls the process The TLB allows for paging to remain fast Programs are no longer limited by amount of real memory much larger problem when computers were first being developed Virtual Memory Drawbacks Addressing is more difficult Conflicts can terminate programs Disk drive is much slower than memory thousands to millions of times How can programs be separated At any given time only a fraction of a program is running 90 10 Rule Only parts needed for immediate execution or that are used repeatedly are loaded Less accessed parts are left on disk and out of memory in order to save space programs can be several gigs these days



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