Virtual memory or virtual memory addressing is a memory management technique, used by multitasking computer operating systems wherein non-contiguous memory is presented to a software application (aka process) as contiguous memory. The contiguous memory is referred to as the virtual address space. Virtual memory addressing is typically used in paged memory systems. This in turn is often combined with memory swapping, whereby memory pages stored in primary storage are written to secondary storage, thus freei ...

Including:

• Virtual memory - Background
• Virtual memory - Segmentation
• Virtual memory - Basic operation
• Virtual memory - Details
• Virtual memory - Paging and virtual memory
• Virtual memory - Additional details
• Virtual memory - History
• Virtual memory - Windows example
• Virtual memory - Misconceptions about the Windows page file
• Virtual memory - Virtual Memory in Linux

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Before the development of the virtual memory technique, programmers in the 1940s and 1950s had to manage two-level storage (main memory or RAM, and secondary memory in the form of hard disks or earlier, magnetic drums) directly. Virtual memory was developed in approximately 1959 - 1962, at the University of Manchester for the Atlas Computer, completed in 1962. However, Fritz-Rudolf Güntsch, one of Germany's pioneering computer scientists and later the developer of the Telefunken TR 440 mainframe, claims to have invented the concept i ...

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Virtual memory, Virtual memory - Background, Virtual memory - Segmentation, Virtual memory - Basic operation, Virtual memory - Details, Virtual memory - Paging and virtual memory, Virtual memory - Additional details, Virtual memory - History, Virtual memory - Windows example, Virtual memory - Misconceptions about the Windows page file, Virtual memory - Virtual Memory in Linux

Read more here: » Virtual memory: Encyclopedia II - Virtual memory - History

Most computers possess four kinds of memory: registers in the CPU, CPU caches (generally some kind of static RAM) both inside and adjacent to the CPU, main memory (generally dynamic RAM) which the CPU can read and write to directly and reasonably quickly; and disk storage, which is much slower, but also much larger. CPU register use is generally handled by the compiler and this isn't a huge burden as data doesn't generally stay in them very long. The decision of when to use cache and when to use main memory is generally dealt with by hardware so generally both are regarded together ...

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Virtual memory, Virtual memory - Background, Virtual memory - Segmentation, Virtual memory - Basic operation, Virtual memory - Details, Virtual memory - Paging and virtual memory, Virtual memory - Additional details, Virtual memory - History, Virtual memory - Windows example, Virtual memory - Misconceptions about the Windows page file, Virtual memory - Virtual Memory in Linux

Read more here: » Virtual memory: Encyclopedia II - Virtual memory - Background

In Linux operating system, it is possible to use a whole partition of the HDD for virtual memory. Though it is still possible to use a file for swapping, it is recommended to use a separate partition, because this excludes chances of fragmentation, which reduces the performance of swapping. A swap area is created using the command mkswap filename/device , and may be turned on and off using the commands swapon and swapoff, respectively, accom ...

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Virtual memory, Virtual memory - Background, Virtual memory - Segmentation, Virtual memory - Basic operation, Virtual memory - Details, Virtual memory - Paging and virtual memory, Virtual memory - Additional details, Virtual memory - History, Virtual memory - Windows example, Virtual memory - Misconceptions about the Windows page file, Virtual memory - Virtual Memory in Linux

Read more here: » Virtual memory: Encyclopedia II - Virtual memory - Virtual Memory in Linux

Virtual memory has been a feature of Microsoft Windows since Windows 3.1 in 1992. 386SPART.PAR (or WIN386.SWP on Windows 3.11 and Windows for Workgroups) is a hidden file created by Windows 3.x for use as a virtual memory swap file. It is generally found in the root directory, but it may appear elsewhere (typically in the WINDOWS directory). Its size depends on how much virtual memory the system has set up under Control Panel - Enhanced under "Virtual Memory." If a user moves or deletes this file, Windows will BSOD the next time it is starte ...

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Virtual memory, Virtual memory - Background, Virtual memory - Segmentation, Virtual memory - Basic operation, Virtual memory - Details, Virtual memory - Paging and virtual memory, Virtual memory - Additional details, Virtual memory - History, Virtual memory - Windows example, Virtual memory - Misconceptions about the Windows page file, Virtual memory - Virtual Memory in Linux

Read more here: » Virtual memory: Encyclopedia II - Virtual memory - Windows example

One additional advantage of virtual memory is that it allows a computer to multiplex its CPU and memory between multiple programs without the need to perform expensive copying of the programs' memory images. If the combination of virtual memory system and operating system supports swapping, then the computer may be able to run simultaneous programs whose total size exceeds the available physical memory. Since most programs have a small subset (active set) of pages that they reference over significant periods of their execution, the pe ...

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Virtual memory, Virtual memory - Background, Virtual memory - Segmentation, Virtual memory - Basic operation, Virtual memory - Details, Virtual memory - Paging and virtual memory, Virtual memory - Additional details, Virtual memory - History, Virtual memory - Windows example, Virtual memory - Misconceptions about the Windows page file, Virtual memory - Virtual Memory in Linux

Read more here: » Virtual memory: Encyclopedia II - Virtual memory - Additional details

When virtual memory is used, or when a main memory location is read or written to by the CPU, hardware within the computer translates the address of the memory location generated by the software (the virtual memory address) into either: the address of a real memory location (the physical memory address) which is assigned within the computer's physical memory to hold that memory item, or an indication that the desired memory item is not currently resident in main memory (a so-called ...

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Virtual memory, Virtual memory - Background, Virtual memory - Segmentation, Virtual memory - Basic operation, Virtual memory - Details, Virtual memory - Paging and virtual memory, Virtual memory - Additional details, Virtual memory - History, Virtual memory - Windows example, Virtual memory - Misconceptions about the Windows page file, Virtual memory - Virtual Memory in Linux

Read more here: » Virtual memory: Encyclopedia II - Virtual memory - Basic operation

The translation from virtual to physical addresses is implemented by an MMU (Memory Management Unit). This may be either a module of the CPU, or an auxiliary, closely coupled chip. The operating system is responsible for deciding which parts of the program's simulated main memory are kept in physical memory. The operating system also maintains the translation tables which provide the mappings between virtual and physical addresses, for use by the MMU. Finally, when a virtual memory exception occurs, the operating system is responsible ...

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Virtual memory, Virtual memory - Background, Virtual memory - Segmentation, Virtual memory - Basic operation, Virtual memory - Details, Virtual memory - Paging and virtual memory, Virtual memory - Additional details, Virtual memory - History, Virtual memory - Windows example, Virtual memory - Misconceptions about the Windows page file, Virtual memory - Virtual Memory in Linux

Read more here: » Virtual memory: Encyclopedia II - Virtual memory - Details

Virtual memory is usually (but not necessarily) implemented using paging. In paging, the low order bits of the binary representation of the virtual address are preserved, and used directly as the low order bits of the actual physical address; the high order bits are treated as a key to one or more address translation tables, which provide the high order bits of the actual physical address. For this reason a range of consecutive addresses in the virtual address space whose size is a power of two will be translated in a corresponding ra ...

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Virtual memory, Virtual memory - Background, Virtual memory - Segmentation, Virtual memory - Basic operation, Virtual memory - Details, Virtual memory - Paging and virtual memory, Virtual memory - Additional details, Virtual memory - History, Virtual memory - Windows example, Virtual memory - Misconceptions about the Windows page file, Virtual memory - Virtual Memory in Linux

Read more here: » Virtual memory: Encyclopedia II - Virtual memory - Paging and virtual memory