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Encyclopedia > Virtual memory
The program thinks it has a large range of contiguous addresses; but in reality the parts it is currently using are scattered around RAM, and the inactive parts are saved in a disk file.
The program thinks it has a large range of contiguous addresses; but in reality the parts it is currently using are scattered around RAM, and the inactive parts are saved in a disk file.

Virtual memory is a computer system technique which gives an application program the impression that it has contiguous working memory, while in fact may be physically fragmented and may even overflow on to disk storage. Systems that use this technique make programming of large applications easier and use real physical memory (e.g. RAM) more efficiently than those without virtual memory. This article is about the TBN game show. ... Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... Look up RAM, Ram, ram in Wiktionary, the free dictionary. ... This article is about the machine. ... Look up RAM, Ram, ram in Wiktionary, the free dictionary. ...


Note that "virtual memory" is not just "using disk space to extend physical memory size". Extending memory is a normal consequence of using virtual memory techniques, but can be done by other means such as overlays or swapping programs and their data completely out to disk while they are inactive. The definition of "virtual memory" is based on tricking programs into thinking they are using large blocks of contiguous addresses. Overlays are a programming method that allow computer programs to be bigger than the amount of virtual address space available to the user. ...


All modern general-purpose computer operating systems use virtual memory techniques for ordinary applications, such as word processors, spreadsheets, multimedia players, accounting, etc. Few older operating systems, such as DOS of the 1980s, or those for the mainframes of the 1960s, had virtual memory functionality - notable exceptions being the Atlas and B5000. An operating system (OS) is a software that manages computer resources and provides programmers with an interface used to access those resources. ... This article is about the family of closely related operating systems for the IBM PC compatible platform. ... For other uses, see Mainframe. ... The Atlas Computer of the University of Manchester, England, became operational in 1962, as a joint development between the University, Ferranti and Plessey. ... Background The B5000 was designed in 1961 by a team at Burroughs under the leadership of Robert (Bob) Barton. ...


Embedded systems and other special-purpose computer systems which require very fast, very consistent response time do not generally use virtual memory. A router, an example of an embedded system. ...

Contents

Implementation techniques

Paged virtual memory

Almost all implementations of virtual memory divide the virtual address space of an application program into pages; a page is a block of contiguous virtual memory addresses. Pages are usually at least 4K bytes in size, and systems with large virtual address ranges or large amounts of real memory (e.g. RAM) generally use larger page sizes. Virtual address space (abbreviated VAS) is a memory mapping mechanism available in modern operating systems such as OpenVMS, Unix, Linux, and Windows NT. Overview When you run an application on a 32-bit operating system (OS), the OS creates a new process for it. ... In a context of computer virtual memory, a page, memory page, or virtual page is a fixed-length block of main memory, that is contiguous in both physical memory addressing and virtual memory addressing. ... For the computer industry magazine, see Byte (magazine). ... Look up RAM, Ram, ram in Wiktionary, the free dictionary. ...


Page tables

Almost all implementations use page tables to translate the virtual addresses seen by the application program into physical addresses (also referred to as "real addresses") used by the hardware to process instructions. Each entry in a page table contains: the starting virtual address of the page; either the real memory address at which the page is actually stored or an indicator that the page is currently held in a disk file (if the system uses disk files to let applications use amounts of virtual memory which exceed real memory). Relationship between pages addressed by virtual addresses and the frames in physical memory, within a simple address space scheme. ... In computer science, a physical address is the address presented to a computers main memory in a virtual memory system, in contrast to the virtual address which is the address generated by the CPU. Virtual addresses are translated into physical addresses by a memory management unit (abbreviated MMU). ...


Systems can have one page table for the whole system or a separate page table for each application. If there is only one, different applications which are running at the same time share a single virtual address space, i.e. they use different parts of a single range of virtual addresses. Systems which use multiple page tables provide multiple virtual address spaces - concurrent applications think they are using the same range of virtual addresses, but their separate page tables redirect to different real addresses. In computing, multitasking is a method by which multiple tasks, also known as processes, share common processing resources such as a CPU. In the case of a computer with a single CPU, only one task is said to be running at any point in time, meaning that the CPU is...


Paging

Paging is the process of saving inactive virtual memory pages to disk and restoring them to real memory when required. In computer operating systems, demand paging is an application of virtual memory. ...



Most virtual memory systems enable programs to use virtual address ranges which in total exceed the amount of real memory (e.g. RAM). To do this they use disk files to save virtual memory pages which are not currently active, and restore them to real memory when they are needed. Pages are not necessarily restored to the same real addresses from which they were saved - applications are aware only of virtual addresses. Usually the real memory to which a page is restored contains another virtual memory page which has been used recently, and which must therefore be saved to disk. Look up RAM, Ram, ram in Wiktionary, the free dictionary. ...


Dynamic address translation

When a CPU fetches an instruction located at a particular virtual address or, while executing an instruction, fetches data from a particular virtual address or stores data to a particular virtual address, the virtual address must be translated to the corresponding physical address. This is done by a hardware component, sometimes called a memory management unit, which looks up the real address (from the page table) corresponding to a virtual address and passes the real address to the parts of the CPU which execute instructions. If the page tables indicate that the virtual memory page is not currently in real memory, the hardware raises a page fault exception (special internal signal) which invokes the paging supervisor component of the operating system (see below). CPU can stand for: in computing: Central processing unit in journalism: Commonwealth Press Union in law enforcement: Crime prevention unit in software: Critical patch update, a type of software patch distributed by Oracle Corporation in Macleans College is often known as Ash Lim. ... This 68451 MMU could be used with the Motorola 68010 MMU, short for memory management unit or sometimes called paged memory management unit as PMMU, is a class of computer hardware components responsible for handling memory accesses requested by the CPU. Among the functions of such devices are the translation... CPU can stand for: in computing: Central processing unit in journalism: Commonwealth Press Union in law enforcement: Crime prevention unit in software: Critical patch update, a type of software patch distributed by Oracle Corporation in Macleans College is often known as Ash Lim. ... In computer storage technology, a page is a fixed length block of memory that is used as a unit of transfer between physical memory and external storage like a disk, and a page fault is an interrupt (or exception) to the software raised by the hardware, when a program accesses... Exception may refer to: An action that is not part of normal operations or standards. ... An operating system (OS) is a software that manages computer resources and provides programmers with an interface used to access those resources. ...


Paging supervisor

This part of the operating system creates and manages the page tables. If the dynamic address translation hardware raises a page fault exception, the paging supervisor searches the page file(s) (on disk) for the page containing the required virtual address, reads it into real physical memory, updates the page tables to reflect the new location of the virtual address and finally tells the dynamic address translation mechanism to start the search again. Usually all of the real physical memory is already in use and the paging supervisor must first save an area of real physical memory to disk and update the page table to say that the associated virtual addresses are no longer in real physical memory but saved on disk. Paging supervisors generally save and overwrite areas of real physical memory which have been least recently used, because these are probably the areas which are used least often. So every time the dynamic address translation hardware matches a virtual address with a real physical memory address, it must put a time-stamp in the page table entry for that virtual address. It has been suggested that page replacement algorithm be merged into this article or section. ...


Permanently resident pages

All virtual memory systems have memory areas that are "pinned down", i.e. cannot be swapped out to secondary storage, for example:

  • Interrupt mechanisms generally rely on an array of pointers to the handlers for various types of interrupt (I/O completion, timer event, program error, page fault, etc.). If the pages containing these pointers or the code that they invoke were pageable, interrupt-handling would become even more complex and time-consuming; and it would be especially difficult in the case of page fault interrupts.
  • The page tables are usually not pageable.
  • Data buffers that are accessed outside of the CPU, for example by peripheral devices that use direct memory access (DMA) or by I/O channels. Usually such devices and the buses (connection paths) to which they are attached use physical memory addresses rather than virtual memory addresses. Even on buses with an IOMMU, which is a special memory management unit that can translate virtual addresses used on an I/O bus to physical addresses, the transfer cannot be stopped if a page fault occurs and then restarted when the page fault has been processed. So pages containing locations to which or from which a peripheral device is transferring data are either permanently pinned down or pinned down while the transfer is in progress.
  • Any other kernel or application areas in which operation is very timing dependent and cannot allow the variation in response time which paging causes.

In computing, an interrupt is an asynchronous signal from hardware or software indicating the need for attention. ... This article is about the computer interface. ... A simple digital timer. ... In computer storage technology, a page is a fixed length block of memory that is used as a unit of transfer between physical memory and external storage like a disk, and a page fault is an interrupt (or exception) to the software raised by the hardware, when a program accesses... In computing, a buffer is a region of memory used to temporarily hold data while it is being moved from one place to another. ... Direct memory access (DMA) is a feature of modern computers that allows certain hardware subsystems within the computer to access system memory for reading and/or writing independently of the central processing unit. ... In computer science, channel I/O is a generic term that refers to an advanced, high-performance input/output architecture that is implemented in various forms on a number of computer architectures, especially on mainframe computers. ... PCI Express bus card slots (from top to bottom: x4, x16, x1 and x16), compared to a traditional 32-bit PCI bus card slot (bottom) In computer architecture, a bus is a subsystem that transfers data or power between computer components inside a computer or between computers, and a bus... Comparison of the I/O memory management unit (IOMMU) to the memory management unit (MMU). ... This 68451 MMU could be used with the Motorola 68010 MMU, short for memory management unit or sometimes called paged memory management unit as PMMU, is a class of computer hardware components responsible for handling memory accesses requested by the CPU. Among the functions of such devices are the translation...

Virtual=real operation

In MVS, z/OS, and similar OSes, some parts of the systems memory are managed in virtual=real mode, where every virtual address corresponds to a real address. Those are: MVS (Multiple Virtual Storage) was the most commonly used operating system on the System/370 and System/390 IBM mainframe computers. ... z/OS Welcome Screen seen through a terminal emulator The title of this article begins with a capital letter due to technical limitations. ...

  • interrupt mechanisms
  • paging supervisor and page tables
  • all data buffers accessed by I/O channels[citation needed]
  • application programs which use non-standard methods of managing I/O and therefore provide their own buffers and communicate directly with peripherals (programs that create their own channel command words).

In IBM's early virtual memory operating systems virtual=real mode was the only way to "pin down" pages. z/OS has 3 modes, V=V (virtual=virtual; fully pageable), V=R and V=F (virtual = fixed, i.e. "pinned down" but with DAT operating).[1] In computing, an interrupt is an asynchronous signal from hardware or software indicating the need for attention. ... In computing, a buffer is a region of memory used to temporarily hold data while it is being moved from one place to another. ... In computer science, channel I/O is a generic term that refers to an advanced, high-performance input/output architecture that is implemented in various forms on a number of computer architectures, especially on mainframe computers. ... A channel command word (CCW) is a command used in the Channel I/O subsystem architecture of the ESA/390 IBM mainframe architecture to initiate a command on a channel-attached device. ...


Segmented virtual memory

Some systems, such as the Burroughs large systems, do not use paging to implement virtual memory. Instead, they use segmentation, so that an application's virtual address space is divided into variable-length segments. A virtual address consists of a segment number and an offset within the segment. The Burroughs large systems were the largest of three series of Burroughs Corporation mainframe computers. ... Segmentation is one of the most common ways to achieve memory protection; another common one is paging. ...


Memory is still physically addressed with a single number (called absolute or linear address). To obtain it, the processor looks up the segment number in a segment table to find a segment descriptor.[2] The segment descriptor contains a flag indicating whether the segment is present in main memory and, if it is, the address in main memory of the beginning of the segment (segment's base address) and the length of the segment. It is checked whether the offset within the segment is less than the length of the segment and, if it isn't, an interrupt is generated. If a segment is not present in main memory, hardware interrupt is raised to the operating system, which may try to read the segment into main memory, or to swap in. The operating system might have to remove other segments (swap out) from main memory in order to make room in main memory for the segment to be read in. In computing, a base address denotes a memory address serving as a reference point (base) for other addresses. ...


Notably, the Intel 80286 supported similar segmentation scheme as an option, but it was unused by most operating systems. AMD 80286 at 12 MHz. ...


It is possible to combine segmentation and paging, usually dividing each segment into pages. In systems that combine them, such as Multics and the IBM System/38 and IBM System i machines, virtual memory is usually implemented with paging, with segmentation used to provide memory protection.[3][4][5] With the Intel 80386 and later IA-32 processors, the segments reside in a 32-bit linear paged address space, so segments can be moved into and out of that linear address space, and pages in that linear address space can be moved in and out of main memory, providing two levels of virtual memory; however, few if any operating systems do so. Instead, they only use paging. Multics (Multiplexed Information and Computing Service) was an extraordinarily influential early time-sharing operating system. ... IBM System 38 The IBM System/38 was a minicomputer. ... i5 Model 570 (2006) The IBM System i (formerly known as iSeries, AS/400, and Application System/400) is a minicomputer platform produced by IBM. It was officially introduced as the AS/400 in 1988. ... 386 DX redirects here. ... It has been suggested that this article or section be merged with X86 assembly language. ... 32-bit is a term applied to processors, and computer architectures which manipulate the address and data in 32-bit chunks. ...


The difference between virtual memory implementations using pages and using segments is not only about the memory division with fixed and variable sizes, respectively. In some systems, e.g. Multics, or later System/38 and Prime machines, the segmentation was actually visible to the user processes, as part of the semantics of a memory model. In other words, instead of a process just having a memory which looked like a single large vector of bytes or words, it was more structured. This is different from using pages, which doesn't change the model visible to the process. This had important consequences. Multics (Multiplexed Information and Computing Service) was an extraordinarily influential early time-sharing operating system. ... The IBM System/38 was a computer. ... Prime Computer was a Natick, Massachusetts-based producer of minicomputers from 1972 until 1992. ...


Segment wasn't just a "page with a variable length", or a simple way to lengthen the address space (as in Intel 80286). In Multics, the segmentation was a very powerful mechanism that was used to provide a single-level virtual memory model, in which there was no differentiation between "process memory" and "file system" - a process' active address space consisted only a list of segments (files) which were mapped into its potential address space, both code and data. [6] It is not the same as the later mmap function in Unix, because inter-file pointers don't work when mapping files into semi-arbitrary places. Multics had such addressing mode built into most instructions. In other words it could perform relocated inter-segment references, thus eliminating the need for a linker completely.[7] This also worked when different processes mapped the same file into different places in their private address spaces.[8] AMD 80286 at 12 MHz. ... In computing, mmap() is a POSIX-compliant Unix system call that maps files or devices into memory. ... Figure of the linking process, where object files and static libraries are assembled into a new library or executable. ...


Avoiding thrashing

All implementations need to avoid a problem called "thrashing", where the computer spends too much time shuffling blocks of virtual memory between real memory and disks, and therefore appears to work slower. Better design of application programs can help, but ultimately the only cure is to install more real memory. For more information see Paging. This article is about computer virtual memory. ...


History

In the 1940s and 1950s, before the development of a virtual memory, all larger programs had to contain logic for managing two-level storage (primary and secondary, today's analogies being RAM and hard disk), such as overlaying techniques. Programs were responsible for moving overlays back and forth from secondary storage to primary. Overlays are a programming method that allow computer programs to be bigger than the amount of virtual address space available to the user. ...


The main reason for introducing virtual memory was therefore not simply to extend primary memory, but to make such an extension as easy to use for programmers as possible.[7]


Many systems already had the ability to divide the memory between multiple programs (required for multiprogramming and multiprocessing), provided for example by "base and bounds registers" on early models of the PDP-10, without providing virtual memory. That gave each application a private address space starting at an address of 0, with an address in the private address space being checked against a bounds register to make sure it's within the section of memory allocated to the application and, if it is, having the contents of the corresponding base register being added to it to give an address in main memory. This is a simple form of segmentation without virtual memory. In computing, multitasking is a method by which multiple tasks, also known as processes, share common processing resources such as a CPU. In the case of a computer with a single CPU, only one task is said to be running at any point in time, meaning that the CPU is... In computing, multitasking is a method by which multiple tasks, also known as processes, share common processing resources such as a CPU. In the case of a computer with a single CPU, only one task is said to be running at any point in time, meaning that the CPU is... The PDP-10 was a computer manufactured by Digital Equipment Corporation (DEC) from the late 1960s on; the name stands for Programmed Data Processor model 10. It was the machine that made time-sharing common; it looms large in hacker folklore because of its adoption in the 1970s by many... Segmentation is one of the most common ways to achieve memory protection; another common one is paging. ...


Virtual memory was developed in approximately 1959–1962, at the University of Manchester for the Atlas Computer, completed in 1962.[9] 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 in 1957 in his doctoral dissertation Logischer Entwurf eines digitalen Rechengerätes mit mehreren asynchron laufenden Trommeln und automatischem Schnellspeicherbetrieb (Logic Concept of a Digital Computing Device with Multiple Asynchronous Drum Storage and Automatic Fast Memory Mode). Affiliations: Russell Group, EUA, N8 Group, NWUA, Worldwide Universities Network (WUN), Association of Commonwealth Universities Website: http://www. ... The Atlas Computer of the University of Manchester became operational in 1962, having been a joint development between the University, Ferranti and Plessey. ... Telefunken is a German radio- and television company, founded in 1903. ...


In 1961, Burroughs released the B5000, the first commercial computer with virtual memory.[10][11] It used segmentation rather than paging. William Seward Burroughs (1857-1898), US inventor William S. Burroughs (1914-1997), author and grandson of William Seward Burroughs Edgar Rice Burroughs (1875-1950), American author of Tarzan fame The Burroughs Corporation began in 1886 as the American Arithmometer Company in St. ... Background The B5000 was designed in 1961 by a team at Burroughs under the leadership of Robert (Bob) Barton. ... Segmentation is one of the most common ways to achieve memory protection; another common one is paging. ... This article is about computer virtual memory. ...


Like many technologies in the history of computing, virtual memory was not accepted without challenge. Before it could be implemented in mainstream operating systems, many models, experiments, and theories had to be developed to overcome the numerous problems. Dynamic address translation required a specialized, expensive, and hard to build hardware, moreover initially it slightly slowed down the access to memory.[7] There were also worries that new system-wide algorithms of utilizing secondary storage would be far less effective than previously used application-specific ones.


By 1969 the debate over virtual memory for commercial computers was over.[7] An IBM research team led by David Sayre showed that the virtual memory overlay system consistently worked better than the best manually controlled systems. Also: 1969 (number) 1969 (movie) 1969 (Stargate SG-1) episode. ... For other uses, see IBM (disambiguation) and Big Blue. ...


Possibly the first minicomputer to introduce virtual memory was the Norwegian NORD-1. During the 1970s, other minicomputers implemented virtual memory, notably VAX models running VMS. Minicomputer (colloquially, mini) is a largely obsolete term for a class of multi-user computers which make up the middle range of the computing spectrum, in between the largest multi-user systems (traditionally, mainframe computers) and the smallest single-user systems (microcomputers or personal computers). ... The NORD-1 was Norsk Datas first minicomputer. ... VAX is a 32-bit computing architecture that supports an orthogonal instruction set (machine language) and virtual addressing (i. ... OpenVMS[1] (Open Virtual Memory System or just VMS) is the name of a high-end computer server operating system that runs on the VAX[2] and Alpha[3] family of computers developed by Digital Equipment Corporation of Maynard, Massachusetts (DIGITAL was then purchased by Compaq, and is now owned...


Virtual memory was introduced to the x86 architecture with the protected mode of the Intel 80286 processor. At first it was done with segment swapping, which became inefficient with larger segments. The Intel 80386 introduced support for paging underneath the existing segmentation layer. The page fault exception could be chained with other exceptions without causing a double fault. Protected mode is an operational mode of x86-compatible CPUs of the 80286 series or later. ... AMD 80286 at 12 MHz. ... 386 DX redirects here. ... Segmentation is one of the most common ways to achieve memory protection; another common one is paging. ... A double fault exception occurs if the processor cannot save the state before calling an exception handler. ...


See also

It has been suggested that this article or section be merged with Primary storage. ... In computer science, a physical address is the address presented to a computers main memory in a virtual memory system, in contrast to the virtual address which is the address generated by the CPU. Virtual addresses are translated into physical addresses by a memory management unit (abbreviated MMU). ... In computer science, a memory address is a unique identifier for a memory location at which a CPU or other device can store a piece of data for later retrieval. ... In computing, an address space defines a range of discrete addresses, each of which may correspond to a physical or virtual memory register, a network host, peripheral device, disk sector or other logical or physical entity. ... Virtual address space (abbreviated VAS) is a memory mapping mechanism available in modern operating systems such as OpenVMS, Unix, Linux, and Windows NT. Overview When you run an application on a 32-bit operating system (OS), the OS creates a new process for it. ... CPU design is the hardware design of a central processing unit. ... In a context of computer virtual memory, a page, memory page, or virtual page is a fixed-length block of main memory, that is contiguous in both physical memory addressing and virtual memory addressing. ... Relationship between pages addressed by virtual addresses and the frames in physical memory, within a simple address space scheme. ... This article is about computer virtual memory. ... Working set is the set of physical memory pages currently dedicated to a specific process. ... This 68451 MMU could be used with the Motorola 68010 MMU, short for memory management unit or sometimes called paged memory management unit as PMMU, is a class of computer hardware components responsible for handling memory accesses requested by the CPU. Among the functions of such devices are the translation... It has been suggested that page replacement algorithm be merged into this article or section. ... It has been suggested that this article or section be merged into cache algorithms. ... Segmentation is one of the most common ways to achieve memory protection; another common one is paging. ... The IBM System/38 was a computer. ... Memory management is the act of managing computer memory. ... It has been suggested that this article or section be merged with Dynamic memory allocation. ... Protected mode is an operational mode of x86-compatible CPUs of the 80286 series or later. ... Intel Pentium 4 (Northwood version), one example out of a huge number of x86 implementations from Intel, AMD, and others. ...

References

  1. ^ z/OS Basic Skills Information Center: z/OS Concepts.
  2. ^ The Operational Characteristics of the Processors for the Burroughs B5000. Retrieved on 2007-11-13. 
  3. ^ (January 1968) GE-645 System Manual, pp21-30. Retrieved on 2007-11-13. 
  4. ^ F. J. Corbató, V. A. Vyssotsky. Introduction and Overview of the Multics System. Retrieved on 2007-11-13.
  5. ^ E. L. Glaser, J. F. Couleur, G. A. Oliver. System Design of a Computer for Time Sharing Applications.
  6. ^ Bensoussan, A. & Clingen, C. T. (May 1972), "The Multics Virtual Memory: Concepts and Design", Communications of the ACM 15 (5): 308-318, <http://www.multicians.org/multics-vm.html> 
  7. ^ a b c d Denning, Peter (1997). "Before Memory Was Virtual". In the Beginning: Recollections of Software Pioneers. 
  8. ^ Organick, E.I. (1972), The Multics System: An Examination of Its Structure, MIT Press 
  9. ^ http://www.computer50.org/kgill/atlas/atlas.html Atlas design includes virtual memory
  10. ^ http://web.mac.com/joynerian/iWeb/Ian%20Joyner/Burroughs.html Ian Joyner on Burroughs B5000
  11. ^ Cragon, Harvey G. (1996), Memory Systems and Pipelined Processors, Jones and Bartlett Publishers, pp. 113, ISBN 0867204745, <http://books.google.com/books?id=q2w3JSFD7l4C> 
  • John L. Hennessy, David A. Patterson, Computer Architecture, A Quantitative Approach (ISBN 1-55860-724-2)
  • Virtual Memory Secrets by Murali

Year 2007 (MMVII) was a common year starting on Monday of the Gregorian calendar in the 21st century. ... is the 317th day of the year (318th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) was a common year starting on Monday of the Gregorian calendar in the 21st century. ... is the 317th day of the year (318th in leap years) in the Gregorian calendar. ... Fernando José Corbató (born July 1, 1926 in Oakland, California) is a prominent computer scientist, notable as a pioneer in the development of time-sharing operating systems. ... Victor A. Vyssotsky (born ) son of Alexander N. Vyssotsky and Emma T. R. Williams is a mathematician and computer scientist. ... Year 2007 (MMVII) was a common year starting on Monday of the Gregorian calendar in the 21st century. ... is the 317th day of the year (318th in leap years) in the Gregorian calendar. ... Peter J. Denning is a computer scientist and one of the team members of the Multics project. ...

External links

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL. This article does not cite any references or sources. ... “GFDL” redirects here. ...


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Virtual Memory and its influences on performance (387 words)
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