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Topic: Virtual Memory System

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	Virtual Memory and Demand Paging: ASP Alliance
		Â Virtual Memory is a concept that addresses this issue by allowing a program than is even larger than the size of the available free memory to be loaded and executed and eliminate the chances of external fragmentation.
		The primary advantage or objective of Virtual Memory systems is the ability to load and execute a process that requires a larger amount of memory than what is available by loading the process in parts and then executing them.
		In virtual memory systems, demand paging is a type of swapping in which pages of data are not copied from disk to RAM until they are needed.
	aspalliance.com /969_Virtual_Memory_and_Demand_Paging (2265 words)

	Virtual memory - Wikipedia, the free encyclopedia
		Virtual memory or virtual memory addressing is a feature of computer systems wherein "permanent" data storage is used to assist in handling processing functions, thereby freeing more RAM or main memory (i.e.
		To properly implement virtual memory the CPU (or a device attatched to it) must provide a way for the operating system to map virtual memory to physical memory and for it to detect when an address is required that does not currently relate to main memory so that the needed data can be swapped in.
		Finally, when a virtual memory exception occurs, the operating system is responsible for allocating an area of physical memory to hold the missing information, bringing the relevant information in from the disk, updating the translation tables, and finally resuming execution of the software that incurred the virtual memory exception.
	en.wikipedia.org /wiki/Virtual_memory (2767 words)

	OpenVMS - Wikipedia, the free encyclopedia
		OpenVMS (Open Virtual Memory System or just VMS) is the name of a high-end computer server operating system that runs on the VAX and Alpha family of computers developed by Digital Equipment Corporation of Maynard, Massachusetts (now owned by Hewlett-Packard), and more recently on Hewlett-Packard systems built around Intel Itanium CPU.
		It offers extremely high system availability including the ability to distribute the physical system over a wide geographical area, allowing the system to be "disaster-tolerant" against earthquakes, weather, and other natural disasters that may disable individual data-processing facilities.
		When the system is restarted, the ordinary VMS 64-bit time value is recomputed based on the time kept by the TOY clock and the last recorded year (stored on the system disk).
	en.wikipedia.org /wiki/Virtual_Memory_System (1449 words)

	Virtual memory Article, Virtualmemory Information (Site not responding. Last check: 2007-09-23)
		Virtual memory is a computer design feature that permits software to use more memory than the computerphysically possesses.
		Simply put, when a memory location is read or written to, hardware within the computer translates the memory address generated by the software (the virtualmemory address) into a, usually distinct, real memory address (the physical memory address) within the computer'smemory.
		The most fundamental advantage of virtual memory is that it allows a computer to multiplex its CPU and memory between multipleprograms without the need to perform expensive copying of the programs' memory images.
	www.anoca.org /address/system/virtual_memory.html (930 words)

	Unix 4.4BSD Virtual Memory System (Site not responding. Last check: 2007-09-23)
		Virtual Memory is when user logical memory is detached from physical memory, providing an enormous virtual memory for programmers when really there is much less physical memory available.
		The new virtual memory system provides algorithms that are more fitting to the large memories that are current today, and has greatly reduced the dependency on the VAX architecture.
		Virtual memory provides for execution of large programs on machines with main memory capacity that are smaller than the size fo the program.
	www-ec.njit.edu /~emm8790 (2615 words)

	The Virtual Memory System
		A virtual memory reference is considered an atomic operation on the Explorer system; that is, the time spent waiting for a virtual memory reference to complete (including time spent waiting for disk paging activity) is not available for use by any other part of the system except interrupt processing.
		The A-memory (the processor memory for data accessed by the microcode) has a dedicated virtual address, which is at the very top of the virtual address space, but consumes none of the physical address space.
		The rest of the local physical memory is used as a transient page area, i.e., the virtual memory system assigns the pages of the virtual memory to physical locations as a part of its management functions.
	www.dridus.com /~nyef/ssdn2/sect05.html (5625 words)

	Basic Virtual Memory Concepts
		In other words, an overlayed program may require less memory to run than a program that is not overlayed, but if the system still does not have sufficient memory for the overlayed program, the end result is the same — an out of memory error.
		Because of this, the amount of memory required to execute the application at any given time is less than 15000 bytes — usually a lot less.
		This is done by structuring the virtual memory subsystem so that it attempts to ensure that those parts of the application currently needed — or likely to be needed in the near future — are kept in RAM only for as long as they are actually needed.
	www.redhat.com /docs/manuals/enterprise/RHEL-4-Manual/admin-guide/s1-memory-concepts.html (788 words)

	Virtual Memory (Site not responding. Last check: 2007-09-23)
		What Virtual Memory Is Virtual memory is a system by which the machine or operating system fools processes running on the machine into thinking that they have a lot more memory to work with than the capacity of RAM would indicate.
		The memory system then checks to see where the data for this virtual address really is, based on a reference to something called the page table.
		From that, the memory system can access the corresponding entry in the page table to find out just where this data is, retrieve it from disk if necessary, and then, using the frame number of the frame to which it was loaded, derive the corresponding physical address of this virtual address.
	www.duke.edu /~twf/cps104/virtual.html (3334 words)

	What is virtual memory?
		Virtual memory is a mapping from the addresses that running programs are using (virtual addresses) to the addresses of physical RAM (physical addresses).
		Virtual memory is allocated in increments of "pages" -- all of the bits inside one page of virtual memory are together in the same page of physical memory.
		Virtual memory is implemented by using "page tables," which list, for every virtual address, what physical page corresponds to that address and some properties of the virtual page.
	www.windowsfordevices.com /articles/AT7967807599.html (1943 words)

	Virtual Memory Tutorial (Site not responding. Last check: 2007-09-23)
		Virtual memory is a technique that allows processes that may not be entirely in the memory to execute by means of automatic storage allocation upon request.
		The term virtual memory refers to the abstraction of separating LOGICAL memory--memory as seen by the process--from PHYSICAL memory--memory as seen by the processor.
		The virtual memory abstraction is implemented by using secondary storage to augment the processor's main memory.
	cne.gmu.edu /modules/vm/green/defn.html (279 words)

	Basic Virtual Memory Concepts
		Virtual memory turns the concept of an application's address space on its head.
		However, even if one instruction required 100 bytes of memory for each type of memory access, the 300 bytes required is still a lot less than the application's 15000-byte address space.
		This is done by structuring the virtual memory subsystem so that it attempts to ensure that those parts of the application that are currently needed — or likely to be needed in the near future — are kept in RAM only for as long as they are needed.
	redhat.activeventure.com /9/systemadminprimer/s1-memory-concepts.html (755 words)

	VMS - Virtual Machine Storage, Virtual Memory System, Voice Message System
		Short for Virtual Memory System, a multi-user, multitasking, virtual memory operating system that runs on DEC's VAX and Alpha lines of minicomputers and workstations.
		Virtual Memory System Official short name for the VMS Timesharing Service: a cluster of two Alpha processors and one VAX processor that use a common VMS disk structure.
		Virtual Memory System (VMS, specifically OpenVMS) is the operating system on the Beecher VAXcluster, node UCBEH.
	www.auditmypc.com /acronym/VMS.asp (594 words)

	1999 USENIX Annual Technical Conference, June 6-11, 1999, Monterey, California, USA
		UVM is the third generation BSD virtual memory system that improves the performance and reduces the complexity of the BSD kernel.
		Anonymous memory is used for a number of purposes in a Unix-like operating system including for zero-fill mappings (e.g., bss and stack), for System V shared memory, for pageable areas of kernel memory, and to store changed pages of a copy-on-write mapping.
		In UVM, we expose the anonymous memory system to the pager-independent code, thus allowing it to be centrally managed and used by all pagers and the IPC and I/O systems.
	www.usenix.org /events/usenix99/full_papers/cranor/cranor_html (10234 words)

	System Software - Virtual Memory (Site not responding. Last check: 2007-09-23)
		For example, a 65k array in MSDOS requires the ``huge'' memory model which is slow as all addresses have to be ``normalised'' to remove ambiguity in addressing.
		All of these memory manipulation algorithms must be run in supervisor mode to gain access to the memory where the tables are kept and to manipulate all processes.
		In a timesharing system it is common for multiple versions of a program to be running (eg many shells).
	pandonia.canberra.edu.au /ssw/memory/vm.html (1815 words)

	NetBSD Documentation: UVM, the new Virtual Memory system
		The UVM Virtual Memory System - a paper by Charles D. Cranor and Gurudatta M. Parulkar.
		In UVM, your total virtual memory is equal to the size of your physical RAM (minus kernel overhead) plus the size of each swap partition.
		The top down memory allocation scheme is currently offered as kernel option on the i386 port, and the PowerPC ports: bebox, macppc, mvmeppc, ofppc, pmppc, prep, and sandpoint.
	www.netbsd.org /Documentation/kernel/uvm.html (506 words)

	What is SWAP (Virtual memory) file system? Recover swap partition - Data Recovery Service
		When the operating system needs data from the disk, it exchanges a portion of data (called a page or segment) in main memory with a portion of data on the disk.
		If you run memory intensive applications on a low physical memory system it is preferable to manually set the size to a value higher than default.
		In Linux operating system, it is possible to use a whole partition of the HDD for virtual memory.
	www.ptdd.com /datarecovery/swap.htm (610 words)

	DECUS '99/Providence SE212 Compaq's Tru64 UNIX Internals - Virtual Memory System
		The setting of various memory management parameters should be such that physical memory is treated as the precious resource that it is. This seminar will examine the details of the Compaq's Digital UNIX Virtual Memory System.
		The seminar will cover the preliminary operations that begin with bootstrapping the system, continue with how memory management structures are used, then go on to discuss virtual memory parameters.
		Although this is not a system performance seminar we will discuss the relationship of various virtual memory parameters and their impact on performance.
	www.decus.org /pr99/seminars/se212.stm.htm (256 words)

	Direct input/output in a virtual memory system (EP0208428B1)
		A virtual memory system is used to control access to I/O device address space in accordance with a preferred embodiment of the present invention.
		In a virtual memory system, access to pages within a processor's address space are assigned to each application program.
		What is meant by global is that physical memory locations map to the same virtual memory space regardless of what process is running on the processor.
	www.delphion.com /details?pn=EP00208428B1 (304 words)

	Virtual Private Servers
		With a VPS you can easily share system resources like the CPU and memory with some other users, whereas a file system is organized in a way that it doesnât have anyone else on it.
		To find out whether Virtual Private Servers are right for you, youâd better look through some frequently asked questions that give detailed characteristics about the advantages of VPS.
		In comparison with a shared system, a virtual private server is much more secure due to file system organization, because it is differently setup.
	www.hostseeq.com /c/virtual_private_servers.htm (532 words)

	Virtual Memory System - a Whatis.com definition - see also: VMS (Site not responding. Last check: 2007-09-23)
		VMS (Virtual Memory System) is an operating system from the Digital Equipment Corporation (DEC) that runs in its older
		VMS originated in 1979 as a new operating system for DEC's new VAX computer, the successor to DEC's PDP-11.
		VMS is a 32-bit system that exploits the concept of virtual memory.
	whatis.techtarget.com /definition/0,,sid9_gci214146,00.html (184 words)

	The Möbius Operating System: Virtual Memory (Site not responding. Last check: 2007-09-23)
		Allocates an area of memory whose mapping is controlled by a callback function.
		Frees an area of memory allocated by the VmmAlloc() function.
		Virtual address at which the area will start.
	mobius.sourceforge.net /documentation/kernel/group__vmm.php (602 words)

	Intoweb Training Virtual Expert
		Users are required to log in to the system and by doing this, their progress can be tracked by the administrator.
		This gives the administrator the power to check that users are using the system and what their main needs are.
		The training system can be customised to a companies needs and additional Virtual Expert systems can be created for training of company’s in-house software.
	www.intowebtraining.co.za /virtualexpert (288 words)

	Virtual Memory System : VMS
		The OpenVMS (Open Virtual Memory System) operating system (OS), also known as VMS, is a multiuser, multiprocessing OS that was designed by Digital (now Hewlett-Packard) in conjunction with their 32-bit VAX processor for use use in time sharing, batch processing, and transaction processing.
		OpenVMS also supports clustering (called VAXcluster or later VMScluster), where multiple systems share processing, job queues, print queues, and disk storage, either over a special computer interconnect (CI) bus or over Ethernet (called a LAVC, for local area network VMS cluster).
		The original name of the operating system was VMS, but it was renamed OpenVMS in the 1990s in a bid by Digital to position the product as an alternative to Unix.
	www.findword.org /vm/vms.html (541 words)

	The Spring Virtual Memory System - Khalidi, Nelson (ResearchIndex) (Site not responding. Last check: 2007-09-23)
		Abstract: : In this document we describe the architecture and the implementation of the Spring virtual memory system.
		A per-node virtual memory manager is responsible for maintaining the mappings on the local machine while external pagers are responsible for managing backing store.
		A novel aspect of the architecture is the separation of the memory abstraction...
	citeseer.ist.psu.edu /khalidi93spring.html (557 words)

	The Rialto Virtual Memory System
		This paper describes the design and implementation of virtual memory in the Rialto kernel.
		The Rialto VM system supports sparse virtual address spaces with mapping between address spaces, copy-on-write faults, and auto-commit (demand-allocation) faults.
		Novel aspects of the design include a VTLB architecture that bounds the memory consumed in machine-dependent mapping structures, real-time requirements for preeemptibility and interruptibility, provision for efficient sharing between user address spaces and the kernel via a user/kernel address bias, and growable kernel-mode thread stacks.
	research.microsoft.com /research/pubs/view.aspx?msr_tr_id=MSR-TR-97-04 (130 words)

	The DASH Virtual Memory System (Site not responding. Last check: 2007-09-23)
		We are now designing a portable operating system kernel for the nodes of this system.
		This report describes the DASH kernel's virtual memory (VM) system.
		The following are key features of the VM system: * A virtual address space is partitioned into three regions, each providing a specific function: 1) private memory, 2) read-only shared memory, and 3) interprocess communication (IPC) buffers.
	sunsite.berkeley.edu /TechRepPages/CSD-88-461 (308 words)

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