HOME

TheInfoList




In
computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and software. It has sci ...

computing
, virtual memory, or virtual storage is a
memory management Memory management is a form of resource management In organizational studies Organizational studies is "the examination of how individuals construct organizational structures, processes, and practices and how these, in turn, shape social relat ...
technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very large (main) memory". The computer's
operating system An operating system (OS) is system software System software is software designed to provide a platform for other software. Examples of system software include operating systems (OS) like macOS, Linux, Android (operating system), Android and Mi ...

operating system
, using a combination of hardware and software, maps
memory address In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softwa ...
es used by a program, called ''
virtual addresses
virtual addresses
'', into ''physical addresses'' in
computer memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and soft ...
. Main storage, as seen by a process or task, appears as a contiguous
address space In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algorithm) for calculating the greatest common ...
or collection of contiguous segments. The operating system manages
virtual address space In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algorithm) for calculating the greatest commo ...

virtual address space
s and the assignment of real memory to virtual memory. Address translation hardware in the CPU, often referred to as a
memory management unit MMU could be used with the Motorola 68010 The Motorola MC68010 processor is a 16/32-bit microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a s ...
(MMU), automatically translates virtual addresses to physical addresses. Software within the operating system may extend these capabilities, utilizing, e.g.,
disk storage Disk storage (also sometimes called drive storage) is a general category of storage mechanisms where data is recorded by various electronic, magnetic, optical, or mechanical changes to a surface layer of one or more rotating disks. A disk drive is ...
, to provide a virtual address space that can exceed the capacity of real memory and thus reference more memory than is physically present in the computer. The primary benefits of virtual memory include freeing applications from having to manage a shared memory space, ability to share memory used by
libraries A library is a collection of materials, books or media that are easily accessible for use and not just for display purposes. It is responsible for housing updated information in order to meet the user's needs on a daily basis. A library provi ...
between processes, increased security due to memory isolation, and being able to conceptually use more memory than might be physically available, using the technique of
paging In computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Computer program, programs. These prog ...

paging
or segmentation.


Properties

Virtual memory makes application programming easier by hiding fragmentation of physical memory; by delegating to the kernel the burden of managing the
memory hierarchy In computer architecture In computer engineering Computer engineering (CoE or CpE) is a branch of engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other i ...
(eliminating the need for the program to handle overlays explicitly); and, when each process is run in its own dedicated address space, by obviating the need to relocate program code or to access memory with relative addressing.
Memory virtualizationIn computer science, memory virtualization decouples volatile random access memory (RAM) resources from individual systems in the data centre, and then aggregates those resources into a virtualized memory pool available to any computer in the cluster ...
can be considered a generalization of the concept of virtual memory.


Usage

Virtual memory is an integral part of a modern
computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer systems. The architecture of a system refers to its structure in terms of separately specifi ...
; implementations usually require hardware support, typically in the form of a
memory management unit MMU could be used with the Motorola 68010 The Motorola MC68010 processor is a 16/32-bit microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a s ...
built into the
CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuit File:PExdcr01CJC.jpg, 200px, A circuit built on a printed circuit board (PCB). An electronic circuit is composed of ...

CPU
. While not necessary,
emulators emulates the command-line interface A command-line interface (CLI) processes commands to a computer program in the form of lines of text. The program which handles the interface is called a command-line interpreter or command-line processor ...
and
virtual machine In computing, a virtual machine (VM) is the virtualization In computing, virtualization or virtualisation (sometimes abbreviated v12n, a numeronym) is the act of creating a virtual (rather than actual) version of something, including virtual co ...
s can employ hardware support to increase performance of their virtual memory implementations. Older operating systems, such as those for the
mainframes A mainframe computer, informally called a mainframe or big iron, is a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can perform generic ...
of the 1960s, and those for personal computers of the early to mid-1980s (e.g.,
DOS DOS (, ) is a platform-independent acronym for disk operating system which later became a common shorthand for disk-based operating systems on IBM PC compatible IBM PC compatible computers are similar to the original IBM Personal Computer, IB ...

DOS
), generally have no virtual memory functionality, though notable exceptions for mainframes of the 1960s include: * the
Atlas Supervisor The Atlas Supervisor was the program which managed the allocation of processing resources of Manchester University's Atlas Computer so that the machine was able to act on many tasks and user programs concurrently. Its various functions included ...
for the
Atlas Blaeu's world map, originally prepared by Joan Blaeu for his ''Atlas Maior">Joan_Blaeu.html" ;"title="world map, originally prepared by Joan Blaeu">world map, originally prepared by Joan Blaeu for his ''Atlas Maior'', published in the first b ...
*
THE multiprogramming system The THE multiprogramming system or THE OS was a computer operating system An operating system (OS) is system software that manages computer hardware, computer software, software resources, and provides common daemon (computing), services for co ...
for the
Electrologica X8
Electrologica X8
(software based virtual memory without hardware support) * MCP for the
BurroughsBurroughs may refer to: * Former spelling of boroughs *Burroughs, Georgia, a historically African American community now a neighborhood of Savannah, Georgia * Burroughs Corporation, a maker of adding machines and computers * Burroughs (surname), peo ...
B5000 The Burroughs Large Systems Group produced a family of large 48-bit mainframes using stack machine instruction sets with dense Syllable (computing), syllables.E.g., 12-bit syllables for B5000, 8-bit syllables for B6500 The first machine in the fami ...
* MTS,
TSS/360 IBM System/360 Model 67-2. This is the computer model on which TSS/360 would have run The IBM Time Sharing System TSS/360 is a discontinued early time-sharing operating system designed exclusively for a special model of the System/360 line of mai ...
and
CP/CMS CP/CMS (Control Program/Cambridge Monitor System) is a discontinued time-sharing operating system of the late 60s and early 70s, known for its excellent performance and advanced features. It had three distinct versions: * IBM CP-40, CP-40/CMS, ...
for the
IBM System/360 Model 67 The IBM System/360 Model 67 (S/360-67) was an important IBM mainframe IBM mainframes are large computer systems produced by IBM since 1952. During the 1960s and 1970s, IBM dominated the large computer market. Current mainframe computers in I ...
*
Multics Multics ("Multiplexed Information and Computing Service") is an influential early time-sharing In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experi ...

Multics
for the
GE 645 The GE 645 mainframe computer A mainframe computer, informally called a mainframe or big iron, is a computer used primarily by large organizations for critical applications like bulk data processing for tasks such as census, censuses, industr ...
* The Time Sharing Operating System for the
RCA Spectra 70 The RCA Spectra 70 was a line of electronic data processing Electronic data processing (EDP) can refer to the use of automated methods to process commercial data. Typically, this uses relatively simple, repetitive activities to process large volum ...
/46 During the 1960s and early '70s, computer memory was very expensive. The introduction of virtual memory provided an ability for software systems with large memory demands to run on computers with less real memory. The savings from this provided a strong incentive to switch to virtual memory for all systems. The additional capability of providing virtual address spaces added another level of security and reliability, thus making virtual memory even more attractive to the marketplace. Most modern operating systems that support virtual memory also run each
process A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic. Things called a process include: Business and management *Business process A business process, business method ...
in its own dedicated
address space In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algorithm) for calculating the greatest common ...
. Each program thus appears to have sole access to the virtual memory. However, some older operating systems (such as
OS/VS1 Operating System/Virtual Storage 1, or OS/VS1, is a discontinued IBM mainframe computer operating system designed to be run on IBM System/370 hardware. It was the successor to the OS/360 and successors#MFT, Multiprogramming with a Fixed number of T ...
and OS/VS2 SVS) and even modern ones (such as
IBM i IBM i (the ''i'' standing for ''integrated'') is an operating system An operating system (OS) is system software System software is software designed to provide a platform for other software. Examples of system software include operating sys ...

IBM i
) are
single address space operating systemIn computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algorith ...
s that run all processes in a single address space composed of virtualized memory.
Embedded system An embedded system is a computer system A computer is a machine that can be programmed to carry out Sequence, sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Co ...
s and other special-purpose computer systems that require very fast and/or very consistent response times may opt not to use virtual memory due to decreased
determinism Determinism is the philosophical Philosophy (from , ) is the study of general and fundamental questions, such as those about existence Existence is the ability of an entity to interact with physical or mental reality Reality is the ...
; virtual memory systems trigger unpredictable traps that may produce unwanted and unpredictable delays in response to input, especially if the trap requires that data be read into main memory from secondary memory. The hardware to translate virtual addresses to physical addresses typically requires a significant chip area to implement, and not all chips used in embedded systems include that hardware, which is another reason some of those systems don't use virtual memory.


History

In the 1940s and 1950s, all larger programs had to contain logic for managing primary and secondary storage, such as overlaying. Virtual memory was therefore introduced not only to extend primary memory, but to make such an extension as easy as possible for programmers to use. To allow for multiprogramming and multitasking, many early systems divided memory between multiple programs without virtual memory, such as early models of the
PDP-10 Digital Equipment Corporation Digital Equipment Corporation (DEC ), using the Digital, was a major American company in the from the 1960s to the 1990s. The company was co-founded by and in 1957. Olsen was president until forced to resi ...
via registers. A claim that the concept of virtual memory was first developed by German
physicist A physicist is a scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empirical method of acquiring knowledge that has characterized the development of science since at leas ...

physicist
Fritz-Rudolf Güntsch
Fritz-Rudolf Güntsch
at the Technische Universität Berlin in 1956 in his doctoral thesis, ''Logical Design of a Digital Computer with Multiple Asynchronous Rotating Drums and Automatic High Speed Memory Operation'' does not stand up to careful scrutiny. The computer proposed by Güntsch (but never built) had an address space of 105 words which mapped exactly on to the 105 words of the drums, ''i.e.'' the addresses were real addresses and there was no form of indirect mapping, a key feature of virtual memory. What Güntsch did invent was a form of
cache memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and s ...
, since his high-speed memory was intended to contain a copy of some blocks of code or data taken from the drums. Indeed, he wrote (as quoted in translation): “The programmer need not respect the existence of the primary memory (he need not even know that it exists), for there is only one sort of addresses (''sic'') by which one can program as if there were only one storage.” This is exactly the situation in computers with cache memory, one of the earliest commercial examples of which was the IBM System/360 Model 85. In the Model 85 all addresses were real addresses referring to the main core store. A semiconductor cache store, invisible to the user, held the contents of parts of the main store in use by the currently executing program. This is exactly analogous to Güntsch's system, designed as a means to improve performance, rather than to solve the problems involved in multi-programming. The first true virtual memory system was that implemented at the
University of Manchester , mottoeng = Knowledge, Wisdom, Humanity , established = 2004 – University of Manchester Predecessor institutions: 1956 – UMIST , mottoeng = By Knowledge and Work , established = 1824 , closed = 2004 (merge ...

University of Manchester
to create a one-level storage system as part of the
Atlas Computer The Atlas Computer was one of the world's first supercomputers, in use from 1962 until 1971. It was considered to be the most powerful computer in the world at that time. Atlas' capacity promoted the saying that when it went offline, half of t ...
. It used a
paging In computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Computer program, programs. These prog ...

paging
mechanism to map the virtual addresses available to the programmer on to the real memory that consisted of 16,384 words of primary
core memory Core or cores may refer to: Science and technology * Core (anatomy) In common parlance, the core of the body is broadly considered to be the torso. Functional movements are highly dependent on this part of the body, and lack of core muscular de ...
with an additional 98,304 words of secondary
drum memory Drum memory was a magnetic data storage device On a reel-to-reel tape recorder (Sony TC-630), the recorder is data storage equipment and the magnetic tape is a data storage medium. Data Data (; ) are individual facts, statistics, ...
. The first Atlas was commissioned in 1962 but working prototypes of paging had been developed by 1959. In 1961, the
Burroughs Corporation The Burroughs Corporation was a major American manufacturer of business equipment. The company was founded in 1886 as the American Arithmometer Company. In 1986, it merged with UNIVAC, Sperry UNIVAC to form Unisys. The company's history paralle ...
independently released the first commercial computer with virtual memory, the
B5000 The Burroughs Large Systems Group produced a family of large 48-bit mainframes using stack machine instruction sets with dense Syllable (computing), syllables.E.g., 12-bit syllables for B5000, 8-bit syllables for B6500 The first machine in the fami ...
, with
segmentation Segment or segmentation may refer to: Biology *Segmentation (biology), the division of body plans into a series of repetitive segments **Segmentation in the human nervous system *Internodal segment, the portion of a nerve fiber between two Nodes of ...
rather than paging. Before virtual memory could be implemented in mainstream operating systems, many problems had to be addressed. Dynamic address translation required expensive and difficult-to-build specialized hardware; initial implementations slowed down access to memory slightly. There were worries that new system-wide algorithms utilizing secondary storage would be less effective than previously used application-specific algorithms. By 1969, the debate over virtual memory for commercial computers was over; an
IBM International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 1911, founded in Endicott, New York, as the C ...

IBM
research team led by
David Sayre David Sayre (March 2, 1924 – February 23, 2012) was an American scientist, credited with the early development of direct methods (crystallography), direct methods for X-ray crystallography#Protein crystallography, protein crystallography and of ...
showed that their virtual memory overlay system consistently worked better than the best manually controlled systems. Throughout the 1970s, the IBM 370 series running their virtual-storage based operating systems provided a means for business users to migrate multiple older systems into fewer, more powerful, mainframes that had improved price/performance. The first
minicomputer A minicomputer, or colloquially mini, is a class of smaller general purpose computers that developed in the mid-1960s and sold for much less than Mainframe computer, mainframe and mid-size computers from IBM and BUNCH, its direct competitors. In ...
to introduce virtual memory was the Norwegian NORD-1; during the 1970s, other minicomputers implemented virtual memory, notably
VAX VAX is a series of computers featuring a 32-bit 32-bit microcomputers are computers in which 32-bit microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrate ...
models running VMS. Virtual memory was introduced to the
x86 x86 is a family of instruction set architecture In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for th ...

x86
architecture with the
protected mode In computing, protected mode, also called protected virtual address mode, is an operational mode of x86 x86 is a family of instruction set architecture In computer science, an instruction set architecture (ISA), also called computer a ...
of the
Intel 80286 The Intel Intel Corporation is an American multinational corporation A multinational company (MNC) is a corporate A corporation is an organization—usually a group of people or a company A company, abbreviated as co., is a Le ...

Intel 80286
processor, but its segment swapping technique scaled poorly to larger segment sizes. The
Intel 80386 The Intel Intel Corporation is an American multinational corporation A multinational company (MNC) is a corporate A corporation is an organization—usually a group of people or a company A company, abbreviated as co., is a ...
introduced paging support underneath the existing
segmentation Segment or segmentation may refer to: Biology *Segmentation (biology), the division of body plans into a series of repetitive segments **Segmentation in the human nervous system *Internodal segment, the portion of a nerve fiber between two Nodes of ...
layer, enabling the page fault exception to chain with other exceptions without
double fault On the x86 architecture, a double fault exception occurs if the central processing unit, processor encounters a problem while trying to service a pending interrupt or Exception handling, exception. An example situation when a double fault would occ ...
. However, loading segment descriptors was an expensive operation, causing operating system designers to rely strictly on paging rather than a combination of paging and segmentation.


Paged virtual memory

Nearly all current implementations of virtual memory divide a
virtual address space In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algorithm) for calculating the greatest commo ...

virtual address space
into
page Page most commonly refers to: * Page (paper) A page is one side of a leaf A leaf (plural leaves) is the principal lateral appendage of the , usually borne above ground and specialized for . The leaves, stem, flower and fruit togethe ...
s, blocks of contiguous virtual memory addresses. Pages on contemporary systems are usually at least 4
kilobyte The kilobyte is a multiple of the unit byte The byte is a unit of digital information that most commonly consists of eight bit The bit is a basic unit of information in computing Computing is any goal-oriented activity requiring, bene ...
s in size; systems with large virtual address ranges or amounts of real memory generally use larger page sizes.


Page tables

Page table A page table is the data structure In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. ...
s are used to translate the virtual addresses seen by the application into
physical address In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softw ...
es used by the hardware to process instructions; such hardware that handles this specific translation is often known as the
memory management unit MMU could be used with the Motorola 68010 The Motorola MC68010 processor is a 16/32-bit microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a s ...
. Each entry in the page table holds a flag indicating whether the corresponding page is in real memory or not. If it is in real memory, the page table entry will contain the real memory address at which the page is stored. When a reference is made to a page by the hardware, if the page table entry for the page indicates that it is not currently in real memory, the hardware raises a
page fault In computing, a page fault (sometimes called PF or hard fault) is an exception that the memory management unit MMU could be used with the Motorola 68010 The Motorola MC68010 processor is a 16/32-bit microprocessor A microprocessor is a ...
exception, invoking the paging supervisor component of the
operating system An operating system (OS) is system software System software is software designed to provide a platform for other software. Examples of system software include operating systems (OS) like macOS, Linux, Android (operating system), Android and Mi ...

operating system
. Systems can have one page table for the whole system, separate page tables for each application and segment, a tree of page tables for large segments or some combination of these. If there is only one page table, different applications running at the same time use different parts of a single range of virtual addresses. If there are multiple page or segment tables, there are multiple virtual address spaces and concurrent applications with separate page tables redirect to different real addresses. Some earlier systems with smaller real memory sizes, such as the
SDS 940 The SDS 940 was Scientific Data SystemsScientific Data Systems (SDS), was an United States, American computer company founded in September 1961 by Max Palevsky and Robert Beck, veterans of Packard Bell Corporation and Bendix Corporation, Bendix, al ...
, used '' page registers'' instead of page tables in memory for address translation.


Paging supervisor

This part of the operating system creates and manages page tables. If the hardware raises a page fault exception, the paging supervisor accesses secondary storage, returns the page that has the virtual address that resulted in the page fault, updates the page tables to reflect the physical location of the virtual address and tells the translation mechanism to restart the request. When all physical memory is already in use, the paging supervisor must free a page in primary storage to hold the swapped-in page. The supervisor uses one of a variety of
page replacement algorithm In a computer A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Computer program, ...

page replacement algorithm
s such as
least recently used In computing, cache algorithms (also frequently called cache replacement algorithms or cache replacement policies) are Optimization (computer science), optimizing instructions, or algorithms, that a computer program or a hardware-maintained struc ...
to determine which page to free.


Pinned pages

Operating systems have memory areas that are ''pinned'' (never swapped to secondary storage). Other terms used are ''locked'', ''fixed'', or ''wired'' pages. For example,
interrupt In digital computer A computer is a machine A machine is a man-made device that uses power to apply forces and control movement to perform an action. Machines can be driven by animals and people A people is a plurality of pe ...

interrupt
mechanisms rely on an array of pointers to their handlers, such as
I/O In computing, input/output (I/O, or informally io or IO) is the communication between an information processing system, such as a computer, and the outside world, possibly a human or another information processing system. Information, Inputs are t ...
completion and
page fault In computing, a page fault (sometimes called PF or hard fault) is an exception that the memory management unit MMU could be used with the Motorola 68010 The Motorola MC68010 processor is a 16/32-bit microprocessor A microprocessor is a ...
. If the pages containing these pointers or the code that they invoke were pageable, interrupt-handling would become far more complex and time-consuming, particularly in the case of page fault interruptions. Hence, some part of the page table structures is not pageable. Some pages may be pinned for short periods of time, others may be pinned for long periods of time, and still others may need to be permanently pinned. For example: * The paging supervisor code and drivers for secondary storage devices on which pages reside must be permanently pinned, as otherwise paging wouldn't even work because the necessary code wouldn't be available. * Timing-dependent components may be pinned to avoid variable paging delays. *
Data buffer In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of , , ...
s that are accessed directly by peripheral devices that use
direct memory access Direct memory access (DMA) is a feature of computer systems that allows certain hardware subsystems to access main system memory Memory is the faculty of the by which or is , stored, and retrieved when needed. It is the retention of inform ...
or I/O channels must reside in pinned pages while the I/O operation is in progress because such devices and the
buses A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a road vehicle A vehicle (from la, vehiculum) is a machine A machine is a man-made device that uses power to apply forces and control movement t ...
to which they are attached expect to find data buffers located at physical memory addresses; regardless of whether the bus has a memory management unit for I/O, transfers cannot be stopped if a page fault occurs and then restarted when the page fault has been processed. In IBM's operating systems for
System/370 The IBM System/370 (S/370) is a model range of IBM mainframe computers announced on June 30, 1970 as the successors to the IBM System/360, System/360 family. The series mostly maintains backward compatibility with the S/360, allowing an easy ...
and successor systems, the term is "fixed", and such pages may be long-term fixed, or may be short-term fixed, or may be unfixed (i.e., pageable). System control structures are often long-term fixed (measured in wall-clock time, i.e., time measured in seconds, rather than time measured in fractions of one second) whereas I/O buffers are usually short-term fixed (usually measured in significantly less than wall-clock time, possibly for tens of milliseconds). Indeed, the OS has a special facility for "fast fixing" these short-term fixed data buffers (fixing which is performed without resorting to a time-consuming
Supervisor Call instruction : ''This article covers the specific instruction on the IBM System/360 and successor mainframe computer A mainframe computer, informally called a mainframe or big iron, is a computer A computer is a machine that can be programmed to car ...
).
Multics Multics ("Multiplexed Information and Computing Service") is an influential early time-sharing In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experi ...

Multics
used the term "wired".
OpenVMS OpenVMS, often referred to as just VMS, is a multi-user Multi-user software is software, computer software that allows access by multiple User (computing), users of a computer. Time-sharing systems are multi-user systems. Most batch processing s ...
and
Windows Microsoft Windows, commonly referred to as Windows, is a group of several proprietary {{Short pages monitor