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The term arose during the early 1970s with the introduction of smaller, less complex computers such as the DEC PDP series, which became known as mini(computer)s. The industry/users then coined the term "mainframe" to describe larger, earlier types (previously known simply as "computers").
Modern mainframe computers' abilities are not so much defined by their CPU speed as by their massive internal memory, large, high-capacity external storage, fast high-throughput I/O, high-quality internal engineering and resulting proven reliability, and expensive but high-quality technical support. These machines can and do run successfully for years without interruption, with repairs taking place whilst they continue to run. Mainframe vendors offer such services as off-site redundancy—if a machine does break down, the vendor offers the option to run customers' applications on their own machines (often without users even noticing the change) whilst repairs go on.
Often, mainframes support thousands of simultaneous users who gain access through "dumb" terminals and early mainframes either supported this timesharing mode or operated in batch mode where users had no direct access to the computing service, it solely providing back officeA back office is a part of most corporations where tasks dedicated to running the company itself take place. Examples of back-office tasks include IT departments that keep the phones and computers running, accounting and human resources. The term comes fr functions. At this time mainframes were so called because of their very substantial size and requirements for specialised HVACHVAC is an initialism that stands for heating, ventilation, and air-conditioning . This is sometimes referred to as climate control . These three functions are closely interrelated, as they control the temperature and humidity of the air within the buildi and electrical power.
Some mainframes have the ability to run (or "host") multiple operating systems and thereby operate not as a single computer but as a number of " virtual machineIn general terms, a virtual machine in computer science is software that creates an environment between the computer platform and the end user in which the end user can operate software. Specifically, the term virtual machine has several distinct meaningss". In this role, a single mainframe can replace dozens or hundreds of smaller PCs, reducing management and administrative costs while providing greatly improved scalability and reliability. The reliability is improved because of the hardware redundancy noted above, and the scalability is achieved because hardware resources can be reallocated among the "virtual machines" as needed. This is much harder to do with PCs, because adding or removing hardware resources often requires the machine to be taken offline, and the hardware limitations are much more restrictive. When running as the host for many "virtual machines" a mainframe can provide the raw power for which they have always been valued, but also the flexibility provided by PC networks.
Currently, IBM mainframeAn IBM mainframe is an International Business Machines (IBM)-made mainframe computer, i. a traditionally "large" high-performance computer (in terms of price tag, physical size, transaction processing and I/O performance), although usually not as expensivs are dominant in the market, with Hitachi, Amdahl, and FujitsuFor the district in Saga, Japan, see Fujitsu, Saga . Fujitsu is a Japanese company specializing in semiconductors, computers ( supercomputers, personal computers, servers) , telecommunications, and services, and is headquartered in Tokyo. Fujitsu Limited also producing machines. Prices start at several hundred thousand dollars.