Editing Mainframe computer (section)

==Design==
Modern mainframe design is characterized less by raw computational speed and more by:
* Redundant internal engineering resulting in high reliability and security
* Extensive input-output ("I/O") facilities with the ability to offload to separate engines
* Strict [[backward compatibility]] with older software
* High hardware and computational utilization rates through virtualization to support massive [[throughput]]
* [[Hot swapping]] of hardware, such as processors and memory

The high stability and reliability of mainframes enable these machines to run uninterrupted for very long periods of time, with [[mean time between failures]] (MTBF) measured in decades.

Mainframes have [[high availability]], one of the primary reasons for their longevity, since they are typically used in applications where downtime would be costly or catastrophic. The term [[reliability, availability and serviceability]] (RAS) is a defining characteristic of mainframe computers. Proper planning and implementation are required to realize these features. In addition, mainframes are more secure than other computer types: the [[National Institute of Standards and Technology|NIST]] vulnerabilities database, [[US-CERT]], rates traditional mainframes such as [[IBM Z]] (previously called z Systems, System z, and zSeries),{{Vague|date=June 2023|reason=Presumably that's IBM Z machines running z/OS or z/VSE or z/VM, rather than running the Linux later credited with "thousands" of vulnerabilities - or is Linux on Z better than Linux on x86?}} [[Unisys]] Dorado, and Unisys Libra as among the most secure, with vulnerabilities in the low single digits, as compared to thousands for [[Windows]], [[UNIX]], and [[Linux]].<ref>{{cite web|title=National Vulnerability Database|access-date=September 20, 2011|url=http://nvd.nist.gov/home.cfm|archive-date=September 25, 2011|archive-url=https://web.archive.org/web/20110925095556/http://nvd.nist.gov/home.cfm|url-status=dead}}</ref> Software upgrades usually require setting up the [[operating system]] or portions thereof, and are non disruptive only when using [[Virtualization|virtualizing]] facilities such as IBM [[z/OS]] and [[IBM Parallel Sysplex|Parallel Sysplex]], or Unisys XPCL, which support workload sharing so that one system can take over another's application while it is being refreshed.

In the late 1950s, mainframes had only a rudimentary interactive interface (the console) and used sets of [[punched card]]s, [[punched tape|paper tape]], or [[magnetic-tape data storage|magnetic tape]] to transfer data and programs. They operated in [[batch processing|batch]] mode to support [[back office]] functions such as payroll and customer billing, most of which were based on repeated tape-based [[Merge sort|sorting and merging]] operations followed by [[line printer|line printing]] to preprinted [[continuous stationery]]. When interactive user terminals were introduced, they were used almost exclusively for applications (e.g. [[Computer reservations system|airline booking]]) rather than program development. However, in 1961 the first<ref>{{cite report |last1=Singh |first1=Jai P. |last2=Morgan |first2=Robert P. |date=October 1971 |title=Educational Computer Utilization and Computer Communications |url=https://files.eric.ed.gov/fulltext/ED057575.pdf |publisher=Washington University |location=St. Louis, MO |id=National Aeronautics and Space Administration Grant No. Y/NGL-26-008-054 |page=13 |access-date=March 8, 2022 |quote=Much of the early development in the time-sharing field took place on university campuses.<sup>8</sup> Notable examples are the CTSS (Compatible Time-Sharing System) at MIT, which was the first general purpose time-sharing system...}}</ref> academic, general-purpose timesharing system that supported software development,<ref>{{cite web |url=http://www.bitsavers.org/pdf/mit/ctss/CTSS_ProgrammersGuide_Dec69.pdf |title=The Compatible Time-Sharing System, A Programmer's Guide |editor-last=Crisman |editor-first=Patricia A. |date=December 31, 1969 |publisher=The M.I.T Computation Center |access-date=March 10, 2022}}</ref> [[Compatible Time-Sharing System|CTSS]], was released at [[Massachusetts Institute of Technology|MIT]] on an [[IBM 709]], later 7090 and 7094.<ref name="50th">{{cite web |url=https://multicians.org/thvv/compatible-time-sharing-system.pdf |title=Compatible Time-Sharing System (1961-1973): Fiftieth Anniversary Commemorative Overview |editor-last1=Walden |editor-first1=David |editor-last2=Van Vleck |editor-first2=Tom |editor2-link=Tom Van Vleck |date=2011 |publisher=IEEE Computer Society |access-date=February 20, 2022}}</ref> [[IBM 2741|Typewriter]] and [[Teletype Corporation|Teletype]] devices were common control consoles for system operators through the early 1970s, although ultimately supplanted by [[Computer keyboard|keyboard]]/[[Display device|display]] devices.

By the early 1970s, many mainframes acquired interactive user terminals<ref group=NB>Some had been introduced in the 1960s, but their deployment became more common in the 1970s</ref> operating as [[timesharing]] computers, supporting hundreds of users simultaneously along with batch processing. Users gained access through keyboard/typewriter terminals and later character-mode text<ref group=NB>Graphics terminals were available but tended to be restricted to niche applications.</ref> [[Computer terminal|terminal]] [[cathode-ray tube|CRT]] displays with integral keyboards, or finally from [[personal computer]]s equipped with [[terminal emulation]] software. By the 1980s, many mainframes supported general purpose graphic display terminals, and terminal emulation, but not graphical user interfaces. This form of end-user computing became obsolete in the 1990s due to the advent of personal computers provided with [[Graphical user interface|GUIs]]. After 2000, modern mainframes partially or entirely phased out classic "[[Monochrome monitor|green screen]]" and color display terminal access for end-users in favour of Web-style user interfaces.{{Citation needed|reason=Reliable source needed|date=April 2015}}

The infrastructure requirements were drastically reduced during the mid-1990s, when [[CMOS]] mainframe designs replaced the older [[Bipolar junction transistor|bipolar]] technology. IBM claimed that its newer mainframes reduced data center energy costs for power and cooling, and reduced physical space requirements compared to [[server farm]]s.<ref>{{cite web |title=Get the facts on IBM vs the Competition- The facts about IBM System z "mainframe" |publisher=IBM |access-date=December 28, 2009 |url=http://www-03.ibm.com/systems/migratetoibm/getthefacts/mainframe.html#4|archive-url=https://web.archive.org/web/20090211133418/http://www-03.ibm.com/systems/migratetoibm/getthefacts/mainframe.html#4|url-status=dead|archive-date=February 11, 2009}}</ref>