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Unibus
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{{short description|Computer bus}} {{Infobox computer hardware bus | name = Unibus | fullname = Unibus | image = Unibus.jpg | caption = Unibus backplane (left) and two [[expansion cards]] | invent-date = {{Start date and age|1969}} | invent-name = [[Digital Equipment Corporation]] | super-name = | super-date = | replaces = | width = 18 address, 16 data | numdev = | speed = | style = p | hotplug = | external = }} [[File:DEC Core Memory 16K x 18 H217C.png|thumb|A DEC [[Core Memory]] Unibus card (16K Γ 18)]] The '''Unibus''' was the earliest of several computer [[bus (computing)|bus]] and [[backplane]] designs used with [[PDP-11]] and early [[VAX]] systems manufactured by the [[Digital Equipment Corporation]] (DEC) of [[Maynard, Massachusetts|Maynard]], [[Massachusetts]]. The Unibus was developed around 1969 by [[Gordon Bell]] and student Harold McFarland while at [[Carnegie Mellon University]].<ref>{{cite web |title= Bell (Gordon) Oral History |interviewer= Gardner, Hendrie |date= June 23, 2005 |url= http://www.computerhistory.org/collections/accession/102702036 |publisher= Computer History Museum |work= Reference number: X3202.2006 |access-date= May 20, 2011}}</ref> The name refers to the unified nature of the bus; Unibus was used both as a [[system bus]] allowing the [[central processing unit]] to communicate with [[main memory]], as well as a [[peripheral bus]], allowing peripherals to send and receive data. Unifying these formerly separate busses allowed external devices to easily perform [[direct memory access]] (DMA) and made the construction of [[device driver]]s easier as control and data exchange was all handled through [[memory-mapped I/O]].<ref>{{cite web |url=http://www.psych.usyd.edu.au/pdp-11/buses.html |title=PDP-11 Buses |website=University of Sydney |access-date=2020-04-30 |archive-date=2020-02-16 |archive-url=https://web.archive.org/web/20200216081243/http://www.psych.usyd.edu.au/pdp-11/buses.html |url-status=dead }}</ref> Unibus was physically large, which led to the introduction of [[Q-bus]], which [[multiplexed]] some signals to reduce pin count. Higher performance PDP systems used Fastbus, essentially two Unibusses in one. The system was later supplanted by [[Massbus]], a dedicated I/O bus introduced on the [[VAX]] and late-model PDP-11s.
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