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{{Short description|Mainframe computer operating system}}
{{Use dmy dates|date=February 2017}}
{{Use British English|date=February 2017}}


The '''Edinburgh Multi-Access System''' ('''EMAS''') was a [[mainframe computer]] [[operating system]] at the [[University of Edinburgh]]. The system went online in 1971.<ref>{{cite journal|title=EMAS—The Edinburgh Multi-Access System|first1=H.|last1=Whitfield|first2=A. S.|last2=Wight|date=1 January 1973|journal=The Computer Journal|volume=16|issue=4|pages=331–346|doi=10.1093/comjnl/16.4.331|doi-access=free}}</ref>

EMAS was a powerful and efficient general purpose [[multi-user]] system which coped with many of the computing needs of the University of Edinburgh and the [[University of Kent]] (the only other site outside Edinburgh to adopt the operating system).

==History==
Originally running on the [[ICL System 4/75]] mainframe (based on the design of the [[IBM 360]]) it was later reimplemented <ref>{{cite web |url=http://www.gtoal.com/athome/edinburgh/docs/Experiment/again.html |title=Archived copy |accessdate=2004-10-09 |url-status=dead |archiveurl=https://web.archive.org/web/20041216160948/http://www.gtoal.com/athome/edinburgh/docs/Experiment/again.html |archivedate=16 December 2004 |df=dmy-all }}</ref><ref>{{cite journal|doi=10.1002/spe.4380101206 | volume=10 | issue=12 | title=The evolution of the operating system EMAS 2900 | journal=Software: Practice and Experience | pages=993–1008 | last1 = Stephens | first1 = P. D.| year=1980 | s2cid=31830708 }}</ref><ref>{{cite journal|doi=10.1002/spe.4380120707 | volume=12 | issue=7 | title=The kernel of the EMAS 2900 operating system | journal=Software: Practice and Experience | pages=655–667 | last1 = Rees | first1 = D. J.| year=1982 | s2cid=22177787 }}</ref> on the [[ICL 2900|ICL 2900 series of mainframes]] (as '''EMAS 2900''' or '''EMAS-2''') where it ran in service until the mid-1980s. Near the end of its life, the refactored version was back-ported (as '''EMAS-3''') to the [[Amdahl Corporation|Amdahl]] 470 mainframe clone, and thence to the IBM [[System/370-XA]] architecture (the latter with help from the [[University of Kent]], although they never actually ran EMAS-3). The [[Hitachi Data Systems|National Advanced System]] (NAS) VL80 IBM mainframe clone followed later. The final EMAS system (the Edinburgh VL80) was decommissioned in July 1992.

The [[University of Kent]] system went live in December 1979, and ran on the least powerful machine in the [[ICL 2900]] range - an ICL 2960, with 2 MB of memory, executing about 290k instructions per second. Despite this, it reliably supported around 30 users. This number increased in 1983 with the addition of an additional 2 MB of memory and a second Order Code Processor (OCP) (what is normally known as a CPU) running with [[symmetric multiprocessing]]. This system was decommissioned in August 1986.

==Features==
EMAS was written entirely in the [[Edinburgh IMP]] [[programming language]], with only a small number of critical functions using embedded assembler within IMP sources. It had several features that were advanced for the time, including [[dynamic linking]],<ref>{{cite web|url=http://history.dcs.ed.ac.uk/archive/os/emas/emas2/subsystem/doc/loader/|title=Index of /archive/os/emas/emas2/subsystem/doc/loader|website=history.dcs.ed.ac.uk}}</ref> multi-level storage, an efficient [[scheduling (computing)|scheduler]],<ref>{{cite book | doi=10.1007/BFb0029366 | chapter=The Edinburgh Multi-Access System scheduling and allocation procedures in the resident supervisor |volume = 16| pages=293–310 | last1 = Shelness | first1 = N. H.| series=Lecture Notes in Computer Science | title=Operating Systems |year = 1974|isbn = 978-3-540-06849-5}}</ref> a separate user-space [[kernel (operating system)|kernel]] ('director'),<ref>{{cite journal|title=The EMAS Director|first=D. J.|last=Rees|date=1 January 1975|journal=The Computer Journal|volume=18|issue=2|pages=122–130|doi=10.1093/comjnl/18.2.122|doi-access=free}}</ref> a user-level [[shell (computing)|shell]] ('basic command interpreter'),<ref>{{cite journal|title=The standard EMAS subsystem|first1=G. E.|last1=Millard|first2=D. J.|last2=Rees|first3=H.|last3=Whitfield|date=1 January 1975|journal=The Computer Journal|volume=18|issue=3|pages=213–219|doi=10.1093/comjnl/18.3.213|doi-access=free}}</ref> a comprehensive archiving system<ref>{{cite journal|title=The EMAS Archiving Program|first=A. S.|last=Wight|date=1 January 1975|journal=The Computer Journal|volume=18|issue=2|pages=131–134|doi=10.1093/comjnl/18.2.131|doi-access=free}}</ref> and a [[memory-mapped file]] architecture.

Such features led EMAS supporters to claim that their system was superior to [[Unix]] for the first 20 years of the latter's existence.{{citation needed|date=December 2016}}

==Legacy==
The Edinburgh Computer History Project is attempting to salvage some of the lessons learned<ref>{{cite web |url=https://history.dcs.ed.ac.uk/archive/os/emas/docs/taste/|title=More Taste: Less Greed?|last=Forsyth|first=C. H.|date=2011|website=history.dcs.ed.ac.uk|publisher=Department of Computer Science [[University of York]]|access-date= |quote=}}</ref> from the EMAS project and has the complete source code of EMAS online for public browsing.<ref>{{cite web|url=http://history.dcs.ed.ac.uk/archive/os/emas/|title=Index of /archive/os/emas|website=history.dcs.ed.ac.uk}}</ref>

==See also==
*[[Atlas Autocode]]

==References==
{{Reflist}}

{{Time-sharing operating systems}}

[[Category:History of computing in the United Kingdom]]
[[Category:Time-sharing operating systems]]
[[Category:1970s software]]
[[Category:University of Edinburgh School of Informatics]]

Edinburgh Multiple Access System - Revision history

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