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{{Other uses|Flex (disambiguation)}}
{{distinguish|FLEX (operating system)|FlexOS}}
{{Use dmy dates|date=September 2019}}
{{Use British English|date=September 2019}}

The '''Flex Computer System''' was developed by Michael Foster and Ian Currie of [[Royal Signals and Radar Establishment]] (RSRE)<ref>{{cite report|url=http://apps.dtic.mil/dtic/tr/fulltext/u2/a201602.pdf|archive-url=https://web.archive.org/web/20140503020404/http://www.dtic.mil/dtic/tr/fulltext/u2/a201602.pdf|url-status=live|archive-date=3 May 2014|title=On The Design And Implementation Of A Secure Computer System|first1=Phil F.|last1=Terry|first2=Simon R.|last2=Wiseman|publisher=Royal Signals and Radar Establishment|date=June 1988 |id=RSRE Memorandum No. 4188}}</ref> in [[Malvern, Worcestershire|Malvern]], England, during the late 1970s and 1980s. It used a tagged storage scheme to implement a [[Capability-based addressing|capability]] architecture, and was designed for the safe and efficient implementation of strongly typed procedures.

The hardware was custom and microprogrammable, with an operating system, (modular) compiler, editor and filing system all written in [[ALGOL 68RS]]. <!-- c.f. http://foldoc.org/flex -->

There were (at least) two incarnations of Flex, implemented using hardware with writable [[microcode]]. The first was supplied by [[Logica]] to an RSRE design,<ref name=rsre-intro>{{cite report |url=http://www.dtic.mil/docs/citations/ADA085010 |archive-url=https://web.archive.org/web/20180720193411/http://www.dtic.mil/docs/citations/ADA085010 |url-status=dead |archive-date=20 July 2018 |title=An Introduction to the FLEX Computer System |author=Foster J M, Moir C I, Currie I F, McDermid J A, Edwards P W |publisher=Royal Signals and Radar Establishment |id=Report No 79016 |date=October 1979 |access-date=12 February 2018}}</ref> and the second used an [[International Computers Limited|ICL]] [[PERQ]].<ref name=rsre-working>{{cite report |url=http://www.dtic.mil/docs/citations/ADA121375 |title=Flex: A Working Computer with an Architecture Based on Procedure Values |author=Foster J M, Currie I F, Edwards P W |publisher=Royal Signals and Radar Establishment |id=RSRE Memorandum No. 3500 |date=July 1982 |access-date=12 February 2018}}{{dead link|date=June 2022|bot=medic}}{{cbignore|bot=medic}}</ref><ref>{{cite report |first1=D. J.|last1=Tombs|first2=D. I.|last2=Bruce|title=The Evolution of Ten15|publisher=Royal Signals and Radar Establishment|date=Nov 1991|url=http://apps.dtic.mil/dtic/tr/fulltext/u2/a247367.pdf|archive-url=https://web.archive.org/web/20150924073146/http://www.dtic.mil/dtic/tr/fulltext/u2/a247367.pdf|url-status=live|archive-date=24 September 2015}}</ref>  The microcode alone was responsible for storage allocation, deallocation and garbage collection. This immediately precluded a whole class of errors arising from the misuse (deliberate or accidental) of pointers.

A notable feature of Flex was the tagged, write-once filestore. This allowed arbitrary code and data structures to be written and retrieved transparently, without recourse to external encodings. Data could thus be passed safely from program to program.

In a similar way, remote capabilities allowed data and procedures on other machines to be accessed over a network connection, again without the application program being involved in external encodings of data, parameters or result values.

The whole scheme allowed [[abstract data type]]s to be safely implemented, as data items and the procedures permitted to access them could be bound together, and the resulting capability passed freely around. The capability would grant access to the procedures, but could not be used in any way to obtain access to the data.

Another notable feature of Flex was the notion of shaky pointers, more recently often called [[weak reference]]s, which points to blocks of memory that could be freed at the next [[Garbage collection (computer science)|garbage collection]].  This is used for example for cached disc blocks or a list of spare procedure [[activation record|work-space]]s.<ref name=rsre-perqflex>{{cite report |url=http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA166398 |title=PerqFlex Firmware |author=Currie I F, Foster J M, Edwards P W |publisher=Royal Signals and Radar Establishment |id=Report No 85015 |date=December 1985 |access-date=6 February 2018}}{{dead link|date=June 2022|bot=medic}}{{cbignore|bot=medic}}</ref>

COMFLEX, a packet switching network capable of transmitting data at magnetic-disc speed, was developed alongside Flex. It made feasible the use of remote file-stores, remote capabilities, and [[remote procedure call]]s.<ref name=iee-mcdermid>{{cite journal |url=https://ieeexplore.ieee.org/document/4647592 |archive-url=https://web.archive.org/web/20180213081742/http://ieeexplore.ieee.org/document/4647592/ |url-status=dead |archive-date=13 February 2018 |title=Design and use of Comflex - a hardware-controlled packet switch |last=McDermid |first=J.A. |journal=[[IEE Proceedings E - Computers and Digital Techniques]] |volume=127 |issue=6 |date=November 1980 |page=233 |access-date=12 February 2018|doi=10.1049/ip-e.1980.0048  |url-access=subscription }}</ref>

== See also ==
* [[Royal Radar Establishment Automatic Computer]]
* [[Ten15]]
* [[PS-algol]]
* [[Rekursiv]]

== References ==
{{Reflist}}

==Further reading==
{{Refbegin}}
* Martin C. Atkins: [http://mca-ltd.com/martin/Ten15/introduction.html<cite> An Introduction to Ten15 - A personal retrospective.</cite>]{{Dead link|date=April 2024 |bot=InternetArchiveBot |fix-attempted=yes }} (includes a section about RSRE's Flex)
{{Refend}}

{{Object-capability security}}

{{DEFAULTSORT:Flex Machine}}
[[Category:Computers designed in the United Kingdom]]
[[Category:Capability systems]]
[[Category:Malvern, Worcestershire]]
[[Category:Mainframe computers]]
[[Category:Science and technology in Worcestershire]]