Editing Colossus computer (section)

==Design and construction==
[[File:GB-ENG - Bletchley - Computers - Buckinghamshire - Milton Keynes - Bletchly - Bletchley Park (4890148011).jpg|thumb|Valves (vacuum tubes) seen on end in a recreation of the Colossus computer]]
Colossus was developed for the "[[Newmanry]]",{{sfn|Good|Michie|Timms|1945|loc = 3 Organisation: 31 Mr Newman's section, p. 276}} the section headed by the mathematician [[Max Newman]] that was responsible for machine methods against the twelve-rotor [[Lorenz cipher|Lorenz SZ40/42 on-line teleprinter cipher machine]] (code-named Tunny, for tunafish). The Colossus design arose out of a parallel project that produced a less-ambitious counting machine dubbed "[[Heath Robinson (codebreaking machine)|Heath Robinson]]".{{sfn|GCHQ|2024}} Although the Heath Robinson machine proved the concept of machine analysis for this part of the process, it had serious limitations. The electro-mechanical parts were relatively slow and it was difficult to synchronise two looped [[punched tape|paper tapes]], one containing the enciphered message, and the other representing part of the keystream of the Lorenz machine.{{sfn|Anderson|2007|p=8}} Also the tapes tended to stretch and break when being read at up to 2000 characters per second.
[[File:COLOSSUS, part of the machine, presented by Director GCHQ to Director NSA in 1986 - National Cryptologic Museum - DSC07890.JPG|thumbnail|[[Stepping switch]] said to be from an original Colossus, presented by the Director of [[GCHQ]] to the Director of the [[National Security Agency|NSA]] to mark the 40th anniversary of the [[UKUSA Agreement]] in 1986<ref>Exhibit in the National Cryptologic Museum, Fort Meade, Maryland, USA</ref>]]

[[Tommy Flowers]] [[Order of the British Empire|MBE]]{{efn|Flowers had been appointed MBE in June 1943.}} was a senior electrical engineer and Head of the Switching Group at the [[Post Office Research Station]] at [[Dollis Hill]]. Prior to his work on Colossus, he had been involved with GC&CS at Bletchley Park from February 1941 in an attempt to improve the [[Bombe]]s that were used in the cryptanalysis of the German Enigma cipher machine.{{sfn|Randell|1980|p=9}} He was recommended to Max Newman by Alan Turing, who had been impressed by his work on the Bombes.{{sfn|Budiansky|2000|p=314}} The main components of the Heath Robinson machine were as follows.
* A tape transport and reading mechanism that ran the looped key and message tapes at between 1000 and 2000 characters per second.
* A combining unit that implemented the logic of [[Cryptanalysis of the Lorenz cipher#Tutte's "1+2 break in"|Tutte's method]].
* A counting unit that had been designed by [[C. E. Wynn-Williams]] of the [[Telecommunications Research Establishment]] (TRE) at Malvern, which counted the number of times the logical function returned a specified [[truth value]].

Flowers had been brought in to design the Heath Robinson's combining unit.{{sfn|Good|Michie|Timms|1945|loc = 1 Introduction: 15 Some Historical Notes, 15A First Stages in Machine Development, (c) Heath Robinson, p. 33}} He was not impressed by the system of a key tape that had to be kept synchronised with the message tape and, on his own initiative, he designed an electronic machine which eliminated the need for the key tape by having an electronic analogue of the Lorenz (Tunny) machine.{{sfn|Flowers|2006|p=96}} He presented this design to Max Newman in February 1943, but the idea that the one to two thousand thermionic valves ([[vacuum tube]]s and [[thyratron]]s) proposed, could work together reliably, was greeted with great scepticism,{{sfn|Flowers|1983|p=244}} so more Robinsons were ordered from Dollis Hill. Flowers, however, knew from his pre-war work that most thermionic valve failures occurred as a result of the thermal stresses at power-up, so not powering a machine down reduced failure rates to very low levels.{{sfn|Copeland "Machine against Machine"|2006|p=72}} Additionally, if the heaters were started at a low voltage then slowly brought up to full voltage, thermal stress was reduced. The valves themselves could be soldered-in to avoid problems with plug-in bases, which could be unreliable.{{citation needed|reason=There is no mention of these newly added points in Copeland (2006)|date=January 2018}} Flowers persisted with the idea and obtained support from the Director of the Research Station, W Gordon Radley.{{sfn|Copeland "Machine against Machine"|2006|p=74}}

Flowers and his team of some fifty people in the switching group{{sfn|Flowers|2006|p=80}}{{sfn|Randell|2006|p=143}} spent eleven months from early February 1943 designing and building a machine that dispensed with the second tape of the Heath Robinson, by generating the wheel patterns electronically. Flowers used some of his own money for the project.<ref>{{Citation | last = Boden | first = Margaret | author-link = Margaret Boden | title = Mind as Machine: A History of Cognitive Science | publisher = Oxford University Press | year = 2000 | page = [https://archive.org/details/mindasmachinehis0001bode/page/159 159] | isbn = 978-0199241446 | url = https://archive.org/details/mindasmachinehis0001bode/page/159 }}</ref><ref>{{Citation | last = Atkinson | first = Paul | title = Computer (Objekt) | publisher = Reaktion Books | year = 2010 | page = [https://archive.org/details/computer0000atki/page/29 29] | isbn = 978-1861896643 | url = https://archive.org/details/computer0000atki/page/29 }}</ref> This prototype, Mark 1 Colossus, contained 1,600 thermionic valves (tubes).{{sfn|Flowers|2006|p=80}} It performed satisfactorily at Dollis Hill on 8 December 1943{{sfn|Copeland|2010}} and was dismantled and shipped to Bletchley Park, where it was delivered on 18 January and re-assembled by [[Harry Fensom]] and Don Horwood.<ref name="The Colossus Computer"/><ref>{{Citation |last=Fensom |first=Jim |title=Harry Fensom obituary |date=8 November 2010 |url=https://www.theguardian.com/theguardian/2010/nov/08/harry-fensom-obituary |access-date=17 October 2012 |location=London |work=The Guardian}}</ref> It was operational in January<ref>{{Citation |editor-last= Sterling |editor-first= Christopher H. |title= Military Communications: From Ancient Times to the 21st Century |publisher= ABC-CLIO |year= 2007 |isbn= 978-1851097326}}</ref><ref name=lorenz>{{Citation |editor-last= Preneel |editor-first= Bart |title= Advances in Cryptology - EUROCRYPT 2000: International Conference on the Theory and Application of Cryptographic Techniques Bruges, Belgium May 14-18, 2000, Proceedings |publisher=  Springer |series= Lecture Notes in Computer Science |year= 2000 |isbn= 978-3540675174|contribution-url=https://www.iacr.org/archive/eurocrypt2000/1807/18070423-new.pdf |archive-url=https://web.archive.org/web/20081120221724/http://www.iacr.org/archive/eurocrypt2000/1807/18070423-new.pdf |archive-date=2008-11-20 |url-status=live |contribution=Colossus and the German Lorenz Cipher – Code Breaking in WW II |page=417 |doi=10.1007/3-540-45539-6_29 |doi-access= free }}</ref> and it successfully attacked its first message on 5 February 1944.{{sfn|Copeland "Machine against Machine"|2006|p=75}} It was a large structure and was dubbed 'Colossus'. A memo held in the National Archives written by Max Newman on 18 January 1944 records that "Colossus arrives today".{{sfn|Gannon|2007|p=283}}

During the development of the prototype, an improved design had been developed – the Mark 2 Colossus. Four of these were ordered in March 1944 and by the end of April the number on order had been increased to twelve. Dollis Hill was put under pressure to have the first of these working by 1 June.{{sfn|Good|Michie|Timms|1945|loc = 1 Introduction: 15 – Some Historical Notes, 15C Period of Expansion, (b) Colossus, p. 35}} [[Allen Coombs]] took over leadership of the production Mark 2 Colossi, the first of which – containing 2,400 valves – became operational at 08:00 on 1 June 1944, just in time for the Allied [[Invasion of Normandy]] on [[Normandy landings|D-Day]].<ref>{{Citation | last1 = Randell | first1 = Brian | author-link = Brian Randell | last2 = Fensom | first2 = Harry | last3 = Milne | first3 = Frank A. | title = Obituary: Allen Coombs | newspaper = The Independent | date = 15 March 1995 | url = https://www.independent.co.uk/news/people/obituary-allen-coombs-1611270.html | access-date = 18 October 2012 | location=London}}</ref> Subsequently, Colossi were delivered at the rate of about one a month. By the time of [[Victory in Europe Day|V-E Day]] there were ten Colossi working at Bletchley Park and a start had been made on assembling an eleventh.{{sfn|Good|Michie|Timms|1945|loc = 1 Introduction: 15 – Some Historical Notes, 15C Period of Expansion, (b) Colossus, p. 35}} Seven of the Colossi were used for 'wheel setting' and three for 'wheel breaking'.{{sfn|Kenyon|2019||p=60}}

[[File:Wartime photo of Colossus 10.png|right|thumbnail|upright=1.35|Colossus 10 with its extended bedstead in Block H at [[Bletchley Park]] in the space now containing the Tunny gallery of [[The National Museum of Computing]] ]]
The main units of the Mark 2 design were as follows.{{sfn|Flowers|2006|p=96}}{{sfn|Flowers|1983|pp=249–252}}
* A tape transport with an 8-photocell reading mechanism.
* A six character [[FIFO (computing and electronics)|FIFO]] [[shift register]].
* Twelve thyratron ring stores that simulated the Lorenz machine generating a bit-stream for each wheel.
* Panels of switches for specifying the program and the "set total".
* A set of functional units that performed [[Boolean algebra|Boolean]] operations.
* A "span counter" that could suspend counting for part of the tape.
* A master control that handled clocking, start and stop signals, counter readout and printing.
* Five electronic counters.
* An electric typewriter.

Most of the design of the electronics was the work of Tommy Flowers, assisted by William Chandler, Sidney Broadhurst and Allen Coombs; with Erie Speight and [[Arnold Lynch]] developing the photoelectric reading mechanism.{{sfn|Flowers|1983|pp=243, 245}} Coombs remembered Flowers, having produced a rough draft of his design, tearing it into pieces that he handed out to his colleagues for them to do the detailed design and get their team to manufacture it.{{sfn|Coombs|1983}} The Mark 2 Colossi were both five times faster and were simpler to operate than the prototype.{{efn|For comparison, later [[stored-program computer]]s such as the [[Manchester Mark 1]] of 1949 used 4,050 valves,<ref>{{citation |last=Lavington |first=S. H. |title=The Manchester Mark 1 and Atlas: a Historical Perspective |journal=Communications of the ACM |volume=21|issue=1| url=http://www.cs.ucf.edu/courses/cda5106/summer03/papers/mark1.atlas.1.pdf |archive-url=https://web.archive.org/web/20040114142723/http://www.cs.ucf.edu/courses/cda5106/summer03/papers/mark1.atlas.1.pdf |archive-date=2004-01-14 |url-status=live |date=July 1977 |access-date=8 February 2009 |doi=10.1145/359327.359331|pages=4–12| s2cid=10301670}}</ref> while [[ENIAC]] (1946) used 17,468 valves.}}

Data input to Colossus was by [[Photoelectric sensor|photoelectric]] reading of a paper tape transcription of the enciphered intercepted message. This was arranged in a continuous loop so that it could be read and re-read multiple times – there being no internal storage for the data. The design overcame the problem of synchronizing the electronics with the speed of the message tape by generating a [[clock signal]] from reading its sprocket holes. The speed of operation was thus limited by the mechanics of reading the tape. During development, the tape reader was tested up to 9700 characters per second (53&nbsp;mph) before the tape disintegrated. So 5000 characters/second ({{convert|40|ft/s|m/s mph||abbr=on|sigfig=3}}) was settled on as the speed for regular use. Flowers designed a 6-character shift register, which was used both for computing the delta function (ΔZ) and for testing five different possible starting points of Tunny's wheels in the five processors.{{sfn|Flowers|1983}}{{sfn|Flowers|2006|p=100}} This five-way parallelism{{efn|This would now be called a [[systolic array]].}} enabled five simultaneous tests and counts to be performed giving an effective processing speed of 25,000 characters per second.{{sfn|Flowers|2006|p=100}} The computation used algorithms devised by [[W. T. Tutte]] and colleagues to decrypt a Tunny message.{{sfn|Copeland|2011}}<ref>{{cite web|url=https://uwaterloo.ca/combinatorics-and-optimization/about/professor-william-t-tutte/biography-professor-tutte#bletchley|title=Biography of Professor Tutte - Combinatorics and Optimization|date=13 March 2015|website=Uwaterloo.ca|access-date=26 October 2017|archive-date=19 August 2019|archive-url=https://web.archive.org/web/20190819115149/https://uwaterloo.ca/combinatorics-and-optimization/about/professor-william-t-tutte/biography-professor-tutte#bletchley|url-status=dead}}</ref>