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Colossus computer
A Colossus Mark 2 computer being operated by Wrens.[1] The slanted control panel on the left was used to set the 'pin' (or 'cam') patterns of the Lorenz. The 'bedstead' paper tape transport is on the right.
DeveloperTommy Flowers, assisted by Sidney Broadhurst, William Chandler and for the Mark 2 machines, Allen Coombs
ManufacturerPost Office Research Station
TypeSpecial-purpose electronic digital programmable computer
GenerationFirst-generation computer
Release date
Units shipped12
  • Electric typewriter output
  • Programmed using switches and plug panels
CPUCustom circuits using thermionic valves and thyratrons. A total of 1,600 in Mk 1 and 2,400 in Mk 2. Also relays and stepping switches
MemoryNone (no RAM)
DisplayIndicator lamp panel
InputPaper tape of up to 20,000 × 5-bit characters in a continuous loop
Power8.5 kW[2]

Colossus was a set of computers developed by British codebreakers in the years 1943–1945 to help in the cryptanalysis of the Lorenz cipher. Colossus used thermionic valves (vacuum tubes) to perform Boolean and counting operations. Colossus is thus regarded[3] as the world's first programmable, electronic, digital computer, although it was programmed by switches and plugs and not by a stored program.[4]

Colossus was designed by research telephone engineer Tommy Flowers to solve a problem posed by mathematician Max Newman at the Government Code and Cypher School (GC&CS) at Bletchley Park. Alan Turing's use of probability in cryptanalysis (see Banburismus) contributed to its design. It has sometimes been erroneously stated that Turing designed Colossus to aid the cryptanalysis of the Enigma.[5] Turing's machine that helped decode Enigma was the electromechanical Bombe, not Colossus.[6]

The prototype, Colossus Mark 1, was shown to be working in December 1943 and was in use at Bletchley Park by early 1944. An improved Colossus Mark 2 that used shift registers to quintuple the processing speed, first worked on 1 June 1944, just in time for the Normandy landings on D-Day.[7] Ten Colossi were in use by the end of the war and an eleventh was being commissioned.[7] Bletchley Park's use of these machines allowed the Allies to obtain a vast amount of high-level military intelligence from intercepted radiotelegraphy messages between the German High Command (OKW) and their army commands throughout occupied Europe.

The existence of the Colossus machines was kept secret until the mid-1970s; the machines and the plans for building them had previously been destroyed in the 1960s as part of the effort to maintain the secrecy of the project.[8][9] This deprived most of those involved with Colossus of the credit for pioneering electronic digital computing during their lifetimes. A functioning rebuild of a Mark 2 Colossus was completed in 2008 by Tony Sale and some volunteers; it is on display at The National Museum of Computing at Bletchley Park.[10][11][12]

Purpose and origins[edit]

A Lorenz SZ42 cipher machine with its covers removed at The National Museum of Computing on Bletchley Park
The Lorenz SZ machines had 12 wheels, each with a different number of cams (or 'pins').
Wheel number 123456789101112
BP wheel name[13]ψ1ψ2ψ3ψ4ψ5μ37μ61χ1χ2χ3χ4χ5
Number of cams (pins) 434751535937614131292623

The Colossus computers were used to help decipher intercepted radio teleprinter messages that had been encrypted using an unknown device. Intelligence information revealed that the Germans called the wireless teleprinter transmission systems 'Sägefisch' (sawfish). This led the British to call encrypted German teleprinter traffic 'Fish',[14] and the unknown machine and its intercepted messages 'Tunny' (tunafish).[15]

Before the Germans increased the security of their operating procedures, British cryptanalysts diagnosed how the unseen machine functioned and built an imitation of it called 'British Tunny'.

It was deduced that the machine had twelve wheels and used a Vernam ciphering technique on message characters in the standard 5-bit ITA2 telegraph code. It did this by combining the plaintext characters with a stream of key characters using the XORBoolean function to produce the ciphertext.

In August 1941, a blunder by German operators led to the transmission of two versions of the same message with identical machine settings. These were intercepted and worked on at Bletchley Park. First, John Tiltman, a very talented GC&CS cryptanalyst, derived a key stream of almost 4000 characters.[16] Then Bill Tutte, a newly arrived member of the Research Section, used this key stream to work out the logical structure of the Lorenz machine. He deduced that the twelve wheels consisted of two groups of five, which he named the χ (chi) and ψ (psi) wheels, the remaining two he called μ (mu) or 'motor' wheels. The chi wheels stepped regularly with each letter that was encrypted, while the psi wheels stepped irregularly, under the control of the motor wheels.[17]

Cams on wheels 9 and 10 showing their raised (active) and lowered (inactive) positions. An active cam reversed the value of a bit (0→1 and 1→0).

With a sufficiently random key stream, a Vernam cipher removes the natural language property of a plaintext message of having an uneven frequency distribution of the different characters, to produce a uniform distribution in the ciphertext. The Tunny machine did this well. However, the cryptanalysts worked out that by examining the frequency distribution of the character-to-character changes in the ciphertext, instead of the plain characters, there was a departure from uniformity which provided a way into the system. This was achieved by 'differencing' in which each bit or character was XOR-ed with its successor.[18] After Germany surrendered, allied forces captured a Tunny machine and discovered that it was the electromechanicalLorenz SZ (Schlüsselzusatzgerät, cipher attachment) in-line cipher machine.[14]

In order to decrypt the transmitted messages, two tasks had to be performed. The first was 'wheel breaking', which was the discovery of the cam patterns for all the wheels. These patterns were set up on the Lorenz machine and then used for a fixed period of time for a succession of different messages. Each transmission, which often contained more than one message, was enciphered with a different start position of the wheels. Alan Turing invented a method of wheel-breaking that became known as Turingery.[19] Turing's technique was further developed into 'Rectangling', for which Colossus could produce tables for manual analysis. Colossi 2, 4, 6, 7 and 9 had a 'gadget' to aid this process.[20]

The second task was 'wheel setting', which worked out the start positions of the wheels for a particular message, and could only be attempted once the cam patterns were known.[21] It was this task for which Colossus was initially designed. To discover the start position of the chi wheels for a message, Colossus compared two character streams, counting statistics from the evaluation of programmable Boolean functions. The two streams were the ciphertext, which was read at high speed from a paper tape, and the key stream, which was generated internally, in a simulation of the unknown German machine. After a succession of different Colossus runs to discover the likely chi-wheel settings, they were checked by examining the frequency distribution of the characters in processed ciphertext.[22] Colossus produced these frequency counts.

Decryption processes[edit]

Notation [23]
Kkey – the sequence of characters used in binary XOR with
the plaintext to give the ciphertext
χ{displaystyle chi }chi component of key
ψ{displaystyle psi }psi component of key
ψ{displaystyle psi '}extended psi – the actual sequence of characters added by
the psi wheels, including those when they do not advance [24]
Dde-chi—the ciphertext with the chi component of the key removed[23]
Δany of the above XOR'ed with its successor character or bit[18]
the XOR operation[25][26]
Bletchley Park shorthand for telegraphy code space (zero)
xBletchley Park shorthand for telegraphy code mark (one)

By using differencing and knowing that the psi wheels did not advance with each character, Tutte worked out that trying just two differenced bits (impulses) of the chi-stream against the differenced ciphertext would produce a statistic that was non-random. This became known as Tutte's '1+2 break in'.[27] It involved calculating the following Boolean function:

∆Z1 ⊕ ∆Z2 ⊕ ∆χ{displaystyle chi }1 ⊕ ∆χ{displaystyle chi }2 =

and counting the number of times it yielded 'false' (zero). If this number exceeded a pre-defined threshold value known as the 'set total', it was printed out. The cryptanalyst would examine the printout to determine which of the putative start positions was most likely to be the correct one for the chi-1 and chi-2 wheels.[28]

This technique would then be applied to other pairs of, or single, impulses to determine the likely start position of all five chi wheels. From this, the de-chi (D) of a ciphertext could be obtained, from which the psi component could be removed by manual methods.[29] If the frequency distribution of characters in the de-chi version of the ciphertext was within certain bounds, 'wheel setting' of the chi wheels was considered to have been achieved,[22] and the message settings and de-chi were passed to the 'Testery'. This was the section at Bletchley Park led by Major Ralph Tester where the bulk of the decrypting work was done by manual and linguistic methods.[30]

Colossus could also derive the start position of the psi and motor wheels, but this was not much done until the last few months of the war, when there were plenty of Colossi available and the number of Tunny messages had declined.

Design and construction[edit]

Colossus was developed for the 'Newmanry',[31] the section headed by the mathematician Max Newman that was responsible for machine methods against the twelve-rotor Lorenz SZ40/42 on-line teleprinter cipher machine (code named Tunny, for tunafish). The Colossus design arose out of a prior project that produced a counting machine dubbed 'Heath Robinson'. Although it proved the concept of machine analysis for this part of the process, it was initially unreliable. The electro-mechanical parts were relatively slow and it was difficult to synchronise two looped paper tapes, one containing the enciphered message, and the other representing part of the key stream of the Lorenz machine,[32] also the tapes tended to stretch when being read at up to 2000 characters per second.

Stepping switch allegedly from an original Colossus presented by the Director of GCHQ to the Director of the NSA to mark the 40th anniversary of the UKUSA Agreement in 1986[33]

Tommy Flowers MBE[34] 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 Bombes that were used in the cryptanalysis of the German Enigma cipher machine.[35] He was recommended to Max Newman by Alan Turing, who had been impressed by his work on the Bombes.[36] 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 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.[37] 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.[38] He presented this design to Max Newman in February 1943, but the idea that the one to two thousand thermionic valves (vacuum tubes and thyratrons) proposed, could work together reliably, was greeted with great scepticism,[39] 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.[40] Additionally, the heaters were started at a low voltage then slowly brought up to full voltage to reduce the thermal stress. The valves themselves were soldered in to avoid problems with plug-in bases, which could be unreliable.[citation needed] Flowers persisted with the idea and obtained support from the Director of the Research Station, W Gordon Radley.[41] Flowers and his team of some fifty people in the switching group[42][43] 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.[44][45]

This prototype, Mark 1 Colossus, contained 1600 thermionic valves (tubes).[42] It performed satisfactorily at Dollis Hill on 8 December 1943[46] and was dismantled and shipped to Bletchley Park, where it was delivered on 18 January and re-assembled by Harry Fensom and Don Horwood.[47][48] It was operational in January [49][50] and it successfully attacked its first message on 5 February 1944.[51] It was a large structure and was dubbed 'Colossus', supposedly by the WRNS operators. However, a memo held in the National Archives written by Max Newman on 18 January 1944 records that 'Colossus arrives today'.[52]

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.[53]Allen Coombs took over leadership of the production Mark 2 Colossi, the first of which – containing 2400 valves – became operational at 08:00 on 1 June 1944, just in time for the Allied Invasion of Normandy on D-Day.[54] Subsequently, Colossi were delivered at the rate of about one a month. By the time of V-E Day there were ten Colossi working at Bletchley Park and a start had been made on assembling an eleventh.[53]

Colossus 10 with its extended bedstead in Block H at Bletchley Park in the space now containing the Tunny galley of The National Museum of Computing

The main units of the Mark 2 design were as follows.[38][55]

  • A tape transport with an 8-photocell reading mechanism.
  • A six character FIFOshift 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 function units that performed 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.[56] 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.[57] The Mark 2 Colossi were both five times faster and were simpler to operate than the prototype.[58]

Data input to Colossus was by 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 store 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 mph) before the tape disintegrated. So 5000 characters/second (40 ft/s (12.2 m/s; 27.3 mph)) 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.[59][60] This five-way parallelism[61] enabled five simultaneous tests and counts to be performed giving an effective processing speed of 25,000 characters per second.[60] The computation used algorithms devised by W. T. Tutte and colleagues to decrypt a Tunny message.[62][63]


Colossus selection panel showing selections amongst others, of the far tape on the bedstead, and for input to the algorithm: ΔZ, Δχ{displaystyle chi } and Δψ{displaystyle psi }.

The Newmanry was staffed by cryptanalysts, operators from the Women's Royal Naval Service (WRNS) – known as 'Wrens' – and engineers who were permanently on hand for maintenance and repair. By the end of the war the staffing was 272 Wrens and 27 men.[53]

The first job in operating Colossus for a new message, was to prepare the paper tape loop. This was performed by the Wrens who stuck the two ends together using Bostik glue, ensuring that there was a 150-character length of blank tape between the end and the start of the message.[64] Using a special hand punch they inserted a start hole between the third and fourth channels ​212 sprocket holes from the end of the blank section, and a stop hole between the fourth and fifth channels ​112 sprocket holes from the end of the characters of the message.[65][66] These were read by specially positioned photocells and indicated when the message was about to start and when it ended. The operator would then thread the paper tape through the gate and around the pulleys of the bedstead and adjust the tension. The two-tape bedstead design had been carried on from Heath Robinson so that one tape could be loaded whilst the previous one was being run. A switch on the Selection Panel specified the 'near' or the 'far' tape.[67]

After performing various resetting and zeroizing tasks, the Wren operators would, under instruction from the cryptanalyst, operate the 'set total' decade switches and the K2 panel switches to set the desired algorithm. They would then start the bedstead tape motor and lamp and, when the tape was up to speed, operate the master start switch.[67]


Colossus K2 switch panel showing switches for specifying the algorithm (on the left) and the counters to be selected (on the right).
Colossus 'set total' switch panel

Howard Campaigne, a mathematician and cryptanalyst from the US Navy's OP-20-G, wrote the following in a foreword to Flowers' 1983 paper 'The Design of Colossus'.

My view of Colossus was that of cryptanalyst-programmer. I told the machine to make certain calculations and counts, and after studying the results, told it to do another job. It did not remember the previous result, nor could it have acted upon it if it did. Colossus and I alternated in an interaction that sometimes achieved an analysis of an unusual German cipher system, called 'Geheimschreiber' by the Germans, and 'Fish' by the cryptanalysts.[68]

Colossus was not a stored-program computer. The input data for the five parallel processors was read from the looped message paper tape and the electronic pattern generators for the chi, psi and motor wheels.[69] The programs for the processors were set and held on the switches and jack panel connections. Each processor could evaluate a Boolean function and count and display the number of times it yielded the specified value of 'false' (0) or 'true' (1) for each pass of the message tape.

Input to the processors came from two sources, the shift registers from tape reading and the thyratron rings that emulated the wheels of the Tunny machine.[70] The characters on the paper tape were called Z and the characters from the Tunny emulator were referred to by the Greek letters that Bill Tutte had given them when working out the logical structure of the machine. On the selection panel, switches specified either Z or ΔZ, either χ{displaystyle chi } or Δχ{displaystyle chi } and either ψ{displaystyle psi } or Δψ{displaystyle psi } for the data to be passed to the jack field and 'K2 switch panel'. These signals from the wheel simulators could be specified as stepping on with each new pass of the message tape or not.

The K2 switch panel had a group of switches on the left hand side to specify the algorithm. The switches on the right hand side selected the counter to which the result was fed. The plugboard allowed less specialized conditions to be imposed. Overall the K2 switch panel switches and the plugboard allowed about five billion different combinations of the selected variables. [64]

As an example: a set of runs for a message tape might initially involve two chi wheels, as in Tutte's 1+2 algorithm. Such a two-wheel run was called a long run, taking on average eight minutes unless the parallelism was utilised to cut the time by a factor of five. The subsequent runs might only involve setting one chi wheel, giving a short run taking about two minutes. Initially, after the initial long run, the choice of next algorithm to be tried was specified by the cryptanalyst. Experience showed, however, that decision trees for this iterative process could be produced for use by the Wren operators in a proportion of cases.[71]

Influence and fate[edit]

Although the Colossus was the first of the electronic digital machines with programmability, albeit limited by modern standards,[72] it was not a general-purpose machine, being designed for a range of cryptanalytic tasks, most involving counting the results of evaluating Boolean algorithms.

A Colossus computer was thus not a fully Turing complete machine. However, University of San Francisco professor Benjamin Wells has shown that if all ten Colossus machines made were rearranged in a specific cluster, then the entire set of computers could have simulated a universal Turing machine, and thus be Turing complete.[73] The notion of a computer as a general purpose machine — that is, as more than a calculator devoted to solving difficult but specific problems — did not become prominent until after World War II.[citation needed]

Colossus and the reasons for its construction were highly secret, and remained so for 30 years after the War. Consequently, it was not included in the history of computing hardware for many years, and Flowers and his associates were deprived of the recognition they were due. Colossi 1 to 10 were dismantled after the war and parts returned to the Post Office. Some parts, sanitised as to their original purpose, were taken to Max Newman's Royal SocietyComputing Machine Laboratory at Manchester University.[74] Tommy Flowers was ordered to destroy all documentation and burnt them in a furnace at Dollis Hill. He later said of that order:

That was a terrible mistake. I was instructed to destroy all the records, which I did. I took all the drawings and the plans and all the information about Colossus on paper and put it in the boiler fire. And saw it burn.[75]

Colossi 11 and 12, along with two replica Tunny machines, were retained, being moved to GCHQ's new headquarters at Eastcote in April 1946, and again with GCHQ to Cheltenham between 1952 and 1954.[76] One of the Colossi, known as Colossus Blue, was dismantled in 1959; the other in 1960.[76] There had been attempts to adapt them to other purposes, with varying success; in their later years they had been used for training.[77]Jack Good related how he was the first to use Colossus after the war, persuading the US National Security Agency that it could be used to perform a function for which they were planning to build a special-purpose machine.[76] Colossus was also used to perform character counts on one-time pad tape to test for non-randomness.[76]

A small number of people who were associated with Colossus—and knew that large-scale, reliable, high-speed electronic digital computing devices were feasible—played significant roles in early computer work in the UK and probably in the US. However, being so secret, it had little direct influence on the development of later computers; it was EDVAC that was the seminal computer architecture of the time. In 1972 Herman Goldstine, who was unaware of Colossus and its legacy to the projects of people such as Alan Turing (ACE), Max Newman (Manchester computers) and Harry Huskey (Bendix G-15), wrote that,

Britain had such vitality that it could immediately after the war embark on so many well-conceived and well-executed projects in the computer field.[78]

Professor Brian Randell, who unearthed information about Colossus in the 1970s, commented on this, saying that:

It is my opinion that the COLOSSUS project was an important source of this vitality, one that has been largely unappreciated, as has the significance of its places in the chronology of the invention of the digital computer.[79]

Randell's efforts started to bear fruit in the mid-1970s, after the secrecy about Bletchley Park was broken when Group Captain Winterbotham published his book The Ultra Secret in 1974.[80] In October 2000, a 500-page technical report on the Tunny cipher and its cryptanalysis—entitled General Report on Tunny[81]—was released by GCHQ to the national Public Record Office, and it contains a fascinating paean to Colossus by the cryptographers who worked with it:

It is regretted that it is not possible to give an adequate idea of the fascination of a Colossus at work; its sheer bulk and apparent complexity; the fantastic speed of thin paper tape round the glittering pulleys; the childish pleasure of not-not, span, print main header and other gadgets; the wizardry of purely mechanical decoding letter by letter (one novice thought she was being hoaxed); the uncanny action of the typewriter in printing the correct scores without and beyond human aid; the stepping of the display; periods of eager expectation culminating in the sudden appearance of the longed-for score; and the strange rhythms characterizing every type of run: the stately break-in, the erratic short run, the regularity of wheel-breaking, the stolid rectangle interrupted by the wild leaps of the carriage-return, the frantic chatter of a motor run, even the ludicrous frenzy of hosts of bogus scores.[82]


A team led by Tony Sale (right) reconstructed a Colossus Mark II at Bletchley Park. Here, in 2006, Sale supervises the breaking of an enciphered message with the completed machine.

Construction of a fully functional rebuild[83][84] of a Colossus Mark 2 was undertaken between 1993 and 2008 by a team led by Tony Sale.[12][11] In spite of the blueprints and hardware being destroyed, a surprising amount of material survived, mainly in engineers' notebooks, but a considerable amount of it in the U.S. The optical tape reader might have posed the biggest problem, but Dr. Arnold Lynch, its original designer, was able to redesign it to his own original specification. The reconstruction is on display, in the historically correct place for Colossus No. 9, at The National Museum of Computing, in H Block Bletchley Park in Milton Keynes, Buckinghamshire.

In November 2007, to celebrate the project completion and to mark the start of a fundraising initiative for The National Museum of Computing, a Cipher Challenge[85] pitted the rebuilt Colossus against radio amateurs worldwide in being first to receive and decode three messages enciphered using the Lorenz SZ42 and transmitted from radio station DL0HNF in the Heinz Nixdorf MuseumsForum computer museum. The challenge was easily won by radio amateur Joachim Schüth, who had carefully prepared[86] for the event and developed his own signal processing and code-breaking code using Ada.[87] The Colossus team were hampered by their wish to use World War II radio equipment,[88] delaying them by a day because of poor reception conditions. Nevertheless, the victor's 1.4 GHz laptop, running his own code, took less than a minute to find the settings for all 12 wheels. The German codebreaker said: 'My laptop digested ciphertext at a speed of 1.2 million characters per second—240 times faster than Colossus. If you scale the CPU frequency by that factor, you get an equivalent clock of 5.8 MHz for Colossus. That is a remarkable speed for a computer built in 1944.'[89]

The Cipher Challenge verified the successful completion of the rebuild project. 'On the strength of today's performance Colossus is as good as it was six decades ago', commented Tony Sale. 'We are delighted to have produced a fitting tribute to the people who worked at Bletchley Park and whose brainpower devised these fantastic machines which broke these ciphers and shortened the war by many months.'[90]

Front view of the Colossus rebuild showing, from right to left (1) The 'bedstead' containing the message tape in its continuous loop and with a second one loaded. (2) The J-rack containing the Selection Panel and Plug Panel. (3) The K-rack with the large 'Q' switch panel and sloping patch panel. (4) The double S-rack containing the control panel and, above the image of a postage stamp, five two-line counter displays. (5) The electric typewriter in front of the five sets of four 'set total' decade switches in the C-rack.[91]

Other meanings[edit]

There was a fictional computer named Colossus in the 1970 movie Colossus: The Forbin Project which was based on the 1966 novel Colossus by D. F. Jones. This was sheer coincidence as it pre-dates the public release of information about Colossus, or even its name.

Neal Stephenson's novel Cryptonomicon (1999) also contains a fictional treatment of the historical role played by Turing and Bletchley Park.

See also[edit]


  1. ^The two operators have been variously identified as Dorothy Du Boisson (left) and Elsie Booker, Vivian Vorster (left) and Catherine Kennedy, and (unknown) and Patricia (Pat) Davis (right).[citation needed]
  2. ^Based on what the National Museum of Computing state is the power consumption of the Colossus rebuild. In the absence of information to the contrary, the original is presumed to be similar.
  3. ^Copeland 2006, Copeland, Jack, Introduction p. 2.
  4. ^Sale 2000.
  5. ^Golden, Frederic (29 March 1999), 'Who Built The First Computer?', Time Magazine, vol. 153 no. 12
  6. ^Copeland, Jack, 'Colossus: The first large scale electronic computer', Colossus-computer.com, retrieved 21 October 2012
  7. ^ abFlowers 1983, p. 246.
  8. ^Barber, Nicola (21 December 2015). Who Broke the Wartime Codes?. Capstone. ISBN9781484635599. Retrieved 26 October 2017 – via Google Books.
  9. ^Preneel, Bart (26 June 2003). Advances in Cryptology - EUROCRYPT 2000: International Conference on the Theory and Application of Cryptographic Techniques Bruges, Belgium, May 14-18, 2000 Proceedings. Springer. ISBN9783540455394. Retrieved 26 October 2017 – via Google Books.
  10. ^'coltalk_2'. Codesandciphers.org.uk. Retrieved 26 October 2017.
  11. ^ abCampbell-Kelly, Martin (31 August 2011). 'Tony Sale obituary'. Theguardian.com. Retrieved 26 October 2017.
  12. ^ abColossus – The Rebuild Story, The National Museum of Computing, archived from the original on 18 April 2015, retrieved 13 May 2017Cite uses deprecated parameter dead-url= (help)
  13. ^Good, Michie & Timms 1945, 1 Introduction: 11 German Tunny, 11B The Tunny Cipher Machine, p. 6.
  14. ^ abGood, Michie & Timms 1945, 1 Introduction: 11 German Tunny, 11A Fish Machines, (c) The German Ciphered Teleprinter, p. 4.
  15. ^Hinsley, F. H.; Stripp, Alan (26 October 2017). Codebreakers: The Inside Story of Bletchley Park. Oxford University Press. ISBN9780192801326. Retrieved 26 October 2017 – via Google Books.
  16. ^Copeland 2006, Budianski, Stephen Colossus, Codebreaking and the Digital Age pp. 55–56.
  17. ^Copeland 2006, Tutte, William T. My Work at Bletchley Park p. 357.
  18. ^ abGood, Michie & Timms 1945, 1 Introduction: 11 German Tunny, 11C Wheel Patterns, (b) Differenced and Undifferenced Wheels, p. 11.
  19. ^Copeland 2006, Copeland, Jack, Turingery pp. 378–385.
  20. ^Good, Michie & Timms 1945, 24 – Rectangling: 24B Making and Entering Rectangles pp. 114–115, 119–120.
  21. ^Good, Michie & Timms 1945, 1 Introduction: 11 German Tunny, 11E The Tunny Network, (b) Wheel-breaking and Setting, p. 15.
  22. ^ abSmall 1944, p. 15.
  23. ^ abGood, Michie & Timms 1945, 1 Introduction: 12 Cryptographic Aspects, 12A The Problem, (a) Formulae and Notation, p. 16.
  24. ^Good, Michie & Timms 1945, 1 Introduction: 11 German Tunny, 11B The Tunny Cipher Machine, (e) Psi-key, p. 7.
  25. ^The Boolean or 'truth' functionXOR, also known as Exclusive disjunction and Exclusive or, is the same as binary modulo 2 addition and subtraction
  26. ^Good, Michie & Timms 1945, 1 Introduction: 11 German Tunny, 11B The Tunny Cipher Machine, (a) Addition, p. 5.
  27. ^Copeland 2006, Budiansky, Stephen, Colossus, Codebreaking, and the Digital Age pp. 58–59.
  28. ^Carter 2008, pp. 18–19.
  29. ^Small 1944, p. 65.
  30. ^Roberts 2009, 34 minutes in.
  31. ^Good, Michie & Timms 1945, 3 Organisation: 31 Mr Newman's section, p. 276.
  32. ^Anderson 2007, p. 8.
  33. ^Exhibit in the National Cryptologic Museum, Fort Meade, Maryland, USA
  34. ^Flowers had been appointed MBE in June 1943
  35. ^Randell 1980, p. 9.
  36. ^Budiansky 2000, p. 314.
  37. ^Good, Michie & Timms 1945, 1 Introduction: 15 Some Historical Notes, 15A First Stages in Machine Development, (c) Heath Robinson, p. 33.
  38. ^ abCopeland 2006, Flowers, Thomas H. Colossus p. 96.
  39. ^Flowers 1983, p. 244.
  40. ^Copeland 2006, Copeland, Jack, Machine against Machine p. 72.
  41. ^Copeland 2006, Copeland, Jack, Machine against Machine p. 74.
  42. ^ abCopeland 2006, Flowers, Thomas H. Colossus p. 80.
  43. ^Copeland 2006, Randell, BrianOf Men and Machines p. 143.
  44. ^Boden, Margaret (2000), Mind as Machine: A History of Cognitive Science, Oxford University Press, p. 159, ISBN978-0199241446
  45. ^Atkinson, Paul (2010), Computer (Objekt), Reaktion Books, p. 29, ISBN978-1861896643
  46. ^Copeland 2010.
  47. ^'Colossus - The Rebuild Story - The National Museum of Computing'. Tnmoc.org. Archived from the original on 18 April 2015. Retrieved 26 October 2017.Cite uses deprecated parameter dead-url= (help)
  48. ^Fensom 2010.
  49. ^Sterling, Christopher H., ed. (2007), Military Communications : From Ancient Times to the 21st Century, ABC-CLIO, ISBN978-1851097326
  50. ^Preneel, Bart (2003), Advances in Cryptology - EUROCRYPT 2000: International Conference on the Theory and Application of Cryptographic Techniques Bruges, Belgium May 14-18, 2000 Proceedings, Lecture Notes in Computer Science, Springer, ISBN978-3540675174
  51. ^Copeland 2006, Copeland, Jack, Machine against Machine p. 75.
  52. ^Gannon 2007, p. 283.
  53. ^ abcGood, Michie & Timms 1945, 1 Introduction: 15 – Some Historical Notes, 15C Period of Expansion, (b) Colossus, p. 35.
  54. ^Randell, Brian; Fensom, Harry; Milne, Frank A. (15 March 1995), 'Obituary: Allen Coombs', The Independent, London, retrieved 18 October 2012
  55. ^Flowers 1983, pp. 249–252.
  56. ^Flowers 1983, pp. 243, 245.
  57. ^Coombs 1983.
  58. ^For comparison, later stored-program computers such as the Manchester Mark 1 of 1949 used 4050 valves,Lavington, S. H. (July 1977), 'The Manchester Mark 1 and Atlas: a Historical Perspective'(PDF), Communications of the ACM, 21 (1): 4–12, doi:10.1145/359327.359331, retrieved 8 February 2009 while ENIAC (1946) used 17,468 valves.
  59. ^Flowers 1983.
  60. ^ abCopeland 2006, Flowers, Thomas H. Colossus p. 100.
  61. ^This would now be called a systolic array
  62. ^Copeland 2011.
  63. ^'Biography of Professor Tutte - Combinatorics and Optimization'. Uwaterloo.ca. 13 March 2015. Retrieved 26 October 2017.
  64. ^ abGood, Michie & Timms 1945, 5 Machines: 53 Colossus 53A Introduction, p.333.
  65. ^Flowers 1983, pp. 241,242.
  66. ^Good, Michie & Timms 1945, 5 Machines: 53 Colossus 53B The Z stream, p.333.
  67. ^ abFensom 2006, p. 303.
  68. ^Flowers 1983, pp. 239–252.
  69. ^Small 1944, p. 108.
  70. ^Good, Michie & Timms 1945, 5 Machines: 53 Colossus, pp. 333–353.
  71. ^Budiansky 2006, p. 62.
  72. ^'A Brief History of Computing. Jack Copeland, June 2000'. Alanturing.net. Retrieved 26 October 2017.
  73. ^Wells, Benjamin (2009). 'Proceedings of the 8th International Conference on Unconventional Computation 2009 (UC09), Ponta Delgada, Portugal: Advances in I/O, Speedup, and Universality on Colossus, an Unconventional Computer'. Lecture Notes in Computer Science. Berlin, Heidelberg: Springer-Verlag. 5175: 247–261. ISBN978-3-642-03744-3. Retrieved 10 November 2009.
  74. ^'A Brief History of Computing'. alanturing.net. Retrieved 26 January 2010.
  75. ^McKay 2010, pp. 270–271.
  76. ^ abcdCopeland 2006, Copeland, Jack, et al. Mr Newman's section pp. 173–175.
  77. ^Horwood 1973.
  78. ^Goldstine 1980, p. 321.
  79. ^Randell 1980, p. 87.
  80. ^Winterbotham, F.W. (2000) [1974], The Ultra secret: the inside story of Operation Ultra, Bletchley Park and Enigma, London: Orion Books Ltd, ISBN9780752837512, OCLC222735270
  81. ^Good, Michie & Timms 1945.
  82. ^Good, Michie & Timms 1945, 5 Machines: 51 Introductory, (j) Impressions of Colossus, p. 327.
  83. ^'Colossus Rebuild - Tony Sale'. Codesandciphers.org.uk. Retrieved 26 October 2017.
  84. ^* Sale, Tony (2008). 'Video of Tony Sale talking about rebuilt Colossus 2008-6-19'. Retrieved 13 May 2017.
  85. ^'Cipher Challenge'. Archived from the original on 1 August 2008. Retrieved 1 February 2012.
  86. ^'SZ42 codebreaking software'. Schlaupelz.de. Retrieved 26 October 2017.
  87. ^'Cracking the Lorenz Code - Ada Answers'. Adacore.com. Archived from the original on 8 February 2012. Retrieved 26 October 2017.Cite uses deprecated parameter dead-url= (help)
  88. ^Ward, Mark (16 November 2007). 'BBC News Article'. News.bbc.co.uk. Retrieved 2 January 2010.
  89. ^'Archived copy'. Archived from the original on 2 January 2013. Retrieved 7 April 2012.Cite uses deprecated parameter deadurl= (help)CS1 maint: archived copy as title (link)
  90. ^'Latest Cipher Challenge News 16.11.2007'. Archived from the original on 18 April 2008.
  91. ^Sale, Tony. 'The Colossus its purpose and operation'. Codesandciphers.org.uk. Retrieved 26 October 2017.


  • Anderson, David (2007), Was the Manchester Baby conceived at Bletchley Park?(PDF), British Computer Society, retrieved 25 April 2015
  • Budiansky, Stephen (2000), Battle of wits: The Complete Story of Codebreaking in World War II, Free Press, ISBN978-0684859323
  • Budiansky, Stephen (2006), Colossus, Codebreaking, and the Digital Age in Copeland 2006, pp. 52–63
  • Carter, Frank (2008), Codebreaking with the Colossus Computer, Bletchley Park Reports, 1 (New ed.), Bletchley Park Trust, ISBN978-1-906723-00-2
  • Chandler, W. W. (1983), 'The Installation and Maintenance of Colossus', IEEE Annals of the History of Computing, 5 (3): 260–262, doi:10.1109/MAHC.1983.10083
  • Coombs, Allen W. M. (July 1983), 'The Making of Colossus', IEEE Annals of the History of Computing, 5 (3): 253–259, doi:10.1109/MAHC.1983.10085
  • Copeland, B. Jack (2011) [2001], Colossus and the Dawning of the Computer Age in Erskine & Smith 2011, pp. 305–327
  • Copeland, B. J. (October–December 2004), 'Colossus: its origins and originators', IEEE Annals of the History of Computing, 26 (4): 38–45, doi:10.1109/MAHC.2004.26
  • Copeland, B. Jack, ed. (2006), Colossus: The Secrets of Bletchley Park's Codebreaking Computers, Oxford: Oxford University Press, ISBN978-0-19-284055-4
  • Copeland, B. Jack (2010), 'Colossus: Breaking the German 'Tunny' Code at Bletchley Park. An Illustrated History', The Rutherford Journal, 3
  • Erskine, Ralph; Smith, Michael, eds. (2011), The Bletchley Park Codebreakers, Biteback Publishing Ltd, ISBN9781849540780 Updated and extended version of Action This Day: From Breaking of the Enigma Code to the Birth of the Modern Computer Bantam Press 2001
  • Fensom, Jim (8 November 2010), 'Harry Fensom obituary', The Guardian, London, retrieved 17 October 2012
  • Fensom, Harry (2006), How Colossus was Built and Operated – One of its Engineers Reveals its Secrets in Copeland 2006, pp. 297–303
  • Flowers, Thomas H. (1983), 'The Design of Colossus', Annals of the History of Computing, 5 (3): 239–252, doi:10.1109/MAHC.1983.10079
  • Gannon, Paul (2006), Colossus: Bletchley Park's Greatest Secret, London: Atlantic Books, ISBN9781843543305
  • Goldstine, Herman H. (1980), The Computer from Pascal to von Neumann, Princeton University Press, ISBN978-0-691-02367-0
  • Good, Jack; Michie, Donald; Timms, Geoffrey (1945), General Report on Tunny: With Emphasis on Statistical Methods, UK Public Record Office HW 25/4 and HW 25/5, archived from the original on 17 September 2010, retrieved 15 September 2010Cite uses deprecated parameter dead-url= (help) That version is a facsimile copy, but there is a transcript of much of this document in '.pdf' format at: Sale, Tony (2001), Part of the 'General Report on Tunny', the Newmanry History, formatted by Tony Sale(PDF), retrieved 20 September 2010, and a web transcript of Part 1 at: Ellsbury, Graham, General Report on Tunny With Emphasis on Statistical Methods, retrieved 3 November 2010
  • Good, I. J. (1979), 'Early Work on Computers at Bletchley', IEEE Annals of the History of Computing, 1 (1): 38–48, doi:10.1109/MAHC.1979.10011
  • Good, I. J. (1980), 'Pioneering Work on Computers at Bletchley', in Metropolis, Nicholas; Howlett, J.; Rota, Gian-Carlo (eds.), A History of Computing in the Twentieth Century, New York: Academic Press, ISBN0124916503
  • Horwood, D.C. (1973), A technical description of Colossus I: PRO HW 25/24(YouTube video)
  • McKay, Sinclair (2010), The Secret Life of Bletchley Park: The WWII Codebreaking Centre and the men and women who worked there, London: Aurum Press, ISBN9781845135393
  • Randell, Brian (1982) [1977], 'Colossus: Godfather of the Computer', The Origins of Digital Computers: Selected Papers, New York: Springer-Verlag, ISBN9783540113195
  • Randell, Brian (1980), 'The Colossus'(PDF), in Metropolis, N.; Howlett, J.; Rota, Gian-Carlo (eds.), A History of Computing in the Twentieth Century, pp. 47–92, ISBN978-0124916500, archived from the original(PDF) on 17 February 2012Cite uses deprecated parameter deadurl= (help)
  • Randell, Brian (2006), Of Men and Machines in Copeland 2006, pp. 141–149
  • Roberts, Jerry (2009). Capt. Jerry Roberts: My Top Secret Codebreaking at Bletchley Park 1941 to 45: Lecture on 11 March 2009 (YouTube). University College London.
  • Sale, Tony (2000), 'The Colossus of Bletchley Park – The German Cipher System', in Rojas, Raúl; Hashagen, Ulf (eds.), The First Computers: History and Architecture, Cambridge, Massachusetts: The MIT Press, pp. 351–364, ISBN0-262-18197-5
  • Small, Albert W. (December 1944), The Special Fish Report describes the operation of Colossus in breaking Tunny messages
  • Tutte, William T. (2006), Appendix 4: My Work at Bletchley Park in Copeland 2006, pp. 352–369
  • Wells, B (2004), 'A Universal Turing Machine Can Run on a Cluster of Colossi', Abstracts of the American Mathematical Society, 25: 441
  • Wells, Benjamin (2006), The PC-User's Guide to Colossus in Copeland 2006, pp. 116–140

Further reading[edit]

  • Campaigne, Howard; Farley, Robert D. (28 February 1990), Oral History Interview: NSA-OH-14-83 Campaigne, Howard, Dr. 29 June 83 Annopalis, MD By: Robert G. Farley(PDF), National Security Agency, retrieved 16 October 2016
  • Colossus: Creating a Giant on YouTube A short film made by Google to celebrate Colossus and those who built it, in particular Tommy Flowers.
  • Cragon, Harvey G. (2003), From Fish to Colossus: How the German Lorenz Cipher was Broken at Bletchley Park, Dallas: Cragon Books, ISBN0-9743045-0-6 – A detailed description of the cryptanalysis of Tunny, and some details of Colossus (contains some minor errors)
  • Enever, Ted (1999), Britain's Best Kept Secret: Ultra's Base at Bletchley Park (3rd ed.), Sutton Publishing, Gloucestershire, ISBN978-0-7509-2355-2 – A guided tour of the history and geography of the Park, written by one of the founder members of the Bletchley Park Trust
  • Gannon, Paul (2007), Colossus: Bletchley Park's Greatest Secret, Atlantic Books, ISBN978-1-84354-331-2
  • Rojas, R.; Hashagen, U. (2000), The First Computers: History and Architectures, MIT Press, ISBN0-262-18197-5 – Comparison of the first computers, with a chapter about Colossus and its reconstruction by Tony Sale.
  • Sale, Tony (2004), The Colossus Computer 1943–1996: How It Helped to Break the German Lorenz Cipher in WWII, Kidderminster: M.&M. Baldwin, ISBN0-947712-36-4 A slender (20 page) booklet, containing the same material as Tony Sale's website (see below)
  • Smith, Michael (2007) [1998], Station X: The Codebreakers of Bletchley Park, Pan Grand Strategy Series (Pan Books ed.), London: Pan MacMillan Ltd, ISBN978-0-330-41929-1

External links[edit]

  • The National Museum of Computing (TNMOC)
  • Tony Sale's Codes and Ciphers Contains a great deal of information, including:
    • Lorenz Cipher and the Colossus
      • Walk around Colossus A detailed tour of the replica Colossus – make sure to click on the 'More Text' links on each image to see the informative detailed text about that part of Colossus
    • IEEE lecture – Transcript of a lecture Tony Sale gave describing the reconstruction project
  • Website on Copeland's 2006 book with much information and links to recently declassified information
  • Walk through video of the Colossus rebuild at Bletchley Park on YouTube
Wikimedia Commons has media related to Colossus computer.
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Colossus_computer&oldid=914428181'

Choosing a Computer Power Supply

Use the following guidelines to choose a power supply appropriate for your system:

Choose the correct form factor.

Above all, make sure the power supply you buy fits your case.

Choose a name-brand power supply.

There are literally scores of brands of power supplies available, many of which are made in the same Chinese factories and simply have different labels attached to them. Most of those are of mediocre quality or worse, but some good name-brand power supplies are made in China. For years we have exclusively used and recommended units from two companies, Antec (http://www.antec.com) and PC Power & Cooling (http://www.pcpowerandcooling.com). Both produce a wide range of models in different capacities. One of them is probably right for your needs.

Choose a power supply with sufficient capacity.

When it comes to power supplies, too much capacity is far better than too little. Using a 450W power supply on a system that draws only 250W does no harm; assuming equal efficiencies, the 450W unit consumes the same amount of power as would a 250W unit. Using a higher-capacity power supply than necessary costs a bit more, but has several advantages. The larger power supply generally runs cooler, because its fans were designed to cool the unit when it runs at full capacity. The larger unit typically provides tighter voltage regulation because it's not being stressed. And when it's time to add a faster processor or video card, the larger power supply has enough excess capacity to handle the additional load.

It's possible to add the maximum current draws for all system components and size the power supply on that basis. The problem with that method is that it can be nearly impossible to determine those draws for all components, especially motherboards and expansion cards. If you want to keep it simple, size your power supply according to the following configurations:

Basic system

For a system with a slow processor, 256 MB to 512 MB of RAM, embedded video, one hard drive, one optical drive, and zero or one expansion card, install a 300W or larger power supply.

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Mainstream system

For a system with a midrange processor, 512 MB to 1 GB of RAM, a midrange video adapter, one or two hard drives, one or two optical drives, and one or two expansion cards, install a 400W or larger power supply.

High-performance system

For a system with a fast processor, more than 1 GB of RAM, one or two fast video adapters, two or three hard drives, one or two optical drives, and two or more expansion cards, install a 500W or larger power supply.

Choose a high-efficiency power supply.

Don't buy any power supply, particularly a high-capacity unit, that is rated at less than 70% efficiency at moderate to high loads. (Power supplies are typically less efficient at very light loads.)

Choose a quiet power supply.

Noise-reduced power supplies used to sell at significant premiums over standard power supplies. That's no longer true. Mainstream 'quiet' power supplies such as the Antec TruePower 2.0 and PC Power & Cooling Silencer series sell for little or no more than standard power supplies of equal quality that produce considerably more noise. Even if your goal is not to produce a quiet PC, there's little point to choosing a noisy unit when quieter units are so readily available.

Installing a power supply

Before you do anything else, verify that the new power supply is set to the correct input voltage. Some power supplies detect the input voltage and set themselves automatically, but some must be set manually. If your power supply is of the latter type, check the position of the slide switch to make sure it's set for the correct input voltage, as shown in Figure 16-11.

Figure 16-11: Verify that the power supply is set for the proper input voltage



If you connect a power supply set for 230V to a 115V receptacle, no harm is done. The system receives half the voltage it requires, and won't boot. But if you connect a power supply set for 115V to a 230V receptacle, the system receives twice the voltage it's designed to use, and is destroyed instantly in clouds of smoke and showers of sparks.

Standard power supplies are secured with four screws. To remove a power supply, disconnect the AC supply cord, the motherboard power cable(s), and all device power cables. Use one hand to hold the power supply in place while removing the four screws that secure it, and then lift it straight out. Some power supplies use a locking tab and slot arrangement, so you may have to slide the power supply a short distance to clear the tab before lifting it out. To install a power supply, reverse that process. Slide the power supply into place, as shown in Figure 16-12, making sure that the locking tab, if present, mates with the slot.

Figure 16-12: Slide the power supply into place

Once the power supply is in place, align the screw holes and insert the screws, as shown in Figure 16-13. If necessary, support the power supply with one hand while you insert screws with the other. Many good cases have a tray that supports the power supply, while other cases simply leave the power supply hanging in mid-air, secured only by the screws. In the latter situation, you may want to get someone to volunteer a second pair of hands to hold the power supply while you insert the screws, particularly if you're working in an awkward position. We've seen at least one motherboard destroyed by a dropped power supply, which ripped the processor, heatsink/fan, and socket right out of the motherboard on its way past.

Figure 16-13: Secure the power supply with the four screws provided

The next step in assembling the system is to connect the power cables from the power supply to the motherboard. The 20-pin or 24-pin main power connector is usually located near the right front edge of the motherboard. Locate the corresponding cable coming from the power supply. The main power connector is keyed, so verify that the cable is aligned properly before you attempt to seat it.

Once everything is aligned, press down firmly until the connector seats, as shown in Figure 16-14. It may take significant pressure to seat the connector, and you should feel it snap into place. The locking tab on the side of the connector should snap into place over the corresponding nub on the socket. Make sure the connector seats fully. A partially seated main power connector may cause subtle problems that are very difficult to troubleshoot.

All recent Intel systems and many AMD systems require the ATX12V +12V power connector. On most motherboards, the +12V power connector is located near the processor socket. Orient the cable connector properly relative to the motherboard connector, and press the cable connector into place until the plastic tab locks, as shown in Figure 16-15.

Figure 16-14: Connect the Main ATX Power Connector

Figure 16-15: Connect the ATX12V Power Connector

After you connect the motherboard power connectors, connect the power cables for the following items:

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  • Any supplementary power connectors present, such as the supplementary Molex connector on the motherboard, the PCI Express graphics power connector, the fan or supplementary power connector on your AGP video card, and so on
  • All hard drives, optical drives, tape drives, floppy drives, and so on
  • Any supplemental fans that connect to the power supply rather than to the motherboard

Once you've verified that everything is installed and connected correctly, dress the cables, reconnect the main power cable, and apply power to the system.

Suspect a power supply problem if you experience any of the following symptoms, particularly in combination:

  • Memory errors. Such errors may be caused by defective or poorly seated memory or by overheating, but insufficient or poorly regulated power from a failing or inadequate power supply is a likely cause. If a memory testing utility such as Memtest86 reports errors at a consistent address or range of addresses, the problem is probably the memory itself. If memory errors occur at random, nonreproducible addresses, the problem is most likely the power supply.
  • Sporadic or regular boot failures. Obviously, such errors may be caused by hard drive, cable, or disk controller problems, but inadequate or poorly regulated power is also a common cause of this problem.
  • Spontaneous reboots or system lockups during routine operations, especially during OS installations, that are not attributable to running a particular program. Numerous other factors can cause this problem, but one common cause is insufficient or poorly regulated power to the memory and/or processor.
  • Lockups after you install a new processor, memory, drive, or expansion card. Driver issues aside, this problem commonly occurs when new components overload a marginal power supply. This problem is particularly likely to occur if you make dramatic changes to the system, such as replacing a slow processor with a fast, high-current processor or adding a high-current video card. The power supplies provided with commercial systems, particularly inexpensive ones, often have very little reserve.

In-depth power supply troubleshooting is impractical unless you have a well-equipped test bench. There are, however, a couple of things you can do to isolate the problem to the power supply:

Most motherboard makers provide a monitoring utility to track system temperatures, fan speeds, and power supply voltages. (Intel, for example, provides the Intel Active Monitor, shown in Figure 16-16.) Install and enable this utility and use it to keep an eye on voltages. Most monitoring utilities allow you to set threshold values. If a voltage drops below or climbs above the acceptable range, the monitoring utility generates an alert. Some monitoring utilities allow you to log data, which can be very helpful in troubleshooting power supply problems.

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Figure 16-16: Use the motherboard monitoring utility to watch voltages

If you have a spare known-good power supply or a second system with a compatible power supply, try installing the known-good unit temporarily. If the problems stop, it's likely that the original power supply is marginal or defective.

Advice from Brian Bilbrey

At home, I keep a new ATX12V Antec power supply on the shelf. That power supply is equivalent to the most powerful one currently in my collection of computers, and so can be used to replace any of them. Power supplies have moving parts to keep the system cool, and also have first exposure to any input power problems, so they are the single most likely component to fail. That invariably happens on Sunday night at 9:03 PM with a presentation due on Monday morning. Spares on hand are good.