Editing Colossus computer (section)

==Programming==
[[File:Colossus computer Q panel.png|right|upright=1.35 |thumbnail |Colossus K2 switch panel showing switches for specifying the algorithm (on the left) and the counters to be selected (on the right)]]
[[File:Picture111Shopped.jpg|upright=1.35|thumb|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".{{Blockquote|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.{{sfn|Flowers|1983|pp=239–252}} }}

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.{{sfn|Small|1944|p=108}} 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.{{sfn|Good|Michie|Timms|1945|loc = 5 Machines: 53 Colossus, pp. 333–353}} 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 <math>\chi</math> or '''Δ'''<math>\chi</math> and either <math>\psi</math> or '''Δ'''<math>\psi</math> 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.{{sfn|Good|Michie|Timms|1945|loc = 5 Machines: 53 Colossus 53A Introduction, p.333}}

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 the 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.{{sfn|Budiansky|2006|p=62}}