Programming Colossus computer

colossus k2 switch panel showing switches specifying algorithm (on left) , counters selected (on right).

howard campaigne, mathematician , cryptanalyst navy s op-20-g, wrote following in foreword flowers 1983 paper design of colossus .

my view of colossus of cryptanalyst-programmer. told machine make calculations , counts, , after studying results, told job. did not remember previous result, nor have acted upon if did. colossus , alternated in interaction achieved analysis of unusual german cipher system, called geheimschreiber germans, , fish cryptanalysts.

colossus not stored program computer. input data 5 parallel processors read looped message paper tape , electronic pattern generators chi, psi , motor wheels. programs processors set , held on switches , jack panel connections. each processor evaluate boolean function , count , display number of times yielded specified value of false (0) or true (1) each pass of message tape.

input processors came 2 sources, shift registers tape reading , thyratron rings emulated wheels of tunny machine. characters on paper tape called z , characters tunny emulator referred greek letters bill tutte had given them when working out logical structure of machine. on selection panel, switches specified either z or Δz, either



χ


{\displaystyle \chi }

or Δ



χ


{\displaystyle \chi }

, either



ψ


{\displaystyle \psi }

or Δ



ψ


{\displaystyle \psi }

data passed jack field , k2 switch panel . these signals wheel simulators specified stepping on each new pass of message tape or not.

the k2 switch panel had group of switches on left hand side specify algorithm. switches on right hand side selected counter result fed. plugboard allowed less specialized conditions imposed. overall k2 switch panel switches , plugboard allowed 5 billion different combinations of selected variables.

as example: set of runs message tape might involve 2 chi wheels, in tutte s 1+2 algorithm. such two-wheel run called long run, taking on average 8 minutes unless parallelism utilised cut time factor of five. subsequent runs might involve setting 1 chi wheel, giving short run taking 2 minutes. initially, after initial long run, choice of next algorithm tried specified cryptanalyst. experience showed, however, decision trees iterative process produced use wren operators in proportion of cases.