In the 1970s a need arose to perform special arithmetic operations on minicomputers much more quickly than had been possible in the past. This paper tells the story of why micro programming was needed for special arithmetic operations on mini computers in the 1970s and how it was implemented. The paper tells how the laboratory in which the first experiment took place had a PDP-9 minicomputer from Digital Equipment Corporation and how the author, with several colleagues, after attending a course for the technical service of a PDP-9 given by a specialist from Digital Equipment, knew exactly which signals flew through the machine at any time. The paper describes how by having ‘programmable’ control memory they were able to make changes in the execution of instructions.