Microprogramming: History and Evolution - Edwardbosworth.com

Wilkes’ Motivations
―As soon as we started programming, we found to our surprise that it wasn’t
as easy to get programs right as we had thought. … I can remember the exact
instant when I realized that a large part of my life from then on was going to
be spent in finding mistakes in my own programs.‖
After his visit to the United States, Wilkes started to worry about the complexity of the
control unit of the EDVAC, then in design. Here is what he wrote later.
“I found that it did indeed have a centralized control based on the use of a matrix
of diodes. It was, however, only capable of producing a fixed sequence of eight
pulses—a different sequence for each instruction, but nevertheless fixed as far as
a particular instruction was concerned. It was not, I think, until I got back to
Cambridge that I realized that the solution was to turn the control unit into a
computer in miniature by adding a second matrix to determine the flow of control
at the microlevel and by providing for conditional microinstructions.”
In other words, have the control signals emitted by a smaller computer that is controlled
by a microprogram. The advantage is that the control unit of this smaller computer is
extremely simple to design, test, and understand.