Where Code Comes From: Architectures of Automatic Control from Babbage to Algol

In our previous Communications column (September 2015) we noted that a celebrated table published by Ada Lovelace in 1843 was not a computer program, despite frequent claims to the contrary. Here we turn to a related question: where did computer code come from? Back in the 1840s nobody talked about "programming" Charles Babbage's planned engines. More importantly, nobody had yet formulated the concept of a program as a series of instructions controlling the operation of a general-purpose computer. The work of Charles Babbage and Ada Lovelace provides an important milestone on the road to this invention, but marks the beginning of the story rather than its end.

In this column we explore the rest of that story, returning briefly to the world of Lovelace and Babbage before moving on to the 1940s when their ideas were independently rediscovered, extended, and finally realized in actual machinery. Developments came thick and fast, moving in just a few years from the earliest relay computers controlled by "coded" arithmetic instructions on tape to ENIAC, the first computer to automatically carry out computations with complex structures including branches and nested loops. This was the context in which the word "programming" was initially applied to a computer, originally to describe the action of the machine's control wires, circuits, and switches when triggering the appropriate sequence of mathematical operations. Before ENIAC was even finished its creators, in collaboration with John von Neumann, had come up with a new approach in which control and arithmetic operations were both represented in a single series of coded instructions stored in an addressable memory unit. That was soon called a computer program, and although the meaning of the term has continued to evolve it has retained this basic sense of a set of instructions that direct the performance of a series of operations, enabling a computer to carry out a task without human intervention.