EDSAC

The Electronic Delay Storage Automatic Calculator (EDSAC) was an early British computer. Inspired by John von Neumann's seminal First Draft of a Report on the EDVAC, the machine was constructed by Maurice Wilkes and his team at the University of Cambridge Mathematical Laboratory in England to provide a service to the university. EDSAC was the second electronic digital stored-program computer, after the Manchester Mark 1, to go into regular service.

Later the project was supported by J. Lyons & Co. Ltd., intending to develop a commercially applied computer and resulting in Lyons' development of the LEO I, based on the EDSAC design. Work on EDSAC started during 1947, and it ran its first programs on 6 May 1949, when it calculated a table of square numbers and a list of prime numbers. EDSAC was finally shut down on 11 July 1958, having been superseded by EDSAC 2, which remained in use until 1965.

The conception of the EDSAC I can be traced back to 1945, during early planning of the EDVAC. In June of that year, John von Neumann wrote his First Draft of a Report on the EDVAC after taking on a consulting role while J. Presper Eckert and John Mauchly were the designers. The document described the concept of a stored-program computer, where both the program and data are stored in the same memory, which is now known as the Von Neumann architecture; it briefly explains the idea that computer instructions, or the program, could be stored in the same memory as the data, allowing for flexibility and automation in computation.

Later in August 1946, when Wilkes participated in the final weeks of the Moore School Lectures, he was exposed to the principles of the ENIAC – Eckert and Mauchly's previous invention – and their proposed next project, the EDVAC. He proposed the concept of microprogramming, a system that simplifies the logical design of computers, which later became widely adopted in the industry. Using the knowledge he gathered about the EDVAC's working concept in the lectures, he began development of the EDSAC I in October of that year.

As soon as EDSAC was operational, it began serving the university's research needs. It used mercury delay lines for memory and derated vacuum tubes for logic. Power consumption was 11 kW of electricity. Cycle time was 1.5 ms for all ordinary instructions, 6 ms for multiplication. Input was via five-hole punched tape, and output was via a teleprinter.

Initially, registers were limited to an accumulator and a multiplier register. In 1953, David Wheeler, returning from a stay at the University of Illinois, designed an index register as an extension to the original EDSAC hardware.

A magnetic-tape drive was added in 1952 but never worked sufficiently well to be of real use.

Until 1952, the available main memory (instructions and data) was only 512 18-bit words, and there was no backing store. The delay lines (or "tanks") were arranged in two batteries providing 512 words each. The second battery came into operation in 1952.