EDSAC

EDSAC was conceived in 1946 by Maurice Wilkes, who had attended the Moore School Lectures in the United States and read the EDVAC report. Construction began in 1947 at the Mathematical Laboratory (now the Computer Laboratory) in Cambridge, with funding from J. Lyons & Co., a catering firm interested in office automation. The machine used mercury delay lines for storage, each holding 32 or 64 words of 17 bits (including a parity bit). The arithmetic unit operated in binary, with a two's complement representation for negative numbers. Input was via five-hole punched tape, and output was via a teleprinter or punched cards. The initial memory of 512 words was later doubled to 1024. The machine consumed about 11 kW of power and occupied a large room, with racks of valves and a control console.

EDSAC was the first computer to have a systematic assembly language, known as "Initial Orders," written by David Wheeler. This allowed programmers to write symbolic instructions that were translated into machine code by a bootstrap program stored in read-only memory. Wheeler also introduced the concept of the "subroutine" and the "Wheeler jump," a technique for calling and returning from subroutines, which became a standard practice. The EDSAC programming manual, written by Wilkes, Wheeler, and Stanley Gill, was the first book on computer programming, published in 1951. The machine ran a library of subroutines for mathematical functions, input/output, and debugging, which users could link into their programs. This pioneering software environment made EDSAC a practical tool for scientific research.

EDSAC was used for a wide range of scientific computations, including calculations for X-ray crystallography, nuclear physics, and number theory. One of the first programs, run in May 1949, computed a table of squares and a list of prime numbers. Later, it was used to solve problems in radio astronomy and to calculate the orbits of particles in particle accelerators. The machine also supported early research in artificial intelligence, including a program that played noughts and crosses (tic-tac-toe) written by A.S. Douglas in 1952. EDSAC's success directly influenced the design of LEO I, the first computer used for commercial business applications, built by J. Lyons & Co. with Wilkes's assistance. The techniques developed on EDSAC, such as microprogramming (later explored by Wilkes), shaped the future of computing.

Beyond its well-known achievements, EDSAC had several obscure facets. The machine's delay-line memory was sensitive to temperature, so the room had to be kept at a constant 20°C, and operators would sometimes adjust the mercury levels manually. The initial orders were stored on a separate set of delay lines and could be overwritten by user programs, leading to occasional crashes. EDSAC also had a primitive debugging facility: a "post-mortem" routine that dumped memory contents to the printer. The machine was used to calculate the first tables of the error function for the British Association, and it helped in the design of the EDSAC 2, its successor. A replica of EDSAC was built at The National Museum of Computing in 2014, and it is now on display, demonstrating the machine's operation to visitors.

Delay line memory

A storage device using acoustic waves in a mercury column to store bits as pulses, with a recirculating loop.

Initial Orders

A bootstrap program that translated symbolic assembly code into machine code on EDSAC.

Wheeler jump

A technique for subroutine call and return, named after David Wheeler, using a modified jump instruction.