Edsac (Electronic Delay Storage Automatic Calculator)

Edsac (Electronic Delay Storage Automatic Calculator)

EDSAC (the Electronic Delay Storage Automatic Calculator) was the first full-scale electronic computer to implement the stored-program principle. The stored-program principle sates that both data and programming are to be stored in the computer's main memory and accessed during run-time. EDSAC executed its first program on May 6, 1949 at Cambridge University in England.

The development of EDSAC was largely due to the efforts of English computer scientist Maurice Vincent Wilkes (1913- ). Educated as a mathematical physicist, Wilkes had been involved in British radar development during World War II. After the war, Wilkes took a position with the computing laboratory at Cambridge University. Wilkes's subsequent development of computers at Cambridge was to be heavily influenced by computer development in America. In his book Memoirs of a Computer Pioneer, Wilkes wrote:

• In the middle of May 1946 I had a visit from L. J. Comrie, who was just back from a trip to the United States. He put in my hands a document written by J. von Neumann on behalf of the group at the Moore school and entitled "Draft report on the EDVAC." In it, clearly laid out, were the principles on which the development of the modern digital computer was to be based: the stored program with the same store [i.e., memory] for numbers and instructions, the serial execution of instructions, and the use of binary switching circuits for computation and control. I recognized this at once as the real thing, and from that time on never had any doubt as to the way computer development would go.

These comments from Wilkes illustrate the extent to which he was indebted to the American computer pioneers of the 1940s. Indeed, only a few months after reading von Neumann's report, Wilkes traveled to the United States in order to attend a workshop on advances in computer technology. The lectures were presented at the University of Pennsylvania's Moore School of Engineering, where ENIAC (the world's first general-purpose electronic computer) had been built, and where a more advanced computer, the EDVAC, was being constructed.

On the voyage back to England, Wilkes thought (as he put it) about constructing a "computer of modest dimensions very much along the lines of the EDVAC Proposal." Wilkes would eventually call his creation "EDSAC" in recognition of the fact that it was in large part based upon the EDVAC design.

EDVAC was being constructed in the United States by a large team of engineers and technical advisors, and its construction consumed a commensurately large budget. By contrast, Wilkes had a very modest budget within which to work and only a few assistants (initially only one). However, he was more interested in using an advanced computer to solve scientific problems than he was in becoming an expert computer-maker as such. For all these reasons, Wilkes decided to construct a scaled-down version of EDVAC and to do it as quickly as his limited resources would allow.

The vacuum tube technology available for Wilkes's EDSAC presented him with little difficulty because of his wartime electronics experience. However, the memory storage of EDSAC, like the American EDVAC, was to be constructed of mercury delay lines (long, sealed tubes, filled with mercury, which represented data as ripples in the mercury). Wilkes was not well-versed in delay-line technology, so he turned to an experienced colleague, T. Gold, for advice. By early 1947, Wilkes had constructed a working mercury delay line. (The phrase "Delay Storage" in "Electronic Delay Storage Automatic Calculator" refers to these mercury delay lines.) After the crucial challenge of the delay lines had been met, Wilkes forged ahead with construction of the entire computer, and by early 1949 EDSAC was ready for operational testing. The computer possessed control and arithmetic units composed of approximately 3,000 vacuum tubes housed in rows of six-foot-high cabinets. Input and output for EDSAC consisted of modified Teletype equipment. At the time of its first successful test in May 1949, EDSAC had only four mercury delay lines for memory storage (though eventually this was increased to 32 delay lines). Upon becoming operational, EDSAC became the world's first full-scale computer that performed electronic binary computations while storing its program and data in the same memory. It should be pointed out that another British binary, electronic computer, the "baby Manchester," employed the stored program concept before EDSAC; however, it was not a full-scale computer, but a very limited prototype. The EDSAC is also notable for having been the first computer to construct programs from subroutines that could be linked together at load time.