EDSAC

The Electronic Delay Storage Automatic Calculator, known as EDSAC, was a pioneering British computer.

It was built at the University of Cambridge Mathematical Laboratory to serve the university's research needs. EDSAC was the second electronic digital stored-program computer to enter regular service. The first was the Manchester Mark 1. The concept of a stored-program computer is very important. It means the computer stores both its instructions, or programs, and its data in the same memory. This allows for much greater flexibility and automation in computation.

The design of EDSAC was inspired by the work of John von Neumann. In 1945, von Neumann wrote a report about the EDVAC. This document described the Von Neumann architecture, which is the idea of storing programs and data together. Maurice Wilkes began developing the EDSAC I in October 1946. He had attended the Moore School Lectures in 1946. There, he learned about the ENIAC and the proposed EDVAC. Wilkes also proposed microprogramming. This is a system that simplifies the logical design of computers. Microprogramming later became a standard practice in the computer industry.

Technically, EDSAC relied on specific components to function. It used mercury delay lines for its memory. These delay lines were arranged in two batteries, which were often called "tanks." The machine also used derated vacuum tubes for its logic.

To provide instructions, users utilized five-hole punched tape. The computer would then output information via a teleprinter. The machine consumed 11 kW of electricity to operate. Most ordinary instructions had a cycle time of 1.5 ms. However, multiplication was slower, taking 6 ms to complete.

The memory system of EDSAC evolved over time. Initially, the main memory was limited to 512 words. Each word contained 18 bits. However, the topmost bit was always unavailable due to timing problems. This meant only 17 bits were actually used for data. The machine used two's complement binary numbers for its calculations. Numbers could be 17 bits long or 35 bits long. By 1952, a second battery of delay lines was added. This eventually allowed for a full 1024-word store. By 1955 or 1956, the full capacity was available. This expansion allowed programs to grow larger than the original 800-word limit.

EDSAC's first successful run occurred on May 6, 1949.

The very first program was written by Beatrice Worsley. She traveled from Canada to study the machine. This first program calculated a table of square numbers from 0 to 99. It also produced a list of prime numbers. In 1953, David Wheeler added an index register to the hardware. This was an extension to the original design. This addition helped programmers manage data more effectively. The machine remained in service until it was shut down on July 11, 1958.

The computer also helped birth the software industry. In May 1949, the machine used the world's first assembler. An assembler is a tool that translates code. This primitive relocating assembler used only 31 words. David Wheeler, who earned the first Computer Science PhD on this project, also invented the subroutine. A subroutine is a sequence of instructions that performs a specific task. Programmers used "Wheeler Jumps" to call these subroutines. They also developed early job queues. Operators would hang paper tapes on a line for the machine to process. This is very similar to how modern computers handle tasks.

EDSAC's influence reached far beyond the University of Cambridge. The design of EDSAC served as a basis for the LEO I. This was a commercial computer developed by J. Lyons & Co. Ltd. In 1951, Wilkes, Wheeler, and Stanley Gill published a book. This was the first programming textbook. It was titled "The Preparation of Programs for an Electronic Digital Computer." This work helped standardize how people learned to interact with computers. The legacy of EDSAC lives on in the fundamental way all modern computers are designed and programmed.