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EDSAC rebuild gets the go-ahead

Cliff Saran

The Computer Conservation Society (CCS) has commissioned a working replica of EDSAC, the world's first operational stored program computer.

The Electronic Delay Storage Automatic Calculator (EDSAC) was a general purpose research tool at Cambridge University, which led directly to the development of the first business computer.

The EDSAC was the brainchild of Sir Maurice Wilkes, who died in November 2010. Wilkes was in charge of the Mathematical Lab in Cambridge after the war - and he attended a series of lectures in the US in 1946. The trip back from the US, which took four days, gave Wilkes the time to sketch out plans for the design of EDSAC.

While the Manchester Baby is considered the first computer, David Hartley, chairman of the Computer Conservation Society, described it as a proof of concept, whereas the EDSAC was "the first practical stored program computer", he said.

It was used to aid scientific research at Cambridge. Hartley said, "EDSAC replaced a human being with a desk calculating machine. It was 1,500 times faster than the human."

The project is being run by Hartley, along with Kevin Murrell, who specialises in post-1945 computer history, Martin Campbell-Kelly, who created the EDSAC emulator and Chris Burton, who rebuilt the Manchester Baby.

The first IT department

EDSAC was not only the first computer, it also heralded the concept of the user, the IT function and reusable programs, known as subroutines.

Murrell, said, "It is a very important project for the museum. EDSAC is the first general purpose machine. It was built to answer the problems of researchers who would never have expertise to program the machine themselves.

"There were people queuing up with their programs [to feed into the computer]".

In fact, Murrell said the the concept of users did not exist before the EDSAC machine.

Programs were loaded using puch tape. A punch card operator would key in the codes for the program, which would be printed out on paper tape. The tape contained the code for the program, which was then fed into the EDSAC, where it was executed. The results would then be printed out.

He said, "There is awful lot we don't know about how it was used on a day to day basis. The rebuild is a fantastic opportunity to understand the engineering."

The coding itself was quite insightful, according to Murrell. The IT function at Cambridge created computer programs that could be reused in more complex programs, such as the square root function. This concept is the basis of modern programming and allows programmers to use programming libraries that contain subroutines that have already been tested and debugged, saving a considerable amount of programming effort, "reinventing the wheel".

On the EDSAC, these subroutines existed on strips of paper tape. The operators literally strung together the subroutines with the main program by feeding in these tapes and the main program tape, to create a master tape with the whole program. The process mirrors how C programs are comiled today, said Murrell. A programmer "#includes" a library like "stdio.h", which provides programs with basic input/output routines; the source code is compiled and the subsequent program is then executed.

Rebuilding EDSAC

The build is a substantial project. "We will need 2,500 valves of a particular type, together with chassis and power supplies." Luckily said Murrell, "Sir Maurice used components readily available at war time and there are still stocks of components left."

The project team will also look at alternatives, like using transistors instead of valves, or even remanufacture valves.

As a far as the chassis goes, the project has had a stroke of good luck. "There are literally two or three chassis left out of the hundreds that were used in the original." These will form the blueprint to remanufacture the EDSAC chassis.

Five facts about EDSAC

  • EDSAC was over two metres high and occupied a ground area of four metres by five metres.
  • Its 3,000+ vacuum tubes used as logic were arranged on 12 racks.
  • Mercury-filled tubes acted as memory.
  • It performed 650 instructions per second.
  • EDSAC ran its first program on 6 May 1949 and soon began nine years of regular service, ending in July 1958 when it was dismantled to enable the re-use of precious space. By then it had been superseded by the faster, more reliable and much larger EDSAC 2.

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