The people who built the world's first fully operational and practical stored-program computer are meeting to celebrate the machine's 50th anniversary.

Edsac - Electronic Delay Storage Automatic Calculator - was a huge contraption that took up a room in what was the university's old Mathematical Laboratory.

It had 3,000 vacuum valves arranged on 12 racks and used tubes filled with mercury for memory. But despite its impressive size, Edsac could carry out just 650 instructions per second.

Punched tape

A program was fed into the machine via a sequence of holes punched into a paper tape. This generated pulses that the computer used to store the program and perform the desired calculation.

The old Maths Lab which has now been demolished

Although Manchester University would claim the very first stored-program computer - "Baby" began operating 11 months before Edsac - the Cambridge machine had the distinction of being a fully-working scientific tool.

From day one, it operated as a complete system. Unlike earlier machines, none of the wiring or switches had to be altered to perform a new calculation.

"The machine wasn't retained by the engineers for experimental use," says Dr Peter Robinson from Cambridge's Computer Laboratory. "Very quickly, it entered general use for science around the university."

Scientific paper

The world's first scientific paper to be published using computer calculations was done on Edsac - a genetics paper by R A Fisher.

The programs were fed in on punched tape

The first successful program was run on 6 May, 1949, computing a table of squares.

Edsac 1 was replaced by Edsac 2 in 1958. Much later, the university turned to Titan, a commercial machine modified for university use.

Many of those who built and used Edsac are being brought back to Cambridge for a celebratory two-day seminar. They include Professor Maurice Wilkes who led the design team, and David Wheeler who wrote the very first program.

Simulator programs

"Very little remains of the original machine," says Dr Robinson. "There is a chassis from the Edsac 1 and the paper tape reader that was used with it, and a thing called a mercury delay line which was the sort of memory used on that computer."

Maurice Wilkes will address the two-day meeting

These will be put on display during the seminar, together with some of the simulator programs that can now be downloaded from the Net and which turn your desktop into a faithful replica of Edsac.

Sadly, hardly any of the original paper tapes survive.

"We've got some from later in the Lab's history but I've got a feeling most of them were used as streamers at children's parties," says Dr Robinson.

Edsac's vital statistics:

• 650 instructions per second
• 17-bit words of memory in mercury ultrasonic delay lines
• Paper tape input and teleprinter output at 6 2/3 characters per second
• 3000 valves
• 12 kW power consumption
• "Operating system" occupied 31 words of read-only memory
• Occupied a room five metres by four metres
• Early use to solve problems in meteorology, genetics and X-ray crystallography
• First book on programming by Wilkes, Wheeler and Gill published in 1951
• Magnetic tape backing store added in 1952
• First course in Computer Science at Cambridge started in 1953, using the EDSAC.