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Leading Edge

Making history

A landmark in the history of computing celebrates its 50th anniversary next month, a landmark so significant that 2,500 people from across the world are going to the party and the Halle Orchestra is providing the music. Hilary Kahn describes the leading edge technology of 1948

The Small Scale Experimental Machine, known as the Baby, was the first machine that had all the components now regarded as characteristic of the basic computer. Most importantly it was the first computer that could store not only data but any (short) user program in electronic memory and process it at electronic speed. Furthermore, the electronic memory was a true random access memory, in contrast to the other computers being proposed at the time.

The machine was designed and built at Manchester University by Professor Freddie Williams and his senior scientific officer, Tom Kilburn, and first successfully ran a program on 21 June 1948. Although the components of the Baby were small and simple, it so clearly demonstrated the potential of the stored-program electronic computer that an immediate decision was made to build a more powerful and useable machine, the Manchester Mark 1.

A letter dated 26 October 1948 from Sir Ben Lockspeiser, chief scientist at the then Ministry of Supply, requested Ferranti in no uncertain terms to build a commercial machine to this new design: 'Construct an electronic calculating machine to the instructions of Professor F. C. Williams.'

During the 1939-45 war Freddie Williams had led a team of engineers at the Telecommunications Research Establishment at Malvern, Worcestershire, working on radar; one member of that team was the young Tom Kilburn. One of the emerging challenges at the time was to find a way of storing information digitally, so Freddie Williams started work in July 1946 on a form of digital storage using a cathode ray tube.

He demonstrated the successful operation of a single-bit memory using the 'anticipation pulse method' in early October 1946, and provisionally patented this system in December.

The bit was stored in the form of a charge in the screen's phosphor, which could be controlled by the electron beam to write a zero or a one. Although the phosphor was an electrical insulator, the charge would leak away in the order of a second. Freddie Williams arranged to read the charge and then rewrite it continuously at electronic speeds so that information could be kept permanently.

In December 1946 Freddie Williams moved to the Manchester University to take up a chair in electro-technics. He continued to work on the system, and Tom Kilburn was seconded to Manchester to work with him. It was known world-wide that the provision of an effective means of electronic storage was crucial to the further development of electronic digital computers.

By December 1947 2,048 bits were being stored on a single six-inch diameter cathode ray tube using a method recorded in a technical report by Tom Kilburn. The storage system became known as the Williams Tube, although Williams-Kilburn Tube might have been a better name. To prove the full capability of this new digital store, it was necessary to test its ability to set and read any required bit at electronic speeds and to remember its value indefinitely between settings. So the next step was to build a small computer around a cathode ray tube memory, to subject it to the most effective and searching tests possible.

This computer, the Small Scale Experimental Machine, included the stored-program concept, so that the random access memory was used to hold not only numbers involved in calculations but also the program instructions. Running a different program involved only resetting part of the memory using a simple keyboard rather than reconfiguring the electronic circuitry (a process that could take days on the earlier ENIAC). This machine was built during the early months of 1948.

The first program to run successfully, on 21 June 1948, was to determine the highest factor of a number. It was written by Tom Kilburn. The test number chosen was initially quite small, but within days Tom Kilburn and his colleagues had built up to trying the program on 218; the correct answer was found in 52 minutes, involving about 2.1m instructions with about 3m store accesses.

Professor Williams later said of the first successful run, 'A program was laboriously inserted and the start switch pressed. Immediately the spots on the display tube entered a mad dance. In early trials it was a dance of death leading to no useful result, and, what was even worse, without yielding any clue as to what was wrong. But one day it stopped, and there, shining brightly in the expected place, was the expected answer. It was a moment to remember. It was in June 1948, and nothing was ever the same again.'

The Baby machine had a computing speed of around 1.2 milliseconds per instruction, 32-bit word length, serial binary arithmetic using two's complement integers, a single address format order code, and a random access main store of 32 words, extendable to 8,192 words.

The main store was a 3232-bit array on a Williams Tube. In addition there were two other Williams Tubes holding special storage registers: one held the accumulator A and the other the address of the current instruction CI ('control instruction') and the instruction itself PI ('present instruction'). A fourth tube, the display tube, could be switched to provide a suitable display of the current contents of any of the Williams Tubes.

The basic order code of the machine was designed with some care by Tom Kilburn. The 32-bit word used 16 bits for the instruction: a three-bit function field and a 13-bit store address. Just seven instructions made up the order code.

The Baby's successor, the Manchester Mark 1, completed by late 1949, not only expanded the size and power of the basic Baby components but also added some important innovations, including a magnetic drum: this was the original ancestor of the disc.

The Ferranti Mark 1 made some further fairly minor enhancements to the Manchester Mark 1, but had much improved engineering. It was the world's first commercially available general purpose computer. The first machine was delivered to Manchester University in February 1951. The second was provided to government code breakers, who had moved to Cheltenham from Bletchley Park. The third was sold to Toronto University in 1952 and was used in an early design of the St Lawrence Seaway.

A few quotations from the early days of computing reveal how difficult it was to predict the phenomenal later development of computing.

'Computers in the future may weigh no more than 1.5 tons,' wrote Popular Mechanics in 1949.

'We have a computer in Cambridge, one in Manchester and one at the National Physical Laboratory; I suppose there ought to be one in Scotland, but that's about all,' said Professor Douglas Hartree in September 1951.

'There is no reason anyone would want a computer in their home,' said Digital Equipment founder, Ken Olsen.

After the Baby and the Mark 1 had been developed in Manchester, the researchers at the university continued to make pioneering advances in computer hardware, architecture and software. Today, 50 years on from those first flickering digits, digital electronics and computing in particular affect every aspect of our lives in ways that no one could have imagined.

Hilary Kahn is professor of computer science at Manchester University, and co-ordinator of the June celebrations of the Baby's 50th birthday.

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Copyright British Computer Society 1998