In the annals of computing history, the Small Scale Experimental Machine (SSEM), affectionately known as the "Manchester Baby", holds a uniquely pivotal place. Developed at the University of Manchester in England in the late 1940s, this diminutive machine, weighing in at a mere half a ton, was the first computer in the world to implement a number of key innovations that would come to define modern computing. Chief among these was the stored program concept, the idea that a computer‘s program instructions and working data could be held in the same modifiable memory.
While it‘s tempting to picture the pioneers of the digital age working with circuits and bits from the very beginning, the reality is that the first electronic computers, like the famous ENIAC in the United States, were programmed by the laborious physical reconfiguration of switches and cables. Each new problem required a new setup, a process that could take days. The stored program concept promised to change all that, allowing a computer to be reprogrammed for a new task as quickly as new instructions could be written to its memory.
The Invention of the Williams Tube
The story of the Manchester Baby begins with Frederic C. Williams, an ingenious electrical engineer who had made substantial contributions to radar technology during World War II. In 1946, Williams took up a chair at the University of Manchester, where he began exploring the potential of cathode ray tubes (CRTs), the display technology used in television sets and oscilloscopes, as a means of digital data storage.
Williams‘ key insight was that the phosphorescent dots that CRTs paint on their screens in response to a focused beam of electrons could serve as a form of memory. A dot could represent a 1, the absence of a dot a 0. By using the electron beam to periodically refresh the charge of each dot, Williams realized, this data could be made to persist indefinitely, providing a form of what we now call random-access memory (RAM).
Working with his student Tom Kilburn and his colleague Geoff Tootill, Williams engineered a system in which bits could be written to the screen by focusing the electron beam, read by detecting the charges with a metal pickup plate, and refreshed by periodically rescanning each location. A working prototype of this "Williams tube" was successfully demonstrated in late 1947. It was, in the words of computer historian Simon Lavington, "the world‘s first high-speed, entirely electronic, digital storage device."
Building the Baby
With a workable memory technology in hand, Williams and Kilburn set out to build a machine that could showcase its capabilities. Over the first half of 1948, with the assistance of the celebrated mathematician Alan Turing and computing pioneer Max Newman, they designed and constructed the Small Scale Experimental Machine.
The Baby, as it came to be known, was a fully functional computer built on a single, 5-foot steel chassis. Its arithmetic unit, responsible for performing calculations, consisted of some 550 vacuum tubes, while its memory was provided by a system of four Williams tubes. One tube held the 32-word main memory, another the single-bit accumulator, a third the program counter and current instruction register, and the fourth served as an output display. Instructions and data were input via a simple keyboard, and the machine boasted a respectable (for the time) clock speed of around 1.2 milliseconds per instruction.
| Component | Details |
|---|---|
| Word length | 32 bits |
| Memory size | 32 words (expandable to 8192) |
| Instruction format | 3-bit opcode, 13-bit address |
| Number of instructions | 7 (later expanded to 30+) |
| Clock speed | ~1.2 milliseconds per instruction |
| Input/Output | 1-bit accumulator, keyboard input, CRT display |
The SSEM‘s 7-instruction repertoire was minimal but powerful, including basic arithmetic (addition and negation), conditional branching, and the ability to read from and write to memory locations specified by an address in the instruction. Crucially, instructions were stored in the same memory as data, meaning the program could modify itself – the essence of the stored program concept.
The Program Heard Round the World
On June 21, 1948, the Manchester Baby executed the first program ever run on a stored-program computer. Written by Tom Kilburn, the program computed the highest proper factor of a number by testing each integer from n-1 downward for divisibility into n, using repeated subtraction. It was a task well-suited to the Baby‘s limited instruction set, as Kilburn explained:
"A program to find the highest proper factor of any number was chosen for two reasons. It was sufficiently simple to be worked by hand and quickly checked, and it was complex enough to provide a thorough test of the machine, since it used all the facilities and exercised all the functions of the machine except multiplication and division."
In 52 minutes of runtime, the program correctly identified the factors of the number 218. Over the following days, Kilburn and his colleagues tested progressively larger numbers, up to 130,000, marveling as the Baby tirelessly worked through millions of instructions to arrive at the correct solutions.
As word of the Baby‘s success spread, its significance was immediately recognized. Here was incontrovertible proof that the stored program concept, up to that point a largely theoretical notion, could work in practice. The implications were enormous: a single machine could, with only a change of program, be adapted to tackle a limitless variety of computational problems. It was, in essence, the birth of the modern computer.
The Legacy of the Baby
The impact of the Manchester Baby was swift and far-reaching. Its success spurred the rapid development of a more powerful successor, the Manchester Mark 1, which boasted an expanded 256-word memory and an instruction set that grew to include over 30 operations. The Mark 1, in turn, served as the template for the Ferranti Mark 1, the world‘s first commercially available general-purpose computer.
More broadly, the Baby established a blueprint for computer architecture that has endured to the present day. The combination of electronic arithmetic logic, separate random-access memory for instructions and data, and a stored program that could be electronically modified, all hallmarks of the SSEM, remain at the heart of even the most advanced computing devices.
While the Williams tube itself was eventually superseded by more compact and reliable memory technologies like magnetic core and solid-state memory, the basic concept of regenerative memory it pioneered, where stored data is continually refreshed, lives on in the dynamic RAM chips that power our laptops and smartphones.
Perhaps most enduringly, the Baby and the Manchester computers established the stored program paradigm as the foundational principle of modern computing. In the words of Alan Turing, who played a key advisory role in the Baby‘s creation:
"This idea [of the stored program computer] was probably first conceived by Babbage around 1840. In 1936 I described a Universal machine which was an embodiment of this idea, and in 1945 von Neumann and others produced a report which described a practical form of the machine. The first machine to be built to this plan was completed a year ago at Manchester University. It was purely experimental."
From a "purely experimental" proof of concept to the bedrock of the digital age, the story of the Manchester Baby is a testament to the transformative power of innovative thinking and engineering brilliance. In a mere 32 words of memory, Williams, Kilburn, and their colleagues not only validated a revolutionary computing paradigm, but set in motion a technological revolution that continues to reshape our world to this day. The Baby may have been small in scale, but its impact was anything but.
