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The Manchester Small-Scale Experimental Machine was the world's first stored-program computer. It was built at the Victoria University of Manchester by Frederic C. Williams, Tom Kilburn and Geoff Tootill, and ran its first program on 21 June 1948. The machine was not intended to be a practical computer but was instead designed as a testbed for the Williams tube, an early form of computer memory. It was considered "small and primitive" compared to its contemporaries, although it did contain all of the elements essential to a modern electronic computer. As soon as the SSEM had demonstrated the feasibility of its design a project was initiated at the university to develop it into a more usable computer, the Manchester Mark 1. The Mark 1 in turn quickly became the prototype for the Ferranti Mark 1, the world's first commercially available general-purpose computer. The SSEM had a 32-bit word length and a memory of 32 words. It was designed to be the simplest possible stored-program computer; the only arithmetic operation it could perform was subtraction. The first of the three programs written for the machine found the highest factor of 218 (262,144), a calculation it was known would take a long time to run—and so prove the computer's reliability. The program consisted of 17 instructions and ran for 52 minutes before reaching the correct answer of 131,072, after the SSEM had performed 3.5 million operations.