On this day June 21, 1948

The Manchester Small-Scale Experimental Machine (SSEM), nicknamed Baby, 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. Although considered “small and primitive” by the standards of its time, it was the first working machine to 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. As it was designed to be the simplest possible stored-program computer, the only arithmetic operation implemented in hardware was subtraction; other arithmetic operations were implemented in software.

The first of three programs written for the machine found the highest proper divisor of 218 (262,144), a calculation it was known would take a long time to run—and so prove the computer’s reliability—by testing every integer from 218 − 1 downwards, as divisions had to be implemented by repeated subtractions of the divisor. 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 (for an effective CPU speed of 1.1 kIPS).

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