August 22, 2010

Small-Scale Experimental Machine (SSEM)- World's First Stored Program Computer

A Williams Tube
Image via Wikipedia
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 test-bed 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
operations implemented in hardware were subtraction and negation; 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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