Small-Scale Experimental Machine

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Image:SSEM Replica.jpg The Manchester Small-Scale Experimental Machine (SSEM), nicknamed Baby, was the first stored-program computer to run a program, on June 21, 1948. It was developed by Frederic C. Williams and Tom Kilburn at the University of Manchester.

The computer was built around a Williams tube, which was developed by Williams and Kilburn. The Williams tube stored 32 words of 32 bits each. This was used for the computer's memory, which had the advantage of allowing random access to memory, rather than the sequential access of the delay line memory units. It was a serial machine, operating on one bit at the time. The input was a bank of switches to set any bit in memory, and the output was a bit pattern on the Williams tube (a cathode ray tube).

The SSEM was a very limited machine, apparently more for the purposes of testing the Williams tube and other hardware than for producing a practical computer. It was limited because (1) it could store a total of only 32 numbers and instructions, and (2) the instruction set was very limited. The instructions in the instruction set were:

take a number from memory, negate it, and load it into the accumulator
subtract a value from the accumulator
write the number in the accumulator back to memory
a conditional branch depending on the value in the accumulator
stop

A division program was written, using pencil-and-paper method, operating on one bit at the time. It was used to divide 2³⁰-1 by 31, giving the answer in about 1.5 seconds. Then this routine was used in a program to show that 314,159,265 and 217,828,183 are relatively prime. Finally, a program was written to find the largest divisor of integers, by testing all numbers from a starting point down as possible divisors, with repeated subtraction used for division. This program was comprised of seventeen instructions and it was written by Kilburn. (A nineteen instruction amended version of it has been published.) It ran successfully on June 21, 1948, first on small integers. Within a few days it was run on 2³⁰-1 by trying every number from 2¹⁸-1 down. It ran for 52 minutes, executing 3.5 million accesses to memory and 2.1 million instructions, and produced the correct answer.

The SSEM developed into the Manchester Mark I, which led to the Ferranti Mark I, the world's second commercially available general-purpose computer. At around the same time EDSAC was being developed at the University of Cambridge Mathematical Laboratory.

A working replica of the SSEM was created in 1998 to celebrate the fiftieth anniversary of its first program. This is on display at the Museum of Science and Industry in Manchester.

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