Manchester Small-Scale Experimental Machine

The Manchester Small Scale Experimental Machine

The Manchester Small-Scale Experimental Machine (SSEM), nicknamed Baby, was the world's first stored-program computer. Developed by Frederic C. Williams and Tom Kilburn at the University of Manchester, it ran its first program on June 21, 1948.

The computer was built around a Williams tube, a particular type of cathode ray tube (CRT) which had been developed by Williams at the Telecommunications Research Establishment in July-November 1946, before he joined the University of Manchester in December 1946. Working with Kilburn at the university they increased the storage capacity of the CRT from one bit to 2048 by October 1947 using a 64 by 32 array. This could be used for a computer's memory, with the advantage of allowing random access to memory, rather than the sequential access of the delay line memory units.

The SSEM was a very limited machine, designed to test the Williams tube and other hardware rather than as a practical computer. The SSEM had a single 32 by 32-bit word store, a second CRT to hold a single 32-bit accumulator, and a third CRT to hold the current instruction and its address. A fourth CRT was the output device, displaying the bit pattern of any chosen storage tube. The inputdevice was a set of 32 buttons with manual switches to set the bit pattern of any word.

A whole word was used for any instruction, with bits 0–12 representing the address and bits 13–15 the code defining the function. An instruction was executed in 1.2 milliseconds and the main store was refreshed every 16 instructions.

It was limited because it could store a total of only 32 numbers and instructions, and the instruction set was very limited. The initial seven instructions were:

* jump indirect
* relative jump indirect
* take a number from memory, negate it, and load it into the accumulator
* write the number in the accumulator back to memory
* subtract a value from the accumulator
* skip next if accumulator is negative
* stop

A division program was written, using pencil-and-paper method, operating on one bit at a 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 consisted 17 instructions and it was written by Kilburn. (A 19-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 50th anniversary of the running of its first program. This is on display at the Museum of Science and Industry in Manchester.