Manchester Baby

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The Manchester Baby (formal name Small-Scale Experimental Machine, sometimes abbreviated to SSEM) was a very early experimental computer, built at the University of Manchester. Exactly like the Memory Test Computer, it was not really intended for computational use; rather, its primary intended purpose was to test out a new form of main memory, in this case the Williams tube.

It was constructed out of vacuum tubes, starting early in 1948; starting later that year, it was re-built into the Manchester Mark I.


The Baby had 32-bit words, with a main memory of only 32 words. Two more Williams tubes held the accumulator ('A') in one, and the program counter ('CI') and the instruction register ('PI') in the other. Its ALU was serial, and used two's complement.

Instructions included a 3-bit operation code, and a 13-bit memory address (the other 16 bits were unused). It had only 7 instructions:

Opcode Instruction Description
0 s,C Copy content of memory to CI
1 c+s,C Add content of memory to CI
2 -s,A Load negated memory content to accumulator
3 a,S Copy content of accumulator to memory
4 a-s, A Subtract content of memory from accumulator
6 Test Skip next instruction if accumulator is negative
7 Stop Light "Stop" light and halt

First computer

It was thought for many years that the Baby had been the first stored-program computer to ever run (on a problem of finding the largest factor of a given integer); on 21st June, 1948. It has recently been brought to wider attention that, on a suggestion from John von Neumann, the ENIAC was re-configured fairly early in its life into a stored-program computer - albeit one of very limited capability. It used the machine's 'function table' (an array of switches) to hold the program, thereby acting as a form of EPROM for ENIAC.

It can be argued that this does not make the ENIAC, so configured, a Turing machine, as it cannot modify its own program. However, contemporary embedded systems often have a microprocessor with all their code contained in ROM, but no-one argues that those machines are not Turing machines.

Planning for that conversion started in the Fall of 1947; several different instruction sets were proposed. The re-configuration actually started at the end of March, 1948; the first run (on a problem related to atomic bomb work) happened at the end of April, 1948, finishing in May - effectively contemporaneous with the first run of the Baby.


The original Baby was not saved, but a replica was produced for the 50th anniversary with the assistance of the Computer Conservation Society.

Further reading

  • Simon H. Lavington, Early Computing in Britain: Ferranti Ltd. and Government Funding, 1948-1958, Springer Nature, Cham, 2019 - the background to the creation of the Baby is covered in appendix A
  • Simon H. Lavington, A History of Manchester Computers, National Computer Centre, Manchester, 1976
  • Simon H. Lavington, Early British Computers, Manchester University, Manchester, 1980
  • Raúl Rojas, Ulf Hashagen (editors), The First Computers - History and Architectures, MIT Press, Cambridge, 2000 - the Baby is covered in Part IV-2, pp. 365-377; the re-build is covered in Part IV-3, pp. 379-386

External links