CUED 2012

Back in April 2012 I submitted this fourth year project to Cambridge University Engineering Department.   Nobody took it up....

Minimal computer

The aim of this project is to build a minimal computer with as much as possible of the processing visible so that people can see how computers work.  It will be constructed either out of relays or transistors and use LEDs to show the state.   The switch count is to be minimised so that it can be easily understood by school children and so that it is affordable to reproduce.   The clock rate should be low enough so that it can be seen how the next state arises from the current state and so it can be seen how operations such as addition and memory access work, as well as constructs such as registers and program counters.   Efficiency is not a major issue although the aim is that it will run small programs in a few minutes which may involve an adjustable clock rate.

The provisional design is to have a four bit instruction set and a twelve bit address space accessing four bit words.   Perhaps one quarter will be RAM or 4096 bits and so the memory design will be critical.   Current thoughts are to use a square grid of capacitors indexed by two address decoders or core memory.  Core memory would be preferable as the memory contents are visible (sic).   The lower bank of memory may be defined by DIP switches to store boot code and commonly used subroutines.

The project requires a wide range of skills: 

* the hardware must be designed and built so that the operation is reliable
* the instruction set must be designed so that the operation of programs is clear
* good experimental skills will be needed to design the capacitor or core memory
* good logic skills will be needed for the circuit design
* there will be considerable experimentation so project planning skills will have to be strong to achieve a working system at the end
* the resulting system must look good, not spaghetti wiring, to maximise the educational impact

Ideally the project would not use silicon for the switches but use relays which would demonstrate that it would have been possible to build a computer in the late 19th century.   This limits the memory capacity and design, electrolytic capacitors and micro-relays.

If time permits then some I/O will be added.  Ideally the machine could be loaded remotely and the operation tracked with a webcam.

Interested students should look at:

* Harry Porter's Relay Computer (http://web.cecs.pdx.edu/~harry/Relay).   This has a more complex CPU than planned for this project but also uses conventional SRAM.
* Relay computer 2 - built with 281 relays http://www.electronixandmore.com/project/relaycomputertwo/index.html
* Tim - the simplest relay computer http://www.northdownfarm.co.uk/rory/tim/
* A transistorised 4-bit digital computer made out of discrete DTL http://neazoi.com/transistorizedcpu/

The plan is that the final machine would be demonstrated in local schools before being donated to The National Museum of Computing at Bletchley Park http://www.tnmoc.org

The budget for this project is £400 donated by Cantab Research.