A Core Memory Shield for the Arduino

Alasdair Allan
3 min readNov 14, 2018


Core memory was the most heavily used type of RAM from the mid-1950s, all the way up until the early-1970s. Which is before even my time, if only just. So I’m not quite old enough to remember magnetic core memory in the wild.

But, like a lot of other people who grew up closer to the ‘golden age’ of computing than the age the black rectangle of glass and brushed aluminium that lives in all of our pockets today, I have a soft spot for retro-computing.

So the arrival of core memory for the Arduino, designed and built by Jussi Kilpelainen, gives me a warm feeling of remembering an age I wasn’t quite part of, if only just.

The Core Memory Shield for Arduino. (📷: Jussi Kilpelainen)

Core uses magnetic toroids, called the cores, threaded on wires, with each core representing just a single bit of information. The cores are magnetised in two different ways, clockwise or counter-clockwise, and the bit stored in a core is zero or one depending on that core’s direction.

Apollo guidance computer’s RAM (made from magnetic cores) was 2K words of 15 bits + parity.

Core memory is actually fairly quirky, or at least different, than modern memory. While it is non-volatile, it preserves memory contents even when power is turned off, the process of reading the core causes the core to erase itself. It’s also really rather power hungry, hilariously so compared to modern memory, and of course is rather vulnerable to magnetic fields.

Top down view of the Core Memory Shield. (📷: Jussi Kilpelainen)

While the whole idea might seem, and really is, impractical there is sense in the madness of building something like this in the modern age of computing. Disregard the nostalgia for simpler times, there is a reason that so many of us, are fans of retro-computing. It’s tactile.

Anything beginners do with a modern computer is almost inevitably now abstracted away from how it works. Reaching deep down into the bottom of the machine, making it do what you want, is harder. Much harder.

Building, using, playing with, something like core memory is more tactile, and gives a different view of how things work than using a modern computer. It helps you get an understand what is going on inside our slabs of glass, silicon, and black plastic. One that’s hard, these days, to get any other way.

All the components included in the kit. (📷: Jussi Kilpelainen)

Based on previous work by Ben North and Oliver Nash, Kilpelainen’s shield comes as a kit, and yes, that means that you have to self-weave the core itself.

The Core Memory Shield for the Arduino cost $39.90 plus shipping and is available now on Kilpelainen’s Tindie Store. Documentation for the kit is available online, including source code, schematics and a full Bill of Materials.