I picked up a beautiful 4096 bit (64x64) core plane on eBay a few years ago:<p><a href="https://geary.com/images/CoreMemory.jpg" rel="nofollow">https://geary.com/images/CoreMemory.jpg</a><p>Zoom all the way in to see the detail of this 512 bytes of <i>hand woven</i> memory. Later core planes were machine woven; this was from an early '60s Burroughs machine.<p>The original image above is 8MB, so here are reduced size versions that load faster, use whichever works best for you...<p><a href="https://www.flickr.com/photos/geary/46893923024/" rel="nofollow">https://www.flickr.com/photos/geary/46893923024/</a><p><a href="https://geary.smugmug.com/Computers/History/i-jb2rgF5/A" rel="nofollow">https://geary.smugmug.com/Computers/History/i-jb2rgF5/A</a>
I actually have core memory that I built myself on my desk right now! It’s only one bit, but consists of three cores, the two additional ones are not memory but there to cancel out the big inductive pulse when “accessing” the core that is separate from the actual memory kickback pulse (real core memory does it by clever wiring across the entire plane of cores, no extra cores needed).<p>You can see the kickback pulse here, the big one is where the bit was “set”. Reading is always destructive, so the next pulse does not have the kickback: <a href="https://live.staticflickr.com/7890/47241409541_5915191605_o_d.jpg" rel="nofollow">https://live.staticflickr.com/7890/47241409541_5915191605_o_...</a><p>It’s great fun and actually very simple once you understood the (already relatively simple) concept. My “magnetic core bit” is literally just a few wires going through the cores (<a href="https://live.staticflickr.com/7874/33365624228_61caed6e71_k_d.jpg" rel="nofollow">https://live.staticflickr.com/7874/33365624228_61caed6e71_k_...</a> ), though I later added a couple of transistors to form a sense amp. I got the cores by just looking for tiny ferrite cores on eBay, highly unlikely that they were meant as memory cores.<p>For driving them, I just set my function generator to its maximum output, 10V at 50Ohm output impedance. Anything less than that and you barely get a kickback, so it’s really not efficient, in any way.
I really enjoy core memory, my PDP 8's have 24K words and 32K words. There is a great process for tuning the drivers in the PDP 8 maintenance manual which results in a matched set of driver boards to a core-plane board.<p>I built a small 4 bit (2 x 2) using #2 iron nuts and LM293D H-bridges as the drivers. The hysteresis band of the nuts was really small so it made it pretty unreliable sadly.<p>Perhaps the most interesting fallout from core memory was the invention of FRAM which looks like a serial EEPROM (in 8 pin PDIPs) but are in fact small ferro-magnetic memories. Unlimited writes and all that.
In addition to this core RAM there was Core Rope Memory[1] which was used as the ROM for the Apollo Guidance computer amongst others. The cores were hand woven to program in the binary, a core could be wired to produce a 0 or 1 and any changes would require the core to be woven again.<p>[1] <a href="https://en.wikipedia.org/wiki/Core_rope_memory" rel="nofollow">https://en.wikipedia.org/wiki/Core_rope_memory</a>
> microcode (stored on metalized mylar sheets read by transformers)<p>First time I hear about this. Sounds wild, couldn't find a good explanation of how this works though.
Funny, I watched the embedded video yesterday. It gives a very good explanation of how the core worked, including a lot of theory and implimentation details. They wire up an actual core, and write a bit, then read it back by detecting core flips with an oscilloscope.<p>It's a bit long, but well worth the time to watch.
I've seen pictures of people assembling core memory by hand, it looks remarkably similar to the repetitive process of hand weaving a high-end persian carpet.