Core Memory
So, way back in the days of yore, memory didn’t come in handy-dandy chips that just plugged in . Memory was an actual thing, with wires and magnets and whatnot. Big and bulky (compared to what we see these days of course!) it was a technological tour-de-force for the time.
For a long long period (mid-50s through the mid-70s), one of the predominant memory types was Core memory, which used little magnetic rings (or cores) as the individual bits.
In particular, Core RAM used ferrite cores as the bits, with the wires used to magnetize the cores in opposing directions. Depending on the direction that you sent current through the wires, you could set them to be clockwise (1), or counterclockwise (0).
In fact, one of the first computers I worked on was a PDP-10, which used CORE RAM. The thing is, these ferrite cores were brittle, and would would frequently break and fall. The sysadmin would sweep up the broken ones, and throw ’em in the trash-bucket, which was — literally — a bit-bucket. (Yeah, not a joke).
Core ROM, on the other hand inverted this entire process. The ferrite cores were used as transformers, with the wires themselves being the bits. Each bit consists of a ferrite core and two wires, the first wire being wrapped around the core for sure. When you passed a current through the first wire, if the second wire was also wrapped around the core, a current would be induced in it like in a transformer (1). If, OTOH, the second wire was not wrapped around the core, then there would be no current induced in it (0). You could even change the bits by rewiring them, in case there was an error. Want to change a 1 to a 0? Unwrap the wire from the core!
Simple, and foolproof — this was called CORE Rope Memory, and was used in the Apollo missions!
Simple, and foolproof — this was called CORE Rope Memory, and was used in the Apollo missions!
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