“Core rope, as contrasted with conventional read/write core memory, does not depend on the magnetization of the cores. Rather, the ferrite rings act as transformers, and the contents of a word are dependent on the wiring scheme of the memory and not on any particular magnetic state .  . . .  it is important to remember that these cores are simply torodial shaped transformers, and are not themselves magnetized.“

This last sentence, although technically correct, was initially confusing, as clearly the cores are required and if they are not themselves magnetized, then what purpose do they fulfill?  Is the answer that they are magnetized, but only during the memory operation of fetching from the ROM – just the “1”s, and only at that moment? Well, as a transformer action, a pulse generator, but this imagery was less helpful.

Some alternative wording is here on page 93 of the MIT report of 1972 – describing the core rope – which says: “After the selected core has switched and the inhibit currents are removed, a reset current is passed through all cores, Only the core that was just set will change state, and the sense amplifiers may be gated on during either the set or the reset part of the cycle to read information out of the memory.”

If we still have difficulty imagining the essence of these two “core” juxtapositions, try this:  1) Core RAM memory can retain its content (AGC RAM memories have been analyzed decades later to find out what the computer was doing and the status of many variables). 2) In a different way, the core rope ‘ROM’ willingly gives up its secrets — because, unless the wires or components in the core rope have been damaged, the rope code can still be “read” because, even though nothing is stored in the cores, information can be regenerated from each address just as it was during operation. ‘Content’ is determined by the threading pattern of the wires, using the cores to detect the ‘content’ of each address — valid only during that memory operation cycle.

Each of these cores is actually much larger than those in the core RAM – each core is used multiple (192) times in the AGC requiring there be multiple sets of wires threaded through or around them (see footnote) – which brings the total wires to the physical space limit of the core. The manufacturing process of the core ropes can be seen in the video above and those linked here and here. Total bits per module is 512 physical cores x 192 sense lines — so x 6 modules gives 589,824 bits = 36,864 words (15 bits + parity).

Photo of the core rope © Raytheon. Diagram from MIT,redrawn in Ken Shirriff’s blog.

CuriousMarc and the team have been able to read a real core rope from an AGC located at the Mountain View (CA) computer museum. The work performed here along with superb video documenting is amazing and such an important addition to our collective history.