Core rope memory
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Core rope memory is a form of read-only memory (ROM) for computers, first used by early NASA Mars probes and then in the Apollo Guidance Computer (AGC) designed by MIT and built by Raytheon.
Contrary to ordinary magnetic core memory, which was used for RAM at the time, all the ferrite cores in a core rope are permanently magnetized in one direction. The signal from a wire passing through a given core is interpreted as a binary "one" while a wire that bypasses the core is read as a "zero". In the AGC, up to 64 wires could be passed through a single core. A relatively large amount of data could be stored in a small volume. It could store 16 bit words at a density of 36,864 per cubic foot (1,301,800/m³) compared to only 2048/ft³ (72,320/m³) of standard read-write core memory.