Core rope memory

Core rope memory is a form of read-only memory (ROM) for computers, first used in the 1960s by early NASA Mars space probes and then in the Apollo Guidance Computer (AGC) designed and programmed by the MIT Instrumentation Lab and built by Raytheon.

Contrary to ordinary coincident-current magnetic-core memory, which was used for RAM at the time, the ferrite cores in a core rope are just used as transformers. The signal from a word line wire passing through a given core is coupled to the bit line wire and interpreted as a binary "one", while a word line wire that bypasses the core is not coupled to the bit line wire and is read as a "zero". In the AGC, up to 64 wires could be passed through a single core.

Software written by MIT programmers was woven into core rope memory by female workers in factories. Some programmers nicknamed the finished product LOL memory, for Little Old Lady memory.

By the standards of the time, a relatively large amount of data could be stored in a small installed volume of core rope memory: 72 kilobytes per cubic foot, or roughly 2.5 megabytes per cubic meter. This was about 18-fold the amount of data per volume compared to standard read-write core memory.

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