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 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.

Memory density

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For its time, a relatively large amount of data could be stored in a small installed volume of core rope memory—72 kilobytes per cubic foot (roughly 2.5 megabytes per cubic meter)—about 18-fold the amount of data per volume compared to standard read-write core memory.

Memory technology	Data units per cubic foot		Data units per cubic meter
	Bytes	Bits	Bytes	Bits
Core rope ROM	72K	576K	~2.5M	~20M
Magnetic core RAM	4K	32K	~140K	~  1M

Notes

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1. The Block II AGC used 36,864 sixteen-bit words of core rope memory (placed within one cubic foot) and 4,096 words of magnetic core memory (within two cubic feet). Other machines will have somewhat different ratios between the two memory types.

External links

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• Photos of core rope memory and its production – By Raytheon; hosted by the Dibner Institute for the History of Science and Technology
• Computers in Spaceflight: The NASA Experience – By James Tomayko (Chapter 2, Part 5, "The Apollo guidance computer: Hardware")

Computer memory | Non-volatile memory

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