The HP 9100 calculator uses core memory to store a user program and data, but the internal read-only control store (microcode) is stored in a novel and patented inductively coupled printed circuit board memory (512 words of 64 bits), and a core rope (or wire braid) memory (64 words of 29 bits).
2/
The amazing inductively coupled PCB memory uses NO electronic or mechanical components per bit. The data is literally stored in the PCB traces. There is one diode per 64-bit word (512 doliodes total), 48 line drivers, and 64 sense amplifiers
This technology was never used again, as bipolar PROM semiconductor memory chips appeared in 1969.
3/
If you have a fediverse account, you can quote this note from your own instance. Search https://mastodon.social/users/brouhaha/statuses/114368418244288453 on your instance and quote it. (Note that quoting is not supported in Mastodon.)