I also like the inductive matrix ROM used in the
HP9100. It works by the
coupling between 512 address tracks and 64 data loops in a 14 layer (?)
PCB. The PCB is about 4" square and stores 512 words, each one 64 bits
long. It is _very_ reliable -- 9100s are now about 30 years old, and
while I've had to replace transistors and diodes in them, I've never
heard of the ROM failing.
I Gotta get me one of those, they are so likeable.