Before the creation of DDR2 memory, even before the creation of RAM there was magnetic core memory. These memory module consisted of a stack of planes containing one bit by however large the module would be, 128x 128 or 16k.
The core is a tiny ring of ferrite that is magnetized in one direction, usually the one state. to read the core they are all, one word, written to a zero state and the changes sensed. After the data is clocked into a register then the data is written back by writing them to a one state and inhibiting the write for zero’s.