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Before semiconductor RAM, most computers used magnetic core memory. This is the 32 KB core stack from the computer on Spacelab, the Space Shuttle's experimental laboratory. Let's take a closer look...

Magnetic core memory uses tiny ferrite rings, one for each bit of storage. The rings are arranged in a grid, with tiny copper wires threaded vertically and horizontally to select a core. To read a value, the green "sense" wire snakes through all the cores. Each ring is 32 mils (0.8mm) in diameter.

Each tiny ring can be magnetized clockwise or counterclockwise to store a 0 or 1. Putting a current through an X wire and a Y wire in the grid magnetizes the core where the two wires cross. Special magnetic materials ensure that the other cores are unaffected. The sense wire reads out the data.

Spacelab used three French-built minicomputers, model Mitra 125 MS. This photo shows the core memory stack in front of the computer. (The computer looks empty because I removed the circuit boards for analysis.) The computer is surprisingly heavy; it has a lot of metal for conduction cooling.

The back side of each core memory board is crammed with chips. 18 sense amplifier chips (yellow) provide 18 bits of output (16 bits+parity+storage protection). A multitude of diode modules are connected to the core plane's vertical wires (red) and horizontal wires (blue).

Oh! Fascinating they used core memories even though they had ICs. I assumed the minute we figured out ICs, core memory was gone. But I guess they hadn't invented DRAM yet, and that much SRAM would have been absurd?