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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 core memory is arranged with 16 horizontal lines of cores to store each of the 18 bits in a word. This photo shows how the tiny wires from the core plane are soldered to the much larger traces on the circuit board.

Core memories usually have an "inhibit" wire for writing data back. The core memory board uses a more complicated "2½D" design, which eliminates the inhibit wire. Three wires—X, Y, and sense—go through each core. The sense wires cross each other in the middle; this reduces the noise they pick up.

The core memory stack has two driver boards that generate the high-current pulses through the X and Y wires. The boards are crammed with integrated circuits that were specially designed for core memory systems.

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?