Cores in space: The core memory module from a 1980 Spacelab computer

Spacelab was a reusable laboratory that could be carried in the Space Shuttle's cargo bay, providing lab space for astronauts and experiments.1 Because Spacelab was a European project, it used a French-built minicomputer, the Mitra 125 MS,2 rather than the Shuttle's main computers, IBM-built AP-101 systems. For storage, the Spacelab computer contained 128 kilobytes of RAM. Rather than silicon memory, the computer used magnetic core memory, with each bit stored in a tiny ferrite ring. In this article, I take a close look at this computer's core memory system. The illustration below shows how Spacelab fit inside the Shuttle's cargo bay. The pressurized laboratory is the cylindrical module in the front of the cargo bay, connected to the Shuttle by a tunnel. Experiments were mounted on pallets behind the laboratory. The laboratory held three identical Mitra computers.3 One computer managed Spacelab itself, while the second computer managed the experiments. The third computer provided a backup in case of failures. The photo below shows the core memory stack, removed from the computer. The core memory stack takes up roughly a third of the computer. The entire side panel of the computer detaches, and the core memory unit slides out. Since the computer is cooled by conduction, firmly attaching the core memory stack to the side panel kept it cool. The core memory stack consists of seven boards: a driver board, four core plane boards, a second driver board, and an interface board. Each board has two 160-pin connectors that plug into a large daughter board on each side, providing extensive connectivity between the boards. The daughter board on the right has another 160-pin connector that links the memory stack to the rest of the computer. (These connectors are the long blue connectors in the photo.) How core memory works One of the hardest problems for early computers was storage. Computers of the late 1940s and early 1950s stored data through techniques such as sound waves in mercury, spots on a CRT screen, or spinning magnetic drums, but these all had limitations. What computers needed was dense, inexpensive storage that was fast, reliable, and could be accessed randomly. During World War II, Germany developed special magnetic alloys that could "flip" from one magnetic state to another. After the war, American researchers realized that these materials could be used for storing binary data: "It was completely obvious that you could make a memory with this material," in the words of Jan Rajchman. Different aspects of core memory were patented by various inventors (including independent inventor Frederick Viehe, An Wang at Harvard, Jan Rajchman at RCA, and Jay Forrester at MIT), leading to expensive patent battles. (IBM ended up paying $400,000 to Wang-who used the money to build the computer company Wang Laboratories-and $13,000,000 to MIT.) I view Jay Forester as the most important inventor, developing the design of practical core memory, researching magnetic materials, and building the first core memory in 1953 for the groundbreaking Whirlwind computer. Core memory is based around a tiny toroidal magnetic core, one per bit.4 A core can be magnetized clockwise or counterclockwise to store a bit. The core can be magnetized by threading a wire through the core: running a current through the wire produces a magnetic field that magnetizes the core, while running a current in the opposite direction produces the opposite magnetization. A key problem with core memory was how to wire the cores without an absurd number of wires: if each core had a separate wire, just 16 KB of storage would require over 100,000 wires. The solution was called "coincident current addressing". The cores are arranged in a grid, with horizontal and vertical wires, as shown below. By running a current through one horizontal wire and one vertical wire, the single core at the intersection was selected. But wouldn't that magnetize all the cores along the horizontal and vertical wires? The key was that the cores were constructed from special magnetic materials with a property called hysteresis: a small current leaves the core completely unchanged, while a larger current flips the core's magnetic state. The currents through the horizontal and vertical wires were carefully selected so each wire had half the current necessary to flip the core; where the wires intersected, the two currents provided sufficient magnetic field to flip the core. The next step was reading the core. A sense wire was threaded through all the cores in the two-dimensional plane. To read a core, the X and Y select wires were driven to flip the desired core to the 0 state. If the core was already in the 0 state, nothing happened. But if the core was originally in the 1 state, the magnetic field changed as the core changed state. This induced a small current in the sense line, indicating that the core held a 1. Note that reading the value of a bit destroys that value. Thus, a core needs to be rewritten after reading, to restore the original data. To access a word of memory at a time, core planes were combined into a three-dimensional stack (below). Since each plane held one bit of the word, a 16-bit word would have a stack of 16 planes. All the planes shared the signals to drive the X and Y lines, so a one-word column through the stack was accessed in parallel. Each plane had a separate sense line to read out the bit. But how do you write different values to the different bits? The trick was to put an "inhibit" line through all the cores in a plane, running the inhibit line in the opposite direction to the X lines. Putting a current through the inhibit line would cancel out the current through the X line, preventing the core in that plane from being modified. To summarize, a read-write cycle consisted of first energizing a pair of X and Y lines to select a word and write a 0 to the column of cores in that word. The sense lines provided a readout of the bit values. Next, the X and Y lines were energized in the opposite direction to write a 1 to the cores. At the same time, the inhibit lines were energized for each plane with a 0 bit. Thus, the cores either flipped back to 1 or stayed at 0, as required. Many core memories, such as the one below, used a shared wire for sense and inhibit, so there were three wires through each core. The final ingredient to make core memory practical was the diode matrix. The X and Y lines require driver circuits that can produce fast, bidirectional high-current (e.g. 600 mA) pulses. A core memory plane can have hundreds of these lines. Providing a separate driver for each wire would be very expensive, especially in the vacuum tube era. The solution was to put separate drivers at each end of the wire, with each driver supporting multiple wires. For a trivial example, suppose you have 9 vertical lines. Put three drivers (A, B, and C) on the top, each connected to three wires, and three drivers on the bottom (1, 2, and 3), each connected to three wires. By energizing a driver at the top and a driver at the bottom (e.g. B and 1), the corresponding wire will be energized. Now, N drivers on each side control N2 wires, supporting N4 cores in total. Unfortunately, it's not quite that easy. Current can take "sneak paths" through the cores, such as the path in pink above. The solution is to add diodes to ensure that current can't take the wrong path. Since a wire needs to be driven with currents in both directions (to flip cores both ways), two diodes are required on each wire, as shown below, one in each direction. Each matrix input (A, B, etc.) is replaced with two inputs, one to drive each direction. (The horizontal wires also require diodes, not shown.) Since each wire requires two diodes, core memories used many diodes. Fortunately, diodes were small and inexpensive, so a large quantity of diodes was manageable. The photo below shows the diode stack for the computer used in the Saturn V rocket, the Launch Vehicle Digital Computer. Originally, core memories were tediously constructed by hand. For the Whirlwind computer, it took a full 40 hours to wire a 64×64 core plane. Companies such as IBM soon developed automated techniques to manufacture core memory, and the price dropped by a factor of two every two years, similar to Moore's Law.5 Core memories became fast, inexpensive, and reliable, and were the most popular form of main-memory storage until semiconductor memory took over in the 1970s. The Spacelab computer's core memory The Spacelab computer's memory was manufactured in 1980, a late date for core memory, so it is advanced and high density. The photo below shows one of the four core plane boards from the computer. Each board holds 16K of 18-bit words (32 KB), so the computer has 128 KB of RAM in total. The computer is a 16-bit computer, but each word also has a parity bit and a "storage protect" bit, bringing the total to 18 bits. (The storage protect bit provided write protection on a word-by-word basis, preventing programs from being accidentally overwritten. Because core memory is nonvolatile, a program could be loaded into memory once and would be immediately available every time the computer was powered on.) The core memory board is arranged with 1024 vertical (Y) wires and 288 horizontal (X) wires, supporting 294,912 lithium ferrite cores. These very thin wires are soldered to tiny pads on the printed-circuit board. The board supports 18 bits, which is visible as 18 alternating stripes of green and copper because alternating sense lines have different colors. The board has 36 sense lines: the left and right halves of the board have independent sense lines to reduce noise, so the board has 36 sense lines for 18 bits. The sense wires pass through four holes in the board (green arrows) and are soldered on the back of the board. The photo below shows a close-up of the cores. Each core is approximately 32 mils (0.8mm) in diameter, the same as the IBM Syste