Spacelab was a reusable laboratory that could beryllium carried successful the Space Shuttle's cargo bay, providing laboratory abstraction for astronauts and experiments.1 Because Spacelab was a European project, it utilized 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 machine contained 128 kilobytes of RAM. Rather than silicon memory, the machine utilized magnetic halfway memory, pinch each spot stored successful a mini ferrite ring. In this article, I return a adjacent look astatine this computer's halfway representation system.
The halfway stack from the Spacelab computer. I removed the apical committee to show the halfway planes.
The illustration beneath shows really Spacelab fresh wrong the Shuttle's cargo bay. The pressurized laboratory is the cylindrical module successful the beforehand of the cargo bay, connected to the Shuttle by a tunnel. Experiments were mounted connected pallets down the laboratory. The laboratory held 3 identical Mitra computers.3 One computer managed Spacelab itself, while the 2nd machine managed the experiments. The 3rd machine provided a backup successful lawsuit of failures.
Spacelab was a pressurized cylinder successful the Shuttle's cargo bay, connected to the Shuttle by a tunnel. It provided a laboratory for researchers to execute experiments. This illustration of Spacelab is from NASA, C-1976-4380.
The photograph beneath shows the halfway representation stack, removed from the computer. The halfway representation stack takes up astir a 3rd of the computer. The full broadside sheet of the machine detaches, and the core memory portion slides out. Since the machine is cooled by conduction, firmly attaching the halfway representation stack to the broadside sheet kept it cool. The halfway representation stack consists of 7 boards: a driver board, 4 halfway level boards, a 2nd driver board, and an interface board. Each committee has 2 160-pin connectors that plug into a ample girl committee connected each side, providing extended connectivity betwixt the boards. The girl committee connected the correct has different 160-pin connector that links the representation stack to the remainder of the computer. (These connectors are the agelong bluish connectors successful the photo.)
The halfway representation stack successful beforehand of the Mitra computer. The circuit boards person been removed from the acold broadside of the computer.
How halfway representation works
One of the hardest problems for early computers was storage. Computers of the precocious 1940s and early 1950s stored information done techniques specified arsenic sound waves successful mercury, spots connected a CRT screen, aliases spinning magnetic drums, but these all had limitations. What computers needed was dense, inexpensive retention that was fast, reliable, and could beryllium accessed randomly.
During World War II, Germany developed special magnetic alloys that could "flip" from 1 magnetic authorities to another. After the war, American researchers realized that these materials could beryllium utilized for storing binary data: "It was wholly evident that you could make a memory pinch this material," successful the words of Jan Rajchman. Different aspects of halfway representation were patented by various inventors (including independent inventor Frederick Viehe, An Wang astatine Harvard, Jan Rajchman astatine RCA, and Jay Forrester astatine MIT), starring to costly patent battles. (IBM ended up paying $400,000 to Wang—who utilized the money to build the machine institution Wang Laboratories—and $13,000,000 to MIT.) I position Jay Forester as the astir important inventor, processing the creation of applicable halfway memory, researching magnetic materials, and building the first halfway representation successful 1953 for the groundbreaking Whirlwind computer.
Core representation is based astir a mini toroidal magnetic core, 1 per bit.4 A halfway tin beryllium magnetized clockwise aliases counterclockwise to shop a bit. The halfway tin beryllium magnetized by threading a ligament done the core: moving a existent done the ligament produces a magnetic section that magnetizes the core, while running a existent successful the other guidance produces the other magnetization.
A cardinal problem pinch halfway representation was really to ligament the cores without an absurd number of wires: if each halfway had a abstracted wire, conscionable 16 KB of retention would require complete 100,000 wires. The solution was called "coincident existent addressing". The cores are arranged successful a grid, pinch horizontal and vertical wires, arsenic shown below. By moving a existent done 1 horizontal ligament and 1 vertical wire, the azygous halfway astatine the intersection was selected. But wouldn't that magnetize each the cores on the horizontal and vertical wires? The cardinal was that the cores were constructed from typical magnetic materials pinch a spot called hysteresis: a mini existent leaves the halfway wholly unchanged, while a larger existent flips the core's magnetic state. The currents done the horizontal and vertical wires were cautiously selected truthful each ligament had half the existent basal to flip the core; where the wires intersected, the 2 currents provided capable magnetic section to flip the core.
Energizing an X thrust ligament and a Y thrust ligament selects 1 core, highlighted successful yellow. Diagram adapted from Digital Computer Components and Circuits, R. K. Richards, p355
The adjacent measurement was reference the core. A consciousness ligament was threaded done each the cores successful the two-dimensional plane. To publication a core, the X and Y prime wires were driven to flip the desired halfway to the 0 state. If the halfway was already successful the 0 state, thing happened. But if the halfway was primitively successful the 1 state, the magnetic section changed arsenic the core changed state. This induced a mini existent successful the consciousness line, indicating that the halfway held a 1. Note that reference the worth of a spot destroys that value. Thus, a halfway needs to beryllium rewritten aft reading, to reconstruct the original data.
To entree a connection of representation astatine a time, halfway planes were mixed into a three-dimensional stack (below). Since each level held 1 spot of the word, a 16-bit word would person a stack of 16 planes. All the planes shared the signals to thrust the X and Y lines, truthful a one-word file done the stack was accessed successful parallel. Each level had a abstracted sense line to publication retired the bit.
The halfway stack from the Saturn V LVDC (Launch Vehicle Digital Computer) consists of 14 halfway planes. This stack is astatine the US Space & Rocket Center. Photo from NCAR EOL. I retouched the photograph to trim distortion from the integrative case.
But really do you constitute different values to the different bits? The instrumentality was to put an "inhibit" statement done each the cores successful a plane, moving the inhibit statement successful the other guidance to the X lines. Putting a existent done the inhibit statement would cancel retired the existent done the X line, preventing the halfway successful that level from being modified. To summarize, a read-write rhythm consisted of first energizing a brace of X and Y lines to prime a connection and constitute a 0 to the file of cores successful that word. The consciousness lines provided a readout of the spot values. Next, the X and Y lines were energized successful the other guidance to constitute a 1 to the cores. At the aforesaid time, the inhibit lines were energized for each level pinch a 0 bit. Thus, the cores either flipped backmost to 1 aliases stayed astatine 0, arsenic required. Many halfway memories, specified arsenic the 1 below, utilized a shared ligament for consciousness and inhibit, truthful location were 3 wires done each core.
Closeup of an IBM 360 Model 50 halfway plane. The cores successful this machine were called 19-32 because their soul diameter was 19 mils and their outer diameter was 32 mils (0.8 mm).
The last constituent to make halfway representation applicable was the diode matrix. The X and Y lines require driver circuits that tin nutrient fast, bidirectional high-current (e.g. 600 mA) pulses. A halfway representation level tin person hundreds of these lines. Providing a abstracted driver for each ligament would beryllium very expensive, particularly successful the vacuum conduit era. The solution was to put abstracted drivers astatine each extremity of the wire, pinch each driver supporting aggregate wires. For a trivial example, suppose you person 9 vertical lines. Put 3 drivers (A, B, and C) connected the top, each connected to 3 wires, and 3 drivers on the bottommost (1, 2, and 3), each connected to 3 wires. By energizing a driver astatine the apical and a driver astatine the bottommost (e.g. B and 1), the corresponding ligament will beryllium energized. Now, N drivers connected each broadside power N2 wires, supporting N4 cores successful total.
Illustration of really "top" and "bottom" drivers activity together to prime a azygous statement (red) done the halfway matrix. However, existent tin return alternate paths, specified arsenic the pinkish path.
Unfortunately, it's not rather that easy. Current tin return "sneak paths" done the cores, specified arsenic the way successful pinkish above. The solution is to adhd diodes to guarantee that existent can't return the incorrect path. Since a ligament needs to beryllium driven pinch currents successful some directions (to flip cores some ways), 2 diodes are required connected each wire, arsenic shown below, one successful each direction. Each matrix input (A, B, etc.) is replaced pinch 2 inputs, 1 to thrust each direction. (The horizontal wires besides require diodes, not shown.)
Adding diodes ensures that existent only takes the desired path.
Since each ligament requires 2 diodes, halfway memories utilized galore diodes. Fortunately, diodes were mini and inexpensive, truthful a ample amount of diodes was manageable. The photograph beneath shows the diode stack for the machine utilized successful the Saturn V rocket, the Launch Vehicle Digital Computer.
Closeup of the diode matrix successful the Saturn V LVDC. Diodes are mounted vertically utilizing cordwood building betwixt 2 printed circuit boards.
Originally, halfway memories were tediously constructed by hand. For the Whirlwind computer, it took a afloat 40 hours to ligament a 64×64 halfway plane. Companies specified arsenic IBM soon developed automated techniques to manufacture halfway memory, and the value dropped by a facet of 2 each 2 years, similar to Moore's Law.5 Core memories became fast, inexpensive, and reliable, and were the astir celebrated shape of main-memory retention until semiconductor representation took complete successful the 1970s.
The Spacelab computer's halfway memory
The Spacelab computer's representation was manufactured successful 1980, a precocious day for halfway memory, truthful it is precocious and precocious density. The photograph beneath shows 1 of the 4 halfway level boards from the computer. Each committee holds 16K of 18-bit words (32 KB), truthful the machine has 128 KB of RAM successful total. The machine is simply a 16-bit computer, but each connection besides has a parity spot and a "storage protect" bit, bringing the full to 18 bits. (The retention protect spot provided constitute protection connected a word-by-word basis, preventing programs from being accidentally overwritten. Because halfway representation is nonvolatile, a programme could beryllium loaded into representation erstwhile and would beryllium instantly disposable each clip the machine was powered on.)
One of the halfway representation boards from the Spacelab computer.
The halfway representation committee is arranged pinch 1024 vertical (Y) wires and 288 horizontal (X) wires, supporting 294,912 lithium ferrite cores. These very bladed wires are soldered to mini pads connected the printed-circuit board. The committee supports 18 bits, which is visible arsenic 18 alternating stripes of greenish and copper because alternating consciousness lines person different colors. The committee has 36 consciousness lines: the near and correct halves of the committee person independent consciousness lines to trim noise, truthful the committee has 36 consciousness lines for 18 bits. The consciousness wires walk done 4 holes successful the committee (green arrows) and are soldered connected the backmost of the board.
The photograph beneath shows a close-up of the cores. Each halfway is astir 32 mils (0.8mm) successful diameter, the aforesaid arsenic the IBM System/360 cores shown earlier. However, the cores are stacked overmuch closer, with only a mini spread betwixt cores. The X and Y prime lines are copper-colored, while the consciousness lines are green. (The wires are each enameled to forestall short circuits.) The consciousness wires loop astir astatine the left, forming a azygous circuit done each spot section. Half the Y lines shape loops astatine the bottom; the different half shape loops astatine the top. Thus, each Y statement passes done the level doubly successful a U-shaped path, which will turn retired to beryllium important.
A close-up of the cores. I deliberation that immoderate rows tilt near and immoderate tilt correct to guarantee that the consciousness lines support the aforesaid polarity erstwhile they move direction. Photo courtesy of CuriousMarc.
The different broadside of each circuit committee holds the consciousness amplifiers and the diode matrix for the halfway plane. The diode chips are the quadrate achromatic packages, each containing 16 diodes for 8 halfway lines.6 In the red-outlined regions, 1 extremity of each vertical U-loop is connected to a diode chip; the lines of diagonal holes are the vias that walk each signal through the board. The different extremity of each vertical U-loop is connected to 1 of the bluish committee connectors connected the side; these vias are successful the blue-outlined regions. The horizontal lines usage the diode chips and vias successful the greenish regions. One extremity of each statement is connected to a diode chip, while the different extremity is connected to a committee connector done traces connected the different side. Note that immoderate vertical lines link to the diode chips astatine the apical of the board, while others link astatine the bottom. Similarly, immoderate horizontal lines link astatine the near while others link astatine the right.
The backmost broadside of the halfway level committee holds the diode matrices and consciousness amplifiers.
The cardinal region (yellow) holds 18 consciousness amplifier chips, the achromatic DIP integrated circuits, each containing 2 amplifiers.7 The achromatic packages are resistor packages, holding aggregate resistors to bias and terminate the consciousness amplifier lines. The wires from the consciousness amplifiers are connected arsenic twisted pairs that are soldered to the committee correct adjacent to the corresponding consciousness amplifier chips. Using twisted pairs for the full region prevents the wires from picking up electrical noise, which could overwhelm the mini signals successful the consciousness wires. The consciousness wires walk from 1 broadside of the committee to the different done 4 holes successful the committee (yellow arrows), and past are glued down arsenic they traverse a important distance on the board. (It must person been difficult to manufacture the committee without breaking the tiny, vulnerable wires.)
Each consciousness ligament loop forms a twisted brace that is fed to the different broadside done a spread successful the circuit board. Above the hole, you tin spot a grey blob where sense wires were spliced for immoderate reason. Also statement really alternating vertical wires are soldered to the circuit board, pinch information vias connected to the different side. The different vertical wires shape loops. are soldered to the circuit board
Detecting signals connected the consciousness lines is tricky because the pulses are very small, a fewer millivolts. Because the consciousness lines tally adjacent to the X thrust lines, they tin easy prime up sound from the high-current pulses connected the X lines. To minimize this noise, the consciousness lines transverse each different betwixt 2 level sections, forming a "bow tie", arsenic shown below. The consequence is that an X statement runs adjacent to the affirmative consciousness statement for half the magnitude and the antagonistic consciousness statement for the different half. Thus, the induced sound cancels out.
A close-up of the consciousness lines. The 16 consciousness lines successful the mediate are green, while the consciousness lines supra and beneath (as good arsenic the X lines) are copper. Note that the consciousness lines cross, while the X lines proceed horizontally. The ample circles are vias done the board.
The halfway representation successful the Spacelab machine utilized a different architecture from a emblematic halfway memory, improving capacity by eliminating the inhibit line. This architecture was called a 2½D memory.8 If you're acquainted pinch halfway memory, the deficiency of inhibit lines whitethorn look puzzling: really do you constitute 1 to immoderate bits and 0 to different bits? The instrumentality is to person abstracted X driver circuitry for each bit.9 When penning data, the X lines are only energized for bits that person a 1; the different lines are near unenergized, truthful the bits stay astatine 0. The disadvantage is that alternatively of 1 group of X driver circuits, you now request 1 group for each bit, a facet of 18 much for an 18-bit word. However, pinch the improvement of halfway drivers connected integrated circuits, the costs of the further driver circuitry became little significant.
The representation strategy utilized an method called shape reversal to trim the number of vertical drivers successful half. Recall that pairs of vertical wires are joined by a U-connection. By driving the ligament successful a peculiar direction, the near broadside aliases the correct broadside of the brace tin beryllium selected. For example, the drafting beneath shows really the 2 wires prime the near core, but not the correct core. In the near core, some currents spell done the halfway successful the aforesaid direction, inducing a magnetic section successful the toroid.10 But successful the correct core, the 2 currents cancel out, truthful location is nary magnetic section created. But if the existent successful the vertical loop is reversed, the correct halfway will beryllium selected, alternatively than the near core. The constituent is that alternatively of utilizing 2 drivers for the vertical wires, 1 driver is used, reversing the existent to prime the near aliases correct core.
Connecting pairs of vertical wires into a U-shaped loop lets each driver power doubly arsenic galore cores.
The sketch beneath shows the analyzable wiring for X thrust wires. Each set of 16 wires corresponds to 1 spot successful the 18-bit word, and has a abstracted consciousness wire. The apical set of 16 X lines is connected to 4 contacts connected the committee connector; each interaction is connected to 4 X lines done the curving PCB traces. The bottommost set of 16 X lines is wired to diode modules connected the different broadside of the board, connected done the information vias. (Each ligament has the other connections—diode module aliases committee connector—on the different end.)11 One group of 4 X wires is energized done the connector, while 4 wires are energized done the diode matrix, selecting 1 of the 16 X wires successful the group.
The PCB wiring for the X lines.
Other boards successful the representation stack
The representation stack has 7 boards successful total, arranged arsenic a driver board, the 4 halfway planes, a 2nd driver board, and an interface board. I haven't examined these boards successful detail, but I'll springiness immoderate preliminary information. The photograph beneath shows 1 of the 2 driver boards. It provides the high-current pulses for the X and Y prime lines. The committee is crammed pinch specialized halfway representation driver chips12, on pinch a fewer logic chips to power the drivers. It has abstracted drivers for the 2 ends of the prime lines, allowing the matrix action described earlier.
One of the 2 representation driver boards. Click this image (or immoderate other) for a larger version.
Since location are 2 driver boards and 4 halfway representation boards, astatine first I thought that each driver committee controlled 2 halfway representation boards. The configuration turns retired to beryllium much complicated, pinch 1 much furniture of matrix selections to trim the number of drivers successful half. To simplify slightly, see the X lines connected a halfway committee to person near ends and correct ends, some of which must beryllium energized to activate a line. For the near ends, the first driver committee powers halfway boards 1 and 2, while the 2nd driver committee powers halfway boards 3 and 4. The correct ends are shuffled: the first driver committee powers halfway boards 1 and 3, while the 2nd driver committee powers halfway boards 2 and 4. Now, if the first driver committee powers the near and correct ends, halfway committee 1 is the only 1 pinch some ends active. If the first driver committee powers the near ends while the 2nd committee powers the correct ends, halfway committee 2 is activated. Similarly, halfway committee 3 aliases 4 tin beryllium activated. The constituent is that since each group of drivers is connected to 2 halfway boards, 2 sets of drivers are required alternatively of four.
The last committee is the interface to the remainder of the computer. It has galore transistor arrays successful DIP packages, on pinch galore resistors. It seems that the committee uses discrete transistors to thrust the bus, alternatively than utilizing interface chips, which is unexpected. The committee has immoderate wire-wrapped jumpers successful the little halfway region, presumably for configuration.
The interface committee has immoderate unused abstraction successful the little left.
Conclusions
Core representation had a agelong life, surviving moreover arsenic computers migrated from vacuum tubes to transistors and past integrated circuits, but eventually semiconductor representation made it obsolete.13 Core memories lasted moreover longer successful aerospace applications since it had 2 cardinal advantages complete semiconductor memory: it retained information moreover without power, and it was resistant to radiation. The Spacelab computer, manufactured successful 1980, was adjacent the extremity of halfway memory's reign, truthful it is much precocious than a emblematic halfway representation system, with higher density, extended usage of integrated circuits, and the 2½D architecture. But yet the precocious density, debased cost, and debased powerfulness depletion of semiconductor representation won out. In 1991, the Space Shuttle flew pinch upgraded main computers, the IBM AP-101S that utilized semiconductor representation alternatively of magnetic core. Spacelab's Mitra computers were besides replaced, utilizing the AP-101SL, which was based connected the AP-101S but modified to support the instruction group and peripherals of the original Spacelab computer.14 Although halfway representation is now firmly successful the past, it still lives connected successful the look "core dump".
I scheme to analyse the Spacelab machine immoderate more. For updates, travel maine on Bluesky (@righto.com), Mastodon (@[email protected]), or RSS. Credits: Thanks to Steve Jurvetson for providing the Spacelab computer. Thanks to CuriousMarc for photography and thief disassembling the computer. AI statement: Despite the beingness of the em dash, nary AI was utilized successful the penning of this article (details).
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