Magnetic Core Memory

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core storage ( ?kör ?störij ) ( computer science ) The main memory of a computer. magnetic core ... the development of magnetic core memory, which enabled the ...
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A Magnetic Memory Element. Arrangement of Magnetic Core Memories ... to describe the principles of magnetic core memories in a simple and clear way. ...
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Magnetic core memory, or ferrite-core memory, is an early form of computer memory. It uses small magnetic ceramic rings, the cores, to store information via the Polarity (physics) of the magnetic field they contain. Such memory is often just called core memory, or, informally, core.

History The earliest work on core memory was carried out by the Shanghai-born United States physicists, An Wang and Way-Dong Woo, who created the pulse transfer controlling device in 1949. The name referred to the way that the magnetic field of the cores could be used to control the switching of current in electro-mechanical systems. Wang and Woo were working at Harvard University's Computation Laboratory at the time, but unlike Massachusetts Institute of Technology, Harvard was not interested in promoting inventions created in their labs. Instead Wang was able to patent the system on his own while Woo took ill.

Jay Forrester's group, working on the Whirlwind (computer) project at MIT, became aware of this work. This machine required a fast memory system for realtime flight simulator use. At first, Williams tubes (more accurately, Williams tube) — a storage system based on cathode ray tubes — were used, but these devices were always temperamental and unreliable.

Two key inventions led to the development of magnetic core memory, which enabled the development of computers as we know them. The first, An Wang's, was the write-after-read cycle, which solved the puzzle of how to use a storage medium in which the act of reading was also an act of erasure. The second, Jay Forrester's, was the coincident-current system, which enabled a small number of wires to control a large number of cores (see Description section below for details).

Forrester's coincident-current system required one of the wires to be run at 45 degrees to the cores, which proved impossible to wire mechanically. Thus core arrays were manually assembled; the work was performed under microscopes and required fine motor control. Initially, garment workers were used.

By the late 1950s, industrial plants had been set up in the Far East to build core. Inside, hundreds of low-paid workers strung cores for cents a day. This lowered the cost of core to the point where it became largely universal as primary storage by the early 1960s, replacing both the low-cost/low-performance drum memory as well as the high-cost/high-performance systems using vacuum tubes as memory. Certain manufacturers also employed Scandinavian seamstresses who had been laid off due to modernizations in the textile industry.

Throughout the development of core memory technology, costs almost followed the not-yet-formulated Moore's Law; over the lifetime of the technology costs began at roughly a dollar a bit and eventually approached roughly US$0.01 per bit. Core was in turn replaced by silicon memory chips (Random Access Memory) in the 1970s.

Dr. Wang's patent was not granted until 1955, and by that time core was already in use. This started a long series of lawsuits, which eventually ended when International Business Machines paid Wang several million dollars to buy the patent outright. Wang used the funds to greatly increase the size of Wang Laboratories which he co-founded with Dr. Ge-Yao Chu, a school mate from China.

Core memory was part of a family of related technologies, now largely forgotten, which exploited magnetic properties of materials to perform switching and amplification. By the 1950s, vacuum-tube electronics was well-developed and very sophisticated, but tubes were fragile, and the use of heated filaments made them short-lived, high in power consumption, and unstable in their operating characteristics. Magnetic devices had many of the virtues of the transistor and solid-state devices that would replace them, and saw considerable use in military applications. A notable example was the portable (truck-based) MOBIDIC computer developed by Sylvania for the United States Army Signal Corps in the late Fifties.

Description (0.04 inch).
The light color vertical and horizontal wires are X and Y wires, the diagonal wires are Sense wires, the dark colored horizontal wires are Inhibit wires.

How core memory works The most common form of core memory, X/Y line coincident-current – used for the main memory of a computer, consists of a large number of small Ferrite (magnet) (ferromagnetic ceramic) rings, cores, held together in a grid structure (each grid called a plane), with wires woven through the holes in the cores' middle. In early systems there were four wires, X, Y, Sense and Inhibit, but later cores combined the latter two wires into one Sense/Inhibit line. Each ring stores one bit (a 0 or 1). One bit in each plane could be accessed in one cycle, so each machine word in an array of words was spread over a stack of planes. Each plane would manipulate one bit of a word in parallel computing, allowing the full word to be read or written in one cycle.

Core relies on the hysteresis of the magnetic material used to make the rings. Only a magnetic field over a certain intensity (generated by the wires through the core) can cause the core to change its magnetic polarity. To select a memory location, one of the X and one of the Y lines are driven with half the current required to cause this change. Only the combined magnetic field generated where the X and Y lines cross is sufficient to change the state; other cores will see only half the needed field, or none at all. By driving the current through the wires in a particular direction, the resulting electromagnetic induction field forces the selected core's magnetic field to point in one direction or the other (north or south).

Reading and writing Reading from core memory is somewhat complex. Basically the read operation consists of doing a "flip to 0" operation to the bit in question, that is, driving the selected X and Y lines at half power in the direction that causes the core to flip to whatever polarity the machine considers to be zero. If the core was already in the 0 state, nothing will happen. However if the core was in the 1 state it will flip to 0. If this flip occurs, a brief pulse of power is induced into the Sense line, saying, in effect, that the memory location used to hold a 1. If the pulse is not seen, that means no flip occurred, so the core must have already been in the 0 state. Note that every read forces the core in question into the 0 state, so reading is destructive, which is one of the oddities of core memory.

Writing is similar in concept, but always consists of a "flip to 1" operation, relying on the memory already having been set to the 0 state in a previous read. If the core in question is to hold a 1, then the operation proceeds normally and the core flips to 1. However if the core is to instead hold a zero, a small amount of current is sent into the Inhibit line, enough to drop the combined field from the X, Y and Inhibit lines below the amount needed to make the flip. This leaves the core in the 0 state.

Note that the Sense and Inhibit wires are used one after the other, never at the same time. For this reason later core systems combined the two into a single wire, and used circuitry in the memory controller to switch the duty of the wire from Sense to Inhibit.

Because core always requires a write after read, many computers included instructions that took advantage of this. These instructions would be used when the same location was going to be read, changed and then written, such as an increment operation. In this case the computer would ask the memory controller to do the read, but then signal it to pause before doing the write that would normally follow. When the instruction was complete the controller would be unpaused, and the write would occur with the new value. For certain types of operations, this effectively doubled the speed.

Other forms of core memory Word line core memory was often used to provide register memory. This form of core memory typically wove three wires through each core on the plane, word read, word write, and bit sense/write, To read or clear words, the full current is applied to one or more word read lines; this clears the selected cores and any that flip induce voltage pulses in their bit sense/write lines. For read, normally only one word read line would be selected; but for clear, multiple word read lines could be selected while the bit sense/write lines ignored. To write words, the half current is applied to one or more word write lines, and half current is applied to each bit sense/write line for a bit to be set. For write, multiple word write lines could be selected. This offered a speed advantage over X/Y line coincident-current in that multiple words could be cleared or written with the same value in a single cycle. A typical machine's register set usually used only one small plane of this form of core memory.

Another form of core memory called core rope memory provided read-only storage. In this case, the cores were simply used as transformers; no information was actually stored magnetically within the individual cores.

Physical characteristics The speed of early core memories can be characterized in today's terms as being very roughly comparable to a clock speed of 1 megahertz (equivalent to early 1980s home computers, like the Apple II and Commodore 64). Early core memory systems had cycle times of about 6 microsecond, which had fallen to 1.2 µs by the early 1970s, and by the mid-70s it was down to 600 nanosecond (0.6 µs). Everything possible was done in order to speed access, including the simultaneous use of multiple grids of core, each storing one bit of a data word. For instance a machine might use 32 grids of core with a single bit of the 32-bit word in each one, and the controller could access the entire 32-bit word in a single read/write cycle.

Core memory is non-volatile storage – it can retain its contents indefinitely without power. It is also relatively unaffected by electromagnetic pulse and radiation. These were important advantages for some applications like military installations and vehicles like fighter aircraft, as well as spacecraft, and led to core being used for a number of years after availability of semiconductor MOS memory (see also MOSFET). For example, the Space Shuttle flight computers initially used core memory, which preserved the contents of memory even through the Space Shuttle Challenger's explosion and subsequent plunge into the sea in 1986.

A characteristic of core was that it is electrical current-based, not voltage-based. The "half select current" was typically about 400 ampere for later, smaller, faster cores. Earlier, larger cores required more current.

Another characteristic of core is that the hysteresis loop was temperature sensitive, the proper half select current at one temperature is not the proper half select current at another temperature. So the memory controllers could include temperature sensors (typically a thermistor) to check the temperature and adjust the current levels to correct for temperature changes. An example of this is the core memory used by Digital Equipment Corporation for their PDP-1 computer; this strategy continued through all of the follow-on core memory systems built by Digital Equipment Corporation for their Programmed Data Processor line of air-cooled computers. Another method of handling the temperature sensitivity was to enclose the magnetic core "stack" in a temperature controlled oven. Examples of this are the heated air core memory of the IBM 1620 (which could take up to 30 minutes to reach operating temperature, about 106 °F, 41 °C) and the heated oil bath core memory of the IBM 709, IBM 7090, and IBM 7030.

Core trivia

Although computer memory long ago moved to silicon chips, memory is still occasionally called "core". This is most obvious in the naming of the core dump, which refers to the contents of memory recorded at the time of a program error.
Core memory is fragile and doesn't support cleaning by blowing compressed air through. If you would be able to lay your hands on a beautiful but dusty specimen, clean it very gently. Otherwise you'll find the cores all over the place and both functional and decorative value will be lost.
Diagnosing hardware problems in core memory required time-consuming diagnostic programs to be run. While a quick test checked if every bit could contain a one and a zero, these diagnostics tested the core memory with worst-case patterns and had to run for several hours. As most computers just had one single core memory board, these diagnostics also moved themselves around in memory, making it possible to test every bit. In many occasions, errors could be resolved by gently tapping the printed circuit board with the core array on a table. This slightly changed the position of the cores to the wires running through and could fix the problem. The procedure was seldom needed, as core memory proved to be very reliable compared to other computer components those days.
In 1980, the price of a 16KW (KiloWord, equivalent to 32KB) core memory board that fitted into a DEC Q-bus computer was around USD 3000. At that time, core array and supporting electronics fitted on a single printed circuit board about 25 x 20cm in size, the core array was mounted a few mm above the PCB and was protected with a metal or plastic plate.