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Magnetic-core memory was the predominant form of random-access
computer memory In computing, memory is a device or system that is used to store information for immediate use in a computer or related computer hardware and digital electronic devices. The term ''memory'' is often synonymous with the term '' primary storag ...
for 20 years between about 1955 and 1975. Such memory is often just called core memory, or, informally, core. Core memory uses
toroid In mathematics, a toroid is a surface of revolution with a hole in the middle. The axis of revolution passes through the hole and so does not intersect the surface. For example, when a rectangle is rotated around an axis parallel to one of its ...
s (rings) of a hard magnetic material (usually a semi-hard ferrite) as
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
cores, where each wire threaded through the core serves as a transformer winding. Two or more wires pass through each core. Magnetic hysteresis allows each of the cores to "remember", or store a state. Each core stores one
bit The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...
of information. A core can be magnetized in either the clockwise or counter-clockwise direction. The value of the bit stored in a core is zero or one according to the direction of that core's magnetization. Electric current pulses in some of the wires through a core allow the direction of the magnetization in that core to be set in either direction, thus storing a one or a zero. Another wire through each core, the sense wire, is used to detect whether the core changed state. The process of reading the core causes the core to be reset to a zero, thus erasing it. This is called ''destructive readout''. When not being read or written, the cores maintain the last value they had, even if the power is turned off. Therefore, they are a type of '' non-volatile'' memory. Using smaller cores and wires, the memory density of core slowly increased, and by the late 1960s a density of about 32 kilobits per cubic foot (about 0.9 kilobits per litre) was typical. However, reaching this density required extremely careful manufacture, which was almost always carried out by hand in spite of repeated major efforts to automate the process. The cost declined over this period from about $1 per bit to about 1 cent per bit. The introduction of the first semiconductor memory chips in the late 1960s, which initially created static random-access memory ( SRAM), began to erode the market for core memory. The first successful dynamic random-access memory ( DRAM), the Intel 1103, followed in 1970. Its availability in quantity at 1 cent per bit marked the beginning of the end for core memory. Improvements in semiconductor manufacturing led to rapid increases in storage capacity and decreases in price per kilobyte, while the costs and specs of core memory changed little. Core memory was driven from the market gradually between 1973 and 1978. Depending on how it was wired, core memory could be exceptionally reliable. Read-only
core rope memory Core rope memory is a form of read-only memory (ROM) for computers, first used in the 1960s by early NASA Mars space probes and then in the Apollo Guidance Computer (AGC) and programmed by the Massachusetts Institute of Technology (MIT) Inst ...
, for example, was used on the mission-critical Apollo Guidance Computer essential to
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...
's successful Moon landings. Although core memory is obsolete, computer memory is still sometimes called "core" even though it is made of semiconductors, particularly by people who had worked with machines having actual core memory. The files that result from saving the entire contents of memory to disk for inspection, which is nowadays commonly performed automatically when a major error occurs in a computer program, are still called "
core dump In computing, a core dump, memory dump, crash dump, storage dump, system dump, or ABEND dump consists of the recorded state of the working memory of a computer program at a specific time, generally when the program has crashed or otherwise termina ...
s".


History


Developers

The basic concept of using the square hysteresis loop of certain magnetic materials as a storage or switching device was known from the earliest days of computer development. Much of this knowledge had developed due to an understanding of
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s, which allowed amplification and switch-like performance when built using certain materials. The stable switching behavior was well known in the electrical engineering field, and its application in computer systems was immediate. For example, J. Presper Eckert and Jeffrey Chuan Chu had done some development work on the concept in 1945 at the
Moore School The Moore School of Electrical Engineering at the University of Pennsylvania came into existence as a result of an endowment from Alfred Fitler Moore on June 4, 1923. It was granted to Penn's School of Electrical Engineering, located in the Town ...
during the
ENIAC ENIAC (; Electronic Numerical Integrator and Computer) was the first programmable, electronic, general-purpose digital computer, completed in 1945. There were other computers that had these features, but the ENIAC had all of them in one pac ...
efforts. Robotics pioneer George Devol filed a patent for the first static (non-moving) magnetic memory on 3 April 1946. Devol's magnetic memory was further refined via 5 additional patents and ultimately used in the first
Industrial Robot An industrial robot is a robot system used for manufacturing. Industrial robots are automated, programmable and capable of movement on three or more axes. Typical applications of robots include welding, painting, assembly, disassembly, pick ...
. Frederick Viehe applied for various patents on the use of
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s for building digital logic circuits in place of
relay logic Relay logic is a method of implementing combinational logic in electrical control circuits by using several electrical relays wired in a particular configuration. Ladder logic The schematic diagrams for relay logic circuits are often calle ...
beginning in 1947. A fully developed core system was patented in 1947, and later purchased by IBM in 1956. This development was little-known, however, and the mainstream development of core is normally associated with three independent teams. Substantial work in the field was carried out by the
Shanghai Shanghai (; , , Standard Mandarin pronunciation: ) is one of the four direct-administered municipalities of the People's Republic of China (PRC). The city is located on the southern estuary of the Yangtze River, with the Huangpu River flowin ...
-born American
physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate cau ...
s
An Wang An Wang (; February 7, 1920 – March 24, 1990) was a Chinese–American computer engineer and inventor, and cofounder of computer company Wang Laboratories, which was known primarily for its dedicated word processing machines. Wang was an impo ...
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; his patent focused on using cores to create delay-line or shift-register memory systems. Wang and Woo were working at
Harvard University Harvard University is a private Ivy League research university in Cambridge, Massachusetts. Founded in 1636 as Harvard College and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of high ...
's Computation Laboratory at the time, and the university was not interested in promoting inventions created in their labs. Wang was able to patent the system on his own. The MIT
Project Whirlwind Whirlwind I was a Cold War-era vacuum tube computer developed by the MIT Servomechanisms Laboratory for the U.S. Navy. Operational in 1951, it was among the first digital electronic computers that operated in real-time for output, and the firs ...
computer required a fast memory system for real-time aircraft tracking. At first, an array of
Williams tube The Williams tube, or the Williams–Kilburn tube named after inventors Freddie Williams and Tom Kilburn, is an early form of computer memory. It was the first random-access digital storage device, and was used successfully in several early co ...
s—a storage system based on
cathode ray tubes A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms (oscilloscope), pictur ...
—was used, but proved temperamental and unreliable. Several researchers in the late 1940s conceived the idea of using magnetic cores for computer memory, but MIT computer engineer Jay Forrester received the principal patent for his invention of the coincident-current core memory that enabled the 3D storage of information. William Papian of Project Whirlwind cited one of these efforts, Harvard's "Static Magnetic Delay Line", in an internal memo. The first core memory of was installed on Whirlwind in the summer of 1953. Papian stated: "Magnetic-Core Storage has two big advantages: (1) greater reliability with a consequent reduction in maintenance time devoted to storage; (2) shorter access time (core access time is 9 microseconds: tube access time is approximately 25 microseconds) thus increasing the speed of computer operation." In April 2011, Forrester recalled, "the Wang use of cores did not have any influence on my development of random-access memory. The Wang memory was expensive and complicated. As I recall, which may not be entirely correct, it used two cores per binary bit and was essentially a delay line that moved a bit forward. To the extent that I may have focused on it, the approach was not suitable for our purposes." He describes the invention and associated events, in 1975. Forrester has since observed, "It took us about seven years to convince the industry that random-access magnetic-core memory was the solution to a missing link in computer technology. Then we spent the following seven years in the patent courts convincing them that they had not all thought of it first." A third developer involved in the early development of core was Jan A. Rajchman at
RCA The RCA Corporation was a major American electronics company, which was founded as the Radio Corporation of America in 1919. It was initially a patent trust owned by General Electric (GE), Westinghouse, AT&T Corporation and United Fruit Comp ...
. A prolific inventor, Rajchman designed a unique core system using ferrite bands wrapped around thin metal tubes, building his first examples using a converted
aspirin Aspirin, also known as acetylsalicylic acid (ASA), is a nonsteroidal anti-inflammatory drug (NSAID) used to reduce pain, fever, and/or inflammation, and as an antithrombotic. Specific inflammatory conditions which aspirin is used to treat inc ...
press in 1949. Rajchman later developed versions of the Williams tube and led development of the Selectron. Two key inventions led to the development of magnetic core memory in 1951. The first, An Wang's, was the write-after-read cycle, which solved the problem of how to use a storage medium in which the act of reading erased the data read, enabling the construction of a serial, one-dimensional
shift register A shift register is a type of digital circuit using a cascade of flip-flops where the output of one flip-flop is connected to the input of the next. They share a single clock signal, which causes the data stored in the system to shift from one loc ...
(of 50 bits), using two cores to store a bit. A Wang core shift register is in the Revolution exhibit at the Computer History Museum. The second, Forrester's, was the coincident-current system, which enabled a small number of wires to control a large number of cores enabling 3D memory arrays of several million bits. The first use of core was in the Whirlwind computer, and Project Whirlwind's "most famous contribution was the random-access, magnetic core storage feature." Commercialization followed quickly. Magnetic core was used in peripherals of the
ENIAC ENIAC (; Electronic Numerical Integrator and Computer) was the first programmable, electronic, general-purpose digital computer, completed in 1945. There were other computers that had these features, but the ENIAC had all of them in one pac ...
in 1953, the
IBM 702 The IBM 702 was an early generation tube-based digital computer produced by IBM in the early to mid-1950s. It was the company's response to Remington Rand's UNIVAC—the first mainframe computer to use magnetic tapes. As these machines ...
delivered in July 1955, and later in the 702 itself. The IBM 704 (1954) and the
Ferranti Mercury The Mercury was an early commercial computer from the mid-1950s built by Ferranti. It was the successor to the Ferranti Mark 1, adding a floating point unit for improved performance, and increased reliability by replacing the Williams tube memory w ...
(1957) used magnetic-core memory. It was during the early 1950s that
Seeburg Corporation Seeburg was an American design and manufacturing company of automated musical equipment, such as orchestrions, jukeboxes, and vending equipment. Prior to manufacturing their signature jukebox suite of products, Seeburg was considered to be one of ...
developed one of the first commercial applications of coincident-current core memory storage in the "Tormat" memory of its new range of jukeboxes, starting with the V200 developed in 1953 and released in 1955. Numerous uses in computing, telephony and industrial process control followed.


Patent disputes

Wang's patent was not granted until 1955, and by that time magnetic-core memory was already in use. This started a long series of lawsuits, which eventually ended when IBM bought the patent outright from Wang for . Wang used the funds to greatly expand Wang Laboratories, which he had co-founded with Dr. Ge-Yao Chu, a schoolmate from China. MIT wanted to charge IBM $0.02 per bit royalty on core memory. In 1964, after years of legal wrangling, IBM paid MIT $13 million for rights to Forrester's patent—the largest patent settlement to that date.


Production economics

In 1953, tested but not-yet-strung cores cost each. As manufacturing volume increased, by 1970 IBM was producing 20 billion cores per year, and the price per core fell to . Core sizes shrank over the same period from around diameter in the 1950s to in 1966. The power required to flip the magnetization of one core is proportional to the volume, so this represents a drop in power consumption by a factor of 125. The cost of complete core memory systems was dominated by the cost of stringing the wires through the cores. Forrester's coincident-current system required one of the wires to be run at 45 degrees to the cores, which proved difficult to wire by machine, so that core arrays had to be assembled under microscopes by workers with fine motor control. In 1956, a group at IBM filed for a patent on a machine to automatically thread the first few wires through each core. This machine held the full plane of cores in a "nest" and then pushed an array of hollow needles through the cores to guide the wires. Use of this machine reduced the time taken to thread the straight X and Y select lines from 25 hours to 12 minutes on a 128 by 128 core array. Smaller cores made the use of hollow needles impractical, but there were numerous advances in semi-automatic core threading. Support nests with guide channels were developed. Cores were permanently bonded to a backing sheet "patch" that supported them during manufacture and later use. Threading needles were butt welded to the wires, so the needle and wire diameters were the same, and efforts were made to entirely eliminate the use of needles. The most important change, from the point of view of automation, was the combination of the sense and inhibit wires, eliminating the need for a circuitous diagonal sense wire. With small changes in layout, this also allowed much tighter packing of the cores in each patch.Creighton D. Barnes, et al., Magnetic core storage device having a single winding for both the sensing and inhibit function, , granted 4 July 1967.Victor L. Sell and Syed Alvi, High Density Core Memory Matrix, , granted Jan. 16, 1973. By the early 1960s, the cost of core fell to the point that it became nearly universal as main memory, replacing both inexpensive low-performance
drum memory Drum memory was a magnetic data storage device invented by Gustav Tauschek in 1932 in Austria. Drums were widely used in the 1950s and into the 1960s as computer memory. For many early computers, drum memory formed the main working memory of ...
and costly high-performance systems using
vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. The type known as ...
s, and later discrete
transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...
s as memory. The cost of core memory declined sharply over the lifetime of the technology: costs began at roughly per bit and dropped to roughly per bit. Core was replaced with integrated
semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
RAM Ram, ram, or RAM may refer to: Animals * A male sheep * Ram cichlid, a freshwater tropical fish People * Ram (given name) * Ram (surname) * Ram (director) (Ramsubramaniam), an Indian Tamil film director * RAM (musician) (born 1974), Dutch * ...
chips in the 1970s. An example of the scale, economics, and technology of core memory in the 1960s was the 256K 36-bit word (1.2 MiB) core memory unit installed on the PDP-6 at the
MIT Artificial Intelligence Laboratory Computer Science and Artificial Intelligence Laboratory (CSAIL) is a research institute at the Massachusetts Institute of Technology (MIT) formed by the 2003 merger of the Laboratory for Computer Science (LCS) and the Artificial Intelligence Lab ...
by 1967. This was considered "unimaginably huge" at the time, and nicknamed the "Moby Memory". It cost $380,000 ($0.04/bit) and was 69 inches wide, 50 inches tall, and 25 inches deep with its supporting circuitry (189 kilobits/cubic foot = 6.7 kilobits/litre). Its cycle time was 2.75 μs.


Description

The term "core" comes from conventional
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s whose windings surround a magnetic core. In core memory, the wires pass once through any given core—they are single-turn devices. The properties of materials used for memory cores are dramatically different from those used in power transformers. The magnetic material for a core memory requires a high degree of magnetic remanence, the ability to stay highly magnetized, and a low
coercivity Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in ...
so that less energy is required to change the magnetization direction. The core can take two states, encoding one bit. The core memory contents are retained even when the memory system is powered down (
non-volatile memory Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retain stored information even after power is removed. In contrast, volatile memory needs constant power in order to retain data. Non-volatile memory typi ...
). However, when the core is read, it is reset to a "zero" value. Circuits in the computer memory system then restore the information in an immediate re-write cycle.


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
toroid In mathematics, a toroid is a surface of revolution with a hole in the middle. The axis of revolution passes through the hole and so does not intersect the surface. For example, when a rectangle is rotated around an axis parallel to one of its ...
al
ferrimagnetic A ferrimagnetic material is a material that has populations of atoms with opposing magnetic moments, as in antiferromagnetism, but these moments are unequal in magnitude so a spontaneous magnetization remains. This can for example occur when ...
ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
ferrites (''cores'') held together in a grid structure (organized as a "stack" of layers called ''planes''), with wires woven through the holes in the cores' centers. 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 toroid stored one
bit The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...
(0 or 1). One bit in each plane could be accessed in one cycle, so each machine
word A word is a basic element of language that carries an objective or practical meaning, can be used on its own, and is uninterruptible. Despite the fact that language speakers often have an intuitive grasp of what a word is, there is no conse ...
in an array of words was spread over a "stack" of planes. Each plane would manipulate one bit of a word in parallel, allowing the full word to be read or written in one cycle. Core relies on the "square loop" properties of the ferrite material used to make the toroids. An electric current in a wire that passes through a core creates a magnetic field. Only a magnetic field greater than a certain intensity ("select") 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 ("half-select") required to cause this change. Only the combined magnetic field generated where the X and Y lines cross (a logical AND function) is sufficient to change the state; other cores will see only half the needed field ("half-selected"), or none at all. By driving the current through the wires in a particular direction, the resulting induced field forces the selected core's magnetic flux to circulate in one direction or the other (clockwise or counterclockwise). One direction is a stored ''1'', while the other is a stored ''0''. The toroidal shape of a core is preferred since the magnetic path is closed, there are no magnetic poles and thus very little external flux. This allows the cores to be packed closely together without allowing their magnetic fields to interact. The alternating 45-degree positioning used in early core arrays was necessitated by the diagonal sense wires. With the elimination of these diagonal wires, tighter packing was possible.


Reading and writing

To read a bit of core memory, the circuitry tries to flip the bit to the polarity assigned to the 0 state, by driving the selected X and Y lines that intersect at that core. * If the bit was already 0, the physical state of the core is unaffected. * If the bit was previously 1, then the core changes magnetic polarity. This change, after a delay, induces a voltage pulse into the Sense line. The detection of such a pulse means that the bit had most recently contained a 1. Absence of the pulse means that the bit had contained a 0. The delay in sensing the voltage pulse is called the access time of the core memory. Following any such read, the bit contains a 0. This illustrates why a core memory access is called a ''destructive read'': Any operation that reads the contents of a core erases those contents, and they must immediately be recreated. To write a bit of core memory, the circuitry assumes there has been a read operation and the bit is in the 0 state. * To write a 1 bit, the selected X and Y lines are driven, with current in the opposite direction as for the read operation. As with the read, the core at the intersection of the X and Y lines changes magnetic polarity. * To write a 0 bit, two methods can be applied. The first one is the same as reading process with current in the original direction. The second has reversed logic. Write 0 bit, in other words, is to inhibit the writing of a 1 bit. The same amount of current is also sent through the Inhibit line. This reduces the net current flowing through the respective core to half the select current, inhibiting change of polarity. The access time plus the time to rewrite is the memory cycle time. The Sense wire is used only during the read, and the Inhibit wire is used only during the write. For this reason, later core systems combined the two into a single wire, and used circuitry in the memory controller to switch the function of the wire. However, when Sense wire crosses too many cores, the half select current can also induce a considerable voltage across the whole line due to the superposition of the voltage at each single core. This potential risk of "misread" limits the minimum number of the Sense wire. Increasing Sense wires requires more decode circuits. Core memory controllers were designed so that every read was followed immediately by a write (because the read forced all bits to 0, and because the write assumed this had happened). Computers began to take advantage of this fact. For example, a value in memory could be read and incremented (as for example by the AOS instruction on the PDP-6) almost as quickly as it could be read; the hardware simply incremented the value between the read phase and the write phase of a single memory cycle (perhaps signalling the memory controller to pause briefly in the middle of the cycle). This might be twice as fast as the process of obtaining the value with a read-write cycle, incrementing the value in some processor register, and then writing the new value with another read-write cycle.


Other forms of core memory

''Word line'' core memory was often used to provide register memory. Other names for this type are ''linear select'' and ''2-D''. 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. In some designs, the ''word read'' and ''word write'' lines were combined into a single wire, resulting in a memory array with just two wires per bit. For write, multiple ''word write'' lines could be selected. This offered a performance 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. Some very large memories were built with this technology, for example the Extended Core Storage (ECS) auxiliary memory in the
CDC 6600 The CDC 6600 was the flagship of the 6000 series of mainframe computer systems manufactured by Control Data Corporation. Generally considered to be the first successful supercomputer, it outperformed the industry's prior recordholder, the IBM ...
, which was up to 2 million 60-bit words. Another form of core memory called
core rope memory Core rope memory is a form of read-only memory (ROM) for computers, first used in the 1960s by early NASA Mars space probes and then in the Apollo Guidance Computer (AGC) and programmed by the Massachusetts Institute of Technology (MIT) Inst ...
provided read-only storage. In this case, the cores, which had more linear magnetic materials, were simply used as
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s; no information was actually stored magnetically within the individual cores. Each bit of the word had one core. Reading the contents of a given memory address generated a pulse of current in a wire corresponding to that address. Each address wire was threaded either through a core to signify a binary or around the outside of that core, to signify a binary As expected, the cores were much larger physically than those of read-write core memory. This type of memory was exceptionally reliable. An example was the Apollo Guidance Computer used for the
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...
Moon landings.


Physical characteristics

The performance of early core memories can be characterized in today's terms as being very roughly comparable to a clock rate of 1
MHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one he ...
(equivalent to early 1980s home computers, like the Apple II and Commodore 64). Early core memory systems had cycle times of about 6
µs A microsecond is a unit of time in the International System of Units (SI) equal to one millionth (0.000001 or 10−6 or ) of a second. Its symbol is μs, sometimes simplified to us when Unicode is not available. A microsecond is equal to 1 ...
, which had fallen to 1.2 µs by the early 1970s, and by the mid-70s it was down to 600 ns (0.6 µs). Some designs had substantially higher performance: the
CDC 6600 The CDC 6600 was the flagship of the 6000 series of mainframe computer systems manufactured by Control Data Corporation. Generally considered to be the first successful supercomputer, it outperformed the industry's prior recordholder, the IBM ...
had a memory cycle time of 1.0 µs in 1964, using cores that required a half-select current of 200 mA. Everything possible was done in order to decrease access times and increase data rates (bandwidth), 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 Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retain stored information even after power is removed. In contrast, volatile memory needs constant power in order to retain data. Non-volatile memory typi ...
—it can retain its contents indefinitely without power. It is also relatively unaffected by EMP and radiation. These were important advantages for some applications like first-generation industrial programmable controllers, military installations and vehicles like fighter aircraft, as well as
spacecraft A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...
, and led to core being used for a number of years after availability of
semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
MOS memory (see also MOSFET). For example, the
Space Shuttle The Space Shuttle is a retired, partially reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program ...
IBM AP-101B flight computers used core memory, which preserved the contents of memory even through the '' Challenger''s disintegration and subsequent plunge into the sea in 1986. Another characteristic of early core was that the coercive force was very temperature-sensitive; the proper half-select current at one temperature is not the proper half-select current at another temperature. So a memory controller would include a temperature sensor (typically a thermistor) to adjust the current levels correctly for temperature changes. An example of this is the core memory used by
Digital Equipment Corporation Digital Equipment Corporation (DEC ), using the trademark Digital, was a major American company in the computer industry from the 1960s to the 1990s. The company was co-founded by Ken Olsen and Harlan Anderson in 1957. Olsen was president un ...
for their
PDP-1 The PDP-1 (''Programmed Data Processor-1'') is the first computer in Digital Equipment Corporation's PDP series and was first produced in 1959. It is famous for being the computer most important in the creation of hacker culture at Massachusett ...
computer; this strategy continued through all of the follow-on core memory systems built by DEC for their PDP 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 The IBM 1620 was announced by IBM on October 21, 1959, and marketed as an inexpensive scientific computer. After a total production of about two thousand machines, it was withdrawn on November 19, 1970. Modified versions of the 1620 were used as ...
(which could take up to 30 minutes to reach operating temperature, about and the heated-oil-bath core memory of the
IBM 7090 The IBM 7090 is a second-generation transistorized version of the earlier IBM 709 vacuum tube mainframe computer that was designed for "large-scale scientific and technological applications". The 7090 is the fourth member of the IBM 700/7000 se ...
, early IBM 7094s, and
IBM 7030 The IBM 7030, also known as Stretch, was IBM's first transistorized supercomputer. It was the fastest computer in the world from 1961 until the first CDC 6600 became operational in 1964."Designed by Seymour Cray, the CDC 6600 was almost three ...
. Core was heated instead of cooled because the primary requirement was a ''consistent'' temperature, and it was easier (and cheaper) to maintain a constant temperature well above room temperature than one at or below it. In 1980, the price of a 16 kW ( kiloword, equivalent to 32 kB) core memory board that fitted into a DEC Q-bus computer was around . At that time, core array and supporting electronics fit on a single printed circuit board about 25 × 20 cm in size, the core array was mounted a few mm above the PCB and was protected with a metal or plastic plate. 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 had just a single core memory board, these diagnostics also moved themselves around in memory, making it possible to test every bit. An advanced test was called a " Schmoo test" in which the half-select currents were modified along with the time at which the sense line was tested ("strobed"). The data plot of this test seemed to resemble a cartoon character called " Schmoo," and the name stuck. In many occasions, errors could be resolved by gently
tapping Tapping is a playing technique that can be used on any stringed instrument, but which is most commonly used on guitar. The technique involves a string being fretted and set into vibration as part of a single motion. This is in contrast to stand ...
the printed circuit board with the core array on a table. This slightly changed the positions of the cores along the wires running through them, and could fix the problem. The procedure was seldom needed, as core memory proved to be very reliable compared to other computer components of the day. File:8 bytes vs. 8Gbytes.jpg, This
microSDHC Secure Digital, officially abbreviated as SD, is a proprietary non-volatile flash memory card format developed by the SD Association (SDA) for use in portable devices. The standard was introduced in August 1999 by joint efforts between SanDis ...
card holds 8 billion bytes (8 GB). It rests on a section of magnetic-core memory that uses 64 cores to hold eight bytes. The microSDHC card holds over one billion times more bytes in much less physical space. File:Magnetic-core memory, 18x24 bits.jpg, Magnetic-core memory, 18×24 bits, with a US quarter for scale File:Magnetic-core memory close-up.JPG, Magnetic-core memory close-up File:Magnetic-core memory, at angle.jpg, At an angle


See also

*
Bubble memory Bubble memory is a type of non-volatile computer memory that uses a thin film of a magnetic material to hold small magnetized areas, known as ''bubbles'' or ''domains'', each storing one bit of data. The material is arranged to form a series o ...
*
Core dump In computing, a core dump, memory dump, crash dump, storage dump, system dump, or ABEND dump consists of the recorded state of the working memory of a computer program at a specific time, generally when the program has crashed or otherwise termina ...
*
Core rope memory Core rope memory is a form of read-only memory (ROM) for computers, first used in the 1960s by early NASA Mars space probes and then in the Apollo Guidance Computer (AGC) and programmed by the Massachusetts Institute of Technology (MIT) Inst ...
*
Delay-line memory Delay-line memory is a form of computer memory, now obsolete, that was used on some of the earliest digital computers. Like many modern forms of electronic computer memory, delay-line memory was a refreshable memory, but as opposed to modern ran ...
* Electronic calculators *
Ferroelectric RAM Ferroelectric RAM (FeRAM, F-RAM or FRAM) is a random-access memory similar in construction to DRAM but using a ferroelectric layer instead of a dielectric layer to achieve non-volatility. FeRAM is one of a growing number of alternative non-vo ...
*
Magnetoresistive random-access memory Magnetoresistive random-access memory (MRAM) is a type of non-volatile random-access memory which stores data in magnetic domains. Developed in the mid-1980s, proponents have argued that magnetoresistive RAM will eventually surpass competing tec ...
* Read-mostly memory (RMM) *
Thin-film memory Thin-film memory is a high-speed alternative to core memory developed by Sperry Rand in a government-funded research project. Instead of threading individual ferrite cores on wires, thin-film memory consisted of 4-micrometre thick dots of per ...
*
Twistor memory Twistor memory is a form of computer memory formed by wrapping magnetic tape around a current-carrying wire. Operationally, twistor was very similar to core memory. Twistor could also be used to make ROM memories, including a re-programmable fo ...


References


External links


Interactive Java Tutorial - Magnetic Core Memory
National High Magnetic Field Laboratory

at Columbia University *



accessed 15 April 2006

'' Byte magazine'', July 1976
Casio ''AL-1000'' calculator
– Shows close-ups of the magnetic core memory in this desktop electronic calculator from the mid-1960s.
Still used core memory
in multiple devices in a German computer museum *

{{Authority control History of electronic engineering Non-volatile memory Types of RAM