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memory,in computing: see computercomputer,
device capable of performing a series of arithmetic or logical operations. A computer is distinguished from a calculating machine, such as an electronic calculator, by being able to store a computer program (so that it can repeat its operations and make logical
..... Click the link for more information. .
memory,in psychology, the storing of learned information, and the ability to recall that which has been stored. It has been hypothesized that three processes occur in remembering: perception and registering of a stimulus; temporary maintenance of the perception, or short-term memory; and lasting storage of the perception, or long-term memory. Two major recognized types of long-term memory are procedural memory, involving the recall of learned skills, and declarative memory, the remembrance of specific stimuli. For long-term memory to occur, there must be a period of information consolidation.
The process of forgetting was first studied scientifically by Hermann Ebbinghaus, a German experimental psychologist, who performed memory tests with groups of nonsense syllables (disconnected syllables without associative connection). Ebbinghaus showed that the rate of forgetting is greatest at first, gradually diminishing until a relatively constant level of retained information is reached. Theories to explain forgetting include the concept of disuse, which proposes that forgetting occurs because stored information is not used, and that of interference, which suggests that old information interferes with information learned later and new information interferes with previously learned information.
In some instances, memory loss is an organic, physiological process. Retrograde amnesiaamnesia
, [Gr.,=forgetfulness], condition characterized by loss of memory for long or short intervals of time. It may be caused by injury, shock, senility, severe illness, or mental disease.
..... Click the link for more information. , i.e., the failure to remember events preceding a head injury, is evidence of interrupted consolidation of memory. In anterograde amnesia, events occurring after brain damage—e.g., in head injury or alcoholism—may be forgotten. Memory loss may also result from brain cell deterioration following a series of strokes, cardiovascular disease, or Alzheimer's disease (see dementiadementia
[Lat.,=being out of the mind], progressive deterioration of intellectual faculties resulting in apathy, confusion, and stupor. In the 17th cent. the term was synonymous with insanity, and the term dementia praecox was used in the 19th cent.
..... Click the link for more information. ).
Physiologically, learning involves modification of neural pathways. PET scansPET scan
or positron emission tomography
, a medical imaging technique that monitors metabolic, or biochemical, activity in the brain and other organs by tracking the movement and concentration of a radioactive tracer injected into the bloodstream.
..... Click the link for more information. and related studies have shown certain parts of the brain, such as the frontal lobe of the cerebral cortex and a structure called the hippocampus, to be particularly active in recall. Computer models of brain memory are called neural networksneural network
or neural computing,
computer architecture modeled upon the human brain's interconnected system of neurons. Neural networks imitate the brain's ability to sort out patterns and learn from trial and error, discerning and extracting the relationships that
..... Click the link for more information. . In a study using genetic manipulation, a mouse with enhanced memory capabilities has been produced.
See M. H. Ashcroft, Human Memory and Cognition (1989, repr. 1994); N. Cowan, Attention and Memory (1995, repr. 1998); J. McConkey, ed. The Anatomy of Memory (1996); D. L. Schacter, Searching for Memory (1996) and The Seven Sins of Memory (2001); J. A. Groegerd, Memory and Remembering (1997); A. Baddeley, Human Memory (rev. ed. 1998); R. Rupp, Committed to Memory (1998).
The ability to store and access information that has been acquired through experience. Memory is a critical component of practically all aspects of human thinking, including perception, learning, language, and problem solving. See Perception
The information-processing approach divides memory into three general stages: sensory memory, short-term memory, and long-term memory. Sensory memory refers to the sensations that briefly continue after something has been perceived. Short-term memory includes all of the information that is currently being processed in a person's mind, and is generally thought to have a very limited capacity. Long-term memory is where all the information that may be used at a later time is kept.
A number of interesting facts are known about sensory memory, including the following: (1) sensory memories appear to be associated with mechanisms in the central nervous system rather than at the sensory receptor level, and (2) the amount of attention that a person pays to a stimulus can affect the duration of the sensory memory. Although all of the functions of sensory memory are not understood, one of its most important purposes is to provide people with additional time to determine what should be transferred to the next stage in the memory system, that is, short-term memory.
Information obtained from either sensory memory or long-term memory is processed in short-term memory in order for a person to achieve current goals. In some situations, short-term memory processing simply involves the temporary maintenance of a piece of information, such as remembering a phone number long enough to dial it. Other times, short-term memory can involve elaborate manipulations of information in order to generate new forms. For example, when someone reads 27 + 15, the person manipulates the symbols in short-term memory in order to come up with the solution. One useful manipulation that can be done in short-term memory is to reorganize items into meaningful chunks. For example, it is a difficult task to keep the letters S K C A U Q K C U D E H T in mind all at once. However, if they are rearranged in short-term memory, in this case reversing them, they can be reduced to a single simple chunk: THE DUCK QUACKS. Short-term memory can accommodate only five to seven chunks at any one time. However, the amount of information contained in each chunk is constrained only by one's practice and ingenuity. In order to increase the amount of information that can be kept in short-term memory at one time, people need to develop specific strategies for organizing that information into meaningful chunks. In addition, many studies have also demonstrated that the transfer of information from short-term to long-term memory is much greater when the information is manipulated rather than simply maintained.
One can keep massive amounts of information in long-term memory. In general, recall from long-term memory simply involves figuring out the heading under which a memory has been filed. Many tricks for effective retrieval of long-term memories involve associating the memory with another more familiar memory that can serve as an identification tag. This trick of using associations to facilitate remembering is called mnemonics. Long-term memory stores related concepts and incidents in close range of one another. This logical association of memories is indicated by subjects' reaction times for identifying various memories. Generally, people are faster at recalling memories if they have recently recalled a related memory. One good way to locate a long-term memory is to remember the general situation under which it was stored. Accordingly, techniques that reinstate the context of a memory tend to facilitate remembering.
Sometimes information may not have been filed in long-term memory in the first place, or if it has, is inaccessible. In these situations, the long-term memory system often fills in the gaps by using various constructive processes. One common component to memory constructions is a person's expectations. Countless studies have also indicated that memories tend to systematically change in the direction of a prior expectation or inference about what is likely to have occurred.
A number of physiological mechanisms appear to be involved in the formation of memories, and the mechanisms may differ for short-term and long-term memory. There is both direct and indirect evidence suggesting that short-term memory involves the temporary circulation of electrical impulses around complex loops of interconnected neurons. A number of indirect lines of research indicate that short-term memories are eradicated by any event that either suppresses neural activity (for example, a blow to the head or heavy anesthesia) or causes neurons to fire incoherently (for example, electroconvulsive shock). More direct support for the electric circuit model of short-term memory comes from observing electrical brain activity. By implanting electrodes in the brain of experimental animals, researchers have observed that changes in what an animal is watching are associated with different patterns of circulating electrical activity in the brain. These results suggest that different short-term memories may be represented by different electrical patterns. However, the nature of these patterns is not well understood. See Electroencephalography
Long-term memories appear to involve some type of permanent structural or chemical change in the composition of the brain. This conclusion is derived both from general observations of the imperviousness of long-term memories and from physiological studies indicating specific changes in brain composition. Even in acute cases of amnesia where massive deficits in long-term memory are reported, often, with time, all long-term memories return. Similarly, although electroconvulsive therapy is known to eliminate recent short-term memories, it has practically no effect on memories for events occurring more than an hour prior to shocking. Thus the transfer from a fragile short-term memory to a relatively solid long-term memory occurs within an hour. This process is sometimes called consolidation.
The nature of the “solid” changes associated with long-term memories appears to involve alterations in both the structural (neural connections) and chemical composition of the brain. One study compared the brains of rats that had lived either in enriched environments with lots of toys or in impoverished environments with only an empty cage. The cerebral cortices of the brains of the rats from the enriched environment were thicker, heavier, endowed with more blood vessels, and contained significantly greater amounts of certain brain chemicals (such as the neurotransmitter acetylcholine). Other researchers have observed that brief, high-frequency stimulation of a neuron can produce long-lasting changes in the neuron's communications across synapses.
Researchers believe that different brain structures may be involved in the formation and storage of long-term memories. The hippocampus, thalamus, and amygdala are believed to be critical in the formation of long-term memories. Individuals who have had damage to these structures are able to recall memories prior to the damage, indicating that long-term memory storage is intact; however, they are unable to form new long-term memories, indicating that the long-term memory formation process has been disrupted. It is not known where long-term memories are stored, but they may be localized in the same areas of the brain that participated in the actual learning. See Brain
the ability to reproduce past experience; one of the fundamental properties of the nervous system, manifested in the ability to store information concerning bodily reactions and events in the environment and to introduce this information repeatedly into the consciousness and behavior over a long period of time.
Neurophysiology. Memory is characteristic of man and of animals that have a sufficiently developed central nervous system. The capacity of the memory and the duration and reliability of information storage, as well as the ability to perceive complex signals from the environment and elaborate appropriate reactions, increase as, in the course of evolution, the number of nerve cells (neurons) in the brain increases and the structure of the brain becomes more complex. Physiological research has shown that there are two principal stages in the formation of memory, each of which is associated with a specific type of memory— short-term or long-term. Characteristic of short-time memory is an information storage time ranging from seconds to tens of minutes and a vulnerability to disruption by factors affecting the coordinated work of the neurons (for example, electric shock, narcosis, and hypothermia). Long-term memory, in which the storage time for information is comparable to the life-span of the organism, is resistant to the factors that disrupt short-term memory. The transition from short-term to long-term memory is gradual. Neurophysiologists hypothesize that short-term memory depends on active mechanisms that support the excitation of certain neuronal systems. When information is transferred to long-term memory, the bonds between the neurons in these systems are fixed by structural changes in certain cells. Experiments involving the removal of portions of the cerebral cortex, as well as electrophysiological research, have shown that the “record” of each event is distributed in more or less extensive zones of the brain. Thus, there are grounds for presuming that information concerning different events is recorded not through the excitation of different neurons but through various combinations of simultaneously stimulated areas and cells in the brain.
Nerve cells do not divide during the organism’s life-span, and new reactions can be elaborated and remembered by the nervous system only through the creation of new bonds between neurons already existing in the brain. New neuronal systems are established as a result of changes in the interneuronal contacts—the synapses, in which the nerve impulse produces the secretion of a special chemical substance, the transmitter, which is capable of attenuating or inhibiting the generation of an impulse by the next neuron. Long-term changes in the effectiveness of the synapses may be caused by changes in the biosynthesis of proteins, on which the synaptic membrane’s sensitivity to the transmitter depends. It has been established that the biosynthesis of proteins is activated when there is excitation of neurons on different levels of organization of the central nervous system. If the synthesis of nucleic acids or proteins is blocked, the formation of long-term memory is hampered or prevented. Evidently, one of the functions of the activation of protein synthesis during excitation is the structural establishment of neuronal systems, a process fundamental to long-term memory. Available experimental data are not yet sufficient to permit scientists to decide whether the paths for the distribution of excitation emerge as a result of an increase in the conductivity of existing synapses or as a result of the development of additional interneuronal bonds. Both of these mechanisms require increased protein synthesis. The first possibility (increased conductivity) is reducible to partially studied phenomena of cellular adaptation and fits in well with the idea of the universality of the cell’s basic biochemical systems. The second explanation (development of additional interneuronal bonds) assumes an oriented growth of the nerve fibers and, in the final analysis, a coding of behavioral information in the structure of the chemical agents in the cell’s genetic apparatus, which regulate such growth.
Memory research uses the methods of a number of branches of science: clinical and experimental psychophysiology, the physiology of behavior, morphology and histochemistry, the electrophysiology of the brain and certain neurons, pharmacology, and analytical biochemistry. Depending on the problem under investigation, research on the mechanisms of memory uses various subjects, ranging from man to cultures of nerve cells.
REFERENCESAnokhin, P. K. Biologiia i neirofiziologiia uslovnogo refleksa. Moscow, 1968.
Beritashvili, I. S. Pamiat’ pozvonochnykh zhivotnykh, ee kharakteristika i proiskhozhdenie, 2nd ed. Moscow, 1974.
Agranoff, B. W. “Pamiat’ i sintez belka.” In Molekuly i kletki, fasc. 4. Moscow, 1969. (Translated from English.)
Sokolov, E. N. Mekhanizmy pamiati. Moscow, 1969.
Konorski, J. Integrativnaia deiatel’nost’ mozga. Moscow, 1970. (Translated from English.)
Hebb, D. O. Organization of Behavior. New York, 1949.
S. I. ROZANOV
Psychology. By establishing a link between past states of the psyche, the present, and processes of preparation for future states, memory imparts cohesiveness and stability to the life experience of man, ensures continuity of existence for the human “ego,” and thus acts as one of the prerequisites for the formation of individuality and personality.
From antiquity, special significance has been attributed to memory. For example, the goddess of memory, Mnemosyne, was known as the mother of the Muses and the patroness of crafts and sciences. According to legend, the ancient Greek poet Simonides (sixth century B.C.) worked out the first mnemonic system.
For a long time, the problem of memory was treated chiefly by philosophy, in close connection with the general problem of knowledge. The first developed concept of memory was propounded by Aristotle in the treatise On Memory and Recollection, which asserts that memory proper is characteristic of both man and animal, but that recollection is found only in man. “A kind of unique searching” for images, recollection “exists only in those who are capable of thinking,” for “he who recollects concludes that he previously saw, heard, or experienced something similar” (453a). Aristotle formulated rules for successful recollection, which were later “rediscovered” as the fundamental laws of association: proximity, similarity, and contrast. He pointed out a number of problems that are still pertinent, including changes in memory with age, characterological differences in memory, and the connection between memory and the segmentation of time.
Accepted by the Stoics and Epicureans, the materialist tendencies established in the Aristotelian doctrine of memory were given a considerably simplified interpretation. Thus, the idea of the active character of recollections was lost, and Aristotle’s metaphor—”impressions” in the soul—was interpreted literally. By contrast, an idealist concept of memory as an activity of the soul was developed by the Neoplatonists (Plotinus). The connection between recollection and the experience of time was observed by Augustine.
In modern times the problem of memory was treated especially by the English empiricists (T. Hobbes, J. Locke), in connection with their discussion of the problem of experience and their criticism of the doctrine of “innate ideas.” Memory, according to Locke, is like a “storehouse” of ideas: “this laying up of our ideas in the repository of the memory signifies no more but this—that the mind has a power in many cases to revive perceptions which it has once had, with this additional perception annexed to them, that it has had them before” (Locke, Izbr. filos, proizv., Moscow, 1960, p. 168). The idea that associations are a factor determining the “movement of ideas” was introduced by Locke. Later, in associationism, Locke’s idea came to be regarded as the universal principle for explaining psychic life. In 1885 the German psychologist H. Ebbinghaus performed the first experimental research on memory, within the framework of associationism.
American behaviorism (E. Thorndike and J. Watson) continued the associationist line of thought, placing the study of memory in the context of the general problem of learning and, in the final analysis, identifying memory with the acquisition of habits. The teachings of the French intuitionist philosopher H. Bergson were directed against the tendency to equate these two phenomena (Matter and Memory; Russian translation, St. Petersburg, 1911). Contrasting the simple reproduction of previously learned material (for example, the text of a poem) with the memory of unrepeatable past events in all their individuality (for example, the act of memorizing), he attempted to show the existence of a special “image” memory, “a sphere of pure recollections, memory of the spirit,” in relation to which the brain can only perform as an instrument for bringing recollections to consciousness. The brain, according to Bergson, is not capable of generating recollections or of acting as their storehouse. This idealist conception, however, exposed the limitations of the associationist doctrine of memory by posing a number of problems with extreme clarity (for example, the problem of recognition, and the question of the connection between memory and attention and between memory and the unconscious).
Gestalt psychology, which was sharply critical of the “atomism” and mechanistic quality of the associationist concept of memory, insisted on the integral and structural character of memory and, in particular, on the idea that traces of memory must be understood as dynamic systems or fields of force. A tendency toward a broader interpretation of memory was also evident in gestalt psychology. In the doctrine of the “mneme” the German psychologist E. Hering (1870) and later, E. Bleuler (1931), attempted to construe memory as not only a mental function but also a “general organic function,” using it specifically to explain the processes of heredity. (Thus, they revived the doctrine associated with C. Carus, a German natural philosopher of the first half of the 19th century.)
All of these developments led to a sharp restatement of the problem of the nature and characteristics of human memory. The French sociological school focused on the historical nature and social causality of human memory. According to P. Janet, human memory is a special act, “specially devised by human beings” and differing in principle from simple reproduction. It is a symbolic reconstruction, a re-creation of the past in the present. Man’s social world, acting as if it were a unique expresser of the “collective memory” of society (M. Halbwachs), turns out to be the source and ordering force of his recollections. In British psychology, a similar point of view was espoused by F. C. Bartlett (1932), who considered memory to be more a reconstruction than a reproduction of past experience.
Drawing on the methodology of dialectical and historical materialism, Soviet psychology has paid special attention to problems of the development of memory (for example, L. S. Vygotskii’s work). Experimental research conducted in 1930 by A. N. Leont’ev on the higher forms of remembering showed that the leading events in the formation of higher voluntary forms of memory are the inclusion, perfecting, and internalization of artificial “means-stimuli,” or symbols, of recall. In later work by Soviet psychologists (P. I. Zinchenko and A. A. Smirnov, for example) the study of human memory was placed in the context of the investigation of man’s object-related activity.
Traditionally, memory has been broken down into the processes of memorization, storage, and reproduction. The last of these subdivisions includes recognition, recollection, and recall proper (P. P. Blonskii). Voluntary memory is distinguished from involuntary memory. In voluntary memory, a person solves a special mnestic problem—he remembers or recalls something. In involuntary memory, remembering or recalling are dependent factors in a broader context of activity. The distinction between immediate and mediated memory (Vygotskii) is based on the method by which a mnestic act is accomplished. Other types of memory have been defined in terms of the form taken by the processes of memory: motor, or habit memory; emotional memory (T. Ribot’s “affective memory”); image memory; and verbal-logical memory. Some scientists (for example, Blonskii) have tried to establish genetic relationships between these types of memory. Finally, depending on the strongest tendency in a particular individual, it is possible to speak of visual, auditory, and verbal memory.
Inasmuch as it is involved in the solution of a particular life problem, a person’s memory is essentially determined by the structure and dynamics of his motives, needs, interests, and attitudes (K. Lewin, USA; B. V. Zeigarnik, USSR). Conversely, the characteristics of a person’s memory affect not only the course of certain forms of his activity and thought but also the entire structure of his life, leaving an imprint on his personality. (This is most clearly manifested in cases of local, clinical disturbances of memory, as well as in cases of “phenomenal,” especially eidetic, memory.)
Research on short-term memory is an independent branch of contemporary psychology and neurophysiology. Emphasis has been placed on the study of the patterns of forgetting. There have been detailed investigations of the phenomena of proactive and retroactive inhibition, reminiscence, and the influence of the emotions on memory (D. Rapaport, USA). It is necessary to distinguish pathological disturbances or disorders of memory from “normal” forgetting.
M. S. ROOOVIN
Disorders. Memory disorders may be quantitative (weakening or sharpening) or qualitative (distortion, false recollections). In psychiatric practice the most commonly observed memory disorders are decreased memory (hypomnesia) and loss of memory (amnesia), whose character is determined by their origin. For instance, forgetting recent events and remembering the remote past are characteristic of disorders associated with old age. An episode of clouded consciousness (in cases of cranial or brain trauma, for example) is usually accompanied by loss of memory of events that preceded the episode (retrograde amnesia) or events that took place immediately after the restoration of consciousness (anterograde amnesia). A patient suffering from Kor-sakov’s psychosis usually remembers what happened to him before the onset of the syndrome but cannot fix in his memory current, or even immediate, impressions.
Purely functional (“protective”) exclusion of recollections that are burdensome to the patient is observed in hysteria and in reactive psychoses (those arising in response to psychic trauma). Unlike amnesia, the intensification and sharpening of memory (hypermnesia), which occurs, for example, in mania or in delirium, is an unstable, rapidly passing phenomenon. Distortions of memory (cryptomnesias) and false recollections (confabulations) often accompany hypomnesia and amnesia: gaps in the memory are filled with fictions, or real events are transposed to another time. Treatment of memory disorders is directed at the underlying disease.
B. I. FRANKSHTEIN
In technology the concept of machine memory is applicable to computer devices designed for the storage of information.
REFERENCESKorsakov, S. S. Boleznennye rasstroistva pamiati i ikh diagnostika. Moscow, 1890.
Ribot, T. Pamiat’ v ee normal’nom i boleznennom sostoianiiakh. St. Petersburg, 1894. (Translated from French.)
Leont’ev, A. N. Razvitie pamiati. Moscow-Leningrad, 1931.
Golant, R. Ia. O rasstroistvakh pamiati. Leningrad-Moscow, 1935.
Vygotskii, L. S. Razvitie vysshikh psikhicheskikh funktsii. Moscow, 1960.
Zinchenko, P. I. Neproizvol’noe zapominanie. Moscow, 1961.
Blonskii, P. P. “Pamiat’ i myshlenie.” In his book Izbrannye psikhologicheskie proizvedeniia. Moscow, 1964.
Smirnov, A. A. Problemy psikhologii pamiati. Moscow, 1966.
Rogovin, M. S. Filosofskie problemy teorii pamiati. Moscow, 1966.
Luriia, A. R. Malen’kaia knizhka o bol’shoi pamiati. Moscow, 1968.
Zeigarnik, B. V. Lichnost’ i patologiia deiatel’nosti. Moscow, 1971.
Ebbinghaus, H. Über das Gedächtnis. Leipzig, 1885.
Janet, P. L’Évolution de la mémoire et de la notion du temps. Paris, 1928.
Bleuler, E. Mechanismus—Vitalismus—Mnemismus. Berlin, 1931.
Bartlett, F. C. Remembering. Cambridge, 1950.
Halbwachs, M. La Mémoire collective. Paris, 1950.
Rapaport, D. Emotions and Memory, 2nd ed. New York, 1959.
memoryIncreasingly, the term memory refers to permanent "non-volatile" storage and not the original meaning. The "flash memory" chips used in USB drives and "memory" cards caused this terminology change because they are permanent storage, not temporary as explained in the following paragraph.
The Original Definition
Starting in the 1960s, memory referred to the computer's temporary workspace, and for the past several decades that means dynamic RAM (DRAM) chips. A major resource in the computer, memory (RAM) determines the size and number of programs that can be run at the same time, as well as the amount of data that can be processed instantly.
To always be clear when communicating, avoid using the term memory, and instead use "RAM" for temporary memory and "storage" for permanent memory. RAM capacity in today's computing devices ranges approximately from 4GB to 32GB (gigabytes). Storage capacities are from 32GB to several terabytes (TB). See dynamic RAM, storage vs. memory, USB drive, memory card and flash memory.
It All Takes Place in Memory
All program execution and data processing takes place in memory, often called "main memory." The program's instructions are copied into memory from storage or the network and then extracted into the CPU's control unit circuit for analysis and execution. The instructions direct the computer or mobile device to input, process and output data.
Calculate, Compare and Copy
As data are entered into memory, the previous contents of that space are lost. Only in memory can data be processed (calculated, compared and copied). The results are copied from memory to a screen, printer, storage device or the network. See 3 C's.
Memory Is an Electronic Checkerboard
Think of a checkerboard with each square holding one byte of data or instruction. Each square (each byte) has a separate address like a post office box that can be manipulated independently. As a result, the computer can break apart programs into instructions for execution and data records into fields for processing. See byte addressable, early memory and RAM.
|A Checkerboard of Bytes|
|Once in memory (RAM), the contents of any single byte or group of bytes can be calculated, compared and copied independently. This is how fields are put together to form records and broken apart when read back in. In storage (hard drive, solid state drive, USB drive, etc.), data reside in sectors at least 512 bytes long, which are the smallest unit that can be read from or written to the drive.|
Computer Memory Does Not Remember
Oddly enough, memory does not "remember" anything when the power is turned off. So why call it memory? Because the first memory did "remember," but today's RAM chips do not. However, there are memory chips for special purposes that do hold their content, but they are not used as the main memory in computers, phones and tablets. See non-volatile memory.
What we think of as "remembering" memory in a computer are the hard drives and solid state drives (SSDs). See storage vs. memory.
Memory Can Get Clobbered!
Memory is an important resource that should not be wasted. It is allocated by the operating system as well as by applications and then released when no longer needed. Errant programs can grab memory and not let go, which results in less memory available to other programs. Restarting the computer gives memory a clean slate, which is why rebooting the computer clears up so many problems with applications.
In addition, if the operating system has bugs, a malfunctioning application can write into the same memory used by another program, causing unspecified behavior such as the system locking up. If one were able to look into and watch how fast data and instructions are written into and out of memory in the course of a single second, it would become obvious that it is a miracle it works at all.
Other terms for the computer's main memory are RAM, primary storage and read/write memory. Earlier terms were core and core storage. See dynamic RAM, static RAM and memory module.