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Encyclopedia > Memory (computers)
For other uses, see memory (disambiguation).

The term computer memory refers to the parts of a digital computer which retain physical state (data) for some interval of time. memory can mean: The ability to retain information in the brain or in nervous system, see memory. ... A digital system is one that uses numbers, especially binary numbers, for input, processing, transmission, storage, or display, rather than a continuous spectrum of values (an analog system) or non-numeric symbols such as letters or icons. ... Computer hardware is the physical parts of a computer, as distinguished from the computer software or computer programs and data that operate within the hardware. ... DATA (Debt, AIDS, Trade, Africa) was established in 2002 by Bono (Paul Hewson) of the Rock band U2, and Bobby Shriver, along with activists from the Jubilee 2000 Drop the Debt Campaign, as an organisaton focused on Justice, not charity. ... A watch Attempting to understand Time has long been a prime occupation for philosophers, scientists and artists. ...


In its most common usage, "memory" refers to very fast storage which does not retain its stored data when the power is turned off. Compare this to "storage", such as hard drive space, which is slow but keeps its data even without power. An analogy is to think of the storage as human memory, with the hard disk as long-term memory, and the memory as short-term memory. Typical hard drives of the mid-1990s. ... // Overview Long-term memory (LTM) is memory that can last as little as 30 seconds or as long as decades. ... Short-term memory, sometimes referred to as primary or active memory, is that part of memory which stores a limited amount of information for a limited amount of time (roughly 30-45 seconds). ...


In a home computer, memory will often take the form of: The home computer is a consumer-friendly word for the second generation of microcomputers (the technical term that was previously used), entering the market in 1977 and becoming common during the 1980s. ...

  • Random access memory, or RAM, which is used to temporarily store things such as programs and data while the computer is using them. Since RAM can be accessed at very high speeds, it is well suited for this task.
  • Cache memory is a small amount of very high speed dedicated memory. Cache memory is used to allow quicker access to data which ordinarily is slow to retrieve. Because of cache memory's high speed nature, storing data into cache memory before it is actually accessed can allow quicker response times. Cache memory is found in microprocessors, hard drives and many other places.

Contents

To meet Wikipedias quality standards, this article or section may require cleanup. ... Look up cache in Wiktionary, the free dictionary. ... Microprocessors, including an Intel 80486DX2 and an Intel 80386 Microprocessor, here an AMD Athlon64 X2 4200+ A microprocessor (sometimes abbreviated µP or uP) is a digital electronic component with miniaturized transistors on a single semiconductor integrated circuit (IC). ... Typical hard drives of the mid-1990s. ...


Different types and different purposes

Memory can be categorized in different ways by technology or properties:

  • primary access by the CPU or secondary (indirect) access by the CPU. This distinction is primarily based on the speed of access to the memory.
  • volatile or non-volatile is a distinction based on technology (magnetic vs. electrical, etc.). Volatile memory requires power to maintain its stored information.
  • Read-only memory, or read-write is a distinction based on properties of the memory. Read only memory, or "ROM", is not modifiable.
  • Random-Access or Sequential-Access, is a distinction based on the mechanism of reading the memory.
  • Mutable (Read-write) vs. Immutable (Read only) storage.

Historically, "memory" referred to "magnetic core memory" in the 1950s, and then to semiconductor-based storage in the 1970s. The evolution of memory is closely tied to the costs of various technologies, as can be seen in the history of computing hardware. Primary storage is a category of computer storage, often called main memory. ... Secondary storage is a category of computer storage. ... Volatile memory refers to computer memory that must be powered to maintain its data. ... Non-volatile memory, or non-volatile storage, is computer memory that can retain the stored information even when not powered. ... Read-only memory (ROM) is used as a storage medium in computers. ... In computer science, random access is the ability to access a random element of a group in equal time. ... In computer science sequential access means that a group of elements (e. ... Rom is also the name of a toy and comic book character Rom (Spaceknight). ... 1950 (MCML) was a common year starting on Sunday (link will take you to calendar). ... 1970 (MCMLXX) was a common year starting on Thursday. ... Computing hardware has been an essential component of the process of calculation and data storage since it became useful for numerical values to be processed and shared. ...


Each type of storage is suited for different purposes, and most computers contain several types: primary, secondary, and volatile.


Primary vs. secondary storage

In traditional parlance, primary storage contains data that are actively being used (for example, the programs currently being run and the data they are operating on). It is typically high-speed, relatively small, is often (but not always) volatile. It is sometimes referred to as "Main Memory." It can be accessed immediately and randomly. A computer program or software program (usually abbreviated to a program) is a step-by-step list of instructions written for a particular computer architecture in a particular computer programming language. ...


Secondary storage, also known as peripheral storage, is where the computer stores information that is not necessarily in current use. It is typically slower and higher-capacity than primary storage. It is almost always non-volatile. It is slow due to serial access (thus it is also termed Serial Access Memory).


Confusingly, these terms are sometimes used differently. Primary storage can be used to refer to local random-access disk storage, which should properly be called secondary storage. If this type of storage is called primary storage, then the term secondary storage would refer to offline, sequential-access storage like tape media. This usage usually occurs in contexts where only the slower, larger forms of storage are being discussed.


Volatile storage

Volatile storage loses its contents when it loses power; non-volatile storage does not.


Mutable vs. immutable storage

Data stored in mutable storage can be overwritten at any time. Data stored in immutable storage cannot be overwritten. Systems can be made more secure by storing programs and static data in immutable storage, where they cannot be changed by an attacker. Dynamic data is stored in mutable storage because it must be changed from time to time. Most operating systems store all programs and data on hard disk drives, which are inherently mutable storage devices. File system permissions can be used to make certain areas of the hard disk logically immutable. However, the superuser is normally not affected by these permissions thus allowing some attacks to succeed.


Some operating systems, such as Linux, extend this logical immutability so data remains immutable even if an attacker gains superuser access. Attackers may be able to destroy the data but they can't change it.


A list of storage devices

Internal storage areas in the computer. The term memory identifies data storage that comes in the form of chips, and the word storage is used for memory that exists on tapes or disks. Moreover, the term memory is usually used as a shorthand for physical memory, which refers to the actual chips capable of holding data. Some computers also use virtual memory, which expands physical memory onto a hard disk. Intel bubble memory module Bubble memory is a type of computer memory that uses a thin film of a magnetic material to hold small magnetized areas, known as bubbles, which each store one bit of data. ... This article is about the computer term. ... A 16×16 cm area core memory plane of 128×128 bits, i. ... Core rope memory is a form of read-only memory (ROM) for computers, first used by early NASA Mars probes and then in the Apollo Guidance Computer (AGC) designed by MIT and built by Raytheon. ... Delay line memory was a form of computer memory used on some of the earliest digital computers, such as the EDSAC and UNIVAC I. The first such systems consisted of a column of mercury with piezo crystal transducers (a combination of speaker and microphone) at either end. ... Holographic memory is a technique that can store information at high density inside crystals or photopolymers. ... 128 MB Memory Stick with MagicGate support Memory Stick is a removable flash memory card format, launched by Sony in October 1998, and is also used in general to describe the whole family of Memory Sticks. ... The Selectron was an early form of computer memory developed by RCA. Development started in 1946 with a planned production of 200 by the end of the year, but production problems meant that they were still not available by the middle of 1948. ... Semiconductor memory is a generic term referring to any computer storage method implemented on a semiconductor-based integrated circuit. ... Dram can mean several things: for the imperial unit of volume see dram (volume) for the imperial unit of weight or mass see avoirdupois and apothecaries system of mass for the Armenian monetary unit see dram (currency) DRAM is a type of RAM and unlike dram is spelled in all... An EPROM, or erasable programmable read-only memory, is a type of computer memory chip that retains its data when its power supply is switched off. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... A USB Flash Memory Device Flash memory is a form of EEPROM (Electrically-Erasable Programmable Read-Only Memory) that allows multiple memory locations to be erased or written in one programming operation. ... Non-Volatile Random Access Memory (NVRAM) is a computer memory chip which will not lose its information when the power is lost. ... Magnetoresistive Random Access Memory (MRAM) is a non-volatile computer memory (NVRAM) technology, which has been in development since the 1990s. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Read-only memory (ROM) is used as a storage medium in computers. ... Static Random Access Memory (SRAM) is a type of semiconductor memory. ... VRAM an acronym for Video RAM. Generally a term used in computers to describe RAM dedicated to the purpose of displaying bitmap graphics in raster graphics hardware. ... Window RAM or WRAM is an obsolete type of semiconductor computer memory that was designed to replace video RAM (VRAM) in graphics adapters. ... Thin film memory is a high-speed variation of core memory developed by Sperry Rand in a government-funded research project. ... The Williams tube or (more accurately) the Williams-Kilburn tube (after Freddie Williams and coworker Tom Kilburn), developed about 1946 or 1947, was a cathode ray tube used to store electronic data. ...


Every computer comes with a certain amount of physical memory, usually referred to as main memory or RAM. You can think of main memory as an array of boxes, each of which can hold a single byte of information. A computer that has 1 megabyte of memory, therefore, can hold about 1 million bytes (or characters) of information.


There are several different types of memory:

  • RAM (random-access memory): This is the same as main memory. When used by itself, the term RAM refers to read and write memory; that is, you can both write data into RAM and read data from RAM. This is in contrast to ROM, which permits you only to read data. Most RAM is volatile, which means that it requires a steady flow of electricity to maintain its contents. As soon as the power is turned off, whatever data was in RAM is lost.
    • SRAM (static RAM): Stores data in flip flops. Only requires steady flow of electricity to maintain its contents.
    • DRAM (dynamic RAM): Stores data in capacitors, which gradually lose charge and "forget". Requires periodic Refresh cycles to restore the data.
  • ROM (read-only memory): Computers almost always contain a small amount of read-only memory that holds instructions for starting up the computer. Unlike RAM, ROM cannot be written to.
    • PROM (programmable ROM): A PROM is a memory chip on which you can store a program. But once the PROM has been used, you cannot wipe it clean and use it to store something else. Like ROMs, PROMs are non-volatile.
    • EPROM (erasable programmable ROM): An EPROM is a special type of PROM that can be erased by exposing it to ultraviolet light. These must be removed from the board, erased, then reprogrammed.
    • EEPROM (electrically erasable programmable ROM): An EEPROM is a special type of PROM that can be erased by electrical signals, often the same ones that program it with new data. These can stay on the board and be reprogrammed in system.

|- ||0||0||0||0 |- ||0||0||1||1 |- ||0||1||X||0 |- ||1||0||X||1 |- ||1||1||0||1 |- ||1||1||1||0 |- |} The J-K flip-flop was named after Jack Kilby, the man who invented integrated circuits in 1958, for which he was awarded the Nobel Prize... A capacitor is a device that stores energy in the electric field created between a pair of conductors on which equal but opposite electric charges have been placed. ...

A list of memory-related software

Aard (Dutch for Earth) is a programming tool to check memory use for C++ programs. ... QEMM, the Quarterdeck Expanded Memory Manager by Quarterdeck, was the most popular memory manager for the DOS operating system. ... Memtest86 is a piece of software designed to stress-test an x86-compatible computers random access memory for errors. ...

See also


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Computers which use 72-Pin SIMM's generally have two sockets as one bank; therefore, two memory modules of equal value must be installed in pairs, unless instructed otherwise by the computer or motherboard manufacturer.
Once the memory appears to have been successfully installed into the computer, place the case back onto the computer and connect the keyboard, monitor and power to the computer (we recommend that you do not connect all the cables yet, if problems are experienced you may have to disconnect all the cables again).
As the computer is booting, enter the BIOS and either set the memory to its proper settings or verify that the computer's memory is being properly detected.
  More results at FactBites »

 
 

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