Computer data storage, often called storage or memory, refers to computer components, devices, and recording media that retain digital data used for computing for some interval of time. A computer is a Machine that manipulates data according to a list of instructions. A data storage device is a device for recording (storing information (data In Computer science, data is anything in a form suitable for use with a Computer. Computer data storage provides one of the core functions of the modern computer, that of information retention. It is one of the fundamental components of all modern computers, and coupled with a central processing unit (CPU, a processor), implements the basic computer model used since the 1940s.
In contemporary usage, memory usually refers to a form of semiconductor storage known as random access memory (RAM) and sometimes other forms of fast but temporary storage. A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that Similarly, storage today more commonly refers to mass storage - optical discs, forms of magnetic storage like hard disks, and other types slower than RAM, but of a more permanent nature. In Computing, mass storage refers to the storage of large amounts of information in a persisting and Machine-readable fashion Magnetic storage and magnetic recording are terms from Engineering referring to the storage of Data on a Magnetized medium A hard disk drive ( HDD) commonly referred to as a hard drive, hard disk, or fixed disk drive, is a Non-volatile storage device Historically, memory and storage were respectively called primary storage and secondary storage.
The contemporary distinctions are helpful, because they are also fundamental to the architecture of computers in general. As well, they reflect an important and significant technical difference between memory and mass storage devices, which has been blurred by the historical usage of the term storage. Nevertheless, this article uses the traditional nomenclature.
Various forms of storage, based on various natural phenomena, have been invented. So far, no practical universal storage medium exists, and all forms of storage have some drawbacks. Therefore a computer system usually contains several kinds of storage, each with an individual purpose.
A digital computer represents each datum using the binary numeral system. A computer is a Machine that manipulates data according to a list of instructions. Debt AIDS Trade in Africa (or DATA) is a Multinational non-government organization founded in January 2002 in London by U2 's The binary numeral system, or base-2 number system, is a Numeral system that represents numeric values using two symbols usually 0 and 1. Text, numbers, pictures, audio, and nearly any other form of information can be converted into a string of bits, or binary digits, each of which has a value of 1 or 0. A bit is a binary digit, taking a value of either 0 or 1 Binary digits are a basic unit of Information storage and communication The most common unit of storage is the byte, equal to 8 bits. A byte (pronounced "bite" baɪt is the basic unit of measurement of information storage in Computer science. A piece of information can be handled by any computer whose storage space is large enough to accommodate the binary representation of the piece of information, or simply data. In Computer science, data is anything in a form suitable for use with a Computer. For example, using eight million bits, or about one megabyte, a typical computer could store a small novel. A megabyte is a unit of Information or Computer storage equal to either 106 (1000000 Bytes or 220 (1048576 bytes depending on
Traditionally the most important part of every computer is the central processing unit (CPU, or simply a processor), because it actually operates on data, performs any calculations, and controls all the other components.
Without significant amount of memory, a computer would merely be able to perform fixed operations and immediately output the result. It would have to be reconfigured to change its behaviour. This is acceptable for devices such as desk calculators or simple digital signal processors. A calculator is device for performing mathematical calculations distinguished from a Computer by having a limited problem solving ability and an interface optimized for interactive Digital signal processing ( DSP) is concerned with the representation of the signals by a sequence of numbers or symbols and the processing of these signals Von Neumann machines differ in that they have a memory in which they store their operating instructions and data. The von Neumann architecture is a design model for a stored-program Digital computer that uses a processing unit and a single separate storage structure In Computer science, an instruction is a single operation of a processor defined by an Instruction set architecture. Such computers are more versatile in that they do not need to have their hardware reconfigured for each new program, but can simply be reprogrammed with new in-memory instructions; they also tend to be simpler to design, in that a relatively simple processor may keep state between successive computations to build up complex procedural results. One of the key concepts in Computer programming is the idea of state, essentially a snapshot of the measure of various conditions in the system Most modern computers are von Neumann machines.
In practice, almost all computers use a variety of memory types, organized in a storage hierarchy around the CPU, as a tradeoff between performance and cost. The Hierarchical arrangement of storage in current Computer architectures is called the memory hierarchy. Generally, the lower a storage is in the hierarchy, the lesser its bandwidth and the greater its access latency is from the CPU. In Computer networking and Computer science, digital bandwidth or just bandwidth is the capacity for a given system to transfer data over a connection This traditional division of storage to primary, secondary, tertiary and off-line storage is also guided by cost per bit.
Primary storage, presently known as memory, is the only one directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them. Any data actively operated on is also stored there in uniform manner.
Historically, early computers used delay lines, Williams tubes, or rotating magnetic drums as primary storage. The history of computer hardware encompasses the hardware, its architecture, and its impact on software. Genesis in radar The basic concept of the delay line originated with World War II Radar research as a system to reduce clutter from reflections from the ground The Williams tube or the Williams-Kilburn tube (after inventors Freddie Williams and Tom Kilburn) developed about 1946 or 1947 Drum memory is a magnetic Data storage device and was an early form of Computer memory widely used in the 1950s and into the 1960s invented by Gustav Tauschek By 1954, those unreliable methods were mostly replaced by magnetic core memory, which was still rather cumbersome. Magnetic core memory, or ferrite-core memory, is an early form of Random access Computer memory. Undoubtedly, a revolution was started with the invention of a transistor, that soon enabled then-unbelievable miniaturization of electronic memory via solid-state silicon chip technology. In Electronics, a transistor is a Semiconductor device commonly used to amplify or switch electronic signals Microchipsjpg|right|thumb|200px|Microchips ( EPROM memory with a transparent window showing the integrated circuit inside
This led to a modern random access memory (RAM). It is small-sized, light, but quite expensive at the same time. (The particular types of RAM used for primary storage are also volatile, i. Volatile memory, also known as volatile storage or primary storage device, is Computer memory that requires power to maintain the stored information e. they lose the information when not powered).
As shown in the diagram, traditionally there are two more sub-layers of the primary storage, besides main large-capacity RAM:
Main memory is directly or indirectly connected to the CPU via a memory bus, today sometimes referred to as a front side bus. In Personal computers the Front Side Bus ( FSB) is the bus that carries data between the CPU and the northbridge. It is actually comprised of two buses (not on the diagram): an address bus and a data bus. An address bus is a Computer bus, controlled by CPUs or DMA -capable Peripherals for specifying the Physical addresses of computer In Computer architecture, a bus is a subsystem that transfers data between computer components inside a Computer or between computers The CPU firstly sends a number through an address bus, a number called memory address, that indicates the desired location of data. In Computer science, a memory address is an identifier for a memory location at which a Computer program or a hardware device can store a piece of data Then it reads or writes the data itself using the data bus. Additionally, a memory management unit (MMU) is a small device between CPU and RAM recalculating the actual memory address, for example to provide an abstraction of virtual memory or other tasks. A memory management unit ( MMU) sometimes called paged memory management unit ( PMMU) is a Computer hardware component responsible for handling Virtual memory is a Computer system technique which gives an application program the impression that it has contiguous working memory while in fact it may be physically
As the RAM types used for primary storage are volatile (cleared at start up), a computer containing only such storage would not have a source to read instructions from, in order to start the computer. Hence, non-volatile primary storage containing a small startup program (BIOS) is used to bootstrap the computer, that is, to read a larger program from non-volatile secondary storage to RAM and start to execute it. Non-volatile memory, nonvolatile memory, NVM or non-volatile storage, is Computer memory that can retain the stored information In Computing, the BIOS (ˈbaɪoʊs In computing bootstrapping ("to pull oneself up by one's bootstraps" refers to techniques that allow a simple system to activate a more complicated system A non-volatile technology used for this purpose is called ROM, for read-only memory (the terminology may be somewhat confusing as most ROM types are also capable of random access).
Many types of "ROM" are not literally read only, as updates are possible; however it is slow and memory must be erased in large portions before it can be re-written. Some embedded systems run programs directly from ROM (or similar), because such programs are rarely changed. An embedded system is a special-purpose Computer system designed to perform one or a few dedicated functions often with Real-time computing constraints Standard computers do not store non-rudimentary programs in ROM, rather use large capacities of secondary storage, which is non-volatile as well, and not as costly.
Recently, primary storage and secondary storage in some uses refer to what was historically called, respectively, secondary storage and tertiary storage. 
Secondary storage, or storage in popular usage, differs from primary storage in that it is not directly accessible by the CPU. The computer usually uses its input/output channels to access secondary storage and transfers desired data using intermediate area in primary storage. In Computing, input/output, or I/O, refers to the communication between an Information processing system (such as a Computer) and the outside In Computing, a buffer is a region of memory used to temporarily hold Data while it is being moved from one place to another Secondary storage does not lose the data when the device is powered down—it is non-volatile. Per unit, it is typically also an order of magnitude less expensive than primary storage. Consequently, modern computer systems typically have an order of magnitude more secondary storage than primary storage and data is kept for a longer time there.
In modern computers, hard disks are usually used as secondary storage. A hard disk drive ( HDD) commonly referred to as a hard drive, hard disk, or fixed disk drive, is a Non-volatile storage device The time taken to access a given byte of information stored on a hard disk is typically a few thousandths of a second, or milliseconds. By contrast, the time taken to access a given byte of information stored in random access memory is measured in thousand-millionths of a second, or nanoseconds. This illustrates the very significant access-time difference which distinguishes solid-state memory from rotating magnetic storage devices: hard disks are typically about a million times slower than memory. Rotating optical storage devices, such as CD and DVD drives, have even longer access times. In Computing, an optical disc drive ( ODD) is a Disk drive that uses Laser light or electromagnetic waves near the Light spectrum A Compact Disc (also known as a CD) is an Optical disc used to store digital data, originally developed for storing digital audio DVD (also known as " Digital Versatile Disc " or " Digital Video Disc " - see Etymology)is
Some other examples of secondary storage technologies are: flash memory (e. Flash memory is non-volatile computer memory that can be electrically erased and reprogrammed g. USB sticks or keys), floppy disks, magnetic tape, paper tape, punch cards, standalone RAM disks, and Zip drives. A floppy disk is an increasingly Obsolete data storage medium that is composed of a disk of thin flexible ("floppy" Magnetic storage medium encased Magnetic tape has been used for Data storage for over 50 years Punched tape or paper tape is a largely obsolete form of Data storage, consisting of a long strip of paper in which holes are punched to store data A RAM disk is a software layer that enables applications to transparently use RAM, often a segment of Main memory, as if it was a Hard disk The Zip drive is a medium-capacity removable Disk storage system introduced by Iomega in late 1994
The secondary storage is often formatted according to a filesystem format, which provides the abstraction necessary to organize data into files and directories, providing also additional information (called metadata) describing the owner of a certain file, the access time, the access permissions, and other information. In Computing, a file system (often also written as filesystem) is a method for storing and organizing Computer files and the data they contain to make Metadata ( meta data, or sometimes metainformation) is "data about data" of any sort in any media
Most computer operating systems use the concept of virtual memory, allowing utilization of more primary storage capacity than is physically available in the system. An operating system (commonly abbreviated OS and O/S) is the software component of a Computer system that is responsible for the management and coordination Virtual memory is a Computer system technique which gives an application program the impression that it has contiguous working memory while in fact it may be physically As the primary memory fills up, the system moves the least-used chunks (pages) to secondary storage devices (to a swap file or page file), retrieving them later when they are needed. In a context of Computer Virtual memory, a page, memory page, or virtual page is a fixed-length block of Main memory, that is contiguous In Computer Operating systems that have their Main memory divided into pages, paging (sometimes called swapping) is a transfer In Computer Operating systems that have their Main memory divided into pages, paging (sometimes called swapping) is a transfer As more of these retrievals from slower secondary storage are necessary, the more the overall system performance is degraded.
Tertiary storage or tertiary memory, provides a third level of storage. Typically it involves a robotic mechanism which will mount (insert) and dismount removable mass storage media into a storage device according to the system's demands; this data is often copied to secondary storage before use. It is primarily used for archival of rarely accessed information since it is much slower than secondary storage (e. g. 5-60 seconds vs. 1-10 milliseconds). This is primarily useful for extraordinarily large data stores, accessed without human operators. Typical examples include tape libraries and optical jukeboxes. In Computer storage, a tape library, sometimes called a tape silo, or tape jukebox, is a storage device which contains one or more Tape drives An optical jukebox is a robotic Data storage device that can automatically load and unload Optical discs such as Compact Disc, DVD, Ultra
When a computer needs to read information from the tertiary storage, it will first consult a catalog database to determine which tape or disc contains the information. A Computer Database is a structured collection of records or data that is stored in a computer system Next, the computer will instruct a robotic arm to fetch the medium and place it in a drive. An industrial robot is officially defined by ISO as an automatically controlled reprogrammable multipurpose manipulator programmable in three or more axes. When the computer has finished reading the information, the robotic arm will return the medium to its place in the library.
Off-line storage, also known as disconnected storage, is a computer data storage on a medium or a device that is not under the control of a processing unit.  The medium is recorded, usually in a secondary or tertiary storage device, and then physically removed or disconnected. It must be inserted or connected by a human operator before a computer can access it again. Unlike tertiary storage, it cannot be accessed without human interaction.
Off-line storage is used to transfer information, since the detached medium can be easily physically transported. Data transmission is the transfer of Data from point-to-point often represented as an Electro-magnetic Signal over a physical point-to-point or point-to-multipoint Additionally in case a disaster, for example a fire, destroys the original data, a medium in a remote location will be probably unaffected, enabling disaster recovery. Disaster recovery is the process policies and procedures of restoring operations critical to the resumption of business including regaining access to data (records hardware software Off-line storage increases a general information security, since it is physically inaccessible from a computer, and data confidentiality or integrity cannot be affected by computer-based attack techniques. Information security means protecting information and information systems from unauthorized access use disclosure disruption modification or destruction Also, if the information stored for archival purposes is accessed seldom or never, off-line storage is less expensive than tertiary storage.
In modern personal computers, most secondary and tertiary storage media are also used for off-line storage. Optical discs and flash memory devices are most popular, and to much lesser extent removable hard disk drives. In enterprise uses, magnetic tape is predominant. Older examples are floppy disks, Zip disks, or punched cards.
Storage technologies at all levels of the storage hierarchy can be differentiated by evaluating certain core characteristics as well as measuring characteristics specific to a particular implementation. These core characteristics are volatility, mutability, accessibility, and addressibility. For any particular implementation of any storage technology, the characteristics worth measuring are capacity and performance.
As of 2008, the most commonly used data storage technologies are semiconductor, magnetic, and optical, while paper still sees some limited usage. Some other fundamental storage technologies have also been used in the past or are proposed for development.
Semiconductor memory uses semiconductor-based integrated circuits to store information. Semiconductor memory is Computer memory implemented on a Semiconductor -based Integrated circuit. A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that Microchipsjpg|right|thumb|200px|Microchips ( EPROM memory with a transparent window showing the integrated circuit inside A semiconductor memory chip may contain millions of tiny transistors or capacitors. In Electronics, a transistor is a Semiconductor device commonly used to amplify or switch electronic signals A capacitor is a passive electrical component that can store Energy in the Electric field between a pair of conductors Both volatile and non-volatile forms of semiconductor memory exist. In modern computers, primary storage almost exclusively consists of dynamic volatile semiconductor memory or dynamic random access memory. Since the turn of the century, a type of non-volatile semiconductor memory known as flash memory has steadily gained share as off-line storage for home computers. Flash memory is non-volatile computer memory that can be electrically erased and reprogrammed Non-volatile semiconductor memory is also used for secondary storage in various advanced electronic devices and specialized computers.
Magnetic storage uses different patterns of magnetization on a magnetically coated surface to store information. Magnetic storage and magnetic recording are terms from Engineering referring to the storage of Data on a Magnetized medium Magnetization is defined as the quantity of Magnetic moment per unit volume In Physics, magnetism is one of the Phenomena by which Materials exert attractive or repulsive Forces on other Materials. Magnetic storage is non-volatile. The information is accessed using one or more read/write heads which may contain one or more recording transducers. A read/write head only covers a part of the surface so that the head or medium or both must be moved relative to another in order to access data. In modern computers, magnetic storage will take these forms:
In early computers, magnetic storage was also used for primary storage in a form of magnetic drum, or core memory, core rope memory, thin film memory, twistor memory or bubble memory. Disk storage is a general category of a Computer storage mechanisms in which data is recorded on planar round and rotating surfaces ( disks, discs, or A floppy disk is an increasingly Obsolete data storage medium that is composed of a disk of thin flexible ("floppy" Magnetic storage medium encased A hard disk drive ( HDD) commonly referred to as a hard drive, hard disk, or fixed disk drive, is a Non-volatile storage device Magnetic tape has been used for Data storage for over 50 years Drum memory is a magnetic Data storage device and was an early form of Computer memory widely used in the 1950s and into the 1960s invented by Gustav Tauschek Magnetic core memory, or ferrite-core memory, is an early form of Random access Computer memory. 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 Twistor is a form of Computer memory, similar to Core memory, formed by wrapping or closing Magnetic tape around a current-carrying wire Prehistory twistor memory Bubble memory is largely the brainchild of a single person Andrew Bobeck. Also unlike today, magnetic tape was often used for secondary storage.
Optical storage, the typical Optical disc, stores information in deformities on the surface of a circular disc and reads this information by illuminating the surface with a laser diode and observing the reflection. Optical storage is a term from Engineering referring to the storage of data on an optically readable medium A laser diode is a Laser where the active medium is a Semiconductor similar to that found in a Light-emitting diode. Optical disc storage is non-volatile. The deformities may be permanent (read only media ), formed once (write once media) or reversible (recordable or read/write media). The following forms are currently in common use:
Magneto-optical disc storage is optical disc storage where the magnetic state on a ferromagnetic surface stores information. A magneto-optical drive is a kind of Optical disc drive capable of writing and rewriting data upon a magneto-optical disc. Ferromagnetism is the basic mechanism by which certain materials (such as Iron) form Permanent magnets and/or exhibit strong interactions with Magnets it The information is read optically and written by combining magnetic and optical methods. Magneto-optical disc storage is non-volatile, sequential access, slow write, fast read storage used for tertiary and off-line storage.
3D optical data storage has also been proposed. 3D optical data storage is the term given to any form of Optical Data storage in which information can be recorded and/or read with three dimensional
Paper data storage, typically in the form of paper tape or punch cards, has long been used to store information for automatic processing, particularly before general-purpose computers existed. Paper data storage refers to the storage on paper of data that can be interpreted by a machine or is the result of the functioning of a machine Punched tape or paper tape is a largely obsolete form of Data storage, consisting of a long strip of paper in which holes are punched to store data Information was recorded by punching holes into the paper or cardboard medium and was read mechanically (or later optically) to determine whether a particular location on the medium was solid or contained a hole.
A secondary or tertiary storage may connect to a computer utilizing computer networks. A computer network is a group of interconnected Computers. Networks may be classified according to a wide variety of characteristics This concept does not pertain to the primary storage, which is shared between multiple processors in a much lesser degree.
Large quantities of individual magnetic tapes, and optical or magneto-optical discs may be stored in robotic tertiary storage devices. In tape storage field they are known as tape libraries, and in optical storage field optical jukeboxes, or optical disk libraries per analogy. In Computer storage, a tape library, sometimes called a tape silo, or tape jukebox, is a storage device which contains one or more Tape drives An optical jukebox is a robotic Data storage device that can automatically load and unload Optical discs such as Compact Disc, DVD, Ultra Smallest forms of either technology containing just one drive device are referred to as autoloaders or autochangers. An autoloader, or autochanger, is a data storage device consisting of at least one Tape drive (the drive a method of loading tapes into the drive (the
Robotic-access storage devices may have a number of slots, each holding individual media, and usually one or more picking robots that traverse the slots and load media to built-in drives. The arrangement of the slots and picking devices affects performance. Important characteristics of such storage are possible expansion options: adding slots, modules, drives, robots. Tape libraries may have from 10 to more than 100,000 slots, and provide terabytes or petabytes of near-line information. Optical jukeboxes are somewhat smaller solutions, up to 1,000 slots.
Robotic storage is used for backups, and for high-capacity archives in imaging, medical, and video industries. In Information technology, backup refers to making copies of Data so that these additional copies may be used to restore the original after a Hierarchical storage management is a most known archiving strategy of automatically migrating long-unused files from fast hard disk storage to libraries or jukeboxes. Hierarchical Storage Management ( HSM) is a data storage technique which automatically moves data between high-cost and low-cost storage media If the files are needed, they are retrieved back to disk.