| Computer storage, computer memory, and
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| | time.)
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| often casually memory refer to computer
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| | Secondary and off-line storage
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| components, devices and recording media
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| | Secondary storage requires the computer
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| that retain data for some interval of
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| | to use its input/output channels to
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| time. Computer storage provides one of
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| | access the information, and is used for
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| the core functions of the modern
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| | long-term storage of persistent
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| computer, that of information retention.
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| | information. However most computer
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| It is one of the fundamental components
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| | operating systems also use secondary
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| of all modern computers, and coupled with
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| | storage devices as virtual memory - to
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| a central processing unit (CPU),
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| | artificially increase the apparent amount
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| implements the basic Von Neumann computer
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| | of main memory in the computer. Secondary
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| model used since the 1940s.
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| | storage is also known as "mass storage",
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| In contemporary usage, memory usually
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| | as shown in the diagram above. Secondary
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| refers to a form of solid state storage
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| | or mass storage is typically of much
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| known as random access memory (RAM) and
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| | greater capacity than primary storage
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| sometimes other forms of fast but
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| | (main memory), but it is also much
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| temporary storage. Similarly, storage
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| | slower. In modern computers, hard disks
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| more commonly refers to mass storage -
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| | are usually used for mass storage. The
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| optical discs, forms of magnetic storage
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| | time taken to access a given byte of
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| like hard disks, and other types of
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| | information stored on a hard disk is
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| storage which are slower than RAM, but of
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| | typically a few thousandths of a second,
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| a more permanent nature. These
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| | or milliseconds. By contrast, the time
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| contemporary distinctions are helpful,
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| | taken to access a given byte of
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| because they are also fundamental to the
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| | information stored in random access
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| architecture of computers in general. As
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| | memory is measured in thousand-millionths
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| well, they reflect an important and
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| | of a second, or nanoseconds. This
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| significant technical difference between
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| | illustrates the very significant speed
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| memory and mass storage devices, which
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| | difference which distinguishes
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| has been blurred by the historical usage
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| | solid-state memory from rotating magnetic
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| of the terms "main storage" (and
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| | storage devices: hard disks are typically
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| sometimes "primary storage") for random
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| | about a million times slower than memory.
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| access memory, and "secondary storage"
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| | Rotating optical storage devices, such as
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| for mass storage devices. This is
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| | CD and DVD drives, are typically even
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| explained in the following sections, in
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| | slower than hard disks, although their
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| which the traditional "storage" terms are
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| | access speeds are likely to improve with
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| used as sub-headings for convenience.
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| | advances in technology. Therefore, the
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| The fundamental components of a
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| | use of virtual memory, which is millions
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| general-purpose computer are arithmetic
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| | of times slower than "real" memory,
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| and logic unit, control circuitry,
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| | significantly degrades the performance of
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| storage space, and input/output devices.
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| | any computer. Virtual memory is
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| If storage was removed, the device we had
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| | implemented by many operating systems
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| would be a simple digital signal
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| | using terms like swap file or "cache
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| processing device (e.g. calculator, media
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| | file". The main historical advantage of
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| player) instead of a computer. The
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| | virtual memory was that it was much less
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| ability to store instructions that form a
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| | expensive than real memory. That
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| computer program, and the information
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| | advantage is less relevant today, yet
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| that the instructions manipulate is what
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| | surprisingly most operating systems
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| makes stored program architecture
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| | continue to implement it, despite the
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| computers versatile.
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| | significant performance penalties.
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| A digital computer represents information
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| | Off-line storage is a system where the
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| using the binary numeral system. Text,
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| | storage medium can be easily removed from
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| numbers, pictures, audio, and nearly any
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| | the storage device. Off-line storage is
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| other form of information can be
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| | used for data transfer and archival
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| converted into a string of bits, or
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| | purposes. In modern computers, CDs, DVDs,
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| binary digits, each of which has a value
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| | memory cards, flash memory devices
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| of 1 or 0. The most common unit of
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| | including "USB drives", floppy disks, Zip
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| storage is the byte, equal to 8 bits. A
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| | disks and magnetic tapes are commonly
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| piece of information can be manipulated
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| | used for off-line mass storage purposes.
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| by any computer whose storage space is
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| | "Hot-pluggable" USB hard disks are also
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| large enough to accommodate the
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| | available. Off-line storage devices used
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| corresponding data, or the binary
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| | in the past include punched cards,
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| representation of the piece of
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| | microforms, and removable Winchester disk
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| information. For example, a computer with
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| | drums.
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| a storage space of eight million bits, or
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| | Tertiary and database storage
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| one megabyte, could be used to edit a
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| | Tertiary storage is a system where a
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| small novel.
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| | robotic arm will "mount" (connect) or
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| Various forms of storage, based on
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| | "dismount" off-line mass storage media
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| various natural phenomena, have been
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| | (see the next item) according to the
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| invented. So far, no practical universal
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| | computer operating system's demands.
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| storage medium exists, and all forms of
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| | Tertiary storage is used in the realms of
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| storage have some drawbacks. Therefore a
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| | enterprise storage and scientific
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| computer system usually contains several
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| | computing on large computer systems and
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| kinds of storage, each with an individual
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| | business computer networks, and is
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| purpose, as shown in the diagram.
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| | something a typical personal computer
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| Primary storage
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| | user never sees firsthand.
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| Primary storage is directly connected to
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| | Database storage is a system where
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| the central processing unit of the
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| | information in computers is stored in
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| computer. It must be present for the CPU
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| | large databases, data banks, data
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| to function correctly, just as in a
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| | warehouses, or data vaults. It involves
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| biological analogy the lungs must be
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| | packing and storing large amounts of
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| present (for oxygen storage) for the
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| | storage devices throughout a series of
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| heart to function (to pump and oxygenate
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| | shelves in a room, usually an office, all
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| the blood). As shown in the diagram,
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| | linked together. The information in
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| primary storage typically consists of
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| | database storage systems can be accessed
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| three kinds of storage:
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| | by a supercomputer, mainframe computer,
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| Processor registers are internal to the
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| | or personal computer. Databases, data
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| central processing unit. Registers
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| | banks, and data warehouses, etc, can only
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| contain information that the arithmetic
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| | be accessed by authorized users.
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| and logic unit needs to carry out the
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| | Network storage
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| current instruction. They are technically
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| | Network storage is any type of computer
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| the fastest of all forms of computer
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| | storage that involves accessing
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| storage, being switching transistors
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| | information over a computer network.
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| integrated on the CPU's silicon chip, and
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| | Network storage arguably allows to
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| functioning as electronic "flip-flops".
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| | centralize the information management in
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| Cache memory is a special type of
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| | an organization, and to reduce the
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| internal memory used by many central
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| | duplication of information. Network
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| processing units to increase their
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| | storage includes:
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| performance or "throughput". Some of the
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| | Network-attached storage is secondary or
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| information in the main memory is
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| | tertiary storage attached to a computer
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| duplicated in the cache memory, which is
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| | which another computer can access at file
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| slightly slower but of much greater
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| | level over a local-area network, a
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| capacity than the processor registers,
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| | private wide-area network, or in the case
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| and faster but much smaller than main
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| | of online file storage, over the
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| memory. Multi-level cache memory is also
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| | Internet.
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| commonly used—"primary cache" being
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| | Storage area network provides other
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| smallest, fastest and closest to the
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| | computers with storage capacity over a
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| processing device; "secondary cache"
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| | network, the crucial difference between
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| being larger and slower, but still faster
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| | network-attached storage (NAS) and
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| and much smaller than main memory.
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| | storage area Networks (SAN) is the former
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| Main memory contains the programs that
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| | presents and manages file systems to
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| are currently being run and the data the
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| | client computers, whilst a SAN provides
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| programs are operating on. In modern
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| | access to disks at block addressing
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| computers, the main memory is the
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| | level, leaving it to attaching systems to
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| electronic solid-state random access
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| | manage data or file systems within the
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| memory. It is directly connected to the
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| | provided capacity.
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| CPU via a "memory bus" (shown in the
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| | Network computers are computers that do
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| diagram) and a "data bus". The arithmetic
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| | not contain internal secondary storage
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| and logic unit can very quickly transfer
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| | devices. Instead, documents and other
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| information between a processor register
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| | data are stored on a network-attached
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| and locations in main storage, also known
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| | storage.
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| as a "memory addresses". The memory bus
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| | Confusingly, these terms are sometimes
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| is also called an address bus or front
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| | used differently. Primary storage can be
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| side bus and both busses are high-speed
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| | used to refer to local random-access disk
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| digital "superhighways". Access methods
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| | storage, which should properly be called
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| and speed are two of the fundamental
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| | secondary storage. If this type of
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| technical differences between memory and
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| | storage is called primary storage, then
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| mass storage devices. (Note that all
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| | the term secondary storage would refer to
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| memory sizes and storage capacities shown
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| | offline, sequential-access storage like
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| in the diagram will inevitably be
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| | tape media.
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| exceeded with advances in technology over
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|
