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