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