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Home , Computer data storage

MEMORY Defination

In , memory refers to the of a computing system, as it is kept active in some physical structure. The term "memory" is used for the information in physical systems which are fast (i.e. RAM), as a distinction from physical systems which are slow to access (ie. storage). By design, the term "memory" refers to temporary state devices, whereas the term "storage" is reserved for permanent data. Advances in storage have blurred the distinction a ²memory kept on what is conventionally a storage system is called "".

Colloquially, memory refers to the physical devices used to store data or programs (sequences of instructions) on a temporary or permanent basis for use in an electronic digital computer. represent information in binary , written as sequences of 0s and 1s. Each binary digit (or "bit") may be stored by any physical system that can be in either of two stable states, to represent 0 and 1. Such a system is called bistable. This could be an on-off switch, an electrical capacitor that can store or lose a charge, a magnet with its polarity up or down, or a surface that can have a pit or not. Today, capacitors and transistors, functioning as tiny electrical switches, are used for temporary storage and either disks or tape with a magnetic coating, or discs with patterns of pits are used for long-term storage.

Computer memory is usually meant to refer to the technology that is used to store information in electronic devices. Current primary makes use of integrated circuits consisting of silicon-based transistors. There are two main types of memory: volatile and non- volatile.

Computer

1 GB of SDRAM mounted in a . An example of primary storage.

40 GB PATA (HDD); when connected to a computer it serves as secondarystorage.

160 GB SDLT tape cartridge, an example of off-line storage. When used within a robotic , it is classified

as tertiary storage instead.

Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data used for computing for some interval of time. 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 (CPU, a ), implements the computer model used since the 1940s.

In contemporary usage, memory usually refers to a form of semiconductor storage known as random-access memory, typically DRAM (Dynamic-RAM) but many times other forms of fast but temporary storage. Similarly, storage today more commonly refers to storage devices and their media not directly accessible by the CPU (secondary or tertiary storage) ² typically hard disk drives, drives, and other devices slower than RAM but more permanent.[1] Historically, memory has been called main memory, real storage or internal memory while storage devices have been referred to as secondary storage, external memory or auxiliary/ storage).

Hierarchy of storage

Various forms of storage, divided according to their distance from the central processing unit. The fundamental components of a general-purpose computer are arithmetic and logic unit, control circuitry, storage space, and input/output devices. Technology and capacity as in common home computers around 2005. Primary storage

Primary storage (or main memory or internal memory), often referred to simply as memory, is the only one directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them as required. Any data actively operated on is also stored there in uniform manner.

Historically, early computers used lines, Williams¶s tubes, or rotating magnetic drums as primary storage. By 1954, those unreliable methods were mostly replaced by magnetic core memory. Core memory remained dominant until the 1970s, when advances in technology allowed to become economically competitive.

This led to 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.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:

 Processor registers are located inside the processor. Each register typically holds a word of data (often 32 or 64 ). CPU instructions instruct the arithmetic and logic unit to perform various calculations or other operations on this data (or with the help of it). Registers are the fastest of all forms of computer data storage.  Processor is an intermediate stage between ultra-fast registers and much slower main memory. It's introduced solely to increase performance of the computer. Most actively used information in the main memory is just duplicated in the cache memory, which is faster, but of much lesser capacity. On the other hand it is much slower, but much larger than processor registers. Multi-level hierarchical cache setup is also commonly used²primary cache being smallest, fastest and located inside the processor; secondary cache being somewhat larger and slower.

Secondary storage

Secondary storage (also known as external memory or auxiliary storage), 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 the desired data using intermediate area in primary storage. Secondary storage does not lose the data when the device is powered down²it is non-volatile. Per unit, it is typically also two orders of magnitude less expensive than primary storage. Consequently, modern computer systems typically have two orders of magnitude more secondary storage than primary storage and data is kept for a longer time there.

In modern computers, hard disk drives are usually used as secondary storage. The time taken to access a given of information stored on a hard disk is typically a few thousandths of a second, or . By contrast, the time taken to access a given byte of information stored in memory is measured in billionths of a second, or nanoseconds. This illustrates the very significant access-time difference which distinguishes solid-state memory from rotating devices: hard disks are typically about a million times slower than memory. Rotating devices, such as CD and DVD drives, have even longer access times. With disk drives, once the disk read/write head reaches the proper placement and the data of interest rotates under it, subsequent data on the track are very fast to access. As a result, in order to hide the initial seek time and rotational , data are transferred to and from disks in large contiguous blocks.

Keyboard

In computing, a keyboard is typewriter keyboard, which uses an arrangement of buttons or keys, to act as mechanical levers or electronic switches. After punch cards and tape, interaction via teletype-style keyboards became the main for computers.

Despite the development of alternative input devices, such as the mouse (computing mouse), touch sensitive screens, pen devices, recognition, voice recognition, and improvements in computer speed and memory , the keyboard remains the most commonly used and most versatile device used for direct (human) input into computers.

A keyboard typically has characters engraved or printed on the keys and each press of a key typically corresponds to a single written symbol. However, to produce some symbols requires pressing and holding several keys simultaneously or in sequence. While most keyboard keys produce letters, numbers or signs (characters), other keys or simultaneous key presses can produce actions or computer commands.

Types Standard

Standard "full-travel" alphanumeric keyboards have keys that are on three-quarter inch centers (0.750 inches, 19.05 mm), and have a key travel of at least 0.150 inches (3.81 mm). keyboards, such as the 101-key US traditional keyboards or the 104-key Windows keyboards, include alphabetic characters, punctuation symbols, numbers and a variety of function keys. The internationally- common 102/105 key keyboards have a smaller 'left shift' key and an additional key with some more symbols between that and the letter to its right (usually Z or Y). Also the key is usually shaped differently. Computer keyboards are similar to electric-typewriter keyboards but contain additional keys. -size

Keyboards on and notebook computers usually have a shorter travel distance for the keystroke and a reduced set of keys. They may not have a numerical keypad, and the function keys may be placed in locations that differ from their placement on a standard, full-sized keyboard.

Thumb-sized

Smaller keyboards have been introduced for laptops, PDAs, , or users who have a limited workspace. The size of a standard keyboard is dictated by the practical consideration that the keys must be large enough to be easily pressed by fingers. To reduce the size of the keyboard, the numeric keyboard to the right of the alphabetic keyboard can be removed, or the size of the keys can be reduced, which makes it harder to enter text.

Another way to reduce the size of the keyboard is to reduce the number of keys and use chording keyer, i.e. pressing several keys simultaneously. For example, the GKOS keyboard has been designed for small wireless devices. Other two-handed alternatives more akin to a , such as theAlphaGrip, are also used as a way to input data and text. Another way to reduce the size of a keyboard is to use smaller buttons and pack them closer together. Such keyboards, often called a "thumbboard" (thumbing) are used in some personal digital assistants such as the Palm Treo and BlackBerryand some Ultra-Mobile PCs such as the OQO.

Numeric

Numeric keyboards contain only numbers, mathematical symbols for addition, subtraction, multiplication, and division, a decimal point, and several function keys. They are often used to facilitate data entry with smaller keyboards that do not have a numeric keypad, commonly those of laptop computers. These keys are collectively known as a numeric pad, numeric keys, or a numeric keypad, and it can consist of the following types of keys:

 arithmetic operators such as +, -, *, /  numeric digits 0±9  cursor arrow keys

Non-standard or special-use types Chorded

While other keyboards generally associate one action with each key, chorded keyboards associate actions with combinations of key presses. Since there are many combinations available, chorded keyboards can effectively produce more actions on a board with fewer keys. Court reporters' stenotype use chorded keyboards to enable them to enter text much faster by typing a syllable with each stroke instead of one letter at a time. The fastest typists (as of 2007) use a stenograph, a kind of chorded keyboard used by most court reporters and closed-caption reporters. Some chorded keyboards are also made for use in situations where fewer keys are preferable, such as on devices that can be used with only one hand, and on small mobile devices that don't have room for larger keyboards. Chorded keyboards are less desirable in many cases because it usually takes practice and memorization of the combinations to become proficient.

Software

Software keyboards or On-Screen Keyboards often take the form of computer programs that display an image of a keyboard on the screen. Another input device such as a mouse or a can be used to operate each virtual key to enter text. Software keyboards have become very popular in touchscreen enabled cell phones, due to the additional cost and space requirements of other types of hardware keyboards. Microsoft Windows, Mac OS X, and some varieties of include on-screen keyboards that can be controlled with the mouse.

Foldable Further information: Flexible electronics

A foldable keyboard.

Foldable (also called flexible) keyboards are made of soft plastic or silicone which can be rolled or folded on itself for travel.[2] When in use, these keyboards can conform to uneven surfaces, and are more resistant to liquids than standard keyboards. These can also be connected to portable devices and smartphones. Some models can be fully immersed in water, making them popular in hospitals and laboratories, as they can be disinfected.

Projection/

Projection keyboards project an image of keys, usually with a laser, onto a flat surface. The device then uses a camera or infrared sensor to "watch" where the user's fingers move, and will count a key as being pressed when it "sees" the user's finger touch the projected image. Projection keyboards can simulate a full size keyboard from a very small projector. Because the "keys' are simply projected images, they cannot be felt when pressed. Users of projected keyboards often experience increased discomfort in their fingertips because of the lack of "give" when typing. A flat, non-reflective surface is also required for the keys to be projected onto. Most projection keyboards are made for use with PDAs due to their small form factor.

Optical keyboard technology

Also known as photo-optical keyboard, light responsive keyboard, Photo-electric keyboard and optical key actuation detection technology.

An optical keyboard technology utilizes light emitting devices and photo sensors to optically detect actuated keys. Most commonly the emitters and sensors are located in the perimeter, mounted on a small PCB. The light is directed from side to side of the keyboard interior and it can only be blocked by the actuated keys. Most optical keyboards require at least 2 beams (most commonly vertical beam and horizontal beam) to determine the actuated key. Some optical keyboards use a special key structure that blocks the light in a certain pattern, allowing only one beam per row of keys (most commonly horizontal beam).

Motherboard

A is the central (PCB) in many modern computers and holds many of the crucial components of the system, while providing connectors for other . The motherboard is sometimes alternatively known as the main board, system board

History

Prior to the advent of the , a computer was usually built in a card-cage case or mainframe with components connected by a backplaneconsisting of a set of slots themselves connected with wires; in very old designs the wires were discrete connections between card connector pins, but printed circuit boards soon became the standard practice. The Central Processing Unit, memory and peripherals were housed on individual printed circuit boards which plugged into the backplane Overview

A motherboard, like a backplane, provides the electrical connections by which the other components of the system communicate, but unlike a backplane, it also connects the central processing unit and hosts other subsystems and devices.

A typical desktop computer has its microprocessor, main memory, and other essential components connected to the motherboard. Other components such as external storage, controllers for video display and sound, and peripheral devices may be attached to the motherboard as plug-in cards or via cables, although in modern computers it is increasingly common to integrate some of these peripherals into the motherboard itself.

An important component of a motherboard is the microprocessor's supporting , which provides the supporting interfaces between the CPU and the various buses and external components. This chipset determines, to an extent, the features and capabilities of the motherboard.

Modern include, at a minimum:

 sockets (or slots) in which one or more may be installed[3]  slots into which the system's main memory is to be installed (typically in the form of DIMM modules containing DRAM chips)  a chipset which forms an between the CPU's front-side , main memory, and peripheral buses  non- chips (usually Flash ROM in modern motherboards) containing the system's firmware or BIOS  a clock generator which produces the system clock signal to synchronize the various components  slots for expansion cards (these interface to the system via the buses supported by the chipset)  power connectors, which receive electrical power from the computer power supply and distribute it to the CPU, chipset, main memory, and expansion cards.[4]

CD-ROM

CD-ROM (pronounced /ÚsiÚÚdiÚÚrMm/, an acronym of " read-only memory") is a pre- pressed compact disc that contains data accessible to, but not writable by, a computer for data storage and music playback. The 1985 ³Yellow Book´ standard developed by Sony and Philips adapted the format to hold any form of binary data

CD-ROMs are popularly used to distribute computer software, including games and applications, though any data can be stored (up to the capacity limit of a disc). Some CDs hold both computer data and audio with the latter capable of being played on a CD player, while data (such as software or ) is only usable on a computer (such as ISO 9660 format PC CD-ROMs). These are called enhanced CDs.

CD-ROM format

A CD-ROM sector contains 2,352 , divided into 98 24-byte frames. Unlike a music CD, a CD-ROM cannot rely on error concealment by interpolation, and therefore requires a higher reliability of the retrieved data. In order to achieve improved error correction and detection, a CD-ROM has a third layer of Reed±Solomon error correction.[4] A Mode-1 CD-ROM, which has the full three layers of error correction data, contains a net 2,048 bytes of the available 2,352 per sector. In a Mode-2 CD-ROM, which is mostly used for video files, there are 2,336 user-available bytes per sector. The net byte rate of a Mode-1 CD-ROM, based on comparison to CDDA audio standards, is 44100 Hz × 16 bits/sample × 2 channels × 2,048 / 2,352 /8 = 153.6 kB/s = 150 KiB/s. The playing time is 74 minutes, or 4,440 seconds, so that the net capacity of a Mode-1 CD-ROM is 682 MB or, equivalently, 650 MiB.

A 1× speed CD drive reads 75 consecutive sectors per second.

Magnetic tape

Magnetic tape is a medium for magnetic recording, made of a thin magnetizable coating on a long, narrow strip of plastic. It was developed in Germany, based on magnetic . Devices that record and play back audio and video using magnetic tape are tape recorders and video tape recorders. A device that stores computer data on magnetic tape is a (tape unit, streamer).

Magnetic tape revolutionized broadcast and recording. When all radio was live, it allowed programming to be prerecorded. At a time when gramophone records were recorded in one take, it allowed recordings in multiple parts, which mixed and edited with tolerable loss in quality. It is a key technology in early computer development, allowing unparalleled amounts of data to be mechanically created, stored for long periods, and to be rapidly accessed.

Data storage

In all tape formats, a tape drive (or "transport" or "deck") uses motors to wind the tape from one reel to another, passing tape heads to read, write or erase as it moves. Magnetic tape was first used to record computer data in 1951 on the Eckert-Mauchly UNIVAC I. The recording medium was a thin strip of one half inch (12.65 mm) wide metal, consisting of nickel-plated bronze (called Vicalloy). Recording density was 128 characters per inch (198 micrometre/character) on eight tracks.

Early IBM tape drives were floor-standing drives that used vacuum columns to physically buffer long U- shaped loops of tape. The two tape reels visibly fed tape through the columns, intermittently spinning the reels in rapid, unsynchronized bursts, resulting in visually-striking action. Stock shots of such vacuum- column tape drives in motion were widely used to represent "the computer" in movies and television.

Most modern magnetic tape systems use reels that are much smaller than the 10.5 inch open reels and are fixed inside a cartridge to protect the tape and facilitate handling. Many late 1970s and early 1980s home computers used Compact Cassettes encoded with the Kansas City standard. Modern cartridge formats include LTO, DLT, and DAT/DDC.

Tape remains a viable alternative to disk in some situations due to its lower cost per bit. Though the areal density of tape is lower than for disk drives, the available surface area on a tape is far greater. The highest capacity tape media are generally on the same order as the largest available disk drives (about 3 TB in 2010). Tape has historically offered enough advantage in cost over to make it a viable product, particularly for , where media removability is necessary.

In 2002, Imation received a US$11.9 million grant from the U.S. National Institute of Standards and Technology for research into increasing the data capacity of magnetic tape.[1]

Hard disk drive

A hard disk drive[2] (hard disk,[3] hard drive,[4] HDD) is a non-volatile storage device for digital data. It features one or more rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is encoded magnetically by read/write heads that float on a cushion of air above the platters.

Hard disk manufacturers quote disk capacity in SI-standard powers of 1000, wherein a terabyte is 1000 and a is 1000 . Withfile systems that report capacity in powers of 1024, available space appears somewhat less than advertised capacity.

Technology

HDDs record data by magnetizing ferromagnetic material directionally. Sequential changes in the direction of represent patterns of binary data bits. The data is read from the disk by detecting the transitions in magnetization and decoding the originally written data. Different encoding schemes, such as Modified Frequency Modulation, Group code recording, Run-length limited encoding, and others are used. A typical HDD design consists of a spindle that holds one or more flat circular disks called platters, onto which the data is recorded. The platters are made from a non-magnetic material, usually aluminum alloy or glass, and are coated with a thin layer of magnetic material, typically 10±20 nm in thickness ² for reference, standard copy paper is 0.07±0.18 millimetre (70,000±180,000 nm) thick[7] ² with an outer layer of carbon for protection. Older disks used iron(III) oxide as the magnetic material, but current disks use a cobalt-based alloy.[8]

The platters are spun at very high speeds. Information is written to, and read from a platter as it rotates past devices called read-and-write heads that operate very close (tens of nanometers in new drives) over the magnetic surface. The read-and-write head is used to detect and modify the magnetization of the material immediately under it.