Reading is an action performed by

computers A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These prog ...

, to acquire data from a source and place it into their volatile memory for processing. Computers may read information from a variety of sources, such as magnetic storage, the Internet, or

audio Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to: Sound *Audio signal, an electrical representation of sound *Audio frequency, a frequency in the audio spectrum * Digital audio, representation of sou ...

and video input

ports A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as H ...

. Reading is one of the core functions of a Turing machine. A read cycle is the act of reading one unit of information (e.g. a byte). A read channel is an electrical circuit that transforms the physical magnetic flux changes into abstract bits. A read error occurs when the physical part of the process fails for some reason, such as dust or dirt entering the drive.

Example

For example, a computer may read information off a floppy disk and store it temporarily in random-access memory before it is written to the

hard drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magneti ...

to be processed at a future date.

Memory types

CMOS

Complementary metal–oxide–semiconductor (CMOS) is a non-volatile medium. It is used in microprocessors, microcontrollers,

static RAM Static random-access memory (static RAM or SRAM) is a type of random-access memory (RAM) that uses latching circuitry (flip-flop) to store each bit. SRAM is volatile memory; data is lost when power is removed. The term ''static'' differen ...

, and other digital logic circuits. Memory is read through the use of a combination of p-type and n-type metal–oxide–semiconductor field-effect transistors (MOSFETs). In CMOS logic, a collection of n-type MOSFETs are arranged in a pull-down network between the output

node In general, a node is a localized swelling (a "knot") or a point of intersection (a vertex). Node may refer to: In mathematics *Vertex (graph theory), a vertex in a mathematical graph *Vertex (geometry), a point where two or more curves, lines, ...

and the lower-voltage power supply rail, named V_ss, which often has

ground Ground may refer to: Geology * Land, the surface of the Earth not covered by water * Soil, a mixture of clay, sand and organic matter present on the surface of the Earth Electricity * Ground (electricity), the reference point in an electrical c ...

potential. By asserting or de-asserting the inputs to the CMOS circuit, individual transistors along the pull-up and pull-down networks become

conductive In physics and electrical engineering, a conductor is an object or type of material that allows the flow of charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. Electric current is gener ...

and

resistive The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallel ...

to electric current, and results in the desired path connecting from the output node to one of the voltage rails.

Flash

Flash memory stores information in an array of memory cells made from floating-gate transistors. Flash memory utilizes either NOR logic or NAND logic. In

NOR gate The NOR gate is a digital logic gate that implements logical NOR - it behaves according to the truth table to the right. A HIGH output (1) results if both the inputs to the gate are LOW (0); if one or both input is HIGH (1), a LOW output (0 ...

flash, each cell resembles a standard

MOSFET The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...

, except the transistor has two gates instead of one. On top is the control gate (CG), as in other MOS transistors, but below this, there is a floating gate (FG) insulated all around by an oxide layer. The FG is interposed between the CG and the MOSFET channel, and because the FG is electrically isolated by its insulating layer, any electrons placed on it are trapped there and, under normal conditions, will not discharge for many years. When current flow through the MOSFET channel

binary code A binary code represents text, computer processor instructions, or any other data using a two-symbol system. The two-symbol system used is often "0" and "1" from the binary number system. The binary code assigns a pattern of binary digits, also ...

is generated, reproducing the stored data. NAND gate flash utilizes tunnel injection for writing and tunnel release for erasing. NAND flash memory forms the core of the removable

USB Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and power supply ( interfacing) between computers, peripherals and other computers. A broad ...

storage devices known as

USB flash drive A USB flash drive (also called a thumb drive) is a data storage device that includes flash memory with an integrated USB interface. It is typically removable, rewritable and much smaller than an optical disc. Most weigh less than . Since first ...

s, as well as most

memory card A memory card is an electronic data storage device used for storing digital information, typically using flash memory. These are commonly used in digital portable electronic devices. They allow adding memory to such devices using a card in a soc ...

formats available today.

Magnetic

The magnetic medium is found in magnetic tape, hard disk drives, floppy disks, and so on. This medium uses different patterns of magnetization in a magnetizable material to store data and is a form of

non-volatile memory Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retain stored information even after power is removed. In contrast, volatile memory needs constant power in order to retain data. Non-volatile memory typic ...

. Magnetic storage media can be classified as either sequential access memory or random-access memory although in some cases the distinction is not perfectly clear. Small polarized ferrous cores in the shape of wires or poles are flipped along the surface of reading and writing into the desired data is stored. Accessing different parts of the medium involves winding the wire forward or backward until the point of interest is found. Data is read through a small reading peripheral which amplifies which side the ferrous cores are arranged.

Mechanical

The mechanical medium utilizes one of the oldest methods of computing and has largely become obsolete. The earliest known method of memory storage and subsequent computerized reading is the

Antikythera mechanism The Antikythera mechanism ( ) is an Ancient Greek hand-powered orrery, described as the oldest example of an analogue computer used to predict astronomical positions and eclipses decades in advance. It could also be used to track the four-yea ...

(c. 100–150

BCE Common Era (CE) and Before the Common Era (BCE) are year notations for the Gregorian calendar (and its predecessor, the Julian calendar), the world's most widely used calendar era. Common Era and Before the Common Era are alternatives to the o ...

) which utilizes over thirty gears that spin a dial indicator. Following the Antikythera mechanism,

Hero of Alexandria Hero of Alexandria (; grc-gre, Ἥρων ὁ Ἀλεξανδρεύς, ''Heron ho Alexandreus'', also known as Heron of Alexandria ; 60 AD) was a Greek mathematician and engineer who was active in his native city of Alexandria, Roman Egypt. He ...

(c. 10–70 CE) designed an entirely mechanical play almost ten minutes in length, powered by a binary-like system of ropes, knots, and simple machines operated by a rotating cylindrical cogwheel.

Punched cards A punched card (also punch card or punched-card) is a piece of stiff paper that holds digital data represented by the presence or absence of holes in predefined positions. Punched cards were once common in data processing applications or to di ...

became popularized from 1900 to 1950 as it became the most common storage medium for computers. The information was read through a method of identifying the holes in the card which was then converted to another medium.

Peripherals

The usage of electronic computer peripherals became popularized as means of linking two computing devices. Information is sent in electronic singles which are interpreted by a Multiplexer or multiplexer.

Optical discs

Optical discs refer to the non-volatile flat, circular, usually polycarbonate discs. Data is stored in pits or bumps arranged sequentially on the continuous, spiral track extending from the innermost track to the outermost track, covering the entire disc surface. Data is read by a means of a laser; when the laser enters a pit, the focus of the laser is changed and interpenetrated by the reader's software.

Random-access memory

Random-access memory (RAM) is a form of computer data storage. A random-access device allows stored data to be accessed directly in any random order. In contrast, other data storage media such as hard disks, CDs, DVDs and magnetic tape, as well as early primary memory types such as drum memory, read and write data only in a predetermined order, consecutively, because of mechanical design limitations. Therefore, the time to access a given data location varies significantly depending on its physical location. Today, random-access memory takes the form of integrated circuits. Strictly speaking, modern types of DRAM are not random access, as data is read in bursts, although the name DRAM / RAM has stuck. However, many types of SRAM, ROM, OTP, and NOR flash are still random access even in a strict sense. RAM is normally associated with volatile types of memory (such as DRAM memory modules), where its stored information is lost if the power is removed. Many other types of non-volatile memory are RAM as well, including most types of ROM and a type of flash memory called NOR-Flash. The first RAM modules to come into the market were created in 1951 and were sold until the late 1960s and early 1970s.

References

{{reflist Computing terminology