# Reading (computer) > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Reading_(computer) > Markdown URL: https://mediated.wiki/source/Reading_(computer).md > Source: https://en.wikipedia.org/wiki/Reading_(computer) > Source revision: 1355067847 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) For other uses, see [Reading (disambiguation)](/source/Reading_(disambiguation)). Action performed by computers This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Reading" computer – news · newspapers · books · scholar · JSTOR (February 2024) (Learn how and when to remove this message) **Reading** is an action performed by [computers](/source/Computers), to acquire [data](/source/Data) from a source and place it into their [volatile memory](/source/Volatile_memory) for [processing](/source/Process_(computing)). Computers may read information from a variety of sources, such as [magnetic storage](/source/Magnetic_storage), the [Internet](/source/Internet), or [audio](/source/Sound) and [video](/source/Video) input [ports](/source/Computer_port_(hardware)). Reading is one of the core functions of a [Turing machine](/source/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](/source/Floppy_disk) and store it temporarily in [random-access memory](/source/Random-access_memory) before it is written to the [hard drive](/source/Hard_drive) to be processed at a future date. ## Memory types ### CMOS Main article: [CMOS](/source/CMOS) Complementary metal–oxide–semiconductor (CMOS) is a [non-volatile medium](/source/Non-volatile_memory).[1] It is used in [microprocessors](/source/Microprocessor), [microcontrollers](/source/Microcontroller), [static RAM](/source/Static_RAM), and other [digital logic](/source/Digital_logic) circuits. Memory is read through the use of a combination of p-type and n-type [metal–oxide–semiconductor field-effect transistors](/source/Metal%E2%80%93oxide%E2%80%93semiconductor_field-effect_transistor) (MOSFETs). In CMOS logic, a collection of n-type MOSFETs are arranged in a [pull-down](/source/Pull-up_resistor) [network](/source/Electrical_network) between the output [node](/source/Node_(circuits)) and the lower-voltage [power supply rail](/source/Voltage_source), named [Vss](/source/IC_power_supply_pin), which often has [ground](/source/Ground_(electricity)) potential. By asserting or de-asserting the inputs to the CMOS circuit, individual transistors along the pull-up and pull-down networks become [conductive](/source/Conductive) and [resistive](/source/Resistivity) to electric current, and results in the desired path connecting from the output node to one of the voltage rails. ### Flash Main article: [Flash memory](/source/Flash_memory) Flash memory stores information in an array of memory cells made from [floating-gate transistors](/source/Floating-gate_transistor). Flash memory utilizes either NOR logic or NAND logic. In [NOR gate](/source/NOR_gate) flash, each cell resembles a standard [MOSFET](/source/MOSFET), 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](/source/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](/source/Electron) 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](/source/Binary_code) is generated, reproducing the stored [data](/source/Data). [NAND gate](/source/Sheffer_stroke#NAND_gate) flash utilizes [tunnel injection](/source/Tunnel_injection) for writing and [tunnel release](/source/Tunnel_release) for erasing. NAND flash memory forms the core of the removable [USB](/source/USB) storage devices known as [USB flash drives](/source/USB_flash_drive), as well as most [memory card](/source/Memory_card) formats available today. ### Magnetic Main article: [Magnetic storage](/source/Magnetic_storage) The magnetic medium is found in magnetic tape, hard disk drives, floppy disks, and so on. This medium uses different patterns of [magnetization](/source/Magnetization) in a magnetizable material to store data and is a form of [non-volatile memory](/source/Non-volatile_memory). Magnetic storage media can be classified as either [sequential access memory](/source/Sequential_access_memory) or [random-access memory](/source/Random-access_memory). [Magnetic-core memory](/source/Magnetic-core_memory) uses toroids (rings) of a hard magnetic material (usually a semi-hard ferrite) as transformer cores, where each wire threaded through the core serves as a transformer winding. Two or more wires pass through each core. Magnetic hysteresis allows each of the cores to store a state. ### 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](/source/Antikythera_mechanism) (c. 100–150 [BCE](/source/BCE)) which utilizes over thirty gears that spin a dial indicator. Following the Antikythera mechanism, [Hero of Alexandria](/source/Hero_of_Alexandria) (c. 10–70 [CE](/source/Common_Era)) 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](/source/Punched_cards) were a common storage medium for computers from 1900 to 1950. The information was read through a method of identifying the holes in the card. ### Optical discs Main article: [Optical disc](/source/Optical_disc) Optical discs refer to the [non-volatile](/source/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](/source/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. ## See also - [Computer storage](/source/Computer_storage) - [Optical character recognition](/source/Optical_character_recognition) - [Optical storage media writing and reading speed](/source/Optical_storage_media_writing_and_reading_speed) - [Turing machine](/source/Turing_machine) ## References 1. **[^](#cite_ref-1)** Durini, Daniel. *[High Performance Silicon Imaging: Fundamentals and Applications of CMOS and CCD Sensors](https://books.google.com/books?id=uMe3DwAAQBAJ&dq=Complementary%20metal%E2%80%93oxide%E2%80%93semiconductor&pg=PA95)* Woodhead Publishing, 2019. 95. --- Adapted from the Wikipedia article [Reading (computer)](https://en.wikipedia.org/wiki/Reading_(computer)) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Reading_(computer)?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.