Reading (computer)
Reading, as computers perform it, means acquiring data from a source and placing it into volatile memory for processing. That simple act underpins every computation ever run, from the earliest geared mechanisms to the chips inside a modern phone. At its core, reading is one of the fundamental operations of a Turing machine, the theoretical model that defines what a computer can do. The questions worth asking are: how do machines actually accomplish this, and what does it look like across such different physical materials as magnetized metal, light-sensitive plastic, and silicon transistors?
A read cycle is the act of reading one unit of information, such as a single byte. That unit moves through a read channel, which is an electrical circuit. The channel converts physical magnetic flux changes into abstract bits that software can use. When that physical process breaks down, a read error results. Dust or dirt entering a drive is a common cause. The gap between the physical world and the logical world of bits is always where failure hides.
Complementary metal-oxide-semiconductor technology, known as CMOS, is a non-volatile medium found in microprocessors, microcontrollers, and static RAM. Reading in CMOS relies on combinations of p-type and n-type metal-oxide-semiconductor field-effect transistors, called MOSFETs. A collection of n-type MOSFETs sits in a pull-down network between the output node and the lower-voltage power supply rail, called Vss, which often sits at ground potential. By asserting or de-asserting inputs, individual transistors along the pull-up and pull-down networks become conductive or resistive, directing current to one of the voltage rails and producing a readable output.
Flash memory takes a different approach, storing information in arrays of memory cells built from floating-gate transistors. Each NOR-gate flash cell resembles a standard MOSFET except it has two gates rather than one. A control gate sits on top; below it, a floating gate is insulated on all sides by an oxide layer. Because the floating gate is electrically isolated, electrons placed on it are trapped and will not discharge for many years under normal conditions. When current flows through the MOSFET channel, binary code is generated, reproducing the stored data. NAND gate flash uses tunnel injection for writing and tunnel release for erasing, and it forms the core of USB flash drives as well as most memory card formats available today.
Magnetic storage appears in hard disk drives, floppy disks, and magnetic tape. The medium encodes data as different patterns of magnetization in a magnetizable material, making it a form of non-volatile memory. Magnetic storage can be classified as either sequential access or random-access memory, depending on how the device reads data. Magnetic-core memory uses toroids, which are rings of a hard magnetic material, usually a semi-hard ferrite, arranged as transformer cores. Two or more wires pass through each core, and magnetic hysteresis allows each core to store a state. Reading information from a floppy disk and storing it temporarily in random-access memory before writing it to a hard drive is one textbook example of how read operations chain together within a single machine.
The mechanical medium represents one of the oldest approaches to reading stored data, and it has largely become obsolete. The earliest known method of memory storage and computerized reading is the Antikythera mechanism, dated to roughly 100-150 BCE, which uses more than thirty gears to spin a dial indicator. Hero of Alexandria, working around 10-70 CE, designed an entirely mechanical theatrical play almost ten minutes long. That production ran on a binary-like system of ropes, knots, and simple machines driven by a rotating cylindrical cogwheel. Punched cards served as a common storage medium from 1900 to 1950, with information read by identifying holes in the card.
Optical discs replaced punched cards for many uses. These are flat, circular, usually polycarbonate discs that store data in pits or bumps arranged sequentially on a continuous spiral track running from the innermost to the outermost edge, covering the entire disc surface. A laser reads that track; when it enters a pit, the focus of the laser changes, and the reader's software interprets that change as data.
Random-access memory, or RAM, allows stored data to be reached directly in any order. That distinguishes it from devices such as hard disks, CDs, DVDs, and magnetic tape, where data is read only in a predetermined consecutive order because of mechanical design limits. Drum memory, an early primary memory type, shared that sequential constraint. The time to access a given location on a sequential device varies significantly depending on its physical position on the medium.
Modern RAM takes the form of integrated circuits. Strictly speaking, modern DRAM is not fully random access, because data is read in bursts, though the name has remained. Many types of SRAM, ROM, OTP, and NOR-Flash are still random access in the strict sense. RAM is normally associated with volatile memory, where information is lost when power is removed. Non-volatile RAM also exists, including most types of ROM and NOR-Flash. The first RAM modules to reach the market appeared in 1951 and were sold until the late 1960s and early 1970s, marking a period when the modern shape of computer memory began to settle into familiar form.
Common questions
What does reading mean in the context of a computer?
Reading, as a computer operation, is the act of acquiring data from a source and placing it into volatile memory for processing. Computers can read from sources including magnetic storage, the Internet, and audio and video input ports. Reading is one of the core functions of a Turing machine.
What is a read cycle and a read error in computing?
A read cycle is the act of reading one unit of information, such as a single byte. A read error occurs when the physical part of the reading process fails, commonly due to dust or dirt entering a drive.
How does CMOS memory reading work?
CMOS memory is read through combinations of p-type and n-type MOSFETs arranged in pull-up and pull-down networks. Asserting or de-asserting inputs makes individual transistors conductive or resistive, directing current to a voltage rail and producing a readable output.
How does flash memory store and read data?
Flash memory stores data in arrays of floating-gate transistors. In NOR-gate flash, electrons are trapped on an electrically isolated floating gate and will not discharge for many years under normal conditions; current flowing through the MOSFET channel generates binary code that reproduces the stored data. NAND flash, which uses tunnel injection for writing, forms the core of USB flash drives and most memory card formats.
What is the oldest known mechanical reading device?
The Antikythera mechanism, dated to approximately 100-150 BCE, is the earliest known method of memory storage and computerized reading. It uses more than thirty gears to spin a dial indicator.
When were the first RAM modules sold commercially?
The first RAM modules to reach the market appeared in 1951 and were sold until the late 1960s and early 1970s.
All sources
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