Editing Bit (section)

== Physical representation <span class="anchor" id="Representation"></span> ==
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A bit can be stored by a digital device or other physical system that exists in either of two possible distinct [[state (computer science)|states]]. These may be the two stable states of a [[Flip-flop (electronics)|flip-flop]], two positions of an [[Switch|electrical switch]], two distinct [[voltage]] or [[electric current|current]] levels allowed by a [[electrical circuit|circuit]], two distinct levels of [[Irradiance|light intensity]], two directions of [[magnetism|magnetization]] or [[electrical polarity|polarization]], the orientation of reversible double stranded [[DNA]], etc.

Perhaps the earliest example of a binary storage device was the [[punched card]] invented by [[Basile Bouchon]] and Jean-Baptiste Falcon (1732), developed by [[Joseph Marie Jacquard]] (1804), and later adopted by [[Semyon Korsakov]], [[Charles Babbage]], [[Herman Hollerith]], and early computer manufacturers like [[IBM]]. A variant of that idea was the perforated [[paper tape]]. In all those systems, the medium (card or tape) conceptually carried an array of hole positions; each position could be either punched through or not, thus carrying one&nbsp;bit of information. The encoding of text by bits was also used in [[Morse code]] (1844) and early digital communications machines such as [[Teleprinter|teletypes]] and [[stock ticker machine]]s (1870).

The first electrical devices for discrete logic (such as [[elevator]] and [[traffic light]] control [[Electronic circuit|circuits]], [[telephone switches]], and Konrad Zuse's computer) represented bits as the states of [[electrical relay]]s which could be either "open" or "closed". These relays functioned as mechanical switches, physically toggling between states to represent binary data, forming the fundamental building blocks of early computing and control systems. When relays were replaced by [[vacuum tube]]s, starting in the 1940s, computer builders experimented with a variety of storage methods, such as pressure pulses traveling down a [[mercury delay line]], charges stored on the inside surface of a [[cathode-ray tube]], or opaque spots printed on [[optical disc|glass discs]] by [[photolithographic]] techniques.

In the 1950s and 1960s, these methods were largely supplanted by [[magnetic storage]] devices such as [[magnetic-core memory]], [[magnetic tape]]s, [[magnetic drum|drums]], and [[Disk storage|disks]], where a bit was represented by the polarity of [[magnetism|magnetization]] of a certain area of a [[ferromagnetic]] film, or by a change in polarity from one direction to the other. The same principle was later used in the [[magnetic bubble memory]] developed in the 1980s, and is still found in various [[magnetic strip]] items such as [[Rapid transit|metro]] tickets and some [[credit card]]s.

In modern [[semiconductor memory]], such as [[dynamic random-access memory]] or a [[solid-state drive]], the two values of a bit are represented by two levels of [[electric charge]] stored in a [[capacitor]] or a [[floating-gate MOSFET]]. In certain types of [[programmable logic array]]s and [[read-only memory]], a bit may be represented by the presence or absence of a conducting path at a certain point of a circuit. In [[optical disc]]s, a bit is encoded as the presence or absence of a [[microscopic]] pit on a reflective surface. In one-dimensional [[bar code]]s and two-dimensional [[QR codes]], bits are encoded as lines or squares which may be either black or white.

In modern digital computing, bits are transformed in Boolean [[logic gate]]s.

=== Transmission and processing ===
Bits are transmitted one at a time in [[serial transmission]]. By contrast, multiple bits are transmitted simultaneously in a [[parallel transmission]]. A [[serial computer]] processes information in either a bit-serial or a byte-serial fashion.  From the standpoint of data communications, a byte-serial transmission is an 8-way parallel transmission with binary signalling.

In programming languages such as [[C (programming language)|C]], a [[bitwise operation]] operates on binary strings as though they are vectors of bits, rather than interpreting them as [[binary number]]s.

Data transfer rates are usually measured in decimal SI multiples.  For example, a [[channel capacity]] may be specified as 8 kbit/s = 1 kB/s.

=== Storage ===

File sizes are often measured in (binary) IEC multiples of bytes, for example 1 KiB = 1024 bytes = 8192 bits.  Confusion may arise in cases where (for historic reasons) filesizes are specified with binary multipliers using the ambiguous prefixes K, M, and G rather than the IEC standard prefixes Ki, Mi, and Gi.<ref>{{cite web |title=UnitsPolicy - Ubuntu Wiki |url=https://wiki.ubuntu.com/UnitsPolicy |access-date=7 January 2025}}</ref>

Mass storage devices are usually measured in decimal SI multiples, for example 1 TB = <math>10^{12}</math> bytes.

Confusingly, the storage capacity of a directly addressable memory device, such as a [[Dynamic Random Access Memory|DRAM]] chip, or an assemblage of such chips on a memory module, is specified as a binary multiple—using the ambiguous prefix G rather than the IEC recommended Gi prefix.  For example, a DRAM chip that is specified (and advertised) as having "1 GB" of capacity has <math>2^{30}</math> bytes of capacity.  As at 2022, the difference between the popular understanding of a memory system with "8 GB" of capacity, and the SI-correct meaning of "8 GB" was still causing difficulty to software designers.<ref>{{cite web |title=Use MB/GB/TB suffix for VM memory input |url=https://github.com/netbox-community/netbox/issues/8437 |website=Github Netbox Community |access-date=8 January 2025 |date=2022}}</ref>