Units of information

Units of information are a way of measuring a quantity of information or an entropy.

Primary units

Small amount of information just as logarithm of number of states.According to Shannon, "The choice of a logarithmic base corresponds to the choice of a unit for measuring information".

The minimal integer, whose logarithm is positive, is 2. The unit, corresponding to it is bit. Bit is the base of information measurement in modern information technologies.The unit, corresponding to the number 3 is trit, which is equal to log_{2} 3 approx 1.585 bit.The unit, corresponding to the number 10: ban = log_{2} 10 approx 3.322 bit.

Since the base of natural logarithm is not an integer, such unit as Nat is usually not applied to digital information, but is well known in theory.

Units derived from bit

There have been some attempts to construct unambiguous terms for bit blocks of some sizes. [ [http://dictionary.reference.com/browse/nybble nybble] reference.com sourced from Jargon File 4.2.0, accessed 2007-08-12] Terms in italic are strictly jargon, not serve as "units" (in the proper sense) and not very common.

* 2 bits: "crumb, quad, quarter, tayste, tydbit"
* 4 bits: "nibble, nybble", sometimes "half-byte"
* 5 bits: "nickel, nyckle"
* 8 bits: byte, octet
* 10 bits: "deckle"
* 16 bits: "plate, playte, chomp, chawmp" (on a 32-bit machine)
* 18 bits: "chomp, chawmp" (on a 36-bit machine)
* 32 bits: "dinner, dynner, gawble" (on a 32-bit machine)
* 48 bits: "gobble, gawble" (under circumstances that remain obscure)

The following therms allows precise meaning on bits quantities:
* 1 bit is binary digit
* 8 bits: byte or octet, 1B(yte) = 8b(its)
* 16 bits: "doublet"
* 32 bits: "quadlet"
* 64 bits: "octlet"


Large amounts of information are almost always counted in bytes rather than bits, and it is generally used as an atomic unit when addressing memory. See What is "byte"?">below a clarification of the term "byte".


Whether used to denote 1024 bytes (a kibibyte, or KiB), or more correctly, 1000 bytes (kB), this is a convenient order of magnitude to express:
* the typical sector of a hard disk: 512 bytes = ½ KiB
* the classical block size in UNIX filesystems: 1024 bytes = 1 KiB
* a CD-ROM sector: 2048 bytes = 2 KiB
* a memory page in x86 (since Intel 80386): 4096 bytes = 4 KiB


Units megabyte MB = 1000000 bytes and mebibyte MiB = 1024 KiB = 1048576 bytes.

Electronic memory is commonly measured in mebibytes, but hard disks are (or were) measured mainly in megabytes.


Units gigabyte GB = 10^9 bytes and gibibyte GiB = 1024 MiB = 2^{30} bytes.


What is "byte"?

copyedited from Byte -->The word "byte" has two closely related meanings:
# A contiguous sequence of a "fixed" number of bits (binary digits).
# A contiguous sequence of bits within a binary computer that comprises the "smallest addressable sub-field" of the computer's natural word-size.In modern computing, the use of a byte to mean 8 bits has become nearly ubiquitous in both senses. The term "octet" is used in very strict speech (such as RFCs) to avoid any ambiguity.

SI vs. Binary prefixes

cutedited from Byte -->The prefixes historically used for byte measurements are usually the same as the SI prefixes used for other measurements, but have slightly different values. The former are based on powers of 1,024 (210), a convenient binary number, while the SI prefixes are based on powers of 1,000 (103), a convenient decimal number. The table below illustrates these differences.

Sometimes "K" is used instead of "k". The use of "K" as a prefix has no meanings for the SI.

In 1998, the IEC, then the IEEE, published a new standard describing binary prefixes:


* [http://swiss.csail.mit.edu/~jaffer/MIXF/MIXF-08 Representation of numerical values and SI units in character strings for information interchanges]

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