Units of information
In computing and telecommunications, a unit of information is the capacity of some standard data storage system or communication channel, used to measure the capacities of other systems and channels. In information theory, units of information are also used to measure the information contents or entropy of random variables.
The most common units are the bit, the capacity of a system which can exist in only two states, and the byte (or octet), which is equivalent to eight bits. Multiples of these units can be formed from these with the SI prefixes (power-of-ten prefixes) or the newer IEC binary prefixes (binary power prefixes). Information capacity is a dimensionless quantity.
In 1928, Ralph Hartley observed a fundamental storage principle, which was further formalized by Claude Shannon in 1945: the information that can be stored in a system is proportional to the logarithm logb N of the number N of possible states of that system. Changing the basis of the logarithm from b to a different number c has the effect of multiplying the value of the logarithm by a fixed constant, namely logc N = (logc b) logb N. Therefore, the choice of the basis b determines the unit used to measure information. In particular, if b is a positive integer, then the unit is the amount of information that can be stored in a system with b possible states.
When b is 2, the unit is the shannon, equal to the information content of one "bit" (a contraction of binary digit). A system with 8 possible states, for example, can store up to log28 = 3 bits of information. Other units that have been named include:
- Base b = 3: the unit is called "trit", and is equal to log2 3 (≈ 1.585) bits.
- Base b = 10: the unit is called decimal digit, hartley, ban, decit, or dit, and is equal to log2 10 (≈ 3.322) bits.
- Base b = e, the base of natural logarithms: the unit is called a nat, nit, or nepit (from Neperian), and is worth log2 e (≈ 1.443) bits.
The trit, ban, and nat are rarely used to measure storage capacity; but the nat, in particular, is often used in information theory, because natural logarithms are sometimes more convenient than logarithms in other bases.
Units derived from bit
Several conventional names are used for collections or groups of bits.
Historically, a byte was the number of bits used to encode a character of text in the computer, which depended on computer hardware architecture; but today it almost always means eight bits — that is, an octet. A byte can represent 256 (28) distinct values, such as the integers 0 to 255, or -128 to 127. The IEEE 1541-2002 standard specifies "B" (upper case) as the symbol for byte. Bytes, or multiples thereof, are almost always used to specify the sizes of computer files and the capacity of storage units. Most modern computers and peripheral devices are designed to manipulate data in whole bytes or groups of bytes, rather than individual bits.
A group of four bits, or half a byte, is sometimes called a nibble or nybble. This unit is most often used in the context of hexadecimal number representations, since a nibble has the same amount of information as one hexadecimal digit.
Word, block, and page
Computers usually manipulate bits in groups of a fixed size, conventionally called words. The number of bits in a word is usually defined by the size of the registers in the computer's CPU, or by the number of data bits that are fetched from its main memory in a single operation. In the IA-32 architecture more commonly known as x86-32, a word is 16 bits, but other past and current architectures use words with 8, 24, 32, 36, 56, 64, 80 bits or others.
Terms for large quantities of bits can be formed using the standard range of SI prefixes for powers of 10, e.g., kilo = 103 = 1000 (as in kilobit or kbit), mega- = 106 = 1000000 (as in megabit or Mbit) and giga = 109 = 1000000000 (as in gigabit or Gbit). These prefixes are more often used for multiples of bytes, as in kilobyte (1 kB = 8000 bit), megabyte (1 MB = 8000000bit), and gigabyte (1 GB = 8000000000bit).
However, for technical reasons, the capacities of computer memories and some storage units are often multiples of some large power of two, such as 228 = 268435456 bytes. To avoid such unwieldy numbers, people have often misused the SI prefixes to mean the nearest power of two, e.g., using the prefix kilo for 210 = 1024, mega for 220 = 1048576, and giga for 230 = 1073741824, and so on. For example, a random access memory chip with a capacity of 228 bytes would be referred to as a 256-megabyte chip. The table below illustrates these differences.
|Symbol||Prefix||SI Meaning||Binary meaning||Size difference|
|k||kilo||103 = 10001||210 = 10241||2.40%|
|M||mega||106 = 10002||220 = 10242||4.86%|
|G||giga||109 = 10003||230 = 10243||7.37%|
|T||tera||1012 = 10004||240 = 10244||9.95%|
|P||peta||1015 = 10005||250 = 10245||12.59%|
|E||exa||1018 = 10006||260 = 10246||15.29%|
|Z||zetta||1021 = 10007||270 = 10247||18.06%|
|Y||yotta||1024 = 10008||280 = 10248||20.89%|
In the past, uppercase K has been used instead of lowercase k to indicate 1024 instead of 1000. However, this usage was never consistently applied.
On the other hand, for external storage systems (such as optical disks), the SI prefixes were commonly used with their decimal values (powers of 10). There have been many attempts to resolve the confusion by providing alternative notations for power-of-two multiples. In 1998 the International Electrotechnical Commission (IEC) issued a standard for this purpose, namely a series of binary prefixes that use 1024 instead of 1000 as the main radix:
|Multiples of bytes|
|Orders of magnitude of data|
|Ki||kibi, binary kilo||1 kibibyte (KiB)||210 bytes||1024 B|
|Mi||mebi, binary mega||1 mebibyte (MiB)||220 bytes||1024 KiB|
|Gi||gibi, binary giga||1 gibibyte (GiB)||230 bytes||1024 MiB|
|Ti||tebi, binary tera||1 tebibyte (TiB)||240 bytes||1024 GiB|
|Pi||pebi, binary peta||1 pebibyte (PiB)||250 bytes||1024 TiB|
|Ei||exbi, binary exa||1 exbibyte (EiB)||260 bytes||1024 PiB|
- 1 bit - answer to a yes/no question
- 1 byte - a number from 0 to 255.
- 90 bytes: enough to store a typical line of text from a book.
- 512 bytes = ½ KiB: the typical sector of a hard disk.
- 1024 bytes = 1 KiB: the classical block size in UNIX filesystems.
- 2048 bytes = 2 KiB: a CD-ROM sector.
- 4096 bytes = 4 KiB: a memory page in x86 (since Intel 80386).
- 4 kB: about one page of text from a novel.
- 120 kB: the text of a typical pocket book.
- 1 MB - a 1024×1024 pixel bitmap image with 256 colors (8 bpp color depth).
- 3 MB - a three minute song (133 kbit/s)
- 650-900 MB - a CD-ROM
- 1 GB - 114 minutes of uncompressed CD-quality audio at 1.4 Mbit/s
- 15 GB - amount of storage Google offers for free.
- 8/16 GB - size of a normal flash drive
- 4 TB - The size of a $150 hard disk (as of late 2014)
- 966 EB - prediction of the volume of the whole internet in 2015
Obsolete and unusual units
|This section needs additional citations for verification. (September 2014)|
Several other units of information storage have been named.
- 1 bit: sniff.
- 2 bits: crumb, quad, quarter, tayste, tydbit, semi-nibble.
- 3 bits: triad, triade
- 5 bits: nickel, nyckle.
- 6 bits: byte (in early IBM machines using BCD alphamerics).
- 10 bits: decle, deckle, dyme.
- 16 bits: wyde, doublet, plate, playte, chomp, chawmp (on a 32-bit machine).
- 18 bits: chomp, chawmp (on a 36-bit machine).
- 32 bits: quadlet, dinner, dynner, gawble (on a 32-bit machine).
- 48 bits: gobble, gawble (under circumstances that remain obscure).
- 64 bits: octlet.
- 128 bits: hexlet.
- 16 bytes: paragraph.
- 6 trits: tryte 
Most of these names are jargon, obsolete, or used only in very restricted contexts.
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- Donald E. Knuth, The Art of Computer Programming, vol.2: Seminumerical algorithms.
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- ISO/IEC standard is ISO/IEC 80000-13:2008. Not publicly available. This standard cancels and replaces subclauses 3.8 and 3.9 of IEC 60027-2:2005. The only significant change is the addition of explicit definitions for some quantities. ISO Online Catalogue
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- Representation of numerical values and SI units in character strings for information interchanges
- Bit Calculator - Convert between bit, byte, kilobit, kilobyte, megabit, megabyte, gigabit, gigabyte
- Paper on standardized units for use in information technology