Binary prefix

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In computing, Binary prefixes are often used to quantify large numbers where powers of two are more useful than powers of ten. They are written identically to the SI prefixes kilo-, mega-, giga-, etc., but each successive prefix is multiplied by 1024 (2¹⁰) rather than 1000 (10³).

Whether these prefixes, and their abbreviations K, M, G, etc., are being used in the binary or the decimal sense depends on context and common usage. This can cause serious confusion.

Examples: a 50 byte text string, a 100 KB (kilobyte) file, 128 MB (megabytes) of RAM, or 30 GB (gigabytes) of disk storage.

Conventional usage

The one-letter abbreviations are identical to SI prefixes, except for "K", which corresponds to "k"; in SI (K stands for Kelvin in SI).

It is widely regarded as confusing that the common usage of "kilobyte" means 1024 bytes, while the SI kilo means 1000. Some have suggested that "k" be used for 1000, and "K" for 1024, but this is not extended to the higher order prefixes and has never been widely recognised.

We can abbreviate these to get the prefixes K, M, and G for kilo-, mega-, and giga-. Mbit/s, kbit/s (kbps) etc. are often used to abbreviate "Megabits per second", "Kilobits per second", etc. However, terms such as "two megabytes" are often abbreviated inaccurately to "2M", and then the prefix in fact appears to become a suffix, but in fact still prefixes the unit (two megabytes).

When dealing with volumes of bytes, and sometimes bits, the binary versions are almost always implied. Notable exceptions are:

• Hard disk manufacturers are the only group in computing that habitually uses the lower SI factors. So what is advertised as a "30 GB" hard disk will actually only hold 30×10⁹ bytes, roughly equal to 28×2³⁰ bytes (i.e. 28 GB proper). However, memory is always measured using the binary prefixes.
• Floppy disk manufacturers are even more confusing. The "1.44 MB" floppy holds exactly 1440 KB (1.44 × 1000 × 1024 bytes).
• Telecommunications always uses the SI factors, so a 1 Mbit/s internet connection transfers 10⁶ bits per second.
• Other measurements of bitrate, including audio and video, also use the decimal prefixes. So a 128kbps MP3 stream consumes 128000 bits/s (15.625 KByte/s)

In the days of 32 KB RAM computers, this confusion was not serious, since the difference between 2¹⁰ and 10³ is approximately 2%. However, as computing equipment has grown in memory capacity, these differences have led to increasingly large errors when expressed as percentages.

The confusion even extends to the symbols for the units of information themselves, since they are not SI units:

• Common practice often uses "b" for bit and "B"; for byte, which is incompatible with SI because B stands for the Bel.
• Others recommend "bit" for bit and "b" for byte, which conflicts with the above
• French-speaking countries sometimes use "o" for "octet", a synonym of byte, which is also unacceptable in SI because of the risk of confusion with the zero.

IEC standard prefixes

In 1999, the International Electrotechnical Commission (IEC) published Amendment 2 to "IEC 60027-2: Letter symbols to be used in electrical technology – Part 2: Telecommunications and electronics";. This standard, which had been approved in 1998, introduced the prefixes kibi-, mebi-, gibi-, tebi-, pebi-, exbi-, to be used in specifying binary multiples of a quantity. The names come from the first two letters of the original SI prefixes followed by bi which is short for "binary". It also clarifies that, from the point of view of the IEC, the SI prefixes only have their base-10 meaning and never have a base-2 meaning.

As of 2004 this naming convention has not yet gained widespread use.

Note that the IEC names extend only up to exbi-, corresponding to the SI prefix exa-. The two SI prefixes zetta- (10²¹) and yotta- (10²⁴) have no corresponding IEC binary prefixes, even though the obvious continuation would be zebi-/Zi (2⁷⁰) and yobi-/Yi (2⁸⁰).

See also

• Integral data type
• Bit
• Nibble
• Byte
• Octet
• Kilobit
• Kilobyte
• Kibibit
• Megabit
• Megabyte
• Mebibit
• Gigabit
• Gigabyte
• Gibibit
• Terabit
• Terabyte
• Tebibit
• Petabit
• Petabyte
• Pebibit
• Exabit
• Exabyte
• Exbibit
• Zettabit
• Zettabyte
• Yottabit
• Yottabyte

External links

• http://members.optus.net/alexey/prefBin.xhtml
• http://physics.nist.gov/cuu/Units/binary.html
• http://www.iec.ch/zone/si/si_bytes.htm
• http://www.quinion.com/words/turnsofphrase/tp-kib1.htm
• http://www.nist.gov/public_affairs/techbeat/tb9903.htm

da:Binært præfiks fr:Préfixe binaire nl:Veelvouden van bytes ja:2進接頭辞 pl:Przedrostek bajtów pt:Prefixo binário

Categories: Computer terminology | Units of measure