Numeral systems | Binary prefixes | Units of information
Binary prefix
A binary prefix is a unit prefix for multiples of units. It is most often used in data processing, data transmission, and digital information, principally in association with the bit and the byte, to indicate multiplication by a power of 2. As shown in the table to the right there are two sets of symbols for binary prefixes, one set established by International Electrotechnical Commission (IEC) and several other standards and trade organizations using two-letter symbols, e.g. Mi indicating 1048576 with a second set established by semiconductor industry convention using one-letter symbols, e.g., M also indicating 1048576. In most contexts, industry uses the multipliers kilo (k), mega (M), giga (G), etc., in a manner consistent with their meaning in the International System of Units (SI), namely as powers of 1000. For example, a 500-gigabyte hard disk holds 500000000000 bytes, and a 1 Gbit/s (gigabit per second) Ethernet connection transfers data at nominal speed of 1000000000 bit/s. In contrast with the binary prefix usage, this use is described as a decimal prefix, as 1000 is a power of 10 (103). The computer industry has historically in citations of main memory (RAM) capacity used the units kilobyte, megabyte, and gigabyte, and the corresponding symbols KB, MB, and GB, in a binary sense: gigabyte customarily means 1073741824 bytes. As this is a power of 1024, and 1024 is a power of two (210), this usage is referred to as a binary measurement. The use of the same unit prefixes with two different meanings has caused confusion. Starting around 1998, the IEC and several other standards and trade organizations attempted to address the ambiguity by publishing standards and recommendations for a set of binary prefixes that refer exclusively to powers of 1024. Accordingly, the US National Institute of Standards and Technology (NIST) requires that SI prefixes be used only in the decimal sense: kilobyte and megabyte denote one thousand bytes and one million bytes respectively (consistent with SI), while new terms such as kibibyte, mebibyte, and gibibyte, having the symbols KiB, MiB, and GiB, denote 1024 bytes, 1048576 bytes, and 1073741824 bytes, respectively. In 2008, the IEC prefixes were incorporated into the ISO/IEC 80000 standard alongside the decimal prefixes of the international standard system of units. In response to litigation over the use of metric prefixes, the United States District Court for the Northern District of California includes a judicial notice that "the U.S. Congress has deemed the decimal definition of gigabyte to be the 'preferred' one for the purposes of 'U.S. trade and commerce.'" (Wikipedia).
