Other Definitions
megabyte (dict)
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Megabyte - This article is about a unit of data measurement. For the cartoon character, see ReBoot.
A megabyte (informally, meg) is a unit of measurement for computer storage, memory and information that is either exactly or approximately equal to one million bytes, depending on the definition used. The most-often used symbol for megabyte is MB, with Mb close behind, although the latter is also used to stand for megabit. Three definitions are currently in use: - 1,048,576 bytes (10242 or 220): Though unofficial, this is the definition used for computer memory and Compact Discs. It is also frequently used for computer hard disk space (as presented by the operating system). According to this definition, one megabyte is 1,024 kilobytes, and one gigabyte is 1,024 megabytes. The reason is that computers use the binary numeral system internally. This is also referred to as a mebibyte (MiB or Mib, whose magnitude is not in doubt).
- 1,000,000 bytes (106): This is the definition used for DVDs and other rewriteable optical drives, and some flash-based memories. It is also often used for hard drives (as presented by the manufacturers). It is consistent with the SI prefix "mega" and is endorsed by international standards bodies, and consistent with the other uses of the "mega" prefix in the computer world.
- 1,024,000 bytes (1,024×1,000): This is a "definition" used by floppy disk and some Flash drive manufacturers.
Hard drives are particularly problematic. They are almost always specified by computer hardware manufacturers as per meaning (2) above, but the size is most often reported in software and referred to by users according to meaning (1) above. For example, all current Apple computers have hard drives specified in base-10 MB, but Mac OS X reports their size in base-2 MB. The same is true of most Windows PCs. To reduce the confusion and distinguish between meanings (1) and (2) above, the International Electrotechnical Commission (IEC), adopted an international standard in December 1998 which reserves the term megabyte for 106 bytes and introduces the new prefix mebi (symbol Mi) for 220. Similarly, the prefixes kibi (Ki, equal to 210) and gibi (Gi, equal to 230) were introduced. This naming convention, strongly endorsed by IEEE and CIPM, is slowly coming into common usage. Note also the distinction between a megabyte (one million bytes) and a megabit (one million bits). A megabit is abbreviated as Mbit (preferably) or as Mb (when the byte is abbreviated by an upper case "B", which stands for Bel in SI). There are eight bits in one byte, so a megabyte is eight times as large as a megabit. Megabits are often used in applications where a serial bitstream is the item of interest, particularly in communications and in specifying the internal data rate of a computer hard drive. In these contexts, one megabit is almost invariably defined as 106 bits. In practice, when the symbol Mb is encountered, an examination of the context is necessary to indicate which unit of measure was intended. In the specific case of USB and Firewire buses, the regular SI base-10 Mb is used. Certain units are always understood as decimal even in computing contexts. For example, hertz (Hz), which is used to measure clock rates of electronic components, and bit/s, used to measure bitrate. So a 1 GHz processor performs 1,000,000,000 clock ticks per second, and a 1 Mbit/s internet connection can transfer 1,000,000 bits (approx. 122 KiB) per second. In colloquial speech, one may say "meg" to mean megabyte. Unicode has a symbol for Megabyte: ㎆, U+3386, part of the CJK compatibility range. See also External links
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