Mebibyte

The mebibyte is a multiple of the unit byte for digital information. The binary prefix mebi means 2²⁰, therefore 1 mebibyte is . The unit symbol for the mebibyte is MiB. The unit was established by the International Electrotechnical Commission (IEC) in 2000 and has been accepted for use by all major standards organizations. It was designed to replace the megabyte used in some computer science contexts to mean 2²⁰ bytes, which is similar to but conflicts with the SI definition of the prefix mega (10⁶).

Definition

The prefix mebi is a binary prefix derived from the words mega and binary, indicating its origin in the closeness in value to the SI prefix mega. One mebibyte (MiB) is 2²⁰ (i.e., 1024 x 1024) bytes, or . One MiB differs from one megabyte (MB), which means 10⁶ (i.e. 1000 x 1000 = ).

Mebibyte is not commonly used. Instead, megabyte is often used to mean 1000 x 1000, 1024 x 1024, and even 1000 x 1024 bytes. Such usage can be confusing and inconsistent. For example, Microsoft's Windows XP operating system shows a file of 2²⁰ bytes as "1.00 MB" in its file properties dialog, while showing a file of 10⁶ () bytes as 976 KB; but Apple's Mac OS X 10.6 would report a 10⁶ byte file as 1 MB.

History

The mebi- prefix was defined by the International Electrotechnical Commission (IEC) in December 1998. It is becoming gradually accepted. For example, the interpretation of a 10⁶ byte file as 1 MB did not appear in versions of Apple's Mac OS X before version 10.6.

The use of mebibyte and related units is presently endorsed by the Institute of Electrical and Electronics Engineers (IEEE) and the International Committee for Weights and Measures (CIPM) in contexts where use of a binary prefix makes sense.

In The Art of Computer Programming, Donald Knuth proposed that the mebibyte be called a large megabyte (abbreviated MMB).

See also

References

Category:Units of information