A Lesson in Megabytes

by Bruce Barrow

(Reprinted with permission from the January 1997 issue of the IEEE Standards Bearer)

Once upon a time, computer professionals noticed that 2.10 was very nearly equal to 1000 and started using the metric prefix "kilo" to mean 1024. That worked well enough for a decade or two because everybody who talked kilobytes knew that the term implied 1024 bytes. But, almost overnight, a much more numerous "everybody" bought computers, and the true computer professionals needed to talk to physicists and engineers and even to ordinary people, most of whom know that a kilometer is 1000 meters and a kilogram is 1000 grams. Then data storage for gigabytes, and even terabytes, became practical, and the storage devices were not constructed on binary trees, which meant that, for many practical purposes, binary arithmetic was less convenient than decimal arithmetic. The result is that today "everybody" does not "know" what a megabyte is. When discussing computer memory, most manufacturers use megabyte to mean 1 048 576 bytes, but the manufacturers of computer storage devices usually use the term to mean bytes. Some designers of local area networks have used megabit per second to mean 1 048 576 b/s, but all telecommunications engineers use it to mean 10.6 b/s. And if two definitions of the megabyte are not enough, a third megabyte of 1 024 000 bytes is the megabyte used to format the familiar 3-1/2 inch, "1.44 MB" diskette. The confusion is real, as is the potential for incompatibility in standards and in implemented systems.

Faced with this reality, the IEEE Standards Board decided that IEEE standards will use the conventional, internationally adopted, definitions of the metric prefixes. Mega will mean 1 000 000, except that the base-two definition may be used during an interim period if such usage is explicitly pointed out on a case-by-case basis.

Standards Coordinating Committee (SCC14) for Quantities, Units and Letter Symbols, has begun to work with the Computer Society, the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC) to find acceptable names for prefixes that are related to powers of two. A proposal being circulated internationally would introduce the new prefixes kibi, mebi, gibi and tebi derived as short unions of the metric prefixes with the word "binary." The proposed new prefix symbols are Ki, Mi, Gi and Ti. Thus we would have a gibibyte of 2.30 bytes and a gigabyte of 10.9 bytes, and the 3-1/2 inch diskette would be formatted for 1440 KiB. (The 3-1/2 inch diskette is really, truly a mm diskette, but that is a question for another day.)

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