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The megabyte is a multiple of the unit byte for digital information storage or transmission with two different values depending on context: Template:Gaps bytes (10242) generally for computer memory;[1][2] and one million bytes (106, see prefix mega-) generally for computer storage.[1][3] The IEEE Standards Board has decided that "Mega will mean 1 000 000", with exceptions allowed for the base-two meaning.[3] In rare cases, it is used to mean 1000×1024 (Template:Gaps) bytes.[3] It is commonly abbreviated as Mbyte or MB (compare Mb, for the megabit). Template:Quantities of bytes

Definition

The term "megabyte" is commonly used to mean either 10002 bytes or 10242 bytes. This originated as compromise technical jargon for the byte multiples that needed to be expressed by the powers of 2 but lacked a convenient name. As 1024 (210) approximates 1000 (103), roughly corresponding SI multiples began to be used for binary multiples. By the end of 2007, standards and government authorities including IEC, IEEE, EU, and NIST proposed standards for binary prefixes and requiring the use of megabyte to strictly denote 10002 bytes and mebibyte to denote 10242 bytes. This is reflected in an increasing number of software projects using the new definitions, but some file managers still show file sizes using the binary interpretation (10242 bytes). The term remains ambiguous and it can follow any one of the following common definitions:

  1. Template:Gaps bytes (10002, 106): This is the definition recommended by the International System of Units (SI) and the International Electrotechnical Commission IEC.[3] This definition is used in networking contexts and most storage media, particularly hard drives, Flash-based storage[4], and DVDs, and is also consistent with the other uses of the SI prefix in computing, such as CPU clock speeds or measures of performance. The Mac OS X 10.6 file manager is a notable example of this usage in software. Since Snow Leopard, file sizes are reported in decimal units.[5]
  2. Template:Gaps bytes (10242, 220): This definition is most commonly used in reference to computer memory, but most software that display file size or drive capacity, including file managers also use this definition. See Consumer confusion (in the "gigabyte" article). The Microsoft Windows file manager is a notable example of this usage in software.
  3. Template:Gaps bytes (1000×1024): This is used to describe the formatted capacity of the "1.44 MB" 3.5 inch HD floppy disk, which actually has a capacity of Template:Gaps bytes.

Semiconductor memory doubles in size each time an address pin was added to an integrated circuit package, and so naturally favored counts that were powers of two. The capacity of a disk drive was the product of the sector size, number of sectors per track, number of tracks per side, (and in hard drives, the number of disk platters in the drive). Changes in any of these factors would not usually double the size. Sector sizes were set as powers of two (256 bytes, 512 bytes and so on) for convenience in processing. It was a natural extension to give the capacity of a disk drive in multiples of the sector size, giving a mix of decimal and binary multiples when expressing total disk capacity.

Examples of use

File:3,5"-Diskette.jpg

Depending on compression methods and file format, a megabyte of data can roughly be:

  • a 1024×1024 pixel bitmap image with 256 colors (8 bpp color depth).
  • 1 minute of 128 kbit/s MP3 compressed music.
  • 6 seconds of uncompressed CD audio.
  • a typical book volume in text format (500 pages × 2000 characters per page).

See also

References

  1. 1.0 1.1 The American Heritage Science Dictionary. Houghton Mifflin Company. 2005. http://www.thefreedictionary.com/megabyte. Retrieved 2009-12-22. "1. A unit of computer memory or data storage capacity equal to 1,048,576 bytes (1,024 kilobytes or 220) bytes. 2. One million bytes. ... prefix mega- often does not have its standard scientific meaning of 1,000,000 ... rate of one megabit per second is equal to one million bits per second ..." 
  2. "What are bits, bytes, and other units of measure for digital information? - Knowledge Base". Indiana University. "1MB is 1,024 kilobytes, or 1,048,576 (1024x1024) bytes, not one million bytes. ... Many hard drive manufacturers use a decimal number system to define amounts of storage space. As a result, 1MB is defined as one million bytes, 1GB is defined as one billion bytes, and so on." 
  3. 3.0 3.1 3.2 3.3 "Definitions of the SI units: The binary prefixes". National Institute of Standards and Technology. http://physics.nist.gov/cuu/Units/binary.html. Retrieved 2009-12-22. "third megabyte of 1 024 000 bytes is the megabyte used to format the familiar 90 mm (3½ inch), "1.44 MB" diskette" 
  4. SanDisk USB Flash Drive "Note: 1 megabyte (MB) = 1 million bytes; 1 gigabyte (GB) = 1 billion bytes."
  5. "How Mac OS X reports drive capacity". Apple Inc. 2009-08-27. http://support.apple.com/kb/TS2419. Retrieved 2009-10-16. 

External links

Template:Computer Storage Volumes


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