Megabyte

The megabyte (abbreviated as Mbyte or MB) is a multiple of the unit byte for digital information storage or transmission with three different values depending on context: 1048576 bytes (2²⁰) generally for computer memory;^[1][2] and one million bytes (10⁶, see prefix mega-) generally for computer storage.^[1][3] In rare cases, it is used to mean 1000�-1024 (1024000) bytes.^[3] The IEEE Standards Board has confirmed that mega- means 1000000, with exceptions allowed for the base-two meaning.^[3]

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. The term remains ambiguous and it can follow any one of the following common definitions:

• 1000000 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]
• 1048576 bytes (1024², 2²⁰): This definition is most commonly used in reference to computer memory (e.g. RAM), but most software that display file size or drive capacity, including file managers also use this definition.^{[citation needed]} See Consumer confusion (in the "gigabyte" article). This definition is synonymous with the unambiguous IEC standard name mebibyte, but as of 2012^[update], adoption of the standard has not been widespread.
• 1024000 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 1474560 bytes.

Semiconductor memory doubles in size for each address line added to an integrated circuit package, which favors counts that are powers of two. The capacity of a disk drive is the product of the sector size, number of sectors per track, number of tracks per side, and 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

1.44 MB floppy disks can store 1,474,560 bytes of data. MB in this context means 1,000�-1,024 bytes.

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).
• a 4 megapixel JPEG image with normal compression is about 1 MB in size.
• About 1 minute of 128 kbit/s MP3 compressed music.
• 6 seconds of uncompressed CD audio.
• a typical English book volume in plain text format (500 pages �- 2000 characters per page).

The human genome consists of DNA representing 800 MB of data. The parts that differentiate one person from another can be losslessly compressed to 4 MB.^[6]

See also

• Timeline of binary prefixes

References

External links

• Historical Notes About The Cost Of Hard Drive Storage Space
• the megabyte (established definition in Networking and Storage industries; from whatis.com)
• International Electrotechnical Commission definitions
• IEC prefixes and symbols for binary multiples