Megabyte
From Wikipedia, the free encyclopedia
The megabyte is a multiple of the unit byte for digital information storage or transmission with two different
values depending on context: 1 048 576 bytes (220) 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 (1 024 000) bytes.[3] It is commonly abbreviated as Mbyte or MB (compare Mb, for the
megabit).
Contents
1 Definition
2 Examples of use
3 See also
4 References
5 External links
Definition
Multiples of bytes
SI decimal prefixes Binary IEC binary prefixes
Name
Value usage
Name
Value
(Symbol)
(Symbol)
kilobyte (kB)
103
210
kibibyte (KiB)
210
megabyte (MB)
106
220
mebibyte (MiB)
220
gigabyte (GB)
109
230
gibibyte (GiB)
230
terabyte (TB)
1012
240
tebibyte (TiB)
240
petabyte (PB)
1015
250
pebibyte (PiB)
250
exabyte (EB)
1018
260
exbibyte (EiB)
260
The term "megabyte" is commonly used to mean either zettabyte (ZB)
1021 270 zebibyte (ZiB)
270
10002 bytes or 10242 bytes. This originated as
yottabyte (YB)
1024 280 yobibyte (YiB)
280
compromise technical jargon for the byte multiples that
needed to be expressed by the powers of 2 but lacked See also: Multiples of bits · Orders of magnitude of data
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:
1 000 000 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]
1 048 576 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). This definition is synonymous with the
unambiguous IEC standard name mebibyte, however as of 2011 adoption of the standard has not
been widespread.
1 024 000 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 1 474 560 bytes.
RAM usage: In modern computer systems, blocks of main working memory (RAM) invariably come in sizes
that are some-power-of-2 bytes, so for this reason, the binary M-means-1024×1024-system will almost always
be used when talking about blocks of RAM. Example: "1 MB of RAM" =1×1024×1024 bytes (1 048 576
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
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 English book volume in plain text format (500 pages
× 2000 characters per page).
See also
Binary prefix
Megabit
Orders of magnitude (data)
Timeline of binary prefixes
1.44 MB floppy disks can store
1,474,560 bytes of data. MB in
this context means 1,000×1,024
bytes.
References
1. ^ a b The American Heritage Science Dictionary (http://www.thefreedictionary.com/megabyte) . 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. ^ a b c d "Definitions of the SI units: The binary prefixes" (http://physics.nist.gov/cuu/Units/binary.html) .
National Institute of Standards and Technology. http://physics.nist.gov/cuu/Units/binary.html. Retrieved 200912-22. "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 (http://apac.sandisk.com/Products/Catalog(1349)SanDisk_Extreme_Ducati_Edition_USB_Flash_Drive.aspx) "Note: 1 megabyte (MB) = 1 million bytes; 1
gigabyte (GB) = 1 billion bytes."
5. ^ "How Mac OS X reports drive capacity" (http://support.apple.com/kb/TS2419) . Apple Inc. 2009-08-27.
http://support.apple.com/kb/TS2419. Retrieved 2009-10-16.
External links
Historical Notes About The Cost Of Hard Drive Storage Space
(http://www.alts.net/ns1625/winchest.html)
the megabyte (http://searchstorage.techtarget.com/sDefinition/0,,sid5_gci212542,00.html)
(established definition in Networking and Storage industries; from whatis.com
(http://whatis.techtarget.com/definition/0,,sid9_gci551931,00.html) )
International Electrotechnical Commission definitions (http://www.iec.ch/zone/si/si_bytes.htm)
IEC prefixes and symbols for binary multiples (http://members.optus.net/alexey/prefBin.xhtml)
Retrieved from "http://en.wikipedia.org/w/index.php?title=Megabyte&oldid=451747673"
Categories:
Units of information
This page was last modified on 21 September 2011 at 21:42.
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