Gigabyte

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Gigabyte is an SI-multiple of the unit byte for digital information storage. Since the giga prefix means 10⁹, gigabyte means 1000000000bytes (1000³, 10⁹). However, this term is also often used meaning 1073741824bytes (1024³, 2³⁰).

Originally the binary use of SI prefixes (kilo, mega, giga, etc.) was confined to contexts where the quantities were inherently determined in powers of two by the computer word or address size, like memory or disk sizes, so that confusion was unlikely. Later, disk design was not constrained by address word sizes or other physical details and disk blocks were numbered consecutively in decimal numbers (logical block addressing), creating the opportunity for confusion when size was still reported with binary interpretation of the prefix.

Today the usage of the word "gigabyte" is ambiguous: the value depends on the context. When referring to RAM sizes it traditionally has a binary interpretation of 1024³ bytes. Some operating systems list file sizes in SI units, but using the binary interpretation. Today, when referring to disk storage capacities it usually means 1000³ bytes. This also applies to data transmission volumes over telecommunication lines, as the telecommunications industry has always used the SI prefixes with their standards-based meaning.

In order to address this confusion, the International Electrotechnical Commission (IEC) has been promoting the use of the term gibibyte for the binary definition. This position is endorsed by other standards organizations including the Institute of Electrical and Electronics Engineers (IEEE), the International Committee for Weights and Measures (CIPM) and the National Institute of Standards and Technology (NIST) but has seen limited acceptance.

Gigabyte is commonly abbreviated GB or Gbyte (not to be confused with Gb, which is used for a gigabit) while Gibibyte is abbreviated GiB.

[edit] Definition and usage

There are two different interpretations of gigabyte in general use:

• 1000000000bytes or 10⁹ bytes is the official definition, based on the metric system (SI) of prefixes for multiples. It is used in telecommunications for network speeds and traffic volume and by most computer storage manufacturers for capacities of hard disks and flash drives. Quote from Seagate: "For drive storage capacity, 1 gigabyte = 1000000000bytes (or one billion bytes).",^[1] Similar quotes are found on the websites and products of other storage manufacturers. The Linux kernel also uses this definition.
• 1073741824bytes, equal to 1024³, or 2³⁰ bytes is the interpretation commonly used for computer memory and often file sizes. Microsoft uses this definition to display hard drive sizes^[2]. Since 1999, the IEC has defined this quantity as gibibyte(abbreviated GiB) and most standards bodies now recommend this usage.

The IEC's recommendations are frequently ignored amongst computer professionals, and "gigabyte" is used colloquially to mean 1024³ bytes. The JEDEC Solid State Technology Association, another standards body, acknowledges the conflict by noting that in light of the existence of the IEC recommendations this usage is deprecated but, in recognition of the widespread colloquial usage, it continues to recognize the usage of 1024³ bytes solely in the context of semiconductor storage capacities.^[3]

[edit] Gigabytes vs. gigabits

In conventional modern usage, a byte is 8 bits. One gigabyte is equivalent to eight gigabits.

In computer networking the conventional SI units are followed. Manufacturers of networking equipment always use 1000-bit kilobits as their basic unit of measurement.

[edit] Consumer confusion

Since the early 2000s most consumer hard drive capacities are grouped in certain size classes measured in gigabytes. The exact capacity of a given drive is usually some number above or below the class designation. Although most manufacturers of hard disk drives and flash-memory disk devices define 1 gigabyte as 1000000000bytes, the computer operating systems used by most users usually calculate size in gigabytes by dividing the total capacity in bytes (whether it is disk capacity, file size, or system RAM) by 1073741824. This distinction can be a cause of confusion, as a hard disk with a manufacturer-rated capacity of 400 gigabytes may be reported by the operating system as only 372 GB large, depending on the type of report. The JEDEC memory standards uses the IEEE 100 nomenclatures which defines a gigabyte as 1073741824bytes.^[4]

The difference between units based on SI and binary prefixes increases exponentially—for example, the SI kilobyte value is nearly 98% of the kibibyte, but a megabyte is under 96% of a mebibyte, and a gigabyte is just over 93% of a gibibyte value. This means that a 300 GB (279 GiB) hard disk drive can appear as 279 GB. As storage sizes increase and larger units are used, this difference becomes even more pronounced. Some legal challenges have been waged over this confusion such as a legal challenge against Western Digital.^[5][6] The settlement of the legal challenge against Western Digital included directions to add a disclaimer that the usuable capacity may differ from the advertised capacity.^[6]

Because of its physical design, computer memory is addressed in multiples of base 2, thus, memory size can always be factored by a power of two (for instance, 384 MiB = 3×2²⁷ bytes). It is thus convenient to use binary units for non-disk memory devices at the hardware level (for example, in using DIMM memory boards). Most software applications have no particular need to use or report memory in binary multiples and operating systems often use varying granularity when allocating it. Other computer measurements, like storage hardware size, data transfer rates, clock speeds, operations per second, etc., do not have an inherent base, and are usually presented in decimal units.

[edit] Examples of gigabyte-sized storage

• One hour of SDTV video at 2.2 Mbit/s is approximately 1 GB.
• One hour of HDTV video at 19.39 Mbit/s is approximately 8.7 GB.
• A basic Unix system installation uses less than 1 GB.
• 114 minutes of uncompressed CD-quality audio at 1.4 Mbit/s is approximately 1 GB.

[edit] See also

• Megabyte
• Terabyte
• Binary prefix
• JEDEC memory standards
• Orders of magnitude (data)

[edit] References

[edit] External links

• http://physics.nist.gov/cuu/Units/binary.html
• http://www.iec.ch/zone/si/si_bytes.htm
• http://www.quinion.com/words/turnsofphrase/tp-kib1.htm
• http://www.nist.gov/public_affairs/techbeat/tb9903.htm