Gigabyte

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A gigabyte (derived from the SI prefix giga-) is a unit of information or computer storage meaning either exactly 1 billion bytes (1000³, or 10⁹) or approximately 1.07 billion bytes (1024³, or 2³⁰).^[1]

The usage of the word "gigabyte" is ambiguous: the value depends on the context. When referring to RAM sizes and file sizes, it traditionally has a binary definition, of 1024³ bytes. For other uses, it means exactly 1000³ bytes. In order to address this confusion, currently the International Electrotechnical Commission (IEC) promotes 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 (CPIM) and the National Institute of Standards and Technology (NIST).

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

[edit] Definition

There are two different definitions of gigabyte in general use:

• 1,000,000,000 bytes or 10⁹ bytes is the decimal definition, used in telecommunications (such as network speeds) and most computer storage manufacturers (such as hard disks and flash drives). This usage is compatible with SI. Quote from Seagate: "For drive storage capacity, 1 gigabyte = 1,000,000,000 bytes (or one billion bytes).",^[2] Similar quotes are found on the websites of other storage manufacturers.
• 1,073,741,824 bytes, equal to 1024³, or 2³⁰ bytes. This is the definition commonly used for computer memory and file sizes. Microsoft uses this definition to display hard drive sizes,^[3] as do most other operating systems and programs by default. By this definition, there are 1,024³, or 1024×1024×1024 bytes in a gigabyte. (This is equivalent to 1,024 megabytes, where one megabyte is 1,048,576 bytes or 2²⁰ bytes.) Since 1999, the IEC recommends that this unit should instead be called a gibibyte (abbreviated GiB). Although there is little other usage of the term "gigabyte" apart from referring to memory, file sizes, or storage capacity, the IEC's recommendations are frequently ignored amongst computer professionals, and "gigabyte" is used colloquially to mean 1024³ bytes. The standard body JEDEC Solid State Technology Association acknowledges the conflict and the existence of the IEC recommendations but, in recognition of the widespread colloquial usage, continues to recognize the definition of 1024³ bytes.^[4]

[edit] Gigabytes vs. gigabits

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

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

[edit] Consumer confusion

As of 2007, most consumer hard drives are defined by their gigabyte-range capacities. The true capacity is usually some number above or below the class designation. Although most manufacturers of hard disks and Flash disks define 1 gigabyte as 1,000,000,000 bytes, the computer operating systems used by most users usually calculate a gigabyte by dividing the bytes (whether it is disk capacity, file size, or system RAM) by 1,073,741,824. This distinction is a cause of confusion, as a hard disk with a manufacturer rated capacity of 400 gigabytes may have its capacity reported by the operating system as only 372 GB, depending on the type of report.

The difference between units based on SI and binary prefixes increases exponentially — in other words, an SI kilobyte is nearly 98% as much as a kibibyte, but a megabyte is under 96% as much as a mebibyte, and a gigabyte is just over 93% as much as a gibibyte. This means that a 500 GB hard disk drive would appear as "465 GB". As storage sizes get larger and higher units are used, this difference will become more pronounced.

Note that computer memory is addressed in base 2, due to its design, so memory size is always a power of two (or some closely related quantity, for instance 384 MiB = 3×2²⁷ bytes). It is thus convenient to work in binary units for RAM at the hardware level (for example, in using DIMM memory units). RAM memory size as seen by application software has no consistent bias towards power of two units, as the operating system will allocate memory in other granularities. 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.

An example, take a hard drive that can store exactly 250×10⁹ or 250 billion bytes after formatting. Generally, operating systems calculate disk and file sizes using binary numbers, so this 250 GB drive would be reported as "232.83 GB". The result is that there is a significant discrepancy between what the consumer believes they have purchased and what their operating system says they have.

Some consumers feel short-changed when they discover the difference, and claim that manufacturers of drives and data transfer devices are using the decimal measurements in an intentionally misleading way to inflate their numbers. Several legal disputes have been waged over the confusion. See Binary prefix — Legal disputes.

To further complicate matters, flash memory chips are organized in multiples of 2, but retail flash memory products have available capacities specified by multiples of 10. Removable flash storage products contain file systems that make the devices behave like hard disks instead of RAM, yet it is called 'memory'. In operating systems like Windows Vista, flash memory can indeed be treated like RAM.

The basis of the problem is that the official definition of the SI units is not well known,^{[citation needed]} and some legal settlements include directions for manufacturers to use clearer info, e.g. by stating a hard disk's size in both GB and GiB. However, JEDEC memory standards still uses the IEEE 100 nomenclatures.

[edit] Gigabytes in use

Trivia sections are discouraged under Wikipedia guidelines. The article could be improved by integrating relevant items and removing inappropriate ones.

• The human genome contains 0.791175 GB of data (the 3.1647×10⁹ base pairs[1] represented as 2-bits).
• A DVD-5 format disc is specified as capable of storing 4.7 gigabytes (4,700,000,000 bytes), or roughly 4.38 gibibytes. A DVD-9 is capable of storing 8.5 gigabytes, or roughly 7.91 gibibytes.
• One gigabyte can hold over 1000 novels (uncompressed at 100,000 words per novel).
• One gigabyte is roughly equal to 17 hours of MP3 music at 128 kbit/s (1,000³ / 128 / 1000 * 8 / 3600 = 17.36).
• One gigabyte is roughly equivalent to 12 hours of Flash Video (at 450x370).
• All 6th generation and 7th generation game consoles have game discs that are around 1 GB or more: Dreamcast (GD-ROM - 1.2 GB), Nintendo GameCube (MiniDVD - 1.5 GB), PlayStation 2, Xbox, Xbox 360, and Wii (DVD - 8.5 GB), and PlayStation 3 (Blu-ray Disc - 50 GB).
• Dual-layer Blu-ray Discs can hold about 50 gigabytes (50,000,000,000 bytes), dual-layer HD DVD discs 30 gigabytes (30,000,000,000 bytes) of data.

[edit] See also

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

[edit] References

1. ^ 1000³ = one billion.
2. ^ Industry Terms Glossary: Gigabyte.
3. ^ HP Notebook PCs - Size of Disk Drive Does not Match Specifications (Windows XP).
4. ^ JEDEC Solid State Technology Association (December 2002), “Terms, Definitions, and Letter Symbols for Microcomputers, Microprocessors, and Memory Integrated Circuits”, JESD 100B.01, <http://www.jedec.org/download/search/JESD100B01.pdf> 

[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