字节

字节的次方单位
十进制前缀; (SI)
二进制前缀; (IEC 60027-2)
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字节（英语：byte）是通常用在电脑、手机及智慧手表等装置上的资讯计量单位，不分数据类型。^[1]^[2] 。一个字节代表八个位元。从历史的观点上，“字节”表示用于编码单个字符所需要的位元数量，因此它是许多电脑架构中最小的可寻址内存单元。历史上字节长度曾基于硬件为1-48 bit不等，最初通常使用6 bit或9 bit为一字节。今日标准以8 bit作为一字节。为了消除常见8 位定义中任意大小的字节的歧义，八个位元在一些规范（例如工业标准、电脑网络、电信技术等）中常被称为八位组（octet）。Internet 协议（RFC 791 ) 将 8 位字节称为八位字节。^[3]

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国际电工委员会(IEC) 和电气与电子工程师协会(IEEE) 将字节的单位符号指定为大写字母 B。例如MB表示兆字节（megabyte）；位元（bit）可缩写成b，例如Mb表示兆位元（ megabit（英语：megabit）），与字节进行区分。国际上，单位八位字节（octet，符号 o）明确定义了八位的序列，消除了术语“字节”的潜在歧义。

字节的大小历来取决于硬件，并且不存在强制规定大小的明确标准。曾经使用过的字节的大小包含 1 到 48 位。六位字符代码是早期编码系统中常用的实现方式，使用六位和九位字节的电脑在 20 世纪 60 年代很常见。这些系统通常具有 12、18、24、30、36、48 或 60 位的存储器字，对应于 2、3、4、5、6、8 或 10 个六位字节。在术语“字节”变得普遍之前，指令流中的位分组通常被称为syllable^[a]或slab 。

ISO/IEC 2382-1:1993 中记录的现代事实上的标准（8位）是相对方便的2 的幂，因为 2 的 8 次方是 256，允许一个字节使用 0 到 255的二进制编码值。国际标准IEC 80000-13定义了这一常见含义。许多类型的应用程式使用可用八位或更少位表示的资讯，并且处理器设计者通常针对这种用法进行优化。主要商业计算架构的普及有助于 8 位字节的普遍接受。现代架构通常使用 32 位字或 64 位字，分别由 4 个或 8 个字节构成。

历史

字节一词是Werner Buchholz于1956年6月在IBM Stretch电脑的早期设计阶段发明的，该电脑的寻址为位和可变字段长度（VFL）指令，指令中编码了字节大小。这是为了避免意外突变为位元而特意重写的。

注释

[a]
术语“syllable”用于包含指令或指令组成部分的字节，而不是数据字节。

参考资料

[1]
Blaauw, Gerrit Anne; Brooks, Jr., Frederick Phillips; Buchholz, Werner, 4: Natural Data Units, Buchholz, Werner (编), Planning a Computer System – Project Stretch (PDF), McGraw-Hill Book Company, Inc. / The Maple Press Company, York, PA.: 39–40, 1962 [2017-04-03], LCCN 61-10466, （原始内容 (PDF)存档于2017-04-03）, […] Terms used here to describe the structure imposed by the machine design, in addition to bit, are listed below.
Byte denotes a group of bits used to encode a character, or the number of bits transmitted in parallel to and from input-output units. A term other than character is used here because a given character may be represented in different applications by more than one code, and different codes may use different numbers of bits (i.e., different byte sizes). In input-output transmission the grouping of bits may be completely arbitrary and have no relation to actual characters. (The term is coined from bite, but respelled to avoid accidental mutation to bit.)
A word consists of the number of data bits transmitted in parallel from or to memory in one memory cycle. Word size is thus defined as a structural property of the memory. (The term catena was coined for this purpose by the designers of the Bull fr computer.)
Block refers to the number of words transmitted to or from an input-output unit in response to a single input-output instruction. Block size is a structural property of an input-output unit; it may have been fixed by the design or left to be varied by the program. […]
[2]
Bemer, Robert William, A proposal for a generalized card code of 256 characters, Communications of the ACM, 1959, 2 (9): 19–23, doi:10.1145/368424.368435
[3]
Postel, J.. Internet Protocol DARPA INTERNET PROGRAM PROTOCOL SPECIFICATION. September 1981: p. 43 [28 August 2020]. RFC 791 （英文）. octet An eight bit byte.

延伸阅读

Tafel, Hans Jörg. 写于RWTH, Aachen, Germany. Einführung in die digitale Datenverarbeitung [Introduction to digital information processing]. Munich, Germany: Carl Hanser Verlag. 1971: 300. ISBN 3-446-10569-7 （德语）. Byte = zusammengehörige Folge von i.a. neun Bits; davon sind acht Datenbits, das neunte ein Prüfbit (NB. Defines a byte as a group of typically 9 bits; 8 data bits plus 1 parity bit.)
Programming with the PDP-10 Instruction Set (PDF). PDP-10 System Reference Manual 1. Digital Equipment Corporation (DEC). August 1969 [2017-04-05]. （原始内容存档 (PDF)于2017-04-05）.
Computer History Museum – Exhibits – Internet History – 1964: Internet History 1962 to 1992. Computer History Museum. 2017 [2015] [2017-04-03]. （原始内容存档于2017-04-03）.
Jaffer, Aubrey. Metric-Interchange-Format. 2011 [2008] [2017-04-03]. （原始内容存档于2017-04-03）.
Kozierok, Charles M. The TCP/IP Guide – Binary Information and Representation: Bits, Bytes, Nibbles, Octets and Characters – Byte versus Octet. 3.0. 2005-09-20 [2001] [2017-04-03]. （原始内容存档于2017-04-03）.

参阅

八字节（octet）

外部链接

ГОСТ 8.417-2002 | Страница 19 （俄文）

[4] [a]
术语“syllable”用于包含指令或指令组成部分的字节，而不是数据字节。

[Buchholz_1962-1] [1]
Blaauw, Gerrit Anne; Brooks, Jr., Frederick Phillips; Buchholz, Werner, 4: Natural Data Units, Buchholz, Werner (编), Planning a Computer System – Project Stretch (PDF), McGraw-Hill Book Company, Inc. / The Maple Press Company, York, PA.: 39–40, 1962 [2017-04-03], LCCN 61-10466, （原始内容 (PDF)存档于2017-04-03）, […] Terms used here to describe the structure imposed by the machine design, in addition to bit, are listed below.
Byte denotes a group of bits used to encode a character, or the number of bits transmitted in parallel to and from input-output units. A term other than character is used here because a given character may be represented in different applications by more than one code, and different codes may use different numbers of bits (i.e., different byte sizes). In input-output transmission the grouping of bits may be completely arbitrary and have no relation to actual characters. (The term is coined from bite, but respelled to avoid accidental mutation to bit.)
A word consists of the number of data bits transmitted in parallel from or to memory in one memory cycle. Word size is thus defined as a structural property of the memory. (The term catena was coined for this purpose by the designers of the Bull fr computer.)
Block refers to the number of words transmitted to or from an input-output unit in response to a single input-output instruction. Block size is a structural property of an input-output unit; it may have been fixed by the design or left to be varied by the program. […]

[Bemer_1959-2] [2]
Bemer, Robert William, A proposal for a generalized card code of 256 characters, Communications of the ACM, 1959, 2 (9): 19–23, doi:10.1145/368424.368435

[3] [3]
Postel, J.. Internet Protocol DARPA INTERNET PROGRAM PROTOCOL SPECIFICATION. September 1981: p. 43 [28 August 2020]. RFC 791 （英文）. octet An eight bit byte.

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