代码页

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-{A|zh:代码页;zh-tw:內碼表;}- code page 是 IBM 公司用来描述特定字符编码表的一个术语：一组位序列的映射，通常由单个八位字节代表值 0-255，与特定字符相关联。IBM 和微软公司通常会分配一个代码页号映射到某个字符集，虽然字符集比它的另一个名称更著名。

术语“代码页”来自于 IBM 的基于EBCDIC的主机系统，该术语通常特指 IBM PC 的代码页。微软，一个 PC 操作系统的制造商家，将“代码页”指定为 OEM 代码页，还将自己的 ANSI 代码页添加到其中。

除了 CJK 和越南语之外，大多数著名的代码页，代表适合 8 位并且其中的每个代码都可以映射到单个位图上（如组合字符，复杂脚本）的字符集。

PC 图形硬件的文本模式标准（兼容 VGA）是建立在 8 位代码页基础上的，虽然可以在损失部分色彩的前提下同时使用两种代码页，到了 8 种就可以存储在显卡上来进行简单地转换。[1]可载入这种硬件的代码页需要对其进行选择。但是，操作系统商家可以提供他们自己的字符编码和翻译系统，它可运行在图形模式下并完全避开此系统，这是很常见的。这些图形系统（特别是 Windows）使用的字符编码有时也叫做“代码页”。

[编辑] Relationship to ASCII

The basis of the IBM PC code pages is ASCII, a 7-bit code representing 128 characters and control codes. In the past, 8-bit extensions to the ASCII code often either set the top bit to zero, or used it as a parity bit in network data transmissions. When this bit was instead made available for representing character data, another 128 characters and control codes could be represented. IBM used this extended range to encode characters used by various languages. No formal standard existed for these ‘extended character sets’; IBM merely referred to the variants as code pages, as it had always done for variants of EBCDIC encodings.

[编辑] IBM PC (OEM) -{A|zh:代碼頁;zh-tw:內碼表;}-

These codepages are most often used under MS-DOS-like operating systems; they include a lot of box drawing characters. Since the original IBM PC code page (number 437) was not really designed for international use, several incompatible variants emerged. Microsoft refers to these as the OEM code pages. Examples include:

437 — 原始的 IBM PC 代碼頁
737 — 希腊语
850 — "Multilingual (Latin-1)" (西歐語系)
852 — "Slavic (Latin-2)" (東歐語系)
855 — 西里尔(Cyril)字母
857 — 土耳其语
858 — "Multilingual" with euro symbol
860 — 葡萄牙语
861 — 冰岛语
863 — 法语加拿大英语
865 — 北欧
866 — 西里尔(Cyril)字母
869 — 希腊语

[编辑] 其他代碼頁的筆記

10000 — Macintosh Roman encoding (followed by several other Mac character sets)
10007 — Macintosh Cyrillic encoding
10029 — Macintosh Central European encoding
932 — 日文
936 — GBK 簡體中文
949 — 韓文
950 — 繁體中文
1200 — UCS-2LE Unicode little-endian
1201 — UCS-2BE Unicode big-endian
65001 — UTF-8 Unicode

In modern applications, operating systems and programming languages, the IBM code pages have been rendered obsolete by international standards, such as ISO 8859-1 and Unicode.

[编辑] Windows (ANSI)代碼頁

Microsoft defined a number of code pages known as the ANSI code pages (as the first one, 1252 was based on an ansi draft of what became ISO 8859-1). Code page 1252 is built on ISO 8859-1 but uses the range 0x80-0x9F for extra printable characters rather than the C1 control codes used in ISO-8859-1. Some of the others are based in part on other parts of ISO 8859 but often rearranged to make them closer to 1252.

1250 — 東歐拉丁文
1251 — 古斯拉夫語
1252 — 西歐拉丁文
1253 — 希臘語
1254 — 土耳其語
1255 — 希伯來語
1256 — 阿拉伯語
1257 — 巴爾
1258 — 越南

Many Microsoft products produce characters in these ranges automatically, notably with ‘smart quotes’. This means that other software has to choose between

not interoperating with documents produced with Microsoft applications
mis-rendering the text in question
adding support for the Microsoft code pages, in effect making Microsoft's implementation a de facto standard.

Microsoft applications also mislabeled text in Windows-1252 as ISO-8859-1 and many Windows-based developers, ignorant of the issues involved, followed their example. Whilst current Microsoft applications seem to correctly label Windows-1252 text as such when they can (such as when sending e-mail), they still allow both reading and writing (e.g., through forms) these characters on websites declared as ISO-8859-1. The most popular competing web browsers do so too, favoring compatibility over standards compliance.

These code pages were sometimes viewed as part of Microsoft's embrace, extend and extinguish strategy towards open standards, though something as simple as an 8 bit character table could never really be kept proprietary. On the other hand, since standards bodies had decided to not assign graphical characters to the upper-half control-character positions 80–9F, which are hardly used in practice for control functions, 12.5% of the available code positions were wasted.

[编辑] Private code pages

When, early in the history of personal computers, users didn't find their character encoding requirements met, private or local codepages were created using Terminate and Stay Resident utilities or by re-programming BIOS EPROMs. In some cases, unofficial code page numbers were invented (e.g., cp895).

When more diverse character set support became available most of those code pages fell into disuse,with some exceptions such as the Kamenický or KEYBCS2 encoding for the Czech and Slovak alphabets.