Code page
From Wikipedia, the free encyclopedia
Code page is the traditional IBM term used for a specific character encoding table: a mapping in which a sequence of bits, usually a single octet representing integer values 0 through 255, is associated with a specific character. IBM and Microsoft often allocate a code page number to a character set even if that charset is better known by another name.
Whilst the term code page originated from IBM's EBCDIC-based mainframe systems, the term is most commonly associated with the IBM PC code pages. Microsoft, a maker of PC operating systems, refers to these code pages as OEM code pages, and supplements them with its own "ANSI" code pages.
Most well-known code pages, excluding those for the CJK languages and Vietnamese, represent character sets that fit in 8 bits and do not involve anything that cannot be represented by mapping each code to a simple bitmap, such as combining characters, complex scripts, etc.
The text mode of standard (VGA compatible) PC graphics hardware is built around using an 8 bit code page, though it is possible to use two at once with some color depth sacrifice, and up to 8 may be stored in the display adaptor for easy switching [1]. There were a selection of code pages that could be loaded into such hardware. However, it is now commonplace for operating system vendors to provide their own character encoding and rendering systems that run in a graphics mode and bypass this system entirely. The character encodings used by these graphical systems (particularly Windows) are sometimes called code pages as well.
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[edit] 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.
[edit] IBM PC (OEM) code pages
These code pages 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 — The original IBM PC code page
- 737 — Greek
- 775 — Estonian, Lithuanian and Latvian
- 850 — "Multilingual (Latin-1)" (Western European languages)
- 852 — "Slavic (Latin-2)" (Eastern European languages)
- 855 — Cyrillic
- 857 — Turkish
- 858 — "Multilingual" with euro symbol
- 860 — Portuguese
- 861 — Icelandic
- 862 — Hebrew
- 863 — French Canadian
- 865 — Nordic
- 866 — Cyrillic
- 869 — Greek
In modern applications, operating systems and programming languages, the IBM code pages have been rendered obsolete by newer & better international standards, such as Unicode.
[edit] Other code pages of note
The following code page numbers are specific to Microsoft Windows. IBM uses different numbers for these code pages.
- 10000 — Macintosh Roman encoding (followed by several other Mac character sets)
- 10007 — Macintosh Cyrillic encoding
- 10029 — Macintosh Central European encoding
- 932 — Supports Japanese
- 936 — GBK Supports Simplified Chinese
- 949 — Supports Korean
- 950 — Supports Traditional Chinese
- 1200 — UCS-2LE Unicode little-endian
- 1201 — UCS-2BE Unicode big-endian
- 65000 — UTF-7 Unicode
- 65001 — UTF-8 Unicode
- ASMO449+ — Supports Arabic
- MIK — Supports Bulgarian and Russian as well
[edit] Windows (ANSI) code pages
Microsoft defined a number of code pages known as the ANSI code pages (as the first one, 1252 was based on an apocryphal 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 — East European Latin
- 1251 — Cyrillic
- 1252 — West European Latin
- 1253 — Greek
- 1254 — Turkish
- 1255 — Hebrew
- 1256 — Arabic
- 1257 — Baltic
- 1258 — Vietnamese
[edit] Criticism
Many products by Microsoft and other companies use the Microsoft code pages to encode their text. This means that other software has to choose between
- not working with documents produced with those applications
- mis-rendering the text in question
- adding support for the Microsoft code pages, in effect making Microsoft's implementation a de facto standard.
The advent of Unicode and XML has solved most of these problems, because it provides and to some extent enforces clear labels for character encoding.
Applications may also mislabel text in Windows-1252 as ISO-8859-1, the default character set for HTML. Fortunately the only difference between these code pages is that the range ISO-8859-1 reserves for control characters, Windows-1252 uses for additional printable characters. Since control characters have no function in HTML, web browsers tend to use Windows-1252 rather than ISO-8859-1.
[edit] Private code pages
When, early in the history of personal computers, users didn't find their character encoding requirements met, private or local code pages 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. Another character set is Iran System encoding standard that was created by Iran System corporation for Persian language support. This standard was in use in Iran in DOS-based programs and after introduction of Microsoft code page 1256 this standard became obsolete. However some Windows and DOS programs using this encoding are still in use and some Windows fonts with this encoding exist.
[edit] See also
- Windows code page
- Character encoding
- CCSID More precise definition of how "codepages" are used in IBM.
[edit] External links
- IBM code pages
- IBM/ICU Charset Information
- Microsoft code page identifiers (Microsoft's list contains only code pages actively used by normal apps on Windows see also Torsten Mohrin's list for the full list of supported code pages)
- Shorter Microsoft list containing only the ANSI and OEM code pages but with links to more detail on each
- Character Sets And Code Pages At The Push Of A Button
- Microsoft Chcp command: Display and set the console active code page
- A list of languages and the codepages they use on windows, from the project Natural Language Support Objects(NLSO)
