|Classification||Extended ASCII,[a][b] Variable-width encoding, DBCS, CJK encoding|
|Extensions||Windows-950, Big5-HKSCS, numerous others|
|Other related encoding(s)||CNS 11643|
Big5 gets its name from the consortium of five companies in Taiwan that developed it.
The original Big5 character set is sorted first by usage frequency, second by stroke count, lastly by Kangxi radical.
The original Big5 character set lacked many commonly used characters. To solve this problem, each vendor developed its own extension. The ETen extension became part of the current Big5 standard through popularity.
The structure of Big5 does not conform to the ISO 2022 standard, but rather bears a certain similarity to the Shift JIS encoding. It is a double-byte character set (DBCS) with the following structure:
|First byte ("lead byte")||0x81 to 0xfe (or 0xa1 to 0xf9 for non-user-defined characters)|
|Second byte||0x40 to 0x7e, 0xa1 to 0xfe|
(the prefix 0x signifying hexadecimal numbers).
Certain variants of the Big5 character set, for example the HKSCS, use an expanded range for the lead byte including values in the 0x81 to 0xA0 range (similar to Shift JIS).
If the second byte is not in the correct range, behaviour is undefined (i.e., varies from system to system).
The numerical value of individual Big5 codes are frequently given as a 4-digit hexadecimal number, which describes the two bytes that comprise the Big5 code as if the two bytes were a big endian representation of a 16-bit number. For example, the Big5 code for a full-width space, which are the bytes 0xa1 0x40, is usually written as 0xa140 or just A140.
Strictly speaking, the Big5 encoding contains only DBCS characters. However, in practice, the Big5 codes are always used together with an unspecified, system-dependent single-byte character set (ASCII, or an 8-bit character set such as code page 437), so that you will find a mix of DBCS characters and single-byte characters in Big5-encoded text. Bytes in the range 0x00 to 0x7f that are not part of a double-byte character are assumed to be single-byte characters. (For a more detailed description of this problem, please see the discussion on "The Matching SBCS" below.)
The meaning of non-ASCII single bytes outside the permitted values that are not part of a double-byte character varies from system to system. In old MSDOS-based systems, they are likely to be displayed as 8-bit characters; in modern systems, they are likely to either give unpredictable results or generate an error.
In the original Big5, the encoding is compartmentalized into different zones:
|0x8140 to 0xa0fe||Reserved for user-defined characters 造字|
|0xa140 to 0xa3bf||"Graphical characters" 圖形碼|
|0xa3c0 to 0xa3fe||Reserved, not for user-defined characters|
|0xa440 to 0xc67e||Frequently used characters 常用字|
|0xc6a1 to 0xc8fe||Reserved for user-defined characters|
|0xc940 to 0xf9d5||Less frequently used characters 次常用字|
|0xf9d6 to 0xfefe||Reserved for user-defined characters|
The "graphical characters" actually comprise punctuation marks, partial punctuation marks (e.g., half of a dash, half of an ellipsis; see below), dingbats, foreign characters, and other special characters (e.g., presentational "full width" forms, digits for Suzhou numerals, zhuyin fuhao, etc.)
In most vendor extensions, extended characters are placed in the various zones reserved for user-defined characters, each of which are normally regarded as associated with the preceding zone. For example, additional "graphical characters" (e.g., punctuation marks) would be expected to be placed in the 0xa3c0–0xa3fe range, and additional logograms would be placed in either the 0xc6a1–0xc8fe or the 0xf9d6–0xfefe range. Sometimes, this is not possible due to the large number of extended characters to be added; for example, Cyrillic letters and Japanese kana have been placed in the zone associated with "frequently-used characters".
An individual Big5 code does not always represent a complete semantic unit. The Big5 codes of logograms are always logograms, but codes in the "graphical characters" section are not always complete "graphical characters". What Big5 encodes are particular graphical representations of characters or part of characters that happen to fit in the space taken by two monospaced ASCII characters. This is a property of double-byte character sets as normally used in CJK (Chinese, Japanese, and Korean) computing, and is not a unique problem of Big5.
(The above might need some explanation by putting it in historical perspective, as it is theoretically incorrect: Back when text mode personal computing was still the norm, characters were normally represented as single bytes and each character takes one position on the screen. There was therefore a practical reason to insist that double-byte characters must take up two positions on the screen, namely that off-the-shelf, American-made software would then be usable without modification in a DBCS-based system. If a character can take an arbitrary number of screen positions, software that assumes that one byte of text takes one screen position would produce incorrect output. Of course, if a computer never had to deal with the text screen, the manufacturer would not enforce this artificial restriction; the Apple Macintosh is an example. Nevertheless, the encoding itself must be designed so that it works correctly on text-screen-based systems.)
To illustrate this point, consider the Big5 code 0xa14b (…). To English speakers this looks like an ellipsis and the Unicode standard identifies it as such; however, in Chinese, the ellipsis consists of six dots that fit in the space of two Chinese characters (……), so in fact there is no Big5 code for the Chinese ellipsis, and the Big5 code 0xa14b just represents half of a Chinese ellipsis. It represents only half of an ellipsis because the whole ellipsis should take the space of two Chinese characters, and in many DBCS systems one DBCS character must take exactly the space of one Chinese character.
Characters encoded in Big5 do not always represent things that can be readily used in plain text files; an example is "citation mark" (0xa1ca, ﹋), which is, when used, required to be typeset under the title of literary works. Another example is the Suzhou numerals, which is a form of scientific notation that requires the number to be laid out in a 2-D form consisting of at least two rows.
In practice, Big5 cannot be used without a matching Single Byte Character Set (SBCS); this is mostly to do with a compatibility reason. However, as in the case of other CJK DBCS character sets, the SBCS to use has never been specified. Big5 has always been defined as a DBCS, though when used it must be paired with a suitable, unspecified SBCS and therefore used as what some people call a MBCS; nevertheless, Big5 by itself, as defined, is strictly a DBCS.
The SBCS to use being unspecified implies that the SBCS used can theoretically vary from system to system. Nowadays, ASCII is the only possible SBCS one would use. However, in old DOS-based systems, Code Page 437—with its extra special symbols in the control code area including position 127—was much more common. Yet, on a Macintosh system with the Chinese Language Kit, or on a Unix system running the cxterm terminal emulator, the SBCS paired with Big5 would not be Code Page 437.
Outside the valid range of Big5, the old DOS-based systems would routinely interpret things according to the SBCS that is paired with Big5 on that system. In such systems, characters 127 to 160, for example, were very likely not avoided because they would produce invalid Big5, but used because they would be valid characters in Code Page 437.
The modern characterization of Big5 as an MBCS consisting of the DBCS of Big5 plus the SBCS of ASCII is therefore historically incorrect and potentially flawed, as the choice of the matching SBCS was, and theoretically still is, quite independent of the flavour of Big5 being used.
The inability of ASCII to support large character sets such as used for Chinese, Japanese and Korean led to governments and industry to find creative solutions to enable their languages to be rendered on computers. A variety of ad-hoc and usually proprietary input methods led to efforts to develop a standard system. As a result, Big5 encoding was defined by the Institute for Information Industry of Taiwan in 1984. The name "Big5" is in recognition that the standard emerged from collaboration of five of Taiwan's largest IT firms: Acer (宏碁); MiTAC (神通); JiaJia (佳佳), ZERO ONE Technology (零壹 or 01tech); and, First International Computer (FIC) (大眾).
Big5 was rapidly popularized in Taiwan and worldwide among Chinese who used the traditional Chinese character set through its adoption in several commercial software packages, notably the E-TEN Chinese DOS input system (ETen Chinese System). The Republic of China government declared Big5 as their standard in mid-1980s since it was, by then, the de facto standard for using traditional Chinese on computers.
The original Big-5 only include CJK logograms from two lists "常用國字標準字體表; cháng yòng gúo zì bīao zhǔn zì tĭ bǐao" (4808 characters) and "次常用國字標準字體表; cì cháng yòng gúo zì bīao zhǔn zì tĭ bǐao" (6343 characters), but not letters from people's names, place names, dialects, chemistry, biology, Japanese kana. As a result, many Big-5 supporting software include extensions to address the problems.
In some versions of Eten, there are extra graphical symbols and Simplified Chinese characters.
Microsoft (微軟) created its own version of Big5 extension as Code page 950 for use with Microsoft Windows, which supports ETEN's extensions, but only the F9D6-F9FE code points. In Windows ME, the euro currency symbol was mapped to Big-5 code point A3E1, but not in later versions of the operating system.
After installing Microsoft's HKSCS patch on top of traditional Chinese Windows (or any version of Windows 2000 and above with proper language pack), applications using code page 950 automatically use a hidden code page 951 table. The table supports all code points in HKSCS-2001, except for the compatibility code points specified by the standard.
Code page 950 used by Windows 2000 and Windows XP maps hiragana and katakana characters to Unicode private use area block when exporting to Unicode, but to the proper hiragana and katakana Unicode blocks in Windows Vista.
ChinaSea fonts (中國海字集) are Traditional Chinese fonts made by ChinaSea. The fonts are rarely sold separately, but are bundled with other products, such as the Chinese version of Microsoft Office 97. The fonts support Japanese kana, kokuji, and other characters missing in Big-5. As a result, the ChinaSea extensions have become more popular than the government-supported extensions.[when?] Some Hong Kong BBSes had used encodings in ChinaSea fonts before the introduction of HKSCS.
The 'Sakura' font (日和字集 Sakura Version) is developed in Hong Kong and is designed to be compatible with HKSCS. It adds support for kokuji and proprietary dingbats (including Doraemon) not found in HKSCS.
Unicode-at-on (Unicode補完計畫), formerly BIG5 Extension, extends BIG-5 by altering code page tables, but uses the ChinaSea extensions starting with version 2. However, with the bankruptcy of ChinaSea, late development, and the increasing popularity of HKSCS and Unicode (the project is not compatible with HKSCS), the success of this extension is limited at best.
Despite the problems, characters previously mapped to Unicode Private Use Area are remapped to the standardized equivalents when exporting characters to Unicode format.
The web sites of the Oriental Daily News and Sun Daily, belonging to the Oriental Press Group Limited(東方報業集團有限公司) in Hong Kong, used a downloadable font with a different Big-5 extension coding than the HKSCS.
The Taiwan Ministry of Education supplied its own font, the Taiwan Ministry of Education font(臺灣教育部造字檔) for use internally.
Taiwan's Council of Agriculture font, Executive Yuan introduced a 133-character custom font, the Taiwan Council of Agriculture font(臺灣農委會常用中文外字集) that includes 84 characters from the 'fish' radical and 7 from the 'bird' radical.
The Chinese Foundation for Digitization Technology(中文數位化技術推廣委員會) introduced Big5+ in 1997, which used over 20000 code points to incorporate all CJK logograms in Unicode 1.1. However, the extra code points exceeded the original Big-5 definition (Big5+ uses high byte values 81-FE and low byte values 40-7E and 80-FE), preventing it from being installed on Microsoft Windows without new codepage files.
To allow Windows users to use custom fonts, the Chinese Foundation for Digitization Technology introduced Big-5E, which added 3954 characters (in three blocks of code points: 8E40-A0FE, 8140-86DF, 86E0-875C) and removed the Japanese kana from the ETEN extension. Unlike Big-5+, Big5E extends Big-5 within its original definition. Mac OS X 10.3 and later supports Big-5E in the fonts LiHei Pro (儷黑 Pro.ttf) and LiSong Pro (儷宋 Pro.ttf).
The Chinese Foundation for Digitization Technology made a Big5 definition and put it into CNS 11643 in note form, making it part of the official standard in Taiwan.
Big5-2003 incorporates all Big-5 characters introduced in the 1984 ETEN extensions (code points A3C0-A3E0, C6A1-C7F2, and F9D6-F9FE) and the Euro symbol. Cyrillic characters were not included because the authority claimed CNS 11643 does not include such characters.
Hong Kong also adopted Big5 for character encoding. However, written Cantonese has its own characters not available in the normal Big5 character set. To solve this problem, the Hong Kong Government created the Big5 extensions Government Chinese Character Set (GCCS) in 1995 and Hong Kong Supplementary Character Set in 1999. The Hong Kong extensions were commonly distributed as a patch. It is still being distributed as a patch by Microsoft, but a full Unicode font is also available from the Hong Kong Government's web site.
There are two encoding schemes of HKSCS: one encoding scheme is for the Big-5 coding standard and the other is for the ISO 10646 standard. Subsequent to the initial release, there are also HKSCS-2001 and HKSCS-2004. The HKSCS-2004 is aligned technically with the ISO/IEC 10646:2003 and its Amendment 1 published in April 2004 by the International Organization for Standardization (ISO).
Similar to Hong Kong's situation, there are also characters that are needed by Macao but is neither included in Big5 nor HKSCS. Therefore, the Macao Supplementary Character Set have been released to public in Macao for information exchange.