GB 18030 is a new Chinese codepage standard that extends GB 2312-1980 and GBK (which itself is an extension of GB 2312-1980).
It is a multi-byte encoding using 1-byte, 2-byte, and 4-byte codes.
The 1-byte and 2-byte codes have the same assignments as in GBK, which itself is a superset of GB 2312-1980.
There are about 1.6 million valid byte sequences.
It is not possible to determine if a byte sequence is either 2 or 4 bytes long by just examining the lead byte — the second byte must be examined as well.
The Chinese Government has mandated that all applications released on or after 2001-Sep-01 must support GB 18030.
The specification refers directly to a mapping of GB 18030 codes to and from ISO 10646/Unicode to define most character assignments. Some characters that used to be mapped for GBK to the PUA (Private Use Area) for Unicode 2.1 are now assigned in Unicode 3.0, and their mappings from GB 18030 use only the Unicode 3.0 code points.
In addition, GB 18030 defines roundtrip mappings for all 1.1 million Unicode code points including unassigned and non-character ones, but excluding single surrogates. This makes GB 18030 functionally very similar to a UTF.
China has confirmed in discussions with major IT companies that it is sufficient to be able to
According to current understanding, this means that processes can use ISO 10646/Unicode internally if they also provide conversion between GB 18030 and ISO 10646/Unicode. This is possible because of the definition of GB 18030 with a mapping table to ISO 10646/Unicode.
A Unicode mapping table for GB 18030 in XML format is available from the ICU website (.xml and .ucm).
Both GB 18030 and ISO 10646 define sets of "user" codes. The User-Defined Areas in GB 18030 do not correspond 1:1 to Private-Use Areas in Unicode.
Some assigned characters are mapped from 2-byte parts of GBK and GB 18030 to the Private-Use Area in the BMP (U+E000..U+F8FF). A small portion of these mappings have changed between GBK and GB 18030, and GB 18030 maps them instead to Unicode characters that were introduced in Unicode 3.0.
The User-Defined Areas in the 2-byte parts of GBK and GB 18030 are mapped to other parts of the Private-Use Area in the BMP. Note that all single-byte and 2-byte codes have defined mappings — they must be mapped according to the standard table.
Similarly, GB 18030 maps all remaining Unicode Private-Use code points to four-byte GB 18030 codes.
GB 18030 also provides a User-Defined Area with 25200 four-byte codes, without specified mappings. Normally, they need to be treated as unassigned codes.
There are some 460000 four-byte codes that are reserved for future use and must be treated as unassigned codes at this point.
As noted above, all Private-Use code points are mapped to GB 18030 codes. This means that they can be exchanged via GB 18030. In addition to the usual agreement about Private-Use characters between processes exchanging them, one must take the GB 18030 assignments into account when exchanging text in GB 18030.
GB 18030 assigns characters to some of the codes corresponding to Private-Use BMP code points. All other such codes are either User-Defined in GB 18030 or not specified other than through the mapping correspondence.
An article with more details and with implementation suggestions is available on the IBM DeveloperWorks site.