The Vietnamese Standard Code for Information Interchange (VISCII) is a character set comprising the Vietnamese alphabet, punctuation, and other graphemes.
A traditional extended ASCII character set adds up-to 128 characters to the ASCII set. Vietnamese requires 134 additional letter-diacritic combinations, which is six too many. There are essentially three different ways to handle this problem:
- Use variable-width encoding
- Use combining diacritical marks, as Windows-1258 does
- Replace six of the basic ASCII control characters
VISCII went for the last option, replacing six of the least problematic (e.g., least likely to be recognised by an application and acted on specially) (C0 control codes STX, ENQ, ACK, DC4, EM, and RS) with six of the least-used uppercase letter-diacritic combinations. While this option may cause programs that use those control codes to malfunction when handling VISCII text, it creates fewer complications than the other two options. However, using up all the extended code points for accented letters left no room to add useful symbols, superscripted numbers, curved quotes, proper dashes, etc., like most other extended ASCII character sets.
VISCII was designed by the Vietnamese Standardization Working Group (Viet-Std Group) based in the Silicon Valley in California in 1992 while they were working with the Unicode consortium to include precomposed Vietnamese characters in the Unicode standard.
VISCII was fully supported by the TriChlor Software Group in California, which released a lot of software packages, libraries, and fonts for MS-DOS and Windows, Unix, and Macintosh. However, it, along with other Vietnamese-specific character sets, fell out of usage after the adoption of Unicode. VISCII-compliant software is still available at many ftp sites.