KOI character encodings
A particular feature of the KOI code pages is that the text remains human-readable when the leftmost bit is stripped, should it inadvertently pass through equipment or software that can only deal with 7 bit wide characters. This is due to characters being placed in a special order (128 codepoints apart from the Latin letter they look most similar to), which, however, does not correspond to the alphabetic order in any language that is written in Cyrillic and necessitates the use of lookup tables to perform sorting.
These encodings are derived from ASCII on the base of some correspondence between Latin and Cyrillic (nearly phonetical), which was already used in Russian dialect of Morse code and in MTK-2 telegraph code.
Modern KOI code pages are 8-bit extensions of ASCII. This family of encodings is also known as KOI8, KOI 8 and KOI-8 (КОИ-8).
The family members are:
- KOI8-R / KOI8-RUSSIA for Russian and Bulgarian. (RFC 1489)
- KOI8-U / KOI8-UKRAINE and KOI8-RU for Ukrainian, Belorussian and Russian. (RFC 2319)
- KOI8-T for Tajik.
- KOI8-E / KOI8-CYRILLIC (GOST 19768-74 Roman / Cyrillic)
- KOI8-CS for Czech and Slovak (ČSN (Czech technical standard) 369103. Devised by the Comecon. This encoded Latin with diacritics, as used in Czech and Slovak, rather than Cyrillic, but the basic idea was the same - text should remain legible with the 8-th bit cleared, thus e.g. Č became C etc.).
- KOI8-O for Old Russian.
There is also an obsolete 7-bit KOI7 (КОИ-7) code page, which does not contain lowercase letters. In KOI7, the codes of the 31 Russian uppercase letters are their KOI8 codes with the most significant bit cleared. Other code points are the same as in ASCII.