KOI-7 (КОИ-7) is a 7-bit character encoding, designed to cover Russian, which uses the Cyrillic alphabet.
In Russian, KOI-7 stands for Kod Obmena Informatsiey, 7 bit (Код Обмена Информацией, 7 бит) which means "Code for Information Exchange, 7 bit".[1]
It was first standardized in GOST 13052-67 (with the 2nd revision GOST 13052-74 / ST SEV 356-76) and GOST 27463-87 / ST SEV 356-86.
Shift Out (SO) and Shift In (SI) control characters are used in KOI-7, where SO starts printing Russian letters (KOI-7 N1), and SI starts printing Latin letters again (KOI-7 N0), or for lowercase and uppercase switching. This version is also known as KOI7-switched aka csKOI7switched.[2][3]
On ISO 2022 compatible computer terminals KOI7-switched can be activated by the escape sequence ESC ( @ ESC ) N LS0
.[2]
KOI-7 was used on machines like the SM EVM (СМ ЭВМ) and DVK (ДВК); KOI-7 N2 was utilized in the machine-language of the Электроника Д3-28 (Elektronika D3-28) as four-digit hexadecimal code, БЭСМ-6 (BESM-6), where it was called ВКД, (internal data code). The encodings were also used on RSX-11, RT-11 and similar systems.[4]
KOI-7 N0[2] (КОИ-7 Н0[1]) is identical to the IRV set in ISO 646:1967.[2] Compared to US-ASCII, the dollar sign ("$") at code point 24 (hex) was replaced by the universal currency sign "¤", but this was not maintained in all cases, in particular not after the fall of the Iron Curtain. Likewise, the IRV set in ISO/IEC 646:1991 also changed the character back to a dollar sign.
KOI-7 N1 (КОИ-7 Н1[1]) was first standardized in GOST 13052-67,[5][6][7] and later also in ISO 5427.[2] It is sometimes referred to as "koi-0" as well.[6]
Compared to ASCII and ISO 646 uppercase and lowercase letters are swapped in order to make it easier to recognize Russian text when presented using ASCII.[6]
To trim the alphabet into chunks of 32 characters the dotted Ё/ë was dropped.[6] In order to avoid conflicts with ASCII's and ISO 646's definition as DEL and its usage as EOF marker (-1) in some systems, it dropped the "CAPITAL HARD SIGN" Ъ that would have naturally resided at this location.[6]
In a Bulgarian variant the unnecessary Russian "CAPITAL YERY" Ы at code point 121 was replaced by the "CAPITAL HARD SIGN" Ъ.[6]
KOI-7 N2 (КОИ-7 Н2[1]), like KOI-7 N1, was also standardized in GOST 13052-67.[7]
Kermit names it SHORT-KOI / short-koi.[8][9]