C0 and C1 control codes

The C0 and C1 control code or control character sets define control codes for use in text by computer systems that use the ISO/IEC 2022 system of specifying control and graphic characters. Most character encodings, in addition to representing printable characters, also have characters such as these that represent additional information about the text, such as the position of a cursor, an instruction to start a new line, or a message that the text has been received.

The C0 set defines codes in the range 00_HEX–1F_HEX and the C1 set defines codes in the range 80_HEX–9F_HEX. The default C0 set was originally defined in ISO 646 (ASCII), while the default C1 set was originally defined in ECMA-48 (harmonized later with ISO 6429). While other C0 and C1 sets are available for specialized applications, they are rarely used.

Encoding interoperability

While the C1 control characters are used in conjunction with the ISO/IEC 8859 series of graphical character sets among others, they are rarely used directly, except on specific platforms such as OpenVMS. When they turn up in documents, Web pages, e-mail messages, etc., which are ostensibly in an ISO-8859-n encoding, their code positions generally refer instead to the characters at that position in a proprietary, system-specific encoding such as Windows-1252 or the Apple Macintosh (Mac OS Roman) character set that use the codes provided for representation of the C1 set with a single 8-bit byte to instead provide additional graphic characters, though this is technically invalid under the ISO encodings.

Control character

In computing and telecommunication, a control character or non-printing character is a code point (a number) in a character set, that does not represent a written symbol. They are used as in-band signaling to cause effects other than the addition of a symbol to the text. All other characters are mainly printing, printable, or graphic characters, except perhaps for the "space" character (see ASCII printable characters).

All entries in the ASCII table below code 32 (technically the C0 control code set) are of this kind, including CR and LF used to separate lines of text. The code 127 (DEL) is also a control character. Extended ASCII sets defined by ISO 8859 added the codes 128 through 159 as control characters, this was primarily done so that if the high bit was stripped it would not change a printing character to a C0 control code, but there have been some assignments here, in particular NEL. This second set is called the C1 set.

These 65 control codes were carried over to Unicode. Unicode added more characters that could be considered controls, but it makes a distinction between these "Formatting characters" (such as the Zero-width non-joiner), and the 65 Control characters.