C0 and C1 control codes explained

The C0 and C1 control code or control character sets define control codes for use in text by computer systems that use ASCII and derivatives of ASCII. The codes 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.

C0 codes are the range 00_HEX–1F_HEX and the default C0 set was originally defined in ISO 646 (ASCII). C1 codes are the range 80_HEX–9F_HEX and the default C1 set was originally defined in ECMA-48 (harmonized later with ISO 6429). The ISO/IEC 2022 system of specifying control and graphic characters allows other C0 and C1 sets to be available for specialized applications, but they are rarely used.

C0 controls

ASCII defined 32 control characters, plus a necessary extra character for the DEL character, 7F_HEX or 01111111_BIN (needed to punch out all the holes on a paper tape and erase it).

This large number of codes was desirable at the time, as multi-byte controls would require implementation of a state machine in the terminal, which was very difficult with contemporary electronics and mechanical terminals.

Only a few codes have maintained their use: BEL, ESC, and the "Format " (FE_n) characters BS, TAB, LF, VT, FF, and CR. Others are unused or have acquired different meanings such as NUL being the C string terminator. Some data transfer protocols such as ANPA-1312, Kermit, and XMODEM do make extensive use of SOH, STX, ETX, EOT, ACK, NAK and SYN for purposes approximating their original definitions; and some file formats use the "Information Separators" (IS_n) such as the Unix info format^[1] and Python's string method.^[2]

The names of some codes were changed in ISO 6429:1992 (or ECMA-48:1991) to be neutral with respect to writing direction. The abbreviations used were not changed, as the standard had already specified that those would remain unchanged when the standard is translated to other languages. In this table both new and old names are shown for the renamed controls (the old name is the one matching the abbreviation).

ASCII control codes, originally defined in ANSI X3.4.^[3] ! ! ! ! Abbreviations! ! Name! ! Description

	0	00	NUL	␀			Does nothing. The code of blank paper tape, and also used for padding to slow transmission.
	1	01	TC1, SOH	␁	Start of Heading		First character of the heading of a message.
	2	02	TC2, STX	␂	Start of Text		Terminates the header and starts the message text.
	3	03	TC3, ETX	␃			Ends the message text, starts a footer (up to the next TC character).
	4	04	TC4, EOT	␄			Ends the transmission of one or more messages.^[4] May place terminals on standby.
	5	05		␅	Enquiry		Trigger a response at the receiving end, to see if it is still present.
	6	06	TC6, ACK	␆			Indication of successful receipt of a message.
	7	07	BEL	␇	Bell, Alert		Call for attention from an operator.
	8	08	FE0, BS	␈	Backspace		Move one position leftwards. Next character may overprint or replace the character that was there.
	9	09	FE1, HT	␉	Character Tabulation, Horizontal Tabulation		Move right to the next tab stop.
	10	0A	FE2, LF	␊	Line Feed		Move down to the same position on the next line (some devices also moved to the left column).
	11	0B	FE3, VT	␋	Line Tabulation, Vertical Tabulation		Move down to the next vertical tab stop.
	12	0C	FE4, FF	␌	Form Feed		Move down to the top of the next page.
	13	0D	FE5, CR	␍	Carriage Return		Move to column zero while staying on the same line.
	14	0E	SO, LS0	␎	Shift Out		Switch to an alternative character set.
	15	0F	SI, LS1	␏	Shift In		Return to regular character set after SO.
	16	10		␐	Data Link Escape		Cause a limited number of contiguously following characters to be interpreted in some different way.^[5] ^[6]
	17	11	DC1, XON	␑	Device Control One		Turn on (DC1 and DC2) or off (DC3 and DC4) devices.Teletype^[7] used these for the paper tape reader and the paper tape punch. The first use became the de facto standard for software flow control.^[8]
	18	12	DC2, TAPE	␒	Device Control Two
	19	13	DC3, XOFF	␓	Device Control Three
	20	14	DC4, TAPE	␔	Device Control Four
	21	15	TC8, NAK	␕			Negative response to a sender, such as a detected error.
	22	16	TC9, SYN	␖	Synchronous Idle		Sent in synchronous transmission systems when no other character is being transmitted.
	23	17	TC10, ETB	␗			End of a transmission block of data when data are divided into such blocks for transmission purposes.
	24	18	CAN	␘			Indicates that the data preceding it are in error or are to be disregarded.
	25	19	EM	␙	End of medium		Indicates on paper or magnetic tapes that the end of the usable portion of the tape had been reached.
	26	1A	SUB	␚	Substitute		Replaces a character that was found to be invalid or in error. Should be ignored.
	27	1B	ESC	␛	Escape		Alters the meaning of a limited number of following bytes. Nowadays this is almost always used to introduce an ANSI escape sequence.
	28	1C	IS4, FS	␜	File Separator		Can be used as delimiters to mark fields of data structures. US is the lowest level, while RS, GS, and FS are of increasing level to divide groups made up of items of the level beneath it. SP (space) could be considered an even lower level.
	29	1D	IS3, GS	␝	Group Separator
	30	1E	IS2, RS	␞	Record Separator
	31	1F	IS1, US	␟	Unit Separator
While not technically part of the C0 control character range, the following two characters can be thought of as having some characteristics of control characters.
	32	20	SP	␠	Space		Move right one character position.
	127	7F	DEL	␡	Delete		Should be ignored. Used to delete characters on punched tape by punching out all the holes.

C1 controls

In 1973, ECMA-35 and ISO 2022 attempted to define a method so an 8-bit "extended ASCII" code could be converted to a corresponding 7-bit code, and vice versa. In a 7-bit environment, the Shift Out would change the meaning of the 96 bytes through (i.e. all but the C0 control codes), to be the characters that an 8-bit environment would print if it used the same code with the high bit set. This meant that the range through could not be printed in a 7-bit environment, thus it was decided that no alternative character set could use them, and that these codes should be additional control codes, which become known as the C1 control codes. To allow a 7-bit environment to use these new controls, the sequences ESC @ through ESC _ were to be considered equivalent. The later ISO 8859 standards abandoned support for 7-bit codes, but preserved this range of control characters.

The first C1 control code set to be registered for use with ISO 2022 was DIN 31626, a specialised set for bibliographic use which was registered in 1979.

The more common general-use ISO/IEC 6429 set was registered in 1983, although the ECMA-48 specification upon which it was based had been first published in 1976 and JIS X 0211 (formerly JIS C 6323).^[9] Symbolic names defined by and early drafts of ISO 10646, but not in ISO/IEC 6429 (and) are also used.

Except for and in EUC-JP text, and in text transcoded from EBCDIC, the 8-bit forms of these codes were almost never used., and are used to control text terminals and terminal emulators, but almost always by using their 7-bit escape code representations. Nowadays if these codes are encountered it is far more likely they are intended to be printing characters from that position of Windows-1252 or Mac OS Roman.

Name

Description^[10]
@	128	80	PAD	Padding Character	Proposed as a "padding" or "high byte" for single-byte characters to make them two bytes long for easier interoperability with multiple byte characters. Extended Unix Code (EUC) occasionally uses this.^[11]
A	129	81	HOP	High Octet Preset	Proposed to set the high byte of a sequence of multiple byte characters so they only need one byte each, as a simple form of data compression.
B	130	82	BPH	Break Permitted Here	Follows a graphic character where a line break is permitted. Roughly equivalent to a soft hyphen or zero-width space except it does not define what is printed at the line break.
C	131	83	NBH	No Break Here	Follows the graphic character that is not to be broken. See also word joiner.
D	132	84	IND	Index	Move down one line without moving horizontally, to eliminate ambiguity about the meaning of LF.
E	133	85	NEL	Next Line	Equivalent to CR+LF, to match the EBCDIC control character.
F	134	86	SSA	Start of Selected Area	Used by block-oriented terminals. In xterm moves to the lower-left corner of the screen, since certain software assumes this behaviour.^[12]
G	135	87	ESA	End of Selected Area
H	136	88	HTS		Set a tab stop at the current position.
I	137	89	HTJ		Right-justify the text since the last tab against the next tab stop.
J	138	8A	VTS		Set a vertical tab stop.
K	139	8B	PLD		To produce subscripts and superscripts in ISO/IEC 6429. Subscripts use PLD ''text'' PLU while superscripts use PLU ''text'' PLD.
L	140	8C	PLU
M	141	8D	RI		Move up one line.
N	142	8E	SS2		Next character is from the G2 or G3 sets, respectively.
O	143	8F	SS3
P	144	90	DCS	Device Control String	Followed by a string of printable characters (0x20 through 0x7E) and format effectors (0x08 through 0x0D), terminated by ST (0x9C). Xterm defined a number of these.^[13]
Q	145	91	PU1	Private Use 1	Reserved for private function agreed on between the sender and the recipient of the data.
R	146	92	PU2	Private Use 2
S	147	93	STS	Set Transmit State
T	148	94	CCH	Cancel character	Destructive backspace, to eliminate ambiguity about meaning of .
U	149	95	MW	Message Waiting
V	150	96	SPA	Start of Protected Area	Used by block-oriented terminals.
W	151	97	EPA	End of Protected Area
X	152	98	SOS	Start of String	Followed by a control string terminated by (0x9C) which (unlike,, or) may contain any character except SOS or ST.
Y	153	99	SGC,^[14] SGCI^[15]	Single Graphic Character Introducer	Intended to allow an arbitrary Unicode character to be printed; it would be followed by that character, most likely encoded in UTF-1.
Z	154	9A	SCI	Single Character Introducer	To be followed by a single printable character (0x20 through 0x7E) or format effector (0x08 through 0x0D), and to print it as ASCII no matter what graphic or control sets were in use.
[||155||9B||CSI||Control Sequence Introducer||Used to introduce control sequences that take parameters. Used for ANSI escape sequences.|- id="ST"|\||156||9C||ST||String Terminator||Terminates a string started by,,, or . |- id="OSC"|]||157||9D||OSC||Operating System Command||rowspan=3|Followed by a string of printable characters (0x20 through 0x7E) and format effectors (0x08 through 0x0D), terminated by ST (0x9C), intended for use to allow in-band signaling of protocol information, but rarely used for that purpose. Some terminal emulators, including xterm, use OSC sequences for setting the window title and changing the colour palette. They may also support terminating an OSC sequence with instead of ST.^[16] Kermit used APC to transmit commands.^[17] |- id="PM"|^||158||9E||PM||Privacy Message|- id="APC"|_||159||9F||APC||Application Program Command|} Other control code sets The ISO/IEC 2022 (ECMA-35) extension mechanism allowed escape sequences to change the C0 and C1 sets. The standard C0 control character set shown above is chosen with the sequence and the above C1 set chosen with the sequence . Several official and unofficial alternatives have been defined, but this is pretty much obsolete. Most were forced to retain a good deal of compatibility with the ASCII controls for interoperability. The standard makes ESC,^[18] SP and DEL "fixed" coded characters, which are available in their ASCII locations in all encodings that conform to the standard.^[19] It also specifies that if a C0 set included transmission control (TCn) codes, they must be encoded at their ASCII locations^[20] and could not be put in a C1 set,^[21] and any new transmission controls must be in a C1 set. Other C0 control code sets ANPA-1312, a text markup language used for news transmission, replaces several C0 control characters. IPTC 7901, the newer international version of the above, has its own variations. Videotex has a completely different set. Teletext also defines a set similar to Videotex. T.61/T.51,^[22] and others^[23] replaced EM and GS with SS2 and SS3 so these functions could be used in a 7-bit environment. Some sets replaced FS with SS2,^[24] (same as ANPA-1312). The now-withdrawn JIS C 6225, designated JIS X 0207 in later sources.^[25] replaced FS with CEX or "Control Extension"^[26] which introduces control sequences for vertical text behaviour, superscripts and subscripts and for transmitting custom character graphics. Replacement C1 character sets A specialized C1 control code set is registered for bibliographic use (including string collation), such as by MARC-8.^[27] ^[28] ^[29] Various specialised C1 control code sets are registered for use by Videotex formats. EBCDIC defines up to 29 additional control codes besides those present in ASCII. When translating EBCDIC to Unicode (or to ISO 8859), these codes are mapped to C1 control characters in a manner specified by IBM's Character Data Representation Architecture (CDRA).^[30] Although the New Line (NL) does translate to the ISO/IEC 6429 (although it is often swapped with LF, following UNIX line ending convention), the remainder of the control codes do not correspond. For example, the EBCDIC control and the ECMA-48 control are both used to begin a superscript or end a subscript, but are not mapped to one another. Extended-ASCII-mapped EBCDIC can therefore be regarded as having its own C1 set, although it is not registered with the ISO-IR registry for ISO/IEC 2022. Unicode See main article: Unicode control characters. Unicode reserves the 65 code points described above for compatibility with the C0 and C1 control codes, giving them the general category (control). These are: (C0 controls) and (DEL) assigned to the C0 Controls and Basic Latin block, and (C1 controls) assigned to the C1 Controls and Latin-1 Supplement block. Unicode only specifies semantics for the C0 format controls HT, LF, VT, FF, and CR (note BS is missing); the C0 information separators FS, GS, RS, US (and SP); and the C1 control NEL. The rest of the codes are transparent to Unicode and their meanings are left to higher-level protocols, with ISO/IEC 6429 suggested as a default.^[31] Unicode includes many additional format effector characters besides these, such as marks, embeds, isolates and pops for explicit bidirectional formatting, and the zero-width joiner and non-joiner for controlling ligature use. However these are given the general category (format) rather than . See also Control Pictures ANSI escape code References The Unicode Standard C0 Controls and Basic Latin C1 Controls and Latin-1 Supplement Control Pictures The Unicode Standard, Version 6.1.0, Chapter 16: Special Areas and Format Characters ATIS Telecom Glossary 2007 De litteris regentibus C1 quaestiones septem or Are C1 characters legal in XHTML 1.0? W3C I18N FAQ: HTML, XHTML, XML and Control Codes International register of coded character sets to be used with escape sequences ]

Notes and References

1. Web site: Adding a new node to Info . Info: The online, menu-driven GNU documentation system . Fox . Brian . Brian Fox (computer programmer) . GNU Project.
2. Web site: Built-in Types § str.splitlines . The Python Standard Library . Python Software Foundation.
3. ISO/TC 97/SC 2 . ISO/IEC JTC 1/SC 2#History . The set of control characters of the ISO 646 . 1975 . 1.
4. Web site: end-of-transmission character (EOT) . . 1996 . https://web.archive.org/web/20160309030543/http://www.its.bldrdoc.gov/fs-1037/dir-014/_2015.htm . 2016-03-09 . dead.
5. Web site: data link escape character (DLE) . . 1996 . https://web.archive.org/web/20160801021749/http://www.its.bldrdoc.gov/fs-1037/dir-010/_1429.htm . 2016-08-01 . dead.
6. Web site: Supplementary transmission control functions (an extension of the basic mode control procedures for data communication systems) . ECMA-37 . . 1972.
7. Web site: Robert McConnell . James Haynes . Richard Warren . Understanding ASCII Codes . NADCOMM . December 2002.
8. Web site: What is the point of Ctrl-S? . Unix and Linux Stack exchange . 14 February 2019.
9. Web site: JIS X 02xx 符号 . ja.
10. Book: Control Functions for Coded Character Sets . ECMA-48:1991 . June 1991 . 5th . ECMA/TC 1 . Ecma International . ECMA.
11. Book: CJKV Information Processing: Chinese, Japanese, Korean, and Vietnamese Computing . O'Reilly . Lunde, Ken . 2008 . 9780596800925 . 244.
12. Web site: VT100 Widget Resources (§ hpLowerleftBugCompat) . xterm - terminal emulator for X.
13. Web site: Device-Control functions . XTerm Control Sequences . Moy . Edward . Gildea . Stephen . Dickey . Thomas.
14. Web site: Name Aliases . Unicode Character Database . Unicode Consortium.
15. Web site: Ada 9x Project Report: Character Set Issues for Ada 9x . 1989 . Ronald F. . Brender . Carnegie Mellon University.
16. Web site: Operating System Commands . XTerm Control Sequences . Moy . Edward . Gildea . Stephen . Dickey . Thomas.
17. Book: Frank da Cruz. Christine Gianone. Using C-Kermit. 1997. Digital Press. 978-1-55558-164-0. 278.
18. 104 . ISO/TC97/SC2/WG-7 . ISO/IEC JTC 1/SC 2#History . ECMA . Ecma International . Minimum C0 set for ISO 4873 . 1985-08-01.
19. Book: ECMA-35 . Character Code Structure and Extension Techniques . 6th . ECMA Standard . 1994 . ECMA . Ecma International . 7 . 6.2: Fixed coded characters.
20. Book: ECMA-35 . Character Code Structure and Extension Techniques . 6th . ECMA Standard . 1994 . ECMA . Ecma International . 11 . 6.4.2: Primary sets of coded control functions.
21. Book: ECMA-35 . Character Code Structure and Extension Techniques . 6th . ECMA Standard . 1994 . ECMA . Ecma International . 11 . 6.4.3: Supplementary sets of coded control functions.
22. ITU . ITU . Teletex Primary Set of Control Functions . 1985 . 106.
23. Úřad pro normalizaci a měřeni . The set of control characters of ISO 646, with EM replaced by SS2 . 1987 . 140.
24. ISO/TC 97/SC 2 . ISO/IEC JTC 1/SC 2#History . The set of control characters of ISO 646, with IS4 replaced by Single Shift for G2 (SS2) . 1977 . 36.
25. Web site: Liaison statement to ISO/TC97/SC2/WG8 and ISO/TC97/SC18/WG8 . ISO/TC97/SC2/WG6 N317.rev . ISO/TC97/SC2/WG6 . ISO/IEC JTC 1/SC 2#History . https://web.archive.org/web/20201026055422/http://original-jpeg.org/Document%20archive/wg8/wg8n0604.pdf . 2020-10-26 . dead.
26. ISO/TC 97/SC 2 . ISO/IEC JTC 1/SC 2#History . The C0 set of Control Characters of Japanese Standard JIS C 6225-1979 . 1982 . 74.
27. 40 . Additional Control Codes for Bibliographic Use according to German Standard DIN 31626 . 1979-07-15 . DIN . DIN.
28. 67 . Additional Control Codes for Bibliographic Use according to International Standard ISO 6630 . 1983-06-01 . ISO/TC 46.
29. 124 . Additional Control Codes for Bibliographic Use according to International Standard ISO 6630 . 1986-02-01 . ISO/TC 46.
30. Web site: 3.3 Step 2: Byte Conversion . UTF-EBCDIC . Unicode Technical Report #16 . Umamaheswaran . V.S. . . 1999-11-08 . The 64 control characters […], the ASCII DELETE character (U+007F)[…] are mapped respecting EBCDIC conventions, as defined in IBM Character Data Representation Architecture, CDRA, with one exception -- the pairing of EBCDIC Line Feed and New Line control characters are swapped from their CDRA default pairings to ISO/IEC 6429 Line Feed (U+000A) and Next Line (U+0085) control characters.
31. Book: https://www.unicode.org/versions/Unicode15.0.0/ch23.pdf#page=3 . 23.1: Control Codes . The Unicode Standard . 15.0.0 . 2022 . . 978-1-936213-32-0 . 914–916.