Control character

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In

standard there are 33 control characters, such as code 7, BEL, which rings a terminal bell.

History

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Procedural signs in Morse code are a form of control character.

A form of control characters were introduced in the 1870 Baudot code: NUL and DEL. The 1901

(LF), and other versions of the Baudot code included other control characters.

The bell character (BEL), which rang a bell to alert operators, was also an early teletype control character.

Some control characters have also been called "format effectors".

In ASCII

There were quite a few control characters defined (33 in ASCII, and the ECMA-48 standard adds 32 more). This was because early terminals had very primitive mechanical or electrical controls that made any kind of state-remembering

. But the number of non-standard variations in use is large, especially among printers, where technology has advanced far faster than any standards body can possibly keep up with.

All entries in the

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.

The

(EBCDIC) character set contains 65 control codes, including all of the ASCII control codes plus additional codes which are mostly used to control IBM peripherals.

ASCII control codes.^[3]

	0x00	0x10
0x00	NUL	DLE
0x01	SOH	DC1
0x02	STX	DC2
0x03	ETX	DC3
0x04	EOT	DC4
0x05	ENQ	NAK
0x06	ACK	SYN
0x07	BEL	ETB
0x08	BS	CAN
0x09	HT	EM
0x0A	LF	SUB
0x0B	VT	ESC
0x0C	FF	FS
0x0D	CR	GS
0x0E	SO	RS
0x0F	SI	US
0x7F	DEL

The control characters in ASCII still in common use include:

• 0x00 (
  C to mark the end of a string
  .
• 0x07 (bell, BEL, \a, ^G), which may cause the device to emit a warning such as a bell or beep sound or the screen flashing.
• 0x08 (backspace, BS, \b, ^H), may overprint the previous character.
• 0x09 (horizontal tab, HT, \t, ^I), moves the printing position right to the next tab stop.
• 0x0A (line feed, LF, \n, ^J), moves the print head down one line, or to the left edge and down. Used as the end of line marker in most UNIX systems and variants.
• 0x0B (vertical tab, VT, \v, ^K), vertical tabulation.
• 0x0C (form feed, FF, \f, ^L), to cause a printer to eject paper to the top of the next page, or a video terminal to clear the screen.
• 0x0D (
  HTTP
  .
• 0x1A (
  Control-Z
  , SUB, ^Z). Acts as an end-of-file for the Windows text-mode file i/o.
• 0x1B (
  GCC only), ^[). Introduces an escape sequence
  .

Control characters may be described as doing something when the user inputs them, such as code 3 (End-of-Text character, ETX, ^C) to interrupt the running process, or code 4 (End-of-Transmission character, EOT, ^D), used to end text input on Unix or to exit a Unix shell. These uses usually have little to do with their use when they are in text being output.

In Unicode

In Unicode, "Control-characters" are U+0000—U+001F (C0 controls), U+007F (delete), and U+0080—U+009F (C1 controls). Their

Display

There are a number of techniques to display non-printing characters, which may be illustrated with the bell character in ASCII encoding:

• Code point: decimal 7, hexadecimal 0x07
• An abbreviation, often three capital letters: BEL
• A special character condensing the abbreviation: Unicode U+2407 (␇), "symbol for bell"
• An ISO 2047 graphical representation: Unicode U+237E (⍾), "graphic for bell"
• Caret notation in ASCII, where code point 00xxxxx is represented as a caret followed by the capital letter at code point 10xxxxx: ^G
• An escape sequence, as in C/C++ character string codes: \a, \007, \x07, etc.

How control characters map to keyboards

ASCII-based

When the control key is held down, letter keys produce the same control characters regardless of the state of the

caps lock

keys. In other words, it does not matter whether the key would have produced an upper-case or a lower-case letter. The interpretation of the control key with the space, graphics character, and digit keys (ASCII codes 32 to 63) vary between systems. Some will produce the same character code as if the control key were not held down. Other systems translate these keys into control characters when the control key is held down. The interpretation of the control key with non-ASCII ("foreign") keys also varies between systems.

Control characters are often rendered into a printable form known as caret notation by printing a caret (^) and then the ASCII character that has a value of the control character plus 64. Control characters generated using letter keys are thus displayed with the upper-case form of the letter. For example, ^G represents code 7, which is generated by pressing the G key when the control key is held down.

Keyboards also typically have a few single keys which produce control character codes. For example, the key labelled "Backspace" typically produces code 8, "Tab" code 9, "Enter" or "Return" code 13 (though some keyboards might produce code 10 for "Enter").

Many keyboards include keys that do not correspond to any ASCII printable or control character, for example cursor control arrows and

that identify the specific physical keys that are pressed; computer software then determines how to handle the keys that are pressed, including any of the four methods described above.

The design purpose

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The control characters were designed to fall into a few groups: printing and display control, data structuring, transmission control, and miscellaneous.

Printing and display control

Printing control characters were first used to control the physical mechanism of printers, the earliest output device. An early example of this idea was the use of Figures (FIGS) and Letters (LTRS) in Baudot code to shift between two code pages. A later, but still early, example was the out-of-band ASA carriage control characters. Later, control characters were integrated into the stream of data to be printed. The carriage return character (CR), when sent to such a device, causes it to put the character at the edge of the paper at which writing begins (it may, or may not, also move the printing position to the next line). The line feed character (LF/NL) causes the device to put the printing position on the next line. It may (or may not), depending on the device and its configuration, also move the printing position to the start of the next line (which would be the leftmost position for

scripts such as the Hebrew and Arabic alphabets). The vertical and horizontal tab characters (VT and HT/TAB) cause the output device to move the printing position to the next tab stop in the direction of reading. The form feed character (FF/NP) starts a new sheet of paper, and may or may not move to the start of the first line. The backspace character (BS) moves the printing position one character space backwards. On printers, including hard-copy terminals, this is most often used so the printer can overprint characters to make other, not normally available, characters. On video terminals and other electronic output devices, there are often software (or hardware) configuration choices that allow a destructive backspace (e.g., a BS, SP, BS sequence), which erases, or a non-destructive one, which does not. The shift in and shift out characters (SI and SO) selected alternate character sets, fonts, underlining, or other printing modes. Escape sequences were often used to do the same thing.

With the advent of computer terminals that did not physically print on paper and so offered more flexibility regarding screen placement, erasure, and so forth, printing control codes were adapted. Form feeds, for example, usually cleared the screen, there being no new paper page to move to. More complex escape sequences were developed to take advantage of the flexibility of the new terminals, and indeed of newer printers. The concept of a control character had always been somewhat limiting, and was extremely so when used with new, much more flexible, hardware. Control sequences (sometimes implemented as escape sequences) could match the new flexibility and power and became the standard method. However, there were, and remain, a large variety of standard sequences to choose from.

Data structuring

The separators (File, Group, Record, and Unit: FS, GS, RS and US) were made to structure data, usually on a tape, in order to simulate punched cards. End of medium (EM) warns that the tape (or other recording medium) is ending. While many systems use CR/LF and TAB for structuring data, it is possible to encounter the separator control characters in data that needs to be structured. The separator control characters are not overloaded; there is no general use of them except to separate data into structured groupings. Their numeric values are contiguous with the space character, which can be considered a member of the group, as a word separator.

For example, the RS separator is used by

protocols.

Transmission control

The transmission control characters were intended to structure a data stream, and to manage re-transmission or graceful failure, as needed, in the face of transmission errors.

The start of heading (SOH) character was to mark a non-data section of a data stream—the part of a stream containing addresses and other housekeeping data. The start of text character (STX) marked the end of the header, and the start of the textual part of a stream. The end of text character (ETX) marked the end of the data of a message. A widely used convention is to make the two characters preceding ETX a checksum or CRC for error-detection purposes. The end of transmission block character (ETB) was used to indicate the end of a block of data, where data was divided into such blocks for transmission purposes.

The escape character (ESC) was intended to "quote" the next character, if it was another control character it would print it instead of performing the control function. It is almost never used for this purpose today. Various printable characters are used as visible "escape characters", depending on context.

The substitute character (

, which are common on other operating systems.

The cancel character (

) is normally used as a flag to indicate no problem detected with current element.

When a transmission medium is half duplex (that is, it can transmit in only one direction at a time), there is usually a master station that can transmit at any time, and one or more slave stations that transmit when they have permission. The enquire character (

).

The device control codes (DC1 to DC4) were originally generic, to be implemented as necessary by each device. However, a universal need in data transmission is to request the sender to stop transmitting when a receiver is temporarily unable to accept any more data. Digital Equipment Corporation invented a convention which used 19 (the device control 3 character (DC3), also known as control-S, or XOFF) to "S"top transmission, and 17 (the device control 1 character (DC1), a.k.a. control-Q, or XON) to start transmission. It has become so widely used that most don't realize it is not part of official ASCII. This technique, however implemented, avoids additional wires in the data cable devoted only to transmission management, which saves money. A sensible protocol for the use of such transmission flow control signals must be used, to avoid potential deadlock conditions, however.

The data link escape character (DLE) was intended to be a signal to the other end of a data link that the following character is a control character such as STX or ETX. For example a packet may be structured in the following way (DLE) <STX> <PAYLOAD> (DLE) <ETX>.

Miscellaneous codes

Code 7 (BEL) is intended to cause an audible signal in the receiving terminal.^[6]

Many of the ASCII control characters were designed for devices of the time that are not often seen today. For example, code 22, "synchronous idle" (SYN), was originally sent by synchronous modems (which have to send data constantly) when there was no actual data to send. (Modern systems typically use a start bit to announce the beginning of a transmitted word— this is a feature of asynchronous communication. Synchronous communication links were more often seen with mainframes, where they were typically run over corporate leased lines to connect a mainframe to another mainframe or perhaps a minicomputer.)

Code 0 (ASCII code name NUL) is a special case. In paper tape, it is the case when there are no holes. It is convenient to treat this as a fill character with no meaning otherwise. Since the position of a NUL character has no holes punched, it can be replaced with any other character at a later time, so it was typically used to reserve space, either for correcting errors or for inserting information that would be available at a later time or in another place. In computing, it is often used for padding in fixed length records; to mark the end of a string; and formerly to give printing devices enough time to execute a control function.

Code 127 (

. Paper tape became obsolete in the 1970s, so this clever aspect of ASCII rarely saw any use after that. Some systems (such as the original Apples) converted it to a backspace. But because its code is in the range occupied by other printable characters, and because it had no official assigned glyph, many computer equipment vendors used it as an additional printable character (often an all-black "box" character useful for erasing text by overprinting with ink).

Non-erasable

, which can only be switched one way, usually from one to zero. In such PROMs, the DEL and NUL characters can be used in the same way that they were used on punched tape: one to reserve meaningless fill bytes that can be written later, and the other to convert written bytes to meaningless fill bytes. For PROMs that switch one to zero, the roles of NUL and DEL are reversed; also, DEL will only work with 7-bit characters, which are rarely used today; for 8-bit content, the character code 255, commonly defined as a nonbreaking space character, can be used instead of DEL.

Many file systems do not allow control characters in filenames, as they may have reserved functions.

See also

• Arrow keys § HJKL keys, HJKL as arrow keys, used on ADM-3A terminal
• C0 and C1 control codes
• Escape sequence
• In-band signaling
• Whitespace character

Notes and references

External links

Look up Appendix:Control characters in Wiktionary, the free dictionary.

Look up Control character or Control code in Wiktionary, the free dictionary.

• ISO IR 1 C0 Set of ISO 646 (PDF)