Control character

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Categories: Control characters

In computing, a control character or non-printing character, is a code point (a number) in a character set that does not in itself represent a written symbol. All entries in the ASCII table below 32 (technically the C0 control code set) are of this kind, including BEL (which is intended to cause an audible signal in the receiving terminal), SYN (which is a synchronization signal), and ENQ (a signal that is intended to trigger a response at the receiving end, to see if it is still present). The Unicode standard has added many new non-printing characters, for example the Zero-width non-joiner. The remainder of this article covers control codes in general and some codes that are in common use. For detailed tables of the C0 and C1 control codes used in ASCII and ISO-8859-n, please see their respective articles.


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In ASCII

The control characters in ASCII still in common use include

  • 7 (bell), which may cause the device receiving it to emit a warning of some kind
  • 8 (backspace), used either to erase the last character printed or to overprint it
  • 9 (horizontal tab)
  • 10 (line feed), used to end lines in most UNIX systems and variants
  • 12 (form feed), to cause a printer to eject a page
  • 13 (carriage return), used to end lines of text in Mac OS, OS-9, FLEX and CP/M-80 derivatives including DOS, and
  • 27 (escape).

Occasionally one might encounter modern uses of other codes such as code 4 (End of transmission) used to end a Unix shell session or PostScript printer transmission.

Code 27 (Escape) is a case worth elaborating. Even though many of these control characters are never used, the concept of sending device-control information intermixed with printable characters is so useful that device makers found a way to send hundreds of device instructions. Specifically, they used a series of multiple characters called a "control sequence" or "escape sequence".

Typically code 27 was first sent to alert the device that the following characters were to be interpreted as a control sequence rather than as plain characters, then one or more characters would follow specifying some detailed action, after which the device would go back to interpreting characters normally. For example, the sequence of code 27, followed by the printable characters "[2;10H", would cause a Digital VT-102 terminal to move its cursor to the 10th cell of the 2nd line of the screen. Some standards exist for these sequences, notably ANSI X3.64 (1979), which was based on the behavior of VT-100 series terminals. 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 even with.

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How control characters map to keyboards

ASCII-based keyboards have a key labelled "Control" or "Ctrl" (sometimes referred to as "Cntl") which is used much like a shift key, being depressed in combination with another letter or symbol key. In this way the control key generates the code 64 places below the code for the (generally) uppercase letter it is pressed in combination with, producing one of the 32 ASCII control codes. So, the octet code produced by a control key combination has the second from leftmost bit (the 27th, i.e. the 64th, place bit) forced to zero and is otherwise the bit pattern generated when the control key is not one of those pressed. For example, pressing "control" and the letter "G" (code 71 in base 10, which is 01000111 in binary, a code which generally also requires the shift key be depressed), produces the code 7 (Bell, 7 in base 10, or 00000111 in binary). A key press combination that produces a code with 0 in the 64th place is unaffected should the control key be held down as well.

Keyboards have single keys that 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").

Modern keyboards have many keys that do not correspond to ASCII characters or control characters, for example cursor control arrows and word processing functions. These keyboards communicate these keys to the attached computer by one of three methods: appropriating some otherwise unused control character for the new use, using some encoding other than ASCII, or using multi-character control sequences. Keyboards attached to stand-alone personal computers typically use one (or both) of the first two methods. "Dumb" computer terminals typically use control sequences.

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The design purpose

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

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Printing and Display control

Printing control characters were first used to control the physical mechanisim of a printers, the earliest output device. Carriage return says to put the character at the edge of the paper at which writing begins (it may or may not also move to the next line). Line feed indicates to put the next character at the next line in the direction new lines occur (and may or may not also move to the beginning of the line). Vertical and horizontal tab request the printer to move the print head to the next tab stop in the direction of reading. Form feed starts a new sheet of paper. Backspace moves the next position one character backwards, so the printer can overprint characters to make special characters. Shift in and shift out were to select alternate character sets, fonts, underlining or other printing modes, although often other escape sequences were used for these purposes.

With the advent of computer terminals that did not physically print on paper and so offered more flexiblity regarding character placement, erasure, and so forth, printing control codes were adapted. Form feeds, for example, cleared the screen instead of feeding the next blank sheet of paper. More complex escape sequences were developed to take advantage of the power of the new terminals, and new printers. There were no longer enough single character codes available to support all the features of the new peripheral devices and the distinction between control characters and escape sequences began to blur.

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Data structuring

The separators (group, record, etc) were made to structure data, usually on a tape, in order to simulate punch cards. End of media warns that the tape (or whatever) is ending.

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Transmission control

The transmission control characters were intended to structure a data packet and control when to retransmit it if it has an error.

The start of header was to mark the non-data section of a data packet--the part of a message with addresses and other housekeeping data. The start-of-text marked the end of the header, and the start of the text. End-of-text marked the end of the data of a message. A standard convention is to make the two characters preceding the end of text the checksum or CRC of the message.

Escape was supposed to preface a binary value in a message that might otherwise be interpreted as a control character. For example, the value for binary 27 would be Escape Escape.

Substitute was intended to request a translation of the next character from a printable character to a binary value, usually by setting bit 5 to zero. This is handy because some transmission media (such as sheets of paper produced by typewriters) only transmit printable characters.

Cancel would stop a transmission of a packet. Negative acknowledge requests a retransmission of a packet. Acknowledge indicates that a transmission was received correctly.

When a transmission medium is half duplex (that is, it can only transmit in 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. Enquiry is used by a master station to ask a slave station to send its next message. A slave station indicates that it has completed its transmission by sending end of transmission.

The device control codes were originally generic, to be defined differently for each device. However, a universal need in data transmission is to request the sender to stop transmitting when a receiver can't take more data right now. Digital Equipment Corporation invented a convention which used 19, (device control 3, also known as control S, or "X-OFF") to "S"top transmission, and 17, (device control 1, AKA control Q, or "X-ON") to start transmission. This lets manufacturers control the transmission without "transmission control" wires in the data cable. This saves money and makes operation more reliable by reducing the number of connections in a cable.

Data link escape tells the other end of the data link to end a session.

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Miscellaneous

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", 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.)

Code 0, null, is a special case. In paper tape, it is the case when there are no holes. It's convenient to treat this as a non-existent character.

Code 127 is likewise a special case. Its code is all-bits-on in binary, which made it easy to erase a section of paper tape, a common storage medium of the day, by punching all the holes. Paper tape became obsolete quickly, so this feature was almost never used.

But because its code is in the range occupied by other printable characters, many computers used it as an additional printable character (often an all-black "box" character useful for erasing text by overprinting).

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See also

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External links

it:Carattere di controllo ru:Управляющие символы zh:控制字符