Caret notation

Caret notation is a notation for control characters in ASCII. The notation assigns ^A to control-code 1, sequentially through the alphabet to ^Z assigned to control-code 26 (0x1A). For the control-codes outside of the range 1–26, the notation extends to the adjacent, non-alphabetic ASCII characters.

Often a control character can be typed on a keyboard by holding down the Ctrl and typing the character shown after the caret. The notation is often used to describe keyboard shortcuts even though the control character is not actually used (as in "type ^X to cut the text").

The meaning or interpretation of, or response to the individual control-codes is not prescribed by the caret notation.

Description

The notation consists of a caret (^) followed by a single character (usually a capital letter). The character has the ASCII code equal to the control code with the bit representing 0x40 reversed. A useful mnemonic, this has the effect of rendering the control codes 1 through 26 as ^A through ^Z. Seven ASCII control characters map outside the upper-case alphabet: 0 (NUL) is ^@, 27 (ESC) is ^[, 28 is ^\, 29 is ^], 30 is ^^, 31 is ^_, and 127 (DEL) is ^?.

Examples are "^M^J" for the Windows CR, LF newline pair, and describing the ANSI escape sequence to clear the screen as "^[[3J".

Only the use of characters in the range of 63–95 ("?@ABC...XYZ[\]^_") is specifically allowed in the notation, but use of lower-case alphabetic characters entered at the keyboard is nearly always allowed – they are treated as equivalent to upper-case letters. When converting to a control character, except for '?', masking with 0x1F will produce the same result and also turn lower-case into the same control character as upper-case.

There is no corresponding version of the caret notation for control-codes with more than 7 bits such as the C1 control characters from 128–159 (0x80–0x9F). Some programs that produce caret notation show these as backslash and octal ("\200" through "\237"). Also see the bar notation used by Acorn Computers, below.

History

The convention dates back to at least the PDP-6 (1964). A manual for the PDP-6 describes Control+C as printing ^↑C, i.e., a small superscript upwards arrow before the C.[1] In the change from 1961 ASCII to 1968 ASCII, the up arrow became a caret.[2]

Use in software

Many computer systems allow the user to enter a control character by holding down Ctrl and pressing the letter used in the caret notation. This is practical, because many control characters (e.g., EOT) cannot be entered directly from a keyboard. Although there are many ways to represent control characters, this correspondence between notation and typing makes the caret notation suitable for many applications.

Usually, the need to hold down ⇧ Shift is avoided, for instance lower-case letters work just like upper-case ones. On a US keyboard layout ctrl+/ produces DEL and ctrl+2 produces ^@. It is also common for ctrl+space to produce ^@.

Caret notation is used to describe control characters in output by many programs, particularly Unix terminal drivers and text file viewers such as more and less commands. Although the use of control-codes is somewhat standard, some uses differ from operating system to operating system, or even from program to program. The actual meaning or interpretation of the individual control-codes is not prescribed by the caret notation, and although the ASCII specification does give names to the control-codes, it does not prescribe how software should respond to them.

Alternate notations

The GSTrans string processing API on the operating systems for the Acorn Atom and the BBC Micro, and on RISC OS for the Acorn Archimedes and later machines, use the vertical bar character | in place of the caret. For example, |M (pronounced "control em", the same as for the ^M notation) is the carriage return character, ASCII 13. || is the vertical bar character code 124, |? is character 127 as above and |! adds 128 to the code of the character that follows it, so |!|? is character code 128 + 127 = 255.

References

"PDP-6 Timesharing Software" (PDF). Digital Equipment Corporation. p. 4.
Haynes, Jim (2015-01-13). "First-Hand: Chad is Our Most Important Product: An Engineer's Memory of Teletype Corporation". Engineering and Technology History Wiki (ETHW). Archived from the original on October 31, 2016. Retrieved 2016-10-31. There was the change from 1961 ASCII to 1968 ASCII. Some computer languages used characters in 1961 ASCII such as up arrow and left arrow. These characters disappeared from 1968 ASCII. We worked with Fred Mocking, who by now was in Sales at Teletype, on a type cylinder that would compromise the changing characters so that the meanings of 1961 ASCII were not totally lost. The underscore character was made rather wedge-shaped so it could also serve as a left arrow.

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[1] "PDP-6 Timesharing Software" (PDF). Digital Equipment Corporation. p. 4.

[Haynes_2015-2] Haynes, Jim (2015-01-13). "First-Hand: Chad is Our Most Important Product: An Engineer's Memory of Teletype Corporation". Engineering and Technology History Wiki (ETHW). Archived from the original on October 31, 2016. Retrieved 2016-10-31. There was the change from 1961 ASCII to 1968 ASCII. Some computer languages used characters in 1961 ASCII such as up arrow and left arrow. These characters disappeared from 1968 ASCII. We worked with Fred Mocking, who by now was in Sales at Teletype, on a type cylinder that would compromise the changing characters so that the meanings of 1961 ASCII were not totally lost. The underscore character was made rather wedge-shaped so it could also serve as a left arrow.