Delete character

In computing, the delete character (sometimes also called rubout) is the last character in the ASCII repertoire, with the code 127 (decimal).[1] Not a graphic character but a control character, it is denoted as ^? in caret notation and has a graphic representation of ␡ in Unicode (as all ASCII control characters have graphic representations).

On modern systems terminal emulators typically turn keys marked "Delete" or "Del" into an escape sequence such as ^[[3~. Terminal emulators may produce DEL when ← Backspace key or Control+← Backspace or Control+? are typed, and some programs such as Notepad may insert this character with the same key presses.

History

  ·    NUL
•·••CR
•·•LF
••••·•••DEL
•·SP
"Delete" along with some other ASCII control characters and space as they appear on punched tape

This code was originally used to mark deleted characters on punched tape, since any character could be changed to all ones by punching holes everywhere. If a character was punched erroneously, punching out all seven bits caused this position to be ignored or deleted, a computer version of correction fluid.[2][3] In hexadecimal, this is 7F to rubout 7 bits, and FF to rubout 8 bits. For teleprinters like the Teletype Model 33, lines were commonly ended by the three characters CR, LF, and rubout, with the rubout allowing time for the print mechanism to physically move to the left margin. On VT100 compatible terminals, this is the character generated by the key labeled Delete, which transmits a delete character (octal 177, hexadecimal 7F) to the host system.[4][5] On VT510 compatible terminals, this is the character generated by the key labeled ?, usually called backspace on modern machines, and does not correspond to the PC "Delete" key.[6]

Current use

Unix-like operating systems are known to use it as erase control character, i.e. to delete the previous character in the line mode. This, though, differs from its original function where this code replaced (physically) characters on a punched tape to be deleted.

DOS/Windows never used this character in any way, using the backspace (0x08, or control-H) to delete the previous character. EGA/VGA fonts, as fonts used by Win32 console, usually have the "house" symbol ⌂ at 127 (0x7F) code point, see Code page 437 for details. However, its legacy can be seen in some applications distributed as part of the Windows operating system: as an example, typing the Control and ← Backspace key combination in Microsoft Notepad will output the delete character.

See also

References

  1. ↑ definition of the rubout character 7F hex
  2. ↑ Robert C. Martin (2011). The Clean Coder: A Code of Conduct for Professional Programmers. Pearson Education. ISBN 9780132542883.
  3. ↑ LaFarr (18 August 2006). ""Paper Tape" First Machinable I/O".
  4. ↑ "VT100 Family of Video Terminals". vt100.net.
  5. ↑ "VT100 User Guide- Chapter 3 – Table 3-4 Function Key Codes". vt100.net.
  6. ↑ "VT510 Video Terminal Programmer Information – Chapter 8 – Keyboard Processing". vt100.net.
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