Editing Control character (section)

==How control characters map to keyboards==
ASCII-based [[keyboard (computing)|keyboards]] have a key labelled "[[control key|Control]]", "Ctrl", or (rarely) "Cntl" which is used much like a shift key, being pressed in combination with another letter or symbol key. In one implementation, the control key generates the code 64 places below the code for the (generally) uppercase letter it is pressed in combination with (i.e., subtract 0x40 from ASCII code value of the (generally) uppercase letter). The other implementation is to take the ASCII code produced by the key and [[bitwise AND]] it with 0x1F, forcing bits 5 to 7 to zero. For example, pressing "control" and the letter "g" (which is 0110 0111 in [[binary numeral system|binary]]), produces the code 7 (BELL, 7 in base ten, or 0000 0111 in binary). The NULL character (code 0) is represented by Ctrl-@, "@" being the code immediately before "A" in the ASCII character set. For convenience, some terminals accept Ctrl-Space as an alias for Ctrl-@. In either case, this produces one of the 32 ASCII control codes between 0 and 31. Neither approach works to produce  the DEL character because of its special location in the table and its value (code 127<sub>10</sub>), Ctrl-? is sometimes used for this character.<ref>{{cite web |url=http://geocities.com/dtmcbride/tech/charsets/ascii.html |title=ASCII Characters |access-date=2010-10-08 |archive-url=https://web.archive.org/web/20091028135111/http://geocities.com/dtmcbride/tech/charsets/ascii.html |archive-date=October 28, 2009}}</ref>

When the control key is held down, letter keys produce the same control characters regardless of the state of the [[Shift key|shift]] or [[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) varies 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 [[word processing]] functions.  The associated keypresses are communicated to computer programs by one of four methods: appropriating otherwise unused control characters; using some encoding other than ASCII; using multi-character control sequences; or using an additional mechanism outside of generating characters. "Dumb" [[computer terminal]]s typically use control sequences.  Keyboards attached to stand-alone [[personal computer]]s made in the 1980s typically use one (or both) of the first two methods.  Modern computer keyboards generate [[scancode]]s 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.