Editing Control character (section)

===Miscellaneous codes===
Code 7 ([[bell character|BEL]]) is intended to cause an audible signal in the receiving terminal.<ref>{{cite IETF |rfc=20 |title=ASCII format for Network Interchange |date=October 1969 |access-date=2013-11-03}} An old RFC, which explains the structure and meaning of the control characters in chapters 4.1 and 5.2</ref>

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" ([[C0 and C1 control codes|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&mdash; 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 [[null character|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 [[Record-oriented filesystem|fixed length records]]; to [[Null-terminated string|mark the end of a string]]; and formerly to [[Output padding|give printing devices enough time to execute a control function]].

Code 127 ([[Delete character|DEL]], a.k.a. "rubout") is likewise a special case. Its 7-bit code is ''all-bits-on'' in binary, which essentially erased a character cell on a [[paper tape]] when overpunched. Paper tape was a common storage medium when ASCII was developed, with a computing history dating back to WWII code breaking equipment at [[Biuro Szyfrów]]. Paper tape became obsolete in the 1970s, so this aspect of ASCII rarely saw any use after that. Some systems (such as the original Apple computers) 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 [[programmable ROM]]s are typically implemented as arrays of fusible elements, each representing a [[bit]], 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 system]]s do not allow control characters in [[filename]]s, as they may have reserved functions.