ASCII control characters and codes

The first 32 codes (numbers 0–31 decimal) are kept by ASCII for control characters. Initially, the codes were designed not to represent some printable information, but to control devices (such as printers) that use ASCII, or to give meta-information about data streams, for example those stored on magnetic tape.

ASCII Table (Control chars)

Ok, Let's see an example. The character 10 represents the "line feed" function (which makes a printer to move forward its paper). The character 8 represents "backspace". RFC 2822 may be referred to control characters that do not include carriage return, line feed or white space as non-whitespace control characters. The only exception make the control characters that prescribe elementary line-oriented formatting. ASCII doesn't set any mechanism, the purpose of which would be a description of text structure or appearance within a document. Some other schemes, such as markup languages, address page and document layout and formatting.

Only brief descriptive phrases were used for each of the control character of the original ASCII. This uncertainty wasn't always accidental. Sometimes it was purposely created, for example in cases, where a character would be used a little bit on a terminal link than on a data stream. However, sometimes it was clearly accidental, for example in case of "delete" meaning.

The Teletype Model 33 ASR can be surely called as the most significant single device in the interpretation sphere of these characters. The Teletype Model 33 ASR was a printing terminal with an available paper tape reader/punch option. Long - term program storage used paper tape a lot until the 1980s. It was pretty cheaper and somehow less fragile than magnetic tape. Especially, the Teletype Model 33 machine assignments for codes 17 (DC1, Control-Q, also known as XON), 19 (DC3, Control-S, also known as XOFF), and char 127 (Delete), that in fact became standards. The Model 33 had one more reason to be popular at that time. It was taking the description of Control-G (code 7, BEL, meaning audible alarm the operator). In fact, it was a kind of the device with the real bell which it rang when received a BEL character. Because the keytop for the O key showed a left-arrow symbol as well (from ASCII-1963, which had this exactly character instead of underscore), an incompatible use of code 15 (Control-O, Shift In) interpreted as "delete previous character" was also borrowed by many timesharing systems of these times. However, some time later it became ignored by them.

When a Control-S (XOFF, an abbreviation for transmit off) was received by Teletype 33 ASR equipped with the automatic paper tape reader, it stopped the tape reader; receiving Control-Q (XON, "transmit on") resumed the tape reader. Some computer operating systems of that time have borrowed such technique. It was called as the "handshaking" signal that warned a sender to cancel the transmission because of impending overflow; currently it can be found in lots of systems as a manual output control technique. Some systems kept the initial function of the Control-S intact but they replaced Control-Q by a second Control-S to resume output. The 33 ASR also could be set to use Control-R (DC2) and Control-T (DC4) to start and stop the punch of the tape; on some devices that had such a function, the appropriate control character lettering on the keycap above the letter was TAPE and TAPE accordingly.

The Teletype didn't have a function that would let move the head backwards. Taking this into consideration, it did not put an additional key on the keyboard in order to send a BS (backspace). There was another thing for this: a key called "rubout" that sent code 127 (DEL). This key was created in order to correct the mistakes in a hand-typed paper tape: the operator just needed to push a button on the tape punch to back it up. Then the operator typed the rubout, and it in its turn punched all the gaps left after the punch and replaced the mistake with a character that was initially supposed to be ignored. The Digital Equipment Corporation used Teletypes for not so expensive computers. This way systems had to use the available key and then the DEL code in order to erase the previous character. This was the reason, why DEC video terminals (by default) sent the DEL code for the key marked "Backspace", while the key marked "Delete" sent an escape sequence, while lots of other terminals sent BS for the Backspace key. Just one code could be used by the Unix terminal driver in order to back up. The back up function here could be adjusted either to BS or DEL, but not to the both of them. The result of the use of the both keys would be a long exhausting period of irritation, where you had to correct the mistake taking into consideration the terminal you were using (modern shells using readline, so they respectively understand the both codes). The supposition that no key sent a BS caused Control+H to be used for other purposes, such as a "help" command in Emacs.

Lots of new meaning were given to the the control codes, that initially had completely different functions than the new ones. One of the brightest examples is the "escape" character (ESC, code 27). Initially its function was to allow sending other control characters as literals instead of calling to their meaning. The same meaning of "escape" character can be met in URL encodings, C language strings, and some other systems where particular characters have their stable meaning. However, time flies, new technologies appear, so this meaning has been co-opted. In plain words, it was internally changed. Currently ESC sent to the terminal usually means the start of a command sequence usually in the form of a so-called "ANSI escape code" (or, better to say, a "Control Sequence Introducer") from ECMA-48 (1972) and its continuers, beginning with ESC followed by a "[" (left-bracket) character. An ESC sent from the terminal is usually used as an out-of-band character. It's function is to terminate an operation, as in the TECO and vi text editors. Usually ESC makes a request in order to interrupt its current operation or to exit (terminate) completely in graphical user interface (GUI) and windowing systems.

In addition to their historical use, the incorporated uncertainly of lots of the control characters, created certain problems when transmitting "plain text" files between systems. We suppose, that the brightest example of it is the newline problem that different operating systems face. Teletype machines need a line of text to be completed with both "Carriage Return" (which moves the printhead to the beginning of the line) and "Line Feed" (which moved forward the paper one line without moving the printhead). The name "Carriage Return" has its own meaning. In connection to it we have to remember the fact that on a manual typewriter the carriage holding the paper moved while the position where the typebars struck the ribbon remained stationary. The whole carriage had to be pushed (taken back.) to the right in order to position the left margin of the paper for the next line.

The both characters were used by DEC operating systems (OS/8, RT-11, RSX-11, RSTS, TOPS-10, etc.) in order to mark the end of a line. It was needed for the console device (usually Teletype machines) to work. The convention was set up so well, that backward compatibility compelled to continue the convention. This happen at the time so-called "glass TTYs" (which were then called CRTs or terminals) came along. Gary Kildall found his inspiration to design CP/M in some command line interface conventions used in DEC's RT-11. Until the introduction of PC DOS in 1981, IBM wasn't influentially involved in this. There was a reason for that. The thing is that 1970s operating systems used EBCDIC, not ASCII. They were oriented in the direction of the punch-card input and line printer output. The concept of carriage return on it was absolutely pointless. IBM's PC DOS (also marketed as MS-DOS by Microsoft) was lucky to be freely based on CP/M, so thanks to this fact it inherited the convention. Windows inherited it from MS-DOS.

Some unwanted complexities and questions (for example, to how to interpret each character when encountered alone) arose due to the requiring two characters to mark the end of a line. In order to make matters plain text data streams simpler, including files, on Multics used line feed (LF) alone as a line terminator. Unix, Unix-like systems and Amiga systems have borrowed this convention from Multics. However, the original Macintosh OS, Apple DOS, and ProDOS used carriage return (CR) alone as a line terminator. It's worth mentioning, that since Apple replaced these operating systems with the Unix-based macOS operating system, the line feed (LF) is used by them currently as well. The Radio Shack TRS-80 also used a lone CR to terminate lines.

Computers that were assigned to the ARPANET included machines running operating systems, for example TOPS-10 and TENEX using CR-LF line endings, machines running operating systems, for example Multics using LF line endings, and machines running operating systems, for example OS/360 that represented lines as a character count followed by the characters of the line and that used EBCDIC rather than ASCII. An ASCII "Network Virtual Terminal" (NVT) was determined by the Telnet protocol. This way the connections between hosts with different line-ending conventions and character sets could be supported with the help of transferring a standard text format over the network. Telnet used ASCII along with CR-LF line endings. The software using other conventions would translate between the local conventions and the NVT. The File Transfer Protocol adopted the Telnet protocol. The use of the Network Virtual Terminal was also a part of it. It was intended for the use in transmitting commands and transferring data in the default ASCII mode. This make the realization of those protocols pretty difficult, as well as the realization of some other network protocols, like those used for E-mail and the World Wide Web, on systems not using the NVT's CR-LF line-ending convention.

The PDP-6 monitor, and its PDP-10 continuer TOPS-10, used Control-Z (SUB) as an end-of-file indication for input from a terminal. CP/M and some other operating systems tracked file length only in units of disk blocks. They used Control-Z to mark the end of the actual text in the file. In order to reach such goals, EOF, or end-of-file, was used colloquially and conventionally as a three-letter acronym for Control-Z instead of SUBstitute. There were a lot of reasons why the end-of-text code (ETX), also known as Control-C, was unsuitable. The using Z as the control code to end a file is analogous to it ending the alphabet and serves as a pretty convenient mnemonic aid. A historically common and still dominating convention uses the ETX code convention to interrupt and halt a program via an input data stream, usually from a keyboard.

In C library and Unix conventions, the null character is used to terminate text strings; such null-terminated strings can be known in abbreviation as ASCIZ or ASCIIZ, where here Z stands for "zero".