C0 controls

File Format
Name	C0 controls
Ontology	Electronic File Formats Character Encodings C0 controls

The C0 controls are the control characters (code positions 0-31 decimal) which are part of the ASCII standard (as adopted in 1967; the earlier 1963 version had somewhat different control character assignments). They are also part of a number of other character sets derived from ASCII, and are in the Unicode set at the same code positions they have in ASCII. The official definition gives particular meanings for each of the characters, though they have also been used in a number of other ways, and some of them are rarely used these days. Some character sets (including Unicode) have a second set of controls called the C1 controls in the equivalent positions with the 8th bit set (128-159 decimal), but those are rarely used in their control meanings (the Windows character sets have printable characters there).

Hex	Dec	Codes	Abbreviation	Name	Description and uses
00	0	^@, \0	NUL	Null character	Marks unused space or padding (e.g., to intentionally slow down terminals or to leave space for added data in memory or storage media). Used in C-based programming languages to mark end of string.
01	1	^A	SOH	Start of Heading	Marks the beginning of a header in a message or data structure. (The SOL-20 computer mapped this onto the left arrow key.)
02	2	^B	STX	Start of Text	Marks the beginning of the body text of a message, and/or the end of the header.
03	3	^C	ETX	End of Text	Marks the end of the body text. Also used as "break character" (Control-C) to terminate a program or process. In most Windows programs, Ctrl-C copies text to a buffer for later pasting.
04	4	^D	EOT	End of Transmission	In Unix-style operating systems, signals end-of-file and is used to log out of a terminal. On Apple II, this character signalled that what followed was a DOS command when it was "printed" to standard output.
05	5	^E	ENQ	Enquiry	Used in transmission protocols to request acknowledgement from the other end to make sure connection is still active. In DEC TOPS-20 mainframes, usually resulted in currently-active application outputing status information to terminal.
06	6	^F	ACK	Acknowledge	Sent as response to ENQ message, or used to positively acknowledge receipt of data or messages (as opposed to NAK).
07	7	^G, \a	BEL	Bell	On some systems, this causes a bell, buzzer, or beep to sound, or flashes inverse video to alert a system operator. The Apple II had "BELL" on the "G" key to remind users that Ctrl-G caused this sound effect. Every time this character is sent to a terminal, an angel gets its wings!
08	8	^H, \b	BS	Backspace	Moves back one space. Usually deletes last character (e.g., from input string), but on some old terminals it just moved backward without deleting and allowed "overstrike" effects overlaying multiple characters.
09	9	^I, \t	HT	Horizontal Tab	The typewriter "tab key", usually moving to the next tab stop as defined in the particular software being used.
0A	10	^J, \n	LF	Line Feed	Move down one line. In Unix-style operating systems, it also moves to the beginning of the next line so that it can be used as a line break (newline) character, while in some other systems and terminals it just moves down without moving to the left, requiring the "CR LF" sequence to break a line.
0B	11	^K, \v	VT	Vertical Tab	Moves to vertical tab stops; not used nearly as often as the more-common horizontal tab.
0C	12	^L, \f	FF	Form Feed	Causes page to eject in printers, and may clear the screen in some terminal emulators. Sometimes used as a logical division of sections of a document.
0D	13	^M, \r	CR	Carriage Return	Moves to the beginning of the line. In some systems (e.g., Apple II, Commodore 64, and TRS-80, and early Macintosh systems before its OS switched to a Unix-based system), also moves to the next line so that it can be used as a line break character, while in other systems it stays on the same line so that it must be accompanied by a LF character to break a line (but on some printing terminals CR with no LF was used for overstrike effects including underlining by printing underscores). Thus the three different line-break conventions (LF, CR, and CR+LF) arose, which bedevil users of text files to this day. As an input character, CR is generally mapped onto the Enter key, signaling the completion of input.
0E	14	^N	SO	Shift Out	Switch to alternate character set (reversed by SI). Used in various systems and terminals to set different characters (e.g., APL or Cyrillic), or change the color or font.
0F	15	^O	SI	Shift In	Return to normal character set (reverses operation of SO).
10	16	^P	DLE	Data Link Escape	Signals the start of a sequence of raw data as opposed to normal printable or control characters.
11	17	^Q	DC1	Device Control 1	One of four device-control codes intended to be system-specific. This one (CTRL-Q, also known as XON) is often used to resume operations of a process, device, or output stream that has been paused with CTRL-S (XOFF).
12	18	^R	DC2	Device Control 2	Another device-control code; not used as much as DC1 and DC3.
13	19	^S	DC3	Device Control 3	The third of the device-control codes; this one (CTRL-S, also known as XOFF) is often used to pause processes, devices, or output streams, with CTRL-Q (XON) resuming them (though in some cases, any keypress causes output to resume). (The SOL-20 computer mapped this onto the right arrow key.)
14	20	^T	DC4	Device Control 4	The fourth device-control code; not used as much as DC1 or DC3. In DEC TOPS-20 mainframes, usually resulted in output of system status to terminal.
15	21	^U	NAK	Negative Acknowledge	In transmission protocols, indicates a failure requiring a re-send, or a negative response to a query of whether the process is ready to proceed.
16	22	^V	SYN	Synchronous Idle	Signals that a correction may now be received in synchronous transmission protocols. In most Windows programs, Ctrl-V pastes text from the cut/copy buffer.
17	23	^W	ETB	End of Transmission Block	Marks the end of a block of data divided into blocks for transmission. (The SOL-20 computer mapped this onto the up arrow key.)
18	24	^X	CAN	Cancel	Cancels an operation and signals that previously-sent data can be disregarded. (The SOL-20 computer mapped this onto the down arrow key. In most Windows programs, Ctrl-X cuts text into a buffer for later pasting.)
19	25	^Y	EM	End of Medium	Marks the end of a physical medium such as a data-storage tape. In most Windows programs, Ctrl-Y redoes a command after Ctrl-Z undid it.
1A	26	^Z	SUB	Substitute Character	Used to mark the spot where garbled, missing, or incomplete characters were received due to transmission errors, or various other uses involving place-holder characters. This character (Ctrl-Z) is also used by MS/PC-DOS to mark the end of a file or input stream, calling it EOF (although CTRL-D, EOT, would have been more standards-compliant and is used by Unix-style OSs for this purpose; however, some DEC operating systems used the CTRL-Z convention and this is what was followed by PC-DOS). In most Windows programs, Ctrl-Z undoes the last command.
1B	27	^[	ESC	Escape	Mapped onto the ESC key on keyboards, this usually signals a user attempting to exit a menu or mode. It is also commonly used in printer and terminal control protocols to signal the beginning of a special "escape sequence" where immediately-following characters are interpreted as commands.
1C	28	^\	FS	File Separator	One of four separator characters intended to delimit structured data. FS is the highest-level separator, intended to separate structures which are in turn internally delimited with GS, RS, and US (in descending order). Also used as a "quit and dump core" signal in Unix shells.
1D	29	^]	GS	Group Separator	The second of four separator characters, subordinate to FS, but higher-level than RS and US.
1E	30	^^	RS	Record Separator	The third of four separator characters, subordinate to FS and GS, but higher-level than US.
1F	31	^_	US	Unit Separator	The lowest-level of the separator characters, used to divide strings of ASCII characters which are the base elements of a data structure. A sequence of such US-delimited strings can in turn be used as a higher-level data element separated by other such elements by the RS character, and this structure in turn can be delimited from other such elements by GS, and finally if a fourth level is needed the FS character separates those elements.