ASCII control characters

Open an ASCII table and the first two columns hold abbreviations instead of characters: NUL, SOH, BEL, ESC. These are the control characters, the 32 codes from 0 to 31 plus DEL at 127. None of them prints anything. They were designed to make a machine do something: ring a bell, move the paper, start a tape reader, or mark where a message begins and ends.

On a keyboard you reach them with Ctrl: Ctrl-G sends BEL, and Ctrl-M sends CR. Why Ctrl works that way is a story of its own.

Hex	Code	Built to	Today
00	NUL	Fill time or blank tape	In use: ends C strings
01	SOH	Start a message heading	Rare
02	STX	Start the message text	Rare
03	ETX	End the text	Repurposed: Ctrl-C interrupts
04	EOT	End the transmission	Repurposed: Ctrl-D ends input
05	ENQ	Ask the far end to identify itself	Rare
06	ACK	Confirm receipt	Rare
07	BEL	Ring a bell	In use: terminal bell
08	BS	Move back one space	In use
09	HT	Jump to the next tab stop	In use
0A	LF	Advance one line	In use
0B	VT	Jump to a preset line	Rare
0C	FF	Start a new page	Rare
0D	CR	Return to the start of the line	In use
0E	SO	Switch to another character set	Rare
0F	SI	Switch back	Rare
10	DLE	Change the meaning of what follows, for link control	Rare
11	DC1	Switch a device on	Repurposed: XON, Ctrl-Q
12	DC2	Control a device	Rare
13	DC3	Switch a device off	Repurposed: XOFF, Ctrl-S
14	DC4	Stop a device	Rare
15	NAK	Reject what was received	Repurposed: Ctrl-U erases the line
16	SYN	Keep a synchronous link in step	Rare
17	ETB	End a block of data	Rare
18	CAN	Disregard the data sent with it	Rare
19	EM	Mark the end of the tape	Rare
1A	SUB	Stand in for a garbled character	Repurposed: Ctrl-Z suspends
1B	ESC	Change the meaning of what follows	In use: terminal sequences
1C	FS	Separate files	Repurposed: Ctrl-\ quits
1D	GS	Separate groups	Rare
1E	RS	Separate records	Rare: JSON text sequences
1F	US	Separate units	Rare
7F	DEL	Erase a character on tape	In use

Running the line

The largest group managed traffic between machines. A message could open with SOH (start of heading) and some routing information, move on to its body with STX (start of text), and close with ETX (end of text). EOT (end of transmission) said the sender was finished.

The receiving station replied with ACK (acknowledge) or NAK (negative acknowledge). On synchronous links, where both ends kept time with each other, SYN filled idle moments to hold them in step. DLE (data link escape) gave the next few characters a special meaning for controlling the link.

ENQ (enquiry) asked the far end to identify itself. Its 1963 name, WRU, put it more plainly: who are you? A Teletype Model 33 answered with a small drum of breakable tabs, coded with the station’s identity, which turned and sent that identity back down the line.

Few of these codes are seen today. One survivor: xterm still answers ENQ, though by default its answer is empty.

Moving the paper

The format effectors controlled layout on a printing terminal. BS (backspace) moved the print head back one position, so a typist could strike over a character, for example to underline it. HT (horizontal tab) jumped to the next tab stop, as on a typewriter. VT (vertical tab) moved down to a preset line, and FF (form feed) advanced to the top of the next page. CR and LF split a new line into two motions, returning the carriage and advancing the paper, a split that still causes trouble.

Vertical tab and form feed are mostly a legacy: xterm, for one, treats both as an ordinary line feed.

Bells and switches

BEL did what its name says. On a Teletype it rang a real bell, so a distant sender could call an operator over to the machine. DC1 to DC4 (device controls) were left general: the standard says only that they switch auxiliary devices on or off, with DC4 the preferred way to stop one. The Teletype Model 33 used DC1 and DC3 to switch an attached device, such as its paper-tape punch, on and off, and those two codes came to be known as XON and XOFF.

Files, groups, records, units

Four codes, FS, GS, RS and US (file, group, record and unit separator), were meant to divide data into nested levels, from the largest to the smallest. In the 1963 chart they were unnamed separators, S4 to S7.

They make ideal delimiters, because they never appear in ordinary text. In practice the comma and the tab won, and CSV has to quote any field that contains a comma. Unicode still gives them separator roles in bidirectional text, where FS, GS and RS end a paragraph and US ends a segment. RS has also found a modern job: RFC 7464 uses it to mark the start of each record in a stream of JSON texts. JSON doesn’t allow a raw control character, so an RS can only mean that a new record begins.

Shifting and escaping

SO (shift out) and SI (shift in) switched to another character set and back, much as telegraph codes had shifted between letters and figures. The pair was still being specified in the 1990s: ISO-2022-KR, a Korean email encoding defined in 1993, used SO to switch from ASCII into Korean characters and SI to switch back.

ESC did the same job more broadly. It changes the meaning of the characters that follow it, which is how terminals still receive colours and cursor movements.

Filler, mistakes and endings

NUL is the all-zero code. On paper tape, blank leader with no holes read as NUL, and the standard used it as filler to pass time or fill space. Today it marks the end of a string in C.

CAN (cancel) told the receiver to disregard data just sent. EM (end of medium) marked the physical end of a tape, or the end of its used part. SUB (substitute) stood in for a character that had arrived garbled.

DEL, the code with every hole punched, has its own article.

The ones you still press

Several codes now do jobs ASCII never gave them. On Linux, the terminal driver watches for certain control characters as you type and acts on them itself:

• Ctrl-C sends ETX (end of text) and interrupts the running program.
• Ctrl-D sends EOT (end of transmission) and signals the end of input.
• Ctrl-Z sends SUB and suspends the program.
• Ctrl-U sends NAK and erases the line you’re typing.
• Ctrl-\ sends FS and quits the program.

Read down the table’s last column and the pattern is clear. Most of the codes that laid out a page, such as tab, line feed and carriage return, are still everywhere. The ones that ran a link or a tape punch either retired or found work under a new name.