Special Codes

Although most kinds of text file only contain text, it’s also possible to send additional information, usually related to the transmission or presentation of the text itself. This is commonly sent in the form of control codes or escape sequences.

Control Codes

All of the two or three-letter mnemonics in a computer’s character set represent control codes. Most modern computers and terminals, automatically generate these codes when you press a key, such as the Ctrl (Control) key, at the same time as a given ASCII character key.

Older terminal emulation software on the Mac OS can use a different modifier key, such as ⌘ (Command) or Option, sometimes with an alternative character key.
In some applications you must type ^ (circumflex) before pressing the character key.

The control codes defined by the ASCII standard are listed below:-

Hex	Dec		Name	Function
00	0	@	NUL	Null *
01	1	A	SOH	Start of header •
02	2	B	STX	Start of text •
03	3	C	ETX	End of text •
04	4	D	EOT	End of transmission •
05	5	E	ENQ	Enquire •
06	6	F	ACK	Acknowledge •
07	7	G	BEL	Bell
08	8	H	BS	Backspace
09	9	I	HT	Horizontal tabulation
0A	10	J	LF	Line feed
0B	11	K	VT	Vertical tabulation
0C	12	L	FF	Form feed (new page)
0D	13	M	CR	Carriage Return (non-marking)
0E	14	N	SO	Shift out
0F	15	O	SI	Shift in
10	16	P	DEL	Delete •
11	17	Q	DC1	Device control 1 •
12	18	R	DC2	Device control 2
13	19	S	DC3	Device control 3 •
14	20	T	DC4	Device control 4
15	21	U	NAK	Negative acknowledge •
16	22	V	SYN	Synchronise •
17	23	W	ETB	End of text block •
18	24	X	CAN	Cancel
19	25	Y	EM	End of medium •
1A	26	Z	SUB	Substitute •
1B	27	[	ESC	Escape
1C	28	\	FS	File separator •
1D	29	]	GS	Group separator •
1E	30	~	RS	Record separator •
1F	31	_	US	Unit separator •
7F	127	!	DEL	Delete

• Rarely used within data files

* Not available on a Mac OS computer

In some character sets, code 255 acts as DEL, in the same way as code 127. Similarly, codes 128 to 159, when not used for characters, can have the same effect as codes 0 to 31.
The characters shown above in the Char column follow the normal ASCII character pattern, with the exception of RS. Note that Ctrl-^ is often used to generate the ^ character itself, whilst RS is sometimes be obtained by pressing Ctrl-＞, with US generated by pressing Ctrl-?. In some instances, DEL is sent as a special sequence of codes.
The modern Mac OS produces FS, GS, RS or US when you press Ctrl at the same time as ⇠, ⇢, ⇡ or ⇣ respectively.

Unusual Codes

Control codes are rarely found inside modern documents, especially SOH, STX, ETX and EOT, as used in a communications link to mark the boundaries of text data, ACK and NAK, which are used to check the presence of data passing over such a link, and ENQ, SYN, ETB and CAN, which maintain basic communications control. In addition, SO and SI can be used to instruct a printer to use double-width, compressed-width or an alternative character set, DC1 to DC4 can select other printer modes, whilst FS, GS, RS and US can be used as a form of punctuation for non-textual data.

Using Control Codes

Some software and computer systems use selected codes as follows:-

Code	Effect
Ctrl-C	Aborts the current operation (Interrupt) •
Ctrl-H	Deletes the last typed character (Backspace)
Ctrl-I	Completes file and folder names
Ctrl-O	Stops current communication (Abort output)
Ctrl-Q	Continues current communication (XON)
Ctrl-S	Pauses current communication (XOFF)
Ctrl-M	Terminates a command entry and starts its execution
Ctrl-Z	Marks the End of file (EOF) for a text file

• Has a similar effect to pressing Break or Ctrl-Scroll Lock on a PC

Ctrl-Q and Ctrl-S, are specifically used in a computer terminal to pause and resume long listings of text on the screen. You simply press Ctrl-S when you spot something you want to read and then press Ctrl-Q to continue the listing, or you can press Ctrl-C to abort the entire process.

Non-Standard Uses for Control Codes

Computer operating systems often use control codes for various purposes ‘behind the scenes’. For example, many of the keys on a Mac OS keyboard generate special character codes that are actually control codes. Similarly, the special font characters that appear in Classic Mac OS menus are identified by control codes, although these are unrelated to the original purposes of the codes.

The following table provides further details, including Mac OS symbols:-

Hex	Dec	Name	Std Keys	Mac Key(s)
01	1	SOH	Ctrl-A	Home
02	2	STX	Ctrl-B	-	⇥
03	3	ETX	Ctrl-C	Enter	⇤
04	4	EOT	Ctrl-D	End
05	5	ENQ	Ctrl-E	Help or Insert	⇧
06	6	ACK	Ctrl-F	-	⌃
07	7	BEL	Ctrl-G	-	⌥
08	8	BS	Ctrl-H	Delete
09	9	HT	Ctrl-I	Tab
0A	10	LF	Ctrl-J	-
0B	11	VT	Ctrl-K	Page Up
0C	12	FF	Ctrl-L	Page Down
0D	13	CR	Ctrl-M	Return
0E	14	SO	Ctrl-N	-
0F	15	SI	Ctrl-O	-
10	16	DEL	Ctrl-P	F1 to F15	⇣
11	17	DC1	Ctrl-Q	-	⌘
12	18	DC2	Ctrl-R	-	√
13	19	DC3	Ctrl-S	-	◆
14	20	DC4	Ctrl-T	-
15	21	NAK	Ctrl-U	-
16	22	SYN	Ctrl-V	-
17	23	ETB	Ctrl-W	-
18	24	CAN	Ctrl-X	-	⇠
19	25	EM	Ctrl-Y	-	⇡
1A	26	SUB	Ctrl-Z	-	⇢
1B	27	ESC	Ctrl-[	Esc	⎋
1C	28	FS	Ctrl-\	⇠	⌧
1D	29	GS	Ctrl-]	⇢
1E	30	RS	-	⇡
1F	31	US	-	⇣
7F	127	DEL	-	Fwd Delete

Escape Sequences

Standard coding systems don’t provide for sending instructions to another device, such as a printer or modem. However, such instructions can be accommodated inside an escape sequence, usually consisting of an ESC control code (decimal 27, hex 1B) followed by a string of data bytes. To be effective, each sequence must include the correct number of bytes.

Any escape code that you send should be suitable for the receiving device. In other words, be aware that sending printer instructions to a modem can give unpredictable results!

The notation used for an escape sequence is quite simple. For example, the instruction ESC ! BS is actually made up of ESC (decimal 27) followed by ! (decimal 33) and BS (decimal 8). Sequences sometimes include control codes, as in, for example, ESC EM R, where EM is the control code.

Terminal Sequences

Escape sequences can be used to accommodate non-standard keys on a keyboard where other control codes can’t be used. For example, a standard VT220 terminal can use the following sequences to accommodate special keys, including the function keys from F6 to F20, as found on modern keyboards:-

Key	Sequence	Key	Sequence
Find	ESC [ 1 ~	F6	ESC [ 17 ~
Ins	ESC [ 2 ~	F7	ESC [ 18 ~
Rmv	ESC [ 3 ~	F8	ESC [ 19 ~
Sel	ESC [ 4 ~	F9	ESC [ 20 ~
Prev	ESC [ 5 ~	F10	ESC [ 21 ~
Next	ESC [ 6 ~	F11	ESC [ 23 ~
		F12	ESC [ 24 ~
		F13	ESC [ 25 ~

Key	Sequence
F14	ESC [ 26 ~
F15	ESC [ 28 ~
F16	ESC [ 29 ~
F17	ESC [ 31 ~
F18	ESC [ 32 ~
F19	ESC [ 33 ~
F20	ESC [ 34 ~

Printer Control Sequences

Escape sequences can be used to control an older type of dot-matrix printer, although there is considerable variation in the interpretation of such codes, some of which operate with both an Epson (ESC/P) printer or an IBM-compatible printer. Other printer models also use some of these sequences or different codes of their own.