ASCII Totally Explained

Everything about Ascii totally explained

American Standard Code for Information Interchange (ASCII), is a character encoding based on the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that work with text. Most modern character encodings — which support many more characters than did the original — have a historical basis in ASCII.
Historically, ASCII developed from telegraphic codes and its first commercial use was as a seven-bit teleprinter code promoted by Bell data services. Work on ASCII began in 1960. The first edition of the standard was published in 1963, a major revision in 1967, and the most recent update in 1986. Compared to earlier telegraph codes, the proposed Bell code and ASCII were both reordered for more convenient sorting (for example, alphabetization) of lists, and added features for devices other than teleprinters. Some ASCII features, including the "ESCape sequence", were due to Robert Bemer.
ASCII includes definitions for 128 characters: 33 are non-printing, mostly obsolete control characters that affect how text is processed; 94 are printable characters (excluding the space). The ASCII character encoding — or a compatible extension — is used on nearly all common computers, especially personal computers and workstations.

History

The Americal Standard Code for Information Interchange (ASCII) was developed by a committee of the American Standards Association, called the X3 committee. The ASA became the United States of America Standards Institute or USASI and ultimately the American National Standards Institute.
   The X3 committee designed ASCII based on earlier teleprinter encoding systems. Like other character encodings, ASCII specifies a correspondence between digital bit patterns and character symbols (for example graphemes and control characters). This allows digital devices to communicate with each other and to process, store, and communicate character-oriented information such as written language. The encodings in use before ASCII included 26 alphabetic characters, 10 numerical digits, and from 11 to 25 special graphic symbols. To include control characters compatible with the Comité Consultatif International Téléphonique et Télégraphique standard, Fieldadata and early EBCDIC, more than 64 codes were required.
   The committee debated the possibility of a shift key function (like the Baudot code), which would allow more than 64 codes to be represented by six bits. In a shifted code, some character codes determine choices between options for the following character codes. This allows compact encoding, but is less reliable for data transmission; an error in transmitting the shift code typically makes a long part of the transmission unreadable. The standards committee decided against shifting, and so ASCII required at least a seven-bit code.
   The committee considered an eight-bit code, since eight bits would allow two four-bit patterns to efficiently encode two digits with binary coded decimal. However this would require all data transmission to send eight bits when seven could suffice. The committee voted to use a seven-bit code to minimize costs associated with data transmission. Since perforated tape at the time could only record eight bits in one position, this also allowed for a parity bit for error checking if desired. Machines with octets as the native data type that didn't use parity checking typically set the eighth bit to 0.
   The code itself was structured so that all control codes were together, and all graphic codes were together. The first two columns (32 positions) were reserved for control characters. The "space" character had to come before graphics to make sorting algorithms easy, so its became position 32. The committee decided it was important to support the upper case 64-character alphabets, and chose to structure ASCII so it could easily be reduced to a usable 64-character set of graphic codes. Lower case letters were therefore not interleaved with upper case. To keep options for lower case letters and other graphics open, the special and numeric codes were placed before the letters, and the letter 'A' was placed in position 65 to match the draft of the corresponding British standard.
   Many of the non-alphanumeric characters were positioned to correspond to their shifted position on typewriters. Thus #, $ and % were placed to correspond to 3, 4, and 5 in the adjacent column. The parentheses couldn't correspond to 9 and 0, however, because the place corresponding to 0 was taken by the space character. Since many European typewriters placed the parentheses with 8 and 9, these correspnding positions were chosen for the parentheses. The @ symbol wasn't used in continental Europe and the committee expected it would be replaced by an accented À in France, so the @ was placed in position 64 next to the letter A.
   The control codes felt essential for data transmission were the start of message (SOM), end of address (EOA), end of message (EOM), end of transmission (EOT), "who are you?" (WRU), "are you?" (RU), a reserved device control (DC0), synchronys idle (SYNC), and acknowledge (ACK). These were positioned to maximize the Hamming distance between their bit patterns.
   With the rest of the special characters and control codes filled in, ASCII was published as ASA X3.4-1963, leaving 28 code positions without assigned meaning, reserved for future standardization. This version didn't specify codes for lower case characters because there was some debate there should be more control characters instead. In late 1963 the International Organization for Standardization voted to assign lower case characters to columns 6 and 7. The X3 committee incorporated this decision and other changes, including other new characters (the curly bracket characers), renaming some control characters (SOM became start of header (SOH)) and moving or removing others (RU was removed). ASCII was subsequently updated as USASI X3.4-1967, then USASI X3.4-1968, ANSI X3.4-1977, and finally, ANSI X3.4-1986.
   The X3 committee also addressed how ASCII should be transmitted (low bit first), and how it should be recorded on perforated tape. They proposed a 9-track standard for magnetic tape, and attempted to deal with some forms of punched card formats.
   ASCII itself first entered commercial use in 1963 as a seven-bit teleprinter code for American Telephone & Telegraph's TWX (Teletype Wide-area eXchange) network. TWX originally used the earlier five-bit Baudot code, which was also used by the competing Telex teleprinter system. Bob Bemer introduced features such as the escape sequence. On March 11, 1968, U.S. President Lyndon B. Johnson mandated that all computers purchased by the United States federal government support ASCII, stating:

I have also approved recommendations of the Secretary of Commerce regarding standards for recording the Standard Code for Information Interchange on magnetic tapes and paper tapes when they're used in computer operations. All computers and related equipment configurations brought into the Federal Government inventory on and after July 1, 1969, must have the capability to use the Standard Code for Information Interchange and the formats prescribed by the magnetic tape and paper tape standards when these media are used.

Other international standards bodies have ratified character encodings such as ISO/IEC 646 that are identical or nearly identical to ASCII, with extensions for characters outside the English alphabet and symbols used outside the United States, such as the symbol for the United Kingdom's pound sterling (£). Almost every country needed an adapted version of ASCII since ASCII only suited the needs of the USA and a few other countries. For example, Canada had its own version that supported French. Other adapted encodings include ISCII (India), VISCII (Vietnam), and YUSCII (Yugoslavia). Although these encodings are sometimes referred to as ASCII, true ASCII is strictly defined only by ANSI standard.
ASCII has been incorporated into the Unicode character set as the first 128 symbols, so the ASCII characters have the same numeric codes in both sets. This allows UTF-8 to be backward compatible with ASCII, a significant advantage.
Asteroid 3568 ASCII is named after the character encoding.

ASCII control characters

ASCII reserves the first 32 codes (numbers 0–31 decimal) for control characters: codes originally intended not to carry printable information, but rather to control devices (such as printers) that make use of ASCII, or to provide meta-information about data streams such as those stored on magnetic tape. For example, character 10 represents the "line feed" function (which causes a printer to advance its paper), and character 8 represents "backspace". Control characters that don't include carriage return, line feed or white space are called non-whitespace control characters. Except for the control characters that prescribe elementary line-oriented formatting, ASCII doesn't define any mechanism for describing the structure or appearance of text within a document. Other schemes, such as markup languages, address page and document layout and formatting.
   The original ASCII standard used only short descriptive phrases for each control character. The ambiguity this left was sometimes intentional (where a character would be used slightly differently on a terminal link than on a data stream) and sometimes more accidental (such as what "delete" means).
   Probably the most influential single device on the interpretation of these characters was the ASR-33 Teletype series, which was a printing terminal with an available paper tape reader/punch option. Paper tape was a very popular medium for long-term program storage up through the 1980s, lower cost and in some ways less fragile than magnetic tape. In particular, the Teletype 33 machine assignments for codes 17 (Control-Q, DC1, also known as XON), 19 (Control-S, DC3, also known as XOFF), and 127 (DELete) became de-facto standards. Because the keytop for the O key also showed a left-arrow symbol (from ASCII-1963, which had this character instead of underscore), a noncompliant use of code 15 (Control-O, Shift In) interpreted as "delete previous character" was also adopted by many early timesharing systems but eventually faded out.
   The use of Control-S (XOFF, an abbreviation for "transmit off") as a handshaking signal warning a sender to stop transmission because of impending overflow, and Control-Q (XON, "transmit on") to resume sending, persists to this day in many systems as a manual output control technique. On some systems Control-S retains its meaning but Control-Q is replaced by a second Control-S to resume output.
   Code 127 is officially named "delete" but the Teletype label was "rubout". Since the original standard gave no detailed interpretation for most control codes, interpretations of this code varied. The original Teletype meaning, and the intent of the standard, was to make it an ignored character, the same as NUL (all zeroes). This was specifically useful for paper tape, because punching the all-ones bit pattern on top of an existing mark would obliterate it. Tapes designed to be "hand edited" could even be produced with spaces of extra NULs (blank tape) so that a block of characters could be "rubbed out" and then replacements put into the empty space.
   As video terminals began to replace printing ones, the value of the "rubout" character was lost. DEC systems, for example, interpreted "Delete" to mean "remove the character before the cursor," and this interpretation also became common in Unix systems. Most other systems used "Backspace" for that meaning and used "Delete" as it was used on paper tape, to mean "remove the character after the cursor". That latter interpretation is the most common today.
   Many more of the control codes have taken on meanings quite different from their original ones. The "escape" character (code 27), for example, was originally intended to allow sending other control characters as literals instead of invoking their meaning. This is the same meaning of "escape" encountered in URL encodings, C language strings, and other systems where certain characters have a reserved meaning. Over time this meaning has been coopted and has eventually drifted. In modern use, an ESC sent to the terminal usually indicates the start of a command sequence, usually in the form of an ANSI escape code. An ESC sent from the terminal is most often used as an "out of band" character used to terminate an operation, as in the TECO and vi text editors.
   The inherent ambiguity of many control characters, combined with their historical usage, created problems when transferring "plain text" files between systems. The clearest example of this is the newline problem on various operating systems. On printing terminals there's no question that you terminate a line of text with both "Carriage Return" and "Linefeed". The first returns the printing carriage to the beginning of the line and the second advances to the next line without moving the carriage. However, requiring two characters to mark the end of a line introduced unnecessary complexity and questions as to how to interpret each character when encountered alone. To simplify matters, plain text files on Unix and Amiga systems use line feeds alone to separate lines. Similarly, older Macintosh systems, among others, use only carriage returns in plain text files. Various DEC operating systems used both characters to mark the end of a line, perhaps for compatibility with teletypes, and this de facto standard was copied in the CP/M operating system and then in MS-DOS and eventually Microsoft Windows. Transmission of text over the Internet, for protocols as E-mail and the World Wide Web, uses both characters.
   The DEC operating systems, along with CP/M, tracked file length only in units of disk blocks and used Control-Z (SUB) to mark the end of the actual text in the file (also done for CP/M compatibility in some cases in MS-DOS, though MS-DOS has always recorded exact file-lengths). Text strings ending with the null character are known as ASCIZ or C strings.

Binary	Oct	Dec	Hex	Abbr	PR	CS	CEC	Description
000 0000	000	0	00	NUL	␀	^@		Null character
000 0001	001	1	01	SOH	␁	^A		Start of Header
000 0010	002	2	02	STX	␂	^B		Start of Text
000 0011	003	3	03	ETX	␃	^C		End of Text
000 0100	004	4	04	EOT	␄	^D		End of Transmission
000 0101	005	5	05	ENQ	␅	^E		Enquiry
000 0110	006	6	06	ACK	␆	^F		Acknowledgment
000 0111	007	7	07	BEL	␇	^G	a	Bell
000 1000	010	8	08	BS	␈	^H		Backspace
000 1001	011	9	09	HT	␉	^I		Horizontal Tab
000 1010	012	10	0A	LF	␊	^J		Line feed
000 1011	013	11	0B	VT	␋	^K	v	Vertical Tab
000 1100	014	12	0C	FF	␌	^L	f	Form feed
000 1101	015	13	0D	CR	␍	^M		Carriage return
000 1110	016	14	0E	SO	␎	^N		Shift Out
000 1111	017	15	0F	SI	␏	^O		Shift In
001 0000	020	16	10	DLE	␐	^P		Data Link Escape
001 0001	021	17	11	DC1	␑	^Q		Device Control 1 (oft. XON)
001 0010	022	18	12	DC2	␒	^R		Device Control 2
001 0011	023	19	13	DC3	␓	^S		Device Control 3 (oft. XOFF)
001 0100	024	20	14	DC4	␔	^T		Device Control 4
001 0101	025	21	15	NAK	␕	^U		Negative Acknowledgement
001 0110	026	22	16	SYN	␖	^V		Synchronous Idle
001 0111	027	23	17	ETB	␗	^W		End of Trans. Block
001 1000	030	24	18	CAN	␘	^X		Cancel
001 1001	031	25	19	EM	␙	^Y		End of Medium
001 1010	032	26	1A	SUB	␚	^Z		Substitute
001 1011	033	27	1B	ESC	␛	^[	e	Escape
001 1100	034	28	1C	FS	␜	^		File Separator
001 1101	035	29	1D	GS	␝	^]		Group Separator
001 1110	036	30	1E	RS	␞	^^		Record Separator
001 1111	037	31	1F	US	␟	^_		Unit Separator

111 1111	177	127	7F	DEL	␡	^?		Delete

Printable Representation, the Unicode characters from the area U+2400 to U+2421 reserved for representing control characters when it's necessary to print or display them rather than have them perform their intended function. Some browsers may not display these properly.
Control key Sequence/caret notation, the traditional key sequences for inputting control characters. The caret (^) represents the "Control" or "Ctrl" key that must be held down while pressing the second key in the sequence. The caret-key representation is also used by some software to represent control characters.
Character Escape Codes in C programming language and many other languages influenced by it, such as Java and Perl (though not all implementations necessarily support all escape codes).
The Backspace character can also be entered by pressing the "Backspace", "Bksp", or ← key on some systems.
The Delete character can also be entered by pressing the "Delete" or "Del" key. It can also be entered by pressing the "Backspace", "Bksp", or ← key on some systems.
The 'e' escape sequence isn't part of ISO C and many other language specifications. However, it's understood by several compilers.
The Escape character can also be entered by pressing the "Escape" or "Esc" key on some systems.
The Carriage Return character can also be entered by pressing the "Return", "Ret", "Enter", or ↵ key on most systems.
[i] The ambiguity surrounding Backspace comes from mismatches between the intent of the human or software transmitting the Backspace and the interpretation by the software receiving it. If the transmitter expects Backspace to erase the previous character and the receiver expects Delete to be used to erase the previous character, many receivers will echo the Backspace as "^H", just as they'd echo any other uninterpreted control character. (A similar mismatch in the other direction may yield Delete displayed as "^?".)

ASCII printable characters

Code 32, the "space" character, denotes the space between words, as produced by the space-bar of a keyboard. The "space" character is considered an invisible graphic rather than a control character. Codes 33 to 126, known as the printable characters, represent letters, digits, punctuation marks, and a few miscellaneous symbols.
Seven-bit ASCII provided seven "national" characters and, if the combined hardware and software permit, can use overstrikes to simulate some additional international characters: in such a scenario a backspace can precede a grave accent (which the American and British standards, but only those standards, also call "opening single quotation mark"), a backtick, or a breath mark (inverted vel).

Binary	Oct	Dec	Hex	Glyph
010 0000	040	32	20	␠
010 0001	041	33	21	!
010 0010	042	34	22	"
010 0011	043	35	23	#
010 0100	044	36	24	$
010 0101	045	37	25	%
010 0110	046	38	26	&
010 0111	047	39	27	'
010 1000	050	40	28	(
010 1001	051	41	29	)
010 1010	052	42	2A	*
010 1011	053	43	2B	+
010 1100	054	44	2C	,
010 1101	055	45	2D	-
010 1110	056	46	2E	.
010 1111	057	47	2F	/
011 0000	060	48	30	0
011 0001	061	49	31	1
011 0010	062	50	32	2
011 0011	063	51	33	3
011 0100	064	52	34	4
011 0101	065	53	35	5
011 0110	066	54	36	6
011 0111	067	55	37	7
011 1000	070	56	38	8
011 1001	071	57	39	9
011 1010	072	58	3A	:
011 1011	073	59	3B	;
011 1100	074	60	3C	<
011 1101	075	61	3D	=
011 1110	076	62	3E	>
011 1111	077	63	3F	?

Binary	Oct	Dec	Hex	Glyph
100 0000	100	64	40	@
100 0001	101	65	41	A
100 0010	102	66	42	B
100 0011	103	67	43	C
100 0100	104	68	44	D
100 0101	105	69	45	E
100 0110	106	70	46	F
100 0111	107	71	47	G
100 1000	110	72	48	H
100 1001	111	73	49	I
100 1010	112	74	4A	J
100 1011	113	75	4B	K
100 1100	114	76	4C	L
100 1101	115	77	4D	M
100 1110	116	78	4E	N
100 1111	117	79	4F	O
101 0000	120	80	50	P
101 0001	121	81	51	Q
101 0010	122	82	52	R
101 0011	123	83	53	S
101 0100	124	84	54	T
101 0101	125	85	55	U
101 0110	126	86	56	V
101 0111	127	87	57	W
101 1000	130	88	58	X
101 1001	131	89	59	Y
101 1010	132	90	5A	Z
101 1011	133	91	5B	[
1011100	134	92	5C
101 1101	135	93	5D
101 1110	136	94	5E	^
101 1111	137	95	5F	_

Binary	Oct	Dec	Hex	Glyph
110 0000	140	96	60	`
110 0001	141	97	61	a
110 0010	142	98	62	b
110 0011	143	99	63	c
110 0100	144	100	64	d
110 0101	145	101	65	e
110 0110	146	102	66	f
110 0111	147	103	67	g
110 1000	150	104	68	h
110 1001	151	105	69	i
110 1010	152	106	6A	j
110 1011	153	107	6B	k
110 1100	154	108	6C	l
110 1101	155	109	6D	m
110 1110	156	110	6E	n
110 1111	157	111	6F	o
111 0000	160	112	70	p
111 0001	161	113	71	q
111 0010	162	114	72	r
111 0011	163	115	73	s
111 0100	164	116	74	t
111 0101	165	117	75	u
111 0110	166	118	76	v
111 0111	167	119	77	w
111 1000	170	120	78	x
111 1001	171	121	79	y
111 1010	172	122	7A	z
111 1011	173	123	7B	{
111 1100	174	124	7C	\|
111 1101	175	125	7D	}
111 1110	176	126	7E	~

Structural features

The digits 0–9 are represented with their values in binary prefixed with 0011 (this means that converting BCD to ASCII is simply a matter of taking each BCD nibble separately and prefixing 0011 to it).

Lowercase and uppercase letters only differ in bit pattern by a single bit, simplifying case conversion to a range test (to avoid converting characters that are not letters) and a single bitwise operation. Fast case conversion is important because it's often used in case-ignoring search algorithms.

This is in contrast to EBCDIC, in which lowercase and uppercase letters each occupy 3 non-contiguous series' of bit-patterns.

Aliases

A June 1992 RFC and the IANA registry of character sets)

IBM367

cp367

csASCII

Everything about Ascii totally explained

History

ASCII control characters

ASCII printable characters

Structural features

Aliases

Variants

Incompatibility vs interoperability

Unicode

Order