ASCII defines the following printable characters, presented here in numerical order of their ASCII value:
(spacebar)
!"#$%&'()*+,-./
0123456789:;<=>?
@ABCDEFGHIJKLMNO
PQRSTUVWXYZ[\\]^_
`abcdefghijklmno
pqrstuvwxyz{|}~(del)
ASCII is, strictly, a seven-Bit code, meaning that it uses the bit patterns representable with seven binary digits (a range of 0 to 127 decimal) to represent character information. At the time ASCII was introduced, many computers dealt with eight-bit groups (bytes or, more specifically, octets) as the smallest unit of information; the eighth bit was commonly used as a Parity bit for error checking on communication lines or other device-specific functions. Machines which did not use parity typically set the eighth bit to zero, though some systems such as Prime machines running PRIMOS set the eighth bit of ASCII characters to one.
ASCII does not specify any way to include information about the conceptual structure or appearance of a piece of text. That requires other standards, such as those specifying markup languages. Conceptual structure can be included using XML and appearance can be specified by using HTML for relatively simple things, SGML for more complex things, or PostScript, Display PostScript, TeX, or XSL-FO for advanced layout and font control.
The American Standards Association (ASA, later to become ANSI) first published ASCII as a standard in 1963. ASCII-1963 lacked the lowercase letters, and had an up-arrow (↑) instead of the caret (^) and a left-arrow (←) instead of the underscore (_). The 1967 version added the lowercase letters, changed the names of a few control characters and moved the two controls ACK and ESC from the lowercase letters area into the control codes area.
Many variations of ASCII exist, but its present, most widely-used form uses the ANSI X3.4-1986 definition, also standardized as ECMA-6, ISO/IEC 646:1991 International Reference Version, ITU-T Recommendation T.50 (09/92), and Request for Comments RFC 20. ASCII has become embedded in its probable replacement, Unicode, as the 'lowest' 128 characters. Some observers consider ASCII the most "successful" software standard ever promulgated.
Code 127 (all seven bits on), another special character, equates to "delete" or "rubout". Though its function resembles that of other control characters, the designers of ASCII used this pattern so that it could "erase" a section of paper tape (a popular storage medium until the 1980s) by punching all possible holes at a particular character position, thus effectively replacing any previous information.
Many of the ASCII control codes serve (or served) to mark data packets, or to control a data transmission protocol (e.g. ENQuiry [effectively, "any stations out there?"], ACKnowledge, Negative AcKnowledge, Start Of Header, Start Of Text, End Of Text, etc). ESCape and SUBstitute permit a communications protocol to, for instance, mark binary data so that if it contains codes with the same pattern as a protocol character, the recipient will process the code as data.
The designers of ASCII intended the separator characters ("Record Separator", etc.) for use with magnetic tape systems.
Two of the device control characters, commonly interpreted as XON and XOFF, generally function as Flow control characters to throttle data flow to a slow device (such as a printer) from a fast device (such as a computer) - so data does not overrun and get lost.
Early users of ASCII adopted some of the control codes to represent "meta information" such as end-of-line, start/end of a data element, and so on. These assignments often conflict, so part of the effort in converting data from one format to another involves making the correct meta information transformations. For example, the character(s) representing end-of-line ("Newline") in text data files/streams vary from Operating system to operating system. When moving files from one system to another, the conversion process must recognize these characters as end-of-line markers and handle them appropriately.
Today, ASCII users use the control characters less and less-with the exception of "carriage return" and/or "line feed". Modern markup languages, modern communication protocols, the move from text-based to graphical devices, and the demise of teleprinters, punch-cards, and paper tapes have rendered most of the control characters obsolete.
| Binary | Oct | Dec | Hex | Abbr | PR[1] | CS[2] | Description |
|---|---|---|---|---|---|---|---|
| 0000 0000 | 000 | 0 | 00 | NUL | ␀ | ^@ | Null character |
| 0000 0001 | 001 | 1 | 01 | SOH | ␁ | ^A | Start of Header |
| 0000 0010 | 002 | 2 | 02 | STX | ␂ | ^B | Start of Text |
| 0000 0011 | 003 | 3 | 03 | ETX | ␃ | ^C | End of Text |
| 0000 0100 | 004 | 4 | 04 | EOT | ␄ | ^D | End of Transmission |
| 0000 0101 | 005 | 5 | 05 | ENQ | ␅ | ^E | Enquiry |
| 0000 0110 | 006 | 6 | 06 | ACK | ␆ | ^F | Acknowledgement |
| 0000 0111 | 007 | 7 | 07 | BEL | ␇ | ^G | Bell |
| 0000 1000 | 010 | 8 | 08 | BS | ␈ | ^H | Backspace[3][7] |
| 0000 1001 | 011 | 9 | 09 | HT | ␉ | ^I | Horizontal Tab |
| 0000 1010 | 012 | 10 | 0A | LF | ␊ | ^J | Line feed |
| 0000 1011 | 013 | 11 | 0B | VT | ␋ | ^K | Vertical Tab |
| 0000 1100 | 014 | 12 | 0C | FF | ␌ | ^L | Form feed |
| 0000 1101 | 015 | 13 | 0D | CR | ␍ | ^M | Carriage return[6] |
| 0000 1110 | 016 | 14 | 0E | SO | ␎ | ^N | Shift Out |
| 0000 1111 | 017 | 15 | 0F | SI | ␏ | ^O | Shift In |
| 0001 0000 | 020 | 16 | 10 | DLE | ␐ | ^P | Data Link Escape |
| 0001 0001 | 021 | 17 | 11 | DC1 | ␑ | ^Q | Device Control 1 (oft. XON) |
| 0001 0010 | 022 | 18 | 12 | DC2 | ␒ | ^R | Device Control 2 |
| 0001 0011 | 023 | 19 | 13 | DC3 | ␓ | ^S | Device Control 3 (oft. XOFF) |
| 0001 0100 | 024 | 20 | 14 | DC4 | ␔ | ^T | Device Control 4 |
| 0001 0101 | 025 | 21 | 15 | NAK | ␕ | ^U | Negative Acknowledgement |
| 0001 0110 | 026 | 22 | 16 | SYN | ␖ | ^V | Synchronous Idle |
| 0001 0111 | 027 | 23 | 17 | ETB | ␗ | ^W | End of Trans. Block |
| 0001 1000 | 030 | 24 | 18 | CAN | ␘ | ^X | Cancel |
| 0001 1001 | 031 | 25 | 19 | EM | ␙ | ^Y | End of Medium |
| 0001 1010 | 032 | 26 | 1A | SUB | ␚ | ^Z | Substitute |
| 0001 1011 | 033 | 27 | 1B | ESC | ␛ | ^[ | Escape[5] |
| 0001 1100 | 034 | 28 | 1C | FS | ␜ | ^\\ | File Separator |
| 0001 1101 | 035 | 29 | 1D | GS | ␝ | ^] | Group Separator |
| 0001 1110 | 036 | 30 | 1E | RS | ␞ | ^^ | Record Separator |
| 0001 1111 | 037 | 31 | 1F | US | ␟ | ^_ | Unit Separator |
| 0111 1111 | 177 | 127 | 7F | DEL | ␡ | ^? | Delete[4][7] |
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 Tilde, or a breath mark (inverted vel).
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Processors can convert uppercase characters to lowercase by adding 32 to their ASCII value; in binary, devices can accomplish this simply by setting the sixth-Least significant bit to 1.
Of these, only the aliases "US-ASCII" and "ASCII" have achieved widespread use. One often finds them in the optional "charset" parameter in the Content-Type header of some MIME messages, in the equivalent "meta" element of some HTML documents, and in the encoding declaration part of the prolog of some XML documents.
ISO 646 (1972), the first attempt to remedy the pro-English-language bias, created compatibility problems, since it remained a 7-bit character-set. It made no additional codes available, so it reassigned some in language-specific variants. It thus became impossible to know what character a code represented without knowing which variant to work with, and text-processing systems could generally cope with only one variant anyway.
Eventually, improved technology brought out-of-band means to represent the information formerly encoded in the eighth bit of each byte, freeing this bit to add another 128 additional character-codes for new assignments. For example, IBM developed 8-bit code pages, such as Code page 437, which replaced the control-characters with graphic symbols such as Smiley faces, and mapped additional graphic characters to the upper 128 bytes. Operating systems such as DOS supported these code-pages, and manufacturers of IBM PCs supported them in hardware.
Eight-bit standards such as ISO/IEC 8859 and MacRoman developed as true extensions of ASCII, leaving the original character-mapping intact and just adding additional values above the 7-bit range. This enabled the representation of a broader range of languages, but these standards continued to suffer from incompatibilities and limitations. Still, ISO/IEC 8859-1 and original 7-bit ASCII remain the most common character encodings in use today.
Unicode and the ISO/IEC 10646 Universal Character Set (UCS) have a much wider array of characters, and their various encoding forms have begun to supplant ISO/IEC 8859 and ASCII rapidly in many environments. While ASCII basically uses 7-bit codes, Unicode and the UCS use relatively abstract "code points": non-negative integer numbers that map, using different encoding forms and schemes, to sequences of one or more 8-bit bytes. To permit backward compatibility, Unicode and the UCS assign the first 128 code points to the same characters as ASCII. One can therefore think of ASCII as a 7-bit encoding scheme for a very small subset of Unicode and of the UCS. The popular UTF-8 encoding-form prescribes the use of one to four 8-bit code values for each code point character, and equates exactly to ASCII for the code values below 128. Other encoding forms such as UTF-16 resemble ASCII in how they represent the first 128 characters of Unicode, but tend to use 16 or 32 bits per character, so they require conversion for compatibility.
The Portmanteau word ASCIIbetical has evolved to describe the Collation of data in ASCII-code order rather than "standard" alphabetical order (which requires some tricky computation, and varies with language).
ASCII contains many characters not in common use (or at least not commonly spoken of) outside of the computing context; the "popularization" of these characters encouraged users to agree on standard names for them. See the pronunciation guide in the external links, below.
The abbreviation ASCIIZ or ASCIZ refers to a null-terminated ASCII string.