ASCII is a seven-bit code, meaning that it uses the integers representable with seven binary digits (a range of 0 to 127) to represent information. Even at that time that ASCII was introduced, most computers dealt with eight-bit bytes as the smallest unit of information; the eighth bit was commonly used for error checking on communication lines or other device-specific functions.
The non-printable control characters:
Decimal Hex Value ------- ---- ----- 000 00 NUL (Null char.) 001 01 SOH (Start of Header) 002 02 STX (Start of Text) 003 03 ETX (End of Text) 004 04 EOT (End of Transmission) 005 05 ENQ (Enquiry) 006 06 ACK (Acknowledgment) 007 07 BEL (Bell) 008 08 BS (Backspace) 009 09 HT (Horizontal Tab) 010 0A LF (Line Feed) 011 0B VT (Vertical Tab) 012 0C FF (Form Feed) 013 0D CR (Carriage Return) 014 0E SO (Shift Out) 015 0F SI (Shift In) 016 10 DLE (Data Link Escape) 017 11 DC1 (XON) (Device Control 1) 018 12 DC2 (Device Control 2) 019 13 DC3 (XOFF)(Device Control 3) 020 14 DC4 (Device Control 4) 021 15 NAK (Negative Acknowledgement) 022 16 SYN (Synchronous Idle) 023 17 ETB (End of Trans. Block) 024 18 CAN (Cancel) 025 19 EM (End of Medium) 026 1A SUB (Substitute) 027 1B ESC (Escape) 028 1C FS (File Separator) 029 1D GS (Group Separator) 030 1E RS (Request to Send)(Record Separator) 031 1F US (Unit Separator)Printable characters:
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Decimal Hex Value Decimal Hex Value Decimal Hex Value
------- ---- ----- ------- ---- ----- ------- ---- -----
032 20 (Space) 064 40 @ 096 60 `
033 21 ! 065 41 A 097 61 a
034 22 " 066 42 B 098 62 b
035 23 # 067 43 C 099 63 c
036 24 $ 068 44 D 100 64 d
037 25 % 069 45 E 101 65 e
038 26 & 070 46 F 102 66 f
039 27 ' 071 47 G 103 67 g
040 28 ( 072 48 H 104 68 h
041 29 ) 073 49 I 105 69 i
042 2A * 074 4A J 106 6A j
043 2B + 075 4B K 107 6B k
044 2C , 076 4C L 108 6C l
045 2D - 077 4D M 109 6D m
046 2E . 078 4E N 110 6E n
047 2F / 079 4F O 111 6F o
048 30 0 080 50 P 112 70 p
049 31 1 081 51 Q 113 71 q
050 32 2 082 52 R 114 72 r
051 33 3 083 53 S 115 73 s
052 34 4 084 54 T 116 74 t
053 35 5 085 55 U 117 75 u
054 36 6 086 56 V 118 76 v
055 37 7 087 57 W 119 77 w
056 38 8 088 58 X 120 78 x
057 39 9 089 59 Y 121 79 y
058 3A : 090 5A Z 122 7A z
059 3B ; 091 5B [ 123 7B {
060 3C < 092 5C \ 124 7C |
061 3D = 093 5D ] 125 7D }
062 3E > 094 5E ^ 126 7E ~
063 3F ? 095 5F _ 127 7F DEL
The first thirty-two codes (numbers 0--31) in ASCII are reserved for control characters: codes that may not themselves represent information, but that are used to control devices (such as printers) that make use of ASCII. For example, character 10 represents the "line feed" function (which causes a printer to advance its paper), and character 27 represents the "escape" key found on the top left of common keyboards.
Note how uppercase characters can be converted to lowercase by adding 32 to their ASCII value; in binary, this can be accomplished simply by setting the sixth-least significant bit to 1.
Code 32 is the "space" character, denoting the space between words, which is produced by the large space bar of a keyboard. Codes 33 to 126 are called the printable characters, which represent letters, digits, punctuation marks, and a few miscelaneous symbols (see Table 1). Code 127 (all seven bits on) is another special character known as "delete" or "rubout". Though its function is similar to that of other control characters, it was placed at this position so that it could be used to erase a section of [paper tape]?, a popular storage medium at the time, by punching out all its holes.
The international spread of computer technology led to many variations and extensions to the ASCII character set, since ASCII does not include accented letters and other symbols necessary to write most languages besides English that use Roman-based alphabets. International standard ISO 646 (1972) was the first attempt to remedy this problem, although it regrettably created compatibility problems as well. ISO 646 was still a seven-bit character set, and since no additional codes were available, some were re-assigned in language-specific variants. For example, the ASCII code 93 (the right square bracket, "]") is used in the German variant ISO 646-DE for the uppercase letter U with umlaut (Ü), and in the Danish variant ISO 646-DK for the uppercase letter A with ring (Å).
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. Eight-bit standards such as ISO 8859 enabled a broader range of languages to be represented, but were still plagued with incompatibilities and limitations. Still, ISO 8859-1 and original 7-bit ASCII are the most common character encodings in use today, though Unicode (with a much larger code set) is quickly becoming standard in many places. These newer codes are backward-compatible: that is, the first 127 code points of each code are the same as ASCII, and the first 256 code points of Unicode are the same as ISO 8859-1.
ASCII does not specify any way to represent information about the structure or appearance of a piece of text. That requires the use of a markup language.
The portmanteau word "ASCIIbetical" evolved to describe data that is sorted in ASCII collated order rather than standard alphabetical order (which requires some human judgment, and varies with language). (See [1].)
See also Extended ASCII, Unicode.