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ASCII is the abbreviation of American Standard Code for Information Interchange. ASCII code is currently the most common coding standard for computers. Because computers can only accept numeric information, ASCII codes represent characters as numbers so that the computer can accept and process them. For example, the ASCII code for the capital letter A is 65. In the ASCII code, Nos. 0 to 32 and No. 127 are control characters, commonly used ones are LF (line feed) and CR (carriage return); Nos. 33 to 126 are characters, of which Nos. 48 to 57 are 0 to 9 Arabic numerals; numbers 65 to 90 are 26 uppercase English letters, numbers 97 to 122 are 26 lowercase English letters, and the rest are some punctuation marks, arithmetic symbols, etc.
| ASCII value | Control characters | ASCII value | Control characters | ASCII value | Control characters | ASCII value | Control characters |
|---|---|---|---|---|---|---|---|
| 0 | NUT | 32 | (space) | 64 | @ | 96 | , |
| 1 | SOH | 33 | ! | 65 | A | 97 | a |
| 2 | STX | 34 | ” | 66 | B | 98 | b |
| 3 | ETX | 35 | # | 67 | C | 99 | c |
| 4 | EOT | 36 | $ | 68 | D | 100 | d |
| 5 | ENQ | 37 | % | 69 | E | 101 | e |
| 6 | ACK | 38 | & | 70 | F | 102 | f |
| 7 | BEL | 39 | , | 71 | G | 103 | g |
| 8 | BS | 40 | ( | 72 | H | 104 | h |
| 9 | HT | 41 | ) | 73 | I | 105 | i |
| 10 | LF | 42 | * | 74 | J | 106 | j |
| 11 | VT | 43 | + | 75 | K | 107 | k |
| 12 | FF | 44 | , | 76 | L | 108 | l |
| 13 | CR | 45 | - | 77 | M | 109 | m |
| 14 | SO | 46 | . | 78 | N | 110 | n |
| 15 | SI | 47 | / | 79 | O | 111 | o |
| 16 | DLE | 48 | 0 | 80 | P | 112 | p |
| 17 | DCI | 49 | 1 | 81 | Q | 113 | q |
| 18 | DC2 | 50 | 2 | 82 | R | 114 | r |
| 19 | DC3 | 51 | 3 | 83 | X | 115 | s |
| 20 | DC4 | 52 | 4 | 84 | T | 116 | t |
| 21 | NAK | 53 | 5 | 85 | U | 117 | u |
| 22 | SYN | 54 | 6 | 86 | V | 118 | v |
| 23 | TB | 55 | 7 | 87 | W | 119 | w |
| 24 | CAN | 56 | 8 | 88 | X | 120 | x |
| 25 | EM | 57 | 9 | 89 | Y | 121 | y |
| 26 | SUB | 58 | : | 90 | Z | 122 | z |
| 27 | ESC | 59 | ; | 91 | [ | 123 | { |
| 28 | FS | 60 | < | 92 | \ | 124 | | |
| 29 | GS | 61 | = | 93 | ] | 125 | } |
| 30 | RS | 62 | > | 94 | ^ | 126 | ~ |
| 31 | US | 63 | ? | 95 | — | 127 | DEL |
The ASCII value is a decimal number, and the meaning of the control characters is as shown in the following table.
| NUL empty | VT vertical tab | SYN idle synchronization |
|---|---|---|
| SOH Title Start | FF paper feed control | End of ETB message group transmission |
| STX text begins | CR carriage return | CAN void |
| ETX End of text | SO shift output | EM paper ends |
| EOY end of transmission | SI shift input | SUB substitution |
| ENQ query character | DLE space | ESC code escape |
| ACK acknowledge | DC1 Device Control 1 | FS text separator |
| BEL alarm | DC2 Device Control 2 | GS group separator |
| BS back one space | DC3 device control 3 | RS record separator |
| HT horizontal list | DC4 Device Control 4 | US unit separator |
| LF line feed | NAK negation | DEL delete |
Since the number of characters in the standard ASCII character set is limited, it often cannot meet the requirements in practical applications. To this end, the International Organization for Standardization has formulated the ISO2022 standard, which stipulates a unified method for extending the ASCII character set to 8-bit codes while maintaining compatibility with ISO646. ISO has successively formulated a batch of extended ASCII character sets suitable for different regions. Each extended ASCII character set can be extended by 128 characters. The encoding of these extended characters is an 8-bit code with a high bit of 1 (that is, decimal numbers 128~255 ), called extended ASCII code.
Extended ASCII characters satisfy the need for more characters. Extended ASCII contains the 128 characters already in ASCII (numbers 0–32 are shown in the image below), plus an additional 128 characters for a total of 256.
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