A-level Computing/AQA/Fundamentals of data representation/ASCII and unicode

Character form of decimal digit Error checking thumb|400px|center|The 104-key PC US English QWERTY keyboard layout evolved from the standard typewriter keyboard, with extra keys for computing. ASCII normally uses 8 bits (1 byte) to store each character. However, the 8th bit is used as a check digit, meaning that only 7 bits are available to store each character. This gives ASCII the ability to store a total of 2^7 = 128 different values. thumb|150px|center|The 95 printable ASCII characters, numbered from 32 to 126 (decimal) There is also extended ASCII that uses the 8th bit to store data, allowing for a much larger character set, but for the exam you'll probably be fine with 7 bit parity ASCII ASCII values can take many forms: Numbers Letters (capitals and lower case are separate) Punctuation (?/|\£$ etc.) non-printing commands (enter, escape, F1) Take a look at your keyboard and see how many different keys you have. The number should be 104 for a windows keyboard, or 101 for traditional keyboard. With the shift function valus (a, A; b, B etc.) and recognising that some keys have repeated functionality (two shift keys, the num pad). We roughly have 128 functions that a keyboard can perform. Binary Dec Hex Abbr 000 0000 0 00 NUL 000 0001 1 01 SOH 000 0010 2 02 STX 000 0011 3 03 ETX 000 0100 4 04 EOT 000 0101 5 05 ENQ 000 0110 6 06 ACK 000 0111 7 07 BEL 000 1000 8 08 BS 000 1001 9 09 HT 000 1010 10 0A LF 000 1011 11 0B VT 000 1100 12 0C FF 000 1101 13 0D CR 000 1110 14 0E SO 000 1111 15 0F SI 001 0000 16 10 DLE 001 0001 17 11 DC1 001 0010 18 12 DC2 001 0011 19 13 DC3 001 0100 20 14 DC4 001 0101 21 15 NAK 001 0110 22 16 SYN 001 0111 23 17 ETB 001 1000 24 18 CAN 001 1001 25 19 EM 001 1010 26 1A SUB 001 1011 27 1B ESC 001 1100 28 1C FS 001 1101 29 1D GS 001 1110 30 1E RS 001 1111 31 1F US 111 1111 127 7F DELBinary Dec Hex Glyph010 0000 32 20 ?010 0001 33 21 !010 0010 34 22 "010 0011 35 23 #010 0100 36 24 $010 0101 37 25 %010 0110 38 26 &010 0111 39 27 '010 1000 40 28 (010 1001 41 29 )010 1010 42 2A *010 1011 43 2B +010 1100 44 2C ,010 1101 45 2D -010 1110 46 2E .010 1111 47 2F /011 0000 48 30 0011 0001 49 31 1011 0010 50 32 2011 0011 51 33 3011 0100 52 34 4011 0101 53 35 5011 0110 54 36 6011 0111 55 37 7011 1000 56 38 8011 1001 57 39 9011 1010 58 3A :011 1011 59 3B ;011 1100 60 3C <011 1101 61 3D =011 1110 62 3E >011 1111 63 3F ?Binary Dec Hex Glyph100 0000 64 40 @100 0001 65 41 A100 0010 66 42 B100 0011 67 43 C100 0100 68 44 D100 0101 69 45 E100 0110 70 46 F100 0111 71 47 G100 1000 72 48 H100 1001 73 49 I100 1010 74 4A J100 1011 75 4B K100 1100 76 4C L100 1101 77 4D M100 1110 78 4E N100 1111 79 4F O101 0000 80 50 P101 0001 81 51 Q101 0010 82 52 R101 0011 83 53 S101 0100 84 54 T101 0101 85 55 U101 0110 86 56 V101 0111 87 57 W101 1000 88 58 X101 1001 89 59 Y101 1010 90 5A Z101 1011 91 5B [101 1100 92 5C \101 1101 93 5D ]101 1110 94 5E ^101 1111 95 5F _Binary Dec Hex Glyph110 0000 96 60 `110 0001 97 61 a110 0010 98 62 b110 0011 99 63 c110 0100 100 64 d110 0101 101 65 e110 0110 102 66 f110 0111 103 67 g110 1000 104 68 h110 1001 105 69 i110 1010 106 6A j110 1011 107 6B k110 1100 108 6C l110 1101 109 6D m110 1110 110 6E n110 1111 111 6F o111 0000 112 70 p111 0001 113 71 q111 0010 114 72 r111 0011 115 73 s111 0100 116 74 t111 0101 117 75 u111 0110 118 76 v111 0111 119 77 w111 1000 120 78 x111 1001 121 79 y111 1010 122 7A z111 1011 123 7B {111 1100 124 7C |111 1101 125 7D }111 1110 126 7E ~ If you look carefully at the ASCII representation of each character you might notice some patterns. For example: Binary Dec Hex Glyph110 0001 97 61 a110 0010 98 62 b110 0011 99 63 c As you can see, a = 97, b = 98, c = 99. This means that if we are told what value a character is we can easily work out the value of subsequent or prior characters. Example: ASCII characters Without looking at the ASCII table above! If we are told that the ASCII value for the character '5' is 011 0101, what is the ASCII value for '8'. We know that '8' is three characters after '5', as 5,6,7,8. This means that the ASCII value of '8' will be three bigger than that for '5': 011 0101 ASCII '5' + 011 -------- 011 1000 ASCII '8' Checking above this is the correct value. If you are worried about making mistakes with binary addition, you can deal with the decimal numbers instead. Take the example where you are given the ASCII value of 'g', 110 0111, what is 'e'? We know that 'e' is two characters before 'g', as e, f, g. This means that the ASCII value of 'e' will be two smaller than that for 'g'. 64 32 16 8 4 2 1 1 1 0 0 1 1 1 = 10310 = ASCII value of 'g' 103 - 2 = 10110 64 32 16 8 4 2 1 1 1 0 0 1 0 1 = 10110 = ASCII value of 'e' 1 Exercise: ASCII Without using the crib table (you won't get it in the exam!) answer the following questions: The ASCII code for the letter 'Z' is 90(base10), what is the letter 'X' stored as 88 - as it is 2 characters down in the alphabet How many ASCII 'characters' does the following piece of text use: Hello Pete, ASCII rocks! 27 or 26. If you said 23 you'd be wrong because you must include the non-printing characters at the end of each line. Each end of line needs a EOL command, and a new line needs a carriage return (CR), making the text like so: Hello Pete,[EOL][CR] ASCII rocks![EOL] For the Latin alphabet ASCII is generally fine, but what if you wanted to write something in Mandarin, or Hindi? We need another coding scheme! 6 Extension: Coding ASCII You might have to use ASCII codes when reading from text files. To see what each ASCII code means we can use the folliwing function ChrW(x) which returns the ASCII code with denary value x. Try out the following code to see the first 128 characters. What is special about character 10? For x = 0 To 127 Console.WriteLine("ASCII for " & x & " = " & ChrW(x)) Next Console.ReadLine()