Text Representation within Computers

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Text Representationwithin Computers

• CS208

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The Binary Digit (Bit)

• One bit can encode a value set that contains two
  elements
• e.g. black, white, up, down, registered, not
  registered, etc.
• What if we need to encode a value set that
  contains three values?
• e.g. red, yellow, green for a traffic signal
• Can use a string of two bits
• 00 red
• 01 yellow
• 11 green

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Binary Bit Strings
2
4
8
16
Each additional bit doubles the number of
possible combinations
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The ASCII Code

• When you type text characters at your keyboard,
  each character is stored as a separate bit code.
• A standard bit code was assigned to each possible
  character that can appear in text.
• The code is called the ASCII (American Standard
  Code for Information Interchange) code.

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The ASCII Code

• Original ASCII was a 7bit code
• ANSI identified a standard set of 128 characters
  (including letters, digits, symbols and special
  control characters)
• 7 bits can provide 128 distinct bit patterns
  (codes)
• Extended ASCII is an 8-bit code
• Additional 128 codes are used to accommodate
  letters from other languages (Arabic, French,
  German, etc)

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7-Bit ASCII Code Table
Rightmost Leftmost Three Bits Four
Bits 000 001 010 011 100 101 110 111 0000 NUL DL
E Space 0 _at_ P p 0001 SOH DC1 ! 1 A Q a q 0010
STX DC2 " 2 B R b r 0011 ETX DC3 3 C S c s 0100
EOT DC4 4 D T d t 0101 ENQ NAK 5 E U e u 01
10 ACK SYN 6 F V f v 0111 BEL ETB ' 7 G W g w
1000 BS CAN ( 8 H X h x 1001 HT EM ) 9 I Y i y 1
010 LF SUB J Z j z 1011 VT ESC K k 1
100 FF FS , lt L \ l 1101 CR GS - M m
1110 SO RS . gt N n 1111 SI US / ? O _ o DEL

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Using the ASCII Code Table

• Find a character in the chart
• Use the 3 bits at the top of the column the
  character is in, as the first 3 bits
• Use the 4 bits in at the far left side of the row
  that the character is in, as the last 4 bits
• This creates a binary 7-bit ASCII code
• NOTE a leading zero is added when the code is
  stored in one byte

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Finding ASCII values for Characters using the
Table

• Example 1 Find the binary ASCII and decimal
  ASCII values for the character.

Rightmost Leftmost Three Bits Four
Bits 000 001 010 011 100 101 110 111 0000 NUL DL
E Space 0 _at_ P p 0001 SOH DC1 ! 1 A Q a q 0010
STX DC2 " 2 B R b r 0011 ETX DC3 3 C S c s 0100
EOT DC4 4 D T d t 0101 ENQ NAK 5 E U e u 01
10 ACK SYN 6 F V f v 0111 BEL ETB ' 7 G W g w
1000 BS CAN ( 8 H X h x 1001 HT EM ) 9 I Y i y 1
010 LF SUB J Z j z 1011 VT ESC K k 1
100 FF FS , lt L \ l 1101 CR GS - M m
1110 SO RS . gt N n 1111 SI US / ? O _ o DEL

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Finding ASCII values for Characters using the
Table

• From the chart
  0100110 (binary ASCII value)
• Convert the binary value to decimal 01001102
  32 4 2 3810
• Therefore 3810 (decimal ASCII value)

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Finding ASCII values for Characters using the
Table

• Example 2 Find the binary , octal, and hex ASCII
  values for the M character.

Rightmost Leftmost Three Bits Four
Bits 000 001 010 011 100 101 110 111 0000 NUL DL
E Space 0 _at_ P p 0001 SOH DC1 ! 1 A Q a q 0010
STX DC2 " 2 B R b r 0011 ETX DC3 3 C S c s 0100
EOT DC4 4 D T d t 0101 ENQ NAK 5 E U e u 01
10 ACK SYN 6 F V f v 0111 BEL ETB ' 7 G W g w
1000 BS CAN ( 8 H X h x 1001 HT EM ) 9 I Y i y 1
010 LF SUB J Z j z 1011 VT ESC K k 1
100 FF FS , lt L \ l 1101 CR GS - M m
1110 SO RS . gt N n 1111 SI US / ? O _ o DEL

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Finding ASCII values for Characters using the
Table

• From the chart M
  1001101 (binary ASCII value)
• Convert the binary value to octal (re-group by
  3s) 001 001 1012 1158 (octal ASCII value)
• Convert the binary value to hexadecimal (by
  4s) 0100 11012 4D16 (hex ASCII value)

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Finding Characters represented using the Table

• Example 3 What does the following binary ASCII
  code represent?
• 1001111 1001011

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Finding Characters represented using the Table

• Example Find 1001111 and 1001011 in the table

Rightmost Leftmost Three Bits Four
Bits 000 001 010 011 100 101 110 111 0000 NUL DL
E Space 0 _at_ P p 0001 SOH DC1 ! 1 A Q a q 0010
STX DC2 " 2 B R b r 0011 ETX DC3 3 C S c s 0100
EOT DC4 4 D T d t 0101 ENQ NAK 5 E U e u 01
10 ACK SYN 6 F V f v 0111 BEL ETB ' 7 G W g w
1000 BS CAN ( 8 H X h x 1001 HT EM ) 9 I Y i y 1
010 LF SUB J Z j z 1011 VT ESC K k 1
100 FF FS , lt L \ l 1101 CR GS - M m
1110 SO RS . gt N n 1111 SI US / ? O _ o DEL

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Finding Characters represented using the Table

• From the table
• 1001111 O
• 1001011 K
• So 1001111 1001011 represents OK in binary ASCII

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Character Representation ASCII Table

• Example 4 What character is represented by the
  decimal ASCII value 88?
• Convert the decimal value to binary 8810
  64 16 8 10110002 (binary ASCII
  value)
• From the chart 101 1000 X
• Therefore 88 X (decimal ASCII value)

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Decimal ASCII Example
H i , H e a t h e r .
72 105 44 32 72 101 97 116 104
101 114 46
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Try It Yourself Text Representation

• How would the text So? be
  represented in binary ASCII?
• How would the SAME text be represented in decimal
  ASCII?
• What text does the following binary ASCII
  represent? 10010002 11010012
• How would you represent the SAME text in octal
  ASCII?
• (Answers on NEXT slide)

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Answers

• So? 10100112 11011112 01111112
• In decimal ASCII 8310 11110 6310
• 10010002 11010012 represents text Hi
• The same text in octal ASCII is 1108 1518