Memory & Storage: Lesson 6 Storing Characters, Slides of Computer science

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Memory & Storage: Lesson 6

Starter

A computers memory and storage only hold binary 1 s and 0 s

• How might it be possible to store letters with only binary?

Learning Outcomes:

• Understand the use of binary codes to represent characters
• Understand the term ‘character set’
• Explain the relationship between the number of bits per character in a character set, and the number of characters that can be represented using: - ASCII - Extended ASCII - Unicode

Memory & Storage: Lesson 6

Storing Characters

Memory & Storage: Lesson 6

Representing characters in

binary

• Every character on the keyboard is represented by a binary value
  • Uppercase letters (capitals) have different values from lowercase characters
  • Punctuation symbols have their own character code How many characters are there on a standard keyboard? How many bits would be required to represent this many combinations?

Memory & Storage: Lesson 6

Characters in binary

• A keyboard needs to contain
  • 26 lowercase letters
  • 26 uppercase letters
  • 10 numbers
  • (around) 36 other characters
• There are around 98 unique characters that are available on a keyboard
  • 6 bits give 64 different combinations – this isn’t enough
  • 7 bits give 128 different combinations which can represent 128 different characters

Memory & Storage: Lesson 6

ASCII & Unicode….?

• What is ASCII?
• What is Unicode?
• What is the difference between the two?

Memory & Storage: Lesson 6

The ASCII character set

• ASCII (American Standard Code for Information Interchange) has become the standard code, used worldwide - It was originally developed in the 1960 s for representing the English alphabet - It encodes 128 characters into 7 - bit binary codes
• Characters include numbers 0 to 9 , uppercase and lowercase letters A-Z, a-z, punctuation symbols and the space character

Memory & Storage: Lesson 5

ASCII- Problems

• Standard ASCII only uses the first 128 binary numbers available out of a possible 256.
• Other languages such as German, French, Finnish, Irish, Icelandic etc take advantage of the another 128 values to include their own special characters.
• This is called the extended ascii character set.

Memory & Storage: Lesson 6

7 - and 8 - bit ASCII

• Numerous different codes for representing characters have been created, but ASCII is commonly used on PCs
• Originally only seven bits were used, but now an eighth bit is used allowing for many more characters such as ©, ® etc. - How many different characters can be encoded using 7 bits, 8 bits or 16 bits?

Memory & Storage: Lesson 6

Using the eighth bit

• Sometimes it is useful to be able to type special characters like á, à, ®
• Here are the codes for some of them: © Alt+ 0169 ® Alt+ 0174 á Alt+ 0225 à Alt+ 0224 â Alt+ 0226 ä Alt+ 0228

Memory & Storage: Lesson 6

Extended ASCII

• There is not a single agreed-upon standard for extended ascii code sets.
• There are many different extended ASCII sets as various countries wanted to use the higher bits for their own alphabet. Example:
• In the 'Latin 1 Western European' set, 11011100 is the German u-with-umlaut (ü).
• In other ASCII sets, like the 'Latin 2 Central European' that covers languages such as Croatian, Serbian, and Czech, this value may correspond to an entirely different character.

Memory & Storage: Lesson 6

Programming with text &

numbers

• The ASCII code for ‘ 7 ’ is 011 0111
  • The binary code for the digit 7 is 0000 0111
• When you write a program in Python, for example, you have to specify whether a variable is text or integer - You cannot do arithmetic with characters - If the character represents a number it must first be converted to an integer before any arithmetic can be carried out

Memory & Storage: Lesson 6

Using different alphabets

• To represent other characters for different languages, a new code allowing for many more characters is needed - Unicode was developed to use 16 bits 65 536 possible combinations - The 32 bit version gives 4 294 967 296 (over 4 billion) possible combinations

Memory & Storage: Lesson 6

Unicode

• In Japanese, konnichiwa is used as a greeting meaning “good day”
• In Unicode this is written as three 16 - bit characters
  • How many bytes does the English ‘good day’ require in ASCII?
  • How many bytes does the Japanese require in Unicode?

Memory & Storage: Lesson 6 Smiling face with sunglasses Unicode: 1 F 60 E

Unicode

• ‘good day’ requires 8 bytes to store
• 今日は requires 6 bytes to store ( 3 characters x 2 bytes)
• Unicode is also used to store emoji - ‘e’ is Japanese for picture - ‘moji’ is Japanese for character or alphabet