Computer Systems Nat 4/5 Computing Science Data Representation Lesson 3: Storing Text

1.What is the system used to store real numbers? 2.Convert into a decimal number 3.Convert 23 into binary. 4.Describe what is mean by a mantissa and exponent

1.Floating Point representation = = In the number 3.0 x10 8 the mantissa would be 3.0 with the exponent being 8

Lesson Aims  By the end of this lesson you will be able to:  Describe how text is stored by the computer.  Explain and give an example of an ASCII character.  Describe how many bytes are needed to represent a single character of text.  Describe what is meant by the term character set.  Give an example of why we need different character sets.

A question…  Computers can only use and understand binary digits which are 0’s and 1’s.  We have already looked at how binary can be used to store numbers.  How can we store text?  By storing them as binary numbers…. Nat 4/5

ASCII Codes  Each character on a keyboard has its own ASCII code. This is a binary value that represents each character that can be seen on the screen.  American Standard Code for Information Interchange  For example A = (65)  Each ASCII value takes up 1 byte of storage. Nat 4/5

ASCII Table  An extract of the ASCII Table is shown below:  The ASCII table has 128 values  52 just for text. 10 for numbers  Spacebar and tab key have codes too  What about the rest? Nat 4/5CodeSymbolCodeSymbolCodeSymbolCodeSymbol 49179O65A98b 50280P66B99c 51381Q67C100d 52482R68D101e 53583S69E102f

Control Characters  The rest of the ASCII code are reserved for control characters.  These are non-printable characters that have an effect such as  The Enter key  What about the trusted delete/backspace keys?  Nat 4/5

Character Set  The character set is the name given to the complete set of characters that the computer can represent.  Different character sets are used to represent different languages  The character set can alter the layout of the keyboard  For example on American layout keyboards sign is above the number 2 not the “ mark. Nat 4/5

 Some languages use the Roman or Latin character set, English, French and German  Many others such as Japanese, use completely different characters Arabic ChineseKorean Nat 4/5

 The Unicode character set is designed to represent the writing schemes of all the world’s major languages  The first 128 characters of Unicode are the same as ASCII  Allowing for compatibility between them Nat 4/5

 Unicode is a 16 bit code and can represent 65,536 different characters  All possible characters can be represented  Unicode is used in all the worlds computers and on mobile phones for text messaging  It can represent many more characters than ASCII Nat 4/5

 Even though it takes up twice the space of ASCII to store (16 bits rather than 8 bits) and messages are twice as long to send this is no longer seen as a disadvantage as storage space became cheaper and bandwidth increased  Unicode allows all the worlds languages to be accessed from the one computer (see Microsoft Word File/Options or Region and Language in Windows) Nat 4/5

Summary  Text is stored in the computer using ASCII values  1 ASCII Value = 1 byte of memory  Control characters are the non-printable characters which have an effect on the screen  Such as the Enter Key  The character set is the name given to the entire set of characters that the keyboard can produce  Unicode is a character set designed to represent all the worlds languages Nat 4/5