ASCII and Unicode

Lesson objective Understand how character sets (ASCII and UNICODE) are used to represent text

Representing text For data to be readable by all computer systems, an agreed method of representing characters and strings is important. One approach is ASCII. (American Standard Code for Information Interchange)

Representing text - ASCII Each character of the alphabet and some special symbols are represented by agreed binary patterns. Normally in computers, bits are group together in eight bit bytes

Representing text - ASCII A byte can hold 256 different combinations of 0’s and 1’s Meaning 256 characters can be represented

Representing text - ASCII One byte is used to hold one unique character

Representing text - ASCII Most personal computer use ASCII (American Standard Code for Information Interchange). Computer will have to have the same characters set to allow documents to be readable from one computer to another. Computer can have more than one character set installed. WHY do you think we need other character sets? Not enough room in ASCII character set to represent every different language (diagrammatic fonts - Chinese)

Representing text - ASCII The standard ASCII character set – used 7 bits (8th bit left for error checking information)– represents 128 separate characters. ASCII is limited – can not display all characters for other alphabets, but can store: upper and lower case English alphabet, digits 0 to 9, punctuation and special characters (e.g. % £), some non display signals (e.g. escape key)

Representing text – Another approach is Unicode Different versions of UNICODE character sets are available but each one is at least 16-bits long allowing for more than 65,536 characters to be represented Imagine that you own a company that sells computers all over the world. Every customer will want to type in their own language. What would you do? You want a system that can handle every possible written language – when the computer gets turned on for the first time they simply choose the language that the operating system needs to handle.

Representing text - Unicode Unicode can handle any language and it does so by the user selecting a specific 'code page' which is one portion of the total Unicode space. Each code page represents the chosen language. For example code page 1253 within Unicode represents the Greek language. If the person in Greece has a Greek keyboard and the Greek ‘code page’ is selected within the operating system then the correct characters appear on the screen when they press a certain key on the keyboard.

Representing text - Unicode Unicode can handle any language and it does so by the user selecting a specific 'code page' which is one portion of the total Unicode space. Each code page represents the chosen language. For example code page 1253 within Unicode represents the Greek language. If the person in Greece has a Greek keyboard and the Greek ‘code page’ is selected within the operating system then the correct characters appear on the screen when they press a certain key on the keyboard.

Representing text - Unicode Unicode gets it name because it is a character set that can be used to universally encode text of nearly all modern and ancient languages. Negative Using a universal character set requires over twice as much space to store all the characters compared to ASCII

Questions The ASCII code in denary for the numeric character ‘0’ is 48, other numeric characters follow this sequence. What is the ASCII code for the character ‘2’ in binary? [1] How many different characters can be represented in ASCII? [1] State one difference between Unicode and ASCII [1] State one advantage and one disadvantage of using ASCII rather than Unicode [2]