Storing Characters: ASCII and Unicode

1 Explain what is meant by a character set. 2

One mark for each of the following, to a maximum of 2.

• A list of characters that a computer system recognises
• Each character is represented by a unique binary code or number

Model answer. A character set is the defined list of characters that a computer system is able to recognise and use, such as letters, digits, punctuation and control characters. Each character in the set is assigned its own unique binary code, so that the computer can store and transmit text as binary values.

2 Describe one advantage and one disadvantage of using Unicode rather than ASCII. 4

One mark for each of the following, to a maximum of 4.

• Advantage: Unicode represents far more characters
• Advantage explained: it supports the writing systems of most world languages
• Disadvantage: Unicode uses more bits per character
• Disadvantage explained: files are larger, using more storage and bandwidth

Model answer. An advantage of Unicode is that it can represent over a million different characters rather than the 128 available in standard ASCII, which means it supports the writing systems of virtually every language including Chinese, Arabic and Cyrillic, as well as symbols and emoji. This makes it essential for any system used internationally. The disadvantage is that Unicode uses at least 16 bits per character compared with 7 or 8 bits in ASCII, so the same piece of text takes up roughly twice as much storage space and takes longer to transmit over a network.

3 The ASCII code for the character 'M' is 77. State the ASCII code for the character 'Q' and explain how you worked it out. 2

One mark for each of the following, to a maximum of 2.

• Correct answer 81
• Explains that characters are stored in sequential order, so counting forward from M gives Q

Model answer. The ASCII code for 'Q' is 81. Characters in ASCII are stored in sequential order, so counting forward from M at 77 gives N as 78, O as 79, P as 80 and Q as 81.

4 A text file contains 4000 characters. Calculate the difference in file size, in bytes, between storing it in 8 bit extended ASCII and storing it in 16 bit Unicode. 3

One mark for each of the following, to a maximum of 3.

• ASCII size calculated as 4000 bytes
• Unicode size calculated as 8000 bytes
• Difference stated as 4000 bytes

Model answer. In 8 bit extended ASCII each character takes one byte, so 4000 characters take 4000 bytes. In 16 bit Unicode each character takes two bytes, so the same 4000 characters take 8000 bytes. The difference in file size is therefore 8000 minus 4000, which is 4000 bytes, meaning the Unicode version is twice the size.

5 A company is developing a website that will be used by customers all over the world. Explain why the company should use Unicode rather than ASCII, and identify one drawback of this decision. 4

One mark for each of the following, to a maximum of 4.

• Customers will write in many different languages
• ASCII only covers English letters, digits and basic symbols
• Unicode can represent the characters of virtually all writing systems
• Drawback: larger files, so more storage and bandwidth is used

Model answer. The company should use Unicode because its customers are spread across the world and will enter names, addresses and messages in many different writing systems. Standard ASCII can only represent 128 characters, which covers English letters, digits and basic punctuation, so any customer writing in Arabic, Chinese, Greek or Russian would find their text could not be stored or displayed correctly. Unicode can represent over a million characters and covers essentially every major script, so all customers are supported equally. The drawback is that Unicode uses at least 16 bits per character rather than 7 or 8, which roughly doubles the size of stored text and increases the bandwidth needed to send pages, adding to hosting costs and slightly slowing page loads.