GCSE · Computer Science · AQA · Spec 8525

Unicode

Type a capital A and your computer doesn't store an A. It stores 65. So how could it ever store a letter from another alphabet, or an emoji?

Try it · 7 bits, one character

Flip a bit, change the letter

7-bit binary 1000001 = 65 in denary

Character code65

Tap the 32. Now tap every 0 to 1.

Code key: A = 65, B = 66, C = 67 … Z = 90, then a = 97, b = 98, c = 99 … z = 122. Codes that aren't on this key belong to other characters, such as digits, punctuation and control codes.

Watch out: Seven 1s make 127, not 128. But 0000000 is a code too, so 0 to 127 is 128 different codes altogether.

Codes come in order

Work it out, don't look it up

You know two codes: A is 65 and a is 97. Letters run in order, one code after another. Drag each letter to where its code sits, then check.

Place BPlace EPlace ZPlace c

ASCII and Unicode, side by side

ASCIIvsUnicode

Read the top row first: it's the fact that links the two sets.

Focus

Codes 0 to 127

ASCII

Its whole set: A is 65, a is 97

Unicode

The same codes: A is still 65, a is still 97

The insight

Unicode didn't renumber anything. It was designed to match ASCII's first 128 codes, then add more on top.

Bits per character

ASCII

Unicode

More than ASCII: up to 32

How many characters

ASCII

128

Unicode

A far larger number

What it covers

ASCII

English letters, digits, punctuation and control codes

Unicode

Almost all the world's writing systems, plus many symbols and emoji

What do you really think?

What's actually stored?

You type a capital A and save it.

Which of these is closest to what you think the computer stores?

A tiny picture of the letter A.The number 1, because A is the first letter of the alphabet.A number, 65, held as a pattern of bits.It depends. A is 65 in ASCII, but Unicode gives it a new, bigger code.

How sure are you?

I'm sureI think soI'm guessing

A tiny picture of the letter A.

This is a common misconception

It's a natural thought, because a letter is what you see. But a computer can only store binary. Every character is given a code number, and that number is what gets stored. For A, it's 65.

The number 1, because A is the first letter of the alphabet.

This is a common misconception

You're right that it's a number, but not that one. The code comes from the character set, not from the alphabet. A's code is 65, and B follows it at 66.

A number, 65, held as a pattern of bits.

This matches the evidence

Spot on. A's character code is 65, stored in binary as 1000001. The same 65 is A's code in ASCII and in Unicode.

It depends. A is 65 in ASCII, but Unicode gives it a new, bigger code.

This is a common misconception

This one catches a lot of people. Unicode was designed to be compatible with ASCII, so its first 128 codes are exactly ASCII's codes. A is 65 in both. Unicode's extra codes are for characters ASCII never had.

Text is stored as numbers: each character has a code, and the code is kept in binary. Unicode keeps ASCII's codes and adds room for many more.

Your turn to write it

Unicode or ASCII?

Give one advantage and one disadvantage of storing text using Unicode instead of ASCII. Explain each one. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Advantage: Unicode can represent characters from almost all of the world's writing systems (and symbols such as emoji), not just English (1 mark)
…so text in many languages can be stored and shared consistently between computers worldwide (1 mark)
Disadvantage: Unicode uses more bits per character than ASCII (ASCII uses 7) (1 mark)
…so the same text can take up more memory / make a larger file (1 mark)

Model answer

An advantage is that Unicode can represent characters from almost all of the world's writing systems, as well as symbols such as emoji. This means text in many different languages can be stored and shared consistently between computers anywhere in the world. A disadvantage is that Unicode uses more bits per character than ASCII, which uses only 7. This means the same text can take up more memory and produce a larger file.

Advantage: far more characters, so text in almost any language can be shared. Disadvantage: more bits per character, so text can take more storage. Always attach the 'so…'.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

A character set (character encoding) gives each character a unique number, called its character code. The computer stores that code in binary.
ASCII uses 7 bits per character, so it can represent only 128 characters. That covers English letters, digits, punctuation and control codes, but not other alphabets.
Unicode uses more bits per character than ASCII (up to 32), so it can represent far more characters: almost all the world's writing systems, plus many symbols and emoji.
Unicode's first 128 codes are the same as ASCII's, so A is 65 in both.
Advantage of Unicode: text in many languages can be stored and shared consistently between computers worldwide. Disadvantage: more bits per character can mean more memory and larger files.

The big picture

A character set gives each character a unique number, its character code, and the computer stores that code in binary. ASCII uses 7 bits per character, so it has only 128 codes: enough for English letters, digits, punctuation and control codes, but not other alphabets. Unicode uses more bits (up to 32), so it covers almost all the world's writing systems, plus many symbols and emoji. Its first 128 codes match ASCII's, so A is 65 in both. The cost: more bits per character can mean more memory and larger files.

Key points

1Letter codes run in order, so one code gives you the rest: A is 65, so B is 66 and Z is 90.

2Every small letter's code is 32 more than its capital's: A is 65 and a is 97.

37 bits give 2⁷ = 128 codes. The highest is 127 (1111111), because the count starts at 0.

4Each extra bit doubles the number of possible codes.

5Unicode didn't replace ASCII's codes. It kept them and added room for many more.

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Worked example

Problem

A program stores the letter G. You know that A has the code 65. Find G's code, write it as a 7-bit binary number, then find the code for small g.

⚠ Watch out

Thinking Unicode gave English letters new codes. It didn't: Unicode's first 128 codes are exactly ASCII's, so A is 65 in both. Unicode adds codes for the characters ASCII never had.

🧠

Memory hook

Seven bits, one-two-eight: ASCII's full. Unicode keeps those 128 and builds the rest of the world on top.

✓

Check yourself

Cover the page. Why do 7 bits give ASCII exactly 128 characters? If a is 97, what is the code for e? And what is one advantage and one disadvantage of Unicode?

Flashcards

(13)

What is a character code?

The unique number a character set gives to one character. The computer stores it in binary.

What does a character set (character encoding) do?

It gives every character its own unique code number, so text can be stored as numbers.

How many bits per character does ASCII use?

7 bits.

How many different characters can ASCII represent, and why?

128, because 7 bits make 2⁷ = 128 different patterns, the codes 0 to 127.

What can ASCII represent, and what can't it?

English letters, digits, punctuation and control codes. It has no codes for other alphabets.

What is Unicode designed to represent?

The characters of almost all the world's writing systems, plus many symbols and emoji.

How many bits per character can Unicode use?

More than ASCII: up to 32 bits per character.

What is the code for A in Unicode?

65, the same as in ASCII. Unicode's first 128 codes match ASCII's exactly.

B is 66. How do you get C's code without a table?

Letter codes run in order, so add 1: C is 67.

How far apart are a capital letter's code and its small letter's code?

32. For example, A is 65 and a is 97.

What happens to the number of possible codes when you add one more bit?

It doubles. 7 bits give 128 codes; 8 bits would give 256.

What is an advantage of Unicode over ASCII?

Text in many languages, plus symbols such as emoji, can be stored and shared consistently between computers worldwide.

What is a disadvantage of Unicode compared with ASCII?

It uses more bits per character, so the same text can take up more memory and make larger files.

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Flip a bit, change the letter

Work it out, don't look it up

What's actually stored?

Unicode or ASCII?

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