Why is "Stating that ASCII can represent 256 characters rather than 128." a common mistake in Character encoding?

Why it happens: Characters are stored in 8-bit bytes and 2⁸ = 256 is a familiar figure, so it gets attached to ASCII — and extended 8-bit ASCII variants genuinely do have 256 characters. How to avoid it: The specification names **7-bit** ASCII, so the answer is 2⁷ = **128** characters with codes **0 to 127**. Note the distinction: a character is *stored* in a byte, leaving one bit unused, but the code itself is only 7 bits wide. Source: AQA 8525 specification 3.3.5 names 7-bit ASCII specifically as the encoding method to describe.

Why is "Adding the full alphabet position instead of the position minus one — calculating `'C'` as 65 + 3 = 68 rather than 67." a common mistake in Character encoding?

Why it happens: `'C'` is naturally described as the 3rd letter, so adding 3 feels correct — the off-by-one comes from forgetting that the anchor itself occupies position 1 with an offset of zero. How to avoid it: The offset is **position − 1**, because `'A'` at position 1 *is* the anchor. Count the **steps** from the anchor rather than the position: from `'A'` to `'C'` is two steps, so 65 + 2 = **67**. Checking on your fingers takes seconds. Source: Sequence-rule questions require the calculation to be shown, so the offset must be applied correctly.

Why is "Saying that Unicode replaced ASCII or uses entirely different codes." a common mistake in Character encoding?

Why it happens: Unicode is presented as the modern successor to ASCII, which suggests a clean break — and the specific compatibility fact is easy to overlook among the range advantages. How to avoid it: Unicode **extends** ASCII: it uses the **same codes up to 127**, so `'A'` is 65 in both. That backwards compatibility is separately examinable and is exactly why existing ASCII files remained readable when Unicode was adopted. Source: AQA 8525 specification 3.3.5: 'Know that Unicode uses the same codes as ASCII up to 127.'

Why is "Treating the character `'7'` as the number 7 when using an encoding table." a common mistake in Character encoding?

Why it happens: They look identical on the page, and in everyday use a typed digit is simply a number — the distinction between the symbol and the value is never visible. How to avoid it: The **character** `'7'` has the code **55** (since `'0'` = 48), whereas the **number** 7 is stored as the binary value `00000111`. They have completely different bit patterns, which is why input from the keyboard must be converted with a string-to-integer operation before any arithmetic. Source: This links 3.3.5 to 3.2.1 data types and to the string conversion operations in 3.2.8.

Why is "Answering 'advantages of Unicode' with only 'it has more characters', without saying what those characters are." a common mistake in Character encoding?

Why it happens: The headline fact feels sufficient, and naming specific alphabets seems like unnecessary detail rather than the substance of the answer. How to avoid it: The specification asks for the need to represent **different alphabets and special symbols**, so name them: Greek, Cyrillic, Arabic, accented characters such as é, non-Latin scripts such as Chinese, and symbols including € and emoji. Concrete examples are what develop the point into full marks. Source: AQA 8525 specification 3.3.5 additional information requires students to explain the need for different alphabets and special symbols.

Why is "Trying to memorise a full ASCII table, or revising UTF-8 and UTF-16." a common mistake in Character encoding?

Why it happens: The table looks like the core knowledge of the topic, and wider reading about Unicode quickly reaches encoding schemes that appear important. How to avoid it: The specification states a table will be **given** when needed, so only the **anchors** — `'A'`=65, `'a'`=97, `'0'`=48, space=32 — and the **sequence rule** are required. It also states explicitly that it is **not necessary to be familiar with UTF-8, UTF-16 or other versions** of Unicode, so revising them earns nothing. Source: AQA 8525 specification 3.3.5 additional information states that students use a given table and that UTF-8/UTF-16 are not required.

AQA GCSE Computer Science (8525)

Character encoding

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Detailed Study Notes

Detailed notes on Fundamentals of Data Representation for AQA GCSE Computer Science, covering key concepts, explanations, examples, and exam-focused revision points.

Character Encoding — AQA GCSE Computer Science 8525 (3.3.5)

Two encoding methods — 7-bit ASCII and Unicode — and one enormously useful property: character codes are grouped and run in sequence, so once you know that 'A' is 65 you can calculate every other capital letter. You must describe both methods, use a given encoding table in both directions, explain the purpose of Unicode and its advantages over ASCII, and know that Unicode uses the same codes as ASCII up to 127. UTF-8 and UTF-16 are explicitly not required.

At a glance

A character set is the collection of characters a computer can represent, each with a unique character code.
ASCII is 7-bit, giving 2⁷ = 128 different characters (codes 0–127).
Character codes are grouped and run in sequence — so 'A'=65 means 'B'=66, 'C'=67 and so on.
The three groups worth knowing: capitals from 65, lower case from 97, digits from 48.
Lower case = upper case + 32 for every letter.
Unicode was created to represent a far greater range of characters — different alphabets and special symbols.
Unicode uses the same codes as ASCII up to 127, so it is backwards compatible.
Unicode uses more bits per character, so files are larger.
UTF-8, UTF-16 and other versions of Unicode are NOT required.
The character '7' (code 55) is not the number 7 — a digit character must be converted before arithmetic.

What you’ll learn

Mapped to the AQA GCSE 8525 syllabus (2027-2029).

Understand what a character set is and be able to describe the character encoding methods 7-bit ASCII and Unicode.
Be able to use a given character encoding table to convert characters to character codes and character codes to characters.
Understand that character codes are commonly grouped and run in sequence within encoding tables, so that codes for other characters in a group can be calculated once one is known.
Describe the purpose of Unicode and the advantages of Unicode over ASCII, explaining the need for representation of different alphabets and special symbols allowing a far greater range of characters.
Know that Unicode uses the same codes as ASCII up to 127.

Character sets and character codes

A character set lists every character the computer can represent, each with a unique binary code.

Computers store everything in binary (3.3.1), so text must be represented as numbers.

A character set is the collection of characters that a computer system can represent, where each character is given a unique numeric code.

The character code is that number, and it is what is actually stored in binary.

How text is stored

The word Cat is stored as three character codes:

Character	ASCII code	Binary (8-bit)
`C`	67	`01000011`
`a`	97	`01100001`
`t`	116	`01110100`

So Cat occupies three bytes in memory. This is why a plain-text document of 2,000 characters is roughly 2,000 bytes, or 2 kB (3.3.3).

Why an agreed standard matters

If one computer stored 'A' as 65 and another as 200, text written on one machine would be unreadable on the other. A standard character set means every system agrees on which code means which character — which is exactly why ASCII and Unicode exist.

A character set includes more than letters

Group	Examples
Capital letters	A–Z
Lower-case letters	a–z
Digits	0–9 (as characters, not numbers)
Punctuation and symbols	`.` `,` `?` `@` `#`
Space	code 32
Control characters	carriage return, tab, delete

Control characters do not print anything — they instruct the system to do something, such as move to a new line. They occupy the low codes 0–31 in ASCII.

AQA tip. The specification says you will be given "a given character encoding table" when you need one, so there is no need to memorise a full table. What you must know is the sequence rule and a few anchor values — 'A'=65, 'a'=97, '0'=48.

A character set is the collection of characters a system can represent.
Each character has a unique character code, stored in binary.
One character occupies one byte, so 2,000 characters is about 2 kB.
A standard is needed so different systems interpret codes identically.
Character sets include letters, digits, punctuation, space and control characters.

See the full worked example for character encoding →

7-bit ASCII

128 characters, codes 0–127. Enough for English, not enough for the world.

ASCII stands for the American Standard Code for Information Interchange.

The specification requires 7-bit ASCII:

$2^7 = 128 \text{ characters, codes } 0 \text{ to } 127$

What ASCII contains

Codes	Contents
0–31	Control characters (non-printing)
32	Space
33–47	Punctuation and symbols
48–57	Digits `0`–`9`
58–64	More symbols
65–90	Capitals `A`–`Z`
91–96	More symbols
97–122	Lower case `a`–`z`
123–127	Symbols and delete

The three anchors to memorise

Character	Code
`'A'`	65
`'a'`	97
`'0'`	48

Everything else in those groups follows from the sequence rule.

Storage: 7 bits or 8?

ASCII is defined as a 7-bit code, but computers store data in bytes (8 bits), so each character normally occupies a full byte with one bit unused — a waste of 1/8, or 12.5%. Historically the spare bit was used for error checking or to extend the character set.

In the exam, treat one ASCII character as occupying one byte unless a question specifically asks about the 7-bit definition.

ASCII's limitation

128 characters is enough for English, but the world needs far more:

Other alphabets — Greek, Cyrillic, Arabic, Hebrew
Accented characters — é, ü, ñ, å
Non-alphabetic scripts — Chinese, Japanese, Korean (tens of thousands of characters)
Symbols and emoji — €, ©, ♫

ASCII simply cannot represent them. There are not enough codes. That limitation is exactly why Unicode was created.

AQA tip. State ASCII's size precisely: 7-bit, 128 characters, codes 0–127. Writing "256 characters" describes extended 8-bit ASCII, which is not what the specification names.

ASCII is 7-bit, giving 2⁷ = 128 characters with codes 0–127.
Anchors: 'A'=65, 'a'=97, '0'=48, space=32.
Codes 0–31 are non-printing control characters.
Each character normally occupies one byte, leaving one bit unused.
128 characters cannot cover other alphabets, accents or non-Latin scripts.

See the full worked example for character encoding →

The sequence rule — the most useful fact here

Codes run in order within a group, so one known value gives you all the others.

The specification highlights this explicitly:

"Students should know that character codes are grouped and that they run in sequence. For example in ASCII 'A' is coded as 65, 'B' as 66, and so on, meaning that the codes for the other capital letters can be calculated once the code for 'A' is known. This pattern also applies to other groupings such as lower case letters and digits."

Calculating within a group

Because the codes are consecutive, you only need one anchor per group:

'A' = 65
'B' = 66      (65 + 1)
'E' = 69      (65 + 4)
'Z' = 90      (65 + 25)

The formula for any capital letter:

$\text{code} = 65 + (\text{position in alphabet} - 1)$

'E' is the 5th letter, so 65 + 4 = 69.

The same works for the other groups:

'a' = 97   →  'e' = 97 + 4 = 101
'0' = 48   →  '7' = 48 + 7 = 55

The +32 relationship

One anchor per group is all you need — every other code in that group can be calculated.

$\text{lower case code} = \text{upper case code} + 32$

'a' (97) − 'A' (65) = 32, and the gap is identical for every letter. This is why:

Case conversion is a simple ± 32 (3.2.8).
String comparison is case-sensitive (3.2.4) — 'A' and 'a' are genuinely different values.

Why sequencing matters practically

Because codes are consecutive, a program can sort text alphabetically simply by comparing the numeric codes, and can test whether a character is a letter by checking whether its code lies between 65 and 90.

AQA tip. When a question supplies a partial table and asks for a code that is not listed, use the sequence rule and show the calculation: "'A' is 65, and 'E' is the 5th letter, so 65 + 4 = 69." The working demonstrates the method the specification is testing.

Character codes are grouped and run in sequence.
One anchor per group gives every other code: 'A'=65, 'a'=97, '0'=48.
Formula: code = anchor + (position in group − 1).
Lower case = upper case + 32 for every letter.
Sequencing allows alphabetical sorting by comparing codes.

See the full worked example for character encoding →

Unicode and its advantages over ASCII

Created to represent a far greater range of characters. Backwards compatible with ASCII.

The purpose of Unicode

Unicode was created to represent a far greater range of characters than ASCII can, including different alphabets and special symbols.

That sentence is close to the specification's own wording and is what a "describe the purpose of Unicode" question wants.

ASCII's 128 codes cover English, but not:

Other alphabets — Greek (α β γ), Cyrillic (д ж я), Arabic, Hebrew
Accented characters — é, ü, ñ, ø, å
Non-alphabetic scripts — Chinese, Japanese and Korean, running to tens of thousands of characters
Symbols — €, ©, ™, ♫, mathematical notation, emoji

Unicode uses more bits per character, so it can represent well over a million distinct characters — enough for every writing system in use.

Backwards compatibility — an examinable fact

Unicode uses the same codes as ASCII up to 127.

So 'A' is 65 in both, 'a' is 97 in both, and '0' is 48 in both.

Why this matters: any file written in ASCII can be read correctly by a Unicode system without conversion. Unicode extends ASCII rather than replacing it, which is why the transition was possible at all — decades of existing ASCII documents remained readable.

The comparison table

	ASCII (7-bit)	Unicode
Bits per character	7	More (16 or more)
Number of characters	128	Over a million
Alphabets covered	English only	All major writing systems
Accents, symbols, emoji	✗	✓
File size	Smaller	Larger
Compatible with the other?	—	✓ Same codes up to 127

The trade-off

Unicode's advantage is range; its disadvantage is size. Using more bits per character makes every text file larger, which increases storage requirements and transmission times. For a document containing only English text, ASCII is more efficient — but it cannot handle anything beyond that.

What is NOT required

"It is not necessary to be familiar with UTF-8, UTF-16 or other different versions of Unicode."

So do not revise encoding schemes, variable-length encoding or byte-order marks. Unicode as a concept is all that is examined.

AQA tip. An "advantages of Unicode over ASCII" question wants the range point developed — different alphabets, special symbols, a far greater number of characters — plus the compatibility point that Unicode matches ASCII up to 127. Mentioning the larger file size as the trade-off shows balance and is credited in longer answers.

Unicode's purpose is to represent a far greater range of characters than ASCII.
It covers different alphabets, accented characters, non-Latin scripts and symbols.
Unicode uses the same codes as ASCII up to 127 — it is backwards compatible.
It uses more bits per character, so files are larger.
UTF-8, UTF-16 and other versions are explicitly NOT required.

See the full worked example for character encoding →

Using a given encoding table

Convert both ways, and use the sequence rule for anything not listed.

The specification requires you to use a given character encoding table to convert in both directions.

Character → code

Look the character up and read off its code. If it is not listed, use the sequence rule.

Given 'A' = 65, find 'K'. 'K' is the 11th letter, so 65 + 10 = 75.

Code → character

Find the code and read off the character, or count from the nearest anchor.

Given 'a' = 97, what is code 104? 104 − 97 = 7, so it is 7 letters after 'a': a, b, c, d, e, f, g, h. Code 104 is 'h'.

Encoding a whole word

Encode Hi! given 'A'=65, 'a'=97, '!'=33.

Character	Working	Code
`H`	65 + 7 (8th letter)	72
`i`	97 + 8 (9th letter)	105
`!`	given	33

Hi! = 72, 105, 33 — three characters, three bytes.

Converting to binary

Character codes are stored in binary (3.3.2):

Character	Code	Binary (8-bit)
`H`	72	`01001000`
`i`	105	`01101001`
`!`	33	`00100001`

The counting trap

Count carefully from the anchor. 'C' is the 3rd letter, so its code is 65 + 2 = 67, not 65 + 3. The offset is (position − 1), because 'A' at position 1 is the anchor itself with offset 0.

A quick check: count the letters between the anchor and your target on your fingers. 'A' to 'C' is two steps.

AQA tip. Always show the calculation rather than just the answer. "'A' = 65, 'K' is 10 letters later, so 65 + 10 = 75" demonstrates the sequence rule, which is the skill being assessed — and it earns method marks if the arithmetic slips.

Use the given table directly; use the sequence rule for anything not listed.
Character → code: anchor + (position in group − 1).
Code → character: subtract the anchor and count on from it.
Offset is position − 1, so 'C' is 65 + 2 = 67.
Show the calculation — the sequence rule is what is being assessed.

See the full worked example for character encoding →

Quick recap

A character set is the collection of characters a system can represent, each with a unique character code.
ASCII is 7-bit, giving 128 characters with codes 0–127.
Character codes are grouped and run in sequence, so one anchor gives every code in the group.
Anchors: 'A' = 65, 'a' = 97, '0' = 48, space = 32.
Lower case = upper case + 32 for every letter.
Unicode's purpose is to represent a far greater range of characters — other alphabets and special symbols.
Unicode uses the same codes as ASCII up to 127, so it is backwards compatible.
Unicode uses more bits per character, so files are larger.
UTF-8, UTF-16 and other versions of Unicode are not required.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

A character set is the collection of characters a computer can represent, each with a unique code.
ASCII is 7-bit: 2⁷ = 128 characters, codes 0–127.
'A' = 65 · 'a' = 97 · '0' = 48 · space = 32.
Codes are GROUPED and run in SEQUENCE — one anchor gives the whole group.
Lower case code = upper case code + 32.
Unicode's purpose: represent a FAR GREATER RANGE of characters — different alphabets and special symbols.
Unicode uses the SAME CODES AS ASCII UP TO 127.
Unicode uses more bits per character, so files are larger.
UTF-8 and UTF-16 are NOT required.

How it’s examined

3.3.5 is assessed on Paper 2 and appears reliably every series, usually across two or three question parts. Four shapes recur. (1) Recall, 1–2 marks: state what a character set is, or how many characters 7-bit ASCII can represent (AO1) — the answer is 128, from 2⁷. (2) Use a given table, 2–4 marks: convert characters to codes and codes to characters, where the table supplied is deliberately incomplete so you must apply the sequence rule and show the calculation (AO2). (3) Explain Unicode, 2–4 marks: describe its purpose and its advantages over ASCII — the range point developed with examples of other alphabets and special symbols, plus the compatibility fact that Unicode matches ASCII up to 127 (AO1). (4) Synoptic use, 2–6 marks: character codes appear inside 3.2.8 string handling questions and in file-size calculations, since one character is one byte. Two boundaries are worth banking: the specification says a table will be given, so no memorisation of a full table is needed — only the anchors and the sequence rule; and UTF-8, UTF-16 and other Unicode versions are explicitly excluded, so revising them is wasted effort.

Sources: AQA GCSE Computer Science 8525 specification (v1.3, 16 June 2025) — 3.3.5 Character encoding; AQA GCSE Computer Science Paper 2 past papers, mark schemes and examiner reports. Last reviewed 2026-07-29.

Take this whole topic with you

Step-by-step worked examples — Character encoding

Step-by-step solutions to past-paper-style questions on character encoding, written exactly the way a tutor would explain them at the board.

Question type:

1Define a character set and state ASCII's capacity (3 marks)
Getting startedIdentify & classify• Paper 2 style — 3 marks (AO1)• character-set, ascii, define
Question
(a) State what is meant by a character set. (b) State how many different characters 7-bit ASCII can represent, showing your reasoning. (c) State the range of codes used by 7-bit ASCII. (3 marks)
Step-by-step solution
1. Step 1
  (a) A character set is the collection of characters a computer system can represent, where each character is given a unique numeric code.
2. Step 2
  (b) With 7 bits, each bit can independently be 0 or 1, giving 2⁷ = 128 different combinations and therefore 128 characters.
  $2^7 = 128$
3. Step 3
  (c) The 128 codes run from 0 to 127, because the count includes zero.
Answer
(a) A character set is the collection of characters that a computer system can represent, each of which is assigned a unique numeric code (1). (b) 2⁷ = 128 characters (1). (c) The codes run from 0 to 127 (1). (3)
Examiner tip
Be precise that ASCII is 7-bit with 128 characters — answering 256 describes extended 8-bit ASCII, which the specification does not name. Part (c) applies the same counting point as 3.3.1: 128 values means a range of 0 to 127, because zero occupies one of the codes.
2Use a given encoding table (4 marks)
Getting startedDirect calculation• Paper 2 style — 4 marks (AO2)• ascii, table, conversion
Question
An encoding table shows that 'A' = 65, 'a' = 97 and '0' = 48.

(a) State the code for 'D'. (b) State the code for 'd'. (c) State the character with code 51. (d) State the code for the character '9'. (4 marks)
Step-by-step solution
1. Step 1
  (a) 'D' is the 4th capital letter, so the offset from the anchor is 3: 65 + 3 = 68.
2. Step 2
  (b) 'd' is the 4th lower-case letter: 97 + 3 = 100. (Check with the +32 rule: 68 + 32 = 100 ✓)
3. Step 3
  (c) Code 51 is in the digit group. 51 − 48 = 3, so it is the character '3'.
4. Step 4
  (d) '9' is 9 places after '0': 48 + 9 = 57. Note this is the character '9', not the number 9.
Answer
(a) 'D' = 65 + 3 = 68 (1) (b) 'd' = 97 + 3 = 100 (1) — which also checks against the +32 rule, since 68 + 32 = 100 (c) 51 − 48 = 3, so the character is '3' (1) (d) '9' = 48 + 9 = 57 (1) (4)
Examiner tip
The offset is position − 1, so 'D' as the 4th letter is 65 + 3, not 65 + 4. Part (d) makes the crucial distinction between the character '9' (code 57) and the number 9, which is exactly why keyboard input must be converted before arithmetic (3.2.8).
3Apply the sequence rule to a missing code (3 marks)
Getting startedDirect calculation• Paper 2 style — 3 marks (AO2)• sequence, ascii, calculate
Question
A partial encoding table shows only that 'A' = 65.

(a) Explain how the code for 'M' can be worked out without the full table. (b) State the code for 'M'. (c) State the code for 'Z'. (3 marks)
Step-by-step solution
1. Step 1
  (a) Character codes are grouped and run in sequence, so the capital letters occupy consecutive codes starting from 'A' = 65.
2. Step 2
  This means any capital letter's code can be found by adding its offset from 'A' — that is, its position in the alphabet minus 1.
3. Step 3
  (b) 'M' is the 13th letter, so 65 + 12 = 77.
  $65 + 12 = 77$
4. Step 4
  (c) 'Z' is the 26th letter, so 65 + 25 = 90.
Answer
(a) Character codes are grouped and run in sequence, so the capital letters occupy consecutive codes beginning at 'A' = 65 (1). The code for any capital letter can therefore be calculated by adding its position in the alphabet minus 1 to the anchor value.

(b) 'M' is the 13th letter: 65 + 12 = 77 (1) (c) 'Z' is the 26th letter: 65 + 25 = 90 (1) (3)
Examiner tip
This sequencing property is stated explicitly in the specification and is what makes a full table unnecessary. Note that 'Z' = 90 confirms the capitals occupy codes 65–90, which is also the range used for the case-conversion guard in 3.2.8.
4Encode a word and convert to binary (5 marks)
Building confidenceMulti-step problem• Paper 2 style — 5 marks (AO2)• encode, binary, ascii
Question
Using 'A' = 65 and 'a' = 97, encode the word Cab.

(a) Give the three character codes. (b) Give each code as an 8-bit binary number. (c) State how many bytes the word occupies. (5 marks)
Step-by-step solution
1. Step 1
  (a) C is the 3rd capital: 65 + 2 = 67. a is the anchor itself: 97. b is the 2nd lower-case letter: 97 + 1 = 98.
2. Step 2
  (b) 67 in binary: 64 + 2 + 1 = 01000011.
3. Step 3
  97 in binary: 64 + 32 + 1 = 01100001.
4. Step 4
  98 in binary: 64 + 32 + 2 = 01100010.
5. Step 5
  (c) Each character occupies one byte, so three characters occupy 3 bytes.
Answer
(a)

C = 65 + 2 = 67

a = 97

b = 97 + 1 = 98

(2 marks for all three codes correct.)

(b)

Character Code Binary
C 67 01000011
a 97 01100001
b 98 01100010

(2 marks)

(c) Each character occupies one byte, so Cab occupies 3 bytes (1).
Examiner tip
This links 3.3.5 to 3.3.2 conversion and 3.3.3 units in one question. Note that 'C' and 'a' are genuinely different values (67 and 97) even though they are the same letter position in their respective groups — which is why string comparison is case-sensitive.
5Use the +32 relationship between cases (4 marks)
Building confidenceMulti-step problem• Paper 2 style — 4 marks (AO2/AO3)• case, ascii, 32
Question
Given 'A' = 65 and 'a' = 97:

(a) State the relationship between the code of a capital letter and its lower-case equivalent. (b) 'P' has the code 80. State the code for 'p'. (c) Explain why this relationship means string comparison is case-sensitive. (4 marks)
Step-by-step solution
1. Step 1
  (a) 97 − 65 = 32, and the same gap holds for every letter, so the lower-case code is always the upper-case code + 32.
2. Step 2
  (b) 80 + 32 = 112.
  $80 + 32 = 112$
3. Step 3
  (c) Because the codes differ, 'P' (80) and 'p' (112) are stored as different values in memory.
4. Step 4
  When two strings are compared, the comparison works on those codes — so 'PASSWORD' and 'password' are treated as entirely different strings.
Answer
(a) The lower-case code is always the upper-case code + 32 — for example 97 − 65 = 32, and the gap is identical for every letter (1).

(b) 'p' = 80 + 32 = 112 (1)

(c) Because upper and lower case letters have different character codes, they are stored as different values in memory (1). String comparison works by comparing those codes, so 'P' (80) and 'p' (112) do not match — which is why 'YES' and 'yes' are treated as completely different strings and a case-sensitive login check would reject a correctly spelled but differently capitalised entry (1).
Examiner tip
The +32 gap is why case conversion in 3.2.8 is a simple addition or subtraction, and why case-sensitivity in 3.2.4 exists at all. A neat extra observation: 32 is 2⁵, so the two cases differ by exactly one bit — 'A' is 1000001 and 'a' is 1100001.
6Explain the purpose and advantages of Unicode (5 marks)
Building confidenceWord problem• Paper 2 style — 5 marks (AO1/AO2)• unicode, ascii, advantages
Question
(a) Describe the purpose of Unicode. (b) Explain two advantages of Unicode over ASCII. (c) State one disadvantage of Unicode compared with ASCII. (5 marks)
Step-by-step solution
1. Step 1
  (a) Unicode exists to represent a far greater range of characters than ASCII can — the specification's own framing.
2. Step 2
  (b) Advantage 1 — range. It can represent different alphabets such as Greek, Cyrillic and Arabic, accented characters, non-Latin scripts such as Chinese, and special symbols including € and emoji.
3. Step 3
  Develop it: ASCII's 128 codes are simply not enough, since Chinese alone uses tens of thousands of characters.
4. Step 4
  (b) Advantage 2 — compatibility. Unicode uses the same codes as ASCII up to 127, so existing ASCII files can still be read correctly without conversion.
5. Step 5
  (c) Disadvantage — size. Unicode uses more bits per character, so text files are larger, requiring more storage and longer transmission times.
Answer
(a) The purpose of Unicode is to allow a far greater range of characters to be represented than is possible with ASCII, covering the writing systems used across the world (1).

(b) Advantage 1 — it can represent many more characters. Unicode includes different alphabets such as Greek, Cyrillic, Arabic and Hebrew, accented characters such as é and ñ, non-Latin scripts such as Chinese and Japanese, and special symbols including € and emoji (1). ASCII's 128 codes cannot cover these — Chinese alone uses tens of thousands of characters (1).

Advantage 2 — it is backwards compatible. Unicode uses the same codes as ASCII up to 127, so a file created using ASCII can be read correctly by a Unicode system without any conversion (1).

(c) The disadvantage is that Unicode uses more bits per character, so text files are larger, requiring more storage space and taking longer to transmit (1).
Examiner tip
The specification asks students to explain the need for representing 'different alphabets and special symbols allowing a far greater range of characters', so naming concrete examples is what develops the range point beyond a bare assertion. The backwards-compatibility fact — same codes up to 127 — is separately examinable and often omitted.
7Decode a message from binary (6 marks)
StretchMulti-step problem• Paper 2 style — 6 marks (AO2)• decode, binary, ascii
Question
A message is stored as the following three bytes:

01001000 01101001 00100001

Given 'A' = 65, 'a' = 97 and '!' = 33, decode the message and state how many bytes it occupies. (6 marks)
Step-by-step solution
1. Step 1
  Convert each byte to decimal using the place values 128, 64, 32, 16, 8, 4, 2, 1.
2. Step 2
  Byte 1: 01001000 = 64 + 8 = 72. Since 'A' = 65, this is 65 + 7, the 8th capital letter — 'H'.
  $64 + 8 = 72 = 65 + 7$
3. Step 3
  Byte 2: 01101001 = 64 + 32 + 8 + 1 = 105. Since 'a' = 97, this is 97 + 8, the 9th lower-case letter — 'i'.
  $64 + 32 + 8 + 1 = 105 = 97 + 8$
4. Step 4
  Byte 3: 00100001 = 32 + 1 = 33, which the table gives directly as '!'.
5. Step 5
  Assemble the message: H, i, ! spells Hi!.
6. Step 6
  Count the bytes. Three characters, one byte each, so 3 bytes.
Answer
Byte 1: 01001000 = 64 + 8 = 72. With 'A' = 65, 72 is 7 letters later — the 8th capital, 'H' (2 marks)

Byte 2: 01101001 = 64 + 32 + 8 + 1 = 105. With 'a' = 97, 105 is 8 letters later — the 9th lower-case letter, 'i' (2 marks)

Byte 3: 00100001 = 32 + 1 = 33, which the table gives as '!' (1)

The message is Hi!, occupying 3 bytes (1). (6)
Examiner tip
This chains three skills — binary to decimal (3.3.2), the sequence rule (3.3.5) and units (3.3.3) — which is exactly how character encoding is examined at the top of the tariff. The check that the second byte is lower case comes from its code exceeding 96, placing it in the 97–122 group.
8Compare ASCII and Unicode for a scenario (6 marks)
StretchWord problem• Paper 2 style — 6 marks (AO3)• compare, unicode, ascii, justify
Question
A company is choosing a character encoding for a new messaging app. The app will be used internationally, and messages will be stored on a server with limited capacity.

Compare ASCII and Unicode for this purpose and recommend which should be used, justifying your choice. (6 marks)
Step-by-step solution
1. Step 1
  ASCII's advantage — size. It uses only 7 bits per character, so messages take less storage and transmit faster.
2. Step 2
  ASCII's fatal disadvantage here. With only 128 characters it cannot represent other alphabets, accented characters or emoji — and the app is international.
3. Step 3
  Unicode's advantage. It represents a far greater range, covering every major writing system plus symbols and emoji.
4. Step 4
  Unicode's disadvantage. More bits per character means larger files, which matters given the limited server capacity.
5. Step 5
  Weigh them against the scenario. Storage is a real constraint, but an international app that cannot display a user's own alphabet is not fit for purpose at all.
6. Step 6
  Recommend Unicode, noting that the storage cost can be mitigated by compression (3.3.8), whereas ASCII's limitation cannot be worked around.
Answer
ASCII. Its advantage is efficiency: using only 7 bits per character it produces the smallest possible files, which would reduce storage on the server and speed up transmission (1). However, ASCII can represent only 128 characters, covering English but not other alphabets such as Greek, Cyrillic or Arabic, accented characters, non-Latin scripts, or emoji (1).

Unicode. Its advantage is that it can represent a far greater range of characters, covering every major writing system as well as symbols and emoji (1). Its disadvantage is that it uses more bits per character, so messages occupy more storage — a genuine concern given the server's limited capacity (1).

Recommendation: Unicode. Because the app is used internationally, users must be able to write in their own alphabets — an app that cannot display a user's language is not fit for purpose, so ASCII's limitation is fatal rather than merely inconvenient (1). The storage cost is a real but manageable problem: it can be reduced through compression, or by adding server capacity, whereas ASCII's inability to represent the required characters could not be worked around at all (1). (6)
Examiner tip
A 'compare and recommend' question needs an advantage and a disadvantage for each option before the judgement. The decisive reasoning is the asymmetry of the two drawbacks: the storage cost has workarounds, whereas the missing characters do not — that distinction is what secures the top band.
9Explain the significance of sequenced codes (6 marks)
StretchWord problem• Paper 2 style — 6 marks (AO3)• sequence, explain, sorting, application
Question
Character codes in ASCII are grouped and run in sequence.

(a) Explain what this means, with an example. (b) Explain two practical benefits this gives to programmers. (c) Explain why this property makes 'Z' compare as less than 'a'. (6 marks)
Step-by-step solution
1. Step 1
  (a) Related characters are assigned consecutive codes within a group — capitals run 65–90, lower case 97–122, digits 48–57.
2. Step 2
  Give an example: 'A' = 65, 'B' = 66, 'C' = 67, so any capital's code can be calculated from the anchor.
3. Step 3
  (b) Benefit 1 — sorting. Text can be sorted alphabetically simply by comparing the numeric codes, with no lookup table needed.
4. Step 4
  Benefit 2 — validation. A program can test whether a character is a capital letter by checking whether its code lies between 65 and 90.
5. Step 5
  (c) 'Z' is 90 and 'a' is 97, and comparison works on the codes — so 90 < 97 makes 'Z' compare as less than 'a'.
6. Step 6
  Note the consequence: in a code-order sort, all capitals come before all lower-case letters, which is not true alphabetical order.
Answer
(a) Grouping and sequencing means related characters are assigned consecutive codes within a group — the capitals occupy 65–90, lower case 97–122 and the digits 48–57 (1). For example 'A' = 65, 'B' = 66 and 'C' = 67, so once the code for 'A' is known, every other capital can be calculated rather than looked up (1).

(b) Benefit 1 — sorting. Text can be sorted alphabetically simply by comparing the numeric codes, since a letter earlier in the alphabet always has a lower code — no lookup table or special logic is required (1).

Benefit 2 — validating character types. A program can test whether a character is a capital letter by checking whether its code lies between 65 and 90, or whether it is a digit by checking 48 to 57 — a simple range check rather than 26 separate comparisons (1).

(c) 'Z' has the code 90 and 'a' has the code 97, and comparison operates on these codes rather than on the letters themselves (1). Since 90 is less than 97, 'Z' compares as less than 'a' — which means that in a straightforward code-order sort all capital letters come before all lower-case letters, so 'Zebra' would be placed before 'apple' even though that is not conventional alphabetical order (1). (6)
Examiner tip
Part (c) is the discriminator and explains a genuinely surprising real-world behaviour: sorting by raw character code is not the same as alphabetical sorting, which is why programs convert to a single case before sorting. This connects 3.3.5 directly to the case-sensitivity of string comparison in 3.2.4.

Character	Code	Binary
`C`	67	`01000011`
`a`	97	`01100001`
`b`	98	`01100010`

Model Answers — Character encoding

High-scoring sample answers for character encoding on the AQA GCSE 8525 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1
Paper 2 style — AO12 marks
Define the term character set. (2 marks)
Model answer
A character set is the collection of characters that a computer system can represent (1), where each character is assigned a unique numeric code that is stored in binary (1).
Why this scores
Both halves are needed: the collection of characters, and the unique code per character. A standard character set matters because it ensures different computers interpret the same code as the same character.
Question 2
Paper 2 style — AO1/AO22 marks
State how many characters can be represented using 7-bit ASCII, and show how this figure is obtained. (2 marks)
Model answer
Each of the 7 bits can be either 0 or 1, giving 2⁷ combinations (1) = 128 characters, with codes running from 0 to 127 (1).
Why this scores
The specification names 7-bit ASCII specifically, so the answer is 128 — writing 256 describes extended 8-bit ASCII. Note that although ASCII uses 7 bits, each character is normally stored in a full byte, leaving one bit unused.
Question 3
Paper 2 style — AO12 marks
State the relationship between Unicode and ASCII character codes, and explain why this is useful. (2 marks)
Model answer
Unicode uses the same codes as ASCII up to 127 (1). This makes Unicode backwards compatible, so any text file created using ASCII can be read correctly by a Unicode system without needing to be converted (1).
Why this scores
This is a directly examinable fact stated in the specification. The practical significance is that Unicode extends ASCII rather than replacing it, which is why decades of existing ASCII documents remained readable during the transition.
Question 4
Paper 2 style — AO24 marks
An encoding table gives 'A' = 65 and 'a' = 97.

(a) State the code for 'G'. (b) State the character with code 110. (c) Explain how you obtained your answers without a full table. (4 marks)
Model answer
(a) 'G' is the 7th capital letter, so its code is 65 + 6 = 71 (1)

(b) 110 − 97 = 13, so the character is 13 letters after 'a' — the 14th lower-case letter, 'n' (1)

(c) Character codes are grouped and run in sequence (1), so all capitals occupy consecutive codes from 65 and all lower-case letters occupy consecutive codes from 97. Once one code in a group is known, any other can be calculated by adding or subtracting the difference in alphabet position (1).
Why this scores
The offset is position − 1, so 'G' as the 7th letter is 65 + 6. Counting carefully from the anchor is where errors arise — checking on your fingers takes seconds and is worth doing.
Question 5
Paper 2 style — AO1/AO24 marks
Describe the purpose of Unicode and explain why it was needed. (4 marks)
Model answer
The purpose of Unicode is to allow a far greater range of characters to be represented than ASCII can manage (1).

It was needed because ASCII provides only 128 characters, which is enough for English but cannot represent the different alphabets used around the world — such as Greek, Cyrillic, Arabic and Hebrew (1) — nor accented characters such as é and ñ, nor non-Latin scripts such as Chinese and Japanese, which between them use tens of thousands of characters (1).

Unicode uses more bits per character, giving it enough codes to represent every major writing system as well as special symbols such as €, mathematical notation and emoji (1).
Why this scores
The specification asks for the need for 'representation of different alphabets and of special symbols allowing a far greater range of characters', so naming concrete examples of alphabets and symbols is what develops the point beyond a bare assertion.
Question 6
Paper 2 style — AO23 marks
Explain the difference between the character '5' and the number 5, referring to how each is stored. (3 marks)
Model answer
The character '5' is stored using its character code, which in ASCII is 53 (since '0' = 48, so 48 + 5 = 53) — and 53 in binary is 00110101 (1).

The number 5 is stored as the binary value of five, which is 00000101 (1).

The two therefore have completely different bit patterns, which is why a digit typed at the keyboard arrives as a character and must be converted using a string-to-integer operation before any arithmetic can be performed on it (1).
Why this scores
This connects 3.3.5 to 3.2.1 data types and 3.2.8 string conversion. The neat structural detail worth knowing is that the low four bits of any digit character give its numeric value directly, which is why subtracting the code of '0' performs the conversion.
Question 7
Paper 2 style — AO26 marks
A file contains the following four bytes:

01000011 01101111 01100100 01100101

Given 'A' = 65 and 'a' = 97, decode the word stored and state the file's size in bits and bytes. (6 marks)
Model answer
Byte 1: 01000011 = 64 + 2 + 1 = 67. With 'A' = 65, this is the 3rd capital — 'C' (1)

Byte 2: 01101111 = 64 + 32 + 8 + 4 + 2 + 1 = 111. With 'a' = 97, 111 − 97 = 14, so it is the 15th lower-case letter — 'o' (1)

Byte 3: 01100100 = 64 + 32 + 4 = 100. 100 − 97 = 3, so the 4th lower-case letter — 'd' (1)

Byte 4: 01100101 = 64 + 32 + 4 + 1 = 101. 101 − 97 = 4, so the 5th lower-case letter — 'e' (1)

The word is Code (1)

Size: 4 characters × 8 bits = 32 bits, which is 4 bytes (1).
Why this scores
A four-stage decode chaining binary conversion, the sequence rule and units. A useful check as you go: codes above 96 are lower case, codes between 65 and 90 are capitals — which confirms only the first character is a capital here.
Question 8
Paper 2 style — AO1/AO36 marks
Compare 7-bit ASCII and Unicode, covering the number of characters, file sizes, compatibility and suitability. (6 marks)
Model answer
Number of characters. ASCII is a 7-bit code providing 2⁷ = 128 characters, which covers the English alphabet, digits, punctuation and control characters (1). Unicode uses more bits per character, allowing well over a million characters — enough for every major writing system, accented characters and symbols including emoji (1).

File sizes. ASCII produces smaller files because each character needs fewer bits, so text takes less storage and transmits more quickly (1). Unicode files are correspondingly larger, which increases storage requirements and transmission times (1).

Compatibility. Unicode uses the same codes as ASCII up to 127, so it is backwards compatible — an ASCII file can be read correctly by a Unicode system without conversion (1).

Suitability. ASCII is adequate only where text is guaranteed to be English-only, such as some configuration files or simple embedded systems. For anything international — websites, messaging apps, documents shared globally — Unicode is essential, because ASCII simply cannot represent the required characters (1).
Why this scores
'Compare' requires both encodings to be addressed under each heading rather than described one after the other. The strongest closing point is that ASCII's limitation is absolute — the characters cannot be represented at all — whereas Unicode's drawback is merely a cost that compression can reduce.
Question 9
Paper 2 style — AO36 marks
A program sorts a list of names by comparing their character codes. The result places 'Zebra' before 'apple'. Explain why this happens and how the program could be improved. (6 marks)
Model answer
Why it happens. Character codes are grouped and run in sequence, with the capital letters occupying codes 65 to 90 and the lower-case letters occupying 97 to 122 (1). This means every capital letter has a lower code than every lower-case letter (1).

When the program compares 'Zebra' with 'apple', it compares the first characters: 'Z' has the code 90 while 'a' has the code 97 (1). Since 90 is less than 97, 'Zebra' is judged to come first — even though alphabetically 'apple' should precede it (1).

How to improve it. The program should convert both strings to the same case — for example converting everything to lower case by adding 32 to the code of any character in the range 65 to 90 — before comparing them (1). This removes the artificial gap between the two groups, so the comparison then reflects genuine alphabetical order regardless of how the names were capitalised (1).
Why this scores
This is a real and frequently encountered bug, and it demonstrates that sorting by raw character code is not the same as alphabetical sorting. The fix links directly to the case-conversion technique in 3.2.8, including the guard that only characters in the 65–90 range should be adjusted.

Key Formulae — Character encoding

The formulae you need to memorise for character encoding on the AQA GCSE 8525 paper, with every variable defined in plain English and a note on when to use it.

Character code from an anchor
$\text{code} = \text{anchor} + (\text{position in group} - 1)$
$anchor$
The known first code of the group — 'A'=65, 'a'=97, '0'=48
$position$
The character's position within its group, counting from 1
When to use
Whenever a supplied encoding table is incomplete. Show the calculation — applying the sequence rule is the skill being assessed.
Example
'M' is the 13th letter: 65 + 12 = 77
Upper to lower case gap
$\text{lower case} = \text{upper case} + 32$
$Capitals$
Codes 65 to 90
$Lower case$
Codes 97 to 122
When to use
Converting between cases, and explaining why string comparison is case-sensitive. The gap is exactly 32 for every letter.
Example
'P' = 80, so 'p' = 80 + 32 = 112
Number of characters in an n-bit code
$\text{characters} = 2^n$
$n = 7$
ASCII: 2⁷ = 128 characters, codes 0–127
$Range$
0 to 2ⁿ − 1, because the count includes zero
When to use
Stating how many characters an encoding can represent, and explaining why ASCII cannot cover other alphabets.
Example
7-bit ASCII: 2⁷ = 128 characters

Key Definitions and Keywords — Character encoding

Definitions to memorise and the exact keywords mark schemes credit for character encoding answers — sharpened from recent examiner reports for the 2026 AQA GCSE 8525 sitting.

Character set
Examiner keyword
The collection of characters that a computer system can represent, where each character is assigned a unique numeric code.
Related: Character code, ASCII, Unicode
Character code
Examiner keyword
The unique number assigned to a character within a character set, stored in binary. In ASCII, 'A' has the character code 65.
Related: Character set, ASCII
ASCII
Examiner keyword
The American Standard Code for Information Interchange. A 7-bit character encoding providing 2⁷ = 128 characters with codes 0 to 127.
Example
'A' = 65, 'a' = 97, '0' = 48, space = 32
Related: Unicode, Character set
Unicode
Examiner keyword
A character encoding created to represent a far greater range of characters than ASCII, covering different alphabets and special symbols. It uses the same codes as ASCII up to 127.
Related: ASCII, Backwards compatibility
Sequenced codes
Examiner keyword
The property that character codes are grouped and run in consecutive order, so once one code in a group is known the others can be calculated.
Example
'A' = 65 means 'B' = 66 and 'Z' = 90.
Related: Character code, ASCII
Backwards compatibility
Examiner keyword
The ability of a newer system to work correctly with data created for an older one. Unicode achieves this by using the same codes as ASCII for the first 128 characters.
Related: Unicode, ASCII
Control character
A non-printing character that instructs a system to perform an action, such as a carriage return or tab. These occupy ASCII codes 0 to 31.
Related: ASCII, Character set
Case gap (+32)
Examiner keyword
The constant difference of 32 between the code of an upper-case letter and its lower-case equivalent. Since 32 is 2⁵, the two differ by exactly one bit.
Example
'A' = 65 and 'a' = 97; 'P' = 80 and 'p' = 112.
Related: Sequenced codes, Case sensitivity
Case sensitivity
Examiner keyword
The fact that upper and lower case letters have different character codes and are therefore treated as different values when compared.
Related: Case gap (+32), Character code
Digit character
Examiner keyword
A character representing a digit, such as '5', stored using its character code (53) rather than as the numeric value. It must be converted before arithmetic can be performed.
Example
'0' = 48, so '5' = 48 + 5 = 53
Related: Character code, String conversion
Encoding table
Examiner keyword
A table listing characters alongside their codes. AQA states that a table will be given when needed, so only the anchor values and the sequence rule need to be known.
Related: Character code, Sequenced codes
Bits per character
Examiner keyword
The number of bits used to store one character's code. Standard ASCII uses 7 bits, extended ASCII 8, and Unicode more — which is why Unicode text files are larger.
Related: ASCII, Unicode

Common Mistakes and Misconceptions — Character encoding

The traps other students keep falling into on character encoding questions — taken from recent AQA GCSE 8525 examiner reports and mark schemes — and how to avoid them.

✕Stating that ASCII can represent 256 characters rather than 128.
AQA 8525 specification 3.3.5 names 7-bit ASCII specifically as the encoding method to describe.
Why it happens
Characters are stored in 8-bit bytes and 2⁸ = 256 is a familiar figure, so it gets attached to ASCII — and extended 8-bit ASCII variants genuinely do have 256 characters.
How to avoid it
The specification names 7-bit ASCII, so the answer is 2⁷ = 128 characters with codes 0 to 127. Note the distinction: a character is stored in a byte, leaving one bit unused, but the code itself is only 7 bits wide.
✕Adding the full alphabet position instead of the position minus one — calculating 'C' as 65 + 3 = 68 rather than 67.
Sequence-rule questions require the calculation to be shown, so the offset must be applied correctly.
Why it happens
'C' is naturally described as the 3rd letter, so adding 3 feels correct — the off-by-one comes from forgetting that the anchor itself occupies position 1 with an offset of zero.
How to avoid it
The offset is position − 1, because 'A' at position 1 is the anchor. Count the steps from the anchor rather than the position: from 'A' to 'C' is two steps, so 65 + 2 = 67. Checking on your fingers takes seconds.
✕Saying that Unicode replaced ASCII or uses entirely different codes.
AQA 8525 specification 3.3.5: 'Know that Unicode uses the same codes as ASCII up to 127.'
Why it happens
Unicode is presented as the modern successor to ASCII, which suggests a clean break — and the specific compatibility fact is easy to overlook among the range advantages.
How to avoid it
Unicode extends ASCII: it uses the same codes up to 127, so 'A' is 65 in both. That backwards compatibility is separately examinable and is exactly why existing ASCII files remained readable when Unicode was adopted.
✕Treating the character '7' as the number 7 when using an encoding table.
This links 3.3.5 to 3.2.1 data types and to the string conversion operations in 3.2.8.
Why it happens
They look identical on the page, and in everyday use a typed digit is simply a number — the distinction between the symbol and the value is never visible.
How to avoid it
The character '7' has the code 55 (since '0' = 48), whereas the number 7 is stored as the binary value 00000111. They have completely different bit patterns, which is why input from the keyboard must be converted with a string-to-integer operation before any arithmetic.
✕Answering 'advantages of Unicode' with only 'it has more characters', without saying what those characters are.
AQA 8525 specification 3.3.5 additional information requires students to explain the need for different alphabets and special symbols.
Why it happens
The headline fact feels sufficient, and naming specific alphabets seems like unnecessary detail rather than the substance of the answer.
How to avoid it
The specification asks for the need to represent different alphabets and special symbols, so name them: Greek, Cyrillic, Arabic, accented characters such as é, non-Latin scripts such as Chinese, and symbols including € and emoji. Concrete examples are what develop the point into full marks.
✕Trying to memorise a full ASCII table, or revising UTF-8 and UTF-16.
AQA 8525 specification 3.3.5 additional information states that students use a given table and that UTF-8/UTF-16 are not required.
Why it happens
The table looks like the core knowledge of the topic, and wider reading about Unicode quickly reaches encoding schemes that appear important.
How to avoid it
The specification states a table will be given when needed, so only the anchors — 'A'=65, 'a'=97, '0'=48, space=32 — and the sequence rule are required. It also states explicitly that it is not necessary to be familiar with UTF-8, UTF-16 or other versions of Unicode, so revising them earns nothing.

Character Encoding — AQA GCSE Computer Science 8525 (3.3.5)

Character

ASCII code

Binary (8-bit)

C

01000011

a

01100001

t

116

01110100

Group

Examples

Capital letters

A–Z

Lower-case letters

a–z

Digits

0–9 (as characters, not numbers)

Punctuation and symbols

. , ? @ #

Space

code 32

Control characters

carriage return, tab, delete

Codes

Contents

0–31

Control characters (non-printing)

Space

33–47

Punctuation and symbols

48–57

Digits 0–9

58–64

More symbols

65–90

Capitals A–Z

91–96

More symbols

97–122

Lower case a–z

123–127

Symbols and delete

Character

Code

'A'

'a'

'0'

ASCII (7-bit)

Unicode

Bits per character

More (16 or more)

Number of characters

128

Over a million

Alphabets covered

English only

All major writing systems

Accents, symbols, emoji

✗

✓

File size

Smaller

Larger

Compatible with the other?

—

✓ Same codes up to 127

Character

Working

Code

H

65 + 7 (8th letter)

i

97 + 8 (9th letter)

105

!

given

Character

Code

Binary (8-bit)

H

01001000

i

105

01101001

!

00100001

Character	Code	Binary
`C`	67	`01000011`
`a`	97	`01100001`
`b`	98	`01100010`

Character

Code

Binary

C

01000011

a

01100001

b

01100010

Character encoding

Detailed Study Notes

At a glance

What you’ll learn

Character sets and character codes

How text is stored

Why an agreed standard matters

A character set includes more than letters

7-bit ASCII

What ASCII contains

The three anchors to memorise

Storage: 7 bits or 8?

ASCII's limitation

The sequence rule — the most useful fact here

Calculating within a group

The +32 relationship

Why sequencing matters practically

Unicode and its advantages over ASCII

The purpose of Unicode

Backwards compatibility — an examinable fact

The comparison table

The trade-off

What is NOT required

Using a given encoding table

Character → code

Code → character

Encoding a whole word

Converting to binary

The counting trap

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Define a character set and state ASCII's capacity (3 marks)

2Use a given encoding table (4 marks)

3Apply the sequence rule to a missing code (3 marks)

4Encode a word and convert to binary (5 marks)

5Use the +32 relationship between cases (4 marks)

6Explain the purpose and advantages of Unicode (5 marks)

7Decode a message from binary (6 marks)

8Compare ASCII and Unicode for a scenario (6 marks)

9Explain the significance of sequenced codes (6 marks)

Character code from an anchor

Upper to lower case gap

Number of characters in an n-bit code

Character set

Character code

ASCII

Unicode

Sequenced codes

Backwards compatibility

Control character

Case gap (+32)

Case sensitivity

Digit character

Encoding table

Bits per character

✕Stating that ASCII can represent 256 characters rather than 128.

✕Adding the full alphabet position instead of the position minus one — calculating 'C' as 65 + 3 = 68 rather than 67.

✕Saying that Unicode replaced ASCII or uses entirely different codes.

✕Treating the character '7' as the number 7 when using an encoding table.

✕Answering 'advantages of Unicode' with only 'it has more characters', without saying what those characters are.

✕Trying to memorise a full ASCII table, or revising UTF-8 and UTF-16.

Character encoding

Detailed Study Notes

At a glance

What you’ll learn

Character sets and character codes

How text is stored

Why an agreed standard matters

A character set includes more than letters

7-bit ASCII

What ASCII contains

The three anchors to memorise

Storage: 7 bits or 8?

ASCII's limitation

The sequence rule — the most useful fact here

Calculating within a group

The +32 relationship

Why sequencing matters practically

✕Adding the full alphabet position instead of the position minus one — calculating `'C'` as 65 + 3 = 68 rather than 67.

✕Treating the character `'7'` as the number 7 when using an encoding table.