AQA A-Level Computer Science AS Paper 2, June 2025: Question 3

Mark scheme

Show the mark scheme

Qu Pt Marking Guidance Marks

03 1 Mark is for AO2 (application) 1

1010100;

Qu Pt Marking Guidance Marks

03 2 Mark is for AO1 (understanding) 1

Assumes English as the default/standardised language;

Number of different characters is limited to 128 (27);

Does not include a wide variety of international character sets (eg Chinese) /

popular characters (eg emojis) // must be extended to represent foreign languages

/ languages other than English;

Max 1

How to answer it

Character Representation: ASCII & Unicode

What this question tests

This question assesses your understanding of data representation in computer systems: specifically how character sets encode alphabetic symbols into binary, navigating sequential ASCII character order without a reference table, and evaluating the limitations of standard 7-bit ASCII compared to modern Unicode standards.

Question 03.1 • 1 Mark

ASCII Binary Offset Calculation

Finding the binary code for 'T' given the representation for 'Q'

📐 Step-by-Step Working

Determine the alphabetic offset:
Count the positions from Q to T :
Q → R (+1) → S (+2) → T (+3).
So, character T is Q + 3 .
Convert 'Q' to denary (or add directly in binary):
1010001₂ = 64 + 16 + 1 = 81₁₀.
Add the offset:
81 + 3 = 84₁₀.
Convert 84₁₀ back to 7-bit binary:
64 + 16 + 4 = 64 + 16 + 4 = 1010100₂ .

✅ Correct Answer

1010100

Marking Guidance (AO2 - Application):
1 mark for the exact binary string 1010100 . An optional leading zero ( 01010100 ) to form an 8-bit byte is also acceptable unless standard 7-bit is specifically requested.

🧠 Exam Technique

Direct binary addition is faster: You don't need to convert to denary first. Simply add binary 3 ( 11₂ ) directly onto the last digits of 1010001₂ :
...0001 + 0011 = ...0100 .
Sequential ordering: Remember that in ASCII, uppercase letters are strictly contiguous from 'A' through 'Z'.

❌ Common Errors

Fencepost / off-by-one errors: Miscounting the difference between Q and T (e.g. counting Q as 1, R as 2, S as 3, getting +4 instead of +3).
Binary addition slip: Forgetting to carry 1 when doing 1 + 1 in binary.
Changing upper bits: Accidentally flipping bits in the upper nibble; only the lower bits change for close letters.

Question 03.2 • 1 Mark

Limitations of ASCII vs Unicode

Explaining why Unicode was developed to replace standard ASCII

💡 Key Knowledge

Standard ASCII is 7-bit: It can only represent 2⁷ = 128 unique character codes (values 0 to 127).
Extended ASCII is 8-bit: Still limited to only 2⁸ = 256 unique characters.
Unicode: Uses 16 to 32 bits (e.g. UTF-8, UTF-16) allowing over 1 million possible code points, enough for every written human language, mathematical symbols, and emojis.

✅ Acceptable Answers (Any One)

ASCII is limited to a maximum of 128 characters (or 2⁷).
ASCII assumes English as the default/standard language.
ASCII cannot represent non-Latin / international alphabets (e.g. Chinese, Arabic, Cyrillic, Japanese script).
ASCII does not support modern graphical symbols such as emojis.

Marking Guidance (AO1 - Understanding):
1 mark for identifying the character capacity constraint OR the lack of international character representation.

❌ Common Errors

Vague answers: Stating just "ASCII is old" or "ASCII has less storage" without referencing the number of characters or language support.
Confusing character count with file size: Unicode characters often take more storage (bytes) than ASCII characters, not less!
Omitting specifics: Writing "can't do other symbols" without clarifying foreign scripts, non-English alphabets, or emojis.

🧠 Examiner Insight

To secure the mark quickly, quote precise numbers: "Standard ASCII is limited to 128 characters (2⁷), which is insufficient for international alphabets like Mandarin or Arabic." Stating the mathematical limitation ( 2⁷ = 128 ) leaves no ambiguity.

Topics

4.5 Fundamentals of data representation · 4.5.5 Information coding systems