OCR GCSE Computer Science Computer systems (01), June 2025: Question 6

6 marks · Medium difficulty · Short Answer

Encode a word in ASCII using a given sequence, evaluate Unicode versus ASCII, and identify two CPU registers and their purposes.

Practise this question

Question

Question 6 provides a table showing ASCII binary codes for characters K (01001011), L (01001100), M (01001101), and N (01001110). Part (a) asks to write the binary code to represent the word POP using ASCII for 2 marks. Part (b) asks for one benefit and one drawback of using Unicode instead of ASCII for 2 marks. Part (c) asks to complete a table by identifying two CPU registers and stating the purpose of each register for 2 marks.
Question text

6 A computer uses the ASCII character set.

Part of the ASCII character set is shown here:

Character Binary code

K 01001011

L 01001100

M 01001101

N 01001110

(a) Write the binary code to represent the word POP using ASCII.

… [2]

(b) A new computer is being built.

Identify one benefit and one drawback of the computer using Unicode instead of ASCII.

Benefit …

Drawback …

[2]

(c) The computer’s CPU contains registers.

Complete the table by identifying two registers and state the purpose of each register.

Register Purpose

[2]

Mark scheme

Show the mark scheme Mark scheme for Question 6: (a) gives 1 mark for any 1 correct binary code, 2 marks for all 3 correct codes in order: 01010000 01001111 01010000. (b) awards 1 mark for a benefit (e.g. represents more characters/languages, supports emojis) and 1 mark for a drawback (e.g. larger file size, requires more storage/processing, backward compatibility issues). (c) awards 1 mark per correct register with purpose: Program Counter (stores address of current/next instruction), MAR (stores address of current/next data/instruction to be fetched/executed), MDR (stores data/instruction fetched from or to be stored in memory), or Accumulator (stores result of calculations).

Question Answer Mark Guidance

6 (a) 1 mark for any 1 correct binary code 2 BOD 7-bit ASCII codes if

2 marks for all 3 correct binary codes in the correct order with no additional bits / addition correct

01010000 01001111 01010000 Accept answer vertically and

horizontally

6 (b) 1 mark for benefit 2 Award any correct point in

• Can represent more/a wider range of characters benefit/drawback

• Can represent characters from all/many/different languages // by example

• Can store emojis

1 mark for drawback

• Larger file size // double the file size of ASCII // more data/binary per character //

more storage/bits used // requires more/additional processing

• Additional characters are not backward compatible to ASCII // some characters will

not be recognised when documents are read/opened in computers that only have

ASCII

6 (c) 1 mark for register with purpose 2 Careful that the purpose is

• Program counter // PC not an action such as

Stores the address of the current/next instruction to be fetched // stores the fetches, takes, retrieves.

address of the instruction for the current/next FE cycle

Accept “points” to/at in place

• Memory address register // MAR of stores

Stores the address of the current/next/required instruction/data

// stores the address of data/instruction about to be fetched/executed Accept

// stores the address where data/instruction (in MAR) is going to be stored • Current instruction

// stores the address of instruction/data being decoded/executed register//CIR//Instruction

register//IR

• Memory data register // MDR Stores the instruction

Stores the data/instruction fetched from memory currently being executed

// stores data/instruction to be stored in memory

// stores the data/instruction located in the memory location in the MAR BOD memory buffer register

for MDR.

• Accumulator // ACC

Stores the result of calculations // stores data currently being processed / by Accept memory address,

example // stores the result from the ALU memory data without

‘register’.

How to answer it

Character Sets (ASCII & Unicode) and CPU Registers

📌 What this question tests

This question assesses your ability to determine binary representations in ASCII using sequential patterns, evaluate the trade-offs between ASCII and Unicode (range of characters vs storage space), and accurately recall the names and specific storage functions of CPU registers (MAR, MDR, PC, ACC).

Part (a): Deducing ASCII Binary Codes for "POP"

Total Marks: 2

📐 Step-by-Step Calculation

ASCII values increase by 1 for each successive letter in the alphabet:

  1. Given: N = 01001110 (denary 78)
  2. Find 'O': Add 1 to N
    01001110 + 1 = 01001111 (denary 79)
  3. Find 'P': Add 1 to O (watch the binary carry!)
    01001111 + 1 = 01010000 (denary 80)
  4. Assemble "POP": P then O then P

✅ Correct Answer

Binary code for POP:

01010000 01001111 01010000

Mark Scheme:
• [1 mark] for any 1 correct 8-bit or 7-bit binary code.
• [2 marks] for all 3 correct binary codes in the correct order without extra bits.

🧠 Exam Technique

  • Keep bits in byte groups: Write them with spaces between bytes or on separate lines so the examiner can clearly distinguish each character.
  • Watch the binary carry: Moving from ...1111 to the next value resets the lower four bits to 0000 and carries a 1 into the 5th bit ( 01010000 ).

❌ Common Errors

  • Binary addition mistakes: Forgetting how to carry when adding 1 to ...1111 , leading to incorrect values like 01001112 or 01002000 .
  • Wrong order: Writing "OPP" or "PPO" instead of P - O - P.
  • Writing only one letter: Stopping after calculating 'P' instead of writing all three codes for the full word.

Part (b): Benefits and Drawbacks of Unicode vs ASCII

Total Marks: 2

💡 Key Knowledge

  • ASCII: Uses 7 or 8 bits (representing 128 or 256 unique characters). Primarily covers English and basic symbols.
  • Unicode: Uses 16 to 32 bits (representing millions of unique code points). It covers characters from all global languages, mathematical symbols, and emojis.

✅ Model Answers

Benefit [1 mark] (any one):

  • Can represent a much wider range of characters.
  • Can represent characters from all/multiple world languages (e.g. Mandarin, Arabic, Cyrillic).
  • Can represent emojis and special symbols.

Drawback [1 mark] (any one):

  • Larger file size / uses more memory and storage space (since each character requires more bits, typically 16 or 32 bits compared to 7/8 bits).
  • Requires more processing power / takes longer to transmit across networks.
  • Not fully backward compatible with older ASCII-only systems.

❌ Common Errors

  • Vague answers: Saying "it is better" or "it holds more information" without specifying more characters / foreign languages / symbols.
  • Confusing speed with storage: Stating "Unicode takes up more RAM speed" rather than correctly identifying increased file size or storage demands.

🧠 Exam Technique

Always pair bits with consequences:

More bits per character → larger file size / more storage needed. Clear and concise phrasing guarantees the drawback mark.

Part (c): CPU Registers and Their Purposes

Total Marks: 2

✅ Accepted Answers (Choose Any Two Rows)

Register Name Purpose (How Mark is Awarded)
Program Counter (PC) Stores the address of the next instruction to be fetched / processed.
Memory Address Register (MAR) Stores the memory address currently being read from or written to (or about to be fetched/executed).
Memory Data Register (MDR) Stores the data or instruction fetched from memory, or waiting to be written to memory.
Accumulator (ACC) Stores the intermediate results of arithmetic and logical operations performed by the ALU.
Current Instruction Register (CIR) Stores the instruction currently being decoded and executed.
Marking rule: 1 mark per correct register with an accurate purpose.

❌ Critical Examiner Trap: Active Verbs

Registers are storage locations, not active processors!

  • Zero Marks: "The PC fetches the next instruction." (The control unit and buses do this, not the register!)
  • Zero Marks: "The MAR retrieves the address."
  • Full Marks: Always start your purpose with "Stores the..." or "Holds the...".

🧠 Quick Memory Tricks

  • PC: Points / counts to the next instruction address.
  • M A R: Holds the A ddress.
  • M D R: Holds the D ata / Instruction.
  • ACC: Accumulates the results from the ALU.

Topics

1.1 Systems architecture · 1.2 Memory and storage · 1.1.1 Architecture of the CPU · 1.2.4 Data storage

Question and mark scheme from the OCR GCSE Computer Science examination, Computer systems (01), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.