GCSE (9-1) Computer Science
Mark Scheme
J277/01: Von Neumann Architecture & Registers
Question Answer Marks Guidance
1 Any two from:
  • Program Counter / PC
  • Memory Address Register / MAR
  • Memory Data Register / MDR
  • Accumulator / ACC
  • Current Instruction Register / CIR
 2
1 mark for each correct register.
Reject: "Cache" or "ALU" (components, not registers).
2
  1. PC / Program Counter
  2. Accumulator / ACC
  3. MAR / Memory Address Register
  4. MDR / Memory Data Register
 4
1 mark per correct row.
Must match definition exactly.
Accept: "Memory Buffer Register" for MDR.
3
  • Stores the address of the next instruction to be fetched. (1)
  • Increments to point to the next instruction. (1)
 2
Misconception Alert: Do not accept "counts the instructions" or "stores the current instruction".
4
  • Stores the result of calculations / arithmetic operations. (1)
  • Stores the result from the ALU. (1)
 2
Reject: "performs the calculation" (that is the ALU).
Must reference the result or intermediate data.
5
  • The MAR stores the address / location of the data. (1)
  • The MDR stores the data / instruction itself. (1)
  • The MAR holds the address fetched from the PC. (1)
  • The MDR holds the data fetched from Memory/RAM. (1)
 4
Examiner’s Comment: Candidates often inaccurately identify the MAR as storing data. Ensure distinction is clear.
6 Indicative Content:
  • The PC holds the address of the next instruction.
  • The MAR holds the address of the current instruction to be fetched.
  • The MDR holds the instruction/data that has been fetched from memory.
  • The PC is incremented during the cycle.
  • The instruction is decoded by the Control Unit (CU).
  • The instruction is executed (e.g., ALU performs calculation).
  • The Accumulator (ACC) stores the results of calculations.

Marking Levels:
Level 3 (5-6 marks): Detailed description of the cycle including correct sequence and correct use of at least 3 registers.
Level 2 (3-4 marks): Description of the cycle with some correct register usage.
Level 1 (1-2 marks): Basic steps (e.g., fetch, decode, execute) without correct register details. 		6
Use a "best fit" approach to marking the levels.
OCR 2026