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AS · Practice questions · The fetch-execute cycle in register transfer notation

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Six original Cambridge-style questions on the fetch-execute cycle and register transfer notation, with mark-scheme style solutions.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Four transfers, in order.

01
Write
[4 marks]

Complete the fetch stage of the fetch-execute cycle using register transfer notation.

  • MAR ← [PC] ✓
  • PC ← [PC] + 1 ✓
  • MDR ← [[MAR]] ✓
  • CIR ← [MDR] ✓
02
Explain
[2 marks]

Explain the difference between [MAR] and [[MAR]] in register transfer notation.

  • [MAR] is the contents of the MAR, which is a memory address. ✓
  • [[MAR]] is the contents of the memory location at the address held in the MAR. ✓
03
Trace
[5 marks]

The PC holds 40. Address 40 holds LDD 60 and address 60 holds 18. State the contents of the PC, MAR, MDR, CIR and ACC after the instruction has been fetched and executed.

  • PC: 41. ✓
  • MAR: 60. ✓
  • MDR: 18. ✓
  • CIR: LDD 60. ✓
  • ACC: 18. ✓
04
Explain
[3 marks]

Describe what happens during the decode and execute stages of the cycle for the instruction ADD 75.

  • The control unit decodes the instruction in the CIR (opcode ADD, operand 75). ✓
  • The address 75 is placed in the MAR and the contents of address 75 are copied into the MDR. ✓
  • The ALU adds the contents of the MDR to the accumulator and the result is stored in the ACC. ✓
05
Apply
[2 marks]

Write the execute stage of the instruction JMP 85 in register transfer notation, and explain why the earlier PC ← [PC] + 1 does not cause a problem.

  • PC ← 85. ✓
  • The jump overwrites the incremented value, so the next fetch uses address 85. ✓
06
Analysis
[2 marks]

A student writes the fetch stage as: MAR ← [PC]; MDR ← [MAR]; CIR ← [MDR]. Identify the two errors.

  • MDR ← [MAR] should be MDR ← [[MAR]] (the contents of the address, not the address). ✓
  • The step PC ← [PC] + 1 is missing. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.