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1.1.1Architecture of the CPU

OCR GCSE Computer Science (J277) · Computer systems (Paper 1) › Systems architecture

Practise Architecture of the CPU. 12 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.

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Identify the component of the CPU that carries out arithmetic and logical operations.
Arithmetic Logic Unit (ALU)

Sample questions

Written for this site in the style of OCR exam questions. They are not taken from real past papers.

Question 1Easy3 marks
Every computer contains a central processing unit (CPU).
(a) State the purpose of the CPU.[1]
(b) Which register holds the address of the next instruction to be fetched?
Tick (✓) one box.[1]
  • Accumulator (ACC)
  • Memory Address Register (MAR)
  • Memory Data Register (MDR)
  • Program Counter (PC)
(c) Identify the component of the CPU that carries out arithmetic and logical operations.[1]
Show the answer and mark scheme
(a)
  • to fetch, decode and execute instructions / to process data and instructions
(b) Answer: Program Counter (PC)
(c) Answer: Arithmetic Logic Unit (ALU)
  • Arithmetic Logic Unit / ALU
Question 2Medium6 marks
(a) Describe what happens during the fetch stage of the fetch–execute cycle.[4]
(b) State what happens to the instruction after it has been fetched.[2]
Show the answer and mark scheme
(a)
  • the Program Counter (PC) holds the address of the next instruction to be fetched
  • this address is copied to the Memory Address Register (MAR) / is used to find the instruction in main memory
  • the instruction stored at that address is fetched / copied from main memory (RAM) into the CPU
  • the instruction is held in the Memory Data Register (MDR)
  • the PC is incremented (by 1) so that it holds the address of the next instruction
(b)
  • it is decoded by the Control Unit (CU)
  • it is then executed / carried out, e.g. the ALU performs a calculation or data is loaded or stored
Question 3Hard5 marks
A CPU is executing an instruction that stores the value in the accumulator into a memory location.
(a) Describe how the CPU uses the MAR and the MDR to store the value in memory.[3]
(b) The CPU contains registers even though the computer also has RAM. Explain why registers are needed.[2]
Show the answer and mark scheme
(a)
  • the address of the memory location to be written to is copied into the MAR
  • the value (from the accumulator) is copied into the MDR
  • the contents of the MDR are written to the memory location whose address is in the MAR
(b)
  • registers are much faster to access than RAM
  • they hold the data / instructions / addresses that the CPU is working on during the current cycle

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