CPU components, fetch–execute cycle & performance – GCSE Computer Science exam questions with mark scheme

1Grade 4–56 marks

(a) Describe the purpose of each of the following CPU components. [3]

(i) Arithmetic logic unit (ALU)
(ii) Control unit
(iii) Clock

(b) Describe the stages of the fetch–execute cycle. [3]

(a)(i) ALU: carries out arithmetic (e.g. addition, subtraction) and logical (e.g. AND, OR, comparison) operations(1)
(a)(ii) Control unit: decodes instructions / sends control signals to coordinate the other components / controls the fetch–execute cycle(1)
(a)(iii) Clock: generates a regular signal / pulse that synchronises / sets the timing of the CPU's operations(1)
(b) Fetch: the next instruction is copied / transferred from main memory to the CPU (using the address bus / data bus); accept RAM / memory for main memory; must say where it is fetched from – 'the instruction is fetched' alone does not score(1)
(b) Decode: the control unit works out / interprets what the instruction means / what needs to be done. 'The instruction is decoded' alone does not score(1)
(b) Execute: the instruction is carried out (e.g. by the ALU; may read from / write to main memory). 'The cycle repeats' is not required(1)
Answer: ALU = arithmetic & logic; CU = decodes & coordinates; clock = timing pulses. Fetch from memory, decode, execute.
Examiner tip: 'Fetch the instruction' alone scores nothing – say it is fetched FROM MAIN MEMORY.

2Grade 6–74 marks

A computer is upgraded from a dual-core to a quad-core processor with the same clock speed.

(a) Explain why the upgrade might not double the performance of the computer. [2]

(b) Explain how increasing the size of the cache can improve performance. [2]

(a) Some programs / tasks cannot be split into parts that run at the same time / are not written to use multiple cores(1)
(a) So some cores may be idle / wait for other cores; allow time is needed to coordinate the cores / share data; allow other components (e.g. main memory / bus speed) limit performance so the cores have to wait(1)
(b) Cache is faster (to access) than main memory / RAM(1)
(b) A larger cache can hold more frequently used data / instructions, so the CPU has to fetch from (slower) main memory less often(1)
Answer: (a) not all tasks can run in parallel (b) more data kept in fast cache, fewer slower RAM accesses
Examiner tip: Always compare cache with main memory – 'cache is fast' gets no credit without the comparison.

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