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Which Part of the Computer?
Processor Components Drill

Computer Systems revision Approx 25–35 min 6 scenarios  ·  justification practice
Recap: The ALU does calculations, comparisons and logic; the control unit fetches and decodes instructions and controls the movement of data; registers are tiny, very fast storage locations inside the processor. The address bus carries a memory address one way, from processor to memory; the data bus carries data both ways.  → Full lesson: CS5 — Computer Structure
How to use this drill: Each scenario describes something happening inside a computer. Choose the part responsible, then justify your choice by naming what that part does and linking it to the detail in the scenario. Reveal the model answer to check your reasoning.
Your choices and justifications are saved automatically.
Section B — Processor Components

6 scenarios. Choose an answer, justify it using the scenario, then check the model answer. Scenario 6 is marked DISCUSS — either answer can be correct if well justified.

Scenario 1 of 6 Pass or fail?

A test-marking program checks whether each pupil's score is greater than or equal to 50 so it can decide whether to display "Pass" or "Fail".

Expected: ALU The ALU (arithmetic and logic unit). Checking whether the score is greater than or equal to 50 is a comparison, and the ALU carries out comparisons and logic operations as well as arithmetic.
Scenario 2 of 6 Running the next instruction

The processor must get the next instruction of a program from memory, work out what kind of instruction it is, and then send signals so that the data it needs moves to the right place.

Expected: Control unit The control unit. It fetches instructions, decodes them and controls the movement of data around the processor — exactly the three things described in the scenario.
Scenario 3 of 6 Holding values for a moment

While carrying out an addition, the processor needs somewhere very fast to hold the instruction it is working on and the two numbers being added, just for a few billionths of a second.

Expected: Registers Registers. They are small, very fast temporary storage locations inside the processor, used to hold the current instruction and the data being processed. Main memory would be far too slow for this.
Scenario 4 of 6 Asking for a location

The processor wants to read the value stored at memory location 1011 0010. It must tell memory which location it wants.

Expected: Address bus The address bus. It carries the unique binary address of a memory location from the processor to memory. It is one-way: memory never sends addresses back to the processor.
Scenario 5 of 6 A value travels both ways

A value is read from memory into the processor, a calculation is done, and the result is then written back to the same memory location.

Expected: Data bus The data bus. It carries data between the processor and memory in both directions: from memory to the processor when reading, and back to memory when writing the result.
Scenario 6 of 6 Price of three tickets Discuss

A cinema kiosk multiplies the ticket price by 3 to find the total, then keeps the answer briefly inside the processor before it is sent to be displayed.

Expected: Either — if justified using the scenario If choosing ALU: the multiplication is an arithmetic calculation, which the ALU carries out.

If choosing Registers: the scenario also says the answer is kept briefly inside the processor — that is the job of registers.

Exam note: read the command word. "Which part calculates the total?" is the ALU; "where is the result held?" is a register. Name the part and say what it does in this scenario.
Marker's eye view — why candidates lose marks

The most common lost mark here is mixing up the two buses. Learn the direction: the address bus is one-way (processor to memory) and carries a location; the data bus is two-way and carries the value.

For the ALU, examiners look for "arithmetic and logic" or a specific operation such as a comparison. "It does the maths" on its own is often too vague for the mark. For the control unit, give one of its specific jobs: fetching, decoding, or controlling the movement of data.