The control unit (CU) is a component inside the CPU.
Describe the role of the control unit in the fetch-decode-execute cycle.
Mark scheme
Award up to two marks for a linked description such as:
- Decodes each (fetched) instruction (1) to work out which operation is needed / which components should carry it out (1)
- Coordinates/directs the other components (1) by sending control signals along the control bus (1)
- Fetches the next instruction (1) by sending its address to main memory along the address bus (1)
- Sends a read/write control signal to main memory (1) so that the instruction or data is transferred during the fetch/execute stage (1)
Do not accept answers about synchronising the CPU or timing the cycles (that is the role of the clock).
Do not accept answers about carrying out arithmetic or logical operations (that is the ALU).
Do not accept answers about holding data or the results of instructions (that is registers/main memory).
Do not accept generic answers that only say the CU controls the CPU or processes instructions, with no linked development.
Only award one mark for a response that only repeats that the CU sends control signals; the second mark needs a linked outcome such as coordination, a memory read/write, or fetching an instruction.
Do not accept 'during the decode stage' as the development for the decoding point, as it restates the question.
Example answers and how they were marked
It decodes the instruction that has been fetched (1) so that it knows which operation the CPU must carry out and which components should do it (1).
Why: Decoding named with the acceptable wording 'each (fetched) instruction' (1), plus the linked development that this identifies the operation and components (1).
It decodes each instruction during the decode stage of the cycle.
Why: Earns (1) for decoding the instruction. 'During the decode stage' restates the question and is not accepted as a development, so the second mark is not earned.
It carries out the calculations and comparisons when the instruction is executed.
Why: Calculations and comparisons are the ALU's role, which the 'Do not accept' line rules out, so no mark is earned.