Award up to two marks for a linked explanation such as:
- Decodes/interprets/translates the instruction (1) to work out what the instruction is/what operation is required (1)
- Sends control signals to the other components (1) so that the correct component carries out the instruction (1), e.g. the ALU/main memory
- Controls the fetch of the instruction from main memory (1) because the address of the next instruction is sent via the address bus (1)
- Controls the fetch of the next instruction from main memory (1) by sending a control signal/read signal to main memory (1)
For both marks, the expansion must follow/associate with the statement.
For the decode point, 'decodes the instruction' alone gains 1 mark; a second mark needs the purpose of decoding, i.e. working out what the instruction is/what operation is required.
Do not accept 'it controls the CPU' on its own, without naming a task in the cycle.
Do not accept 'it fetches the instruction from memory' on its own, as this describes the fetch step rather than the control unit's role; credit only if linked to controlling/signalling the fetch.
Do not accept carrying out arithmetic/logical operations, as these are carried out by the ALU, or holding the address of the next instruction, as this is the program counter.
Do not accept storing the instruction in a register (e.g. MAR/MDR), as this is not the role of the control unit.