The fetch-decode-execute cycle is used by the CPU.
State three things that happen during the fetch stage.
Mark scheme
Award one mark for any of the following up to a maximum of three marks:
- The address (of the next instruction) is sent from the CPU to main memory / placed on the address bus (1)
- The (next) instruction is fetched/read from main memory (RAM) (1)
- The instruction is brought/copied into the CPU / received into the MDR (1)
- The program counter (PC) is incremented/updated to hold the address of the next instruction (1)
Allow 'the PC/MAR is placed on the address bus' for the address being sent to main memory.
Allow 'the MDR receives the instruction' for the instruction being brought into the CPU.
Do not award a second mark for the instruction being fetched from main memory and the instruction being brought into the CPU; these are the same point.
Do not award 'the instruction is transferred along the data bus' on its own; this is the same point as the instruction being fetched from main memory.
Do not accept answers about decoding the instruction, e.g. 'the instruction is fetched from RAM and decoded', as decoding happens in the decode stage.
Do not accept answers about the execute stage, e.g. 'the ALU carries out the calculation' or 'the result of a calculation is stored in a register', as these happen in the execute stage.
Example answers and how they were marked
The address of the next instruction is sent to main memory, the instruction is fetched from RAM, and the program counter is incremented.
Why: Address sent to main memory (1), instruction fetched from RAM (1) and program counter incremented (1).
The instruction is fetched from main memory and then decoded.
Why: Fetching the instruction from main memory (1); decoding happens in the decode stage, so it is not creditworthy in the fetch stage.
The ALU carries out the calculation and the result is stored in a register.
Why: The ALU carrying out a calculation and storing a result happen in the execute stage, not the fetch stage, so no point is creditworthy.