Full marks6/6
The control unit controls the other parts of the CPU in each of the college's new computers. In the fetch stage, the address of the next instruction is copied from the program counter into the memory address register and sent to main memory over the address bus. The instruction comes back over the data bus and is held in the memory data register. The control unit then decodes the instruction, working out which operation has to be carried out. In the execute stage it sends control signals to the ALU or to main memory so that the instruction is carried out. The ALU performs arithmetic operations, such as adding two values, and logical operations, such as comparing two values, on data held in registers, and the result is stored in a register such as the accumulator. Registers are small, very fast memory locations inside the CPU, so the CPU holds the instruction and the data it is working on without going back to slower RAM every time. The program counter is then incremented so that it holds the address of the next instruction, and the cycle repeats. The clock sends a regular pulse so that the stages of the cycle happen in step.
Why: Comprehensive understanding of the control unit, ALU and registers, with each component linked to the fetch, decode or execute stage and applied to the college's new computers, and sustained lines of reasoning through the cycle. Level 3 (5–6): all three bullets covered and linked, so 6.
Some marks3/6
The control unit is part of the CPU in the college's new computers. When an instruction is fetched, the control unit decodes it to find out what has to be done, and later it signals the other components to carry out the instruction. The ALU does arithmetic and logical operations, so it is used in the execute stage when a calculation is needed. Registers are fast memory locations inside the CPU that hold the data being worked on. For example, the program counter holds the address of the next instruction, so the CPU knows where to fetch from next.
Why: Adequate understanding: the control unit and ALU are linked to the decode and execute stages and to the college's computers, and the program counter is linked to the fetch stage, so some linkages and lines of reasoning are present. The role of registers is thin and there is no sequence through the whole cycle, so it does not reach Level 3. Level 2: 3 marks.
Some marks1/6
The control unit is the brain of the CPU and makes all of the decisions. The ALU is used to do arithmetic, for example adding numbers together. Registers are like RAM but they are inside the CPU, and they store the data a program needs. The fetch-decode-execute cycle fetches, decodes and then executes an instruction.
Why: One basic, independent point: the ALU carries out arithmetic operations (1). 'The control unit is the brain' and 'registers are like RAM' are the named misconceptions and gain no credit, and restating the three stage names gives no role. Level 1: 1 mark.