Edexcel GCSE Computer Science (1CP2) Exam Questions

• A Address bus (1)
• The only correct answer is A. The correct answer is address bus. Do not accept 'address' on its own.
• B is not correct because the control bus carries control signals in both directions between the CPU and other components
• C is not correct because the data bus carries data in both directions between the CPU and main memory
• D is not correct because the system bus is the collection of all three buses

Award up to two marks for a linked description such as:

• It reads/monitors the temperature/humidity/light level using sensors (1) to switch the heaters/vents/lamps/watering system on or off when the reading is outside the set range (1)
• It compares the readings from the sensors with the set values/thresholds stored in its memory (1) to decide whether the conditions need to be changed (1)
• It monitors the light level using a light sensor (1) and turns the lamps on when the light falls below the level the plants need (1)
• It monitors the soil moisture using a soil moisture sensor (1) and turns the water pump/irrigation on when the soil is too dry (1)

For the second mark, the action/response must be linked to the sensor reading given in the first point.

Allow named sensors (temperature/humidity/light/soil moisture) and named outputs (heater, fan, vent/window, lamp, irrigation/water pump).

Do not accept answers that only name a sensor or only name an output, e.g. 'it has a temperature sensor and a heater', as there is no link between the reading and the control action.

Do not accept answers where a person switches the equipment on or off, as the embedded system controls the conditions automatically.

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.

Indicative content

Fetching and decoding an instruction

• The register that holds the address of the next instruction (the program counter) contains the address of the instruction to be fetched.
• This address is sent to main memory (RAM) and the instruction is copied from main memory into a register inside the CPU.
• The program counter is updated so that it holds the address of the next instruction in the program.
• The control unit decodes the instruction that has been fetched, working out which operation is required and which data or registers are needed.
• The control unit sends control signals to coordinate the other components as the instruction is carried out.
• The control unit decides what happens next, for example after a comparison it decides whether the next instruction is the following one or a jump.

The arithmetic logic unit (ALU) executes an instruction

• The control unit directs the arithmetic logic unit (ALU) to carry out the operation.
• The ALU performs arithmetic operations such as addition, subtraction, multiplication and division, for example adding two values in a program written by the class.
• The ALU performs logical operations such as comparing two values to see if they are equal or whether one is greater than the other.
• The result of the operation carried out by the ALU is passed back and stored in a register.
• The ALU operates on data that has been fetched from main memory; it does not fetch the instruction and it does not decide which instruction is next.

Registers

• Registers are small, very fast memory locations inside the CPU.
• They hold the data, addresses and results that the CPU is working on during the cycle.
• Registers are used in every stage of the cycle, not only when the ALU produces a result.
• Because registers are inside the CPU they can be read and written much faster than main memory during the cycle.

Levels

• Level 0 (0 marks): No rewardable content.
• Level 1 (1–2 marks): Basic, independent points are made, showing elements of understanding of key concepts/principles of computer science. (AO1) The discussion will contain basic information with little linkage between points made or application to the context. (AO2)
• Level 2 (3–4 marks): Demonstrates adequate understanding of key concepts/principles of computer science. (AO1) The discussion shows some linkages and lines of reasoning with some structure and application to the context. (AO2)
• Level 3 (5–6 marks): Demonstrates comprehensive understanding of key concepts/principles of computer science to support the discussion being presented. (AO1) The discussion is well developed, with sustained lines of reasoning that are coherent and logically structured, and which clearly apply to the context. (AO2)
• A Level 3 answer must link the roles of all three components to the fetch, decode and execute stages. An answer that links only one component to a stage, with the other points listed separately, is Level 1 (1–2 marks); an answer that links two of the components to stages is Level 2 (3–4 marks). A point repeated under more than one heading is credited once only.
• Accept 'the program counter', 'the address register' or 'a register holding the address of the next instruction' as the register that holds the address of the next instruction. Do not accept 'the control unit performs the calculations', 'the ALU controls the timing of the CPU' / 'the ALU sends the clock signal' or 'registers are in main memory (RAM)'.