Edexcel GCSE Computer Science (1CP2) Exam Questions

• C Main memory (RAM) (1)
• The only correct answer is C. The correct answer is main memory (RAM); accept 'main memory', 'RAM' or 'random access memory'.
• A is not correct because the arithmetic logic unit performs arithmetic and logical operations
• B is not correct because the clock sends regular pulses to synchronise the components of the CPU
• D is not correct because the program counter holds the address of the next instruction, not the instructions and data

Award one mark for any of the following up to a maximum of two marks:

• An arithmetic operation, e.g. addition/subtraction/multiplication/division (1)
• A logical operation, e.g. comparing two values (is A > B?) / using AND, OR, NOT (1)

Do not accept decoding instructions, fetching instructions or controlling the timing of the CPU, as these are carried out by the control unit.

Indicative content

Control unit and ALU

• The control unit fetches and decodes each instruction that the new computers are running, so it works out which operation has to be carried out.
• The control unit sends control signals to the ALU and to main memory over the control bus so that the other components carry out the instruction.
• The control unit receives the clock signal, so each stage of the cycle begins on a clock tick.
• The ALU carries out arithmetic operations, such as adding two values, and logical operations, such as comparing two values, during the execute stage.
• The ALU is the component that performs the calculations that a program running on the new computers needs.
• The data the ALU works on is held in registers, and the result of the operation is stored in a register such as the accumulator.

Registers

• Registers are small, very fast memory locations inside the CPU that hold the instructions and data the CPU is working on.
• The program counter holds the address of the next instruction to be fetched, and it is incremented each time an instruction is fetched.
• The memory address register (MAR) holds the address of the main memory location that is to be read from or written to during the fetch stage.
• The memory data register (MDR) holds the instruction or data brought from main memory (RAM) over the data bus before it is decoded.
• The accumulator holds the result of an operation carried out by the ALU.
• Because registers are inside the CPU, the CPU can keep working on a value without going back to slower main memory during the cycle.

Working together as the cycle repeats

• 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, and the instruction comes back over the data bus into the memory data register.
• In the decode stage the control unit interprets the instruction and works out which components are needed for it.
• In the execute stage the control unit signals the ALU or main memory so that the instruction is carried out, and the result is held in a register.
• The clock sends a regular pulse so that the fetch, decode and execute stages of each instruction happen in step.
• The program counter is then incremented, so the cycle repeats with the address of the next instruction to be fetched.

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)

Additional guidance

• A list of correct but independent roles, with no linkage between the components and no reference to the stages of the cycle, is limited to Level 1; an answer that gives the roles of the components but does not link them to the fetch, decode and execute stages is limited to Level 2; a Level 3 answer must cover the control unit/ALU and registers and link them to the stages of the cycle.
• Award no credit for 'the control unit is the brain of the CPU', 'the ALU stores data' or 'registers are the same as RAM'; accept the program counter, MAR, MDR and accumulator as examples of registers, accept the clock, address bus, data bus and control bus where they are linked to a stage of the cycle, and accept 'main memory' or 'RAM' as equivalent.

Indicative content

What embedded systems are used for in the drone

• An embedded system is a computer built into a larger device that performs a single, dedicated function as part of that device.
• The drone contains several embedded systems, and each one is dedicated to a task such as flight control, navigation, obstacle detection or battery monitoring rather than being a general-purpose computer.
• The flight controller embedded system processes data from the drone's sensors and changes the signals sent to the rotors, so the drone stays stable and can be steered while carrying a parcel.
• The GPS receiver embedded system works out the drone's position and uses it to navigate along the route to the customer's address.
• The camera embedded system captures images or video that are processed to detect obstacles, such as trees, buildings or overhead cables, in the drone's path.
• Sensor embedded systems, for example ultrasonic or infrared sensors, measure the distance to the ground and to obstacles so the drone can land and avoid collisions.
• The battery management and communication embedded systems monitor the charge and send the drone's status and location to the control centre, and the rotor-control embedded systems drive the rotors so the drone can take off, hover and land.

Benefits of using embedded systems in the drone

• Embedded systems use little power, so the drone can fly for longer and deliver more parcels on one charge.
• They are small and light, which is important because a heavier drone needs more power to stay in the air.
• Because each embedded system is dedicated to one task, it can respond quickly and reliably, so the drone can change direction in time to avoid an obstacle.
• Combining the processor, memory and input/output in one unit reduces the number of separate components and the cost per drone, so a fleet of delivery drones is cheaper to build.
• Embedded systems allow the drone to fly autonomously, so the company does not need a human pilot for every delivery.
• Programs are held in ROM, so the drone's behaviour is consistent and predictable each time it flies.

Any drawbacks or limitations of using embedded systems in the drone

• Each embedded system has limited processing power, so the drone may not be able to run complex obstacle-recognition software and could fail to identify an unusual obstacle.
• They have limited storage and RAM, so the drone cannot store much flight data or video on board and must send it to the control centre.
• The software is held in ROM and is difficult to update, so fixing a bug or adding a new feature may mean returning the drone or connecting it to a computer.
• The embedded systems are dedicated to the drone, so they cannot easily be reused for a different task if the drone is retired.
• If an embedded system fails, the drone may crash, which could injure people or damage property, so robustness is essential.
• A general-purpose computer would be too large and heavy for the drone, so the drone has to use these limited dedicated systems and cannot run general-purpose software.

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)

Additional guidance

• Level 3 responses must cover what embedded systems are used for in the drone and both benefits and drawbacks/limitations, with reasoning applied to the delivery drone. Do not accept general-purpose computer uses such as connecting to Wi-Fi to play videos, installing new software, apps or games, or using a server; do not accept the drone itself as a single embedded system unless a specific dedicated function is explained.
• A definition of an embedded system on its own, with no application to the drone, cannot lift an answer above Level 1; limited storage and inability to be repurposed must be linked to the drone, for example little flight data can be stored or the system cannot be reused for another task, to gain credit.