Edexcel GCSE Computer Science (1CP2) Exam Questions

• B RAM is faster to access than secondary storage (1)
• The only correct answer is B. The correct answer is RAM is faster to access than secondary storage.
• A is not correct because RAM is volatile
• C is not correct because RAM has a smaller capacity than secondary storage
• D is not correct because RAM is more expensive than secondary storage

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

• A temperature sensor measures/reads the temperature in the greenhouse (1) which the processor compares with the value stored in the system (1)
• The heater is switched on (1) when the temperature reading goes below the value stored in the system (1)
• A fan/vent/window is opened/a cooler is switched on (1) when the temperature reading goes above the value stored in the system (1)
• The temperature is checked/measured continuously (1) so the heater/fan is switched on or off as soon as the temperature changes (1)

For both marks, the expansion must follow/associate with the statement.

Allow one mark for a correct point on its own without a linked reason, e.g. 'a temperature sensor measures the temperature'.

Do not award 'the embedded system controls/keeps the temperature correct' as this repeats the question.

Do not accept naming a device without saying what it does, e.g. 'the greenhouse has a thermostat'.

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

Stored program concept

• Instructions and data are stored together in the same main memory (RAM).
• Instructions are fetched one at a time from memory and executed in sequence.
• The program can be changed by loading different instructions into RAM.
• The CPU can only execute instructions that are in main memory.

Role of RAM

• RAM holds the program instructions and the data being used.
• The address of the next instruction is held in the program counter (PC).
• The instruction is fetched from RAM using the address bus.
• The data bus carries the instruction from RAM to the CPU.
• RAM is volatile, so programs are lost when power is off.
• RAM is directly accessible by the CPU, unlike secondary storage.

Benefits

• The same hardware can run many different programs.
• Programs can be loaded and changed quickly.
• No need to rewire the computer for a new task.
• Makes general-purpose computers possible.

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)