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Edexcel GCSE Computer Science (1CP2) Exam Questions

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3 questions · 10 marks

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1Explain2 marksPrimary Memory Swap Remove

A computer follows the von Neumann stored program concept. A program's instructions are stored on secondary storage before the program is run.

Explain one reason why the instructions of a program must be held in RAM before they can be executed.

[2 marks]
Mark scheme

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

  • The CPU can only fetch/read instructions from main memory/RAM (1) so the instructions of a running program must be copied/loaded into RAM before they can be executed (1)
  • RAM is faster to access than secondary storage (1) so the instructions can be fetched quickly enough for the CPU to execute them (1)
  • The CPU needs the instructions in RAM to fetch and execute them (1) because the fetch-decode-execute cycle reads each instruction from main memory/RAM (1)
  • The address of the next instruction is sent to RAM on the address bus (1) so the instruction can be transferred to the CPU/registers for decoding/execution (1)

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

Allow the point and reason in either order if they are linked.

Do not accept answers that only say the CPU needs instructions, or that RAM is faster, without linking this to fetching/executing the instructions.

Do not accept answers that only say RAM holds data and instructions without linking this to the fetch-decode-execute cycle.

Do not accept 'RAM is permanent/non-volatile storage, so the instructions are kept safe' - RAM is volatile.

The response does not need to name the address bus or the fetch stage for full marks; a linked reason that explains why instructions must be in RAM (for example, the CPU can only fetch from RAM, or RAM is faster than secondary storage) is enough.

Example answers and how they were marked
Full marks2/2

The CPU can only fetch instructions from main memory, so the instructions of a running program must be copied into RAM before they can be executed.

Why: Linked point (1) for the CPU only being able to fetch instructions from main memory/RAM, and reason (1) for instructions being copied into RAM before execution.

Some marks1/2

The CPU can only fetch instructions from main memory.

Why: Point (1) only; there is no linked reason that explains why this means the instructions must be held in RAM before execution, such as RAM being faster than secondary storage.

No marks0/2

Instructions are kept in RAM because RAM is permanent storage and does not lose the instructions when the power is turned off.

Why: Misconception: RAM is described as permanent/non-volatile, which the mark scheme's 'Do not accept' line rules out; no creditworthy linked point.

2Explain2 marksFetch Decode Execute Cycle Swap Remove

The von Neumann architecture is used in most computers.

Explain one feature of the von Neumann stored program concept.

[2 marks]
Mark scheme

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

  • The instructions/program instructions and data are stored together in main memory/RAM (1) so the CPU can fetch each instruction (from its address) and execute it (1)
  • The instructions/program instructions are held in main memory/RAM (1) rather than being built into/part of the hardware, so the computer does not need to be rewired/rebuilt to run a different program (1)
  • The next instruction to be executed/instruction is held in main memory/RAM (1) so the CPU can fetch it using its address before it is decoded and executed (1)

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

A point/statement on its own can only earn one mark.

Do not accept a response that only refers to data being stored in main memory/RAM without referring to the instructions/program instructions.

Do not accept instructions/programs being stored on the hard disk/backing store.

Do not accept 'the program is stored in memory' without saying that it is the instructions/program instructions that are stored there.

Example answers and how they were marked
Full marks2/2

Both the instructions of the program and the data it uses are stored together in the same main memory, so the CPU can fetch each instruction in turn and execute it.

Why: Linked point (1) and reason (1).

Some marks1/2

The instructions of the program are stored in main memory.

Why: Point only (1); no linked reason about fetching/executing the instructions.

No marks0/2

The program is stored on the hard disk and is copied into the CPU when it is run.

Why: Misconception covered by the 'Do not accept' line: the instructions must be stored in main memory/RAM, not on the hard disk/backing store, for the CPU to fetch them.

3Discuss6 marksFetch Decode Execute Cycle Swap Remove

A small robotics company is designing a new robot. The robot's computer uses von Neumann architecture. The engineers are explaining how programs are stored and run.

Discuss how the von Neumann stored program concept and main memory (RAM) are used during the fetch-decode-execute cycle.

Your answer should include:

  • what is meant by the stored program concept
  • the role of RAM in the cycle
  • why the stored program concept is useful.
[6 marks]
Mark scheme

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)
Example answers and how they were marked
Full marks6/6

The von Neumann stored program concept means that both the program instructions and the data are stored together in the same main memory, RAM. During the fetch-decode-execute cycle, the CPU fetches an instruction from RAM, decodes it in the control unit, and then executes it. The program counter holds the address of the next instruction, and this address is sent along the address bus to RAM. RAM then returns the instruction along the data bus to the CPU. RAM is volatile, so it only holds the program while the robot is powered on. The stored program concept is useful because the robot's computer can run different programs without changing any hardware; new instructions can simply be loaded into RAM. This makes the computer general-purpose and flexible.

Why: Comprehensive understanding: defines stored program concept, explains RAM role with PC, address bus and data bus, applies to robot, and gives benefits. Sustained lines of reasoning. Level 3.

Some marks3/6

The stored program concept is when instructions and data are stored in RAM. The CPU fetches instructions from RAM. The program counter keeps the address of the next instruction. RAM holds the program. The benefit is that you can change the program without rewiring the computer.

Why: Adequate understanding: correct basic points about stored program concept and RAM, with some linkage, but little development or application to the robot's fetch-decode-execute cycle. Level 2.

Some marks1/6

The von Neumann stored program concept is that the CPU uses RAM to store the program. The fetch-decode-execute cycle fetches, decodes and executes. RAM is random access memory. The benefit is that it is fast.

Why: Basic independent points only; no real linkage or application to the robot. Level 1.

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