Edexcel GCSE Computer Science (1CP2) Exam Questions

• A Blu-ray Disc (1)
• The only correct answer is A. The correct answer is Blu-ray Disc.
• B is not correct because a CD stores less data than a Blu-ray Disc, up to about 700 MB
• C is not correct because a DVD stores less data than a Blu-ray Disc, up to about 4.7 GB (single-layer) or 8.4 GB (dual-layer)
• D is not correct because magnetic tape is not an optical storage medium

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

• It is a computer/processor built into a larger device (1) to carry out a single dedicated task/function (1)
• It is a combination of a processor, memory and input/output in one unit (1) that controls the device it is part of (1)
• It is a computer that forms part of another device (1) so it is not used as a general purpose computer (1)

Allow 'forms part of another device' or 'inside another device' for the first mark.

Do not award the second mark for a separate fact that is not linked to the first point, e.g. 'It is built into a device and it uses little power' would earn only the first mark (1).

Do not accept 'a small computer' on its own, with no reference to being part of another device or performing a dedicated task.

Do not accept 'a computer that controls a device' on its own, with no reference to it being built into/part of a larger device or carrying out a dedicated task.

Do not accept 'it performs a single dedicated task' on its own, with no reference to it being built into/part of a larger device.

Do not accept descriptions of a general purpose computer, e.g. 'it can run many different programs' or 'new software can be installed'.

Do not accept 'it is a system that is embedded in another system' as this repeats the term in the question.

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

• Carries out arithmetic operations/calculations, such as addition or subtraction (1) so that the result can be stored in a register (1)
• Carries out logical operations, such as comparing two data items (1) to see which one is greater/larger (1)
• Compares two data items (1) to decide whether a condition is true / which path a program takes next (1)

Allow the result of an arithmetic or logical operation being held/stored in a register, but only where the operation is named.

Do not accept answers that say the ALU stores data or results without naming an operation (registers/main memory store data).

Do not accept answers about controlling components, decoding instructions or timing (that is the control unit or the clock).

Do not award credit for 'it processes data' on its own; an arithmetic or a logical operation must be named.

An example on its own, e.g. 'it adds two numbers', does not earn a second mark; the second mark needs a linked development.

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

• Decodes/interprets/translates the instruction (1) to work out what the instruction is/what operation is required (1)
• Sends control signals to the other components (1) so that the correct component carries out the instruction (1), e.g. the ALU/main memory
• Controls the fetch of the instruction from main memory (1) because the address of the next instruction is sent via the address bus (1)
• Controls the fetch of the next instruction from main memory (1) by sending a control signal/read signal to main memory (1)

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

For the decode point, 'decodes the instruction' alone gains 1 mark; a second mark needs the purpose of decoding, i.e. working out what the instruction is/what operation is required.

Do not accept 'it controls the CPU' on its own, without naming a task in the cycle.

Do not accept 'it fetches the instruction from memory' on its own, as this describes the fetch step rather than the control unit's role; credit only if linked to controlling/signalling the fetch.

Do not accept carrying out arithmetic/logical operations, as these are carried out by the ALU, or holding the address of the next instruction, as this is the program counter.

Do not accept storing the instruction in a register (e.g. MAR/MDR), as this is not the role of the control unit.

Indicative content

The role of the CPU and registers

• The control unit decodes the instruction fetched from main memory and controls the other components during the cycle.
• The ALU carries out arithmetic and logical operations during the execute stage, such as adding two values held in registers.
• Registers hold the data, addresses and results that the CPU is working on during the cycle.
• The program counter holds the address of the next instruction to be fetched, so it is incremented after each fetch.
• The memory address register holds the address of the memory location that is about to be read or written.
• The memory data register holds the instruction or data that has been fetched from or is being written to main memory.

The role of the address bus, data bus and control bus

• The address bus carries the address of the memory location that the CPU needs to access from the CPU to main memory.
• The address bus is one-way, so addresses are only sent from the CPU to main memory.
• The data bus carries instructions and data between the CPU and main memory, so it is bi-directional.
• The width of the data bus affects how many bits can be transferred at one time.
• The control bus carries control signals such as read and write signals, so main memory knows whether the CPU wants to read an instruction or write data.
• The control bus carries signals from the control unit to coordinate the components during the fetch-decode-execute cycle.

The role of the clock

• The clock sends regular pulses that synchronise the actions of the CPU and main memory.
• Each clock pulse triggers the next step of the fetch-decode-execute cycle, so the stages happen in the correct order.
• The clock provides a regular timing signal so that the fetch-decode-execute cycle repeats at a steady rate.
• The clock is separate from the control bus; the control bus carries control signals, while the clock provides timing pulses.
• Do not credit the address bus storing addresses, the data bus storing data, or the control bus controlling the computer without naming a control signal; do not credit clock speed affecting performance as a role in the cycle.
• Level 3 responses must cover all three bullets and link them to the fetch-decode-execute cycle; Level 2 answers may identify roles but link them only partly to 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)