Edexcel GCSE Computer Science (1CP2) Exam Questions

• A To store data and programs permanently (1)
• The only correct answer is A. The correct answer is to store data and programs permanently.
• B is not correct because storing the data and instructions currently in use is the role of primary storage/RAM, which is volatile, not the role of secondary storage
• C is not correct because calculations on data are carried out by the CPU, not by secondary storage
• D is not correct because secondary storage is not directly accessible by the CPU; data must first be copied into RAM

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.

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.