The CPU contains an arithmetic logic unit (ALU).
Describe the purpose of the arithmetic logic unit (ALU).
Mark scheme
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.
Example answers and how they were marked
The ALU carries out arithmetic operations such as addition and subtraction (1), and the result of the calculation is stored in a register (1).
Why: Linked point (1) for arithmetic operations with an example, and development (1) for the result being stored in a register.
It does all the calculations in the CPU, for example adding two numbers together.
Why: Point (1) for arithmetic operations/calculations named with an example; no linked development, so the second mark is not earned.
It stores the results of calculations so that they can be used by the CPU later.
Why: Storing results with no arithmetic or logical operation named, which the 'Do not accept' line rules out, so no mark is earned.