Common CPU components and their function ALU (Arithmetic Logic Unit) CU…

The idea

The CPU contains components that work together to run instructions. The arithmetic logic unit performs arithmetic such as addition and subtraction and logical operations such as AND, OR and comparisons. The control unit decodes instructions and sends control signals that coordinate the other components and the timing of data movement. Cache is fast memory close to the CPU that stores frequently used instructions and data to reduce slower main memory access. Registers are very small, very fast storage locations inside the CPU, such as the program counter, memory address register, memory data register, current instruction register and accumulator, each holding a current value during processing.

For example: During an addition the control unit decodes the instruction, the ALU calculates, and the result may be held in the accumulator.

What earns the marks

• States that the ALU carries out arithmetic operations and logical operations or comparisons.
• Explains that the control unit decodes instructions and sends control signals to coordinate the CPU and other components.
• Describes cache as fast memory that holds frequently used instructions and data to reduce access time to main memory.
• Describes registers as small, fast storage locations inside the CPU that hold current instructions, addresses or data during processing.
• Links the components together, for example the control unit directing data between registers while the ALU performs an operation.

Common mistakes and how to put them right

• Treating cache as the same as main memory.

  Put it right: Cache is smaller, faster and closer to the CPU, and it stores copies of frequently used data rather than the whole program.

• Saying the ALU only does arithmetic.

  Put it right: It also performs logical operations such as AND, OR and comparisons.

• Describing registers as large storage.

  Put it right: Registers are very small and very fast, holding only the values needed at that moment.

Exam tips

• Match each component to its function using the exact terms ALU, control unit, cache and registers.
• Give one clear example for the ALU, such as adding two numbers or testing whether a value is zero.
• When comparing cache and registers, mention both speed and size to show the difference.