Systems architecture: arithmetic logic unit

The idea

The arithmetic logic unit, usually shortened to ALU, is the part of the central processing unit where calculations and logical decisions are carried out. 'Arithmetic' covers numerical operations such as addition and subtraction; 'logic' covers comparisons and Boolean operations such as AND, OR and NOT, which underpin decisions like testing whether one value is greater than another. The ALU works on data held in registers and returns a result, and it sets status flags that record outcomes such as a zero result. It does not store programs, fetch instructions or hold the main memory; those belong to other parts of the system. In a multiple-choice question, look for the option describing where arithmetic and logical operations are performed, and reject options describing control, storage or input/output.

What earns the marks

• The ALU is part of the CPU where arithmetic operations such as addition and subtraction are performed.
• The ALU also performs logical operations such as comparisons and Boolean operations.
• The ALU operates on data and produces a result used elsewhere in the processor.
• The ALU does not store programs or act as main memory.
• An option describing control, fetching or storage rather than arithmetic and logic is incorrect.

Common mistakes and how to put them right

• Confusing the ALU with the control unit.

  Put it right by saying the ALU performs arithmetic and logic while the control unit coordinates instruction execution.

• Believing the ALU stores data permanently.

  Put it right by noting it processes data and returns results rather than acting as storage.

• Thinking the ALU only does arithmetic.

  Put it right by including logical operations such as comparisons and Boolean operations.

Exam tips

• Look for the option that names both arithmetic and logical operations.
• Reject options that describe storage, fetching or overall control of the processor.
• Link the term to a concrete example, such as adding two numbers or testing whether a value is greater than another.