This topic summary covers Knowledge Organiser: Arithmetic Logic Unit (ALU) within Arithmetic Logic Unit (ALU) for GCSE Computer Science. Revise Arithmetic Logic Unit (ALU) in 3.4 Computer Systems for GCSE Computer Science with 15 exam-style questions and 12 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 7 of 7 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Knowledge Organiser: Arithmetic Logic Unit (ALU)
Key Terms
- ALU: Arithmetic Logic Unit — the component that performs all calculations and logical decisions
- Arithmetic operation: Calculation involving numbers: addition, subtraction, multiplication, division
- Logic operation: Boolean operation: AND, OR, NOT, XOR
- Comparison operation: Evaluates relationships: equals, greater than, less than
- Accumulator (ACC): Register that stores the result of every ALU operation
Must-Know Facts
- The ALU performs three types of operations: arithmetic, logic, and comparison
- Always say "arithmetic AND logic operations" — not just "calculations"
- All ALU results are stored in the Accumulator (ACC)
- The ALU is activated during the EXECUTE stage of the FDE cycle
- The Control Unit tells the ALU which operation to perform
- Comparison operations return TRUE (1) or FALSE (0) stored in the ACC
Key Concepts
- Arithmetic examples: total = price + tax; discount = price - saving
- Logic examples: IF door_closed AND start_pressed THEN run_microwave
- Comparison examples: IF score > high_score THEN new_record = TRUE
- Flow: Control Unit sends instruction to ALU → ALU performs operation → result stored in ACC
Common Mistakes
- Saying the ALU only does arithmetic: The ALU performs three types of operations — arithmetic (add, subtract), logic (AND, OR, NOT), AND comparison (equals, greater than) — omitting logic and comparison operations loses marks
- Confusing the ALU with the Control Unit: The Control Unit directs the operation of the CPU and sends instructions to the ALU; the ALU actually carries out the calculations — they have distinct roles
- Forgetting the Accumulator stores ALU results: Every ALU operation result is stored in the Accumulator (ACC) register — saying results go "back to RAM" or "to the CPU" is incorrect
- Placing ALU activity in the wrong FDE stage: The ALU is active during the EXECUTE stage, not the Fetch or Decode stages — always connect ALU work to the Execute phase
- Describing comparison operations as "making decisions": The ALU evaluates a comparison and returns 1 (true) or 0 (false) — it is the Control Unit that then acts on that result; the ALU itself does not "decide" anything
Practice questions for Arithmetic Logic Unit (ALU)
What does ALU stand for?
Explain how the role of the ALU differs from the role of the Control Unit (CU).
Quick recall flashcards
What does ALU stand for?
Arithmetic Logic Unit
Where are ALU results stored?
Accumulator (ACC)