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