Arithmetic Logic Unit (ALU)

Computer ScienceOCRGCSEUnit: 3.4 Computer Systems
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The basics

The Super Calculator

The Super Calculator

Imagine a calculator that can not only do maths (5+3, 10×2) but can also make decisions ("Is 7 greater than 5?", "Are both conditions true?"). That's the Arithmetic Logic Unit. Every time your computer adds numbers in a spreadsheet, checks if a password is correct, or decides whether to run an IF statement - the ALU is doing the work. It's called the "calculator of the CPU" but it's so much more powerful than that!
What does ALU stand for?: Arithmetic Logic Unit
Key terms

Computer Science glossary

What does ALU stand for?
Arithmetic Logic Unit
Spotlight
Deep Dive: Why Logic Operations Matter

Arithmetic is only half the story. The "L" in ALU stands for Logic, and this is what makes computers smart! Logic operations allow computers to make decisions: "IF password is correct AND user has permission THEN allow access." Without logic operations, computers would just be expensive calculators!

Exam tip

Earn the mark scheme marks

Exam Tips - The ALU

Always mention BOTH arithmetic AND logic:

  • When asked "What does the ALU do?" → "Performs arithmetic AND logic operations"
  • Don't just say "does calculations" - you'll lose marks!
  • It's called Arithmetic Logic Unit - both words matter

Give specific examples:

  • Arithmetic: addition, subtraction, multiplication, division
  • Logic: AND, OR, NOT operations
  • Comparisons: equals, greater than, less than

Remember where results go:

  • ALU results are stored in the Accumulator (ACC)
  • This happens during the EXECUTE stage
  • The result overwrites the previous ACC value

Now try it yourself

Quiz · Question 1 of 15

What does ALU stand for?

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