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Logic gates and truth tables
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At the lowest level, computers make decisions with logic gates. Three gates are the building blocks.
AND, OR, NOT
AND: output is 1 only if both inputs are 1.
OR: output is 1 if at least one input is 1.
NOT: has one input and flips it: 1 becomes 0, 0 becomes 1.
OR: output is 1 if at least one input is 1.
NOT: has one input and flips it: 1 becomes 0, 0 becomes 1.
Truth tables
A truth table lists every possible input and the output.
Combining gates
Work through combined expressions one step at a time, like brackets in maths.
NOT (A AND B) with A = 1, B = 1: A AND B = 1, so NOT 1 = 0.
NOT (A AND B) with A = 1, B = 1: A AND B = 1, so NOT 1 = 0.
Find (A OR B) AND C when A = 0, B = 1, C = 1.
- Brackets first: A OR B = 0 OR 1 = 1.
- Then 1 AND C = 1 AND 1 = 1.
Answer: 1
AND needs both inputs to be 1. OR needs at least one. NOT flips the input. Work out combined expressions bracket by bracket.
The interactive lesson includes the diagrams for this topic.
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