Every operation inside a processor is built from logic gates: tiny circuits that take binary inputs and give one binary output by a fixed rule. Learn the six standard symbols, define what each gate does, and construct the truth table for each one.
A logic gate takes binary inputs (0 or 1) and produces one binary output. NOT has one input and inverts it; in 9618 every other gate has exactly two inputs. AND outputs 1 only if both inputs are 1; OR outputs 1 if at least one input is 1; XOR (EOR) outputs 1 if the inputs are different. NAND and NOR are AND and OR with the output inverted, shown by a small circle on the symbol. A truth table lists the output for every combination of inputs: 2 rows for one input, 4 for two, 8 for three.
Pick a gate in the lab and switch its inputs on and off. The output lamp lights for 1. Check every row of each truth table against the lab.
| A | X = NOT A |
|---|---|
| 0 | 1 |
| 1 | 0 |
| A | B | A AND B | A OR B |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 0 | 1 | 0 | 1 |
| 1 | 0 | 0 | 1 |
| 1 | 1 | 1 | 1 |
Draw the shapes carefully: AND has a straight back, OR a curved back. A NOT gate must have its circle; a triangle alone is a buffer, not a NOT gate.
NAND means NOT AND, and NOR means NOT OR. Work out AND or OR first, then invert every output. The circle on the symbol is the inversion.
| A | B | A AND B | A NAND B | A OR B | A NOR B |
|---|---|---|---|---|---|
| 0 | 0 | 0 | 1 | 0 | 1 |
| 0 | 1 | 0 | 1 | 1 | 0 |
| 1 | 0 | 0 | 1 | 1 | 0 |
| 1 | 1 | 1 | 0 | 1 | 0 |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which gate outputs 1 only when both of its inputs are 0?
In the lab, an XOR gate has both input switches on. What is the output?
What does the small circle on the output of a NOR gate symbol show?
How many rows does the truth table for a circuit with three inputs need?
XOR (exclusive OR, also written EOR) is like OR, except that it outputs 0 when both inputs are 1. Its symbol is the OR shape with an extra curved line behind it. Compare OR and XOR row by row in the lab.
| A | B | A OR B | A XOR B |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 0 | 1 | 1 | 1 |
| 1 | 0 | 1 | 1 |
| 1 | 1 | 1 | 0 |
Learn the output columns in the order 00, 01, 10, 11: AND 0001, OR 0111, NAND 1110, NOR 1000, XOR 0110. Writing rows in binary counting order means you never miss a combination.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
For inputs (A, B) in the order 00, 01, 10, 11, which output column belongs to a NAND gate?
A gate has two inputs. Its output is 1 when the inputs are different and 0 when they are the same. Which gate is it?
Which statement about the gates in the 9618 syllabus is correct?
An OR gate and a NOR gate both receive inputs A = 1 and B = 0. What are the two outputs?
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