Build a CPU
Step 3 of 9
Half adder
Where arithmetic starts
Add two single bits and there are only four sums: 0+0=0, 0+1=1, 1+0=1, and 1+1=10. That last one needs two digits, which is why this circuit has two outputs.
Sum is the digit you write down. Carry is the one you carry over — exactly what you do adding 7+5 on paper.
You have XOR now, because you built it.
This step builds on the ones before it, and will make a lot more sense after them. Go to Logic gates
Tap a part to add it. Tap an output pin, then the part it feeds. Tap a switch to flip it.
Something has an unconnected input. Every input pin needs a wire before the circuit can run.
| A | B | Sum | Carry | |
|---|---|---|---|---|
| 0 | 0 | 0· | 0· | |
| 0 | 1 | 1· | 0· | |
| 1 | 0 | 1· | 0· | |
| 1 | 1 | 0· | 1· |
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