Build a CPU
Step 4 of 9
Full adder
Adding a carry back in
A half adder cannot be chained, because it has nowhere to put a carry coming in from the column to its right. Adding real numbers means columns, and columns pass carries along.
This one takes three inputs — A, B and a carry in — and still produces a Sum and a carry out. Chain eight of these and you can add any two bytes.
Your half adder is now a single block in the palette. Use it.
This step builds on the ones before it, and will make a lot more sense after them. Go to Logic gates
You built
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 | Cin | Sum | Cout | |
|---|---|---|---|---|---|
| 0 | 0 | 0 | 0· | 0· | |
| 0 | 0 | 1 | 1· | 0· | |
| 0 | 1 | 0 | 1· | 0· | |
| 0 | 1 | 1 | 0· | 1· | |
| 1 | 0 | 0 | 1· | 0· | |
| 1 | 0 | 1 | 0· | 1· | |
| 1 | 1 | 0 | 0· | 1· | |
| 1 | 1 | 1 | 1· | 1· |
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