Discussion
In a full adder, how many inputs are there?
Answer: D
Correct answer: 3
A full adder has three inputs - the two bits to be added and a carry-in from the previous stage - and two outputs, the sum and the carry-out. The carry-in is what distinguishes it from a half adder, which has only two inputs and cannot be used beyond the least significant bit of a multi-bit addition. Its Boolean expressions are \[ S = A \oplus B \oplus C_{in}, \qquad C_{out} = AB + C_{in}(A \oplus B), \] and it can be built from two half adders and an OR gate. The sum output is 1 when an odd number of the three inputs is 1, which is why it is a cascade of XOR gates; the carry is 1 when at least two of the three are. Chaining \( n \) full adders gives a ripple-carry adder for \( n \)-bit numbers, simple but slow because each stage waits for the carry below it. Carry-lookahead adders compute the carries in parallel to remove that delay.
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