A full adder can be implemented in many different ways such as with a custom transistor-level circuit or composed of other gates.
12.
We can partially specialize the full adders to 2-input and, or, xor, and xnor because the L input is constant.
13.
The full adder is usually a component in a cascade of adders, which add 8, 16, 32, etc . bit binary numbers.
14.
The carry-out signal from the second full adder ( C _ 1 ) would drive the select signal for three 2 to 1 multiplexers.
15.
Breaking this down into more specific terms, in order to build a 4-bit carry-bypass adder, 6 full adders would be needed.
16.
This kind of adder is called a " ripple-carry adder ", since each carry bit " ripples " to the next full adder.
17.
The bottom level of the tree consists of pairs of 2-bit adders ( 1 half adder and 3 full adders ) plus 2 single-bit multiplexers.
18.
Note that the first ( and only the first ) full adder may be replaced by a half adder ( under the assumption that " C " in = 0 ).
19.
The XOR is used normally within a basic full adder circuit; the OR is an alternate option ( for a carry lookahead only ) which is far simpler in transistor-count terms.
20.
The total delay is two full adder delays, and four mux delays . We try to make the delay through the two carry chains and the delay of the previous stage carry equal.