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Full adder[ edit ] Logic diagram for a full adder. Full adder in action. A full adder gives the number of 1s in the input in binary representation. Schematic symbol for a 1-bit full adder with Cin and Cout drawn on sides of block to emphasize their use in a multi-bit adder A full adder adds binary numbers and accounts for values carried in as well as out. A one-bit full-adder adds three one-bit numbers, often written as A, B, and Cin; A and B are the operands, and Cin is a bit carried in from the previous less-significant stage.
The circuit produces a two-bit output. A full adder can be implemented in many different ways such as with a custom transistor -level circuit or composed of other gates. In this implementation, the final OR gate before the carry-out output may be replaced by an XOR gate without altering the resulting logic. Using only two types of gates is convenient if the circuit is being implemented using simple integrated circuit chips which contain only one gate type per chip.
NOR Full adder A full adder can also be constructed from two half adders by connecting A and B to the input of one half adder, then taking its sum-output S as one of the inputs to the second half adder and Cin as its other input, and finally the carry outputs from the two half-adders are connected to an OR gate.
The sum-output from the second half adder is the final sum output S of the full adder and the output from the OR gate is the final carry output Cout.
The critical path of a full adder runs through both XOR gates and ends at the sum bit s. Assumed that an XOR gate takes 1 delays to complete, the delay imposed by the critical path of a full adder is equal to T.
74LS83 - 74LS83 4-bit Binary Full Adder Datasheet
7483 - 7483 4-bit Full Adder Datasheet