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Half-Adder:
A combinational circuit which adds two one-bit binary numbers is called a half-adder.
o The sum column resembles like an output of the XOR gate.
o The carry column resembles like an output of the AND gate.
Limitations (disadvantages) of half-adder:
o In multi-digit addition we have to add two bits along with the carry of previous digit addition. Such addition requires addition of 3 bits. This is not possible in half-adders.
Full Adder:
o In a full adder, three bits can be added at a time. The third bit is a carry from a less significant column.
Alternate Representation of Full-Adder:
Parallel Binary Adder
o While we add the LSB, there is no carry to be handled and thus, only a half adder is used for this stage.
o For any other bit addition, there may be a carry from the preceding stage and thus, full adders are used for these stages.
Example: