Definition: half-adder

An elementary electronic circuit in the arithmetic logic unit (ALU) of the CPU that adds one bit to another. The output of the half-adder is a result of 0 or 1 and a carry of 0 or 1. Additional circuits shift the operation to the next binary digit on the left until the entire number has been added. For the history of digital circuits, see integrated circuit.

AND, OR and NOT
All circuits comprise transistors wired together in the form of Boolean logic gates. Following are the rules for their operation. All the gates have one output. Both AND and OR have two inputs while NOT has only one. See binary, Boolean logic and ALU.


   BOOLEAN LOGIC GATES

   Gate  Input 1  Input 2  Output
   (Both inputs must be present)
    AND    1         1       1
    AND    0         1       0
    AND    1         0       0
    AND    0         0       0

   (Only one input is required)
     OR    1         1       1
     OR    0         1       1
     OR    1         0       1
     OR    0         0       0

   Gate  Input   Output
   (Output is the reverse of the input)
    NOT    1       0
    NOT    0       1

Trace the Circuit Yourself

Select one of the four additions above the circuit diagram and place the two bits to be added at Input 1 and 2. Then trace the pulse (or no pulse) down the lines to the transistors. This circuit uses only seven transistors. Imagine the complexity of a chip today that contains billions of transistors in an area no larger than a postage stamp. See active area and chip.

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Definition: half-adder