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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. All circuits comprise transistors wired together in the form of AND, OR and NOT Boolean logic gates. See binary, Boolean logic and ALU.


   Gate  Input 1  Input 2  Output
    AND    1         1       1
    AND    0         1       0
    AND    1         0       0
    AND    0         0       0

     OR    1         1       1
     OR    0         1       1
     OR    1         0       1
     OR    0         0       0

   Gate  Input   Output
    NOT    1       0
    NOT    0       1

Trace the Circuit Yourself
Select one of the four binary additions above and place the two bits at Input 1 and 2. Then trace the pulse (or no pulse) down the lines to the transistors. If the transistor is pulsed, the switch closes and the source current flows through. At the NOT gate, the source current normally flows through the transistor unless pulsed, at which time it opens and impedes the current. This circuit shows the interconnections of 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.