Dale - Computer Science Illuminated 131

Dale - Computer Science Illuminated 131 - 104 Chapter 4...

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Multiplexer A circuit that uses a few input control signals to deter- mine which of several input data lines is routed to its output 104 Chapter 4 Gates and Circuits Figure 4.10 A full adder Carry in Logic Diagram Truth Table Sum Carry out A B 0 0 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 1 0 1 0 1 0 1 0 1 1 0 1 0 0 1 0 0 0 1 0 1 1 1 AB Carry- in Sum Carry- out values. That is, we add the sum from the half adder to the carry-in. Both of these additions have a carry out. Is it possible for both of these carry- outs to be 1, thus having a further carry? Fortunately, no. Look at the truth table for the half adder. There is no case where the sum and carry are both 1. The logic diagram and the truth table for the full adder are shown in Figure 4.10. Keep in mind that there are three inputs to this circuit: the original two digits (A and B) and the carry-in value. Thus the truth table has eight rows. We leave the corresponding Boolean expression as an exercise. To add two eight-bit values, we can duplicate a full-adder circuit eight
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This note was uploaded on 01/13/2011 for the course CSE 1550 taught by Professor Marianakant during the Fall '10 term at York University.

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