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Computer organization Notes

Full Subtractor using 1:8 Demultiplexer

As similar to the multiplexers, de-multiplexers are also used for Boolean function implementation as well as combinational circuit design. We can design the de-multiplexer to produce any truth table output by correspondingly controlling the select lines. Consider the case for implementing a de-multiplexer circuit in order to produce the full subtractor output. The truth table below shows the output of a full subtractor.

From the above table, the full subtractor output D can be written as
D = f (A, B, C) = ∑m (1, 2, 4, 7)
And the borrow output can be expressed as
Bout = F (A, B, C) = ∑m (1, 2, 3, 7)
From these Boolean functions, a de-multiplexer for producing full subtractor output can be built by properly configuring the 1-to-8 DEMUX such that with input D=1 it gives the min-terms at the output. And by logically ORing these min-terms, the outputs of difference and borrow can be obtained as shown in figure.

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