Digital Design Fundamentals·Combinational Circuits

The 2-to-1 MUX

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Choosing between two inputs

A trapezoid 2-to-1 MUX symbol with two data inputs a and b stacked on the left, one output y on the right

A multiplexer (MUX) is a selector switch built from gates. The 2-to-1 MUX is the smallest one:

Two data inputs, a and b.
One select line, s.
One output, y.

The rule: when s = 0 the output copies a, and when s = 1 it copies b. Selection shows up everywhere in a processor, choosing which value flows next, so this block is worth knowing cold.