Multiplexer ($\mathrm{mux}(s,a,b)=(\neg s\wedge a)\vee (s\wedge b)$)

A selector that routes one of several inputs to its output according to control bits. The displayed two-input version selects $a$ when $s=0$ and $b$ when $s=1$. A word selector applies the same control bit to parallel one-bit multiplexers; its circuit size grows with the word width while its added circuit depth remains constant.