A decoder converts an n$-$bit input code into an m$-$bit output code with

n$<=$m$<=2$n such that each valid code word produces a unique output code. Below is an example of a $2-$to$-4$

line decoder, where A$1$A$0$ is a $2-$bit input code, and D$3$D$2$D$1$D$0$ is a $4-$bit output code.