Truth Tables

A truth table lists every possible input combination and the output the circuit produces for each. The left columns hold the inputs; the right column holds the output. Reading one is just looking up a row.

The number of rows is set by the number of inputs: each input doubles the row count, so two inputs give four rows, three inputs give eight, and so on. Listing the input combinations in counting order — 00, 01, 10, 11 — keeps a table easy to scan and hard to leave a gap in.

Truth tables are the bridge between intent and code. Once you can fill a table for the behavior you want, choosing the matching gate or operator in Verilog becomes a lookup rather than a guess.