A combinational circuit produces outputs determined only by its current inputs. It has no stored internal state.

Building functions from gates

Truth tables and Boolean expressions can be implemented with networks of logic gates. For example, an XOR output is true when two inputs differ.

Multiplexers

A multiplexer selects one of several input signals according to control bits. It is a general building block for choosing which data should flow to a later circuit.

Arithmetic circuits

Half adders and full adders combine XOR, AND and OR operations to add binary digits and propagate carries. Repeating these blocks produces multi-bit adders.

Propagation delay

Outputs do not change instantaneously when an input changes. Signals pass through gates with finite propagation delay, so different paths can temporarily produce intermediate values before the circuit settles.

Combinational logic can calculate and select information, but it cannot remember a previous value by itself. Memory requires feedback and sequential logic.