Registers and the Bus
Graham A. Lee · 1982
In combinational circuits, the outputs at any one time depend only on the inputs at that time, and the relationship between input values and output values can be described by a truth table. A sequential circuit, on the other hand, is one whose outputs depend on the sequence of past input values, as well as on the current inputs. A sequential circuit can effectively remember past input values, and use these to determine its current outputs.