A digital circuit can consist of sequential logic and combinational logic. Sequential logic refers to circuits whose output depends on previous states. In other words, it involves memory that stores previous states and allows a decision to be made based on these previous states and the current input signals. In the digital realm, flip-flops are the standard devices used for storing previous logic states. In Verilog, we can define a flip-flop by using the reg command:

The above line defines an 8-bit flip-flop. Flip-flops, which are sensitive to clock transitions rather than clock logic states, are the most basic element of synchronous designs.

Combinational logic refers to a circuit that computes an output based only on the current input signals.

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