Logic simulation is the use of a computer program to simulate the operation of a igital circuit. Logic simulation is the primary tool used for verifying the logical correctness of a hardware design. In many cases logic simulation is the first activity performed in the process of taking a hardware design from concept to realization. Modern hardware description languages are both simulatable and synthesizable. Designing hardware today is really writing a program in a hardware description language. Performing a simulation is just running that program. When the program (or model) runs correctly, then one can be reasonably assured that the logic of the design is correct, for the cases that have been tested in the simulation.