Simulation method, system and program for simulating physical unit controlled by electronic control unit

In a normal operation, a physical unit simulator is allowed to speculatively perform high-speed continuous execution. Only when an actual input comes in, a speculative input and the actual input are compared with each other. Thereafter, in response to inconsistency between the inputs, the physical unit simulator is returned to a point closest to the point of the actual input and is allowed to execute a variable step module to reach the point of the actual input. Upon arrival at the point of the actual input, the simulator is shifted back to the high-speed continuous execution from there. Thus, a processing speed of the simulator can be significantly improved.