A flicker filter is adjusted according to degree of alpha blending performed on a display signal. The hardware verification system receives a circuit design and generates a variable period clock from the system clock by analyzing propagation delays in different signal paths of the circuit design. When the current capability of the MOS transistor is reduced owing to a reduction of the power supply voltage, an adjustment is carried out automatically to lower a threshold voltage of the MOS transistor and thus the operating speed can be compensated. Left-to-right curved surfaces facing contiguously along both sides of the curved surface of the left-to-right facing boss are provided on the left-to-right walls of the knob fixture. The sequence valve is disposed in a piston that is axially movable within the tool.