PURPOSE: To prevent an accident that a program is interrupted halfway by setting a specific bit and enabling video blanking and hindering all input key codes and blanking the display till input of a complete password.

CONSTITUTION: When a signal NOBLANK is in the low level, a signal NOSYNC is asserted to the low level or a keyboard blanking function is started to assert a signal PWEN, and therefore, a buffer 298 is made tristate. In this state, a register 299 pulls an input BLANK of a RAMDAC 220 to the low level to start blanking. That is, when an operator writes the specific bit to start the keyboard blanking function of a video controller, screen blanking is controlled. When a password lock hot key sequence is inputted, the operator depresses a hot key to blank the display.