Data processing system with battery discharge function

A data processing system employs a rechargeable battery and supports a full discharge of the battery and is not shut down even if a user mistakenly removes a battery while the system is being operated in a full discharge mode. This is accomplished by, in a full discharge mode, the external power source 1 constantly and steadily supplying power to first electric circuitry, whilst a battery 3 supplies power to second electric circuitry until the discharge is completed. The first electric circuitry is designed to include components, such as a CPU and a main memory, that may force the entire system to be shut down if the supply of power is abruptly halted to those components. The second electric circuitry is designed to include components, such as a backlight of a liquid crystal display device, that do never cause the entire system to be shut down even when the power supply to only these components is abruptly terminated. Therefore, if a user mistakenly removes a battery while a full discharge of the battery is in progress, only the power supply to the second electric circuitry is stopped, and the whole system will not be shut down. Therefore, only a backlight is turned off at the moment a battery is removed, and this can be immediately turned on again by switching in external AC power.