The present invention relates to a method of reducing a clock speed of a
host bus to extend battery life and its operating time when a battery is
supplying electric energy for a portable computer. A bus clock
controlling apparatus according to the present invention includes power
mode detecting means detecting a current power mode, the power mode
indicative of which power source supplies the portable computer with
electric energy; and clock adjusting means adjusting frequency of an
applied clock from a clock generator based on the detected power mode by
said power mode detecting means, and applying the frequency-adjusted
clock to one or more controlling devices. Due to this invention, an
electric energy stored in a battery equipped in a portable computer is
saved, as a result, the battery life is extended.