Interruption signal generating apparatus

A computer system comprising:

• an interruption signal generating apparatus that receives a first clock and a second clock and outputs an interruption signal for requesting an execution of an interrupting process at a time interval corresponding to a frequency obtained by dividing in frequency the second clock, wherein the first clock is provided according to an operation mode of the computer system, and the second clock has a period longer than that of the first clock, is used to generate the interruption signal, and is provided irrespective of the operation mode of the computer system, the interruption signal generating apparatus including:
• a counter unit that counts the time interval based upon the second clock and outputs a count-up signal indicating an end of the counting,
• a first generating unit that detects the end of the counting indicated by the count-up signal while the first clock is being provided and generates a first interruption signal indicating the detection according to the first clock,
• a second generating unit that detects the end of the counting indicated by the count-up signal while the first clock is not being provided and generates a second interruption signal indicating the detection according to the second clock, and
• a selecting unit that selectively outputs the first interruption signal and the second interruption signal according to the operation mode of the computer system;
• a central processing unit that stops the supply of the first clock in accordance with a predetermined program while the first clock is being supplied;
• and
• a system controller that starts the supply of the first clock in response to the interruption signal from said interruption signal generating apparatus while the first clock is not provided.