Abstract: A computer includes a main processing unit having arithmetic logic, memory address control, input/output address control, and execution control. The computer also has an operating system program which includes an interrupt handler module. A variable frequency clock oscillator is disclosed which is controlled by the operating system via the execution control unit of the main processing unit. Machine level instructions in the executing program change the clock speed whenever it is necessary to keep the computer in synchronism with one of its I/O adapters or to speed up the clock when slow speed circuits are not being utilized. Clock speed is changed by the interrupt handling module. Circuits and programs, which require longer cycle times to execute properly, are placed together on interrupt levels. For example, the machine check program requiring the slowest clock speed is assigned to interrupt level 0. I/O adapters using low-speed circuitry are assigned to interrupt levels 1, 2 and 3.