In a controller which generates a clock to be output to a CPU on the basis of a fundamental clock, and incorporates control logic circuits such as DRAM control, CPU cycle control, and the like, the internal operation clock is switched to either an internally generated clock or a clock obtained...http://www.google.de/patents/US5774699?utm_source=gb-gplus-sharePatent US5774699 - System controller for controlling switching operations of various operation clocks for CPU, DRAM, and the like 
