This week TechARP discusses the functionality of the (old) K7 CLK-CTL Select BIOS option, you can read it over here.
As the name suggests, this is an AMD-specific BIOS feature. It controls the Clock Control (CLK_CTL) Model Specific Register (MSR) which is part of the AMD Athlon's power management control system.
Unlike the Intel Pentium 4 processor, the AMD Athlon processor saves power by actually reducing its internal clock speed. The Athlon bus clock speed remains constant but by using an internal clock divider, the Athlon processor can reduce its internal clock speed to 1/64th (Palomino cores and older) or 1/8th (Thoroughbred cores and newer) of its nominal clock speed..