Guys, I'm done with CoolBook which has not been updated for a while and the latest version 2.14 doesn't support my Mac Mini 2.26. So I decided to look around to see if there is any open source SpeedStep driver. And I found xnu-speedstep. It was stated that undervolt function was implemented but I find that the kext has some problem remembering the user defined voltage. I decided to modify the kext FOR MY OWN CPU. I ran CPUTest to load up the CPUs and undervolted from 1.175V to 0.975V and let it run for a while. 0.95V would give kernel panic immediately. Then I rewrote the Auto-throttle section to disable frequency stepping and fix the frequency at the highest and undervolt. Basically everytime the timer runs it checks if the voltage and frequency is correct and recorrect if neccesary. The compiled kext would 1) fix the CPU at the highest frequency 2) fix the voltage at 0.975V At full load the power consumption is lowered from 14W to just 9W. If you are certain your CPU can run at 0.975V, try using it at your own risk. If not, edit the attached XCode project. The kext doesn't give consistent readings via sysctl. The debugger message loads the register directly from the CPU and confirms the buggy sysctl interface. Here is how the hex digits are read: 815 in binary is 1000 0001 0101 First 4 bits represent the frequency ID (multiplier). You would notice that the kext truncates the half multiplier. FID = 8 8 X 266 = 2128 Next 8 bit represent the voltage ID VID = 21 Voltage = 21 X 125 + 7125 = 10000 --> 1000mV This is basically it. I could not think of anything else to post here at the moment.