NOTE: This guide is somewhat outdated- in order to enable SpeedStep for Core i5 750 and Core i7 860, all you need to do is use iMac11,1 smbios.plist. It's available in Multibeast.
By adding correct SSDT.aml code to your DSDT, you can enable the native Intel SpeedStep technology of your CPU in Mac OS X. According to Wikipedia, "SpeedStep is a trademark for a series of dynamic frequency scaling technologies (including SpeedStep, SpeedStep II, and SpeedStep III) built into some Intel microprocessors that allow the clock speed of the processor to be dynamically changed by software. This allows the processor to meet the instantaneous performance needs of the operation being performed, while minimizing power draw and heat dissipation."
This is not a necessary fix, but you can gain noticeable power savings and/or performance gains. This fix may also extend the life of your hardware by enabling OS X to use advanced features that your Intel CPU was made for.
I added a new set of edits to the tonymacx86-DSDT-FIXES.dsl with speedstep information for the Core i5 750 and Core i7 860. Simply download the updated tonymacx86-DSDT-FIXES.zip, search for SpeedStep, and you'll find code additions for your CPU. You can copy and paste the CPU information from there. The example P55M-UD2 DSDT included has SpeedStep fixes, as well as every other edit that is referenced.
If you're feeling adventurous, you can check the values of your own CPU and make the edits yourself. The easiest way to view this information is to-open DSDTSE, go into the sidebar, and choose SSDT.aml, For example, the edits are made as in the pictures below. The left hand code is from the UD2 SSDT.aml, and the fixed code is on the right. You can then see the values to use when editing. Use the DSDT-FIXES guide for the rest of the code block.
MSR Tools. In order for it to run, you must start up Snow Leopard in 32-Bit mode. Restart your computer, pause the Chameleon countdown and type arch=i386. Download it here.
WARNING: Improper use of SpeedStep code in your DSDT can cause major problems with your system including hardware damage and failure. Again, you do not need to do this fix for a stable system. If you're having any issues, take the SpeedStep code out immediately and reboot. Also, do NOT overclock with SpeedStep- it's known to cause performance decreases. Use with care!
Special thanks to the Insanelymac P55 Thread for the Core i7 edits!
SPEEDSTEP EXAMPLES: Due to popular demand, here is a zip of 2 versions of the GA-P55M-UD2 DSDT- 1 with Core i5 750 SpeedStep and the other with i7 860 SpeedStep. Use these as an example to modify your own DSDT, or if you have a GA-P55M-UD2, you can use this, but be sure to use the correct version! Have fun!
tonymacx86 DSDT-P55M-UD2 - SpeedStep