If we did not report procedures or information related to CPU overclock since the Intel CPU transition, it does mean we stopped investigating new possibilities.
Of course with Apple adopting the EFI, it is more difficult to modify CPU or FSB voltage; however one can still overclock Mac Pro Xeon via Windows as shown below. You can simply use SysTool allowing you to directly modify the clock generator:
SysTool can even force the clock generator to change its frequency without rebooting. As shown below, the Mac Pro generator is identified as an ICS932S401.
With SysTool and Apple memory modules we successfully overclocked the FSB from 333 x 4 (1333MHz) to 380 x 4 (1521MHz).
The test machine, a Mac Pro powered by two Dual Core Xeon clocked at 3.0GHz, is now an overclocked to 3.424GHz. The performance gain is proportional to the overclocked Xeon.
With Dane-Elec memory modules, we could push the FSB a bit further to 389 x 4 MHz, Xeon are then overclocked to 3.5GHz.
With both Apple and Dane-Elec modules, the overclocked limit is defined by the memory modules. Indeed, as indicated by SysTool, the CPU - Memory ratio is set to 1:1, and we did not find ways to desynchronize them.
However Apple seemed to have found ways to prevent users to perform such software-based overclock. Indeed when rebooting the overclocked Mac Pro, while the new frequency is saved in a volatile register, OSX refuse to boot and deliver a memory alarm. To bring it back to life, you simply need to switch it off and on.
It is currently not possible to perform such software-based overclock via OS X. The application Reggie SE from the CHUD Tools could have done it, but it has been locked by Apple, and if you attempt to modify registers it immediately generate Kernel Panics.
One will need to look further, or investigate other strategies. The Linux command line i2cget might be of help? We might need a way to directly access the clock generator ICS932S401 in OS X.
If you want to join our effort, do not hesitate to react on our forum: hardmac forum.
[translation by Linathael]
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