iMac 12,x Turbo Overclocking
This firmware patch enables CPU Turbo Overclocking and allows setting the Max Turbo Multipliers of the CPU.
After working for some months in my own overclocking code, I found out that large parts of the Intel reference code to setup Turbo Overclocking were already included in the iMac firmware. While some parts are missing, it's way better to use this code and patch the missing bits.
The number of Turbo Bins (Intel terminology for a 100MHz multiplier) is fused at manufacture time, and the CPU will not run above its fused value.
For Sandy Bridge non-K cpus Turbo Overclocking is limited to 4 Bins above the Processor Base Frequency.
For -K CPUs the number of Bins is unlimited. For Xeon CPUs the number of Bins is 0.
Also, on standard setup the Turbo Bins depend on the number of active physical cores: 4 Bins are used when only one core is used, 3 Bins when 2 cores are used, 2 Bins when 3 cores are used and 1 Bin when all 4 cores are used.
As an example, an i7-2600 CPU with Processor Base Frequency rated at 3400 MHz will Turbo run at 3500 MHz when 4 cores are used and at 3800 MHz when just one core is used.
This patch allow independently setting the Max Core multipliers depending on the number of physical cores used. You will mostly always set the four max core multipliers to the same value.
For ease of use I have provided two version of the patch, one sets the 4 max multipliers to x40, and another that sets them to x44, otherwise they are identical. You can also look inside the patch and set your own max multiplier values.
This patch also works for non-K cpus. Setting the max multiplier to the same value on a non-K cpu will allow all four cores to run 4 Bins above the Processor Base Frequency (instead of just one Bin). This means an i7-2600 can run 4 cores at 3800 MHz instead of 3500 MHz without the patch, a nice ~8% speed increase in multi-core use.
For non-K cpus you can set the max multiplier higher than the cpu supports, but it will not run faster than the fused value, so you can use the x40 patch for non-K cpus safely.
Real world speed increase can range from around 8% for an i7-2600 when using 4 cores, up to above 20% when using an i7-2700K. Higher speed ram may also help there.
I have also tried to delay setup of the max multipliers after bios init, to see if they could be setup using an Opencore module, but for some reason Intel put a watchdog timer that resets the PCH if all power management of the CPU is not done early enough after boot. This means the max multipliers have to be setup at a very precise moment during EFI boot.
This firmware patch enables CPU Turbo Overclocking and allows setting the Max Turbo Multipliers of the CPU.
Background
After working for some months in my own overclocking code, I found out that large parts of the Intel reference code to setup Turbo Overclocking were already included in the iMac firmware. While some parts are missing, it's way better to use this code and patch the missing bits.
How it works
Turbo Overclocking allows the CPU to reach core multipliers above its rated Processor Base Frequency.The number of Turbo Bins (Intel terminology for a 100MHz multiplier) is fused at manufacture time, and the CPU will not run above its fused value.
For Sandy Bridge non-K cpus Turbo Overclocking is limited to 4 Bins above the Processor Base Frequency.
For -K CPUs the number of Bins is unlimited. For Xeon CPUs the number of Bins is 0.
Also, on standard setup the Turbo Bins depend on the number of active physical cores: 4 Bins are used when only one core is used, 3 Bins when 2 cores are used, 2 Bins when 3 cores are used and 1 Bin when all 4 cores are used.
As an example, an i7-2600 CPU with Processor Base Frequency rated at 3400 MHz will Turbo run at 3500 MHz when 4 cores are used and at 3800 MHz when just one core is used.
This patch allow independently setting the Max Core multipliers depending on the number of physical cores used. You will mostly always set the four max core multipliers to the same value.
For ease of use I have provided two version of the patch, one sets the 4 max multipliers to x40, and another that sets them to x44, otherwise they are identical. You can also look inside the patch and set your own max multiplier values.
This patch also works for non-K cpus. Setting the max multiplier to the same value on a non-K cpu will allow all four cores to run 4 Bins above the Processor Base Frequency (instead of just one Bin). This means an i7-2600 can run 4 cores at 3800 MHz instead of 3500 MHz without the patch, a nice ~8% speed increase in multi-core use.
For non-K cpus you can set the max multiplier higher than the cpu supports, but it will not run faster than the fused value, so you can use the x40 patch for non-K cpus safely.
Expected speed increase
Sandy Bridge is one of the most overclock-friendly CPUs intel made, the -K CPUs can reach above 5GHz when properly cooled and setup. In an iMac 12,2 with standard cooling 4x44 up to 4x46 speeds have been tested and proved stable without the need of overvolting the CPU (We are lucky that the iGPU is not used in the iMac).Real world speed increase can range from around 8% for an i7-2600 when using 4 cores, up to above 20% when using an i7-2700K. Higher speed ram may also help there.
Possible improvements
While setting the max multipliers at firmware level is not ideal, it would be possible to develop an UEFI firmware module that reads values from NVRAM and sets them at runtime, but I currently have no time for it.I have also tried to delay setup of the max multipliers after bios init, to see if they could be setup using an Opencore module, but for some reason Intel put a watchdog timer that resets the PCH if all power management of the CPU is not done early enough after boot. This means the max multipliers have to be setup at a very precise moment during EFI boot.
Notes
Many thanks to all people who have helped testing this!!!- Tested on iMac 12,2 with bootrom 87.0.0.0.0.
- May work on other macs with Sandy Bridge CPUs, pending to be tested.
- Not all CPUs are made the same. To be sure yours is stable after patching, run a stress/torture test, using Prime95 or similar, around one hour to be sure the iMac does not hang. Keep an eye on temperatures, according to Intel TJuncMax for Sandy Bridge is 98°C, so I'd try to keep temps in the low 80s at most.
How to apply patch
I have started a Github repository to better keep track of firmware patches, this patch is there.
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