Other P-State Features

Super Low Frequency Mode

This technology is only implemented in mobile Core 2 Duo processors (Merom core). In the normal low-frequency mode, the minimum clock ratio supported by Intel is 6x. If the Front Side Bus base clock is 200 MHz, then the minimum CPU clock speed would be 1.2 GHz. Intel wants the clock speed to go even lower, as power consumption is directly related to (voltage² x frequency).

Since a new PLL design isn't a good idea, Intel reduces the Front Side Bus speed to to only 100 MHz. This forces the CPU to run at only 600 MHz with a lower core voltage than the normal low frequency mode. Desktop Conroe-core processors cannot use this mode because it requires chipset support.

Combining CPU C-state & P-state

Imagine you are only running WinAmp using your Core 2 Duo E6300 processor. The first core would be processing the equalizer band but the second core will be doing nothing at all. If C-state and P-state are both enabled on this processor, the first core will go into its lowest P-state (P1), while the second core will go into the C1 halt state.

If you recall details from the previous pages, the C1 state does not cause the CPU to run at lower clock speed. Therefore, when the first core runs at 1.6 GHz @ 1.225 V, the second core in C1 state will request for 1.83 GHz @ 1.25 V. Since we only have one PLL and Vcore in the Core 2 Duo processor, it will be forced to run at 1.83 GHz @ 1.25 V, even though WinAmp requires very little processing power!

To counter this problem, the C1E (C1 Enhanced state) was introduced to allow the C1 state to associate with the P-state ratio/voltage table. When C1E is enabled, the Core 2 Duo processor can now run both cores at 1.60 GHz @ 1.225 V. C1E has some benefits over traditional C1 state :

• It allows the sleeping core to follow the active core frequency and core voltage.
• The power consumption of processor with C1E enabled is lower than with C1 enabled, because power is directly related to (voltage² x frequency).
• The wake-up time from certain C1E to C0 states improves, so clock and voltage restore can restore faster.

In upcoming Core 2 Duo steppings and future-generation processors, C2E and C3E may be implemented to obtain the three benefits mentioned above. C4, C5 and C6 may not implement the enhanced state because most of the functional units in CPU have their clocks gated and the processor itself is using a lower core voltage compared to the lowest Vcore supported by the minimum P-state.

Incidentally, Intel has branded its P-state management as the Enhanced Intel Speedstep Technology (EIST), but the C1E feature doesn't have a special name. AMD lumped both P-state and C1E together and called it Cool n Quiet.

I noted a good response in the forum thread :

“EIST also works the same way when C1E is disabled in the BIOS. C1E must be seen independently of EIST and there are CPUs by Intel which only have C1E but no EIST”

Let’s view it this way. EIST was originally a mechanism to transition the CPU into various frequency / voltage pairs in the processor active state (C0 state). We can see this in the Pentium 4 family. However, EIST has evolved in Core 2 Duo processor, where this piece of logic can work even in other processor power states, such as C1 and C2.

When Intel says EIST is disabled but C1E is enabled, this means the processor will not change its clock speed and voltage when it is executing instructions, no matter whether it is a 10% load or a 100% load. It will change its frequency and voltage only when it starts sleeping (C1, C2 and etc.).

CPU Thermal Monitor

The main purpose of the thermal monitor is to decrease the processor power consumption when it is running too hot. A few years back, THG demoed this functionality when Pentium 4 was introduced. When the CPU cooler was unplugged in the middle of a game, the Pentium 4 processor did not burn up but caused the game to slow down tremendously.

Basically, there are two types of thermal-throttling mechanisms - one managed by the CPU itself and another managed by the ICH (chipset). The CPU thermal-throttling mechanism is better because it is fast and efficient, compared to thermal throttling by the ICH. Besides, the CPU supports two TM states, while the ICH only supports one.