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It is important to stress that the stability during the switch to this mode and back is granted by the gradual transition to the lower working frequency. When the CPU switches to power-saving mode, the clock frequency multiplier is at first reduced to 14x and then the Vcore is little by little dropped down to the appropriate value. When the CPU switches back from the power-saving mode to normal, the same steps are undertaken in the reverse order: at first the core voltage increases and then the multiplier is set to the nominal value.

All three Demand Based Switching technologies, C1E, TM2 and EIST, use this power-saving mode. But they differ by the moments this mode gets enabled.

C1E (Enhanced Halt State) Technology activated power-saving mode when the CPU received the Halt command, which indicates the wait state for the processor. The OS sends this command to the CPU, when there are no other instructions to be performed. In other words, Pentium 4 processors used to switch their execution units into idle mode when they received the Halt command. Now the C1E mode allows reducing the heat dissipation and power consumption even more due to the additional reduction in the CPU clock frequency in this case.

TM2 (Thermal Monitor 2) Technology is intended to protect the CPU against overheating, but uses the same 2.8GHz power-saving mode. TM2 switches the CPU into this mode upon a special command received by the thermal diode built into the processor core. If the die temperature exceeds a certain barrier (and this value is calibrated for each CPU individually), the CPU frequency drops down to 2.8GHz together with the processor core voltage. This mechanism allows reducing the processor temperature down to the acceptable level 40% faster than the TM1 mechanism based on the internal clock frequency modulation algorithm did.

EIST (Enhanced Intel SpeedStep) Technology. Strange as it might seem, this technology also switches the CPU into the 2.8GHz power-saving mode. Here the switch is initiated by the system OS. If the processor driver reports low CPU utilization at a given moment of time, the OS transfers the CPU into the power-saving mode by means of a corresponding ACPI command. In other words, EIST allows reducing the processor heat dissipation not only in idle mode but also under light workload.

Now I have to point out a pretty disappointing observation of ours. Since the minimal allowed clock frequency multiplier for Prescott and Prescott 2M cores is 14x, C1E, TM2 and EIST technologies can only work for the CPUs featuring higher nominal clock multiplier. For instance, the new Pentium 4 Extreme Edition 3.73Ghz CPU doesn’t support these new technologies at all, because its nominal clock frequency multiplier equals 14x. So, the higher is the nominal clock frequency multiplier of the CPU, the more efficient are all Demand Based Switching technologies described above.

 
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