This system has been running idle for hours. The operating system will send the HALT command when there is nothing to do, lowering thermal loss considerably.

Normally, older operating systems such as Windows 95/98 first edition or DOS always cause the processor to run at its full clock speed, with all processor areas active even though this gives no advantage to the user. If you want to make use of modern processors' halt mode on these OSes, please check out this little tool .

As a multi-threading, multi-tasking operating system, Windows NT introduced the concept of an idle task to Windows for the first time. This task is always running and becomes active whenever the other tasks do not require system time. In the case of office systems running on fast processors, the CPU is idle over 90% of the time.

The idle task does nothing other than sending a special command (HALT) to the CPU. This causes parts of the processor to shut down for a short period of time in order to save energy and reduce thermal loss. The feature has been available since the Pentium III.

CPUCooL can send the HALT command even under older Windows systems.

The Enhanced Halt Mode C1E includes the following changes:

• The VRM specification was upgraded from 9.0 to 10.1.
• The PROCHOT (Processor Hot) signal is used to have the CPU reduce the core clock to 2.8 GHz. This is triggered by the idle task, and also causes the core voltage to drop from 1.4 V to 1.2 V.

Using C1E requires a motherboard running a suitable BIOS version, and an operating system supporting the feature. In the case of Windows XP this means SP2 is needed.

Due to the clock speed and core voltage changes, the C1E mode is much more efficient the older halt mode. We measured as little as 16.5 W minimum power required in idle mode, which makes the whole P4 Prescott family for Socket 775 far more attractive than ever before.

The C1E feature will be available for all Socket 775 Pentium 4 processors. However, be sure to check the SL-SPEC number on the processor box before buying. We put together a short list in this article; please check the corresponding page for details.