The HLT signal which is used by the normal idle-loop of the Linux kernel doesn't normaly bring any significant power-saving on processors of the Athlon/Duron family (hereafter referred to as Athlon processors). To get real power-saving on an Athlon processor, you have to put the processor in the STPGNT-Mode (Stop Grant Mode). In this mode, the processor is disconnected from the front-side bus (FSB), and can turn off parts of the CPU as well as reduce the clock to save power.
Some modern Motherboards/Chipsets also support a Function to make a STPGNT when a HLT signal is detected.
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