The CpuIdle Approach
Under normal circumstances the CPU isn't always active but spends much time waiting for the keyboard, harddisk or CD-ROM. What would be more logical than to turn off the CPU for that period? That's exactly what the HLT machine instruction (Opcode F4) does. Whenever the CPU encounters a HLT instruction the clock is halted and the CPU enters suspend mode until an interrupt, NMI, or reset happens. With the advent of power saving microprocessors like the Cyrix Cx486S the HLT instruction elicits an additional benefit. When "Suspend on HLT" is enabled in the configuration register the processor not only stops on HLT but also enters the power saving suspend mode.
Why Doesn't Windows Do That?
Modern operating systems like Linux execute the HLT instruction in an idle priority thread. This thread is always executed when the CPU is otherwise idle. No additional execution time for HLTing is needed, the CPU will not run slower.
While other operating systems like Linux always used this mechanism, Windows only learned it with NT. But even with NT and following versions it is only enabled when the BIOS and ACPI implementation is recognized by the OS.
