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Idle is a state that a computer processor is in when it is not being used by any program. Every program or task that runs on a computer system occupies a certain amount of processing time on the CPU. If the CPU has completed all tasks it is idle. Modern processors use idle time to save power.
In Windows NT operating systems, the System Idle Process contains one or more kernel threads which run when no other runnable thread can be scheduled on a CPU. In a multiprocessor system, there is one idle thread associated with each CPU core. For a system with hyperthreading enabled, there is an idle thread for each logical processor.
AMD Cool'n'Quiet is a CPU dynamic frequency scaling and power saving technology introduced by AMD with its Athlon XP processor line. [1] It works by reducing the processor's clock rate and voltage when the processor is idle.
For example, hardware timers send interrupts to the CPU at regular intervals. Most operating systems execute a HLT instruction when there is no immediate work to be done, putting the processor into an idle state. In Windows NT, for example, this instruction is run in the "System Idle Process". On x86 processors, the opcode of HLT is 0xF4.
ACPI 1.0 (1996) defines a way for a CPU to go to idle "C states", but defines no frequency-scaling system.. ACPI 2.0 (2000) introduces a system of P states (power-performance states) that a processor can use to communicate its possible frequency–power settings to the OS.
In many applications, the CPU and other components are idle much of the time, so idle power contributes significantly to overall system power usage. When the CPU uses power management features to reduce energy use, other components, such as the motherboard and chipset, take up a larger proportion of the computer's energy.
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The Ivy Bridge-EP processor line announced in September 2013 has up to 12 cores and 30 MB third level cache, with rumors of Ivy Bridge-EX up to 15 cores and an increased third level cache of up to 37.5 MB, [45] [46] although an early leaked lineup of Ivy Bridge-E included processors with a maximum of 6 cores.