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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.
However, the idle process does not use up computer resources (even when stated to be running at a high percent). Its CPU time "usage" is a measure of how much CPU time is not being used by other threads. In Windows 2000 and later the threads in the System Idle Process are also used to implement CPU power saving.
The CPU's clock speed and VCore are automatically decreased when the computer is under low load or idle, to save battery power, reduce heat and noise. The lifetime of the CPU is also extended because of reduced electromigration, which varies exponentially with temperature. [1] The technology is a concept similar to Intel's SpeedStep technology.
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.
CPU Idle: 0x05: Requests system suspend. 0) Clock halted until timer tick interrupt. 1) Slow clock [1] CPU Busy: 0x06: Driver tells system APM to restore clock speed of the CPU. Set Power State: 0x07: Set system or device into Suspend/Standby/Off state. Enable/Disable Power Management: 0x08: Restore APM BIOS Power-On Defaults: 0x09: Get Power ...
TM2 reduces processor temperature by lowering the CPU clock multiplier, and thereby the processor core speed. [2] In contrast, Thermal Monitor 1 inserts an idle cycle into the CPU for thermal control without decreasing multipliers. TM1 and TM2 are associated with DTS/PECI — Digital Temperature Sensor/Platform Environment Control Interface. [3]
An idle computer has a load number of 0 (the idle process is not counted). Each process using or waiting for CPU (the ready queue or run queue) increments the load number by 1. Each process that terminates decrements it by 1. Most UNIX systems count only processes in the running (on CPU) or runnable (waiting for CPU) states.
With this technology, a 1.6 GHz Pentium M can effectively throttle to clock speeds of 600 MHz, 800 MHz, 1000 MHz, 1200 MHz, 1400 MHz and 1600 MHz; these intermediate clock states allow the CPU to better throttle clock speed to suit conditions. The power requirements of the Pentium M varies from 5 watts when idle to 27 watts at full load.