AutoHalt Power Down state

AutoHALT state or AutoHalt Power Down state is a low-power state supported by majority of modern x86 microprocessors. To switch to this state the operating system executes HALT instruction. When in this state, the CPU has internal clock running and may respond to interrupts and other signals. The CPU may transition from AutoHalt state to Normal state, Stop Grant low-power state and for some micro-architectures to other low-power states. Power savings in this mode can be significant - from 50% for Pentium 4 Northwood and higher for other processors.

Please note that although processor's clock is not stopped in this mode, the CPU effectively is not running, i.e. not executing any instructions.