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Some APM implementations execute the HLT (Halt CPU until an interrupt occurs) instruction in the "Idle CPU" call, while others do not. Thus enabling this may result in redundant HLT executions because "Idle CPU" is called from the kernel context switch routine that inherently executes HLT. This may reduce peak system performance.
Also the system hangs up if HLT instruction is disabled in the kernel context switch routine, and if the APM implementation of the machine does not execute HLT in "Idle CPU". On some implementations that do not support CPU clock slowdown, APM might not execute HLT. apm disables APMIO_NOTHALTCPU operation on such machines.
The current version of apm does not call "Idle CPU" from the kernel context switch routine if clock slowdown is not supported, and it executes HLT instruction by default. Therefore, there is no need to use these two operations in most cases.