Requesting the Low-Power Idle States

The primary software interfaces for requesting low-power idle states are through the MWAIT instruction with sub-state hints and the HLT instruction (for C1 and C1E). However, the software may make C-state requests using the legacy method of I/O reads from the ACPI-defined processor clock control registers, referred to as P_​LVLx. This method of requesting C-states provides legacy support for operating systems that initiate C-state transitions using I/O reads.

For legacy operating systems, P_​LVLx I/O reads are converted within the processor to the equivalent MWAIT C-state request. Therefore, P_​LVLx reads do not directly result in I/O reads to the system. The feature, known as I/O MWAIT redirection, should be enabled in the BIOS..

The BIOS can write to the C-state range field of the PMG_​IO_​CAPTURE MSR to restrict the range of I/O addresses that are trapped and emulate MWAIT like functionality. Any P_​LVLx reads outside of this range do not cause an I/O redirection to MWAIT(Cx) like the request. They fall through like a normal I/O instruction.

When P_​LVLx I/O instructions are used, MWAIT sub-states cannot be defined. The MWAIT sub-state is always zero if I/O MWAIT redirection is used. By default, P_​LVLx I/O redirections enable the MWAIT 'break on EFLAGS.IF’ feature that triggers a wake up on an interrupt, even if interrupts are masked by EFLAGS.IF.