Intel CM8064601466003 Requesting Low-Power Idle States, Thread and Core C-State Entry and Exit

Power Management—Processor

Figure 13. Thread and Core C-State Entry and Exit

C0

MWAIT(C1), HLT		MWAIT(C7),
MWAIT(C1), HLT		P_LVL4 I/O Read
		MWAIT(C6),
(C1E Enabled)	MWAIT(C3),	P_LVL3 I/O Read

P_LVL2 I/O Read

C1 C1E C3 C6 C7

While individual threads can request low-power C-states, power saving actions only take place once the core C-state is resolved. Core C-states are automatically resolved by the processor. For thread and core C-states, a transition to and from C0 is required before entering any other C-state.

Table 18.	Coordination of Thread Power States at the Core Level
	Processor Core C-State				Thread 1
			C0	C1	C3	C6	C7
		C0	C0	C0	C0	C0	C0
		C1	C0	C11	C11	C11	C11
	Thread 0	C3	C0	C11	C3	C3	C3
		C6	C0	C11	C3	C6	C6
		C7	C0	C11	C3	C6	C7
	Note: 1. If enabled, the core C-state will be C1E if all cores have resolved a core C1 state or higher.

4.2.3Requesting 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, 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, must 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 request. The reads fall through like a normal I/O instruction.

Desktop 4th Generation Intel® Core™ Processor Family, Desktop Intel® Pentium® Processor Family, and Desktop Intel® Celeron® Processor Family

December 2013	Datasheet – Volume 1 of 2
Order No.: 328897-004	53