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Figure 13. Thread and Core C-State Entry and Exit
C1 C1E C7C6C3
C0
MWAIT(C1), HLT
C0
MWAIT(C7),
P_LVL4 I/ORead
MWAIT(C6),
P_LVL3 I/ORead
MWAIT(C3),
P_LVL2 I/ORead
MWAIT(C1), HLT
(C1E Enabled)
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
Thread 0
C0 C0 C0 C0 C0 C0
C1 C0 C1
1
C1
1
C1
1
C1
1
C3 C0 C1
1
C3 C3 C3
C6 C0 C1
1
C3 C6 C6
C7 C0 C1
1
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.
Requesting 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.
4.2.3
Power Management—Processor
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