Datasheet

Power Management
Intel
®
Xeon
®
and Intel
®
Core™ Processors For Communications Infrastructure
May 2012 Datasheet - Volume 1 of 2
Document Number: 327405
-001 51
Entry and exit of the C-States at the thread and core level are shown in Figure 6-3.
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.
6.2.3 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 via 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. .
Note: The P_LVLx I/O Monitor address needs to be set up before using the P_LVLx I/O read
interface. Each P-LVLx is mapped to the supported MWAIT(Cx) instruction as follows.
Figure 6-3. Thread and Core C-State Entry and Exit
C1 C1E C7C6C3
C0
MWAIT(C1), HLT
C0
MWAIT(C7),
P_LVL4 I/O Read
MWAIT(C6),
P_LVL3 I/O Read
MWAIT(C3),
P_LVL2 I/O Read
MWAIT(C1), HLT
(C1E Enabled)
Table 6-7. 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
1. If enabled, the core C-state will be C1E if all actives cores have also resolved a core C1 state or higher.
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