Core C-States; Core C0 State; Core C1/C1E State; Core C3 State - Intel 2ND GENERATION CORE PROCESSOR FAMILY DESKTOP - DATASHEET VOLUME 1 01-2011 Datasheet

4.2.4

Core C-states

The following are general rules for all core C-states, unless specified otherwise:
• A core C-State is determined by the lowest numerical thread state (such as Thread
0 requests C1E while Thread 1 requests C3, resulting in a core C1E state). See
Table
• A core transitions to C0 state when:
— An interrupt occurs
— There is an access to the monitored address if the state was entered using an
MWAIT instruction
• For core C1/C1E, core C3, and core C6, an interrupt directed toward a single thread
wakes only that thread. However, since both threads are no longer at the same
core C-state, the core resolves to C0.
• A system reset re-initializes all processor cores.
4.2.4.1

Core C0 State

The normal operating state of a core where code is being executed.
4.2.4.2

Core C1/C1E State

C1/C1E is a low power state entered when all threads within a core execute a HLT or
MWAIT(C1/C1E) instruction.
A System Management Interrupt (SMI) handler returns execution to either Normal
state or the C1/C1E state. See the Intel
Developer's Manual, Volume 3A/3B: System Programmer's Guide for more information.
While a core is in C1/C1E state, it processes bus snoops and snoops from other
threads. For more information on C1E, see
4.2.4.3

Core C3 State

Individual threads of a core can enter the C3 state by initiating a P_LVL2 I/O read to
the P_BLK or an MWAIT(C3) instruction. A core in C3 state flushes the contents of its
L1 instruction cache, L1 data cache, and L2 cache to the shared L3 cache, while
maintaining its architectural state. All core clocks are stopped at this point. Because the
core's caches are flushed, the processor does not wake any core that is in the C3 state
when either a snoop is detected or when another core accesses cacheable memory.
4.2.4.4

Core C6 State

Individual threads of a core can enter the C6 state by initiating a P_LVL3 I/O read or an
MWAIT(C6) instruction. Before entering core C6, the core will save its architectural
state to a dedicated SRAM. Once complete, a core will have its voltage reduced to zero
volts. During exit, the core is powered on and its architectural state is restored.
4.2.4.5

C-State Auto-Demotion

In general, deeper C-states such as C6 have long latencies and have higher energy
entry/exit costs. The resulting performance and energy penalties become significant
when the entry/exit frequency of a deeper C-state is high. Therefore, incorrect or
inefficient usage of deeper C-states have a negative impact on power. To increase
residency and improve power in deeper C-states, the processor supports C-state auto-
demotion.
48
4-7.
®
64 and IA-32 Architecture Software
"Package C1/C1E".
Power Management
Datasheet, Volume 1