Data Sheet

Power Management
74 Datasheet, Volume 1 of 2
A System Management Interrupt (SMI) handler returns execution to either Normal
state or the C1/C1E state. Refer the Intel 64 and IA-32 Architectures Software
Developer’s Manual for more information.
While a processor IA core is in C1/C1E state, it processes bus snoops and snoops from
other threads. For more information on C1E, Refer Section 4.2.5.
Processor IA core C3 State
Individual threads of a processor IA core can enter the C3 state by initiating a P_LVL2
I/O read to the P_BLK or an MWAIT(C3) instruction. A processor IA core in C3 state
flushes the contents of its L1 instruction cache, L1 data cache, and L2 cache to the
shared LLC, while maintaining its architectural state. All processor IA core clocks are
stopped at this point. Because the processor IA core’s caches are flushed, the processor
does not wake any processor IA core that is in the C3 state when either a snoop is
detected or when another processor IA core accesses cacheable memory.
Processor IA core C6 State
Individual threads of a processor IA core can enter the C6 state by initiating a P_LVL3
I/O read or an MWAIT(C6) instruction. Before entering processor IA core C6 state, the
processor IA core will save its architectural state to a dedicated SRAM. Once complete,
a processor IA core will have its voltage reduced to zero volts. During exit, the
processor IA core is powered on and its architectural state is restored.
Processor IA core C7-C10 States
Individual threads of a processor IA core can enter the C7, C8, C9, or C10 state by
initiating a P_LVL4, P_LVL5, P_LVL6, P_LVL7 I/O read (respectively) to the P_BLK or by
an MWAIT(C7/C8/C9/C10) instruction. The processor IA core C7-C10 state exhibits the
same behavior as the processor IA core C6 state.
C-State Auto-Demotion
In general, deeper C-states, such as C6 or C7, 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 battery life
and idle power. To increase residency and improve battery life and idle power in deeper
C-states, the processor supports C-state auto-demotion.
There are two C-State auto-demotion options:
• C7/C6 to C3
• C7/C6/C3 To C1
The decision to demote a processor IA core from C6/C7 to C3 or C3/C6/C7 to C1 is
based on each processor IA core’s immediate residency history. Upon each processor IA
core C6/C7 request, the processor IA core C-state is demoted to C3 or C1 until a
sufficient amount of residency has been established. At that point, a processor IA core
is allowed to go into C3/C6 or C7. Each option can be run concurrently or individually. If
the interrupt rate experienced on a processor IA core is high and the processor IA core
is rarely in a deep C-state between such interrupts, the processor IA core can be
demoted to a C3 or C1 state. A higher interrupt pattern is required to demote a
processor IA core to C1 as compared to C3.