Intel® 64 and IA-32 Architectures Optimization Reference Manual

POWER OPTIMIZATION FOR MOBILE USAGES• C3 — This level provides greater power savings than C1 or C2. In C3, theprocessor stops clock generating and snooping activity. It also allows systemmemory to enter self-refresh mode.The basic technique to implement OS power management policy to reduce staticpower consumption is by evaluating processor idle durations and initiating transitionsto higher-numbered C-state types. This is similar to the technique of reducing activepower consumption by evaluating processor utilization and initiating P-state transitions.The OS looks at history within a time window and then sets a target C-statetype for the next time window, as illustrated in Figure 10-3:Figure 10-3. Application of C-states to Idle TimeConsider that a processor is in lowest frequency (LFM- low frequency mode) and utilizationis low. During the first time slice window (Figure 10-3 shows an example thatuses 100 ms time slice for C-state decisions), processor utilization is low and the OSdecides to go to C2 for the next time slice. After the second time slice, processor utilizationis still low and the OS decides to go into C3.10.3.1 Processor-Specific C4 and Deep C4 StatesThe Pentium M, Intel Core Solo, Intel Core Duo processors, and processors based onIntel Core microarchitecture 4 provide additional processor-specific C-states (andassociated sub C-states) that can be mapped to ACPI C3 state type. The processorspecificC states and sub C-states are accessible using MWAIT extensions and can bediscovered using CPUID. One of the processor-specific state to reduce static powerconsumption is referred to as C4 state. C4 provides power savings in the followingmanner:• The voltage of the processor is reduced to the lowest possible level that stillallows the L2 cache to maintain its state.4. Pentium M processor can be detected by CPUID signature with family 6, model 9 or 13; Intel CoreSolo and Intel Core Duo processor has CPUID signature with family 6, model 14; processors basedon Intel Core microarchitecture has CPUID signature with family 6, model 15.10-4