Copyright by William Lloyd Bircher 2010 - The Laboratory for ...

power adaptations as c-states. These terms represent adaption operating points as defined
in the ACPI specification. ACPI [Ac07] “…is an open industry specification co-
developed by Hewlett-Packard, Intel, Microsoft, Phoenix, and Toshiba. ACPI establishes
industry-standard interfaces enabling OS-directed configuration, power management, and
thermal management of mobile, desktop, and server platforms.”
3.4.2 Active Power Management: P-states
A p-state (performance state) defines an operating point for the processor. States are
named numerically starting from P0 to PN, with P0 representing the maximum
performance level. As the p-state number increases, the performance and power
consumption of the processor decrease. Table 3.7 shows p-state definitions for a typical
processor. The state definitions are made by the processor designer and represent a range
of performance levels that match expected performance demand of actual workloads. P-
states are simply an implementation of dynamic voltage and frequency scaling. The
resultant power reduction is obtained using these states is largely dependent on the
amount of voltage reduction attained in the lower frequency states.
Table 3.7 Example P-states Definition
P-State Frequency (MHz) VDD (Volts)
P0 FMax × 100% VMax × 100%
P1 FMax × 85% VMax × 96%
P2 FMax × 75% VMax × 90%
P3 FMax × 65% VMax × 85%
P4 FMax × 50% VMax × 80%
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