CPU / Device Performance States (PStates)
Pstates define the power management state while the CPU / device is within its executing state, C0 for CPU and D0 for device.
P0 : Minimum pstate, highest power consumption.
P1, P2, P3…. : P1 > P2 > P3 and so on, in terms of power consumption.
Pn : Maximum pstate with the lowest power consumption.
Pstate power management can be seen in modern CPUs and GPUs. It allows them to control their active power according to the load at any particular moment. The number of Pstates is implementationspecific.
For instance, lowend GPUs like the NVIDIA GeForce 7300 GS has only one Pstate, where the 2D and 3D clocks remain constant over time. On the other hand, highend GPUs like the NVIDIA GeForce 7900 GT has at least two Pstates  P0 state for running at maximum clock and voltage while playing 3D games, and a P1 state for running at the minimum clock and voltage in 2D mode.
However, Pstate power management in CPU is far more complex. Different CPU models have their own unique Pstate lookup tables. Take for example, the Pstate lookup tables for these CPUs.
Core 2 Extreme X6800 
Athlon A64 X2 4800+ 

PStates 
Clock Ratio 
Clock 
Voltage 
Load 
PStates 
Clock Ratio 
Clock 
Voltage 
Load 

P0 
11x 
2.93 GHz 
1.2875 V 
81100 % 
P0 
12x 
2.4 GHz 
1.35 V 
81100 % 

P1 
10x 
2.67 GHz 
1.2500 V 
7180 % 
P1 
11x 
2.2 GHz 
1.35 V 
6180 % 

P2 
9x 
2.40 GHz 
1.2250 V 
5170 % 
P2 
10x 
2.0 GHz 
1.325 V 
5160 % 

P3 
8x 
2.13 GHz 
1.2125 V 
3150 % 
P3 
9x 
1.8 GHz 
1.30 V 
4150 % 

P4 
7x 
1.87 GHz 
1.2000 V 
1130 % 
P4 
8x 
1.6 GHz 
1.25 V 
3140 % 

P5 
6x 
1.60 GHz 
1.1750 V 
010 % 
P5 
7x 
1.4 GHz 
1.20 V 
2130 % 

P6 
6x 
1.2 GHz 
1.15 V 
1120 % 

P7 
5x 
1.0 GHz 
1.10 V 
010 % 

Core 2 Duo E6300 
Athlon 64 X2 3600+ 

PStates 
Clock Ratio 
Clock 
Voltage 
Load 
PStates 
Clock Ratio 
Clock 
Voltage 
Load 

P0 
7x 
1.87 GHz 
1.2500V 
31  100 % 
P0 
9x 
1.8 GHz 
1.30 V 
81100 % 

P1 
6x 
1.60 GHz 
1.2250V 
0  30 % 
P1 
8x 
1.6 GHz 
1.25 V 
6180 % 

P2 
7x 
1.4 GHz 
1.20 V 
4160 % 

P3 
6x 
1.2 GHz 
1.15 V 
2140 % 

P4 
5x 
1.0 GHz 
1.10 V 
020 % 
As you can see in the examples above, a CPU with higher multiplier has more Pstates than a CPU with a lower multiplier. The smallest clock ratio supported by Intel is 6x, while AMD has a minimum multiplier of 4x. However, their Cool n Quiet feature uses 5x as the minimum clock ratio.
Note :
Different processor stepping may implement a different set of clock ratio / voltage lookup table. The clock ratio / voltage tables shown above are just examples to describe the differences in Pstates used in different CPU models, even of the same family.
The CPU load incorporated with various Pstates is one of the ways to utilize the Pstate power management. Windows XP may transit a processor to P1 but Linux, on the other hand, may transit a processor into P2 at the same CPU load. It therefore depends on the operating system implementation.
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