Undervolting Your Phenom II And Core 2 Processors

Undervolting AMD's Phenom II X4 955

AMD’s Phenom II X4 955 has been the firm’s flagship desktop processor model since its introduction April. With its DDR3 support and 3.2 GHz core clock speed, AMD finally managed to threaten Intel’s Core 2 Quad in some benchmarks, but at somewhat lower cost both for the processor and an average platform. However, the Core i7’s reign remains untouched.

The Phenom II X4 is available at between 2.5 and 3.2 GHz (see an overview on the AMD Web site). The 800-series processors have 4 x 512 KB L2 cache per core and a shared 4 MB L2 cache, while the 900-series offers 50% more L3 cache. All Phenom II processors are manufactured by Globalfoundries using AMD’s 45 nm DSL SOI process, which allows for low power consumption and nice overclocking capabilities. We were curious to look at the undervolting results.

Default Voltage: 1.32 V

The automatic BIOS setting would run the Phenom II X4 955 at 1.32 V, as reported by CPU-Z. This resulted in a system peak power of up to 216 W at full CPU load. Clearly there is some room for improvement here.

Idle Voltage: Always 0.960 V (BIOS Configured)

All AMD processors with Cool’n’Quiet enabled will switch to a clock speed of 800 MHz when idle, and they will all switch to the default core voltage at this speed: 0.96 V. As you can see in the summary table below, the Phenom II switched to 0.96 V in Cool’n’Quiet mode when idle regardless of our BIOS CPU voltage settings. Consequently, the system idle power was the same at all times: 99 W. In this case there is nothing to improve here, unless the BIOS allowed modifying the idle voltage.

Optimized Peak Voltage: 1.152 V

We tried several voltage settings (see table below) and stressed all of them using Prime95 for at least 30 minutes. It turned out that the default voltage of 1.32 V could be lowered by as much as 12%, to 1.1175 V. This lowered system peak power from 216 W to only 179 W, which represents a 17.2% drop in system power consumption. Not bad at all.

Summary Table

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AMD Phenom II X4 955
BIOS VoltageEffective Idle VoltageEffective Idle PowerEffective Load VoltageEffective Load PowerStable
Auto0.96 V*99 W1.32 V216 WYes
1.31250.96 V*99 W1.288 V205 WYes
1.28750.96 V*99 W1.264 V199 WYes
1.26250.96 V*99 W1.24 V196 WYes
1.23750.96 V*99 W1.216 V192 WYes
1.21250.96 V*99 W1.192 V186 WYes
1.18750.96 V*99 W1.168 V181 WYes
1.1750.96 V*99 W1.152 V179 WYes
1.16250.96 V*99 W1.136 V177 WNo

*managed by Cool’n’Quiet