Mobile CPUs: Penryn vs Core

Posted at: Thursday 13th March 2008 by Clive Webster

We put Intel's new 45nm, Penryn-based mobile CPU to the test against a compartively priced, older 65nm Core-based mobile CPU.

Intel has taken only three months to port its 45nm manufacturing process from desktop to laptop chips. We delve into the silicon to find out why we’ve had to wait at all, and discover how Intel has optimised the Penryn architecture for use in dinky laptops.

Shrinking the manufacturing process usually results in faster switching transistors, which enables higher clock frequencies. However, as Intel wants to make its mobile CPUs as cool-running and power-efficient as possible, the new 45nm T8000- and T9000-series CPUs are built for power saving rather than just speed. Rather than designing a smaller transistor that just switches faster, Intel has designed mobile Penryn transistors to have five times less source-to-drain power leakage than their predecessors.

On top of this, shrinking transistors yields an inherent power saving advantage, thanks to lowering the power required for switching. With differences in cache and core speed, we wanted to test the T9300 of the Rock Xtreme 7700-T9300-8800 against a 65nm Core-based CPU. The 2.4GHz Core 2 Duo T7700 costs roughly the same, so we swapped that into the Rock to ensure a fair test. The difference between the clock speeds is 4 per cent, but the T9300 is about eight per cent faster, so Penryn’s extra cache and design tweaks clearly work well.

As well as optimising the transistors for increased power savings, Intel has added several new technologies to the mobile Penryn CPUs. There’s a new power-down state called Deep Power Down (or C6), which not only shuts down the processing core, as with other power-saving technologies, but can also flush the processor’s cache into system memory. The whole CPU can then shut down, and when it wakes up, it can fetch its cache data from system memory and be back in full operation in less than 150ms. With the inherent power saving that smaller transistors bring, plus the large decrease in source-to-drain leakage, a laptop with a Penryn-based mobile Core 2 Duo should be usable for longer than a Core-based CPU with 65nm transistors.

To find out how much difference these new power-saving technologies make, we tested the Core 2 Duo T9300 against the similarly priced Core 2 Duo T7700. We used the smallfft test in Orthos to load both cores fully for an absolute worst-case scenario, and then left the laptop idle in Windows to provide a best-case scenario. In both cases, we dimmed the screen to minimum to exaggerate the effects of the CPUs. We also ran our usual gaming battery test with Oblivion, using the usual half-screen brightness.

As you can see, the Penryn-based T9300 is much more power-efficient than the Core-based CPU. It’s around 17 per cent better when under full load (the Orthos test), and 7 per cent better during our Oblivion game test.