Does powering down a Core Duo core help power consumption?

In a comment to MacBook Pro in single-core mode, x86 power management, a reader says:

The real question is how much more runtime (regardless of what the battery meter says) can a dual core MacBook or MacBook Pro obtain in single core mode?

If shutting down one core on my 2.0GHz MacBook will give me an extra hour of battery life, heck even an additional 30 minutes, it would be a great tool for times when processing power isn't all that important.

When I wrote the referenced entry back in March, running with a single core made the machine unstable during sleep/wake cycles. I wasn't able to do a true dawn-to-dusk test. But now that I've got the latest OS and firmware, the commenter made me wonder whether testing now would reveal a difference. On average, I go through two full charge cycles during workdays when I'm not in my office and working on the Power Mac G5 Quad. So I'll do the test tomorrow.

I'm guessing that it won't make a huge difference in battery running time. The entire shared Level 2 cache keeps burning when one of Core Duo's cores is shut down, and cache is a big factor in power consumption. That, incidentally, is one key to my dislike of Intel's shared cache.

Like Intel's shared bus, shared cache is not used because it's the smarter approach. It's a design expedient, and, I think, one that will haunt Intel down the road. Traditional partitioned cache allows core and cache to power up and down independently. With multiple cores writing to the full range of addresses in Level 2 cache, you can never power down one core's portion of cache.

If Intel sticks with shared cache into four-core designs, we might have a full 8 MB of clock-speed cache glowing away even when one or more cores are deactivated. While Intel is digging ever deeper into fine-grained power control, AMD will show the greater savings of better coarse-grained control, where entire "city blocks" of circuitry power down while idling or explicitly shut down. AMD's process cross-licensing with IBM will eclipse--sooner than you think--Intel's work on power efficiency.

I'll keep you posted.