Microsoft has fixed the battery drain bug that has been affecting a number of computer owners with Intel Core Duo-based notebooks. However, two of the three causes of the drain are not fixed by a patch posted by Microsoft yesterday.

The problem emerged in January. At least some Core Duo machines connected to USB peripherals suffer a persistent loss of battery charge above and beyond what you'd expect. Originally thought to be an issue with Intel's chipset, the bug was later identified as a glitch in Windows XP Service Pack 2's power management code that had, in fact, affected older, pre-Core Duo notebooks running the OS.

In February, Microsoft said it was working on a fix and the patch has now arrived. Well, sort of. The patch fixes faster-than-usual battery drains arising from the CPU's inability to enter Advanced Configuration and Power Interface (ACPI) processor idle sleep states as a result of the USB 2.0 driver leaving "the asynchronous scheduler component running continuously".

However, the patch doesn't prevent the USB host controller from finding itself unable to shut down when devices are removed from nested USB hubs, or when said controller is hooked up to USB 2.0 devices that make use of its periodic scheduler. Both instances can also cause the exceptional power drain, but neither are fixed by the patch.

And Microsoft also warns: "Various factors can affect the battery life of different portable computers that use different types of batteries. Therefore, you may not notice any improvement in battery life after you apply this update."

You can get the patch here. ®