AHCI stands for Advanced Host Controller Interface. Advanced Host Controller Interface was designed purposefully with SATA hard-discs in mind: The Advanced Host Controller Interface standard allows SATA discs to fully utilise their advanced features; such as hot-swapping and native command queuing.

I have no information about this subject with regard to Linux operating systems, and the same goes for a Mac and OS X, so I’ll stick with Windows only for now. Perhaps a Linux guru and/or a Mac user in the know would comment and fill me in on the blanks I’ve just outlined?

STOP PRESS: In fact, newer mainstream Linux kernels support AHCI natively… See here.

Staying with Windows… AHCI can be activated in the BIOS screen of newer motherboards by changing the SATA ports’ mode from ATA to AHCI; but this action alone may well cause your Windows installation to produce a BSOD on boot. – Simply changing the mode in the BIOS with a Windows operating system already installed on the hard-drive is that the correct storage driver isn’t installed – therefore the existing ATA driver is looking for ATA and finds AHCI; can’t make any sense of it, and crashes Windows.

The way to put this right is to set the BIOS for AHCI (Only if you’re using at least 1 relatively modern SATA drive, of course: The setting will have no effect on their PATA counterparts, as those use a totally separate controller.) and do a repair install, AKA maintenance reinstall, of Windows.

This will cause Windows to set itself up again, this time with the correct drivers. You won’t be able to simply switch back from AHCI to ATA again without performing the repair installation again.

This operation will work with Windows Vista, SP1, 2. It shouldn’t cause any problems in Windows 7 either. With Windows XP I’m not sure, as my sources of information on this subject appear to indicate that it will work in XP SP2 onwards, but possibly only with the inclusion of a vendor-specific driver. It’s probably best to leave the BIOS on a computer with an existing Windows XP installation set to ATA, unless you have a lot of time to experiment. It is possible that it’ll work in XP: Maybe; maybe not – ? Again I’d ask for expert comment on this.

“Windows Vista natively supports both AHCI and NCQ. FreeBSD fully supports AHCI and NCQ since version 8.0.” – Wikipedia.