The Tenuous Link

Advanced Host Controller Interface (AHCI) mode is a BIOS option in my computer for the SATA hard disk controller. It lets the OS use Native Command Queuing (NCQ/aka SATA mode) to reorder disk read or write requests to make the reading and writing more efficient, particularly if the disk is very busy.

My BIOS has options IDE, RAID, or AHCI mode. RAID includes all of the ACHI features and Intel recommend that you choose the RAID before you install an OS because changing it afterward can be painful (or "unsupported" if you have an Intel motherboard!).

The process in Vista seems to be less painful than in Windows XP because VISTA includes AHCI drivers by default. However, if AHCI mode was not enabled at the time you installed Vista then the new drivers will not be loaded, and if you change the BIOS setting then Windows doesn't boot.

Simple solution:

1. Start with IDE mode selected in the BIOS, and boot Windows Vista.
2. Exit all Windows-based programs.
3. Click Start, type regedit in the Start Search box, and then press ENTER.
4. If you receive the User Account Control dialog box, click Continue.
5. Locate and then click the following registry subkey: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Msahci
6. In the right pane, right-click Start in the Name column, and then click Modify.
7. In the Value data box, type 0, and then click OK.
8. On the File menu, click Exit to close Registry Editor.
9. Reboot, and change to AHCI mode in the BIOS. Windows will detect some changed hardware (anything connected to a SATA port).
10. For good measure I downloaded and (re)installed the chipset drivers for my motherboard. My motherboard uses an Intel G33 chipset and I wasn't sure if I needed to install the Intel Matrix Storage Manager, but when I tried it told me that I already had a more recent version of the software installed. I guess part of the chipset driver package overlaps with the Storage Manager software.

This process worked fine for me - I have AHCI mode enabled. Choosing RAID mode in the BIOS (which provides a superset of the AHCI features) still stops Windows from booting, but I'm not worried about that!

Finally, I'm not sure if it is to do with the change to AHCI or not, but I've had intermittent trouble shutting down the PC - Vista seems to hang on the "Shutting down" screen and the hard disk light keeps flashing. According to my event log the machine reboots itself after 20 minutes of that, but there were no other clues in the event log. (Someone with a related problem? http://episteme.arstechnica.com/eve/forums/a/tpc/f/77909774/m/433004204831) [Update 2008/12/31: I haven't noticed this problem for a long time now, so it must have been fixed with SP1 or driver upgrades.]

Labels: AHCI, vista

// posted by Steven @ 9:19 AM 2 comments