AHCI

The Advanced Host Controller Interface (AHCI) is a technical standard defined by Intel that specifies the operation of Serial ATA (SATA) host bus adapters in a non-implementation-specific manner.

The specification describes a system memory structure for computer hardware vendors to exchange data between host system memory and attached storage devices. AHCI gives software developers and hardware designers a standard method for detecting, configuring, and programming SATA/AHCI adapters. AHCI is separate from the SATA 3 Gbit/s standard, although it exposes SATA's advanced capabilities (such as hot swapping and native command queuing) such that host systems can utilize them.

As of October 2010, the current version of the specification is v. 1.3.

Operating modes
Many SATA controllers offer selectable modes of operation: legacy Parallel ATA emulation, standard AHCI mode, or vendor-specific RAID (which generally enables AHCI in order to take advantage of its capabilities). Intel recommends choosing RAID mode on their motherboards (which also enables AHCI) rather than AHCI/SATA mode for maximum flexibility. Legacy mode is a software backward-compatibility mechanism intended to allow the SATA controller to run in legacy operating systems which are not SATA-aware or where a driver does not exist to make the operating system SATA-aware.

Operating system support
AHCI is supported out of the box on Windows Vista and newer versions of Windows, Windows XP does not provide support out of the box.

Some operating systems, notably Windows Vista and Windows 7, do not configure themselves to load the AHCI driver upon boot if the SATA-drive controller was not in AHCI mode at the time of installation. This can cause failure to boot with an error message if the SATA controller is later switched to AHCI mode. For this reason, Intel recommends changing the drive controller to AHCI or RAID before installing an operating system. (It may also be necessary to load chipset-specific AHCI or RAID drivers — from a USB Flash drive, for example — at installation time.)

On Windows Vista and Windows 7, this can be fixed by setting non-AHCI mode in the BIOS then booting and changing the registry, then changing the BIOS setting to AHCI.