AHCI stands for Advanced Host Controller Interface. One of the main reasons for switching to AHCI-mode is to take advantage of the NCQ-Feature of your SATA harddrive. NCQ (Native Command Queuing) allows ATA drives to accept more than one command at a time and dynamically reorder the commands for maximum efficiency. NCQ, when used in conjunction with a hard drive that supports NCQ, can increase storage performance on random workloads.
forum.msi.com.tw
Native Command QueuingNative Command Queuing (NCQ) is a technology designed to increase performance of SATA hard disks under certain situations by allowing the individual hard disk to internally optimize the order in which received read and write commands are executed. This can reduce the amount of unnecessary going back-and-forth on the drive's heads, resulting in increased performance (and slightly decreased wear of the drive) for workloads where multiple simultaneous read/write requests are outstanding, most often occurring in server-type applications. However, the current technology actually slows down HD access in certain applications (but not always) like games and sequential reads & writes, because of the added latency induced by NCQ logic. en.wikipedia.org/wiki/Native_Command_Queuing
AHCI is fully supported in Microsoft Windows Vista and the Linux operating system from kernel 2.6.19. Older operating systems like Windows XP require drivers written by the host bus adapter vendor in order to support AHCI. Windows XP requires the installation of a vendor-specific driver even if AHCI is present on the host bus adapter because Windows XP was released before Serial ATA was invented.
Links :
Switch AHCI to IDE from BIOS Guide
Windows XP support AHCI
Enable AHCI Without reinstalling Windows XP (link)
UPDATE :
Install Enable AHCI Without reinstalling Windows XP (link)
Error message when you start a computer Windows Vista after having modified mode SATA of the reader of starting: "STOP 0X0000007B INACCESSABLE_BOOT_DEVICE" (link)
