To fix AHCI registry, open the Registry Editor to make necessary changes. Switch from IDE to AHCI if your system is running in the legacy IDE mode.

How to Solve AHCI Registry in Windows 7

Advanced Host Controller Interface or AHCI is an Intel technical standard, that specifies the operation of Serial ATA (SATA) host bus adapters. AHCI enables many features such as increasing read-write speed, native command queuing, and hot swapping.

The top benefit of enabling AHCI mode for the hard disk is that it can run at a higher speed than the disk adopting IDE mode. It boosts the performance of the hard disk significantly by enabling this mode. 

However, it must be supported by your operating system and PC hardware to get the benefits of features. Older operating systems, like Windows XP, do not support the AHCI mode. You have to get OEM-supplied drivers in that case. 

1. Switch From IDE to AHCI

If your system is running in the legacy IDE mode, switching to the AHCI mode on Windows will not boot after you switch from IDE to AHCI in the BIOS.

First, you have to switch SATA to the legacy/IDE mode. Then, Check your motherboard manual to get the exact location of the SATA options in your BIOS.

2. Open the Registry Editor

First, press the Win+R keys on your keyboard to open the “Run” dialog. Type ‘regedit’ without quotes and press Enter. Continue with the UAC prompt. This method works for any Windows version you have installed.

Next, navigate to the key- HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\msahci

and change the Start DWORD value from 3 to 0. Restart your PC and set the SATA mode to AHCI. 

For more advanced OS, the following additional tasks are to be performed.

• Restart your system and enter the system BIOS. Change the ATA Drive setting back to ATA Mode. Click Enter to save the changes and restart the computer again.
• Click Yes to the Warning regarding the detected mode change on the embedded ATA controller. The system will boot normally to the Start screen.

Make sure to secure the Local Admin account and password before proceeding.

• Open an elevated command prompt. Run the following command to enable Safe Mode boot- bcdedit /set {current} safeboot minimal
• Restart the computer and boot to the system BIOS. Next, change the ATA Drive setting from ATA/IDE Mode to AHCI Mode. Click enter to save the change. The system should boot normally to the Start screen in Safe Mode.
• Open an elevated command prompt and run the following command to remove the Safe Mode boot option-bcdedit /deletevalue {current} safeboot. 
• Restart the computer. Your system should boot successfully to the Start screen.

To Conclude

Not that Windows XP as it has no native AHCI support. You might need to rewire the power to your OS drive when activating AHCI mode. Even for the old SATA 2 controller, you will notice a significant difference in read-write speeds after enabling AHCI mode.