The BIOS Setup Utility provides an option to enable AHCI, this can be found under the Advanced | ATA Controller setup page.
Enabling the AHCI provides access to all 6 SATA ports on the server board. If this is not enabled, the user will only have access to 4 ports on the server board.
When AHCI is enabled, under Microsoft Windows*, an AHCI driver is required to be loaded at install time to ensure proper operation. On Linux* supported operating systems, a driver is not required.
For details on the AHCI Specification, please check the AHCI Webpage.