Advanced Technology Attachment Packet Interface (ATAPI) is a protocol that allows a greater variety of devices to be connected to a computer than ATA (PATA / IDE and SATA) on its own would allow. ATAPI allows IDE and SATA controllers to support optical drives.  ATA on its own doesn't allow for specific functions required by optical drives, such as a "media eject" command or a way for the controller to determine whether media is present in the optical drive. Optical drives include CD-ROM drives, DVD-ROM drives, and Blu-ray players.