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ATA Packet Interface (ATAPI) / IDE
The most common interface used in modern CD-ROM drives is the AT Attachment Packet Interface, more commonly called just ATAPI. This is a special protocol that was developed to allow devices like CD-ROM drives and tape drives to attach to regular IDE controllers normally used for hard disks. CD-ROM drives that use ATAPI are often called "IDE CD-ROMs" but this terminology is not strictly correct.
The ATAPI interface is a derivative of the standard IDE interface; regular IDE commands cannot be used properly for CD-ROM drives, so a modified command structure was created. A special driver is used to control the CD-ROM drive. ATAPI itself is described in more detail here, in the general discussion of the IDE/ATA device interface.
Physically, ATAPI CD-ROM drives connect to the system in about the same way that IDE hard disks do. They are normally configurable to act as master or slave drives, with slave often being the default. See this section for a full discussion of configuring IDE and ATAPI devices.
Warning: One problem that is
often encountered when configuring ATAPI devices is accidentally connecting them to a
connector on a sound card that appears to be an IDE channel but really is not. Many older
sound cards include several connectors for the proprietary
interfaces discussed here. These may appear physically identical to an IDE port, but
an ATAPI CD-ROM will not function if connected to one.