The circuits that control data transfer to and from the disk drive (floppy disk, hard disk, optical disc). In a PC, an IDE disk controller is typically built into the motherboard, and a cable plugs into the controller socket on one end and connects to the drive on the other. On earlier PCs, the disk controller was a separate card plugged into a PCI or ISA slot. However, when SCSI drives are used, a SCSI disk controller (SCSI host adapter) is typically not built in, but available on a separate card. See hard disk, IDE and SCSI.