Disk controller

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The disk controller (or "hard disk controller") is the circuit which allows the CPU to communicate with a hard disk, floppy disk or other kind of disk drive.

Early disk controllers were identified by their storage methods and data encoding. They were typically implemented on a separate controller card. Modified frequency modulation (MFM) controllers were the most common type in small computers, used for both floppy disk and hard disk drives. Run length limited (RLL) controllers used data compression to increase storage capacity by about 50%. Priam created a proprietary storage algorithm that could double the disk storage. Shugart Associates Systems Interface, (SASI) was a predecessor to SCSI.

Modern disk controllers are integrated into the disk drive. The interface between the computer and hard disk is called a host adapter. The most common types in use are IDE, Serial ATA, and SCSI host adapters. Most home personal computers use an IDE or Serial ATA interface. High end PCs, workstations and network file servers mostly have SCSI adapters.

The correct term for the component that allows a computer to talk to a SCSI bus is host adapter or host bus adapter. The component that allows a disk to talk to a SCSI bus is called a controller. Disks called "SCSI disks" have built-in SCSI controllers. In the past, before most SCSI controller functionality was implemented in a single chip, separate SCSI controllers interfaced disks to the SCSI bus. There are other ways of partitioning the functionality that can make applying this terminology difficult or inappropriate.

In the IDE standards, the terminology is like that of SCSI.

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.