{{unreferenced, date=May 2010 Seagate ST11R

The disk controller is the controller circuit which enables the CPU to communicate with a

hard disk A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnet ...

floppy disk A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined w ...

or other kind of

disk drive Disk storage (also sometimes called drive storage) is a general category of storage mechanisms where data is recorded by various electronic, magnetic, optical, or mechanical changes to a surface layer of one or more rotating disks. A disk drive is ...

. It also provides an interface between the disk drive and the bus connecting it to the rest of the system. 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 Run-length limited or RLL coding is a line coding technique that is used to send arbitrary data over a communications channel with bandwidth limits. RLL codes are defined by four main parameters: ''m'', ''n'', ''d'', ''k''. The first two, ''m''/ ...

(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 Shugart Associates (later Shugart Corporation) was a computer peripheral manufacturer that dominated the floppy disk drive market in the late 1970s and is famous for introducing the -inch "Minifloppy" floppy disk drive. In 1979 it was one of the f ...

Systems Interface ( SASI) was a predecessor to

SCSI Small Computer System Interface (SCSI, ) is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, electrical, optical and logical interface ...

. Modern disk controllers are integrated into the disk drive as peripheral controllers. For example, 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. These integrated peripheral controllers communicate with a

host adapter In computer hardware, a host controller, host adapter, or host bus adapter (HBA), connects a computer system bus, which acts as the host system, to other network and storage devices. The terms are primarily used to refer to devices for conne ...

in the host system over a standardized, high-level storage bus interface. The most common types of interfaces provided nowadays by host controllers are PATA (IDE) and

Serial ATA SATA (Serial AT Attachment) is a computer bus interface that connects host bus adapters to mass storage devices such as hard disk drives, optical drives, and solid-state drives. Serial ATA succeeded the earlier Parallel ATA (PATA) standard t ...

for home use. High-end disks use

Parallel SCSI Parallel SCSI (formally, SCSI Parallel Interface, or SPI) is the earliest of the interface implementations in the SCSI family. SPI is a parallel bus; there is one set of electrical connections stretching from one end of the SCSI bus to the othe ...

Fibre Channel Fibre Channel (FC) is a high-speed data transfer protocol providing in-order, lossless delivery of raw block data. Fibre Channel is primarily used to connect computer data storage to servers in storage area networks (SAN) in commercial data cen ...

or Serial Attached SCSI. Disk controllers can also control the timing of access to

flash memory Flash memory is an electronic non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for the NOR and NAND logic gates. Both us ...

which is not mechanical in nature (i.e. no physical disk).

Disk controller versus host adapter

The component that allows a ''computer'' to talk to a peripheral bus is host adapter or

host bus adapter In computer hardware, a host controller, host adapter, or host bus adapter (HBA), connects a computer system bus, which acts as the host system, to other computer network, network and computer storage, storage devices. The terms are primarily ...

(HBA, e.g.

Advanced Host Controller Interface The Advanced Host Controller Interface (AHCI) is a technical standard defined by Intel that specifies the register-level interface of Serial ATA (SATA) host controllers in a non-implementation-specific manner in its motherboard chipsets. The s ...

or AHDC). A disk controller allows a ''disk'' to talk to the same bus. Signals read by a

disk read-and-write head A disk read-and-write head is the small part of a disk drive which moves above the disk platter and transforms the platter's magnetic field into electrical current (reads the disk) or, vice versa, transforms electrical current into magnetic fi ...

are converted by a disk controller, then transmitted over the peripheral bus, then converted again by the host adapter into the suitable format for the

motherboard A motherboard (also called mainboard, main circuit board, mb, mboard, backplane board, base board, system board, logic board (only in Apple computers) or mobo) is the main printed circuit board (PCB) in general-purpose computers and other expand ...

's bus, and then read by the CPU into the memory (RAM). Sometimes there may be yet another controller between a host adapter and a disk controller - a

disk array controller A disk array controller is a device that manages the physical disk drives and presents them to the computer as logical units. It almost always implements hardware RAID, thus it is sometimes referred to as RAID controller. It also often provides a ...

that allows hardware RAID to be formed. Sometimes it may be even physically integrated with an HBA.

Disk controller versus host adapter

See also