{{unreferenced, date=May 2010 Seagate ST11R

The disk controller is the controller circuit which enables the CPU to communicate with a hard disk, floppy disk 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. 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 for home use. High-end disks use Parallel SCSI, Fibre Channel or Serial Attached SCSI. Disk controllers can also control the timing of access to flash memory 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 network and storage devices. The terms are primarily used to refer to devices for conne ...

(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 ...

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'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 ...

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

Disk controller versus host adapter

See also