A disk controller is a controller circuit that enables a

CPU A central processing unit (CPU), also called a central processor, main processor, or just processor, is the primary processor in a given computer. Its electronic circuitry executes instructions of a computer program, such as arithmetic, log ...

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 hard disk drive platter, pla ...

floppy disk A floppy disk or floppy diskette (casually referred to as a floppy, a diskette, or a disk) is a 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 with a ...

or other kind of

disk drive Disc or disk may refer to: * Disk (mathematics), a two dimensional shape, the interior of a circle * Disk storage * Optical disc * Floppy disk Music * Disc (band), an American experimental music band * ''Disk'' (album), a 1995 EP by Moby Other ...

. It also provides an interface between the disk drive and the bus connecting it to the rest of the system.{{Cite book , url=https://ieeexplore.ieee.org/document/955038 , access-date=2023-10-18 , date=2001 , doi=10.1109/ICCD.2001.955038 , s2cid=3084914 , language=en-US , last1=Jeppesen , first1=J. , last2=Allen , first2=W. , last3=Anderson , first3=S. , last4=Pilsl , first4=M. , title=Proceedings 2001 IEEE International Conference on Computer Design: VLSI in Computers and Processors. ICCD 2001 , chapter=Hard disk controller: The disk drive's brain and body , pages=262–267 , isbn=0-7695-1200-3 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 (RLL) 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''/''n'', r ...

(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 ...

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, best known for its use with storage devices such as hard disk drives. SCSI was introduced ...

. Modern disk controllers are integrated into the disk drive as peripheral controllers. For example, ''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 connecting ...

in the host system over a standardized, high-level

storage bus Storage may refer to: Goods Containers * Dry cask storage, for storing high-level radioactive waste * Food storage * Intermodal container, cargo shipping * Storage tank Facilities * Mail storage, storage by mail or delivery service * Self storag ...

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 adapter, 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 (PAT ...

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 ot ...

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 Server (computing), servers in storage area networks (SAN) in ...

Serial Attached SCSI In computing, Serial Attached SCSI (SAS) is a point-to-point serial Communications protocol, protocol that moves data to and from Computer storage, computer-storage devices such as hard disk drives, solid-state drives and tape drives. SAS replac ...

. The peripheral controller integrated on PATA (IDE),

hard disk is usually a

microcontroller A microcontroller (MC, uC, or μC) or microcontroller unit (MCU) is a small computer on a single integrated circuit. A microcontroller contains one or more CPUs (processor cores) along with memory and programmable input/output peripherals. Pro ...

or an

ASIC An application-specific integrated circuit (ASIC ) is an integrated circuit (IC) chip customized for a particular use, rather than intended for general-purpose use, such as a chip designed to run in a digital voice recorder or a high-efficien ...

. Disk controllers can also control the timing of access to

flash memory Flash memory is an Integrated circuit, electronic Non-volatile memory, 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 t ...

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

Disk controller versus host adapter

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

or host bus adapter (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 that moves above the disk platter and transforms the platter's magnetic field into electric current (reads the disk) or, vice versa, transforms electric current into magnetic field ...

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 a mainboard, a system board, a logic board, and informally a mobo (see #Nomenclature, "Nomenclature" section), is the main printed circuit board (PCB) in general-purpose computers and other expandable systems. It ho ...

's bus, and then read by the

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 often implements hardware RAID, thus it is sometimes referred to as RAID controller. It also often provides additio ...

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

Disk controller versus host adapter

References

See also