A disk controller is a controller circuit that enables a 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 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. 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 (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 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 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 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), Serial ATA, Parallel SCSI or

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. 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 host adapter or host bus adapter (HBA, e.g. Advanced Host Controller Interface 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 CPU into the memory (RAM). Sometimes there may be yet another controller between a host adapter and a disk controller - a disk array controller 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