A floppy-disk controller (FDC) has evolved from a discrete set of components on one or more circuit boards to a special-purpose integrated circuit (IC or "chip") or a component thereof. An FDC directs and controls reading from and writing to a computer's

floppy disk drive 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 ...

(FDD). The FDC is responsible for reading data presented from the host computer and converting it to the drive's on-disk format using one of a number of encoding schemes, like FM encoding (single density) or MFM encoding (double density), and reading those formats and returning it to its original binary values. Depending on the platform, data transfers between the controller and host computer would be controlled by the computer's own

microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circ ...

, or an inexpensive dedicated microprocessor like the MOS 6507 or

Zilog Z80 The Z80 is an 8-bit microprocessor introduced by Zilog as the startup company's first product. The Z80 was conceived by Federico Faggin in late 1974 and developed by him and his 11 employees starting in early 1975. The first working samples were ...

. Early controllers required additional circuitry to perform specific tasks like providing clock signals and setting various options. Later designs included more of this functionality on the controller and reduced the complexity of the external circuitry; single-chip solutions were common by the later 1980s. By the 1990s, the floppy disk was increasingly giving way to

hard drive 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 magne ...

s, which required similar controllers. In these systems, the controller also often combined a microcontroller to handle data transfer over standardized connectors like SCSI and IDE that could be used with any computer. In more modern systems, the FDC, if present at all, is typically part of the many functions provided by a single

super I/O Super I/O is a class of I/O controller integrated circuits that began to be used on personal computer motherboards in the late 1980s, originally as add-in cards, later embedded on the motherboards. A super I/O chip combines interfaces for a vari ...

chip. __TOC__

History

The first floppy disk drive controller (FDC) like the first floppy disk drive (the IBM 23FD) shipped in 1971 as a component in the IBM 2385 Storage Control Unit for the IBM 2305 fixed head disk drive, and of the System 370 Models

155 Year 155 ( CLV) was a common year starting on Tuesday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Severus and Rufinus (or, less frequently, year 908 ''Ab urbe condita'' ...

and

165 Year 165 ( CLXV) was a common year starting on Monday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Orfitus and Pudens (or, less frequently, year 918 ''Ab urbe condita'' ...

. The IBM 3830 Storage Control Unit, a contemporaneous and quite similar controller, uses its internal processor to control a 23FD. The resultant FDC is a simple implementation in IBMs’

MST The Master of Studies or Master in Studies (M.St. or MSt; ) is a postgraduate degree at the University of Oxford, University of Cambridge, University of St Andrews, the Australian National University, University of Dublin and the University of Ne ...

hybrid circuits on a few printed circuit cards. The drive, FDC and media were proprietary to IBM and although other manufacturers provided early FDDs prior to 1973 there were no standards for FDCs, drives or media. IBM's 1973 introduction of the 3740 Data Entry System created the basic media standard for the 8-inch single sided floppy disk, IBM's "Type 1" diskette, which coupled with rapidly increasing requirements for inexpensive, removable direct access storage for many small applications caused a dramatic growth in drive and controller shipments. Prior to the introduction of special purpose integrated circuit versions, most FDCs consisted of at least one printed circuit implemented with 40 or more ICs. Examples of such FDCs include: *1973: The FDC in IBM's 3741 is a type of microcontroller that accepts commands from the system's microprocessor ("MPU" in IBM's terminology) and executes them on the attached 33FD as independently as possible. It accepts and executes the following commands, select/stop, write check, seek lower, seek higher, read data, read I D, write data, write control, write I D, set ready, reset access counter, and nothing (no-op). It was implemented using IBM's MST hybrid circuits on the motherboard plus a separate data separator (VFO) pcb. This IBM FDC did establish the IBM Type 1 diskette as the first industry standard floppy disk medium but neither its interface to the host microprocessor nor its interface to the 33FD were adopted as industry standards. *1974: iCOM’s FD360 contained an early FDC, the CF 360, that generated industry standard media, connected to industry standard host busses, and supported industry standard FDDs.Its FDC was implemented on a pcb approximately 12x9 inches as a state machine using 30 ICs. *1976: Scientific Micro Systems’ FD0300 FDC built on an 8-inch by 12-inch contains a microprocessor and approximately 50 integrated circuits and is designed to provide easy attachment to a number of host buses. *1976:

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 ...

introduced the first 5¼-inch floppy disk drive along with an associated and first FDC for this form factor, the SA4400. The SA4400 performs control functions to transfer data between 1 to 3 disk drives and a host system using an 8-bit general purpose host interface which format disks according to a modified IBM 3740 type media format specifications. The FDC is microprocesser controlled and implemented on a 5.75 by 9.50 inch pcb with 45 ICs. The drive interface and media form factors became industry standards with the media then evolving over time to support a number of different formats. *1977: The Apple Disc II FDC, the “Woz Machine”, is built with only 8 ICs. It, like the much earlier IBM 3830 FDC, achieved the reduction in components thru use of the host processor and firmware. Its interface to the Apple host as well as its interface to the Apple 5¼-inch floppy disk drive is unique and it was not adopted as an industry standard. The first FDC implemented as a special purpose integrated circuit is the

Western Digital FD1771 The FD1771, sometimes WD1771, is the first in a line of floppy disk controllers produced by Western Digital. It uses single density FM encoding introduced in the IBM 3740. It is packaged in a 40-pin DIP."The FD1771 is a single-chip floppy disk ...

announced on July 19, 1976. The initial design supported a single format and required additional circuitry but over time, as a family, the design became multi-sourced and

evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

to support many formats and minimize external circuitry. The NEC µPD765, announced in 1978 became a quasi-industry standard when it was adopted in the original IBM PC (1981); the FDC was physically located on its own adapter card along with support circuitry. Other vendors such as Intel produced compatible parts. This design evolved over time into a family offering an almost complete FDC on a chip. In early 1987, Intel introduced the 82072 CHMOS High Integrated Floppy Disk Controller for use in industry standard PC computers. Ultimately in most computer systems the FDC became a part of a

Super I/O Super I/O is a class of I/O controller integrated circuits that began to be used on personal computer motherboards in the late 1980s, originally as add-in cards, later embedded on the motherboards. A super I/O chip combines interfaces for a vari ...

chip or a Southbridge chip.

Overview

A floppy disk stores binary data not as a series of values, but a series of ''changes'' in value. Each of these changes, recorded in the polarity of the magnetic recording media, causes a voltage to be induced in the

drive 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 f ...

as the disk surface rotates past it. It is the timing of these polarization changes and the resulting spikes of voltage that encode the ones and zeros of the original data. One of the functions of the controller is to turn the original data into the proper pattern of polarizations during writing, and then recreate it during reads. As the storage is based on timing, and that timing is easily affected by mechanical and electrical disturbances, accurately reading the data requires some sort of reference signal, the

clock A clock or a timepiece is a device used to measure and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month and t ...

. As the on-disk timing is constantly changing, the clock signal has to be provided by the disk itself. To do this, the original data is modified with extra transitions to allow the clock signal to be encoded in the data and then use

clock recovery In serial communication of digital data, clock recovery is the process of extracting timing information from a serial data stream itself, allowing the timing of the data in the stream to be accurately determined without separate clock information. ...

during reads to recreate the original signal. Some controllers require this encoding to be performed externally, but most designs provide standard encodings like FM and MFM. The controller also provides a number of other services to control the drive mechanism itself. These typically include the movement of the drive head to center over the separate tracks on the disk, tracking the location of the head and returning it to zero, and sometimes functionally to format a disk based on simple inputs like the number of tracks, sectors per track and number of bytes per sector. To produce a complete system, the controller has to be combined with additional circuitry or software that acts as a bridge between the controller and the host system. In some systems, like the Apple II and IBM PC, this is controlled by software running on the computer's host

and the drive interface is connected directly to the processor using an

expansion card In computing, an expansion card (also called an expansion board, adapter card, peripheral card or accessory card) is a printed circuit board that can be inserted into an electrical connector, or expansion slot (also referred to as a bus sl ...

. On other systems, like the Commodore 64 and Atari 8-bit family, there is no direct path from the controller to the host CPU and a second processor like the MOS 6507 or

is used inside the drive for this purpose. The original Apple II controller was in the form of a plug-in card on the host computer. It could support two drives, and the drives eliminated most of the normal onboard circuitry. This allowed Apple to arrange a deal with

for a simplified drive that lacked most of its normal circuitry. This meant that the combined cost of a single drive and controller card was roughly the same as on other systems, but a second drive could be connected for a smaller additional cost. The IBM PC took a more conventional approach, their adaptor card could support up to four drives; on the PC

direct memory access Direct memory access (DMA) is a feature of computer systems and allows certain hardware subsystems to access main system memory independently of the central processing unit (CPU). Without DMA, when the CPU is using programmed input/output, it is ...

(DMA) to the drives was performed using DMA channel 2 and IRQ 6. The diagram below shows a conventional floppy disk controller which communicates with the CPU via an Industry Standard Architecture (ISA) bus or similar bus and communicates with the floppy disk drive with a 34 pin ribbon cable. An alternative arrangement that is more usual in recent designs has the FDC included in a

chip which communicates via a

Low Pin Count The Low Pin Count (LPC) bus is a computer bus used on IBM-compatible personal computers to connect low-bandwidth devices to the CPU, such as the BIOS ROM (BIOS ROM was moved to the Serial Peripheral Interface (SPI) bus in 2006), "legacy" I/O d ...

(LPC) bus. Most of the floppy disk controller (FDC) functions are performed by the integrated circuit but some are performed by external hardware circuits. The list of functions performed by each is given below.

Floppy disk controller functions (FDC)

* Translate data bits into FM, MFM, M²FM, or GCR format to be able to record them * Interpret and execute commands such as seek, read, write, format, etc. * Error detection with

checksums A checksum is a small-sized block of data derived from another block of digital data for the purpose of detecting errors that may have been introduced during its transmission or storage. By themselves, checksums are often used to verify dat ...

generation and verification, like CRC * Synchronize data with

phase-locked loop A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. There are several different types; the simplest is an electronic circuit consisting of a ...

(PLL)

External hardware functions

* Selection of which

(FDD) to address * Switching-on the floppy drive motor * Reset signal for the floppy controller IC * Enable/disable interrupt and DMA signals in the floppy disk controller (FDC) * Data separation logic * Write pre-compensation logic * Line drivers for signals to the controller * Line receivers for signals from the controller

Input/output ports for common x86-PC controller

The FDC has three I/O ports. These are: * Data port * Main status register (MSR) * Digital control port The first two reside inside the FDC IC while the Control port is in the external hardware. The addresses of these three ports are as follows.

Data port

This port is used by the software for three different purposes: * While issuing a command to the FDC IC, command and command parameter bytes are issued to the FDC IC through this port. The FDC IC stores the different parameters and the command in its internal registers. * After a command is executed, the FDC IC stores a set of status parameters in the internal registers. These are read by the CPU through this port. The different status bytes are presented by the FDC IC in a specific sequence. * In the programmed and interrupt mode of data transfer, the data port is used for transferring data between the FDC IC and the CPU IN or OUT instruction.

Main status register (MSR)

This port is used by the software to read the overall status information regarding the FDC IC and the FDD's. Before initiating a floppy disk operation the software reads this port to confirm the readiness condition of the FDC and the disk drives to verify the status of the previously initiated command. The different bits of this register represent :

Digital control port

This port is used by the software to control certain FDD and FDC IC functions. The bit assignments of this port are:

Interface to the floppy disk drive

A controller connects to one or more drives using a flat ribbon cable, 50 wires for 8" drives and 34 wires for 3.5" & 5.25" drives. A "universal cable" has four drive connectors, two each for 3.5" & 5.25" drives. In the IBM PC family and compatibles, a twist in the cable is used to distinguish disk drives by the socket to which they are connected. All drives are installed with the same drive select address set, and the twist in the cable interchanges the drive select lines at the socket. The drive that is at the far end of the cable would also have a

terminating resistor In electronics, electrical termination is the practice of ending a transmission line with a device that matches the characteristic impedance of the line. Termination prevents signals from reflecting off the end of the transmission line. Reflect ...

installed to maintain signal quality. :More detailed descriptions of the interface signals including alternative meanings are contained in manufacturer's specifications for drives or host controllers. When the controller and disk drive are assembled as one device, as it is the case with some external floppy disk drives, e.g.,

Commodore 1540 The Commodore 1540 (also known as the VIC-1540) introduced in 1982 is the companion floppy disk drive for the VIC-20 home computer. It uses single-sided 5¼" floppy disks, on which it stores roughly of data utilizing Commodore's GCR data enco ...

and USB floppy disk drives, the internal floppy disk drive and its interface are unchanged, while the assembled device presents a different interface such as

IEEE 488 IEEE 488 is a short-range digital communications 8-bit parallel multi-master interface bus specification developed by Hewlett-Packard as HP-IB (Hewlett-Packard Interface Bus). It subsequently became the subject of several standards, and is ...

, parallel port or

USB Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and power supply (interfacing) between computers, peripherals and other computers. A broad ...

Format data

Many mutually incompatible floppy disk formats are possible; aside from the physical format on the disk, incompatible file systems are also possible. 090504 gelato.unsw.edu.au Sides: * SS (or 1S) – Single sided * DS (or 2S) – Double sided Density: * SD (or 1D) –

Single density Disk density is a capacity designation on magnetic storage, usually floppy disks. Each designation describes a set of characteristics that can affect the areal density of a disk or the efficiency of the encoded data. Such characteristics include ...

( FM) * DD (or 2D) –

Double density Disk density is a capacity designation on magnetic storage, usually floppy disks. Each designation describes a set of characteristics that can affect the areal density of a disk or the efficiency of the encoded data. Such characteristics include ...

(most often MFM) * QD (or 4D) – Quad density * HD –

High density Disk density is a capacity designation on magnetic storage, usually floppy disks. Each designation describes a set of characteristics that can affect the areal density of a disk or the efficiency of the encoded data. Such characteristics include ...

* ED – Extra-high density * TD – Triple density

"3mode" floppy drive

Primarily in Japan there are 3.5" high-density floppy drives that support three modes of disk formats instead of the normal two – 1440 KB (2 MB unformatted), 1.2 MB (1.6 MB unformatted) and 720 KB (1 MB unformatted). Originally, the high-density mode for 3.5" floppy drives in Japan only supported a capacity of 1.2 MB instead of the 1440 KB capacity that was used elsewhere. While the more common 1440 KB format spun at 300 rpm, the 1.2 MB format instead spun at 360 rpm, thereby closely resembling the 1.2 MB format with 15 sectors per track previously found on 5.25" high-density floppy drives or 1.2 MB format with 8 sectors per track previously found on 8" Double-density floppy drives. Later Japanese floppy drives incorporated support for both high-density formats (as well as the double-density format), hence the name 3mode. Some BIOSes have a configuration setting to enable this mode for floppy drives supporting it.rojakpot.com – 3mode floppy support
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References

External links

viralpatel.net A Tutorial on Programming Floppy Disk Controller

Computer storage devices Floppy disk computer storage Integrated circuits