Floppy-disk controller

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Zilog Z765A
5 1/4 Diskette Drive Adapter found on the IBM PC (IBM 5150)

A floppy-disk controller (FDC) is a special-purpose chip and associated disk controller circuitry that directs and controls reading from and writing to a computer's floppy disk drive (FDD). This article contains concepts common to FDCs based on the NEC µPD765 and Intel 8272A or 82072A and their descendants, as used in the IBM PC and compatibles from the 1980s and 1990s. The concepts may or may not be applicable to, or illustrative of, other controllers or architectures.


A single floppy-disk controller (FDC) board can support up to four floppy disk drives. The controller is linked to the system bus of the computer and appears as a set of I/O ports to the CPU. It is often also connected to a channel of the DMA controller. On the x86 PC the floppy controller uses IRQ 6, on other systems other interrupt schemes may be used. The floppy disk controller usually performs data transmission in direct memory access (DMA) mode.

The diagram below shows a floppy disk controller which communicates with the CPU via an Industry Standard Architecture (ISA) bus. An alternative arrangement which is more usual in recent designs has the FDC included in a super I/O chip which communicates via a Low Pin Count (LPC) bus.

Block diagram showing FDC communication with the CPU and the FDD.

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)[edit]

  • 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 generation and verification, like CRC
  • Synchronize data with phase-locked loop (PLL)

External hardware functions[edit]

  • Selection of floppy disk drive (FDD)
  • 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[edit]

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.

Port Address
Port Name Location Port type
3F5 Data port Bidirectional I/O
3F4 Main status register FDC IC Input
3F2 Digital control port External hardware Output

Data port[edit]

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)[edit]

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 :

Bit Representation
0 FDD 0: Busy in seek mode
1 FDD 1: Busy in seek mode
2 FDD 2: Busy in seek mode
3 FDD 3: Busy in seek mode
4 FDC Busy; Read/Write command in progress
5 Non-DMA mode
6 DIO; Indicates the direction of data transfer between the FDC IC and the CPU
7 MQR; Indicates data register is ready for data transfer
MQR 1 = data register ready, 0 = data register not ready
DIO 1 = controller has data for CPU, 0 = controller expecting data from CPU
Non-DMA 1 = Controller Not in DMA Mode, 0 = Controller in DMA Mode
FDC Busy 1 = Busy, 0 = Not Busy
FDD 0,1,2,3 1 = Running, 0 = Not Running

Digital control port[edit]

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

Bit Representation
0 and 1 Device number to be selected
3 Enable FDC interrupt and DMA request signals
4 to 7 Turn ON the motor in disk drive 0, 1, 2 or 3 respectively

Format data[edit]

Many mutually incompatible floppy disk formats are possible; aside from the physical format on the disk, incompatible file systems are also possible.

Drive Format Capacity Transfer
RPM Tracks TPI Comment
8-inch SD 8-inch SD 80 KB 33.333 360 32 48 Only on old controllers.[1]
5.25-inch SD 5.25-inch SD 160 KB 125 40 Only on old controllers.
5.25-inch SD 5.25-inch SD 171 KB 250–308 300 35 48[2] Only on C1541 compatibles.
5.25-inch SD 5.25-inch SD 180 KB 150 40 Only on old controllers.
5.25-inch DD 5.25-inch DD 320/360/400 KB 250 300 40 48 [3] 8/9/10 512K sectors respectively.
5.25-inch DD (96 tpi) 5.25-inch QD (2DD) 800 KB 250 300 80 96 [4]
5.25-inch HD 5.25-inch DD 360 KB 300 360 40 48 [5][6]
5.25" HD 5.25" HD 1200 KB 500 360 80 96 Up to 83 tracks. Different biasing current.[5][6]
5.25" HD 5.25" HD 720 KB 300 360 80 Up to 83 tracks.[3]
3.5" DD 3.5" DD 720 KB 250 300 80 135 Up to 83 tracks.[3][7]
3.5" DD 3.5" DD 800 KB 394–590 80 Used by Apple Macintosh.[8]
3.5" DD 3.5" DD 800 KB 250 300 80 Used by Commodore 1581.
3.5" DD 3.5" DD 880 KB 250 300 80 Up to 83 tracks. Used by Amiga computers.
3.5" DD 3.5" DD 360 KB 250 300 40 [3]
3.5" HD 3.5" DD 720 KB 250 300 80 Up to 83 tracks.[3]
3.5" HD 3.5" HD 1440 KB 500 300 80 135 Up to 83 tracks.[3][9]
3.5" HD 3.5" HD 1760 KB 250 150 80 Used by Amiga computers.
3.5" ED 3.5" ED 2880 KB 1000 300 80 135 Up to 83 tracks.[7][10]




"3-mode" floppy drive[edit]

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.[12] 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. Later Japanese floppy drives incorporated support for both high-density formats (as well as the double-density format), hence the name 3-mode. Some BIOSes have a configuration setting to enable this mode for floppy drives supporting it.[13]

See also[edit]


  1. ^ hypertextbook.com – Angular Speed of a Floppy Disk
  2. ^ "C 64 Workshop / C= 8 Bit & Peripherals". 1998-05-19. Retrieved 2016-04-18.
  3. ^ a b c d e f "unifr.ch – sys/src/kernel/floppy.c". Archived from the original on 19 July 2011. Retrieved 5 May 2011.
  4. ^ "Product specification single-sided and double-sided TM100 series 5 1/4-inch flexible disk drives 48, 96, and 100 tracks per inch" (PDF). Retrieved 2014-11-05.
  5. ^ a b iesleonardo.info – This diskette tutorial provides technical information concerning diskettes
  6. ^ a b oldskool.org – Let HD 5,25" FDDs operate at 300 rpm instead of 360 rpm
  7. ^ a b intel.com – Intel 82077SL for Super Dense Floppies Archived 8 October 2012 at the Wayback Machine.
  8. ^ Johnson, Herbert R. (2016-12-22). "Floppy Drive Tech Info". Retrieved 2017-01-14.
  9. ^ yi.org – High Density Floppy Disks Mf2hd Disk 3 5 1 Pk[permanent dead link]
  10. ^ mcamafia.de – IBM Personal system/2, 3,5"-inch Diskette Drives, Technical Reference
  11. ^ "Linux-2.6.17/drivers/block/floppy.c".[permanent dead link] 090504 gelato.unsw.edu.au
  12. ^ books.google.com – Fix Your Own PC by Corey Sandler
  13. ^ rojakpot.com – 3mode floppy support

Further reading[edit]

External links[edit]