The Floppy Disk Controller

The IBM 5.25" Diskette Drive Adapter

IBM PC/XT systems outfitted with a floppy disk drive had an IBM 5.25“ Diskette Drive Adapter card installed in one of the available expansion slots.

At a Glance

Controller Overview

The IBM floppy drive controller, as we’ll refer to it here, was a collection of 74-series logic chips and a 16.0MHz clock crystal supporting the “brain” of the card, a NEC µPD765A (NEC 765) floppy drive controller chip (or the more-or-less identical Intel P8272A).

Note

Microscope photography of the Intel P8272A has revealed the numbers ‘765’ are present on the die, implying Intel licensed NEC’s design and the controllers can be considered basically identical.

The IBM floppy controller could support up to four floppy disk drives, although configurations of more than two were uncommon. Drives 0-3 would be assigned the drive letters A-D up until MS-DOS 5.0, whereupon DOS started to reserve drive letter C for hard disks. It might feel a bit cursed to have a floppy disk as drive C, but if you did indeed have three drives connected and an earlier version of MS-DOS, that’s what you’d end up with.

The NEC 765 takes an 8MHz clock, divided once from the card’s 16MHz crystal. It has a maximum data rate of 250kbps, limiting it to support of double-density diskettes only.

The 765 doesn’t perform every function needed by a floppy controller. Notably, the controller’s voltage-controlled oscillator (VCO), a crucial part of the controller’s phase-locked-loop (PLL) is external to the 765, although the 765 has pins for interacting with it. Also external to the 765 is the data separator circuitry.

Operation

On the IBM PC/XT, the floppy drive controller is operated by the BIOS in DMA mode exclusively. It is possible to operate the controller in polled-io mode (PIO) in a manual fashion, but there are severe disadvantages to doing so - as was seen on the IBM PCjr which lacked a DMA controller. The lack of DMA prevented such operations as transferring data via the serial ports and floppy disk drive at the same time.

The 765 operates as a state machine with three basic operational phases:

• During the command phase the controller receives bytes from the host that instruct it to perform a specific operation.
• During the execution phase the controller carries out the operation specified in the command phase.
• During the result phase the controller provides status information to the host describing the result of the execution phase.

The 765 is capable of running some operations such as Seek and Recalibrate on multiple drives simultaneously.

The 765 maintains an internal register holding the current cylinder, called the Present Cylinder Number (PCN), for each of the four supported disk drives.

I/O Ports

The IBM Diskette Drive Adapter decodes the following I/O port addresses:

Floppy Controller Cable Pinout

The IBM floppy controller has two connectors, an internal and external connector, each supporting two drives. The external connector will not be discussed here as it was rarely used.

The internal connector is of the edge connector type, and contains 34 contacts. Other floppy controllers may have a 34-pin, double-row male header.

All of the odd pins of the connector — one entire side — consist of ground connections. That leaves the other side with the following pinout:

IBM floppy drive cables famously have a twist that reverses conductors 10—16. This allows every drive used in an IBM PC to be configured as drive 1 (DS1). This means that the drive desired to be the first floppy drive (A) should be at the end of the floppy cable.

Floppy Drive Communication

The electrical interface to a floppy drive is rather simple in concept. The controller manipulates the drive select and motor lines to activate the desired floppy drive and spin its motor up to its operating RPM. While rotating, regular, active-low pulses of the INDEX signal occur as the floppy drive’s index sensor detects the index hole. As the drive detects flux transitions, active-low pulses of the RD_DATA signal occur. If the controller wishes to write data, it will assert WR_GATE and then provide the appropriate pulses on the WR_DATA.

The controller specifies which head should be active via the HEAD1 output. If this signal is asserted, then head 1 is specified for all operations.

The controller can detect when the drive is positioned at the outermost track, track 0, via an active-low TRACK0 signal.

The Digital Output Register

The IBM controller card adds a main control register external to the 765, called the Digital Output Register or DOR. The DOR has several functions — it selects a specific drive as the target of operations, it can reset the 765, it can enable or disable interrupts and DMA, and it can turn on and off the attached floppy drive motors.

The DOR is a write-only register implemented with a 74LS273 8-bit register chip.

Drive Selection Bits

The two least significant bits (Bits 0—1) of the DOR control which floppy drive is selected.

Important

If a drive’s motor is not on, selecting it in the DOR will do nothing until the motor is turned on.

To avoid confusion, be aware that the DOR is the only drive selection method used by the IBM floppy drive controller. The NEC 765 command set includes fields that select which drive the operation is intended to target. Under IBM’s controller design, these bits do nothing externally — the 765 is not in control of which drive is selected. You can verify this yourself by noting the 765’s unit select pins, 28 and 29, are not connected on the schematic.

Important

Even though the 765 does not control drive selection, a programmer should ensure that the drive selected in the DOR matches the drive number specified in commands. A mismatch in the drive selected in the DOR and command phase can cause incorrect operation of the Seek and Recalibrate commands.

Reset Bit

The DOR’s RESET bit 2 drives the 765’s active-high RST pin (1) via an inverter. When a 1 is written to bit 2, the 765’s RST pin is pulled low, holding the controller in reset.

Note

The IBM floppy controller card ties the 765’s RDY pin (35) high. This causes the 765 to generate an interrupt after coming out of the RESET state after “1-25ms”, according to the datasheet. The controller is reset quite often during normal operation.

The 765 Registers

The 765 contains five status registers in total. The first of these is the main status register (MSR). The MSR is the only status register that may be read at any time. The contents of the other status registers, ST0—ST3, are sent to host during the controller result phase.

The Main Status Register

The BUSY bit

The BUSY bit is set when the controller is executing a command, and is only cleared when the last byte of the result phase has been read by the host CPU. It can be used as a general indication that a new command should not be sent to the CPU.

The DIO bit

The DIO bit controls the direction of transfer to or from the controller. The CPU should only send data to the controller when DIO is 0. When DIO is 1, the controller is not ready to receive data — it expects to have data to send to the CPU.

The MRQ bit

The MRQ bit can be usefully combined with the DIO bit to control data transfer to and from the controller.

Note

This bit may also be labelled as RQM in technical references.

Status Register 0 (ST0)

Note

The IBM floppy controller does not connect the typical floppy drive READY indication line, pin 34. Therefore the NR bit will typically not be set, even if the drive door is open and/or no disk is inserted into the drive. If you emulate this bit anyway, you may receive a “General Failure” error message in DOS when no disk is inserted instead of the expected “Not Ready Reading Drive” error.

Correspondingly, since a drive cannot report not-ready during the execution phase, an IC code of 11b will not be encountered.

Note

The IBM floppy controller does not accommodate an external fault signal, nor do most PC floppy drives supply one. Therefore the EC field can only report failure of the Track 0 sensor during a Seek or Recalibrate operation.

Status Register 1 (ST1)

The No-Data Bit

The No-Data bit is set under the following conditions:

• The FDC could not find the requested sector during execution of READ DATA, WRITE DELETED DATA or SCAN
• The FDC could not read the ID field during execution of READ ID
• The FDC could not find the starting sector during execution of READ CYLINDER.

Status Register 2 (ST2)

Status Register 3 (ST3)

Important

Remember that on the IBM floppy controller, the 765’s unit select pins are not connected and do not drive selection of floppy drives.

Controller Commands

The 765 supports 15 different commands:

Commands consist of 2—9 command bytes, sent during the command phase. The command will then enter the execution phase. When complete (or on error) the command enters the result phase and various status register bytes will be made available to read out from the data register.

The first command code byte 0 is common to all commands and contains the command code as given in the table above, along with operational flags. These flags are not always applicable to each command. The flags applicable to each command are noted in the table above.

Command Code Byte 0

Command Code Byte 1

The Read Data Command

The Read Data command is opcode 06h.

The Read Data command searches for the sector ID matching the specified cylinder, head, sector and sector size. The matching sector must have a normal data address mark (DAM).

In DMA mode, it will read successive sectors that have a normal DAM until terminal count. In PIO mode, it will read successive sectors that have a normal DAM until EOT is reached.

If the MT flag is set, once the last sector of side 0 is transferred, the FDC proceeds to repeat the operation on side 1.

The Read Data command behaves as follows depending on the value of the SK flag:

After the data is transferred, the command enters the result phase.

Status Flags Affected

Command Format

The Read Deleted Data Command

The Read Deleted Data command is opcode 0Ch.

The Read Deleted Data command searches for the sector ID matching the specified cylinder, head, sector and sector size. The matching sector must have a deleted data address mark (DDAM).

In DMA mode, it will read successive sectors that have a DDAM until terminal count. In PIO mode, it will read successive sectors that have a DDAM until EOT is reached.

If the MT flag is set, once the last sector of side 0 is transferred, the FDC proceeds to repeat the operation on side 1.

The Read Deleted Data command behaves as follows depending on the value of the SK flag:

After the data is transferred, the command enters the result phase.

Status Flags Affected

Command Format

The Read ID Command

The Read ID command is opcode 0Ah.

The FDC returns the fields of the next Sector ID encountered on the track.

The MT and SK flags are not valid for this command and should be 0.

Status Flags Affected

Command Format

The Read Track Command

The Read Track command is opcode 02h.

Unlike what its name might suggest, Read Track does not actually read a raw track. Instead, it reads all sectors on a track, regardless of ID or CRC validation.

The Read Track command first waits for the index signal. It then returns the data for each sector encountered, in order. The Sector ID of the first sector is checked against the values given to the command for the purposes of setting the ND bit in ST1, but a mismatch is not an error and does not stop command execution.

The MT and SK flags are not valid for this command and should be 0.

Status Flags Affected

Command Format

The Scan Commands

The three scan commands, Scan Equal, Scan High or Equal, and Scan Low or Equal, behave identically other than the mathematical expression they evaluate for each pair of bytes processed.

In each of the three commands, the FDC performs a comparison (the scan condition) between bytes read from the disk and bytes provided by the host. If this comparison is true for the entire sector, the scan condition is said to have been satisfied or that a scan hit has occurred, at which point the scan operation will enter the result phase.

The value of FFh is special, and is treated as a wildcard. If either the byte from the disk or the byte from the host is FFh then the comparison will be considered true. This allows use of FFh by the host to mask the comparison bytes provided to the FDC.

If a sector does not satisfy the scan condition, the FDC will continue with the next sector to scan based on the STP step parameter. If STP == 1, this will be the next consecutive sector number. If STP == 2, then the FDC will skip the next consecutive sector number.

Important

Regardless of the STP parameter, the FDC must be able to read the last sector on the track as specified by EOT. Unexpectedly encountering the index pulse during the scan operation will result in abnormal termination.

If EOT is reached or terminal count occurs before a scan hit occurs, then the scan operation ends with a scan not satisfied condition. In the case of a terminal count, the operation ends after the current byte being compared.

Scan Command Operations

Status Flags Affected

Command Format

The Specify Command

The Specify command sets three timing variables for the drive as well as setting the FDC operational mode to DMA or PIO modes.

The Specify command has no result phase.

Command Format

Command Byte 1: Specify Timing 0

Command Byte 2: Specify Timing 1

The Write Data Command

The Write Data command is opcode 05h.

The Write Data command searches for the sector ID matching the specified cylinder, head, sector and sector size. Immediately after the matching IDAM, the controller will write a data field containing a normal data address mark (DAM).

In DMA mode, it will write data to consecutive sectors that have a normal DAM, starting at the initial matching sector ID, until terminal count. In PIO mode, it will write data to consecutive sectors that have a normal DAM up to and including the sector number specified by EOT.

If the MT flag is set, once the last sector of side 0 is transferred, the FDC proceeds to repeat the operation on side 1. A multi-track operation must begin on side 0 — the controller will not continue to side 0 from side 1.

The SK flag is not valid with Write Data and should be 0.

After the data is transferred, the command enters the result phase.

Status Flags Affected

Command Format

The Write Deleted Data Command

The Write Deleted Data command is opcode 09h.

The Write Deleted Data command searches for the sector ID matching the specified cylinder, head, sector and sector size. Immediately after the matching IDAM, the controller will write a data field containing a deleted data address mark (DDAM).

In DMA mode, it will write data to consecutive sectors that have a DDAM, starting at the initial matching sector ID, until terminal count. In PIO mode, it will write data to consecutive sectors that have a DDAM up to and including the sector number specified by EOT.

If the MT flag is set, once the last sector of side 0 is transferred, the FDC proceeds to repeat the operation on side 1. A multi-track operation must begin on side 0 — the controller will not continue to side 0 from side 1.

The Write Deleted Data command behaves as follows depending on the value of the SK flag:

After the data is transferred, the command enters the result phase.

Status Flags Affected

Command Format

The Seek Command

The Seek command is opcode 0Fh.

The Seek command instructs the controller to seek to the specified cylinder number provided in the command phase. The controller will issue the required number of step pulses to move the head to the requested cylinder. If the cylinder position is indeterminate, a Recalibrate Command should be issued first.

Important

The 765 maintains an internal register holding the current cylinder — called the Present Cylinder Number (PCN) — for each of the four supported disk drives. Although on the IBM PC the 765’s unit select pins are nonfunctional, a mismatch in the drive selected in the DOR and command phase can cause incorrect Seek operation.

The rate at which the step pulses are issued is controlled by the SRT field of the first command byte provided to the Specify command.

Note

If more than 150 microseconds elapse during the command phase, the period between the first and second step pulse issued by the controller may be shortened by up to 1ms compared to the value of SRT.

Multiple Seek commands can be issued at the same time. Once the first Seek command enters the execution phase, the controller becomes non-busy and can receive another Seek command. In this manner, all four supported drives can be executing a Seek command at the same time, but no other command is valid when any Seek operation is in progress.

The Seek command sets bits D0B—D3B in the Main Status Register according to which drive was selected in the command phase. These bits are only cleared by a Sense Interrupt Status command.

If the drive is not ready at the end of the command phase, or if the drive becomes not ready during the execution phase before completion of the seek operation, the NR bit is set in ST0 and the operation terminates with the abnormal termination interrupt code.

Note

The IBM floppy controller does not connect the READY line, so it has no way of detecting the not-ready condition.

The Seek has no result phase.

Status Flags Affected

Command Format

The Recalibrate Command

The Recalibrate command is opcode 07h.

The Recalibrate command issues step pulses to the drive until the track 0 sensor indicates the drive has reached track 0. Issuing the Recalibrate command immediately clears the present cylinder number (PCN) register for the specified drive.

Multiple Recalibrate commands can be issued at the same time. Once the first Recalibrate command enters the execution phase, the controller becomes non-busy and can receive another Recalibrate command. In this manner, all four supported drives can be executing a Recalibrate command at the same time, but no other command is valid when any Recalibrate operation is in progress.

The Recalibrate command sets bits D0B—D3B in the Main Status Register according to which drive was selected in the command phase. These bits are only cleared by a Sense Interrupt Status command.

If the drive is not ready at the end of the command phase, or if the drive becomes not ready during the execution phase before completion of the recalibrate operation, the NR bit is set in ST0 and the operation terminates with the abnormal termination interrupt code.

Note

The IBM floppy controller does not connect the READY line, so it has no way of detecting the not-ready condition.

The Recalibrate has no result phase.

Status Flags Affected

Command Format

The Format Track Command

The Format Track command is opcode 0Dh.

The Format Track command is used to write out the initial sector layout of a track, including sector ID address marks (IDAMs) and data address marks (DAMs).

The controller first detects the index pulse before writing, to synchronize the newly-written track against the index position.

Unlike most other FDC commands, additional data must be provided to the controller during the execution phase. For each sector to be written (as specified by command byte 3, SPT), four bytes must be transferred to the controller providing the values C, H, S and N for each IDAM that must be written. These bytes are transferred via DMA when DMA mode is enabled.

Command byte 5, the data pattern, is used to fill the data field of the sector written.

The inter-sector gap, or GAP 3, is controlled by command byte 4.

Command Format

The Sense Interrupt Status Command

The Sense Interrupt Status command is opcode 08h.

The Sense Interrupt Status command is used to retrieve the controller status after certain interrupt signals have been issued.

A controller interrupt is generated in the following conditions:

1. Entering the result phase of the following commands:
   • Read Data
   • Read Deleted Data
   • Read Track
   • Write Data
   • Write Deleted Data
   • Format Track
   • any Scan command
2. Upon termination of commands without a result phase:
   • Seek
   • Recalibrate
3. A change in the READY line status for any drive*
4. Regularly during the execution phase when in PIO mode.

Interrupts from source 1 are cleared by reading or writing to the controller, typically reading out the bytes from the result phase. Interrupts from source 2 have no result phase, therefore a Sense Interrupt Status command must be issued to terminate the command and clear the interrupt.

Recall that for condition 2, up to four simultaneous Seek or Recalibrate operations may be in progress, therefore, up to four Sense Interrupt commands may need to be issued, one for each operation.

Note

*The IBM floppy controller ties the 765’s RDY pin high, and thus it cannot respond to changes in drive READY status.

Command Format

The Sense Drive Status Command

The Sense Drive Status command is opcode 04h.

The Sense Drive Status command retrieves the status of the drive indicated by the command byte 1, returned in the form of ST3.

Command Format

Invalid Commands

Any unrecognized or unsupported command code will return a single ST0 byte with a IC of 10b, or invalid command. The value of the ST0 byte in hexadecimal will be 80h. Some software looks for this code to be returned in response to specific commands in order to properly identify the type of controller present.

Datasheets

• (archive.org) μPD765A Single/Double Density Floppy Disk Controller
• (archive.org) IBM 5-1/4“ Diskette Drive Adapter

Primary References

• (debs.future.easyspace.com @ web.archive.org) Programming Floppy Disk Controllers