Click above to get retro games delivered to your door ever month!

               NEC .PD765  -  Floppy Disk Controller  -  8272A

      PS/2 FDC Diskette Status Register A at 3F0h

        |7|6|5|4|3|2|1|0|  3F0h  PS/2 Disk Status Register A (read-only)
         | | | | | | | +---- direction
         | | | | | | +----- write protect
         | | | | | +------ index
         | | | | +------- head 1 select
         | | | +-------- track 0
         | | +--------- step
         | +---------- second drive installed
         +----------- interrupt pending

      PS/2 FDC Diskette Status Register B at 3F1h

        |7|6|5|4|3|2|1|0|  3F1h  PS/2 Disk Status Register B (read-only)
         | | | | | | | +---- motor enable 0
         | | | | | | +---- motor enable 1
         | | | | | +---- write enable
         | | | | +---- read data (toggles w/positive transition in -RD DATA)
         | | | +---- write data (toggles w/positive transition in WR DATA)
         | | +---- drive select
         +------ reserved

      FDC Digital Output Register at 3F2h  (all systems)

        |7|6|5|4|3|2|1|0|  port 3F2h  (write only)
         | | | | | | +------ floppy drive select (0=A, 1=B, 2=floppy C, ...)
         | | | | | +------- 1 = FDC enable, 0 = hold FDC at reset
         | | | | +-------- 1 = DMA & I/O interface enabled  (reserved PS/2)
         | | | +--------- 1 = turn floppy drive A motor on
         | | +---------- 1 = turn floppy drive B motor on
         | +----------- 1 = turn floppy drive C motor on; (reserved PS/2)
         +------------ 1 = turn floppy drive D motor on; (reserved PS/2)

        - used to control drive motors, drive selection, and feature enable
        - PS/2 only uses bit 0 for floppy drive select; bit 1 is reserved
        - PS/2 only uses bits 5 & 4 for  motor enable; bits 7&6 are reserved
        - all DOR bits are cleared during controller reset

      FDC Main Status Register at 3F4h  (all systems)

        |7|6|5|4|3|2|1|0|  port 3F4h  (read only)
         | | | | | | | +---- floppy drive 0 in seek mode/busy
         | | | | | | +----- floppy drive 1 in seek mode/busy
         | | | | | +------ floppy drive 2 in seek mode/busy (reserved PS/2)
         | | | | +------- floppy drive 3 in seek mode/busy (reserved PS/2)
         | | | +-------- FDC read or write command in progress
         | | +--------- FDC is in non-DMA mode
         | +---------- I/O direction;  1 = FDC to CPU; 0 = CPU to FDC
         +----------- data reg ready for I/O to/from CPU (request for master)

      FDC Command Status Register 0 at 3F5h  (all systems)

        |7|6|5|4|3|2|1|0|  Command Status Register 0 at port 3F5h
         | | | | | | +------ unit selected at interrupt (0=A, 1=B, 2=...)
         | | | | | +------- head number at interrupt (head 0 or 1)
         | | | | +-------- not ready on read/write or SS access to head 1
         | | | +--------- equipment check (see note)
         | | +---------- set to 1 when FDD completes a seek command
         +------------- last command status (see below)

      Bits
       76  Last Command Status
         00  command terminated successfully
         01  command execution started but terminated abnormally
         10  invalid command issued
         11  command terminated abnormally due to a change in state of
             the Ready Signal from the FDC  (reserved on PS/2)

        - equipment check can occur if FDD signals a fault or track zero is
          not found after 77 steps on a recalibrate command
        - PS/2 only uses bits 1-0 for drive (values are 01b and 10b)

      FDC Command Status Register 1 at 3F5h  (all systems)

        |7|6|5|4|3|2|1|0|  Command Status Register 1 at port 3F5h
         | | | | | | | +---- FDC cannot find ID address mark (see reg 2)
         | | | | | | +----- write protect detected during write
         | | | | | +------ FDC cannot find sector ID
         | | | | +------- unused (always zero)
         | | | +-------- over-run;  FDC not serviced in reasonable time
         | | +--------- data error (CRC) in ID field or data field
         | +---------- unused (always zero)
         +----------- end of cylinder; sector# greater than sectors/track

        - bit 0 of Status Register 1 and bit 4 of Status Register 2 are
          related and mimic each other

      FDC Command Status Register 2 at 3F5h  (all systems)

        |7|6|5|4|3|2|1|0|  Command Status Register 2 at port 3F5h
         | | | | | | | +---- missing address mark in data field
         | | | | | | +----- bad cylinder, ID not found and Cyl Id=FFh
         | | | | | +------ scan command failed, sector not found in cylinder
         | | | | +------- scan command equal condition satisfied
         | | | +-------- wrong cylinder detected
         | | +--------- CRC error detected in sector data
         | +---------- sector with deleted data address mark detected
         +----------- unused (always zero)

        - bit 0 of Status Register 1 and bit 4 of Status Register 2 are
          related and mimic each other

      FDC Command Status Register 3 at 3F5h (FDD status, all systems)

        |7|6|5|4|3|2|1|0|  Floppy Disk Drive Status at port 3F5h
         | | | | | | +------ FDD unit selected status (0=A, 1=B, 2=...)
         | | | | | +------- FDD side head select status (0=head 0, 1=head 1)
         | | | | +-------- FDD two sided status signal
         | | | +--------- FDD track zero status signal
         | | +---------- FDD ready status signal
         | +----------- FDD write protect status signal
         +------------ FDD fault status signal

      PS/2 FDC Digital Input Register at 3F7h

        |7|6|5|4|3|2|1|0|  3F7h  PS/2 Digital Input Register (read only)
         | | | | | | | +--- high density select
         | +-------------- reserved
         +--------------- diskette change

      PS/2 FDC Configuration Control Register at 3F7h

        |7|6|5|4|3|2|1|0|  3F7h  PS/2 Config. Control Register (write only)
         | | | | | | +---- DRC1, DRC0  (see below)
         +--------------- reserved

        DRC1 DRC0
          0   0    500000 bit per second mode
          0   1    reserved
          1   0    250000 bit per second mode
          1   1    reserved


        - Digital Input Register is used to sense the state of the
          (-diskette change) and the (-high density select) signals
        - Configuration Control Register is used to set the transfer rate


                       FDC Programming Considerations

      Three phases of command execution:

        1.  Command phase; commands are sent from the CPU to the FDC via
            port 3F5h;  bit 6 of the Status Register at 3F4h must be zero
        2.  Execution phase; FDC executes instruction & generates INT 6
        3.  Result phase; status and other information is available to CPU;
            INT 6 sets bit 7 of BIOS Data Area location 40:3E which can
            be polled for completion status


      Example of a read operation:

        1. turn disk motor on and set delay time for drive spin up
        2. perform seek operation;  wait for disk interrupt
        3. prepare DMA chip to move data to memory
        4. send read command and wait for transfer complete interrupt
        5. read status information
        6. turn disk motor off


            Floppy Diskette Controller Operations (15 commands)

      Read Data          D7  D6  D5  D4  D3  D2  D1  D0

        command byte 0:    MT  MF  SK   0   0   1   1   0
        command byte 1:     ?   ?   ?   ?   ?  HD  US1 US0
        command byte 2:    cylinder number
        command byte 3:    head number
        command byte 4:    sector number
        command byte 5:    bytes per sector
        command byte 6:    end of track (last sector in track)
        command byte 7:    gap 3 length
        command byte 8:    data length (if cmd byte 5==0)
        result byte 0:     status register 0
        result byte 1:     status register 1
        result byte 2:     status register 2
        result byte 3:     cylinder number
        result byte 4:     head number
        result byte 5:     sector number
        result byte 6:     bytes per sector

      Read Deleted Data  D7  D6  D5  D4  D3  D2  D1  D0

        command byte 0:    MT  MF  SK   0   1   1   0   0
        command byte 1:     ?   ?   ?   ?   ?  HD  US1 US0
        command byte 2:    cylinder number
        command byte 3:    head number
        command byte 4:    sector number
        command byte 5:    bytes per sector
        command byte 6:    end of track (last sector in track)
        command byte 7:    gap 3 length
        command byte 8:    data length (if cmd byte 5==0)
        result byte 0:     status register 0
        result byte 1:     status register 1
        result byte 2:     status register 2
        result byte 3:     cylinder number
        result byte 4:     head number
        result byte 5:     sector number
        result byte 6:     bytes per sector

      Write Data         D7  D6  D5  D4  D3  D2  D1  D0

        command byte 0:    MT  MF   0   0   0   1   0   1
        command byte 1:     ?   ?   ?   ?   ?  HD  US1 US0
        command byte 2:    cylinder number
        command byte 3:    head number
        command byte 4:    sector number
        command byte 5:    bytes per sector
        command byte 6:    end of track (last sector in track)
        command byte 7:    gap 3 length
        command byte 8:    data length (if cmd byte 5==0)
        result byte 0:     status register 0
        result byte 1:     status register 1
        result byte 2:     status register 2
        result byte 3:     cylinder number
        result byte 4:     head number
        result byte 5:     sector number
        result byte 6:     bytes per sector

      Write Deleted Data D7  D6  D5  D4  D3  D2  D1  D0

        command byte 0:    MT  MF   0   0   1   0   0   1
        command byte 1:     ?   ?   ?   ?   ?  HD  US1 US0
        command byte 2:    cylinder number
        command byte 3:    head number
        command byte 4:    sector number
        command byte 5:    bytes per sector
        command byte 6:    end of track (last sector in track)
        command byte 7:    gap 3 length
        command byte 8:    data length (if cmd byte 5==0)
        result byte 0:     status register 0
        result byte 1:     status register 1
        result byte 2:     status register 2
        result byte 3:     cylinder number
        result byte 4:     head number
        result byte 5:     sector number
        result byte 6:     bytes per sector

      Read a Track       D7  D6  D5  D4  D3  D2  D1  D0
      (Diagnostic)
        command byte 0:     0  MF  SK   0   0   0   1   0
        command byte 1:     ?   ?   ?   ?   ?  HD  US1 US0
        command byte 2:    cylinder number
        command byte 3:    head number
        command byte 4:    sector number
        command byte 5:    bytes per sector
        command byte 6:    end of track (last sector in track)
        command byte 7:    gap 3 length
        command byte 8:    data length (if cmd byte 5==0)
        result byte 0:     status register 0
        result byte 1:     status register 1
        result byte 2:     status register 2
        result byte 3:     cylinder number
        result byte 4:     head number
        result byte 5:     sector number
        result byte 6:     bytes per sector

      Read ID            D7  D6  D5  D4  D3  D2  D1  D0

        command byte 0:     0  MF   0   0   1   0   1   0
        command byte 1:     ?   ?   ?   ?   ?  HD  US1 US0
        result byte 0:     status register 0
        result byte 1:     status register 1
        result byte 2:     status register 2
        result byte 3:     cylinder number
        result byte 4:     head number
        result byte 5:     sector number
        result byte 6:     bytes per sector

      Format a Track     D7  D6  D5  D4  D3  D2  D1  D0
      (Write Sector IDs)
        command byte 0:     0  MF   0   0   1   1   0   1
        command byte 1:     ?   ?   ?   ?   ?  HD  US1 US0
        command byte 2:    bytes per sector
        command byte 3:    sectors per track
        command byte 4:    gap 3 length
        command byte 5:    filler pattern to write in each byte
        result byte 0:     status register 0
        result byte 1:     status register 1
        result byte 2:     status register 2
        result byte 3:     cylinder number
        result byte 4:     head number
        result byte 5:     sector number
        result byte 6:     bytes per sector

      Scan Equal         D7  D6  D5  D4  D3  D2  D1  D0

        command byte 0:    MT  MF  SK   1   0   0   0   1
        command byte 1:     ?   ?   ?   ?   ?  HD  US1 US0
        command byte 2:    cylinder number
        command byte 3:    head number
        command byte 4:    sector number
        command byte 5:    bytes per sector
        command byte 6:    end of track (last sector in track)
        command byte 7:    gap 3 length
        command byte 8:    scan test (1=scan contiguous, 2=scan alternate)
        result byte 0:     status register 0
        result byte 1:     status register 1
        result byte 2:     status register 2
        result byte 3:     cylinder number
        result byte 4:     head number
        result byte 5:     sector number
        result byte 6:     bytes per sector

      Scan Low or Equal  D7  D6  D5  D4  D3  D2  D1  D0

        command byte 0:    MT  MF  SK   1   1   0   0   1
        command byte 1:     ?   ?   ?   ?   ?  HD  US1 US0
        command byte 2:    cylinder number
        command byte 3:    head number
        command byte 4:    sector number
        command byte 5:    bytes per sector
        command byte 6:    end of track (last sector in track)
        command byte 7:    gap 3 length
        command byte 8:    scan test (1=scan contiguous, 2=scan alternate)
        result byte 0:     status register 0
        result byte 1:     status register 1
        result byte 2:     status register 2
        result byte 3:     cylinder number
        result byte 4:     head number
        result byte 5:     sector number
        result byte 6:     bytes per sector

      Scan High or Equal D7  D6  D5  D4  D3  D2  D1  D0

        command byte 0:    MT  MF  SK   1   1   1   0   1
        command byte 1:     ?   ?   ?   ?   ?  HD  US1 US0
        command byte 2:    cylinder number
        command byte 3:    head number
        command byte 4:    sector number
        command byte 5:    bytes per sector
        command byte 6:    end of track (last sector in track)
        command byte 7:    gap 3 length
        command byte 8:    scan test (1=scan contiguous, 2=scan alternate)
        result byte 0:     status register 0
        result byte 1:     status register 1
        result byte 2:     status register 2
        result byte 3:     cylinder number
        result byte 4:     head number
        result byte 5:     sector number
        result byte 6:     bytes per sector

      Recalibrate        D7  D6  D5  D4  D3  D2  D1  D0

        command byte 0:     0   0   0   0   0   1   1   1
        command byte 1:     ?   ?   ?   ?   ?   0  US1 US0
        returns nothing

      Sense Interrupt    D7  D6  D5  D4  D3  D2  D1  D0
          Status
        command byte 0:     0   0   0   0   1   0   0   0
        result byte 0:     status register 0
        result byte 1:     present cylinder number

      Specify Step &     D7  D6  D5  D4  D3  D2  D1  D0
        Head Load
        command byte 0:     0   0   0   0   0   0   1   1
        command byte 1:    step rate time | head unload time
        command byte 2:    ------head load time------   ND
        returns nothing

      Sense Drive        D7  D6  D5  D4  D3  D2  D1  D0
        Status
        command byte 0:     0   0   0   0   0   1   0   0
        command byte 1:     ?   ?   ?   ?   ?  HD  US1 US0
        result byte 0:     status register 3

      Seek               D7  D6  D5  D4  D3  D2  D1  D0

        command byte 0:     0   0   0   0   1   1   1   1
        command byte 1:     ?   ?   ?   ?   ?  HD  US1 US0
        command byte 2:    new cylinder number
        returns nothing

      .PD765 Version     D7  D6  D5  D4  D3  D2  D1  D0

        command byte 0:     ?   ?   ?   1   0   0   0   0
        result byte 0:     status register 0
                           90h = .PD765B;  80h = .PD765A or .PD765A-2

      Invalid Command

        result byte 0:     status register 0 (value of 80h)


      Key to Abbreviations
        HD = Head Number Selected       SK = SKip Deleted-data address mark
        MT = Multi-Track                US0 = drive select bit 0
        MF = MFM mode                   US1 = drive select bit 1
        ND = Non-DMA mode

        Head Load Time = 2 to 254ms in 2ms increments
        Head Unload Time = 16 to 240ms in 16ms increments
        Step Rate Time = 1 to 16ms in 1ms increments


        - PS/2 systems use the 8272A diskette controller which is software
          and port compatible with the NEC .PD765
        - accessed through ports 3F0h-3F7h;  NEC .PD765 is accessed through
          ports 3F2h, 3F4h and 3F5h;   the 8272A uses ports 3F0h, 3F1h,
          3F2h, 3F4h, 3F5h and 3F7h
        - data, command and status registers are all accessed through
          port 3F5h a register stack with one address presented to the bus
        - bit 7 of BIOS Data Area byte 40:3E can be polled to determine
          if a disk operation has completed;  this bit is set by the
          interrupt handler when the operation has completed;  it should
          be reset before continuing on with the next FDC operation

        - see  BIOS Data Area   INT TABLE   INT 13