NEC µPD765 - 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
µPD765 Version D7 D6 D5 D4 D3 D2 D1 D0
command byte 0: ? ? ? 1 0 0 0 0
result byte 0: status register 0
90h = µPD765B; 80h = µPD765A or µPD765A-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 µPD765
- accessed through ports 3F0h-3F7h; NEC µPD765 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