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I/O Ports

Port (hex)	Direction	Peripheral / Function	Notes
00h	Read/Write	Floppy Disk Controller (uPD765A) - Main Status Register	FDC status register
01h	Read/Write	Floppy Disk Controller (uPD765A) - Data Register	FDC data register
80h	Read	AMX Mouse - Vertical Movement (alternate)	Alternate mapping of A0h. Stop Press relocates the mouse to 80h-83h if 0A2h reads 10h at startup (believed to avoid an ASD hard drive conflict)
81h	Read	AMX Mouse - Horizontal Movement (alternate)	Alternate mapping of A1h
82h	Read/Write	AMX Mouse - Button State & Reset (alternate)	Alternate mapping of A2h
83h	Write	AMX Mouse - PPI Mode Register (alternate)	Alternate mapping of A3h
84h	Read	Standalone Centronics (CEN) Port - Status	Bit 0: 1=busy / 0=ready
85h	Write	Standalone Centronics (CEN) Port - Strobe	Write byte to 87h then 85h then 87h to toggle STROBE
87h	Write	Standalone Centronics (CEN) Port - Data	Write data byte here as part of output sequence
9Fh	Read	Kempston Joystick Interface	Bit 4=Fire, Bit 3=Down, Bit 2=Up, Bit 1=Left, Bit 0=Right
A0h	Read	AMX Mouse - Vertical Movement (8255 PPI Port A)	Low nibble=moves up, high nibble=moves down
A0h	Read/Write	Timatic Winchester / SASI (NCR 5380) - SCSI Data	Also used by ASD PCWHD10/20 as Data register (see A0h conflict)
A0h	Read/Write	ASD PCWHD10/PCWHD20 - Data Register	Conflicts with Timatic and AMX Mouse
A1h	Read	AMX Mouse - Horizontal Movement (8255 PPI Port B)	Low nibble=moves right, high nibble=moves left
A1h	Read/Write	Timatic Winchester / SASI (NCR 5380) - Initiator Command Register	Also used by ASD PCWHD10/20 as Error/Features
A1h	Read/Write	ASD PCWHD10/PCWHD20 - Error / Features	Conflicts with Timatic
A2h	Read/Write	AMX Mouse - Button State & Reset (8255 PPI Port C)	Input: bits 0-2=button states, Output: write FFh then 0 to reset
A2h	Read	EMR MIDI Interface - Status Register	~31250 baud serial-style interface. Bit 1=ready to receive data. Requires EMR's supplied init software to be run first. Conflicts with AMX Mouse, Timatic and ASD at A2h
A2h	Read/Write	Timatic Winchester / SASI (NCR 5380) - Mode Register	Also used by ASD PCWHD10/20 as Sector Count
A2h	Read/Write	ASD PCWHD10/PCWHD20 - Sector Count	Conflicts with Timatic
A3h	Write	AMX Mouse - PPI Mode Register	Write 93h at startup for basic I/O mode
A3h	Read/Write	Timatic Winchester / SASI (NCR 5380) - Target Command Register	Also used by ASD PCWHD10/20 as Sector Number
A3h	Read/Write	ASD PCWHD10/PCWHD20 - Sector Number	Conflicts with Timatic
A3h	Read/Write	EMR MIDI Interface - Data I/O Register	MIDI data in/out. Note: Fairhurst (systemed.net) lists the interface as A0h-A2h rather than A2h-A3h
A4h	Read/Write	Timatic Winchester / SASI (NCR 5380) - SCSI Bus Status / Select Register	Also used by ASD PCWHD10/20 as Cylinder Low
A4h	Read/Write	ASD PCWHD10/PCWHD20 - Cylinder Low	Conflicts with Timatic
A5h	Read/Write	Timatic Winchester / SASI (NCR 5380) - Bus and Status Register	Also used by ASD PCWHD10/20 as Cylinder High
A5h	Read/Write	ASD PCWHD10/PCWHD20 - Cylinder High	Conflicts with Timatic
A6h	Read	Electric Studio Light Pen - Position Low 8 Bits	12-bit position counter low byte
A6h	Read/Write	Multilink Network Interface - Control / Status	Input: Bit 7=1 (8251 DSR), Bits 6-2=0 (error flags), Bit 1=byte waiting to send to PCW, Bit 0=ready to receive from PCW. Output: write 48h to reset the card
A6h	Read/Write	Timatic Winchester / SASI (NCR 5380) - Start DMA Target Receive / Input Data	Not used by PCW, also used by ASD PCWHD10/20 as Drive/Head
A6h	Read/Write	ASD PCWHD10/PCWHD20 - Drive / Head	Conflicts with Timatic and Light Pen and Multilink
A7h	Read	Electric Studio Light Pen - Position High 4 Bits + Retrace Flags	Bits 6=V.Retrace, Bit 5=H.Retrace, Bits 3-0=high 4 bits of position
A7h	Read/Write	Multilink Network Interface - Data	Read/write data byte
A7h	Read/Write	Timatic Winchester / SASI (NCR 5380) - Start DMA Initiator Receive / Reset Parity	Not used by PCW, also used by ASD PCWHD10/20 as Status/Command
A7h	Read/Write	ASD PCWHD10/PCWHD20 - Status / Command	Conflicts with Timatic and Light Pen and Multilink
A8h	Read/Write	Cirtech Gem (XTA) - Data Register	XTA hard drive interface, ATA variant maps A8h-AFh
A8h	Read/Write	Cirtech Gem (ATA) - Data	ATA variant. Cirtech Insyder / Hardpak use the same register layout but additionally write 0 to 0FF94h before each transfer, believed to reset the mechanism that converts between 8-bit and 16-bit transfers
A9h	Read	Cirtech Gem (XTA) - Controller Status	Also: Write=Reset controller
A9h	Read/Write	Cirtech Gem (ATA) - Error / Features	ATA variant
A9h	Read	DKTronics Sound Generator - Read Selected AY-3-8912 Register	Read currently selected register
AAh	Read/Write	Cirtech Gem (XTA) - Controller DIP Switches / Generate Controller-Select Pulse	Read=DIP switches, Write=select pulse
AAh	Read/Write	Cirtech Gem (ATA) - Sector Count	ATA variant
AAh	Write	DKTronics Sound Generator - Select AY-3-8912 Register	Write register number to select it
ABh	Write	Cirtech Gem (XTA) - DMA and Interrupt Mask	Not used on PCW
ABh	Read/Write	Cirtech Gem (ATA) - Sector Number	ATA variant
ABh	Write	DKTronics Sound Generator - Write to Selected AY-3-8912 Register	Write value to selected register
ACh	Read/Write	Cirtech Gem (ATA) - Cylinder Low	ATA variant only
ADh	Read/Write	Cirtech Gem (ATA) - Cylinder High	ATA variant only
AEh	Read/Write	Cirtech Gem (ATA) - Drive / Head	ATA variant only
AFh	Read/Write	Cirtech Gem (ATA) - Status / Command	ATA variant only
C8h	Read/Write	Fax Link Interface	CPS8256-compatible serial circuitry (cf. E0h-E7h) but with different baud-rate values
C9h	Read/Write	Fax Link Interface	CPS8256-compatible serial circuitry (cf. E0h-E7h) but with different baud-rate values
CAh	Read/Write	Fax Link Interface	CPS8256-compatible serial circuitry (cf. E0h-E7h) but with different baud-rate values
CBh	Read/Write	Fax Link Interface	CPS8256-compatible serial circuitry (cf. E0h-E7h) but with different baud-rate values
CCh	Read/Write	Fax Link Interface	CPS8256-compatible serial circuitry (cf. E0h-E7h) but with different baud-rate values
CDh	Read/Write	Fax Link Interface	CPS8256-compatible serial circuitry (cf. E0h-E7h) but with different baud-rate values
CEh	Read/Write	Fax Link Interface	CPS8256-compatible serial circuitry (cf. E0h-E7h) but with different baud-rate values
CFh	Read/Write	Fax Link Interface	CPS8256-compatible serial circuitry (cf. E0h-E7h) but with different baud-rate values
D0h	Read	Kempston Mouse - X Position	0-255, also readable at D2h
D1h	Read	Kempston Mouse - Y Position	0-255, also readable at D3h
D2h	Read	Kempston Mouse - X Position (mirror)	Same value as D0h
D3h	Read	Kempston Mouse - Y Position (mirror)	Same value as D1h
D4h	Read	Kempston Mouse - Button State	Bit 0=Left, Bit 1=Right, 0=pressed
D8h	Read	Cirtech Flash Drive - Page In	Reading pages the drive into memory; high byte of the 16-bit port address selects the 32k bank. Bank appears at 00h-7Fh, 0100h-017Fh … 0FF00h-0FF7Fh. Flash-based (Intel E28F008SA) replacement for Gem drive + boot ROM
D9h	Read	Cirtech Flash Drive - Page Out	Reading pages the drive back out of memory
DFh	Read	MasterScan Optical Scanner	Bit 0: 0=light, 1=dark
E0h	Read	Spectravideo Joystick Interface	Bit 4=Right, Bit 3=Up, Bit 2=Left, Bit 1=Fire, Bit 0=Down
E0h	Read	Cascade (JoyceStick) Joystick Interface	Bit 7=Fire, Bit 4=Up, Bit 2=Down, Bit 1=Right, Bit 0=Left, conflicts with Spectravideo
E0h	Read/Write	CPS8256 - DART Channel A Data	Serial/comms interface
E1h	Read/Write	CPS8256 - DART Channel A Control
E2h	Read/Write	CPS8256 - DART Channel B Data
E3h	Read/Write	CPS8256 - DART Channel B Control / CEN Port Status & Control	Also used as CPS8256 CEN port: write 10h to select status mode, bit 5=printer ready
E4h	Read/Write	CPS8256 - 8253 Counter 0	Used for baud rate setting (transmit)
E5h	Read/Write	CPS8256 - 8253 Counter 1	Used for baud rate setting (receive)
E7h	Write	CPS8256 - 8253 Write Mode Word	Write mode byte here (e.g. 36h or 76h for baud rate)
E8h	Write	CPS8256 CEN Port - Data Output	Write character to this port before toggling STROBE via E3h
E9h	Read/Write	ProScan Interface - Base Port	Interface for Naksha hand scanner / fax
EAh	Read/Write	ProScan Interface - Fax Device Select (FX1)	Select fax as image source
EBh	Read/Write	ProScan Interface - Fax Device Select (FX2)	Select fax (second type) as image source
ECh	Read/Write	ProScan Interface - Hand Scanner Select	Select hand scanner as image source
EDh	Read/Write	ProScan Interface - Unknown Device Select	Possibly a second type of hand scanner
EFh	Read/Write	ProScan Interface - Upper Bound Port	Upper port of ProScan range
F0h	Write	Memory Management - Bank Select for 0000h-3FFFh	PCW paging: bit 7=1, bits 6-0=block number
F1h	Write	Memory Management - Bank Select for 4000h-7FFFh	PCW paging
F2h	Write	Memory Management - Bank Select for 8000h-BFFFh	PCW paging
F3h	Write	Memory Management - Bank Select for C000h-FFFFh	PCW paging
F4h	Write	CPC Paging Mode - Lock Register	Bits lock memory ranges from being read from alternate block, not used by CP/M or LocoScript
F4h	Read	Timer Counter	Bottom 4 bits: counter incremented by timer interrupt (0-15), reading resets it
F5h	Write	Video Controller - Roller-RAM Address	Bits 7-0 specify block and offset / 512
F6h	Write	Video Controller - Vertical Scroll Origin	Specifies which Roller-RAM line is at top of screen
F7h	Write	Video Controller - Display Control	Bit 7=inverse video, Bit 6=screen on/off
F8h	Write	System Control Port	Multi-function: video enable/disable, FDC interrupt mode, boot sequence end (value 0 ends boot)
F8h	Read	System Control Port - Status	Bit 6=frame flyback, Bit 5=FDC interrupt pending, Bit 4=60Hz PCW flag
F9h	Read	Prototype Serial Interface (IM6403 UART) - Status Register	Only on prototype PCW, not on production hardware
FCh	Read	PCW8256/8512/9256/10 Dot-Matrix Printer Controller - Error Status	Returns controller error number. Note: the PCW9512 has different controller hardware and decodes 16 bits here - its 00FCh is instead a data register (read: data returned by a command), and 01FCh is its status register (read twice in succession for a stable value)
FCh	Write	PCW8256/8512/9256/10 Dot-Matrix Printer Controller - Command Port	Used to send commands during reset. Note: the PCW9512 has different controller hardware and decodes 16 bits here - its 00FCh is instead a data register (write: data byte of a two-byte command), and 01FCh is its command register (write: command byte)
FDh	Read	PCW8256/8512/9256/10 Dot-Matrix Printer Controller - Printer Status	Bit 7=bail bar, Bit 6=command done, Bit 1=busy, Bit 0=fault, also used by PCW9512 at 00FDh
FDh	Write	PCW8256/8512/9256/10 Dot-Matrix Printer Controller - Command/Data Port	Sends printer commands during normal operation
FEh	Read/Write	LocoLink Parallel Transfer Interface	Bit 1=Data 1 (input) / ACK (output), Bit 0=Data 0 (input) / BUSY (output)
FEh	Read/Write	Prototype Serial Interface (IM6403 UART) - Data Register	Only on prototype PCW, not on production hardware, conflicts with LocoLink
FFh	Read/Write	PCW Linkit Parallel Transfer Interface	Two-bit communication channel, value read is binary inverse of value written by other PCW

The above information is derived from John Elliott's excellent PCW Hardware manual. Please note that the hard drive, and SCA Mark 2 Interface information have not been verified.