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                      PORTS   Common I/O Port Addresses

        Port addresses are not always constant across PC, AT and PS/2
        Unless marked, port addresses are relative to PC and XT only

      000-00F  8237 DMA controller
        000 Channel 0 address register
        001 Channel 0 word count
        002 Channel 1 address register
        003 Channel 1 word count
        004 Channel 2 address register
        005 Channel 2 word count
        006 Channel 3 address register
        007 Channel 3 word count
        008 Status/command register
        009 Request register
        00A Mask register
        00B Mode register
        00C Clear MSB/LSB flip flop
        00D Master clear temp register
        00E Clear mask register
        00F Multiple mask register

      010-01F  8237 DMA Controller (PS/2 model 60 & 80), reserved (AT)

      020-02F  8259A Master Programmable Interrupt Controller
        020 8259 Command port  (see 8259)
        021 8259 Interrupt mask register  (see 8259)

      030-03F  8259A Slave Programmable Interrupt Controller (AT,PS/2)

      040-05F  8253 or 8254 Programmable Interval Timer (PIT, see ~8253~)
        040 8253 channel 0, counter divisor
        041 8253 channel 1, RAM refresh counter
        042 8253 channel 2, Cassette and speaker functions
        043 8253 mode control  (see 8253)
        044 8254 PS/2 extended timer
        047 8254 Channel 3 control byte

      060-067  8255 Programmable Peripheral Interface  (PC,XT, PCjr)
        060 8255 Port A keyboard input/output buffer (output PCjr)
        061 8255 Port B output
        062 8255 Port C input
        063 8255 Command/Mode control register

      060-06F  8042 Keyboard Controller  (AT,PS/2)
        060 8042 Keyboard input/output buffer register
        061 8042 system control port (for compatability with 8255)
        064 8042 Keyboard command/status register

        070 CMOS RAM/RTC, also NMI enable/disable (AT,PS/2, see RTC)
        071 CMOS RAM data  (AT,PS/2)

        080 Manufacturer systems checkpoint port (used during POST)
      080-090  DMA Page Registers
        081 High order 4 bits of DMA channel 2 address
        082 High order 4 bits of DMA channel 3 address
        083 High order 4 bits of DMA channel 1 address

      090-097  POS/Programmable Option Select  (PS/2)
        090 Central arbitration control Port
        091 Card selection feedback
        092 System control and status register
        094 System board enable/setup register
        095 Reserved
        096 Adapter enable/setup register
        097 Reserved

        0A0 NMI Mask Register (PC,XT) (write 80h to enable NMI, 00h disable)
      0A0-0BF  Second 8259 Programmable Interrupt Controller (AT, PS/2)
        0A0 Second 8259 Command port  (see 8259)
        0A1 Second 8259 Interrupt mask register  (see 8259)

        0C0 TI SN76496 Programmable Tone/Noise Generator (PCjr)
      0C0-0DF  8237 DMA controller 2 (AT)
        0C2 DMA channel 3 selector  (see ports 6 & 82)

        0E0-0EF  Reserved

      0F0-0FF  Math coprocessor (AT, PS/2)
      0F0-0F5  PCjr Disk Controller
        0F0 Disk Controller
        0F2 Disk Controller control port
        0F4 Disk Controller status register
        0F5 Disk Controller data port

      0F8-0FF  Reserved for future microprocessor extensions

      100-10F  POS Programmable Option Select (PS/2)
        100 POS Register 0, Adapter ID byte (LSB)
        101 POS Register 1, Adapter ID byte (MSB)
        102 POS Register 2, Option select data byte 1
            Bit 0 is card enable (CDEN)
        103 POS Register 3, Option select data byte 2
        104 POS Register 4, Option select data byte 3
        105 POS Register 5, Option select data byte 4
            Bit 7 is (-CHCK)
            Bit 6 is reserved
        106 POS Register 6, subaddress extension (LSB)
        107 POS Register 7, subaddress extension (MSB)

      110-1EF  System I/O channel

      170-17F  Fixed disk 1 (AT)
        170 disk 1 data
        171 disk 1 error
        172 disk 1 sector count
        173 disk 1 sector number
        174 disk 1 cylinder low
        175 disk 1 cylinder high
        176 disk 1 drive/head
        177 disk 1 status

      1F0-1FF  Fixed disk 0 (AT)
        1F0 disk 0 data
        1F1 disk 0 error
        1F2 disk 0 sector count
        1F3 disk 0 sector number
        1F4 disk 0 cylinder low
        1F5 disk 0 cylinder high
        1F6 disk 0 drive/head
        1F7 disk 0 status

      200-20F  Game Adapter (see GAME PORT or ~JOYSTICK~)

      210-217  Expansion Card Ports (XT)
        210 Write: latch expansion bus data
            read:  verify expansion bus data
        211 Write: clear wait,test latch
            Read:  MSB of data address
        212 Read:  LSB of data address
        213 Write: 0=enable, 1=/disable expansion unit
        214-215  Receiver Card Ports
        214 write: latch data, read: data
        215 read:  MSB of address, next read: LSB of address

        21F Reserved

      220-26F  Reserved for I/O channel

      270-27F  Third parallel port (see ~PARALLEL PORT~)
        278 data port
        279 status port
        27A control port

      280-2AF  Reserved for I/O channel

      2A2-2A3  MSM58321RS clock

      2B0-2DF  Alternate EGA, or 3270 PC video (XT, AT)

        2E0 Alternate EGA/VGA
        2E1 GPIB Adapter  (AT)

      2E2-2E3  Data acquisition adapter (AT)

      2E8-2EF  COM4 non PS/2 UART (Reserved by IBM) (see ~UART~)

      2F0-2F7  Reserved

      2F8-2FF  COM2 Second Asynchronous Adapter (see UART)
                 Primary Asynchronous Adapter for PCjr

      300-31F  Prototype Experimentation Card (except PCjr)
                 Periscope hardware debugger
      320-32F  Hard Disk Controller  (XT)
        320 Read from/Write to controller
        321 Read: Controller Status, Write: controller reset
        322 Write: generate controller select pulse
        323 Write: Pattern to DMA and interrupt mask register
            (see ports 0F,21,C2)
        324 disk attention/status

      330-33F  Reserved for XT/370

      340-35F  Reserved for I/O channel

      360-36F  PC Network

      370-377  Floppy disk controller (except PCjr)
        372 Diskette digital output
        374 Diskette controller status
        375 Diskette controller data
        376 Diskette controller data
        377 Diskette digital input

      378-37F  Second Parallel Printer (see ~PARALLEL PORT~)
                 First Parallel Printer (see PARALLEL PORT)
        378 data port
        379 status port
        37A control port

      380-38F  Secondary Binary Synchronous Data Link Control (SDLC) adapter
        380 On board 8255 port A, internal/external sense
        381 On board 8255 port B, external modem interface
        382 On board 8255 port C, internal control and gating
        383 On board 8255 mode register
        384 On board 8253 channel square wave generator
        385 On board 8253 channel 1 inactivity time-out
        386 On board 8253 channel 2 inactivity time-out
        387 On board 8253 mode register
        388 On board 8273 read: status; Write: Command
        389 On board 8273 write: parameter; read: response
        38A On board 8273 transmit interrupt status
        38B On board 8273 receiver interrupt status
        38C On board 8273 data

      390-39F  Cluster Adapter

      3A0-3AF  Primary Binary Synchronous Data Link Control (SDLC) adapter
        3A0 On board 8255 port A, internal/external sense
        3A1 On board 8255 port B, external modem interface
        3A2 On board 8255 port C, internal control and gating
        3A3 On board 8255 mode register
        3A4 On board 8253 counter 0 unused
        3A5 On board 8253 counter 1 inactivity time-outs
        3A6 On board 8253 counter 2 inactivity time-outs
        3A7 On board 8253 mode register
        3A8 On board 8251 data
        3A9 On board 8251 command/mode/status register

      3B0-3BF Monochrome Display Adapter (write only, see ~6845~)
        3B0 port address decodes to 3B4
        3B1 port address decodes to 3B5
        3B2 port address decodes to 3B4
        3B3 port address decodes to 3B5
        3B4 6845 index register, selects which register [0-11h]
            is to be accessed through port 3B5
        3B5 6845 data register [0-11h] selected by port 3B4,
            registers 0C-0F may be read.  If a read occurs without
            the adapter installed, FFh is returned.  (see 6845)
        3B6 port address decodes to 3B4
        3B7 port address decodes to 3B5
        3B8 6845 Mode control register
        3B9 reserved for color select register on color adapter
        3BA status register (read only)
        3BB reserved for light pen strobe reset

      3BC-3BF  Primary Parallel Printer Adapter (see ~PARALLEL PORT~)
        3BC parallel 1, data port
        3BD parallel 1, status port
        3BE parallel 1, control port

      3C0-3CF  EGA/VGA
        3C0 VGA attribute and sequencer register
        3C1 Other video attributes
        3C2 EGA, VGA, CGA input status 0
        3C3 Video subsystem enable
        3C4 CGA, EGA, VGA sequencer index
        3C5 CGA, EGA, VGA sequencer
        3C6 VGA video DAC PEL mask
        3C7 VGA video DAC state
        3C8 VGA video DAC PEL address
        3C9 VGA video DAC
        3CA VGA graphics 2 position
        3CC VGA graphics 1 position
        3CD VGA feature control
        3CE VGA graphics index
        3CF Other VGA graphics

      3D0-3DF Color Graphics Monitor Adapter (ports 3D0-3DB are
                write only, see 6845)
        3D0 port address decodes to 3D4
        3D1 port address decodes to 3D5
        3D2 port address decodes to 3D4
        3D3 port address decodes to 3D5
        3D4 6845 index register, selects which register [0-11h]
            is to be accessed through port 3D5
        3D5 6845 data register [0-11h] selected by port 3D4,
            registers 0C-0F may be read.  If a read occurs without
            the adapter installed, FFh is returned.  (see 6845)
        3D6 port address decodes to 3D4
        3D7 port address decodes to 3D5
        3D8 6845 Mode control register (CGA, EGA, VGA, except PCjr)
        3D9 color select palette register (CGA, EGA, VGA, see 6845)
        3DA status register (read only, see 6845, PCjr VGA access)
        3DB Clear light pen latch (any write)
        3DC Preset Light pen latch
        3DF CRT/CPU page register (PCjr only)

      3E8-3EF  COM3 non PS/2 UART (Reserved by IBM) (see ~UART~)

      3F0-3F7  Floppy disk controller (except PCjr)
        3F0 Diskette controller status A
        3F1 Diskette controller status B
        3F2 controller control port
        3F4 controller status register
        3F5 data register (write 1-9 byte command, see INT 13)
        3F6 Diskette controller data
        3F7 Diskette digital input

      3F8-3FF  COM1 Primary Asynchronous Adapter  (see ~UART~)

        3220-3227  PS/2 COM3 (see UART)
        3228-322F  PS/2 COM4 (see UART)
        4220-4227  PS/2 COM5 (see UART)
        4228-422F  PS/2 COM6 (see UART)
        5220-5227  PS/2 COM7 (see UART)
        5228-522F  PS/2 COM8 (see UART)

        - many cards designed for the ISA BUS only uses the lower 10 bits
          of the port address but some ISA adapters use addresses beyond
          3FF.  Any address that matches in the lower 10 bits will decode
          to the same card.   It is up to the adapters to resolve or ignore
          the high bits of the port addresses.   An example would be the
          Cluster adapter that has a port address of 390h.  The second
          cluster adapter has a port address of 790h which resolves to
          the same port address with the cards determining which one
          actually gets the data.