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.