Port addresses are not always constant across PC, AT and PS/2
Unless marked, port addresses are relative to PC and XT only
| 000h | Channel 0 address register |
| 001h | Channel 0 word count |
| 002h | Channel 1 address register |
| 003h | Channel 1 word count |
| 004h | Channel 2 address register |
| 005h | Channel 2 word count |
| 006h | Channel 3 address register |
| 007h | Channel 3 word count |
| 008h | Status/command register |
| 009h | Request register |
| 00Ah | Mask register |
| 00Bh | Mode register |
| 00Ch | Clear MSB/LSB flip flop |
| 00Dh | Master clear temp register |
| 00Eh | Clear mask register |
| 00Fh | Multiple mask register |
| 020h | 8259 | Command port |
| 021h | 8259 | Interrupt mask register |
| 040h | 8253 | channel 0, counter divisor |
| 041h | 8253 | channel 1, RAM refresh counter |
| 042h | 8253 | channel 2, Cassette and speaker functions |
| 043h | 8253 | mode control (see 8253/8254) |
| 044h | 8254 | PS/2 extended timer (see 8253/8254) |
| 047h | 8254 | Channel 3 control byte |
| 060h | 8255 | Port A keyboard input/output buffer (output PCjr) |
| 061h | 8255 | Port B output |
| 062h | 8255 | Port C input |
| 063h | 8255 | Command/Mode control register |
| 060h | 8042 | Keyboard input/output buffer register |
| 061h | 8042 | system control port (for compatability with 8255) |
| 064h | 8042 | Keyboard command/status register |
| 070h | CMOS RAM/RTC, also NMI enable/disable (AT,PS/2, see RTC) |
| 071h | CMOS RAM data (AT,PS/2) |
| 080h | Manufacturer systems checkpoint port (used during POST) |
| 081h | High order 4 bits of DMA channel 2 address |
| 082h | High order 4 bits of DMA channel 3 address |
| 083h | High order 4 bits of DMA channel 1 address |
| 090h | Central arbitration control Port |
| 091h | Card selection feedback |
| 092h | System control and status register |
| 094h | System board enable/setup register |
| 095h | Reserved |
| 096h | Adapter enable/setup register |
| 097h | Reserved |
| 0A0h | | NMI Mask Register (PC,XT) (write 80h to enable NMI, 00h disable) |
| 0A0h | 8259 | Second 8259 Command port |
| 0A1h | 8259 | Second 8259 Interrupt mask register |
| 0C0h | TI SN76496 Programmable Tone/Noise Generator (PCjr) |
| 0C2h | DMA channel 3 selector (see ports 6 & 82) |
| 0F0h | Disk Controller |
| 0F2h | Disk Controller control port |
| 0F4h | Disk Controller status register |
| 0F5h | Disk Controller data port |
| 100h | POS Register 0, Adapter ID byte (LSB) |
| 101h | POS Register 1, Adapter ID byte (MSB) |
| 102h | POS Register 2, Option select data byte 1
* Bit 0 is card enable (CDEN) |
| 103h | POS Register 3, Option select data byte 2 |
| 104h | POS Register 4, Option select data byte 3 |
| 105h | POS Register 5, Option select data byte 4
* Bit 7 is (-CHCK)
* Bit 6 is reserved |
| 106h | POS Register 6, subaddress extension (LSB) |
| 107h | POS Register 7, subaddress extension (MSB) |
| 170h | disk 1 data |
| 171h | disk 1 error |
| 172h | disk 1 sector count |
| 173h | disk 1 sector number |
| 174h | disk 1 cylinder low |
| 175h | disk 1 cylinder high |
| 176h | disk 1 drive/head |
| 177h | disk 1 status |
| 1F0h | disk 0 data |
| 1F1h | disk 0 error |
| 1F2h | disk 0 sector count |
| 1F3h | disk 0 sector number |
| 1F4h | disk 0 cylinder low |
| 1F5h | disk 0 cylinder high |
| 1F6h | disk 0 drive/head |
| 1F7h | disk 0 status |
| 210h | write | latch expansion bus data
read: verify expansion bus data |
| 211h | write | clear wait,test latch
Read: MSB of data address |
| 212h | read | LSB of data address |
| 213h | write | 0=enable
1=/disable expansion unit |
| 214h-215h | | Receiver Card Ports |
| 214h | write | latch data |
| 214h | read | data |
| 215h | read | MSB of address,
next read: LSB of address |
| : | | |
| 21Fh | | Reserved |
| 278h | data port |
| 279h | status port |
| 27Ah | control port |
| 2E0h | Alternate EGA/VGA |
| 2E1h | GPIB Adapter (AT) |
Periscope hardware debugger
| 320h | | Read from/Write to controller |
| 321h | Read | Controller Status |
| | Write | controller reset |
| 322h | Write | generate controller select pulse |
| 323h | Write | Pattern to DMA and interrupt mask register (see ports 0F,21,C2) |
| 324h | | disk attention/status |
| 372h | Diskette digital output |
| 374h | Diskette controller status |
| 375h | Diskette controller data |
| 376h | Diskette controller data |
| 377h | Diskette digital input |
First Parallel Printer (see PARALLEL PORT)
| 378h | data port |
| 379h | status port |
| 37Ah | control port |
| 380h | On board 8255 | port A, internal/external sense |
| 381h | On board 8255 | port B, external modem interface |
| 382h | On board 8255 | port C, internal control and gating |
| 383h | On board 8255 | mode register |
| 384h | On board 8253 | channel square wave generator |
| 385h | On board 8253 | channel 1 inactivity time-out |
| 386h | On board 8253 | channel 2 inactivity time-out |
| 387h | On board 8253 | mode register |
| 388h | On board 8273 | read: status
Write: Command |
| 389h | On board 8273 | write: parameter
read: response |
| 38Ah | On board 8273 | transmit interrupt status |
| 38Bh | On board 8273 | receiver interrupt status |
| 38Ch | On board 8273 | data |
| 3A0h | On board 8255 | port A, internal/external sense |
| 3A1h | On board 8255 | port B, external modem interface |
| 3A2h | On board 8255 | port C, internal control and gating |
| 3A3h | On board 8255 | mode register |
| 3A4h | On board 8253 | counter 0 unused |
| 3A5h | On board 8253 | counter 1 inactivity time-outs |
| 3A6h | On board 8253 | counter 2 inactivity time-outs |
| 3A7h | On board 8253 | mode register |
| 3A8h | On board 8251 | data |
| 3A9h | On board 8251 | command/mode/status register |
| 3B0h | | port address decodes to 3B4 |
| 3B1h | | port address decodes to 3B5 |
| 3B2h | | port address decodes to 3B4 |
| 3B3h | | port address decodes to 3B5 |
| 3B4h | 6845 | index register, selects which register [0-11h] is to be accessed through port 3B5 |
| 3B5h | 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. |
| 3B6h | | port address decodes to 3B4 |
| 3B7h | | port address decodes to 3B5 |
| 3B8h | 6845 | Mode control register |
| 3B9h | | reserved for color select register on color adapter |
| 3BAh | | status register (read only) |
| 3BBh | | reserved for light pen strobe reset |
| 3BCh | parallel 1, data port |
| 3BDh | parallel 1, status port |
| 3BEh | parallel 1, control port |
| 3C0h | VGA attribute and sequencer register |
| 3C1h | Other video attributes |
| 3C2h | EGA, VGA, CGA input status 0 |
| 3C3h | Video subsystem enable |
| 3C4h | CGA, EGA, VGA sequencer index |
| 3C5h | CGA, EGA, VGA sequencer |
| 3C6h | VGA video DAC PEL mask |
| 3C7h | VGA video DAC state |
| 3C8h | VGA video DAC PEL address |
| 3C9h | VGA video DAC |
| 3CAh | VGA graphics 2 position |
| 3CCh | VGA graphics 1 position |
| 3CDh | VGA feature control |
| 3CEh | VGA graphics index |
| 3CFh | Other VGA graphics |
see EGA/VGA registers
ports 3D0h-3DBh are write only, see 6845
| 3D0h | 6845 | port address decodes to 3D4 | |
| 3D1h | 6845 | port address decodes to 3D5 | |
| 3D2h | 6845 | port address decodes to 3D4 | |
| 3D3h | 6845 | port address decodes to 3D5 | |
| 3D4h | 6845 | index register, selects which register [0-11h] is to be accessed through port 3D5 | |
| 3D5h | 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. | |
| 3D6h | 6845 | port address decodes to 3D4 | |
| 3D7h | 6845 | port address decodes to 3D5 | |
| 3D8h | 6845 | Mode control register | (CGA, EGA, VGA, except PCjr) |
| 3D9h | 6845 | color select palette register | (CGA, EGA, VGA) |
| 3DAh | 6845 | status register | (read only, PCjr VGA access) |
| 3DBh | 6845 | Clear light pen latch (any write) | |
| 3DCh | 6845 | Preset Light pen latch | |
| 3DFh | 6845 | CRT/CPU page register | (PCjr only) |
| 3F0h | Diskette controller status A |
| 3F1h | Diskette controller status B |
| 3F2h | controller control port |
| 3F4h | controller status register |
| 3F5h | data register (write 1-9 byte command, see INT13 ) |
| 3F6h | Diskette controller data |
| 3F7h | Diskette digital input |
| 3220h-3227h | 8250 UART | PS/2 COM3 |
| 3228h-322Fh | 8250 UART | PS/2 COM4 |
| 4220h-4227h | 8250 UART | PS/2 COM5 |
| 4228h-422Fh | 8250 UART | PS/2 COM6 |
| 5220h-5227h | 8250 UART | PS/2 COM7 |
| 5228h-522Fh | 8250 UART | PS/2 COM8 |
