Ports 000H-0ffH are reserved for and used by the system board
  Ports 100H-3ffH are decoded on the I/O bus for use by adapter cards
  Ports 400H and up are not available on the system bus

The following table summarizes I/O port usage for the PC/XT and AT.
Individual ports covered in greater detail are marked with a hyperlink.
Additional info is covered in I/O Port Detail.  See Cables and Pin Outs
for info on hardware connectors.

AT      PC/XT   Description                              More Information
                                                                             
000-01f  000-00f  DMA controller #1, 8237A-5.              DMA Ports
020-03f  020-021  Interrupt controller, 8259A.             Int Controller
040-05f  040-043  Timer 8253-5 (AT: 8254.2).               Timer Ports
060-063  PPI (Programmable Peripheral Interface)  PPI Ports
keyboard, equipment switches, timer
060-06f           Keyboard controller 8042                 AT Keyboard
070-07f           Real-time clock, CMOS Memory, NMI mask   CMOS Memory
080               manufacture's diagnostic checkpoint
080-09f  080-083  DMA page register 74LS612                DMA Ports
0a0      NMI (Non-maskable Interrupt) mask
0a0-0bf           Interrupt Controller #2, 8259A
0c0-0df           DMA controller #2, 8237A-5               DMA Ports
0f0-0ff           Math Coprocessor

102               PS/2 Programmable Option Select

170-177           Hard disk (secondary)                    AT Hard Disk Ports
1f0-1f7           Hard disk                                AT Hard Disk Ports
200-207  200-20f  Game I/O (joystick A/D port)             Joystick Port
210-217  Expansion Box
278-27f  278-27f  Parallel Printer #2 (#3 with MDA)        Printer Ports
2c0-2df  2c0-2df  EGA #2                                   EGA I/O Ports
2e8-2ef           Asynchronous Adapter Port #4             Serial Ports
2f8-2ff  2f8-2ff  Asynchronous Adapter Port #2             Serial Ports
300-31f           Prototype card
320-32f  Hard disk                                XT Hard Disk Ports
370-377           Diskette Controller (secondary)          FDC Ports
378-37f  378-37f  Parallel Printer #1 (#2 with MDA)        Printer Ports
380-38f  380-38f  Synchronous Data Link Control (SDLC)
3a0-3af  3a0-3a9  Bisynchronous port #1

3b0-3df  3b0-3df  Video Graphics Array (VGA)               VGA I/O Ports
3b0-3bf  3b0-3bf  Monochrome Display Adapter/Printer #1    Printer Ports
3c0-3cf  3c0-3cf  Enhanced Graphics Adapter (EGA) #1       EGA I/O Ports
3d0-3df  3d0-3df  Color/Graphics Adapter (CGA) and EGA     CGA I/O Ports

3f0-3f7  3f0-3f7  Diskette controller                      FDC Ports
3ef-3ff           Asynchronous Adapter Port #3             Serial Ports
3f8-3ff  3f8-3ff  Asynchronous Adapter Port #1             Serial Ports

See Also: I/O Port Detail
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I/O Port Map