Introduction

The Intel 32-bit processors can access 65536 (16 bit) different I/O ports. The table below is some of the I/O ports used today. For a more detailed list download Ralf Brown's interrupt list.

I/O Port Deconding

On early PC system the system components only decoded the lowest 10 bit on the I/O port bus. Therefor the component could only differentiate 1024 different I/O ports (0000h to 0400h). Never system will do the full 16 bit decode, but the 10 bit decoding can still be a problem. When a system component only decode the lower 10 bits for an I/O port, it will positive decode 64 (the top 6 bits) different I/O ports in the full I/O range.

Fx. a system component with a port at 0021h, and doing 10 bit decoding, would react to the following I/O ports:


Where ? can be any value between 0 and F

Fixed I/O Ports

This is a list of the most generic and fixed I/O ports in the IBM PC:

I/O Port Ranges

This is a list of the I/O port ranges which can be used by adapters: