Many devices have a concept of ``I/O space''. In general, multiple devices cannot share the same range of I/O ports. Unfortunately, there are a few exceptions, most notably the keyboard and PS/2 mouse, and the Floppy and IDE controllers.
Many of the device drivers assume they may access port 0x80, for use in timing loops. This is not used in most computers, although POST cards are used to display the last value written to that port.