
Not completly true.  For AT class and later machines, IRQ5 is
reserved for LPT2.  Since it's rare to have a second parallel
port in a PC, it's usually a good safe choice if you need an
interrupt.

On the other hand, we just ran into a problem with that here
at work on a Gateway computer (4DX-33V).  It has a Modem on COM1,
a Mouse on COM2, and the other serial port was set to COM3 (which
normally uses the same interrupt as COM1).  We had a real fight
with a board when trying to use IRQ5, and discoverd the problem
was that Gateway had set it up such that COM3 used IRQ5.  As soon
as we disabled COM3, our problems went away.  Grumble ... after
several days of trying to figure out why the interrupt didn't work.