> It goes like this.
>
> By default (in software and therefore to some extent hardware),  COM1 is
> always on IRQ4, COM2 is always on IRQ3.  Then if you add a COM3 it
> piggybacks onto IRQ4 and COM4 piggybacks onto IRQ3.
>
> Many of the Com and modem boards cannot accept any other assignment for
> IRQs.  There are however four default address groups assigned for the four
> COM ports.

There are two things that may help those who have a modem and a mouse, and
wish to hook up a PIC as well:

[1] For serial transmission, and for low-speed reception, interrupts are
    not needed.  Note that if you do not use interrupts you must be sure
    to poll the FIFO often enough that you do not miss any characters; on
    a 16C550 this may not be much of an issue, but on the single-FIFO
    units it often helps if your transmitting device doesn't send the data
    too fast [a high baud rate is okay if there is space between the char-
    acters or if it's running in a character-by-character handshaking mode].

[2] On many older multi-I/O cards, it's possible to configure the serial
    port as IRQ5 even if the board was not particularly designed for such.
    In particular, a lot of the older boards have two sets of jumpers that
    look like this:

           3 IRQ 4
  COM-A :  *  *  *
  COM-B :  *  *  *

     and this:

           5 IRQ 7
  LPT:     *  *  *

    On many of these boards, running a wire from the top center pin on the
  former connector to the left pin on the latter will assign COM port A to
  IRQ5.  Unfortunately, many of the newer "high-integration" boards don't
  work with this technique, but I've used 3 com ports on a number of PC's
  without particular difficulty.