On 11 Jul 98 at 0:10, Martin Irish wrote:

> At 10:45 AM 7/10/98 -0400, Robert Hale wrote:
>
> >believeit or not, it has COM1 IRQ4 and COM3 IRQ4 setup as defaults.  Looks
> >crazy to me seeing how those 2 IRQ's conflict, but that's the way it was
> >setup.
>
> FYI, COM1 and COM3 have always shared IRQ4, just like COM2 and COM4 have
> always shared IRQ3.  It's been that way since the invention of COM ports.

  Not quite that far back.

  The original PC BIOS reserved four memory locations for COM port
"base addresses", but none for IRQs.  IBM only defined the first two
addresses, and assigned IRQs 4 and 3 to them respectively.

  At some point, IBM internally defined addresses for COM3-COM8(!),
which were all supposed to share IRQ3 with COM2.  They just didn't
bother to TELL anybody this, and in the meantime makers of peripheral
cards started shipping boards that provided COM3 and COM4 with the
addresses and IRQs that the industry continues to regard as standard.
  Not only did IBM not tell anybody about their proposed addresses,
they also didn't read what anybody else was doing.  They went ahead
and created the 8514/A video adapter, using an address that conflicts
with the industry-standard range for COM4.  S3 chipsets and most ATI
video cards continue to include 8514/A emulation as a feature, and so
conflict with COM4.

  There was a long period during which BIOSes would only detect and
set up the first two COM ports.  If you search any shareware/freeware
archive from about ten years ago, you'll see lots of little programs
you can add to AUTOEXEC.BAT to detect and initialize COM3 and COM4.

David G