On 4/3/99 Tommy Holmes, Jr. wrote on PCBUILD:
>
>. . . .  However, if the comm port were disabled at its
>physical address, then would it not remain disabled when one returned to
>install the modem.  Does not the CMOS instruction actually disable the
>external serial jack physically connected to the comm port and otherwise
>logically entitled to this address?  If that be the case, it certainly is
>easier for us to conceptually understand how this onboard comm port then
>might be available for the modem installation having been relieved of the
>customary duty of serving the external serial port.

Tom:

A serial device will require a COM port, which is
merely a designated "device" to the operating system.
System resources are IRQ (Interrupt ReQuest channels),
I/O (Input/Output) port addresses (memory locations,
where the CPU communicates with devices), and DMA
("Direct Memory Access" lines, wherein devices can
read/write to RAM without CPU involvement)

Serial devices only use IRQs and I/O port addresses.
There are "standard" resource allocations to each
COM port, 1-4, as Jim Killmaster has outlined. It
is possible to use non-standard IRQs and I/O port
addresses for each COM port but this is not advisable.

The COM port(s) on the motherboard can be disabled
or may be assigned to whichever resources are designed
in the on-board chipset(s). If the resource is not used,
then another device can use the COM port designation
(COM #), the IRQ, and the I/O port address.

If the device is an internal modem, you might disable
COM1, IRQ4, I/O port 3F8 on the motherboard and set
the modem for those values.

Add-on cards, such as modems, other serial cards,
etc., can use these resources SO LONG AS nothing
is assigned the same COM port # or I/O address. Serial
devices can SHARE the same IRQ, as long as they are not
USED at the same time.

A PCI modem is unique because the PCI bus can dynamically
allocate resources to adaptor cards, as a Plug and Play
feature on a modern motherboard pursuant to the PnP OS or
the programming of the chipset. Usually, the order of the
PCI card installation from the PCI bridge will be the
order of priority of resource assignment. Whatever values
are assigned, either by the OS or the PCI chipset or both,
these values can be saved although, sometimes, upon bootup,
you may discover a "new" re-allocation of resources which
is contrary to the last "saved" settings. Check your CMOS
and Device Manager in Win9x to reconcile settings.

If you wish to deviate from the default resource allocation,
you can go into PCI/PnP settings in CMOS and MANUALLY set
the use of each IRQ from PCI (dynamic) to LEGACY. That will
(hopefully) preserve those IRQs for legacy devices, and the
PCI bus will not dynamically allocate those IRQs among PCI
devices.

I hope this suffices for a concise explanation. If you want
the long story I recommend:

    "IRQ, DMA & I/O" by Jim Aspinwall (MIS:Press 3d Ed.)
and
    "I/O Subsystems" (check out the ADAPTEC website)

But usually one learns from lots of trial and error.

Best,

John Chin
Adjunct Professor Engineering - MDCC
http://profchin.virtualave.net/

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