🚀 go-pugleaf

>>>>> On Tue, 27 Oct 1992 16:09:57 GMT, MSUORANTA@finabo.abo.fi (Mika Suoranta Tkkk) said:
Mika>         Does anyone know how one could possibly have
Mika>         more than 4 Com-ports in one computer? I've an 286
Mika>         machine running under Ms-dos 5.0 and as far as I
Mika>         understand the operating system sets the limit.
Mika>  
Mika>         Could there be some special device driver which would
Mika>         permit more ports in expansion cards? I don't quite
Mika>         like the idea of a big switch  for some external
Mika>         devices -  even if I don't have to have all of them
Mika>         connected and running the 

BIOS and DOS have limit to 4 ports, because their I/O addresses
are stored at 0x40:0 to 7 (4 words) and there is no space for more.
But I suppose not every COM port is to be serviced by BIOS or DOS,
for example mouse driver doesn't use any of them to access port,
it must access hardware directly. Can consider the following:
load mouse driver from a batch file, which contains a command,
which modifies COM1 port address at 0x40:0 (use DEBUG or write
a small program to store the mouse port address there), then
driver is loaded and it can get the port address (I suppose driver
will save the address inside it to avoid reaching so far from its
code), then modify COM port addresses at 0x40:0..7 again to get
there all addresses besides this used by mouse.
Possibly mouse driver is not the only software which bypasses BIOS.

A problem may be interrupt numbers, but COM interrupts are not used
by BIOS. But: seems printer is the only device which can work
without interrupts. Since configuring the interrupt numbers may be
most hard problem (or configuring software to share COM interrupts).