On 23 Feb 99, 22:00 Paul B. Webster VK2BZC wrote: > Mark Willis wrote: > > > Some VGA cards REALLY don't like COM4 using IRQ02/09 much, so mainly > > I use just 3 COM ports. > I don't think VGA cards use IRQ2/9. I think it is a problem with port > address $2E8. I'm still using this lightning-struck motherboard on > which any attempt to use COM3 *or* COM4 generates events on them > (presumably writes to the primary port address) in time with cursor or > mouse action on the screen. If memory servers me right, these are two independant problems. Long ago, at CGA times, IRQ2 was the "vertical retrace interrupt". For that reason, most later cards implemented this for compatibility reasons, but allowed disabling that interrupt (or had it disabled by default), because very few programs needed it, and because of the interrupt shortage. COM4 is someting completely different. The original 8514 graphics adapter made by IBM, and its much better known enhanced clones, for example the "accelearated" video cards made by S3 unfortunately used the very same IO address that was allocated by default for the fourth serial line, COM4. Or more precise, there is a conflict if the serial adapter only decodes the lowest 12 bits of the ea address, and so responds to every address whcih ends in 2e8. -- o ( Wolfgang.Strobl@gmd.de (+49 2241) 14-2394 /\ * GMD mbH #include _`\ `_<=== Schloss Birlinghoven, __(_)/_(_)___.-._ 53754 Sankt Augustin, Germany ________________