Dual VGA in consumer II using ISA VGA

pehite · ‎2006-03-08

Posted on March 08, 2006 at 19:31

pehite · ‎2006-01-12

Posted on January 12, 2006 at 23:50

I would like to use DOS to switch between VGA adapters - the internal one and an external ISA adapter. The programming manual implies that this can be done in the VGA section saying:

''If an external add-in VGA card is placed in the system, the on-chip VGA controller can be disabled in order

to work with this external card. It is possible to enable / disable the system back to dual use VGA controller

if necessary.''

Has anyone done this or give me any information as how to do it. I have had no luck with the registers so far.

Thanks!!

marios · ‎2006-03-08

Posted on March 08, 2006 at 19:31

If the STPC Industrial VGA controller is similar, at index 29H there is the VGA decode register. This is what is used by the BIOS to enable or disable the VGA subsystem. The BIOS also has to disable the decoding of the local VGA BIOS portion, in order to use the BIOS of the add-in VGA card.

Then there are the general VGA registers, where you can enable or disable accessses to the I/Os and memory of the VGA subsystem.

Running on two screens is easier said than done, as the BIOS will only initialize one VGA subsystem at power on. I can not think of an easy way, such as a BIOS call, to make it initialize the second one.