Re: [9fans] OT: supporting multiple VGA cards (was "A shot in the dark")

["ron minnich" <rminnich@gmail.com>]

On Wed, May 28, 2008 at 11:34 AM, Digby Tarvin <digbyt@acm.org> wrote:
>
> I am interested in connecting more than one graphics adapter so that
> one can be dedicated to low level diagnostics (rather than the more
> usual dual head user interface).. but I am not sure how the hardware
> deals with that given that the VGA standard seems to imply that all
> the cards would be using identical memory addresses and IO ports.

There are Rules such that only one terminal gets to own the "legacy" ports.

> Otherwise I would expect PCI to be able to automatically
> map resources to non-conflicting addresses/IO ports.

They do. It's just the legacy stuff that gets weird.

>
> In any case my BIOS has a setting which allows selection between AGP and
> PCI as 'primary' Video, suggesting that it is indeed possible to have at
> least one of each.

it is.

>
> The only thing I can think of is that perhaps the VGA/CGA/EGA etc modes
> are suppressed in the non-primary card, leaving only the more advanced
> and less standard mode available. But this would be less ideal, as text
> mode would be more useful for low overhead diagnostics.

if you can convince the OS to use the non-legacy addresses, you could
configure BIOS to put your desired debug terminal at legacy addresses
and use it for debug.

ron