Re: Devices using VGA Palette snooping?

["John R Pierce" <pierce@scruznet.com>]

Robert Elliott wrote...

> > > Can anyone identify some graphics devices which actually use the 
> > > PCI VGA Palette Snoop feature?   
> > 
> > Yeah, a whole pile of ISA video-in-a-window and capture cards that used the
> > original VGA Feature connector, such as the C&T "PC-TV" chips, the Sigma
> > Designs Reel Magic card, etc.
> > 
> > If used with a PCI VGA, the PCI VGA has to be put into snoop mode so that
the
> > RAMDAC writes make it thru to the duplicate RAMDAC on these ISA cards.
> 
> Are there any PCI cards or chips (excepting actual VGA graphics
> controllers) that use this feature, or is this just an ISA bus
> legacy support issue?  

As far as I know, no.  Everything I've seen that used the old VGA feature
connector has been a ISA card.
 
> If the system VGA controller lies behind a PCI-to-PCI bridge, then
> it seems like the ISA cards are out of luck.  The PCI-to-PCI
> bridge has to claim the palette cycles for the VGA controller, 
> preventing the ISA bridge from claiming them for the ISA card.
> The PCI-to-PCI bridge doesn't have a true "snoop" mode.

Yes, using snooping this way pretty much requires the VGA be on the primary PCI
bus and the 'other' ramdac be on the ISA bus connected thru that same PCI bus. 
It is a feature put in just for this specific situation, and is already become
obsolete as the VGA Feature connector is pretty much useless with most modern
video cards (it only has 8 bit digital video on it, so it doesn't work with
modern 32bit or 64bit ramdacs, and it never worked with video modes much over
800x600 256 color due to the excessive data rates).  Most newer cards
attempting to do the same thing use a external VGA analog passthru, its more
reliable and doesn't require 'shadowing' or 'snooping' the VGA ramdac palette
tables.

-jrp
Eˆ	v