Yeah, video BIOS and drivers must have support for the high color RAMDACs. You may have to use a different video BIOS from a card that has a high color RAMDAC, and ideally using the RAMDAC you intend to use. They're not all compatible but there are not many different ways they do stuff.

EDIT: If I were to upgrade, I would find an example of a card with same chipset that already uses a high color RAMDAC, then buy the same type of chip and locate video BIOS dump of the card. That should provide highest chance for success. It is also possible that there's an external pixel clock generator chip, and those are also not all compatible and may present another level of complication.

Tiido wrote on 2024-11-15, 01:29:

Yeah, video BIOS and drivers must have support for the high color RAMDACs. You may have to use a different video BIOS from a card that has a high color RAMDAC, and ideally using the RAMDAC you intend to use. They're not all compatible but there are not many different ways they do stuff.

EDIT: If I were to upgrade, I would find an example of a card with same chipset that already uses a high color RAMDAC, then buy the same type of chip and locate video BIOS dump of the card. That should provide highest chance for success. It is also possible that there's an external pixel clock generator chip, and those are also not all compatible and may present another level of complication.

i have recently found this from vgadoc:

Apparently Trident BIOS version 3.xx or later on a 8900C will support Sierra HiColor DACs (SC11483 or SC11487). No check is made for the existence of such a DAC, the mode is just set as if it was present, resulting in 1024x480, 1280x480 and 1600x600 256color modes if an ordinary DAC is installed.

for me, i have never seen any 8900c/cl with hicolor ramdacs, and i doubt if there were any. only newer 8900d are equipped with tkd8001 truecolor ramdac.
anyone interested to give it a try?

please move this topic to the video forum...

It would most likely need some video BIOS hacking, unless your video BIOS already allows you to set 16bpp modes (which would display incorrect colors with an 8-bit DAC). Although I wonder if UniVBE has some way of automatically detecting the type of DAC.